a manual for conservation - p2 infohouse · a manual for conservation , sri nc department of...

ENERGY PREVENTIVE MAINTENANCE COST REDUCTION PROGRAM

ERS A MANUAL FOR CONSERVATION

ver 2,000 North Carolina 0 Maintenance and Engineering people have attended and profited from these workshops in the last 60 months and have benefited from over $10,000,000/year in recommended savings.

PRESENTEDBY

ENERGY DIVISION, NORTH CAROLINA DEPARTMENT OF COMMERCE

INDUSTRIAL EXTENSION SERVICE NORTH CAROLINA STATE UNIVERSITY

EMA I WITH -I I-- m IU ===-=-E =E North Carolina Department of Commerce

- - - -- - -

CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation -- 3 Y

NC Department of Commerce

ENERGY DIVISION

NORTH CAROLINA DEPARTMENT OF COMMERCE

Histo y and Purpose

The Energy Division, which was created as a permanent state agency in 1974, is charged with:

Developing emergency plans to handle sudden energy shortages;

Maintaining energy consumption data and making projections for future energy use;

Providing staff support to the Energy Policy Council, which develops and recommends state energy policy for the effective management and use of current and future sources of energy; and

Developing and coordinating energy conservation programs.

t ,] Energy Conservation Programs

The Institutional Consemation Program provides 50 percent matching grants to schools and not-for-profit hospitals for energy audits, technical assistance and other energy-saving improvements in buildings constructed prior to April 1977. To date, the Energy Division has awarded 468 grants totaling $21.7 million.

The Low-Income Weatherization Program provides for the purchase and installation of insulation, weatherstripping, caulking, storm windows and doors, and other energy conservation materials in homes in which the total household income is at or below 150 percent of the poverty level. Since the program's inception in 1977, more than 54,000 homes have been weatherized.

The State Energy Conservation Program and Energy Extension Service give personalized informa tion and technical assistance to develop specialized energy conservation programs for small-scale energy users in various sectors. Some specific programs include:

J (more)

NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

DNiSbll

Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation

NC Department of Commerca

Residential

The North Carolina Solar Center at North Carolina State University which distributes information on solar energy; and

A pilot project providing rebates to residential customers who purchase energy efficient central heating and cooling equipment.

Aericul tura 1

The distribution of information on energy efficient techniques farmers can use to cool products after harvest; and

Energy audits and evaluation of irrigation systems.

Transuortat ion

Two-day car care clinics to help motorists improve their cars’ engine performance; and

Traffic signal timing analysis designed to reduce traffic delay and congestion, vehicle emissions, and unnecessary fuel consumption.

Education

Assisting local school boards identify and select energy efficient building designs; and

Developing a system to computerize the scheduling and routing of school buses.

Local Government

Seminars to help local governments identify and implement solid waste management options as an alternative to landfills;

Technical assistance in establishing energy accounting systems and building audits aimed at identifying no- and low-cost measures to reduce energy consumption in public buildings; and

Development of a manual of North Carolina governmental units that have successfully utilized available alternative rates to trim their electrical costs.

(more)

- NORTH CAROLINA STATE UNWERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Division Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation


Commercial

Rebates for relamping and replacement of inefficient lighting sources in commercial facilities; and

Bulletins outlining recent technological innovations by high energy using commercial enterprises, primarily hotels/motels, laundries and dry cleaners, restaurants, and retail stores.

Jndustr ial

Rebates for relamping and replacement of inefficient lighting sources in commercial facilities;

Technical assistance in preventive maintenance techniques; and

Workshops on measuring and improving boiler efficiency.

3 - NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

TABLE OF CONTENTS

INTRODUCTION

Chapter 1. CHILLERS

Chapter 2. MAINTENANCE

Chapter 3.

Chapter 4. CHILLED WATER SYSTEMS

Chapter 5.

Chapter 6. COOLING TOWER PIPING

Chapter 7. COST OF OWNERSHIP

CHILLER OPTIMIZATION AND FREE COOLING

AIR HANDLERS / COOLING COILS

QiILLElcs AM) COOLING TOWERS WORKSHOP A Manual fir Conservation

Nc DeparDnent of commerce

INTRODUCTION

As technology and industry change at an ever increasing rate, we are doing more, I

faster and with less new materials. Of all the resources necessary to continue this industrial transition, energy is the most critical. Without an adequate supply of energy, all the raw materials in the world are of little value. To insure that we can continue to have the energy resources available to us, it is imperative that we understand how each energy consuming system in our facilities operates. It is only through a thorough understanding of how we use energy that we can develop plans and strategies that will minimize our energy usage.

It is the intent of this workshop to provide you with an understanding of one of the majorusers of energy in a facility today - chiller and cooling tower systems. Specifically, water-cooled chillers and their supporting equipment are the focus of the workshop material. With this information, you will be able to determine if your chiller plant, cooling tower and other equipment are operating effectively and efficiently. In addition, energy conserving strategies will be discussed that will help you in developing plans to minimize energy use and operating costs.

NORTH CAROLINA STATE UNIVERSlTY - WllW THE NORlH CAROLINA ENERGY DIVISION

Introduction - 1

w i Energy CHnLm AND COOLING TOWERS WORKSHOP A Manual for Consemtion

NC D e m t of Commerce

REDUCING ENERGY CONSUMPTION

CHILLERS AND WATER TOWERS

1 NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DnrrSaON

Introduction - 2

Chapter I

CHILLERS

3

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Division CHILLERS AND COOLING TOWERS WORKSHOP Eiferav A Manual for Conservation , s r i


CHILLERS

NOTES

A water-cooled chiller is a mechanical device that uses a centrifugal impeller to compress refrigerant gas, and thus the use of the refrigerant to produce cold water.

The motor-driven impeller imparts its energy to the refrigerant gas, and compressing the gas produces heat. The hot compressed gas is then cooled by water flowing through the condenser, generally from a cooling tower. In this press it gives up its heat and condenses to a liquid. This warm liquid then passes through a metering device into the evaporator. The evaporator is at a lower pressure. The liquid absorbs heat from the water passing through the evaporator, thus cooling the water and evaporating the refrigerant. The low pressure gas passes into the compressor through the compressor inlet vanes and starts the cycle again.

A centrifugal chiller has a centrifugal compressor, a condenser, evaporator, compressor inlet control device, and an evaporator liquid refrigerant control device. The performance of a chiller is impacted by:

e

e

e

e

e

e

0

e

e

e

e

efficiency and horsepower of the motor, design of the evaporator and the condenser, number of passes in the condenser and the evaporator, quantity, thickness and type of tubes, refrigerant levels, tube enhancement, operating capacity of the chiller, condition of the water flowing through the chiller, condition of the heat exchanger ,surfaces, temperature of the condenser and the chilled water, and type of refrigerant used.

NORTH CAROLINA STATE UMVERSlTY - WlTH ‘IHE NORTH CAROLINA ENERGY DIVISION

Chapter 1-1

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation


The condenser and the evaporator are shell and tube heat exchangers. The cooled water is then pumped through the chiller water piping system and is used for cooling air and for process cooling.

The capacity of the chiller is controlled by the inlet vanes to the compressor. They restrict the flow of refrigerant gas to the compressor, thereby controlling the compressor capacity.

The compressor motor under a restricted flow of refrigerant gas does not have as much work to do and uses less energy at lower load conditions (Figures 1-1 through 1-41.

NOTES

NORTH CAROLlNA STATE UMVERsITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-2

CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation

NC Department oi Commerce

CENTRAVAC OPERATING CYCLE: @ Liquid refrigerant is distributed along the length of the evaporator and uniformly sprayed through orifices to coat each evaporator tube with refrigerant. The refrigerant absorbs heat from system water being circulated through the evaporator tubes and changes to a gas. @ Refrierant gas flows through the droplet eliminators to the compressor. 0 Refrigerant vapor is drawn through the first-stage inlet vanes and into the centrifugal compressor impeller. The inlet vanes modulate the flow of gas to match system capacity requirements and also prerotates the gas to enter the impeller wheel at the proper angle, minimizing losses at all load conditions. 0 Discharge gas from the first compressor stage flows through the second-stage inlet vanes and into the second-stage impeller. @ Compressed refrigerant gas from the second stage is introduced into the condenser and distributed across the condenser tubes. Condenser water circulating through the condenser tubes removes heat from the refrigerant causing it to condense. @ Liquid refrigerant passes through the patented Trane orifice system to the economizer. Some of the refrigerant flashes to gas as a result of the pressure drop across the orifice system, coding the remainder of the liquid refrigerant. The flash gas is drawn directly to the second stage of the compressor. This reduces the refrigeration cycle energy requirements by avoiding the necessity of compressing all of the refrigerant gas through two stages of compression. The remaining liquid flows through additional orifices to the evaporator section of the machine

v;& f l pd$ ,;;..,$.e-

% y s&’\cl Gd’YA ’ 4U.L”” p o d / pJC

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Figure 1-1

NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-3

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t=¶ :w: NC Department of Commerce

CHnLERS AND COOUNG TOWERS WORKSHOP A Manual for Conservation

Fnarm3.f 391

Inaddition tothrottling, inlstguide vanes at each stage prerotate refrigerant gas for more efficient entry into the impeller. This results in improved part-load performanca and in increased compressor range.

F

Figure 1-2.

first stage inlet vanes control the flow of refrigerant gas to the centrifugal compressor.

These Vanes can control the flow from 10 to lOO?! of the gas flow.

I N L R GUIDE VANE

Figure 1-3.

NORTH CAROLlNA STATE UrJrVERSlTY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 1-4

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Compressor Operation Schematic

- ... -1

Figure 1-4.

NORTH CAROIJNA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

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Division CHILLERS AND COOLlNC TOWERS WORKSHOP A Manual for Conservation


WATER FLOW

The amount of water circulated through a chiller is generally constant. The design temperature rise in the condenser water and the chilled water are normally loo F. Occasionally the design of a system may require a greater or smaller temperature rise, or delta T (AT).

The design inlet and outlet temperatures of a chiller are important. The chiller is designed around these parameters. Any change from these parameters effects the operation of the chiller mechanically and impact the efficiency of the chiller.

Condenser water design inlet temperature to the chiller is normally 8 5 O F and a discharge temperature of 9 5 O F. The chilled water leaving temperature can be from 420 F to SO0 F with a 100 F AT. In most cases, less than 420 F design is imprudent because it is possible to get a cold spot in the chiller evaporator where ice might form. Ice in a chiller is usually disastrous. A chiller can be designed to operate with a glycol solution to prevent freezing in the evaporator.

One ton of air conditioning is 12,000 BTU. The specific heat of water is 1: The weight of water is 8.33 pounds per gallon. One gallon per minute circulated with one degree rise equals 500 BTU per hour (10 AT x 8.33 lbs x 60 min = 500).

Therefore, BTU = gpm x temp diff x 500 12,000 BTU = 2.4 gpm x loo F x 500

12,000 BTU @ 100 F temp rise is 2.4 gpm circulated per ton of air conditioning

So it is necessary to circulate 2.4 gpm through the evaporator constantly for each ton of cooling at a loo F temperature drop. A 500- ton chiller requires 1200 gpm water circulating rate.

NOTES

f

NORTH CAROIJNA STATE UNlVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-6

Division Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conscroution


The design of the chiller will determine the pressure drop of water through the heat exchangers - normally between 3 and 20 psi (or 6.9 ft and 48 ft of heat).

In the design of a chiller, manufacturers use minimum and maximum velocities with the heat exchangers. If the velocity is too high, the water will erode the tubes. If it is too slow, there will be poor heat transfer.

The selection of the condenser and the evaporator are a compromise of cost and design efficiency. The most efficient condenser and evaporator may have a high cost and require a relatively high waterside pressure drop. A less efficient selection may cost less but still have a relatively high pressure drop. A third design may be more expensive but have a lower pressure drop (Figure 1-8).

The cost of operating the system will be affected by the selection

i

of the various components. If the pressure drop through the condenser, for example, can be reduced, the reduced pressure through the heat exchangers affects the pump motor horsepower. A reduction in static pressure is the relationship of AP* = BHP3.

If the pressure drop through a chiller could be reduced 50 percent, the BHP to drive the water through the chiller would be reduced to 25 percent.

1 1 As 1 is to 2 is to 4, (T)* = .25, ( T ) ~ = .125

If a 20-ft heat required 5 brake HP,

20 R A P = 5 BHP 10 FT'AP = 1.25 BHP

NOTES

NORTH CAROIJNA STATE Uh'lVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-7

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CHnLERS A Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC Department of Commerce

So the selection of heat exchangers must be a compromise of efficiency versus cost of opera tion.

Figures 1.5 and 1.6 show the relationship of some different condensers and evaporators which are available for similar sized units but with different pressure drops and operating characteristics.

COMPRESSOR MOTORS PRODUCE HEAT

In most centrifugal chillers the motor is cooled by refrigerant passing through the windings.

The compressor motor as well as the bearings add heat to the refrigeration cycle which must be allowed for. The result is that in designing water-cooled equipment, the condenser water flow is calculated at 15,000 BTU per ton and requires 3 gpm circulated to provide a 100 F temperature rise across the condenser (Figures 1-7 and 1-8). For this reason cooling towers are rated at 15,000 BTU per ton.

One of the first ways that manufacturers used to reduce the KW per ton in 1975-1980 was to remove the compressor motor from the refrigerant circuit and go to an open drive. This arrangement required a seal for the drive to enter the sealed refrigerant system.

TUBE ENHANCEMENT

The efficiency of the thermal transfer through a chiller tube is a function of the turbulence or velocity of the water through the tube and the amount of surface available for contact with the medium being heated and the medium providing the heat. For some time prior to 1975, chiller tubes were enhanced on the refrigerant side.

NOTES

? NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 1-8

Division



The tubes had very fine fins machined into their exterior surfaces, greatly increasing the square inches of surface available for contact to the refrigerant in rela'tionship to the inside surface of the tube. This is referred to as "tube enhancement". In an effort to further improve heat transfer and efficiency and to allow for fewer tubes in heat exchangers, manufacturers now will provide in temally enhanced tubes.

The tube is rifled internally in a spiral fashion to increase surface area, turbulence and actual water velocity on the tube surface. There was a considerable amount of resistance to this procedure. It was felt that internal enhancement would cause premature tube fouling. However, experience has not borne this out, nor is this type of tube any more difficult to clean.

CHILLER PERFORMANCE ..J

As we have indicated, most chillers are designed for about a 100 F differential between the inlet and outlet of the condenser and the evaporator. When a system is designed, the engineer will select a chilled water discharge temperature based on the use of the system. For the purpose of this discussion, we will use 450 F leaving chilled water temperature.

Once a chiller is designed and manufactured, the temperature differences, inlet and outlet temperatures ut full caaucity are constant.

If the chiller performance or use is to be modified, it is wise to consult the manufacturer before making any changes.

3

NOTES

NORTH CAROLINA STATE UNIVERSITY - w r r ~ THE NORTH CAROLINA ENERGY DMSION

Chapter 1-9

Division Energy C H n L E R S AND COOLING TOWERS WORKSHOP A Manual fir Consewation

CHART 45-1 Extended L e n p Condensers E31 Through J1L CHART 45-2 Evaporators K1 through S1 NC Department of Commerce

FLOW mN)

1 0 0 90 80 70 Bo 50

40

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w 8.0 5 7.0

6.0

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ginb$& Gb a@kqw*mnts am den0t.d as tollom

Figures 1-5 UL-1 OnPurE3LCon6.mr -2 Two PY E3S C o n d w i ~ €3-3 Thry P.u E3 Ev.gora1w

%+

ROLINA STATE UMVERSlTY - WITH THE NOmH CAROLINA ENERGY DIVISION

Chapter 1-10

DNlSbtl

A CHILLERS Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC Department of Commerce

UNISHELL CONDENSER UNISHELL COOLER FLOW RATE ( L I S ) FLOW RATE (LIS1

FLOW RATE (GPM I 001

-3

P . . . . . . . , . . . , . . . , . . ?o 40 60 80 0 P 40 60 80

ISQ

m

50

0 4 0 I2

NOTES: 1. Solid lines show pressure drop with standard Water box. Dashed lines show pressure dro with marine water box option. 2. Cooler pressure drop is based on ULTC tubing. Condenser pressure drop is based on TZTS tubing. 3. To determine pressure drops more aCCUrately and to COmpenSate for actual water temperature, use the computerized selection sewice

availabk through your loul U r r W NIn o f f i i .

NORTH CAROLINA STATE UNIVERSrrY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-11



Hermetic Chiller Motor. The Trane Centravac Hermetic Motor is cooled with liquid refrigerant which flows through passages around the motor windings. The motor bearings are lubricated and cooled with oil. The oil is pumped to the bearings and is cooled by a water- cooled heat exchanger.

Figure 1-7.

NORTH CAROLINA STATE UNIVERSITY - WlTH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-12

‘‘I

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CHILLERS A Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC Department of Commerce

Figure 1-8. Carrier Corporation manufactures both hermetic and open drive chillers.

