high performance chilled water systems · vpf system limits (consult ... system pump(s) p-1, p-2,...

© 2006 American Standard Inc.

© 2012 Trane

High Performance Chilled Water Systems

High Performance Chilled Water Systems

Mick Schwedler, PEApplications EngineeringTrane

Normal Performance Chilled Water Plants ASHRAE/IESNA 90.1 (LEED Prerequisite)

System configuration

Design parameters

System control

Chiller Efficiency Requirements

Both full and part load efficiencies are required

Type Efficiency

< 150-ton water-cooled 4.45 COP 0.790 kW/tonscroll and screw chiller 5.20 IPLV 0.676 IPLV

150-ton < 300 tons water-cooled 4.90 COP 0.718 kW/tonscroll and screw chiller 5.60 IPLV 0.628 IPLV

300 tons water-cooled 5.50 COP 0.639 kW/tonscroll and screw chiller 6.15 IPLV 0.572 IPLV

300-ton water-cooled centrifugal 6.10 COP 0.576 kW/tonchiller (ARI Standard conditions) 6.40 IPLV 0.549 IPLV

Heat rejection equipment Fan speed control 7.5 hp

and greaterCapability to operate at 2/3

fan speed or less

ExceptionsClimates > 7200 CDD50

(e.g. Miami)

1/3 of fans on multiple fan application

ConfigurationNormal Performance Chilled Water

productionpumps

two-way valve

distributionpump

distributionloop

productionloop

Design ParametersNormal Performance Chilled Water Plant AHRI 550/590-2011 Standard Conditions44°F chilled water

2.4 gpm/ton chilled water (10°F (5.6°C T)

85°F entering condenser water

3.0 gpm/ton condenser water (10°F [9.3] T)

ControlNormal Performance Chilled Water Plant Chilled water distribution pump

P at most remote load

Cooling tower fans55°F (12.8°C) (as cold as possible)

Somewhere else

Constant volume condenser water pumps

High Performance Chilled Water Plants System configurations

Design parameters

System control

Variable-Primary-Flow Systems

variable-flowpumps

controlvalve

checkvalves

Flow requirementsVPF System Limits (consult manufacturer)

Absolute flows - Minimum and maximum

Flow rate changes 2% of design flow per minute

not good enough 10% of design flow per minute

borderline 30% of design flow per minute

many comfort cooling applications 50% of design flow per minute

best

Always need a bypass

Chiller ControlVariable W ater Flow

30

40

50

60

70

80

90

100

110

120

130

3:50:00 3:55:00 4:00:00 4:05:00 4:10:00Tim e (hour:m in:sec)

Wat

er T

emp

[deg

F]

-500

-300

-100

100

300

500

700

900

1100

1300

1500

Flow

[gpm

]Evaporator W ater F low

Evap Entering W ater Tem p

Evap Leaving W ater Tem p

BenefitsVPF System Installed costs

Fewer pumps

Reduced piping connections

Reduced electrical connections

Be aware of possible VFD additional costs

Operating Reduced pump operating costs

VPF Savings First cost: 4-8%

Annual energy: 3-8%

Life-cycle cost: 3-5%

http://www.arti-21cr.org/research/completed/finalreports/20070-final1.pdf

Specify and install proper flow control devices

Possible challengesVPF system Chiller control

capabilities

System complexity

Lack of training

Flow measurement -possible costs and accuracy

Series Counterflow

Evaporators in Series Condensers in Series Arranged in Series Counterflow

41°F(5°C)

50°F(10°C)

95.1°F(35°C)

88.1°F(31.2°C)

More informationVPF System Http:/trane.com/commercial

/library/newsletters.asp (1999 and 2002)

“Don’t Ignore Variable Flow,” Waltz, Contracting Business, July 1997

“Primary-Only vs. Primary-Secondary Variable Flow Systems,” Taylor, ASHRAE Journal, February 2002

“Comparative Analysis of Variable and Constant Primary-Flow Chilled-Water-Plant Performance,” Bahnfleth and Peyer, HPAC Engineering, April 2001

“Campus Cooling: Retrofitting Systems,” Kreutzmann, HPAC Engineering, July 2002

Ice Storage System

heat-transferfluid

heat exchanger

or cooling coil

pump

chillerice storagetanks

• Schematic representation of plant layout

• At a glance view of key system operating parameters

• Ability to set key setpoints and weather forecast

System Schematic

Mode Mode Scheduled Hours

System Pump(s)P-1, P-2, (P-3)

Chiller(s)CH-1, (CH-2)

