Understanding Head Pressure Control

Walter H Langille, M.A.Sc., P.Eng

Sales Engineer

KeepRite Refrigeration

1. Why We Need Head Pressure Control ?

2. How Do We Control Head Pressure - Methods ?

• Energy Consumption

• Sounds Levels

4. Examine Head Pressure Control Methods In

Terms Of:

WE WILL LOOK AT:

3. EC – Electronically Commutated Motors

• Performance & Reliability

What is an EC Motor ? Why use an EC Motor in a Commercial Refrigeration Application ?

5. Compare Methods and Quantify Energy Savings

6. Floating Head Pressure Design/Concepts

Constant Head Pressure Is Needed To Ensure:

WHY WE NEED HEAD PRESSURE CONTROL

• Proper TXV Operation

• Optimized System Performance

• Good Oil Return

2. HOW DO WE CONTROL HEAD PRESSURE ?

1. Fan Cycling

4. Condenser Splitting

3. Condenser Flooding

2. Variable Fan Seed Control

(Lead Fan with Fan Cycling or Speed Control ALL Fans)

2 (ORI/ORD) Flooding Valves 1 Flooding Valve

• Virtually eliminates Heat transfer in off cycled cells

• There is a lower ambient T limit ( works in warmer regions)

• Many flare fittings

• Shock to system when bank cycles off/on

• Still requires Variable Speed on last Fan (Low Ambient) closest to header

FAN CYCLING

SOURCE:

PARKER

HANNIFIN

Liquid is backed up in the condenser eliminating effective heat transfer area.

Section of condenser filled with liquid does not act as a condenser.

• Obvious choice for 1 fan condensers

Should NOT Cycle Lead Fan – Thermal Shock to Header

CONDENSER FLOODING

• Provides constant Head Pressure

• Fan Motors run @ 100% speed

• Requires More Refrigerant and Larger Receivers

1 (Fixed)

or

2 (ORI/ORD-Variable)

Flooding Valves

Condenser Splittting

• Eliminates Half the tubes and the Secondary Surface SOURCE: PARKER HANNIFIN

• Slightly more money than fan cycling (Piping & Valves)

• Saves some refrigerant when only having to flood half the condenser

• Can be combined with fan cycling

Variable Fan Speed Control

• Reduced Air flow reduces coil heat transfer effectiveness

• Benefits are:

• Reduced Watt consumption ( Energy saving )

• Sound reduction ( Lower Sound Power Levels )

• Less refrigerant ( Cost Saving )

• Technology has limited the confidence in this method

• Triac controls, VFDs, and now…ECM

3. ECM – ELECTRONICALLY COMMUTATED MOTORS

What is an EC Motor?

EC Motors are DC Motors that connect direct to AC mains, EC = Electronically Commutated

The ECM (Electronically Commutated Motor) is:

Integrated AC to DC Conversion and Motor Commutation within the motor body

Programmable - Connect to Controller / BMS

Ultra High Efficiency

DC motors are significantly more energy efficient than AC motors and much easier

to control. Typically 0 to 10v DC Signal

Brushless

DC motor which uses a permanent magnet rotor and a built in inverter.

AC Motor Construction

Rotor

Stator

Rotor conductors

Stator windings

Air gap

DC Motor Construction

Rotor

Permanent magnet

Stator Stator windings

Commutation

Electronically Commutatted ( EC) Motor

AC to DC conversion

Permanent magnet

Rotor

Stator

Commutation AC mains input

Why use an EC Motor in a Commercial Refrigeration Application ?

Typical motor efficiency for a 50 W motor

0

20

40

60

80

100

Shaded pole single phase

capacitor

three phase EC

Motor type

Eff

icie

ncy

2. Energy Efficiency EC motors are much more efficient than PSC or Shaded Pole motor offerings. EC motors are up to 75%

to 80 % efficient—that’s a 51-59% increase over shaded-pole motors and a 30-35% increase over

permanent split-capacitor (PSC) motors. Additionally, these motors run cooler than PSC or shaded pole

motors, introducing less heat into the refrigerated space and further increasing energy savings.

1. Regulatory Compliance Effective January 1, 2008, California Energy Commission (CEC) Title 20 will require all new unit coolers used in

walk-in coolers and freezers to be equipped with EC motors. Other states are also considering this legislation and

will likely adopt similar language within the next few years..