Fig. 1-8 Open Drive Chiller Motor Separated from the Refrigerant Cyde



CHILLER DESIGN KW PER TON

Up until 1975, most chillers were designed for .85 KW per ton. Then came the oil crisis and the cost of energy rose dramatically. From 1980-1985 we saw KW of .70 to .75 per ton, and now the designs are in the .60-.65 KW per ton range. Chiller design is now approaching the maximum theoretical efficiency for centrifugal chillers of .579 KW per ton.

HOW HAS CHILLER PERFORMANCE BEEN IMPROVED SO DRAMATICALLY?

There are several things that were done. The performances of heat exchangers were improved through tube enhancement and water flow velocity. The exteriors of the tubes have been finned for years; now the interiors, the water side of the tubes have also been enhanced with grooves to increase the surface area and to improve the flow of heat through the tubes. It is usual to have internally enhanced tubes in new chillers now. The design of compressors has been improved to provide more capacity at lower motor horsepower (Figure 1-1 2).

The design of condensers and evaporators has been improved to add to the overall operating efficiency.

The flow of refrigerant through the chillers has been improved to reduce restrictions in chillers, improve fluid flow, reduce pressure drops, and decrease energy losses.

The design of motors has been improved to increase their efficiency (see pages 16 and 17 for a discussion of design versus performance efficiency).

NOTES

~ - _ _ NOR77 I CAROI.INA STATE UMVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 1-14

Division



Figure 1-12. The Trane Three-Stage Direct Drive Chiller has been designed to improve the chiller Kw per ton. The chiller performance is in the .6Q to .65 Kw per ton range. The chiller provides much quieter and smoother operation than has been available from the Tram Company in prior models. The chillers have solid state controls and are available with unit mounted starters.


Chapter 1-15

Division CFULLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation

Em&=- NC Department of Commerce

The CenTraVac" Chiller Engineering Story

Low Kw P u Ton - Roducod Enorgy cortr Just u a multi stage turbine is more efficient than a singb stage turbine, the CenTraVac three-stage compressor is more efficient than single-stage designs This higher efficiency produces savings in direct t onorgy costs for typical applications of 5 to 20%. Consider a typical 320-ton a p p l i i t h . Cataloged kw per ton at ARI cocrditi is ranges from .63 to .67, an efficiency increase of 5 to 15.5% wor previously available chilkn. Cataloged ratings for tvpical 5oo-ton capacity chillers, at ARI c o n d i i , range from .6l to .64 Irw per ton. These are typical - even mon efficient selections are possible. Thw st8g.r of compmuion The thW-8-e c ~ m p r ~ w -rates mom efficiently over a wide range of caprcitias, virhrrlly eliminating the need for w r g y wasting hot gas by- as typically found on single stage chillem. Adrigemnt gas leaves the impeller at an angle having tangential and radial The tangential component (V,) is determined by impeller tip speed, apd the radial component (VJ is determined by volume of gas flow and c t o u sectional area of the impeller discharge. The three-stage design takes advantage of lower tip s p e d to discharge the gas with u large a radial component as possible.

The radial componont of velocity determines the ability of the chiller to resist intemption of smooth refrigerant flow when oponting at light loads and with h q h condensing temperatures. This interruption in flow and unstable operation, called "surge" is avoided with the threo-stage dosign.

Thm sot^ of InM Guido VIM Part load performance is further improved through use of carefully designed variabb inlet guide vanes. Inlet guide vanes improve performance two wayr: 1) by throttling refrigerant gas fiow to exactly meet part load mquircmwnts and 2) by premtating r ~ m t gas for optimum entry into tho impeller. Prerotation of refrigemnt g u minimizas turbulence and inc" efficiency. nlvo-stag. E c o " k 8 r The CVHE ch i l l r also has a two-stage ecconomizer - providing up to wen percent greater o f f i c i i than designs with no economizer. Since the CVHE uses three impellem it is possible to flash refrigerant gas at two intermediate pressures between the evaporator and condenser pressures, signifmntly increasing chiller efficiency. This improvement in efficiency is not possible in . single-rtage chillers since all comprouion is done by one impeller. oknt Dlhro b i g n - Noowrl.oaaa The dinct drive compressor operates without speed increasing gears, thus eliminating gear energy losses. Comprosoor8 wing gears suffer mesh loss08 and extra bearing losses in the nng. of t h m to five percent at full load. Since these losses are fairh/ constant over the load range, increasingly larger percentage losses result as load decreases. For a typical building load profile, power consumed by gear losses can be considerably greater than the thrqe to five pdrcent at full load tona

Dosign Simplktty Impellers are keyed directly to the motor shaft for high reliability and performaw and low l ie cycle costa Rolkblo Motor Cooling Tho motor is engulfed in liquid ntrig.rant to w i d e efficient, complote d i n g at all load Canditiona Refrigerant is delivered to the motor from the liquid sump on the condensor through a fixed orifice system. Reffrig.ront is then mumed to the two-staga ownomiter through a gravity drain. This system is reliable and easy to maintain.

F l X . d o l i ( k . F k W C W t t d For proper d r ig " t flow control at all load conditiorw, the CenTraVac design incorpontes the Tram patented fixed orifice system. It eliminates float valves, thermat expbcrsion valves and other moving pa- A series of exprnsion and c o n t n c t h C h u n h effmiv* controls tho flow of refrigerant in the compreuor, condenser and evaporator to procirely meet all cooling lord conditions. As system cooling load d o c " , liquid refrigerant flow to the condenser is reduced. This h e m the hydrostatic head, causing more refrigerant to vaporize in the downstream chamber and reducing the amount of refrigerant entering the evaporator. This smaller amount of liquid refrigerant in the evaporator matches the cooling load requirements for efficient operation. Since there are no moving parts, reliability is increased.


Chapter 1-16

CHILLERS AND COOUNC TOWERS WORKSHOP A Manual for Cascwafion

"3 *i.tOp.ntion With only one moving component - the rotor and impeller assembly - the Trane low speed, direct dnve design operates exceptionally quietly. Gear driven compressors, by contrast may generate objectionable noiw in their high S& drive train. The smoothly rotating CenTraVac compressor IS inherently quieter than other compressor types. Typical CenTraVac chiller sound ratings arg krr than 80 dBA, measured according to ARI Standard 575. Cow R.uun R-11 Rdrigonnt for Hlgh Hfickncy and Rollablo

All CVHE and CVHB CenTraVac chiikn use safe, efficient R-1 1 nfrigerant allowing a number of kmficil faarwec A-11 is well suited to the Tram kw-spoed, direct drive design. It's mora efficient than hgh density, high p " ~ r e mfrigerants

cunpnrror dmign has inherently highor cycb efficiency compared to other chiller &signs.

p d d d R-11 in Btu's per pound +> o f n f r i g . n n t r " b m r a n t

is mquind t o produce a given capacity. Again, R-11 is a better d u b for grwter m n c y . Sinco krr mfrigmnt is mquind, nfrigerant replacement costs are less expensive than for hgh preuure chilhn. And sinca R-11 is liquid at atmospheric pc8uum, ku of R-1 1 is less likely to occur.

Tha A-1 1 mutti-stage CenTraVac

.Thr large r ~ n t i o n effect

ConlhVat P-H Diagram The preuureanthalphy fP-HI diagram describer refrigerant flow through the major CVHE chiller components. This diagram confirms the superior operating cycle efficiency of the three-stage compressor and two-stage economizer. Evaporator - A liquidgar refrigerant mixture enters the evaporator at state point 1. Liquid refrigerant is vaporized to state point 2 as it absorbs heat from the system cooling load. The vaporized refrigerant then flowr into the

Compressor First Stage - Refrigerant gas is drawn from the evaporator into the first stage compressor. The first stage impeller accelerates the gas increasing its temperature and pressure to m t e point 3.

Refrigerant gas leaving the fim stage compressor is mixed with coder drigerant gar from the low pressure side of the two-stage economizer. This mixing lowen the enthalpy of the mixtum entering the second stage. The second stage impeller accelerates the gas, further increasing its temperature and promure t o stare point 4. Camp" Third Stage - Refrigerant gas leaving the comprerun second stage is mixed with cooler refrigorant gas from the high pnrurre ride of the two-stage economizer. This mixing lowers the enthalpy of the gas mixture entering the third stage compomor. The third stage impeller accelerates the gas, further increasing its temperature

compmssor first stage.

c o m p r ~ secorrd Stage -

Nc Department of commerce

and pressure to state point 5, then discharges it to the condenser.

Condenser - Refrigerant gas entem the condenser where the system cooling load and heat of compression are rejected to the condenser water circuit. This heat rejection cools and condenses the refrigerant gas to a liquid at state point 6. Patented T w o - S t e Economizer and Refrigerant Orifice System-Liquid refrigerant leaving the condensor at state point 6 flows through the first d c e and enten the high pressure side of the e c " u e r . The purpose of this orifice and economizer is to prefiash a small amount of refrigerant at an intermediate pressure cslled Pl . P1 is between the evaporator and condenser pressures. Proflashing some liquid refrigerant cools the remaining liquid to state point 7. Refrigerant W i the first stage economizer flows through the ldcond orifice and enters the second stage economizer. Some refrigerant is proflashed at i n t e r m d i e pressure P2. Froflashing the liquid refrigerant cools the remaining liquid to state point 8. Another benefit of preflrshing refrigerant is to increase the total evaporator mfngeration effect from R E to RE. The two-stage economizer provides a seven percent energy savings compared to chillers with no economizer. To complete the operating cycle, liquid refrigerant leaving the economizer at state point 8 flows through a third orifice system. Here. refrigerant pressure and temperature are reduced to evaporator conditions at state point 1.

NOWW CAROLINA STATE UMVERSITY - WKH THE NOKTH CAROLINA ENERCY DMSION

Chapter 1-17

Division CHILLERS AND COOLlNG TOWERS WORKSHOP A Manual for Conseruution


The search for chiller efficiencv J

Available options to increase the efficiency cf cent rifugal packaged chillers

By WILLIAM J. LANDMAN, Manager. Applications Engineering. The "kme Co., Le Crosse, Wis.

In 1972, the average cataloged Kwper ton value of the most popular hermetic centrifugal water chillers was 0.817. A recent catalog shows an average value of 0.699. This change represents an efficiency increase of over 14 percent.

This significant change brings three questions to mind:

Why did this change occur now?

How are higher efficiencies being achieved?

0 What are the efficiency limits? Answers to the first question are

x3

x 2

x 1.5

Base surface

0 6 0 7 08 09 Chiller KW per ton*

*Based on constant mechanical efficiency of

Motor 93 percent Transmission 97 percent Compressor 75 percent

45195 F terminal water'temperatures foi evaporator and condenser

both technical and economic. rich- nology has always had the capability to improve chiller efficiency. However, only economics can un- leash technology so that it can perform. In this case , economic changes are driven by rapid increases in energy costs. These costs are increasing faster than hardware costs. Consequently, it is cost effective to fund technology in hopes of improving performance.

Any analysis of chiller efficiency quickly focuses on the four key in- gredients that influence machine performance:

1) Water-to-refrigerant heat transfer efficiency.

2) Refrigerant cycle efficiency. 3) Drive train efficiency. 4) Compressor efficiency. No other factor has any effect on

overall chiller efficiency. Obviously, then, any improvement in KW per ton values at any load must occur in one or more of these four categories.

Advances in technology improve efficiency in two basic ways. Firstly, they provide better heat transfer techniques and surfaces. This area has the greatest potential. Sec- ondly, the mechanics of com- pressible fluid flow are receiving close scrutiny. In this case, the potential for improvement, while real, is relatively small.

Heat t ransfer The technology of heat transfer

has been especially active over the past 10 years. Still, the funda- mentals have not changed. The classic heat transfer equation remains:

where Q = L'A(1T)

1 Surface area vs chiller K W per ton.

180 160

x 140

=: 120 100

s 80 xo

60

h LL

+ - -

3 4 5 6 7 6 9 10 11 Water velocity. fps I

~~

2 Typical values of Uc based on optimistic heat transfer efficiencies.

Q = the rate-quantity of heat transferred

U = the overall heat transfer coefficient

A = the heat transfer surface area

A7' = the "log mean temperature difference"btween the two materials

The goal is to improve the efficiency of this process. This is ac- complished by decreasing AT for a given Q. This can only be achieved by increasing either A, U, or both. Area is increased by simply 'throw- ing moneyn a t the problem. As more surface is added, diminishing effectiveness occurs. The plot in Fig. 1 shows this relationship. Curve A is based on a Ybase surface" performance of a 10 F approach between the leaving water and the saturated refrigerant temperature. Curve B uses an 8 F approach temperature base surface. As surface is increased to 300 percent of the original value, the chiller K W per ton value decreases. Clearly, the additional surface results in very little efficiency improvement at this point.

Bchnology, then, can do some-

)

Chapter 1-18

3

CHILLERS AND COOlJhC TOWERS WORKSHOP e E! 2'2 ! q J y A Manual for Conservation Nc Department of commerce

The search for chi l ler eff ic iency

thing about U. The overall heat transfer coefficient is comprised of four individual factors. See the ac- companying sidebar for an explanation.

Technology can improve two of the factors shown in the sidebar. Work has recently focused on h,and h,, the fluid film heat transfer coefficients. T h e term h, is helped by "external tube enhance- ments." Essentially, all manufacturers employ 'high flux tubes." This is another way of describing something better than conventional extruded integrally finned tubing. The term h,, being primarily a physical property of water, is at the mercy of velocity. High velocity helps h, but eats up water pumping power. Recent 'internal tube en- hancements" are simply configurations to the normally smooth tube bore. They boost velocity at the expense d increased pressure loss just as turbulators d o in water coils. While this'works,"an equally effective strategy might be to use a greater number of waterside passes to increase velocity.

Combining these improvements, Fig. 2 shows typical values of U. based on optimistic heat transfer efficiencies. All that is needed to find the refrigerant temperature is the equivalent external tube surface ana. To produce the chart Shawn in

'Ihble 1, the equation shown in the sidebar is used to solve for LMTD. Refrigerant temperatures relate to LMTD as follows:

LMTD - (GTD - LTDV In (GTDILTD)

where GTD = the greater tempera-

LTD = the lesser tempera-

This is pictured in Fig. 3. The lesser tempera ture difference (LTD) is sometimes referred to as the 'approach" temperature. With infinite heat transfer surface, this value would approach zero.

The water-to-refrigerant heat transfer efficiency is now defined. We can calculate real numbers

ture difference

ture difference

based on the most optimistic technical information available.

Refr igerant cycle The second category is the refrig-

erant cycle efficiency. It is a physical property of the refrigerant itself. Table 2 shows relative theoretical KW per ton values for various refrig- erants commonly used in centrifugal chillers. These values are based on data from Table 8, Chapter 16, ASHRAE Handbook, 1931 f inda- mentals. Once the saturated refrigerant temperatures are established, the refrigerant efficiency is fixed. The only variables that can change the cycle efficiency are cycle configurations such as condenser sub- coolers and economizers (interstage flash coolers). Subcoolen are seldom used with water cooled centrif- ugals because their effectiveness is limited. Their additional expense would be more effectively applied to increase condenser surface. Econo- mizers are confined to chillers using

multistage compressors because they are used to separate saturated gas and liquid at the interstage pressures.

Fig. 4 shows a typical refrigerant pressure-enthalpy chart for a centrifugal chiller. The theoretical KW per ton value is calculated on the basis that compression occurs is- entropically (without a change in gas entropy, S). The heat content of compressor discharge (/a2) can be calculated from t h e t h e r m o - dynamic properties of any refrigerant. The theoretical KW per ton value is found from:

Kwlton [ (h , - hJ1

The term 3.517 converts the units of enthalpy (Btu per Ib) to KW per ton by:

(hi - h,)] X 3.517

(12,000 Btu per ton-hr) X ( ~ ~ / 3 4 1 3 Btu) = 3.517 Fig. 5 is a plot ofthe results of this

calculation for various Refrigerant 11 suction and condensing temperatures. Since R-11 is the most effi-

Heatmg/Piping/Air Conditioning July 1983

- NORTI 1 CAROI.INA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 1-19

Division E F9 iE 9-q a 1

i L I 9 - 4 r CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation “ - 1


2 H W

cient refrigerant in common use, Fig, 5 shows the lowest theoretical cycle KW per ton values we can expect to see.

Drive t ra in The third category, drive train ef-

ficiency, involves all of the energy conversions between the incoming power wiring and the rotating compressor shaft. Fig. 6 shows a representation of the various elements and their power losses.

Motor starters generally lose very little power. Resistance heating of the wiring and contactors is min- imal. Total losses are well under 1 percent of the input electrical power. Solid state devices are sometimes part of the drive train. Solid state reduced voltage starters pass essentially the complete wave form in the “run” mode. Therefore, very little loss occurs. Total losses are about the same as mechanical starters.