Ice ValveVAL-1

Bypass ValveVAL-3

Diverting ValveVAL-2

Notes

Chiller Only

7am – 11am6pm – 11pm April to October

Modulate on system DPT-2 [setpoint] psid

Enabledto [temp] setpoint

100% to bypass ice

Modulate on chiller flow (DPT-1) to maintainchiller minimum [setpoint] psid

Fully open to building load

Use VPF add routine for number of chillers to operate

Tank Discharge Only

11am – 11pm November through March

Modulate on system DPT-2 [setpoint] psid

Off Modulate on system supply temp [setpoint] °F

Closed Fully open to building load

Simultaneous Chiller & Tank Discharge

11am-6pm April to October

Modulate on remote DP [setpoint] psid

Enabledto [temp] setpoint or [%RLA] limit or operator intervention

Modulate on system supply temp [setpoint] °F

Modulate on chiller flow (DPT-1) to maintainchiller minimum [setpoint] psid

Fully open to building load

Use VPF add routine for number of chillers to operate

Make Ice 11pm to 7am or until terminationsetpoint is reached.

Modulate to ice making flow using DPT-1 [xxx gpm]

Ice Making Modeenabled

Modulate on system supply temp of 15°F (100% to ice)

Closed Fully open to bypass

Make Ice and Cool

11pm to 7am when humidity alarm level

Modulate to ice making flow using DPT-1

Ice Making Modeenabled

Modulate on system supply temp

Closed Modulate to building DPT, starting at min

Control Mode Matrix

High Performance Chilled Water Plants System configurations

Design parameters

System control

a history ofChiller Performance

8.0

ASHRAE Standard 90

chill

er e

ffic

ien

cy,

CO

P

6.0

4.0

2.0

0.0NBI “best”

available90-75(1977)

90-75(1980)

90.1-89 90.1-99

centrifugal>600 tons

screw150-300 tons

scroll<100 tons

reciprocating<150 tons

chilled water plant design …ProvocationAre our “rules of thumb” …

44 F (6.7°C) chilled water supply

10 F T (5.6°C) for chilled water system

3 gpm/ton condenser water flow

… in need of repair?

High Performance Design Parameters ASHRAE GreenGuide

Chilled water T: 12°F to 20 °F (6.7 to 11.1 °C)

Condenser water T: 12°F to 18 °F (multi-stage) (6.7 to 10°C)

Kelly and Chan Chilled water T: 18°F (10°C)

Condenser water T: 14.2°F (7.9°C)(3.6 - 8.3% energy savings in various climates)

High Performance Design Parameters Taylor (ASHRAE Journal December 2011)

Chilled water:“To reiterate: our analysis suggests that it nevermakes sense to use the traditional 10⁰F or 12⁰F ∆Ts that are commonly used in standard practice.” (p.26)

Condenser water:“…life cycle costs were minimized at the largest of the three ∆Ts analyzed, about 15⁰F. This was true for office buildings and datacenters and for both single-stage centrifugal chillers and two-stage centrifugal chillers.” (p.34)

Article provides a simple method to arrive at job-specific flow rates

chilled water plant design …A Paradigm Shift New “rules of thumb”

41 F (5C) chilled water supply

16 F (8.9C) T across the evaporator—that’s at 1.5 gpm/ton

15 F (8.3C) T across the condenser—that’s at 2.0 gpm/ton

Resize the cooling tower accordingly

(We won’t change the coil, chiller or piping for now)

chilled water plant …humid climate

Base Design: 450 Tons 0.5% design

wet bulb: 78 F (25.6C)

Entering condenser water temperature (ECWT): 85 F (29.4 C)

Evaporator and condenser temperature differences: 10 F (5.6C)

Coil, valve and chilled water piping pressure drop: 80 ft

Condenser water piping pressure drop: 30 ft

Pump efficiency: 75%

Pump motorefficiency: 93%

traditional design …humid climate

System Energy Consumption

2.4/3.0

Chilled /Condenser Water Flows, gpm/ton

Ener

gy C

onsu

mpt

ion,

kW

TowerCondenser Water PumpChilled Water PumpChiller (100% Load)

0

50

100

150

200

250

300

350

traditional vs. low-flow design …

System Summary At Full Load

0

50

100

150

200

250

300

350

2.4/3.0 1.5/2.0


Ener

gy C

onsu

mpt

ion,

kW


comparison …humid climate

System Summary At 75% Load

0

50

100

150

200

250

300

350

2.4/3.0 1.5/2.0


Ener

gy C

onsu

mpt

ion,

kW




0

50

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150

200

250

300

350

2.4/3.0 1.5/2.0


Ener

gy C

onsu

mpt

ion,

kW


traditional vs. low-flow design …humid climate

Savings Summary

0

10.0

20.0

25% 50% 75% 100%

Load

Ope

ratin

g C

ost S

avin

gs, %

3.8%

6.7%

10.3%

16.5%

Reduced flow works for all chiller manufacturers Logan Airport - Boston:

$426,000 Construction cost savings

7.3% operating cost savings

Large Chemical Manufacturer -Greenville $45,000 Excavation and concrete savings

6.5% Operating cost savings

Computer Manufacturer - San Francisco Existing tower, pipe savings

2% Operating cost savings (tower not changed)

Low flow works for retrofit applications Chilled water side

Coil It’s a simple heat transfer device Reacts to colder entering water

by returning it warmer

Ideal for system expansion

Cooling Coil PerformanceMBH

EWT

GPM

GPM/ton

LWT

WTR

504

44°F

101

2.4

54°F

10°F

504

41°F

63

1.5

57°F

16°F

GPM reduction of 37.6%

Low flow works for retrofit applications

Condenser side retrofit opportunity Chiller needs to be

replaced

Cooling needs haveincreased by 50%

Cooling tower wasreplaced two years ago

Condenser pump and pipes are in good shape

Condenser side retrofit opportunity

Existing Retrofit

Capacity (tons) 500 750

Flow rate (gpm) 1500 1500

Condenser Entering Water Temperature (F)

85 (29.4 C)

88 (31.1 C)

Condenser Leaving Water Temperature (F)

95 (35 C)

103 (39.4 C)

Design Wet Bulb (F) 78 (25.6 C)

78 (25.6 C)

High Performance Design Parameters Low flow benefits systems - no

matter whose chiller is being used

Low flow works extremely well on existing systems

Low flow works on short piping runs

High PerformanceDesign OptionsEither …

Take full energy (operating cost) savings

Or …

Reduce piping size and cost

Experienced designers use pump,piping and tower savings to select aneven more efficient chiller

always, always,Always Remember …

Oh, by the way...

You may also do this with air

High PerformanceChilled Water Plants System configurations

Design parameters

System control

High PerformanceChilled Water Pump Control

Valve position Pump Pressure Sensor

Communicating BAS Pump Speed

Position (% open)of critical valve

75%

65%

Increase pump static pressure setpoint

Reduce pump static pressure setpoint

No action

pump-pressure optimizationControl Logic

Required by ASHRAE/IES 90.1-2010

High Performance Chiller-Tower Control

Plant Power vs CWS

0.0

200.0

400.0

600.0

800.0

1,000.0

1,200.060 62 64 66 68 70 72 74 76 78 80 82 84 86 88

Condenser Water Setpoint (°F)

Pow

er (k

W)

Lowest condenser water temperature available from tower at this load and wet-bulb temperature

Chillers cannot meet load above this condenser water temperature

Optimal operation

1,550 tons, 65°F Wet-bulb T t

1,160 tons, 59°F Wet-bulb T

730 tons, 54°F Wet-bulb Temperature

Hydeman, et. al. Pacific Gas and Electric. Used with permission.

High Performance Chiller-Tower Control

0

100

200

300

400

72 74 76 78 80 82 84Condenser water temperature (°F)

kW

Chiller kWTower kWTotal kW

High Performance Chiller-Tower Control Braun, Diderrich

Hydeman, Gillespie, Kammerud

Schwedler,ASHRAE Journal

Cascia

Crowther & Furlong, ASHRAE Journal

Chiller-tower optimizationEstimated Savings Crowther

(“Optimized” vs. driving water to 65°F) Chicago: 5.4%

Las Vegas: 2.6%

Miami: 8.5%

Based on chiller+tower annual energy consumption

High performance chilled water plant

Winchester Medical Center

Medical CenterWinchester, Virginia Five 750-ton chillers

0.571 kW/ton full load

Chilled water 58 to 42°F

Condenser water84 to 95°F(missed opportunity)

VFD’s

Variable primary flow

VFD’s on Chilled water pumps

Condenser water pumps

Cooling tower fans

Sophisticated control system with lots of Programming

Commissioning

WMC - August 12Chiller plant

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WMC - Sept 1-7Chiller plant Weekly-Summary

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Remember...Without controls,it’s not a system.Without controls,it’s not a system.

The meter is on the building!

It’s a great time to be in this business!

high performance chilled water systems · vpf system limits (consult ... system pump(s) p-1, p-2,...

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