Features and Benefits of EC motors

> Efficiency

> Noise (Low Sound Power Level)

> Straight Forward Speed Control (DC)

> Energy Savings

• Energy Consumption

• Sounds Levels

4. Head Pressure Control Methods

In Terms Of:

• Performance & Reliability

SOURCE: EBM PAPST

ENERGY CONSUMPTION

SOURCE: EBM PAPST

SOUND LEVELS

PERFORMANCE & RELIABILITY

Condenser Mains (230V or 460V or 575)

TRIAC (P-66) CONTROL & FAN CYCLING

• Most cost ( capital ) effective

• Poorest performance (High heat generation at low speeds)

• Capillary tube leak potential issues

AC INVERTER SYSTEM (VFD’s)

inverter

Motor

Filter

Pressure

Sensor

mains

EMC

filter

Line

Filter

0-10V /

4..20mA

Condenser

• Most complex

• Good Energy savings

• Can be very labour intensive on the jobsite, higher installation cost

• Motor reliability (Especially for 575V) an issue

Fan

Motor

pressure

sensor

EC MOTOR SYSTEM (Factory Controls)

0-10V /

4..20mA

0-10V

Condenser mains

• No Filters

• Easier to understand & setup

• More reliable

• Best energy savings

0-10V

0-10V

0-10V / 4..20mA

0-10V

Condenser mains

0-10V

0-10V

From Rack

Controller

EC MOTOR SYSTEM (Controls by others)

LETS INVESTIGATE FURTHER

MOST COMMON APPLICATIONS

AIR COOLED CONDENSERS

FAN CYCLING vs. EC MOTORS

AIR COOLED CONDENSING UNITS

FLOODED SYSTEM vs. EC MOTORS

4.5 X less energy or

80% Savings

66 dBA

52 dBA

59 dBA

65 dBA

Payback on 8 EC Fan Condenser < 1 Year

SOURCE: KEEPRITE REFRIGERATION ENGINEERING

76 dBA

68 dBA

60 dBA

56 dBA

SOURCE: KEEPRITE REFRIGERATION ENGINEERING

-85% vs. 1140

-70% vs. 850

-50% vs. 550

@ Typical Op

Range

Let’s Quantify

ENERGY SAVINGS (BIN ANALYSIS)

EC Fan Motors with

Fan Speed Control ( All Fans ) AC Motors with

Fan Cycling

VS.

5. Let’s Quantity Further

67% LESS ENERGY = 58,307 kWh

@ $0.10/kWh = $5,831

Payback ≈ 2 years***

***Depending on location

Philadelphia, PA

Let’s Quantity Further

AIR COOLED CONDENSING UNITS

2 HP System - Cooler

Flooded System

for HPC

VS.

Variable Speed

EC Fan Motor

for HPC

ENERGY SAVINGS (BIN ANALYSIS)

ENERGY SAVINGS (BIN ANALYSIS) 2 HP COOLER w/ FLOODED VALVE

-2934

-$235

+32%

SAME BIN ANALYSIS AS BEFORE ENERGY SAVINGS (BIN ANALYSIS) 2 HP COOLER w/ Variable Speed EC

WHAT MAKES EC MOTOR SO SPECIAL

• HIGHEST EFFICIENCY AT REDUCED SPEED

• LOW HEAT GENERATION AT LOW SPEEDS

• ABILITY TO REDUCE TO LOWER SPEEDS

• SOFT STARTS AND NO START UP TORQUES

• ABILITY TO BE CONTROLLED BY LOW VOLTAGE SIGNAL

• HIGHEST RELIABILITY IN LOW AMBIENTS

• GIVES MORE ENERGY SAVINGS AND MORE RELIABILITY

• SIMPLE TO INSTALL AND UNDERSTAND

• CONVENIENCE OF SETTING ADJUSTABLE HEAD PRESSURE…

6. Floating Head Pressure Design / Concepts

• NEEDED TO MEET ENERGY AND GREEN DEMANDS

• NEED TO MEET CURRENT AND FUTURE LEGISLATION (CALIFORNIA TILTE 20 & 24, EISA 2007 etc)

Capacity / Power Input at +40°F Evap T

10

11

11.5

12

12.5

13

13.5

85 105 125

Condensing T °F

Cap

ac

ity T

ON

S

6

7

8

9

10

11

12

13

14

Po

we

r in

pu

t k

W

Q P

Allowing Head Pressure ( Condensing Temperature ) in Refrigeration Systems to operate at reduced

levels ( “Float Down” ) during periods of Low Ambient can result in:

Increased Compressor

Efficiency

Lower Compressor

Motor Amperage

(Power Input )

X ELIMINATE

FLOODING VALVE

FOCUS ON

CONDENSER

FIND WAY

TO OPERATE

AS LOW AS

POSS.