Solid state frequency inverters

3 1.6 15.0 10.0 124 9.677 44 40.25 24 10.0 10.0 124 9.677 44 38.4 4.8 5.0 10.0 124 9.677 44 36.61

2 3.0 10.0 10.0 161 9.317 95 100.20 1 6.0 5.0 10.0 161 9.317 95 102.04

*Calculated lrom Fa. 2 at Uton = 10.

Condensing temperature I

Evaporating temperature

Heat transfer distance (inlet tooutlet)

3 kfrlgerant temperature vs. heat transfer distance.

T a w P-ReIathe theoretical KW per ton values for various m- friaerants.

Theoretiul. KW per ton values

No OW Two 7F Refrigerant economuer economizer economucrs Subcooling

113 0.405 0.466 0.451 0.472 11 0.463 0.444 0.431 0.453

114 0.671 0.644 0.624 0.644 12 0.492 0.472 0.458 0.477 22 0.507 0.487 0.472 0.491

-----

*Based on saturated suctm and condensing temperatures of 40 F and 100 F respectively.

Enthalpy (h). Btu per Ib I 4 Ressure-enthalpy chart for a centrlfugal chiller.

NORTH CAROLINA STATE UMVERSlTY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 1-20

CHILLERS AND COOLrNC TOWERS WORKSHOP A Manual for Conseroation

T h e search for chiller efficiency

I

L windage I seals

C o o l q Elecfrlcal rys!rm losses Eearlngs

and sezls

NC Deoaitment of Commerce

mission efficiency usually ranges between 95 and 97 percent. O&n, the compressor shaft bearing m d seal losses are covered in the b ~ . mission efficiency.

Present day technology can pro. duce a drive train efficiency as high as 95 percent. The other end of this range could be as low as 85 percent, depending on the configuration and the selection &components. Future technological improvementa are unlikely to improve significantly the highest efficiency that is achievable today. Instead, primary effort is focused on using the configurations and components tha t reach the highest overall efficiency. Compressor

The final energy converter is the compressor itself. The conversion process involves the basic dynamics d turbomachinery. The ability d a compressor to make this conversion eficiently embraces complex a” binations d technology and experience. NASA, for example, haa con- d u c t e d extensive s t u d i e s on compressor performance. Ita work indicates a maximum state-d-theart efficiency dabout 86 percent for centrifugal compressors that operate a t compression ration d3.5 to 1. Multistate compressors, for thia total pressure ratio, can achieve 88 percent efficiency. Neither d these values, haarwer, includes any rys- tem loeeee. The chiller system lames absorb power from the compressor by transporting gas and liquid through the cycle.

llansporation losses include the energy needed to m& refrigerant from one place to another. Further, system lasses due to imperfect flow patterns and spurious heat transfer and mixing must be acknawledged. W e n together, the miscellaneous loseee amount to between 1 and 2 percent d compressor input -1. It is a relatively constant loss a t all loads.

Compressors rarely operate a t their maximum efficiency point at design air conditioning conditions. Thus, the actual observed compressor efficiency will fall several points short ofthe calculated “sweet

NORTH CAROLINA S T A T UNlVERsITY - WITH THE NORTH CAROLINA ENUlCY DMSION

Chapter 1-21

Dwision

Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conseruation


spot"va1uee. According to the ASH- RAE Handbook, 1979 Equipment, Chapter 12, the adiabatic efficiency of centrifugal compressors generally varies from 62 to 83 percent.

NASA studies forecast an eventual theoretical maximum compressor efficiency between 90 and 92 percent for our pressure range. It doesn't tell us how to achieve this or when it will occur, if ever. Summary

The following simple equation accounta for the total chiller system KW per ton value:

K W per ton = (theoretical is- e n t r o p i c K W p e r t o n Val- ue) X ( l /d r ive t r a in eff ic iency) X I/compreseor efficiency)

ARI Standard 550-77 specifies 44 and 95 F as the leaving chilled and condenser water temperatures for rating purposes. A minimum KW per ton value can be calculated by using this developed data as follows:

Step 1-Assuming a surface-to- capacity ratio d 10 sq ft per ton d refrigeration, the saturated suction and condensing temperaturee are 38.48 F and 100.2 F.

Step 2-At these saturated refrigerant conditions, the isentropic K W p e r ton value is 0.4925. A double economizer (three-stage compressor) would reduce this by 7 percent to 0.458 KW per ton.

Step 3-A direct drive hermetic chiller could demonstrate the highest drive train efficiency of 95 percent.

Step 4-A multistage compressor could show a 'sweet spot" efficiency d about 88 percent, lesa 2 percent for transporation and labyrinth seal IOSSeS.

S t e p 5-KW p e r t o n = (0.458) X (1b.95) X (1/0.86) = 0.561.

Obviously, we do not see KW per ton values this low quoted in cata- logs. Only an extraordinary combination of machine components could be put together to achieve this efficiency. However, this gives some indication of the theoretical potential for efficient performance. Fur- ther, we now have a way to measure the future's latent capability. Q

Heating/Piping/Air Conditioning July 1983

NORTH CAROLINA STATE UNIVERSITY - W THE NORTH CAROLINA ENERGY DIVISION

Chapter 1-22

Chapter 2

MAINTENANCE

3

--)

3

"'3

Division CHILLERS A N D COOLING TOWERS WORKSHOP A Manual for Conservation

E p a rr-re \ I L S SGPgy NC Department of Cornmerca

MAINTENANCE // I\

IMPROVING CHILLER PERFORMANCE!{),

'* Some of you may be the proud owners of .60 KW/torl,chille$s,

but the majority of you have the .85 to.90 KW/ton machL9. Si ce most chillers operate in excess of 30 years, there will be a lot 07% KW chillers around for a number of years. These are the less efficient workhorses of industry.

i

How can we improve the performance of these chillers? As we consider improving them, we should also consider how we can maintain the performance of the newer units as well as the older machines. The key word is MAINTENANCE.

Preventive maintenance is the cheapest way to reduce the operating cost of a chiller, and it is the only way to keep a chiller system efficient after an energy plan has been implemented.

Chillers are prone to tube fouling, the buildup of dirt or calcium on tubes. Chillers also lose capacity because of low or reduced water flow.

The most effective way to save money is with good maintenance and the opposite is also true. The easiest way to waste money is with poor maintenance.

The first step in maintaining a chiller is having an accurate and timely log. A log is a record of tests taken periodically on the chiller that indicate its performance and operating condition.

Some of the tests recorded are inlet and outlet water temperatures, condenser and evaporator pressures, motor operating amps, operating capacity, ambient temperature, and any addition information you feel is pertinent to your chiller (see Figure 2-1).

NOTES


Chapter 2-1



Generally, each manufacturer has a sample log sheet in the startup instructions. You can obtain a sample from their nearest office.

A handy item to include on your log sheet is a list of normal readings for comparison purposes. These readings can be taken off the startup log on the chiller.

This will give the technician who is recording the log a reference point to know when something may be going wrong or if he should recalibrate the measurement devices. It is very important to use calibrated thermometers and pressure gauges.

One of the most reliable ways to take water pressure drops is to use a single pressure gauge on a manifold as shown in Figure 2-2.

Since the same gauge is used for both readings, the calibration of the gauge is not as important, so long as it is functional and repeatable.

NOTES


Chapter 2-2

Division


NC Department of Co"erce

TRANE DATE LOG

UNIT # PLANT

-3 NORTH CAROLMA S A T E UMVERSlTY - WITH THE NOmH CAROLINA ENERGY DIVISION

Chapter 2-3

CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consemation LI 5-5


Fig. 2-2

PRESSURE

DISCHARGE SUPPLY WATER CON~€EC?-lON

NORTH CAROLINA STATE UNIVERSITY - WlTH THE NORTH CAROLINA ENERGY DIVISION

Chapter 2-4

Division

CHILLERS A Manual A N D for COOLING Consernation TOWERS WORKSHOP Energy NC Department of Commerce

CONDENSER MAINTENANCE

The cost of cleaning a chiller condenser depends a great deal on how well the water treatment is maintained. In many areas, dirt can be a major contributor to fouling condensers and in some cases silicate sand is a major contributor to the problem.

When dirt is a contributor to the annual cost of cleaning condensers, it is also causing wasted energy. A bypass filter may be the answer. A bypass filter will remove solids from your condenser water system.

A bypass filter can be automated to back flush dirt out of the system. Most tower manufacturers will provide these as an option. It will be worth checking into if dirt is part of your chiller maintenance problem.

The chiller is such an integral and important part of your operation that it requires good periodic maintenance, both preventive and predictive. Sometimes it is difficult to get technicians to think of themselves as preventive maintenance people rather than a s service technicians who repair things after they break.

Reliable efficient chiller opera tion requires preventive and predictive maintenance.

Any discussion of increasing the efficiency of or decreasing the energy usage of a chiller must start with a properly performing machine. You cannot optimize a system that is sick (see References 2-1,2-2, and 2-31,

NOTES

NORTH CAROLINA STATE UMVERSlTY - WITH THE NORTH CAROLINA ENERGY DIVISION '

Chapter 2-5

Division

CHILLERS A Manual AND for COOLING Conservation TOWERS WORKSHOP Energy Nc Department of commerce

14 -r 11,1989 Ai r Condrfrorrry, Hairry & Re/rycrmloi

Corrosion, scak a d fouling

Operating and maintaining a refrigerated facility by Jamm MaW&

Them'r an old adage. "!t takes The differenca in electrical money to d e money. Wh.t potential lets a cumnt p u r h' thir b v e to &I mth ra ter through the metal from OIU point trmtment? The o d y muon for to another. "ha poinu ua ullod rater treatment uta u v e money. a common cell. They have two Tbw, it mha mow (buying r a . a n u : tha an& and the cathode tar trutment b m i a l ) to nuke (tha "elam" parr). mow (uw 011 water -1. Tha an& ia tha of lower

If we k l i a w h t . by +ding potmud. when m o d iotu will g0 m o ~ c y up h n t wa rill n v e in rb. into lolution whon an aloctmlyt.

ir prrunt. The uthodr is the m a of higher potential.

El.euonr flow from tha anodic ana to the I-tivs uthodic area. The anodic and cathodic mu rhrR w w n t l y , w cormion spnadr evanly over the mtim metal rurt~w. The extent of cor. rooion is the direct m u l t of the metah ionr and elactronr. which u n trawl thmunh d. md the

a compkdy diUmnt aubjcf om- riding d "impingement" and "urit.riop" we rill not diwusl it bere.)

&de and f o a l 4 SCALE u s &Me. white. ult-

like coating 011 the h u t cruder rurtaca. It co- ouinly dim. p n i c InAtwiaL La., uleium ur-

magnesium ult~. FOULING u tb ununul.tion

ofwli& otbrr thanomlo. It QP ba dirt udt or und r" th. .it. Q c O " i 0 n ~ A n d " a - gmirmr. i.s .. dpo,,fu~@. rad butaria. b f i c " F m t b u n uuy mrc.l QlRQiop It J.0 can attack tb mod in cooling towam.

9ulr and fouling inum with t h . h t t n d U ~ i n L 7 a m

PUnrP bawpaa mui- me* and rdua rb. Ibr d d - ing wataT in or - tb bnt trudu rurf.or, th illdgatiag mor, Dede aod/ahhll#. Aa little Y a 0.1-in "g ddcium

fer surface a n rrLsa tho but dm much m 40%.

What C.UII d e ?

M e u u d by opmpouod. ~minamla u l d ulwm the cooling water that brcoma boluble - thy prripit.t. oucdth rdution - at a h i n " n t m t e levels, tempatrtum. .Ird PH

T b t m o l t c o m U K m & f ~ m i ~ m l b ulcillm a r b o ~ w ccaco3. TbL mianl. in e** 001's -tar mppiy. is mhtod to Lbr hudnm d r.tcr (the .mount at ulcium d o r myncrium mine& prarnt).

We mad this hardness as CaCO, por milligram por liter (m(/L). or purr pr million (ppm). depotding on rho u doing the Ueing. Mat mca-nttQ kiu mad ppm; laboratmy testing is uaually mgL.

A proper water rule-pmvention program can work only when the budncv lave1 nrmi~ within a prwcribed mop. Tbe range will depend on tempemtw. p R the amount of wale-fornag minerals. Tempcratura and pH can cunderobly deet ruL for. mation on heat e x b . w equip ment.

We nlbte rater pH to it. 'ah- linity" r h n talking .bout b..t- meat. The alkalinity d W L d 1 . r and mar CMlbd-thr

bOM.1.. and Pb0lph.t.. d

&MU On* h.t

Ref, 2-1

"ENERGY-CONSCIOUS" OX-

chmgrr.

upper p.rt d tbc pH d. me Wioi ty numbadtbe w . b rc lates .Iluliw (b.u) m i n e d to a point whore "Is in the water precipitate out and farm d on the equipment.

Wa know that at a d i n level of l"a (CaCO,). at a given tempomtun d alkalinity. d e will form due to water evmpom- tion. For exynplr, u the d k d b t y

number m a m a , tba C&O,'a lolubility drauwr. and the =a- tar lorer itl ability to hold the prorant amount. When t h e amount of CaCO, e x c w d ~ i t s u t w t i o n point. d e will rrdt - with or without rater beat- ment.

It is dm imparrnt b under. a n d that, if alkalinity b m a roo low. it incnvcrthc c h u m for wrmion. Wa mud maintain a happy medium.

F e r FOULING u M rrumulation d

mlid material, other than d e . that deemmer the efficiency and/or life span of hest rejection quipment. Tbe mort common foulant. M rilf und , corrosion byproductr, and biological micmorgmirmr.

BidoqLil fouling of tower water iscommon in 111 putr ofthe wun- try. It dtl trom growth of lower

and bcLcria. A tower water rys. tem is an i d 4 u e a for powth of

fOrmr ofplant life - algae. fun@.

th.w o r p l h 6 .


Chapter 2-6

r)

J

CHILLERS AND COOLIh'C TOWERS WORKSHOP A Manual for Conscruation . .

We QII mbmt tbr larvr life f amr by treating the water with a microbiocide. Th key to su- is a rlow rate of fecd and longer coatact time.

Switching types of microbiocidc far a faater rate of kill ia important. When looking for a miaobi- ocide. shop for a wide kill spc. trum. uuble at a wide pH range .ad. hopefully. low in cost.

Cooling towerr and evaporative condensers are the best air. wuhing rystems money can buy. k air paosa through the tower, it is ruhd cloan of all put~culatc matter. "hie wouldn't be so bad if wo h d a once-through system.

However. if the water is recircu- Iatad. it becomes m open. recir. d a t i n g water ryrtem. Whatever caoling rater doesn't evaporate trpr recirculating: particulate matter thus bocomes concern trued. Ifdlowed. waterborne pu- W a t e rcttlos on oquipment sur- f.cw in tho sump, and will do a11 rortr of temble things to the sys- 1.m. To provent &is, um n o d to treat

tbs tower rater system with a C b . W dlprrunt. alro known u a drporit inhibitor. Thu chem- icaakould nat be wrd in phcr of a aideat" wring ayatetn

N&o C h d d dorribcr tho cbrmiul u a chuge-miafolcc moat and rr#inq agent. "he CbrgW0inlarrnvntdirpavntr Q.pI klUhwtonp61 ow mother by incroaring tho electrical c b w u the^ an^. k n p atatrhNlpmion.pmcW &om brom nttlM on motd sur- Lac Tadirpaunt rlro ia .wetting

ylrrt; it IMkOa tho r a m "rot- tr" by ducing d a c e tomion. 7&h hop tho puticloa in the kiL rator flow, to bo mmovod Ihm the w m through blow. dmrn (blood) or filtration. The blood ia r c t iva td by totrl dic dvod solids.

cOacenb.tion cy& We r u d cycla d-ntration

(COCtuTDS,orbycoodminty. Candwtivity u a quantitative ex. m n d m aqcuour rolution'r JUIw topwrkcrr*:nr. d w Wtbs ~IWOM ddholvod dids.

COC is the ratio of the makeup to the point at which we

umt to maintain TDS in the mol- iag tm.r sump. We maintain thu b e l by activating the bleed.

bt'r find the COC by using a TDS motor. Cbiago. for eumple. uy W e Michigan water, otuch h u a TDS reading of between 100 and 150 ppm. If *re maintain a TDS reading of 450 ppm in the 4 - r -tar mmp. m a d a 150 ppm rnding in tho nul.up rater. m d d b v o a COC of 3.

All rater h u diU0lv.d solids. Cooling torerr and evaporative eondenrcrr use water M the oml- ing media by evaporation. Olrly pure water - YO - is evapo- rated off, leaving behind the TDS. All water solutions hold a spccfic amount of diruolved solids before they precipitate out of solution. and rtvL to foul and .cnle the sys tem.

Stick to basics To rum it up. treat the areas

that may p v e problems. The cor. mion proeers, for example. must be interfered with or slowed down with an inhibitor that deals dimctly with either the anode or uthode. Then treat for rule. Check ti&

to -ine the water'r hudnes~. then maintain an alkalinity level that will not scale the system. or eat it away from too low of M alkalinity. Then troat for fouling. Tbir is

done by using a chemical diaper- unt and maintaining the TDS Ievel in the coohg rater sump. Remember, TDS ia dl the imsu- rities or soli& diaaolved in the water.

We aood to maintain a e m i n pH and temperature range, depending on the d i ~ ~ l v d .nd a w pendad soli& in the water. To do this, we nod to take into aumunt the quality of water makeup and the pH. Know what you have to work with, and get a water treat. ment company that believes in educating itr Eultomers.

I " m e n d that you obtain the following booka and make them put of your pemnal library. The Noln, Water H o d b o o k .

M&w Hill, 1979 Drew Princi- p k 8 of ~ndwbrol Wokr Treat- ment. H."oada McEntyn & MumuL lac., 1981; urd solrrtionr b Boiler h Cooling Water R o b lema. The F.irmont b, IM., 1986.

Division Energy NC Department oCCommerce

NOTES


Chapter 2-7

Division CHILLERS AND COOLING TOWERS WORKSHOP C n a u a p I--icd-# A Manual for Conservation - - w g


WINTER INSPECTION Pressure test for leaks. Repair leaks Detailed inspection of purge system and thorough cleaning of purge compressor, purge oil separator, purge drum, and purge condensing coil. Change purge oil as required. Check condition of contacts for wear, pitting, etc. Check and calibrate safety controls. Meg compressor motor and oil pump motor. Record readings. Check dash-pot oil in main starter, tighten all starter terminals and check contacts for wear. Check overloads. Calibrate safety controls.

Ref. 2-2

NOTES

i

NORTH CAROLINA STATE UNIVERSlTY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 2-8

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Casenation

NC Department o ~ & " e r c e

EDDY CURRENT TUBE ANALYSIS

3

What Is Eddy Cumnt Tuba A ~ . l y . l ~ ? A n0ndestruct)Vs method of tssting heat exchange tuber In Centrifugal and & sorption chNkm. Over a period of time verious operating condltlons can cause damage and wtmequent failure of these tubesthroughwear,vibratkn,erosion and amosion. If undetected, the damage CM lead to wrkrw breakdowm Involving not only the t u b , but boarings, motor windings, etc. Bemwe of these inherent rbk there k addlnlte need to know as much about tube QondltloM BI possible in order to propdy pkn your preventive maintenance.

HOW Wlll Eddy Cumnt Tuba AM- S w o You MoImJ? Since the Eddy Cumnt tube analysis is P d O m 3 d byquaHfbd technicians, the analysiis can detect hazardous conditions before they becomo critical and cause failure. This procedure can reduce repair costs and virtually eliminate downtime due lo tube failures.

Tho analysis permits tubes to be removed from service or replaced before a costly fWjOf failure can mur . A failure which could indve complete replacement of refrigerant or tho Pbsorbent sabtions. not to mention the d k.rings and motor windlgr.

What Type of Tube Defects Can Eddy Current Tube Anatpi8 Oetoct?

lnrlck Munetor Pfttlng CIUud by Emion and/or Cor" of tho M e r SI&

StrouConooion Cnck Causod by8 ComMmUon of TukStrou and a

Fatlour Cncklng Cauwd by Tub Aexlng or V l h t b n Duo to 1mpmp.r

Fnur Cracks cluwd by Wator Froulna In tho TLk..

Surtoco Du. to Acldlc Water.

Cormah Envlronmont.

0P.nting Condltlons.

Who b OurlW to Porfom Eddy Cumnt Tub Analysis?

Brady Tmne Services Eddy Current tube analysis is a sophistic nondestructive testing technique which should only be performed by a qualified technician. The Trane comp.nY has the latest stateof- theart equipment and mom importantly, a full time staff of qualified technicians who can properly interpret the test results. Tram's Eddy Current technicians can detect tube problems, measure their severity and determine their cause. Equipped with thii accurate information Srady Tram Sewices wili work with you Io det"inowh.1 course of action is best forywrbuilding.

For proper preventive maintenance of Centrifugal and Absorption units, we recommend that the tubes be tested a minimum of every three to five @am.

Where severe problems exist in a unit. more frequent t u b analysis intervals may be required for proper preventive maintenance.

Occasionally. maintenance checks may reveal indications of tube problems, in which case an immedliafe fesi shoulb be 8ChOdUl.d.

Ref. 2-3 NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 2-9

3

Chapter 3

. -1 ..*

CHILLER OPTIMIZATION

AND FREE COOLING

CHILLERS AND COOLING T O W WORKSHOP A Manual for Conservation


c CHILLER OPTIMIZATION AND FREE COOLING

NOTES

A rule of thumb used with chiller optimization is that a reduction of 1.5 percent of operating KW is achieved with each degree of discharge water temperature reset. Therefore, if the design for a chiller is 450 F leaving water temperature and 550 F return water temperature, a reduction of 1.5 percent in energy usage can be obtained by operating with a 460 F leaving water temperature.

If the chiller plant is used for comfort cooling, temperature reset should be a viable option.

For the purpose of this discussion, we are going to use a typical system application.

A. Assume a system design load of 200 tons.

8. A chiller plant with a 200-ton chiller.

C A load composed of 80% building load and 20% intemal heat gain.

D. A 200 F design temperature differential for chilled water and condenser water.

Based on this example, the only time that 200 percent of the one chiller capacity is required is when the outdoor temperature is at design conditions of 950 F dry bulb, 780 F wet bulb, and the building is occupied.

Since this occurs about 10 percent of the year, the balance of the operating hours the unit is running at, less than full capacity.

3 NORTH CAROLINA STATE UNIVERSlTY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 3-1



Let’s assume, for this discussion, that on May 15th the load is 100 tons, or 50 percent of the system design. Since, as mentioned earlier, the normal design system Delta T (AT) is 100 F, then the unit operating a t 450 F discharge water temperature would have a return water temperature of 50° F.

In this situation the chiller could operate at 500 F discharge and 55O F return. The energy savings would be approximately 7.5 percent.

This method of operation may not be available to many users for a variety of reasons. One of the most prevalent reasons given by physical plant personnel is that the facility requires 450 F water all of the time because one or more areas in the facility cannot maintain temperature of humidity control with a higher discharge water temperature.

This, unfortunately, is a common problem. The most undersized piece of equipment drives the discharge temperature of the chiller plant. This most likely is an area with a high internal load or a room that is being used for a purpose other than it was originally intended.

In some severe cases, the load can be as small as 5 percent of the total building load, and the chiller plant operates for extended periods of time with little or no load in cool months.

A possible solution to this problem is the introduction of an auxiliary chiller for this load, or retrofitting the area with a large air handler to better match the load.

If the load is year-round, i! small reciprocating chiller for light load conditions might be part of the solution.

NOTES

NORTH CAROLINA STATE UNIVERSITY - WlTH THE NOFTH CAROLINA ENERGY DIVISION

Chapter 3-2

Division CHILLERS AND COOLING TOWERS WORKSHOP L: E v-q% I A Manual for Consewation L:


AUTOMATION

Most chillers are designed with discharge temperature control which does not lend itself to temperature reset. The use of DDC controls is required for discharge temperature control. Many chilled water systems are now operated by DDC control systems. The use of discharge temperature reset should be investigated for these systems.

OPERATING KW

Assuming now that you have maintained your machine, that it is clean, and that it is operating reliably and properly, we can analyze reducing the energy usage of your chiller plant.

The chart, CHILLER OPERATION ANALYSIS, (Reference 3-11 will be used in developing a typical chiller optimization analysis. This information can be used to provide payback information for return on investment calculations and for energy usage reduction informa tion.

First, analyze the hours of operation with the CHILLER OPERATION ANALYSIS chart.

How many hours per day, week and year does your chiller plant operate?

What impacts the operation?

What affects the operation?

3

In many cases outdoor ambient condition is a prime mover, In a knitting mill the internal heat gain is the major consideration, and the use of good economizer is very important inmost mills, while in many office buildings sun load and ambient tempgrabre can be the primary factors that will require starting stopping chillers.

NOTES


Chapter 3-3



Next, determine what the annual hours of operation per year, are from minimum capacity to maximum capacity. A sample of typical annual usage curves follows.

Analyzing the load characteristics of your plant will allow you to determine which of the curves best represents the operating condition of your chiller facility.

The curve in Figure 3-1 demonstrates a chiller plant which seldom operates a full load. The chiller is oversized for the connected load or the chiller plant has redundant chiller capacity. This is the curve which offers the potential for the greatest savings with chiller optimization or, in other terms, discharge water temperature reset.

Select the curve that most closely simulates your operation:

FIGURE 3-1. This curve represents the percentage of time a chiller operates a t a given capacity per year when the chiller is lightly loaded or oversized for the load. The curve is used to estimate ton-hours of operation. It si used in developing energy saving analysis information.

FIGURE 3-2. This curve illustrates the percentage of time per year a chiller operates at a given capacity when the load is moderate and the chiller is adequately sized.

FIGURE 3-3. This curve illustrates the percentage of time per year a chiller operates a t a given load when the chiller is heavily loaded and is close to the minimum size necessary to handle the load.

The use of these curves will provide valuable insight into how much savings may be available through chiller optimization.

Use the CHILLER OPERATION ANALYSIS chart to calculate the annual KWH for each of your chillers.

NOTES

NORTH CAROIJNA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 3-4

“3

.>

Division


NC Department of Commw LOAD DISTRIBUTION CURVES

Primarily Light Loads Primarily Moderate Loads

Y 8

8

F

I- z W

W a

2 8

5

F

I- z W 0

Q

24

22

20

I8

16

14

I2

lo

8

6

4

2

0

PERCENT OF FULL CAPACITY PERCENT OF FULL CAPACITY Figure 3-1

York Division Bord-Wamer Figure 3-2

Primarily Heavy Loads

Y 8

6

F

I- z W 0

a

0 2 0 4 S 60 80 1.9

PERCENT OF FULL CAPACITY Figure 3-3


Chapter 3-5

A To tal

Operating Months Hrs/Month

January

February

March

April

May

June

July

August

September

October

November

December

TOTAL

AVERAGE

chiller Load Distribution Curve A, B, C - Select the load distribution curve that most closely matches your load profile. l?J rill 8 :Jz !i @iT 3 7

A Part Load Performance (PLP) = .05 (y) + 35 (y) + 5 c3) + .1D

B. PL -.10(?)+35 (F)+5t+)+.lr3

c. PL=: 25 e+) + 35 (y) + .5 (y) + .1D

A = Chiller power consumption/evaporator load at ~WO.

B = Chiller power consumption/evaporator load at 75%.

C =Chiller power consumption/evaporator load at 50%. 13 8

C C C D E F G

KW/Ton KW/Mon Mini" Maximum Avg Chiller TOn Capacity Capacity Cap Rating inTons Hou S

% Hrs % Hrs % AxCxD ExF

- - - - - - - - - -

D = Chiller power consumption/evaporator load at 25%.

Division

Energy CHILLERS A N D COOLING TOWERS WORKSHOP A Manual for Conseruation

Nc Department of commerce

.. 1

3

Reference 3-2.

Ton Hours PL KW/Ton x Year = KW/Year

Solve for your KW/Ton for the curve you select. Use chart Number 4 for a typical curve. The actual curve for your chiller can be obtained from the manufacturer of the chiller.

EXAMPLE: Chiller Rate KW .85

A = .85 KW

B = .85 - (.85 x .06) = -799

C = .85 - (.85 x .lo) = .765

D = 3 5 + (.85 x .02) = 367

CurveA,PLP=.O5( 3 5 + ,799 )+.35( .799 + .765 )+ .5( .765 + .86 )+.1(867)

Curve A, PLP = .05(.8245) + .35(.782) + .5(.816) + .1(.867)

PLP = .0412 + .2737 + .4080 + .0867

A - PLP = .81 KW/Ton

N O R T H CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-7

DNlsion CJ-ULLERS AND COOLMC TOWERS WORKSHOP A Manual /or Conservation " - 8

Enerav NC Department of CMmorca

Performance data (cont)

0.37

0.37 on Pump

AUXILIARY RATINGS - 3 PHASE, 50/60 HZ DE*'GN SUPPLY SEALED INRUSH KVA - Kilo volt-amps %&:YE V-PH-HZ KVA UVA

NOTES: 1. Average power consumed by auxllieries is included in the values ahown

2. Control circuit ratings provide for 32OOMP controls ESPll optiom com- &reasor nne opentongrge unit, two 1CR relays (15-VA each). a id one

3 relays. The purge opyatm only when non-

230 220/240-350 410 380/440-3-50 o'*l 522 in the Selsction Example tables. 220 200/240-3-80 480 *u)/400-380 0.81 5.22 575 550/800-560 2. PR-1. P R I and

INRUSH KVA 0.30

ITEM

carcm* oYn4d.r -

TYPICAL PART-LOAD PERFORMANCE CURVE

LCWT - Leaving Chilled Water Temperature ERWT - Entering Condenser Water

Temperature U R I P A M LOAD)

all applications follow this exact curve. Part -W pedomanca for specific applkatipn & i s may bedetermined from a canputenzed W o n .

0 ¶o r5 % OESION CAPACITY

SEALED AVERAGE KVA kw 0.23 0.05 1 .m 1 .m

Integrated part-load performance The Integrated Part-Load Value (IPLV) is the weighted average of chiller input power consumption per evaporator load over the complete part-load range of chiller operation. The PLV was established by ARI and is included in the latest version of ARI 550 Standards. Calculate IPLV as

Where: A = chiller power consumption/evaporator load at 100%

B = chiller power consumption/evaporator load at 75%

C = chiller power consumption/evaporator load' at 50%

load point.

load point.

load mint.

To determine specific full-load and part-load machine performance you must obtain a computerized selection from your local Carrier sales office. IPLV calculation example A computerized selection for a 400 ton high-efficiency machine produces the following information: Model: 19DK73353CM 100% load at 400 tons = .63 kW/ton = A 75% load at 300 tons = .55 IkW/ton = B 50% load at 200 tons = .53 IkW/ton = C 25% load at 100 tons = .68 kW/ton = D IPLV

D = chille; power consumption/evaporator load at 25%

(AI water conditions as defined by ARI Standard 550).

= 0.1 (59) + 0.5 (.%) + 0.3 (.605) + 0.1 (.a) = 0.059 + 0.27 + 0.182 + 0.068 = 0.579

load point.

Figure 3-5 NORTH CAROLINA STATE UhWERSlTY -WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 3-8

Figure 3-6. CHILLER OPERATION ANALYSIS

Total

Months Hrs/Month % Hrs % Hrs

January 40 25 40 25 0 I

8 0 5 4 0 3 100 561

100 I 561

AVERAGE I

C I I E I

% AxCxD

200 I 5,000 I C I

G

KW/Mon

chiller b d Distribution Curve A, B, C - Select the load distribution curve that most closely matches your load profile.

A PartILoadPdomance(PLP)=.05 (?)+35 (F)+SC+)+.1D

B. PL =.10(?)+35(?)+5(?)+.1D

C. PL = 25 ( ?) + 35 ( y) + .5 e+) + .1D

A = Chiller power consumption/evaporator load at 100%.

B = Chiller power consumption/evaporator load at 75%.

C = Chiller power consumption/evaporator load at 50%.

8

(Min % x Min Hrs + Max % x Max Hrs) / tIrsper month = Avg Cap D = Chiller power consumption/evaporator load at 25%.

Dwisum Energy CHILLERS A N D COOLING TOWERS WORKSFIOP A Manual for Conservation

NC Depertment of Commerce

In many cases with older chillers, it is difficult to determine the KW per ton at various operating capacities. This is particularly true when there has been a motor failure and the replacement motor is not a direct replacement. In fact, it would be quite unlikely that the KW would be the same. In these cases it may be possible to determine the KW/ton from information available in the operating log.

The formula for calculating the KW/ton is:

KW/ton = operating amps/1000 x voltage x 1.732/ton x % operating ( KW/ton = 100 x 460/1000 x 1.732/200 x .5 KW/ton = .7958

NOTES

lacity

NORTH CAROLrNA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-10

Division CHILL= AND COOLING TOWERS WORKSHOP A Manual for Conservation ,


J

THE MAXIMUM CHILLER OPERATING TEMPERATURE NOTES

A chiller efficiency can be increased by reducing the differences between the inlet condenser water and the outlet chiller water temperature. There are minimum limits to this differential. Each chiller is different and you should consult the manufacturer on your particular piece of equipment. The following graph shows minimum condenser water temperatures for a Carrier chiller with one-, two-, three-, and four-pass condensers.

Figure 3-7. Typical Part Load Performance Cooling Only Chiller

The curves are a representative composite.