ENSURE 100%

LIQUID @ TXV

ENSURE PROPER

APPLICATION /

BALANCE AT

EVAP

ADEQUATE

SUPERHEAT

AT COMP

REFRIGERANT

SAVINGS

WHAT NEEDS TO BE DONE?

OPTIMIZE

BTUH/W

Floating Head Pressure Design / Concepts

Floating Head Pressure Design Concepts

When Ambient T is below the design Ambient T we can take advantage of the greater condenser capacity.

We can benefit by lowering the head pressure ( “Float Down”) and get more compressor capacity

To a point !

IF Head Pressure is allowed to fall below certain Minimum Values

System Performance can be adversely affected in the following areas:

1. Starving Evaps by Underfeeding TXV’s

2. Oil Return / Oil Logging

3. Compressor Efficiency and Higher Discharge T’s / Super Heat

Floating Head Pressure Design/Concepts

1. Starving Evaps by Underfeeding TXV’s

Lowering Head Pressure

(Cond T) results in a

ΔP reduction which will

DECREASE

TXV Capacity

Lower Condensing T

( Lower Liquid T) will

INCREASE

TXV Capacity

The effect of

LowerΔP - (Reduced Valve Capacity) and

Lower Liquid Temperature ( Increased Valve Capacity)

Will tend to offset each other without any significant change in TXV Capacity

Lower Head Pressure requires less motor current and increases compressor efficiency.

There is a Limit

If Lowered Too Far the TXV Capacity will not be able to meet Evaporator load

Starving the Evap of Liquid Refrigerant Reducing Evap Capacity.

Floating Head Pressure Design/Concepts

2. Oil Return / Oil Logging

• Refrigerant and Oil do not mix completely

• For all the Oil to return properly to the compressor requires a

minimum refrigerant velocity in the suction line (particularly the riser).

• If the Evap is starved ( from lower head pressure, available ΔP) the

refrigerant mass flow in the evaporator will start decreasing.

• If the velocity is too low refrigerant will not return to the suction

riser – It will LOG in the Evaporator.

• Oil logged in the evaporator will coat the inner wall of the coil and

reduce heat transfer through the walls. This will cause a loss of

capacity and poor performance and may rob the compressor of oil

for lubrication.

Floating Head Pressure Design/Concepts

3. Reduced Compressor Efficiency and Higher Discharge T’s

If TXV is underfed

high Evap superheats

can result

High superheats

@ Evap outlet

Means

Increasing the suction

vapour T will result in

higher Discharge T’s

Suction vapour

entering

the compressor

Will be warmer as well.

Underfeeding the TXV will reduce the Load on the Compressor

HOWEVER

It will cause the Compressor to operate

Less Efficiently

at the Higher Discharge Temperatures

Specific Volume of the

refrigerant vapour will

decrease as the T rises.

Compressor will pump the

same”volume” of refrigerant

but the mass flow will decrease

reducing the effective pumping

capacity

• Reducing Head Pressure Lowers the Operating Expense of the Compressor

FLOATING HEAD CONSIDERATIONS

• The determining factor for deciding what the minimum allowable Head Pressure should be is the minimum TXV ΔP required for it’s capacity to meet the demands

of it’s Evaporator Load

• Potential For Lots Of Savings (Up to Approx. 30%)

• System Head Pressure Controls then adjusted to maintain that minimum.

Standard JCI Controls

“A SYSTEM CONFIGURATION THAT CAN OPERATE IN A WIDE RANGE OF AMBIENTS

AND

SAVE ENERGY AND REFRIGERANT”

FLOATING HEAD CONSIDERATIONS / FINDINGS

Potential for Reduced

Amount

Of Refrigerant –

EC Fans + Condenser

Splitting

Be Aware Of Your Ambient T

and Compressor Limitations

Lowest condensing temp is

not necessary optimal

Liquid -Suction Heat Xer

Needed To Ensure

Pure (100%) Liquid at Evap

Balanced Port TXV Will Work

Many Locations & Applications

HOWEVER

EEV Is Recommended for

System Optimization &

Especially For Proper Operation

in Low Ambient T

Low Ambient T’s May Need To Still

Consider Use of Flooding Valve(s)

40

• Standard fixed port valve does not cut it, balance port is better

• Additional hpc needed for lower ambient (EC as variable)

• Lowest condensing temp is not necessary optimal

• Too much capacity leads to high TD’s and low humidity levels

• High TD’s will effect product integrity and amount of

condensate on the coil (icing issues)

• SLHX needed to ensure 100% liquid

• EEV’s increase operating envelope of operation but does not

resolve the issue of too much capacity!

• More system modification needed to allow to operate in lower

ambient...ORI/ORD?

Thank You For Your Attention

NOW

Questions / Discussion / Input