3

The performance data in this graph demonstrates the relationship of the entering condenser water temperature to the percentage of operating capacity. This chart is for a Carrier chiller. The chiller is able to operate at lower condenser water temperatures a9 Iower loads than is permissible at high operating capacity. A temperature of 580 F entering water temperature on a two-pass chilkr is permissible up to 44% of the rated capacity. The condenser water temperature must be elevated to operate at a higher rate of capacity.

NORTH CAROL~NA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-11

Division Eneigy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation


MINIMUM CHILLED WATER TEMPERATURE

In most cases the discharge temperature from your chiller is mandated by the most undersized piece of equipment in your facility. A typical example of this is a hospital. Many operating rooms were designed for 720 F with four to eight air changes. Now the requirements are 680 F with ten to fifteen air changes. This is a dramatic increase in outdoor air as well as indoor load. Seldom has this condition been addressed by replacing the air handling unit. Usually the discharge temperature of the chiller is lowered to attempt to increase the air handler capacity.

Since the operating KW is affected directly by the chiller discharge water temperature, it may be to your benefit to change out the piece of equipment that requires a lower operating temperature with a properly sized unit design for a higher inlet water temperature.

Generally, the first objection to increased chiller discharge water temperature is a loss in dehumidifying capability.

The average water temperature in the cooling coil does not change drama tically if the chilled water supply temperature changes based on load conditions. That is, if the load goes down, the entering water temperature goes up. Note the following example.

NOTES

NORTH CAROLINA STATE UMVEFSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 3-12

‘3 CHILLERS A N D COOLING TOWERS WORKSHOP A Manual for Conscroation

EXAMPLE

COIL A ~

CFM EDB / EWB LDB / LWB TH SH EWT / LWT GPM w v WPD FV APD

4000 80.00 / 70.2 59.5 / 58.7

155,561 88,872

43.0 / 56.0 24.0 3.3 6.7

500.0 0.58

COIL B

4000 75.0 / 66.8 56.0 / 55.3

144,590 82,570

43.0 / 55.0 24.0 6.5

39.9 500.0 0.58


NOTES

4000 75.0 / 66.8 59.1 / 58.4

109,870 69,083

49.0 / 58.2 24.0 6.5

39.2 500.0 0.52

Coils A, B, and C are the same coils. These three examples are for the same coil a t varying inlet air temperatures and entering water tempera tu res.

In COIL A, the entering dry bulb (EDB) is 800 F and the entering water temperature (EWT) is 430 F. The work it will perform is 155,561 /BTUH total cooling.

In COIL B, the EDB is 750 F and the EWT is 430 F. The work it will perform is 144,590.

In COIL C, the EBD is 750 F and the EWT is 490 F. The work it will perform is 109,870.

COIL c

Reference 1-7 is a psychometric chart which shows the work required a t the same conditions. The chart shows that the coil at 490 F EWT will provide adequate cooling to satisfy the load at Point B.

NORTI+ CAROLINA STATE UNlVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-13



Since the machine is operating under low load conditions, a major percentage of the time, the chiller has the capacity at 5 percent to 80 percent of its rated capacity to carry the load 98 percent of the time. It will also dehumidify as well as the coil under the conditions A. Note the leaving wet bulb temperature - in COIL A, 58.70 F, in COIL C, 58.40 F.

NOTES


Chapter 3-14

''3

a

I

t

n a 3 L

m

Figure 3-8 N O M CAROLINA S A T E UNIVERSITY - WITH THE NORTH CAROLJNA ENERGY DIVISION

Chapter 3-15

Division



Since the chilled water circulating rate of the chiller is constant, the amount of water circulated exceeds the load requirements 98 percent of the time.

The load BTU = gpm circulated times the temperature rise across the air conditioning coils or the press load times the constant 500.

BTU - GPM x DT x 500

The air conditioning coils will perform less work when the temperature difference between the entering air or return air and the entering water temperature is decreased.

The new EWT of 490 F can be achieved with chilled water temperature reset.

The improvement in efficiency is a result of improved thermal transfer in the evaporator and condenser. This assumes that the water flow rates in the evaporator and the condenser are unchanged. In most cases it is impractical to try to increase the water flow rates through the evaporator or the condenser to improve thermal transfer because they are usually selected at their optimum velocity.

Is it possible to increase the discharge chilled water temperature or reduce the entering condenser water temperature? We will discuss the condenser water reset during the discussion of water towers (see Figure 3-7 and 3-9).

NOTES

NORTH CAROLrNA STATE UNlVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-16

e Dvision rnQrgy CHnLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation


‘3 OPERATING CONDITIONS

The next thing to consider is chiller cycling. In multiple chiller systems it is logical to operate the most efficient chiller as the lead chiller. In chiller plants with multiple chillers of various capacities, a careful analysis of the daily load conditions and what impacts that load is necessary. In order to adequately use multiple chiller strategies, it is necessary to be able to service all parts of your plant load with any and all chillers. We will discuss this subject under variable chilled water flow system.

The most efficient unit is usually the newest chiller; however. a thorough analysis of all of your chillers is necessary to properly address varying load conditions. Most chiller manufacturers will assist you with this information. (Refer to Figure 1-10 for part load KW informa tion.)

This graph shows the relationship between percent of capacity and KW. You will see that this chiller has a lower KWlton at 50 percent than a t 30 percent or 100 percent and that it is most efficient between 40 and 80. For this reason it is not unusual to find two chillers in a system running with the capacity limiter set at 80 percent in both chillers.

A thorough understanding of the performance characteristics of all of the chillers in your plant is necessary to properly plan an energy reduction strategy.

Reference 1-3 is a reference from a 1977 Trane manual. It shows the range of KW/ton for a compressor, a condenser and an evaporator. The KW/ton runs from a low of .65 KW/ton to a high of .78 KW/ton a t full capacity.

NOTES

NORTH CAROI.WA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 3-17

Division CHILLERS AND C001.1NG TOWERS WORKSFIOP A Manual for Conservation


This graph shows the relationship between the entering condenser water temperature and the KW per ton required to operate the chiller at a particular capacity. The efficiency of the chiller is increased by reducing the entering condenser water temperature.

Performance data

- lb - Y 3 I- a a W a. J W c a W

3 a t W v) 2 W P 2

u :: z a W t 2 W

TYPICAL MINIMUM ENTERING CONDENSER WATER TEMPERATURE

63 - -.

62

61

-.

60

59

58

-,

57 A

20 30 40 50 60 70 80 90 100

PERCENT OF FULL LOAD TONS

NOTE: Curveisbased~waterasthecoolerfluid. Lowtemperaturebrine applications result in a slightly lower minimum entering condenser water 1.mpenture cum

Figure 3-9

17

- .u W - a a

a

x W t

t 16

B s c Q H B 0 z a W t 2 W

' 14


Chapter 3-18

Division CHnLERS A Manual A N D for COOLINC Conseroation TOWERS WORKSHOP Energy


Table 3-10 shows evaporator correction factors. Note that at 44O F leaving water temperature and 950 F adjusted leaving condenser temperature it requires 1015 KW to develop 1331 tons or .7625 KW/ton. At 500 F E.L.W.T., 950 F C.L.W.T. it requires 1015 KW to develop 1497 tons or .678 KW/ton or 11 percent less power per ton for a 50 F change in evaporator leaving water temperature.

The 1977 catalogue was used for this comparison since the CVHE chillers are all selected by computer now and on graphs have been published for these new chillers.

Most new chillers have solid-state control systems. Some of these allow the user to optimize the operation.

.,” Chiller optimization is the ability to reset the supply water

jemperature based on the return water deviation from a set point such as 550 F. It may also allow multiple machines to operate on a parallel basis at an equal percent of capacity (see Figure 3-11).

If such a strategy is desirable and available, it is necessary to operate the machines in such a manner that they do not set a demand peak when starting an additional chiller. The operating chiller or chillers should cut back in capacity while the new chiller is brought on line. Many digital control systems have this capability.

It is necessary to be sure that any control strategy is compatible with your chiller capability and controls. Some early digital control systems and control strategies did things that were damaging to a chiller such as duty cycling chillers. Duty cycle is several stop-starts per hour or per day. Most chillers cannot by cycled more than once per hour, and it is best not to cycle them more than once per day. Many chillers have cycle counters’that are helpful to catch short cycling when attended.

NOTES


Chapter 3-19

Division C n r r r u


CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation :=iyy

PERFORMANCE EXAMPLE NO. 14

MODEL CVHB 125 V P1 P1S

COND ? I S

b I 4 0

Performance

90 95 I O 0

I273 I240 I201 TONS 1015 1 0 1 5 1015 K W

ADJUSTED LEAVING CONDENSER - C

EVA? PI I I

0 1 : I I 1 1 1 3 I IMPELLER

1327 1 3 0 2 I u z TONS

42 1015 1 0 1 5 1015 K W

U

1433

i o i a

I509 I497 972 101 I

0 8

12 IMPELLER

1287 TONS 1015 K W

IMPELLER

IMCELLER

lM?ELLER

I429 TONS

1011 K W

01 IMPELLER

Refrigerant (Ib5 R-11) 2200 Oil Charge (Gallona) 10 Auxlllary Water Required (GPM) 4 Evaporator Insulation Area (Square Feet) Shipping Weight (Ibs) 43,650

Maximum Rlgging Weight (Ibr) 23.Ooo

360

Optratlng Welght (Iba) 49,100

Shell i3 Water Box Volume (Gallons) Evaporator 259 Condenaor ne

NOTE All dlmsnslonl and physical mformalion applies only Io tlm cunpo. nenl combinallon ldenMud on this page

.m I

NORTH CAROLINA STATE UNIVERSlTY - WlTH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-20

CF"" ergy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Cascruation

NC Department of Comma

+*--

Table 3-11

S" Chilled Watei Temperature Sensor


Chapter 3-21

Division CHILLERS AND COOLING T O W E WORKSHOP A Manual for Consernation

S??6fSra_r V i 5 2 IYI


Some older machines have failed because of a fluttering water flow switch. It is unwise to save a thousand dollars in energy use and spend five thousand on chiller repairs.

Every motor has a minimum no-load KW requirement. Every chiller has a minimum amp draw. Occasionally because of motor selection, the minimum amp draw of the chiller may be at 30 percent to 50 percent of its full load amps.

Chiller manufacturers have several motor selections for each size compressor and occasionally this matchup is not good. Sometimes the problem occurs when a motor is replaced after a failure, on an emergency basis. This generally happens because you don’t have any choice. The chiller goes down on a Friday or a

weekend and the replacement is on an emergency basis. Production is screaming.

It is changed out on an emergency basis and you cannot get a

rewind done in time or the motor is beyond repair. Whatever the reason, the eventual outcome can be a 400 HP motor in a 250-ton machine. The minimum KW may exceed 50 percent of the maximum design KW of the unit. This obviously will impact any energy reduction strategy.

Low-end surge will also impact the general operation of a chiller. The unit becomes unbalanced because there is not enough gas being compressed by the impeller and the gas starts to rush back through the impeller. The result is call low-end surge. This results in a

practical minimum capacity for a chiller.

NOTES

NORTI 1 CAROI.INA STAW UNlVWSlTY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-22



There is a minimum condenser water temperature and a maximum chilled water temperature at which your chiller will operate. It is necessary that the condenser water temperature in order to start your chiller, and the chiller must be in this condition long enough for the refrigerant in the machine to migrate to the proper location before starting.

Also, the lower the machine operating capacity is, the more critical these operating conditions become.

One way to overcome this low operating condition is with free cooling.

FREE COOLING

One of the other alternatives being used is free cooling. Free cooling in this context means using the cold water from the water tower to cool refrigerant directly and to use the thermal properties and the design of the chiller to cool the chilled water without the use of the centrifugal compressor.

There are several other ways to produce free cooling with the use of cold water from a cooling tower when there is a need for cooling and the outdoor temperature is low enough, generally below 50° F (see Figures 1-21 through 1-26).

NOTES

NORTH CAROLlNA STATE UMVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-23



Free Cooling operation is based on the principle that refrigerant migrates to the area of lowest temperature. When condenser water is available at temperatures lower than the required leaving chilled water temperature (typically 50 to 55 F), the System Control Panel starts the Free Cooling cycle automatically. Up to 45 percent of nominal chiller capacity is available without operating the compressor. When the Free Cooling cycle can no longer provide sufficient capacity to meet cooling requirements, mechanical cooling is restarted automatically by the System Control Panel.

For example, a building with a high internal cooling load is located in a

climate with cold winters. It is possible to cool the building exclusively with Free Cooling three to four months out of the year! Free Cooling payback can easily be less than a year. Free Cooling is completely factory installed and requires no more floor space or piping than the standard CenTraVac chiller (unlike the plate frame heat exchanger).

Compressor Operation Not Needed

I I I

J I I

Rdrigorant Flow

Figure 1-21. This illustration shows the flow of refrigerant in a Trane chiller during the free cooling cycle. The use of low condenser water temperature allows the unit to condense refrigerant gases at a low pressure. At the same time the higher chilled water temperature boils the refrigerant , at this low pressure. In this operation the condenser water must be colder than the chilled water.

NORTH CAROLINA STATE UMVERSITY - WlTH TI-IE NORTH CAROLINA ENERGY DIVISION

Chapter 3-24

3

er Diision Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation

NC D e p a m " of Commerce

Figure 1-22. Free cooling performance to determine the required condenser water temperature for a given chilled water temperature and capacity enter the graph at the desired leaving evaporator liquid temperature. Follow the line vertically to the desired percent base capacity line. To determine the required condenser water temperature follow a line parallel to the applicable solid or broken line to the desired temperature information.

Example: The required condenser water temperature for 48°F leaving chilled water temperature at 25% capacity is 41°F for a standard condenser.

.

Free Cooling Performance

TEMP E R ATU R E ENTERING CONDENSER LIQUID 4OoF 5OOF 55OF

LEAVING EVAPORATOR LIOUID TEMPERATURE (OF) - STANDARD CONDENSER ---- EXTENDED CONDENSER

Fig. 'os - NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-25

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual far Conservation


Fig. 1-24 - Compnuor 0p.r.tiOn schonutk 1-25 - F m Cooling 0p.ration Schomotk

These figures demonstrate the differences in refrigerant fbws between the two methods of chiller operation.


Chapter 3-26

Division CHILLERS A N D COOLING TOWERS WORKSHOP A Manual for Conservation


- H0.t R O C O V O ~ ~ Performance Eumpks

Wnh Heat Cooling RecoVW only

CVHEO144CE4DE-lDE 133 Tonr 140 Tom

2000 MBh - .84 KwKon .80 KwKon

147 Tom 160 Tons 2200 MBh -

CVHM16-XE-3DE-1 DE

.a Kw&m .77 KwKon CVHEOl8-2CE-2DE-1 DE

161 Tom 180 Tonr 2400 MBh - BO KwK' .75 KwKon

CVHEO2O-1CE-1 DE-1DE

Money Saving Options Performance Examples

- Auxiliiry Condenser Performance Examples With Auxiliary coding Typical Annual

Coodenrer only Energy Savings ($1 CVHE0144CE4DE

140 Tons 5341 0 - 140 7iw 1515 MBh .72 K w K i .79 KwKon

CVHEOl6-XE-3DE 160 %It8 160 Tons 53895

1574 MBh - .69 K w K i .76 K w K i

CVHMIBZCE-PDE 180 kns 180 T w 34380

1627 MBh - .69 KwKon .74 KwKon

CVHE020-1CE-1 DE

, - ~m cooing ~ e r f o r n u ~ b m p b Frw coding T w Typical

Model Ent Cond. FAvg. Evrp. F Annual Energy Nomid b s w48 43/52 45/56 Savings ($1

49 57 63 $2930 CVHEOlWCE4DE * A l l .1

CVHMlSXE-IDE 54 63 72 $3160 160

60 71 81 w 2 0 CVHEOlE2CE-2DE 180

65 77 90 uslro CVHEO20-1CE-1 DE 2 M

Fig. 1-26

NORTH CAROIJNA STATE UMVERSITY - MrITH THE NORTH CAROLINA ENERGY DMSION

Chapter 3-27

Chapter 4

3

CHILLED WATER

c

SYSTEMS

3

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Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consewation


CHILLED WATER SYSTEMS

Up to this point we have reviewed the use of chillers and how to conserve energy in their use.

The chilled water systems that they are used with, also use energy. The way these systems are operated is an important part of your total use of electrical energy.

The basic chilled water system includes the chiller, a chilled water circulating pump, and the load.

The load is any heat exchanger that is connected to the chiller. This will be an air handler or a fan coil or such in an office building, or a press heat exchanger in an industrial plant.

In this system, the usual method for controlling the temperature of the space or the process is a three-way modulating valve. The three-way valve will provide a constant flow of water either through the heat exchanger or around it through the by-pass. Constant flow is required for the safe operation of the chiller.

Most older systems were designed with this type of control. It wasn’t until the cost of electricity rose sharply that other types of control were seriously considered.

In the mid-eighties, energy management systems became more prevalent and grew into total building automation systems. With the advent of DDC control, many system operating strategies became practical.

NOTES

One of the most useful designs, that was introduced during this period, is the primary-secondary loop system, which is also referred to as the coupled, decoupled system. This system empbys a primar;.

system pump and a dedicated chiller pump (see Figure 4-1).

NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION ‘I c

Chapter 4-1

Division CHILL= AND COOLING TOWERS WORKSHOP A Manual for Consemation Li 7 I=narm.(cr


HkAI EXCHANGER

TWO WAY VALVE

There are several major attributes of this system. The system allows for the use of two-way control valves, since the flow of water through the chiller can be constant and the flow through the building system can be variable. The operation of chillers can be automated since the system does not require the use of isolating valves to allow one or more chillers to be on- or off-line at a time. This reduces the requirement for automatic valves or for attended chiller startup.

b

The system layout in Figure 4-1 shows the basic components of a primary-secondary loop system.

NOTES

Figure 4-1


Chapter 4-2

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual fir Consemution


3 An example of a typical application should provide a clearer

understanding of the benefits of thissystem. For this example, let us consider the following:

CHILLER Design capacity Minimum capacity GPM EWT LWT AP

SYSTEM Design capacity Minimum capacity GPM EWT LWT Ar BHP

250 tons 62.5 tons 600 550 F 450 F 10 feet

250 tons Variable 600 450 F 550 F 50 feet 9.95

The system is inwlled in a multi-level office buildin air handlers on each floor.

with tw

During normal operation, the chiller capacity will follow the load by controlling the discharge water temperature at 450 F. the chiller dedicated pump will provide 1200 GPM to the chiller at sufficient pressure to overcome the resistance of the evaporator and the piping to and from the main system.

The system pump is sized for the design pressure of the distribution piping, heat exchangers, and control valves at the required GPM.

.

NOTES

NORTH CAROLINA S A T E U M W I T Y - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-3

Division CHILLERS AM) COOLING TOWERS WORKSHOP A Manual for Conservation


The use of a variable speed system pump can greatly reduce the operating KW. In our example, the design conditions are only met a few hours per year. The rest of the time less water flow is required to provide adequate cooling. The reduction in flow results in a reduced system pressure drop. The relationship is:

Change in Flow = (Change in Resistance12

A reduction to 50 percent in flow will result in a reduction in system resistance to .25 percent.

600 GPM x .5 = SR x (.5 x .5) 600 GPM x .5 = 50 ft x .25 300 GPM = 12.5 ft (See Table 4-1 on the following page for examples.)

The same reduction results in a dramatic lowering in motor brake horsepower.

A change in GPM results in a cubed effect on horsepower.

Change in GPM = (BHP)3 GPM x.50 = BHP x (.50 x .50 x .50) 600 x .50 = 9.95 x (.50 x .50 x .50) 300 = 9.95 x .125 300 = 1.24 BHP

While these reductions are dramatic, they may not ue obtainah-,, since with a variable flow system some pressure is required to move the water through the control valves to maintain control. Since the pump speed must be controlled in some manner, one control strategy uses a system pressure sensor located about one-fourth of the total length from the end of the supply line. The sensor rridrntains a coiisiaiit system pr2ssure at this point by varying the

pump RPM.

-_ - * _ _

NOTES

NORTH CAROLINA STATE UMVERsITY - WITH THE NORTH CAROLINA ENERGY DMSlON

Chapter 4-4

3

GPM


NC Depattment of Commerce

Directly

NOTES

Since the flow of water will vary with the system head, the fluid flow laws still apply. The flow will more closely follow the pump curve since the static pressure is being maintained. The static pressure at the pump will still drop as the flow is reduced since the pressure required to move the water to the sensing point will also be reduced (see Figures 4-2 and 4-3).

HEAD

BHP

quare

cube

GPM Directly 1 -

CHANGE OF SPEED (RPM)

EXAMPLES ~~

Double

Triple

RPM = (2) (RPM) = (2) (GPM)

RPM = (3) (RPM) = (3) (GPM)

Double

Triple

RPM = (2) (RPM) = (2)2 = (2) (2) = (4) Head RPM = (3) (RPM) = (3)2 = (3) (3) = (9) Head

:E IN IMPELLER DIAMETER (DIA.)

EXAMPLES

Double

Triple

Dia. = (2) (Dia.) = (2) (GPM)

Dia. = (3) (Dia.) = (3) (GPM)

Double

Triple

Dia.= (2) (Dia.) = (212 = (2) (2) = (4) Head

Dia.= (3) (Dia.) = (3)2 = (3) (3) = (9) Head

Double

Triple

Dias (2) (Dia.) = (a3 = (2) (2) (2) = (8) BHP

Dia.= (3) (Dia.) = (313 = (3) (3) (3) = (27) BHP

Table 4 1

NORTH CAROLINA STATE UNIVERSITY - WlTH THE NORTH CAROLINA ENERGY DMSlON

Chapter 4-5

Division CHILLERS AND COOLING T O W WORKSHOP A Manual for Conservation


Since we are operating the system on a constant system pressure, the pressure at the outlet of the pump will decrease slightly and the GPM will drop because of the reduced load. Since the brake horsepower is a function of GPM, the brake horsepower required to pump the water is reduced whenever the rate drops off from 600 GPM.

The control of the rate of flow in this arrangement is the control valve and the amount of water it allows to flow to the coil and back to the pump. Remember that the variable in this arrangement is the pump RPM.

The brake horsepower for 600 GPM @ 50 ft Head is 9.95. The brake horsepower for 300 GPM at 38 is 4.5 BHP.

NOTES


Chapter 4-6

Division



b

VARIABLE VOLUME SYSTEM with pump speed control and constant system pressure at remote system point

A - B C - D D - E E-F

Distribution pipe to last system load System load (air handler chilled water coil) Two-way control valve Return piping to pump at full load = 600 GPM

A - B 15APat60GPM B-C 5.0 8 600 GPM C-D 10’ 8 600 GPM D-E 15’ 8 600 GPM E.F 5’ 8 600 GPM TOTAL 50’ 8 600 GPM

N O R T H CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 4-7


Calculation for Reduced Flow:

AP 8 300 GPM A - B 8 600 GPM = 15 ft A - B

15 4 = y

15 X = 7 x = 3.75

AI’ = 3.75 ft A = 35 + 3.75 = 38.75 ft

5 ft Point c = 35 - (T) = 33.75 ft

Point D = 33.75 - - = 31.25 ft (Iqo) 5 4 - Point E = - - 1.25 = 1.25 f t

Pressures at Points:

600 GPM 300 GPM

A 50 ft 38.75 ft B 35 f t 35 f t C 30 f t 33.75 ft D 20 ft 31.25 ft E 5ft 1.25 ft F Oft Oft

Division

Energy NC Department of Commerca

NOTES


Chapter 4-8

",.. '-l

. ....

3

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Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consrruafion

Nc Depertment of Commercr,

MAX. DIA.: 8.W IMPELLER NO.: 105

Horsepower curves do not include motor service factor. Figure 4-2

A =

B =

c =

600 GPM 8 50 ft Head with 9.95 BHP

Same pump and impeller 8 300 GPM. The reduced GPM results in a lower system static pressure of 12.50 ft Head and a reduced brake HP of 1.24 BHP.

300 GPM (00 GpM)2 x 50 ft = 12.50 f t

300 GPM (00 GpMy x 9-95 BHP = 1.24 BHP

Curve A-B represents a change in flow as a result of reduced GPM. The same curve will result if the RPM were reduced to pump an equal GPM.

- NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-9

Division

=: :w: NC Department of Commerce


There are a number of ways to control a variable volume system. As we have just discussed, the use of a variable RPM and a constant system pressure is one way. This method is workable. However, if it is necessary to keep a fixed head of pressure in a system to provide water to the top story of a multi-level building, this can become a problem if not understood and properly maintained.

Another method is to use Multiple System Pumps. The same system could use two 300 GPM pumps rated at 70 ft head piped in parallel and controlled by a differential pressure switch in the system at point F-A. If the pressure at point B decreases to X then pump #2 is energized, and if the pressure rises to Y then #2 is de-energized.

NOTES

X = The pressure required to operate the system in excess of 50 %.

Y = The pressure the system will see at 100% flow and less than 50% load.

IO0

80 STATIC PRESSURE 60 IN FEET

4 0

20

0 0 100 200 300 400 500

-.

GPM SYSTEM FLOW

Figure 4-3 NORTH CAROLINA STATE UNlVERSlTY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-10

CHILLERS AND COOLING TOWERS WORKSHOP A Manual /or Caseroution

NC Department of Comwca

Differential pressure: Pump #1 should be on with system. Pump #2 should be on if system pressure is less than zero and less than 25 psi - off when system pressure is greater than 60 psi.

A bypass at the end of the loop is required for the primary- secondary loop system for at least two reasons:

1. A centrifugal pump can be damaged when there is no flow. The bypass will guarantee flow at all times.

2. Loads a t all points on the supply header will have cold water when they are turned on, as long as flow is continuous in the mains.

One bypass can be a fixed orifice such as a section of 1-inch pipe or can be a back press regulating valve (relief valve).

In a primary-secondary system, we can look at the systems as separate entities. Control of the primary system or load system and the secondary or production system is independent. The operation of the primary system controls the use and flow of water for the building loads. The secondary system controls the operation of the chillers and cooling towers.

In many central systems there is more than one chiller and there can be several chiIIers of varying capacities.

NOTES

In the primary-secondary loop system, the starting and stopping of chillers as well as the control of the water temperature is open to a variety of scenarios. Chillers are usually operated by controlling the discharge water temperature. As discussed earlier, they can also be operated by resetting to "off" the return water temperature variation when using DDC control system. That allows the discharge temperature to vary with the load.

NORTH CAROLINA STATE UNlVERslTY - WITH THE NORTH CAROLINA ENERGY DMSION 3

Chapter 4-11

Division E ?.t n r . q . . ~ ~ &c;s iec;s-- I CHILLERS AND COOLING TOWERS WORKSHOP

A Manual for Conservation JI NC Department of Commerce

RETURN WATER TEMP. SENSOR

f

C H I L L E R #1 A

D BYPASS I C

P l 0-

SYSTEM BY PASS II

SECONDARY LOOP

PRIMARY LOOP

Figure 4-6

Chiller #1 - 250 tons Chiller #2 - 250 tons P1= 600 GPM M = 600 GPM CPl = 600 GPM CM = 600 GPM P1 and M are constant speed pumps controlled by DPC-I.

NORTH CAROLINA STATE UNlvERsrrY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-12

Division CHILLERS AND COOUNG TOWERS WORKSHOP CCL,bi” ;PI i4:Li;\c Y

J r A Manual fir Conservation -- NC Department of Commerce

’3 In Figure 4-6, the return water temperature(RWT1 sensor will

react to increases or decreases in return water temperature. If the return water temperature goes down, the amount of chiller capacity has exceeded the load and the chiller capacity can be decreased. If the return water temperature goes up, the load has exceeded the chiller capacity and the chiller capacity should be increased.

Suppose that the system is operating with chiller #1 and pump P1 ”on”. It is mid-morning. The chiller is operating at 50 percent capacity, the return water temperature is 550 F and the supply water temperature is 500 F.

. .>

A-B 600GPM B-C 600GPM A-D zeroGPM RWT = 550 F SWT = 500 F

The chiller is operating a t 50 percent capacity and the system pump is providing 100 percent of its capacity to the building.

The load increases in the building. This causes several things to happen. Flow A-B remains constant. The return water temperature increases above 550 F. The chiller capacity increases due to the change in return water temperature. The chiller capacity will continue to increase until the return water temperature comes back to 550 F.

The load continues to increase and the chiller percent of capacity also increases until the machine reaches 100 percent of capacity.

RWT = 550 F SWT = 450 F

3

1 NOTES

NORTH CAROLlNA STATE UMVERSITY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 4-13

Division CHILLERS AND COOLING T O W WORKSHOP A Manual for Conservation


When this occurs, it may be necessary to start a second chiller. If the DDC control determines that it is allowable, then CP2 starts. Starting CM causes several things to happen. CP2 will pump 600 GPM. The flows are now:

A-B = 600 GPM A-D = 600 GPM

The temperature a t point A will increase to 500 F. The temperature at point D will decrease to less than 55O F or 52.5O F. The DDC control system ignores this for a predetermined period of time and continues the startup sequence. The system differential pressure will continue to drop, and pump P2 will be energized. This will cause the flow in the bypass to stop and the total 1200 GPM to flow through the building system. The temperature at point D will rise to above 550 F. The startup sequence will continue until both chillers are on-line and the return water temperature is controlled at 55O F.

NOTES


Chapter 4-14



VARIABLE FLOW CHILLED WATER SYSTEMS

In the June 1983 issue of the Engineers Newsletter, the basic decoupled primary distributionlsecondary production chilled water system wasshown in Figure 3. We see this same arrangement here in Figure 1. In this newsletter we will explore some of the reasons for this system’s performance. Further. we will speculate on some possible applications for its use.

------

, DISTRIBUTION LOOP II

FIGURE 1

The basic arrangement of pumps and chillers is also shown in the 1980 ASHRAE Systems Handbook. Notice how Figure 3 from Chapter 18, shown here as Figure 2, parallels Figure 1. In fact, they are identical, except for the chiller pump location. Unfortunately, the handbook does not go on to explain how the system works. The only explanation provided is that this scheme allows variable flow at the terminals and constant flow through the chillers, simultaneously.

VARIABLE FLOW CIRCUIT

FIGURE 2

HOW IT WORKS Technically, this arrangement provides far more

capabilities. (Before exploring some of these applications; we need to discover why the decoupler scheme perfprms as it does.

Figure 1 can be divided into its two halves, as shown by Figures 3 and 7. Figure 3 displays “production” functions. The purpose of each pump

NORTH CAROLINA STATE UMVERsITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-15

CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consemation 1 w: :w:

NC Depenment o~Canmerce

is to move water from point A to point E, by way of the chiller. The amount of power consumed in this process is:

KW(pp) = gpm x DP x 1.8763/EFF(p) x EFF(m) where: gpm is the pump flow rate in gallons per

minute

DP is the totai dynamic pressure loss between A and B in feet of water

EFF(p) is the percent pump efficiency

EFF(m) is the percent motor efficiency.

This resulting power is relatively small because so little pressure is needed to move water from A to E. The only significant loss is through the chiller itself. Further, only those pumps actually running (in the case of multiple chillers) actually consume power.

I

PR0WCTY)NLOOP TS1 I '

t t

FIGURE 3

PRODUCTION

forms. Figure 3 displays multiple chillers with dedicated pumps. Each chiller operates in combination with its own pump as a single unit. Temperature control of each chiller is separate from all others. The only function of the temperature control system is to provide a constant temperature of chilled water at the chiller exit. It does not provide any sequencing or other integrating functions.

One objection to this arrangement might be the possibility of either a pump or chiller outage causing the loss of one complete segment of system capacity. Gang pumping avoids thisshortcoming, but presents aproblem in providing increments of chiller flow and capacity. The arrangement shown in Figure 4, while more complex, solves the prob!em. Chiller flow and resulting system capacity increments are added or subtracted by the action of both pumps and valves Coordination of the pumps, valves and chillers is needed to match flows properly. This is certainly not terribly difficult to accomplish, but it adds it measure of complexity to the controls.

The production (secondary) can take several

Studies of the expected mean times between equipment failures reveal that very little system security is gained by ganging pumps per Figure 4, compared to Figure 3.

I t ROW SENSORS

I BYPASS I FIGURE 4

The production side (secondary) can consist of any number of individual chillers and pumps. They can be of any size or type. However, all chillers must be selected to produce their design capacityat the same entering and leaving chilled water temperatures, since all chillers use common return and supply water mains.

Commonly, chillers are located in a single central plant or machine room. Occasionally, chillers are remote or separated from each other. This, too, can be accommodated by the basic decoupler concept. Figure 5 shows one method that has proven popular in retrofitting "incremental" campuses to central plants without forfeiting existing chillers as presently locatedand piped. With thisscheme,valveVl isopen if the local chillers are secured and the central plant furnishes capacity. Thisvalve is closed if the building is to operate on local chillers only.

The thermostatically operated valve V2 meters water as it leaves the building secondary pumping system. This prevents the return of this water until it has reached an appropriate chilled water return ( C H W R ) temperature. Consequently, the building will appear to the distribution system as a large two-way valve controlled load. The "bridge" piping between valves V1 and V2 decouples the constant flow building circuit and the variable flow distribution piping.

absorbed by the central system, the building's existing pumping arrangement requires revision. Figure 6 shows a possible method. A new chiller pump P2 must overcome the differen!ial pressure between the supply and return mains in the central plant through the building supply main connection. This constant flow connection provides surplus water (not used by the building) to the central SuPPlY main. Valve v1 must open when the chillerr are active and be closed when they are not.

In order to allow surplus local chiller capacity to be


Chapter 4-16

Division

CHlLLm A Manual AND for COOLING Consemation TOWERS WORKSHOP Energy Nc Department of cofnmm

WLDmt LOADS 1

I I

EXISTING CHILLERS

- NEW

I K I I X 1134N30NE:

FIGURE 6

DISTRIBUTION The distribution (primary) portion of the decoupler

system is the hydraulic equivalent of a shut-off VAV system. As shown in Figure 7, water flows only when individual terminal control permits it. If all terminals are closed, no water will flow. Some form of relief is necessary to prevent the main disiribuiion pump from attempting to operate at zero flow. This is the Purpose of the pressure actuated relief valve V1

FIGURE 7

loo 1

m

m

FER OF mu)

20

tm 200 300 U x ) m PLMP FLOW - GPM

FIGURE 8

Pumping power varies as system flow changes. Various methods of pumpvolume control are used to match pumping capacity with system demand. Operation along a constant speed pump curve, Figure 8, is possible. The rising pressure characteristic of most pumps can be counterproductive, however. Not only does the pump pressure increase, but the differential pressure between the suppJy and return mains increases dramatically. In simple terms, this can be demonstrated by applying the pressure loss characteristics of pipes at various flow rates, Figure 9. For example, at 50 percent flow (load) the pressure losses decrease by 10.4 feet from P to A causing the differential at A-A' to increase by 20.8 feet. In aaaiiion, the pump curve snows another 24 ieei pressure rise for a total change of nearly 45 feet. In large distribution systems this variation can produce


Chapter 4-17

Division

A CHnLEJtS Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC Department d Cwnmerco

difficult control problems at the terminals. Consequently, system pressure control is an important aspect of system design.

I--?-, BYPASS PUMP

A

T

FIGURE 9

Pump staging is an effective and inexpensive way to reduce overpressurization at partial load conditions. In the previous example, at 50 percent system demand one pump could have been turned off. The remaining pump, at 100 percent of its design flow produces 58 feet of dynamic head rather than 82 feet. The total pressure rise at A-A', then, will be 10.4 feet, thus limiting the differential increase at A-A' to 20.8 feet.

A more effective (and expensive) method is pump speed control. On large systems this technique is cost effective because it greatly lowers the distribution pumping costs. A critical distribution point, A-A' for example, is chosen as the sensor location. Pump speed is varied in response to changes in this pressure differential. This technique allows the pump dynamic pressure, which is proportional to the square of its speed, to meet the reduction in system losses, which also vary approximately with the square of the water flow rate.

Again, we see the similarity to variable speed fans applied to VAV systems. As in that application, this method of pump pressure control approaches the classic flow vs power relationship . . . power is proportional to the cube of flow. More precisely:

KWpl FLOWpl 3 (m) = (m) Clearly, the potential for pumping power reduction

is substantial if, at 50 percent flow, the theoretical power is only one eighth of the full flow power.

It is important to recognize that the distribution ( p r i ma ry) c i :c u it operates complete I y i nd e pe nd en t I y from the production (secondary) circuits. They are hydraulically decoupled by the bypass line common to both circuits. Flow in this line is unrestricted and is free to move in either direction. Thus, direction of flow is purely an indicator of t,he relationship between flow supply and demand.

PREFERENTIAL LOADING Any number of good reasons for preferential chiller

loading can be cited. For example, a system of multiple chillers could involve machines of different type. make, age or eff iciency. Newer or more efficient units might be base loaded to take advantage 6f their life-cycle cost effectiveness. Or, it might be desirable to preferentially load absorption chillers during times of peak electrical demand.

This system easily accommodates complex energy management strategies by allowing any chiller to be started and operated in any desired sequence. But, once a chiller is in operation, its load is based on the system delta-T and its individual flow rate. In effect, all operating units are in parallel, operating at identical "percent design capacity" values.

At first look, this appears inalterable. However, there is a rather simple way to rearrange the system so that a specific chiller can be preferentially loaded, even as it operates in parallel with others.

Figure 10 shows such an arrangement. This scheme might be used to preferentially load a heat recovery machine. In fact, the heat recovery unit could be a small reciprocating chiller that only heats. Cooling becomes a useful byproduct instead of the main event.

' C- B Y P U S - . A

H

- CHWR

AIR FLOW

VARIABLE FLOW DISTRIBUTION

FIGURE 10

4 CHWS

I

?he key ?o understanding this arrangement requires an analysis of available chilled water return temperatures under various load conditions. Notice that, hydraulically, chiller (H) obtains returning water before any mixing occurs at "tee" (B). Thus, the highest return chilled water temperature in the system is available to this chiller.


Chapter 4-18

'I

”1 . .

3

CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consewation


One of the characteristics of the variable flow primary (distribution) system is the inverse return temperature relationship to load. Normally, we .expect a decrease in CHWR temperature as system loads decrease. This is due to mixing at three-way terminal control valves or the action of face and bypass air damper control. However, with variable flow, the CHYR temperature rises as loads decrease. This can be seen by imagining a very low load and flow rate condition at a cooling coil. Eventually. the CHWR temperature approaches the temperature of the air entering and leaving the coil. Clearly, this temperature is higher than the design CHWR temperature for that coil.

can be effected by this arrangement. If the heat recovery chiller is loaded on the basis of heating demand rather than chilled water supply (CHWS) temperature, the leaving chilled water temperature “floats.” If the chiller is large compared to other operating machines, total CHWS temperature control is not maintained.

The arrangement shown as Figure 11 overcomes this problem by feeding chiller (H) supply water back into the CHWR main. Thus, chiller (H) does not help providesystem flow. Instead, it becomesadevice that lowers system CHWR temperature.

Conceivably, CHWR temperature control precision

evpIss

FIGURE 11

DOUBLE-ENDED PLANTS Large campus-type projects often involve more

than one central chiller facility The decoupler system is uniquely versatile in its ability to be applied in this configuration Figure 12 shows a campus involving two distantly separated central chiller plants Each plant employs its own decoupler bypass line Water flows into the CHWS main from either or both ends

WKLLR PLANT A

BVPASSA - tnws

I

UNlVERSm

I CHILLER PUNT e

flGURE 12

With this scheme, an entire plant can standby for another, provided the mains are large enough. A great amount of system redundance is available. Further, plants can be operated to minimize many aspects of operating costs, such as pumping power, chiller power, operating personnel, etc.

Campuses that are expanding with limited “in place” distribution systems can use this doubleended arrangement. In this way, older facilities can be served by newer and more efficient plants without tearing out perfectly adequate underground piping.

BEST DESIGN CONCEPT Is the decoupled primarylsecondary system really

bulletproof? Probablynot. But. it represents the most useful and versatile multiple chiller scheme that our industry has applied to date. A number of perplexing design and operating problems seem to have found solutions.

Operating experience with these systems has been extremely successful. An infinite variety of design errors can be made. This concept does not prevent errors. But, i t overcomes a large number of the more common ones without burdening such a facility with additional owning and operating costs.


Chapter 4-19

VARIABLE FLOW CHILLED WATER SYSTEMS

This topic has been the subject of six previous Engineers Newsletters: 1974 Vol. 3. No. 1 - Control Systems That Save

1976 Vol. 5. No. 2 - Chilled Water Storage 1977 Vol. 6. No. 1 - Large Chillers: Series or Parallel

1977 Vol. 6, No. 9 - A Need for Variable Flow Chilled

1979 Vol. 8, No. 2 - Control of a Reciprocating

1980 Vol. 9, No. 1 - Why Must Chillers Be Constant

With all of this attention, you might think that we

Energy

Flow

Water Systems

Compressor Water Chiller

Flow Devices?

have analyzed every aspect and exhausted the subject. But, we haven't. In fact, considerable controversy still exists in the system design community about the "right way" to design chilled water systems.

Apparently, there are many "right ways." They depend on several variables, or system parameters:

1. The system's mission. 2. The type of equipment; i.e. chillers, terminals,

3. System load profiles and time durations. pumps, control valves, etc.

New Controls Technology Chiller control hardware continues to improve.

Many of the limitations of proportional electric and pneumatic controllers are being set aside by advances in microelectronics. New generations of microprocessor-based chiller controls are quite capable of handling some previously unaddressed aspects of variable flow. Industry jargon refers to the new controllers as providing "PID" response. "PID" stands for "proportional + integral + derivative." In simple terms, this means that the controller operates on the basis of sensing the deviation between chiller water temperature and the set point in terms of:

1 . The amount of deviation - to give an output signal that is proportional to the deviation.

2. The duration of the deviation - to modify the output signa! to account for adeviat!on that does not respond lo a proportional correction (time integral).

3. The speed of change of a deviation - to modify the output signal in response to the rate of deviation change (first derivati,ve).

Controllers with PID capability are properly applied as supply chilled water temperature controllers when the following conditions exist:

1. Capacity control is fully modulated. 2. Chilled water flow variations fall within the

minimum and maximum flow rates permitted by the chiller manufacturer.

3. Flow variations are small and gradual.

Modulation vs Stepped Output PID-type controllers cannot, however, cope with

the physical consequences of "stepped capacity" output. For example, a reciprocating chiller produces a stepped reaction. As each step of capacity is engaged, the AT produced by the chiller changes substantially and rapidly. This, of course, changes the amount (and, likely, thedirection) of the deviation fromsetp0int.A PIDcontrollersees thisasa need for drastic action; i.e. removal of the last step of capacity that was just added. Consequently, the controller is never satisfied and is always in search of a new equilibrium position: one which it can never find.

The control strategy for "stepped capacity" controllers is totally different from PID controllers. Basically, the controller is "educated" with regard to ihe expected AT for each step of additional capacity. Under this scenario, the controller takes action only when the calculated result puts the temperature within the setpoint target. The latest generation of sophisticated reciprocating chiller controllers employ similar strategies to this. Earlier, proportional controllers were simply located in the return chilled water stream.

)

Variable Flow and Heat Transfer Excessively low chilled water flow rates cause poor

heat transfer between the water and the evaporator tube surfaces Poor heat transfer causes the refrigerant temperature to be lower than normal, at a constant supply chilled water temperature More compressor power is then needed to produce a given refrigeration effect Further the refrigerant temperature can fall so far as to endanger the evaporator to freezing and consequent structural damage Therefore, low flows must surely be avoided

On the other hand, high tube velocities above 11 feet per second are traditionally considered to be poor practice Any impurities (including air) in high velocity water can erode the tube metal ana reduce i ts life Consequently manufacturers place empirical high velocity limits on evaporator tubes generally I C

the neighborhood of 11 feet per second I NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 4-20

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Division Energy CHILLERS AM) COOLING TOWERS WORKSHOP A Manual fir Conservation

Nc Department otcommerce

As long as water tube velocities are within these two extremes, chiller performance is not greatly compromised. Many designers then conclude that variationswithin the high and low limits are okay. But, they are not. A variety of conditions can produce operating conditions that are quite unacceptable.

Abrupt Flow Variations An example of rapid flow changes within the high

and low limits will underscore the problem. Suppose a chiller, Figure 1, is producing its full rated capacity at the following conditions:

400 tons 960 gpm, 52.0 to 42.0 F 9.6 feeffsecond evaporator tube velocity.

CONTROLLER -

FIGURE 1

The system allows variable flow through the application of two-way control valves on the numerous air handlers. An energy management system executes a scheduling strategy by operating specific air handlers according to a time clock. At 5:OO p.m., two large air handlers. . . representing 40 percent of the system flow . . . are shut down. The two-way valves close instantly, reducing the system flow to 576 gpm. The flow is still well within the allowable chiller range, Figure 2. An immediate consequence of this event is an instantaneous decrease in the supply chilled water temperature. Until the capacity control mechanisms adjust. the chiller will continue to produce full capacity. But the reduced flows increase the temperature drop from 10 F (52-42) to 16 F (52-36). At the new supply temperature of 36 F, i t is quite likely that the chiller safety controls will shut the machine down. If they don't, a freeze up is possible.

In any event, we see the potential for a nuisance trip-out and possibly a disaster. This represents an

unacceptable method of operation. No control system can be expected to manage a physical circumstance such as this. Therefore, large or abrupt flow rate changes must be avoided, even i f the temperature control system i,s capable. of handling variable flow.

CONTROLLER

L

FIGURE 2

Modest Flow Variations

to the system, especially i f controllers with only proportional action are used. Such a controller operates on the principle that each specific deviation from setpoint produces a certain signal (air pressure, voltage, etc.). Table 1 shows such an arrangement. Clearly, a deviation of 2 F is necessary to position the capacity control mechanism half way in its physical travel. A change of 1 F will move the mechanism through 25 percent of its total travel.

Even small or slow flow changes can be upsetting

The actual "sensitivity" setting (outputhnput) isset as high as possible without inducing "hunting." When this adjustment is made, the chiller flow rate is constant at a high value. Decreases in the flow rate have the effect of increasing sensitivity. This occurs because the lower flow rate causes a larger system "AT" at a given chiller capacity. In other words, a given change in capacity control will create a larger reaction in system temperature deviation.

This characteristic of proportional controllers makes them generally unsuitable for variable flow applications, even i f the variations occur gradually.


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CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conseruation

I 1 PRODUCTION LOOP

Controllers that emulate PID characteristics often can overcome much of this disadvantage. Given adequate time, the integrating function compensates for a long-term deviation. Therefore, the "sensitivity" can be set very low and the time integrating feature is allowed to make the necessary corrections.

Whilewe have the capabilityforapplying chillers to restricted types of variable flow systems, the general rule is to avoid them. This is particularly appropriate for multiple chiller applications where the flow changes can be both abrupt and large. No controller can handle this assignment.

A Solution That Works

is a hydraulically "decoupled" primarylsecondary pumping system. Figure 3 shows this basic arrangement. In concept, the production of chilled water is hydraulically decoupled from its distribution by an open bypass line between supply (production) water flow and demand (distribution). The direction and amount of bypass line flow becomes an indicator of system behavior. Flow in the reverse direction signals a call for more production (pumps and chillers) whileanexcessflow in the forward direction can call for less production (fewer pumplchiller combinations).

Consequently, the only known and proven solution

- - - - - - FLOW SENSORS

DISTRIBUTION

I I

I j DlSTRlBUllON LOOP , I

11

FIGURE 3

Chillers are thereby isolated from any system flow variations, regardless of their size or abruptness.

i


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Chapter 5


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Energy CHILLERS A N D COOLING TOWERS WORKSHOP A Manual far Conservation



NOTES

As we discussed earlier, the chilled water discharge temperature is generally based on the weakest link - an undersized air handler.

The capacity of an air handler can be increased in some cases with the addition of cooling coil surface. Sometimes air handlers have room for an additional coil and others do not allow for the addition of coil surface. It may be necessary to replace or modify an air handler or a cooling system to allow chiller optimization.

If it is determined that an additional coil can be installed, care should be taken to be sure the fan and motor have the additional capacity necessary to overcome the additional pressure drop of the cooling coil. The fans on most air handlers have additional capacity. Motors are generally selected more closely and will probably require replacement.

There are a number of consulting engineers specializing in industrial air conditioning design who can assist you with design, if needed.

FILTRATION

One of the major causes for reduced air conditioning capacity is dirty cooling coils. The dirt passes through or around filters and attaches to the heating and cooling coils, thus reducing the thermal capacity of the coils. Typically throw-away or blanket-type filters have very little effect on keeping the coils clean. The result is a gradual reduction of coil efficiency. Thicker coils and tighter fin spacing increase the problem. it is virtuaiiy impossibie to adequateiy clean a cooling coil eight rows thick and ten or more fins per inch.


Chapter 5-1

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consamation


Ellter housings with tight gaskets and pleated filters will dramatically improve the ability of the AHU to maintain its capacity (see Figure 3-1).

It has been estimated that at least 90 percent of the air handlers in North Carolina bypass more than 25 percent of the air around the filters when they are dirty. Most existing filter systems should be reworked.

Pleated filters of 2-inch size will dramatically improve the performance of air handlers.

i f j

NOTES

NORTH CAROLINA STATE UNIVERSlTY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 5-2

Division CHILL= AND COOLING TOWERS WORKSHOP A Manual for Consemation

NC Dep&t of commerce

AIR SEAL CONSTRUCTION FEATURES ARE I I n A MARK OF QUALITY Every Air Seal Housing IS built to exacting specilications by a company that takes pride in producing a quality w\ poduct. Housings are constructed of 16 gauge galvanized steel reinlorced with corner gussets and vertical leg supports Doors are fully gasketed and lilted with quick action positive pressure latches Housings are sealed by the application 01 silicone compound to areas where metal meets metal

Filters are sealed by the use of extruded aluminum tracks combined with a woven nylon pile seal

These, and other quallty leatures 01 Air Seal Housings are detailed below. W

1" FLANGES FOR EASY INSTALLATION Turnedout flanges all lour sides both lront and rear.

ALL METAL TO METAL- COMPONENTS SEALED After fabrication is complete, a silicone compound is applied to areas where metal meets metal. assuring a sealed housing.

CENTER ON 24"

.

RUGGED CONSTRUCTION The Air Seal Housing is factory assembled in a complete "one piece" unit. constructed Of 16 gauge galvanized steel

COMPLETE GASKETING - OF FILTERS Urelhane loam gasketing (two- pound density) is installed on "inside of doors" When doors close, gasket seals against the edge 01 filters. eliminating by-pass

FULLY GASKETED - ACCESS DOORS Perimeter 01 doors is gasketed with resiliant rubber gasketing, assuring a complete seal when doors are closed.

CORNER GUSSETS MAKE 2 AN EXTRA RIGID UNIT Corner gussets are standard on front 01 Air Seal Housings. adding extra stability.

n

---.-IN POSITIVE ^...^..

These members add overall rigidity to the housing and sews as supports lor the tracks.

EASY ACCESS FROM TWO SIDES Access lor litter installation is available from either side. . or both.

EXTRUDED ALUMINUM FILTER TRACKS

LATCHES UUILK ACllU LOCK DOOR. Access doors are filled with po;&"e piessuie ii,p ixi laiches that assures a tight seat and easy access

Both prefilter and linal filter tracks are extruded aluminum combined with reinlorced nylon plle seal to create a corrosion reststanl seal W'

NORTH CAROLINA STATE UNIVERSITY - WITH THE N O m CAROLINA ENERGY DIVISION

Chapter Si3

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Chapter 6

COOLING TOWER PIPING

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COOLING TOWER PIPING 0

IMPROVING CHILLER PERFORMANCE

There are three general types of cooling towers - forced draft, induced draft, and hyperbolic.

A forced draft tower uses a fan, usually a squirrel cage centrifugal type, to force air through the tower and the wet decking to evaporate the water.

The induced draft tower uses a fan, usually a propeller type to draw air through the tower.

The hyperbolic tower is used on very large cooling loads. Generally these towers are used for cooling water for power plants. For the purpose of this discussion, we will not consider this type of tower.

The forced draft-type towers use a centrifugal-type wheel and are used for low tonnage up to about 500 tons. These towers are usually less expensive initially than induced draft towers. The required motor horsepower is generally greater than the induced draft type.

Some designs for forced draft towers generate a very large amount of turbulence in the water sumps which may cause difficulty in controlling the tower water level. This has caused excessive use of chemicals and water loss for some applications, especially those with multiple towers. This is not a problem for all forced draft towers. It is more prevalent on towers with small sumps and surface areas.

. Induced draft towers use less motor horsepower per ton and can be used for much larger tonnages. The induced draft tower uses propeller-type fans which are more efficient at low static pressures.

NOTES 2'

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Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation Energy


Towers are manufactured from several types of materials. The first towers were built of wood. Many of these towers are still being installed. They provide some advantages not available in other materials. They can be built in place and this is important when accessibility is difficult. They can also perform in very large tonnage in the thousands of tons per cell. These towers do have some liabilities, however. They are difficult to keep tight. They have numerous joints in the water basin and require caulking or other sealing methods which add to the maintenance cost. They are generally physically much larger than towers of other materials.

Galvanized metal towers were the next type of towers entering the marketplace. Galvanized metal towers are the most common type in use today. These towers use various ”fills” - materials used to allow the thorough mingling of water and air. Two of these “fill” materials are wood, as mentioned before, and PVC. The PVC is formed into honeycomb patterns which provide good air passages and large surface areas. The major maintenance in these towers is at the water line in the sump which is constantly wetted and dried causing oxidation.

With the development of plastics came the third type - polyethylene. These towers are available in forced and induced draft design and range in capacity up to 250 tons per cell. The advantage of a polyethylene tower is durability. The tower material is impervious to corrosion and does not degenerate. These towers will provide good service for a long period of time - perhaps as much as 40 years. The fill is PVC which seems to last as long or longer than wood. These towers are more expensive than galvanized towers in initial cost. There seems to be a change in thinking on galvanized towers, and the use of the polyethylene towers is becoming more common. They have been used successfully for some years in harsh environments such as paper manufacturing and chemical plants.

NOTES


Chapter 6-2

Wish Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation


The newest material to be used in cooling towers is preformed fiberglass. Towers made of fiberglass panels with PVC fill are manufactured up to 1000 tons. These towers were introduced in this country about ten years ago but they have been used outside of this country since the mid-1950s. They have had a major impact on the tower market. They have gained rapid acceptance. Since these towers are usually built in place, some problems have occurred from assembly practices by inexperienced personnel. These problems have been declining as the numbers of towers have increased.

There are several other tower designs which have had some limited acceptance but these are the major types available.

Each of these offers special advantages which may impact your selection process. It is important to consider the cost of ownership of any tower. Since the initial cost is low in comparison to the annual operating and maintenance cost, these costs should be carefully analyzed before selecting a new cooling tower.

Energy use in a cooling tower is a function of the type of tower - forced draft versus induced draft - and the internal design of the air flow through the tower.

The brake horsepower required by a cooling tower should be one of the items considered in the selection of a cooling tower.

Cooling towers are designed to work with and take advantage of the existing wet bulb temperature. The wet bulb temperature is the temperature above which water wili evaporate in an air stream. If we wet our skin, the temperature we feel is the wet bulb temperature.

Figure 6-1 is a representation of a psychometric chart.

NOTES

NOR'IH CAROLINA STATE UNIVERSITY - WITH THE NORTH CAROLINA ENERGY DMSION

Chapter 6-3

Division Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conscroation


At 95O F dry bulb, the temperature a thermometer will read, and 50 percent relative humidity, the wet bulb temperature is 78O F.

The wet bulb temperature in North Carolina vanes dramatically from east to west. The wet bulb design for Charlotte is 780 F, for Raleigh it is 790 F, for Plymouth it is 800 F, and for Wilmington it is 820 F.

Cooling towers are usually rated at 95O F DB and 780 F WB by the manufacturer.

The graph (Figure 6-2) shows the effect of the change in wet bulb on cooling tower capacity. There is nearly a 30 percent difference between a tower's capacity at 950 F/780 F and at 950 F/820 F. This is a dramatic decrease in a tower's capacity and may explain the high head pressure experienced in hot humid summer weather.

A cooling tower rated a 100 tons at 950 DB, 780 WB, 950 EWT, 85O DB would have a 70-ton capacity at 820 WB or would only drop the temperature of the leaving water to 88O F.

It is therefore very important to be careful in selecting a replacement cooling tower. The operating condenser pressure of your chiller greatly affects its operating cost. Keep in mind that any increase in wet bulb temperature above the design condition of the tower will result in an equal increase in the discharge water temperature from the tower. It will also mean an increase in operating KW and electrical cost.

NOTES

NORTH CAROLINA STATE UNIVERSITY - WITH THE N O K H CAROLINA ENERGY DIVISION

Chapter 6-4

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i - 9 Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consswation

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NORTH CAROLINA S A T E UNIVERSITY - WITH THE N0KI'I-I CAROLINA ENERGY DMSION

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1 BULLETIN 01-303-10

D E LTA cooling tower selection data ( 1 0 ' ~ Range) EWT/LWT

The capacity of a DELTA cooling tower is fixed. It can function properly under reduced load conditions; however, it cannot be increased above its fixed rating.

Therefore, selection should always be based on maximum load conditions and the particular design wet bulb temperature of the area in which the tower is to be located.

Selecting a DELTA tower depends on four variables: 1. Gallons per minute of water to be cooled. 2. Entering warm water temperature from process (heat load) to be cooled. 3. Cooled water temperature leaving cooling tower to return to process. 4. Ambient design wet bulb temperature.

To aid in selection for varying conditions a set of charts is provided in order to determine the correction factor that

is to be applied to the calculated heat load. interpolation is appropriate between range tables and temperature curves.

Example: cool 500 GPM of water from 95'F to 85.F with 78.F wet bulb. 1. Establish Range

2. Establish tower ton load Range=water in at 95.F minus water off at 85'F=lO*F

Load= = 167 tons 3. Select Appropriate Range Chart (1O'F) 4. Enter chart using Wet Bulb temperature (78'F) and proceed horizontally to cold water temperature line (85'F). From this point proceed vertically to correction factor (1.0). 5. Multiply correction factor x tons previously calculated then refer to model selection tables in Bulletin At-303.

500 GPM X 500 (constant) X 10' Range 15,000 BTUl Hr./Ton

,go W B L - ... - .......... -. . . . . . . - . . . . .-. . . . . . . . . . . . . . . . . . . . .

I

_- .

.. - ......

Fig. 6-2

NORTH CAROLINA STATE UNIVERSITY - IhrITH "E NOKTH CAROLINA ENERGY DMSION

Chapter 6-6

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NC Dep"ent of C 0 " e r c a

This is the point at which your chiller will set a demand peak if

you do not have some means of limiting the peak. A demand limiting system is a must in every air-conditioned facility that uses electricity for production. The application of demand-limiting equipment will not be covered in this seminar.

Cooling towers may be the least understood piece of mechanical equipment you have in your facility. A small decrease in air flow from worn belts, algae growth in the tower, or a plugged water distribution system will result in reduced tower operating capacity.

The cooling tower is a prime candidate for good preventive maintenance and a competent technician can, with the proper tools, save large amounts of energy in chiller operation. The tools are basic. You will need a hand-operated sling psychrometer, accurate thermometers, and an amprobe. The sling psychrometer will indicate the ambient wet bulb temperature so that the approach temperature can be checked. The approach temperature and the temperature difference across the inlet and outlet of the tower will show exactly how efficiently the tower is running. When these parameters start to change, it is time to locate the cause. An accurate record of motor amps indicates the fan motor operation. An increase or decrease in motor amps may indicate a problem.

In selecting a cooling tower, it is important to be conservative. It will be wise to have some cushion in the size of the tower. The cost of this added reserve capacity is relatively inexpensive in comparison to the cost of energy.

The increased cost of the cooling tower will generally be offset in the first year of operation.

NOTES


Chapter 6-7

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*s 1-1


CHnLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation

CONDENSER WATER PIPING

Figure 6-3 shows a typical piping diagram for a single tower. The piping is straightforward. A bypass system will allow the chillers to be started and operated during low ambient conditions.

The use of a condenser water bypass will allow the condenser water temperature to be high enough to permit the chiller refrigerant pressures to equalize and the refrigerant liquids to migrate to the proper locations for a chiller to start during low ambient conditions.

Multiple towers and piping become more complicated. However, a primary-secondary loop system still allows flexibility in operation. Care should be taken to be sure that the system will provide trouble- free operation. The height of the tower basin in comparison to the pump inlets and piping and chillers is important.

The use of the primary-secondary loop system will allow for the automatic start-stop sequence we discussed earlier. It may be functional to use individual pumps as long as crossover piping is used for emergency opera tion.

Since most of you are not planning on changing towers in the near future, there is not a great deal you can do to improve the energy performance of a cooling tower. However, you may be able to improve its performance with regard to the other energy-using equipment in the chiller system, such as condenser water pumps and chiller operation.

NOTES


Chapter 6 8

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CHILLElls A Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC b p M l M t l t of Commerce

WATER COOLING TOWER

Fig. 6-3

CONDENSER

I

. CHILLED WATER

1 1 3-WAY CHILLED WATER VALVE

NORTH CAROLINA STATE UMVERslTY - WITH THE NORTH CAROLINA ENERGY DIVISION

Chapter 6-9

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A CHILLERS Manual AND for COOLING Conservation TOWERS WORKSHOP Energy NC Department of Commerce

CONDENSERS

MULTIPLE COOLING TOWERS

EQUALIZING LINE

%V TOWER BYPASS PIPING

J-

CONDENSER WATER PUMPS

Fig. 6-4


Chapter 6-10

Division CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Conservation Em=F:-a Li i-lgf


:I>

Fig. 6-5


Chapter 6-11



The water treatment being used on your condenser water system will affect your chiller plant operating cost. Your chemical program should keep your chiller clean and corrosion-free, control algae growth and provide an acceptable level of cycles of concentration. A common idiom in the industry is that your water treatment is only as good as the person supervising the treatment. Here again, proper care and understanding are required.

There have been towers with two inches of moss flourishing on the top of the fill in a tower with little to no air passing through the tower and masses of algae growing in the dark confines of towers. There is no question that competent maintenance is fundamental in conserving energy.

.2

G Y J

P @"

TOWERBLEED [

It is necessary to remove water from the condenser water system. This is called a bleed. The bleed rate is usually 10 percent of the makeup water rate. The purpose of a bleed is to control the cycles of concentration of solids in the tower water. A typical rate for the number of cycles is three to five. The number of cycles depends on the amount of desolved solids in the makeup water and the ability of your water treatment program to maintain these solids in suspension.

Solids will usually come out of suspension in heat exchangers, in condensers of chillers.

NOTES

, ,i

NORTH CAROLINA STATE UNIVERslTY - WlTH THE NORTH CAROLINA ENERGY DIVISION

Chapter 6-12

3 e Energy CHILLERS AND COOLING TOWERS WORKSHOP

A Manual for Conscwation NC Department of Commerce

Good water treatment is mandatory for efficient chiller operation and an automated chemical program is almost a necessity. An automated system will include a chemical pump, some type of conductivity sensor, or the use of a timer to cycle the pump on automatic or a continuous bleed. The type of system and the components of your system will depend on your experience and the recommendations of your water treatment chemical company.

Whatever, the case, reliability is important.

NOTES

NORTH CAROLINA D A T E UNIVERSITY - WITH THE NORTH CAROLINA ENE?lGY DMSION

Chapter 6-13

'''..-I . _..

Chapter 7

COST OF OWNERSHIP

Division Energy CHILLERS AND COOLING TOWERS WORKSHOP A Manual for Consewation


COST OF OWNERSHIP

The major consideration for most new construction is first cost. There are only a few firms that make ease of maintenance, energy efficiency and cost of ownership a major part of the design function.

In many cases, if not most, the facility maintenance department is not consulted or given a role in the design of new facilities. It is important that the maintenance department be aggressive early on in a new project. Your views must be made clear to the design firm responsible for the mechanical design. In many cases you will get what the engineer designed the last time unless you are aggressive.

Since time is money, an engineering firm will use as much as is practical of their standard specification. they will also specify the same manufacturers they have been using. There is strong resistance to researching and analyzing new products that they are not familiar with. This is uncharted ground for a conservative-minded group. It is to your benefit to make it clear to an engineering firm what your thoughts are regarding any mechanical standards you have developed, the make or type of mechanical equipment you want in your facility and the level of efficiency you expect on equipment such as fans, boilers, chillers, cooling towers, air conditioning equipment, and many others.

The maintenance department in a facility can make or break a

company. In many companies the maintenance department is considered a major player in the generation of profits. However, in others it is a necessary evil. Frequently the maintenance of production equipment is the primary responsibility of the maintenance department. The care and maintenance of the facility is performed by contractors and service companies.

3

NOTES


Chapter 7-1

Division Energy CHILLERS AM) COOLING TOWERS WORKSHOP A Manual for Conscruufion


In either case, the maintenance department is still responsible for the operation and cost of the facility maintenance.

A capable, well-administered maintenance department can be the most valuable asset a company has. The down time of production equipment, the cost of operating a facility, the efficiency of maintenance, the cost of energy, the life cycle cost of mechanical equipment are all costs of doing business and are major factors in the cost of the product or service provided by a company.

NOTES ')


Chapter 7-2

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Enerav CHlLLERS A N D COOLING TOWERS WORKSHOP A Manual for Conscwation G I

Nc D e p a " of Commerce

3 Fig. 7-1

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NORTH CAROLINA STATE UNIVERSITY - WITH THE NORTH C A R O W A ENERGY DIVISION

Chapter 7-3

Bibliography

“Cen Tra Vac Controls.” The Trane Company, December 1985, Form PL-RF-CTV-000- TS-12-1285.

“Cen Tra Vac Liquid Chillers.” The Trane Company, January 1988, PL-RF-CTV-000- DS-1-983.

Landman, William J. “The Search for Chiller Efficiency.” Heating, Piping and Air Conditioning, July 1983.

Marella, James. “Corrosion, Scale and Fouling - Operating and Maintaining a Refrigerated Facility. Air Conditioning, Heating and Refrigeration News, Sept. 11, 1989.

“Packaged Hermetic Centrifugal Liquid Chiller D-1 000 Series 50/60 Hz.“ Carrier Heating and Cooling, Sept. 1988, Form #19DK, DM-lPD, Form #17DK, DM-1 PD.

Trane Air Conditioning Centravac Chillers Two-Stage Centrifugal Liquid Chillers, 80 to 1630 Tons, August 1977.

Trane Air Conditioning Psychometric Chart, 1960.

‘Variable Flow Chilled Water Systems.” Trane Air Conditioning Engineering News- letter, June 1983, Volume 12, Number 5, and October 1983, Volume 12, Number 8.

“Winter Maintenance.” Brady Trane Service Update, Fall 1984.

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