floating head pressure

Floating Head Pressure Control

• Evaporative condensers are sized for rejecting

heat at the highest ambient condition, typically the

"ASHRAE summer 1% design" wet bulb temperature. • Conventional head pressure control maintains

condensing pressure corresponding to this fixed

wet bulb temperature condition. • For example, a system in Toronto may have a

condenser sized for following conditions:

ASHRAE summer 1% design wet bulb: 72ºF

Condensing pressure: 180 psig (95ºF)

Conventional Head Pressure Control

• Head pressure typically controlled by using

pressure switches to turn condenser fan(s) and

pump(s) on and off.

• Pressure switches work on a differential, which is

the difference between cut-in and cut-out pressures.

Conventional Head Pressure Control

High Pressure Control

Cut-in pressure 170 psig (91ºF) 180 psig (95ºF)

Cut-out pressure 145 psig (82ºF) 160 psig (88ºF)

Condenser pump Condenser fan

Conventional Head Pressure Control

150 (82°F)

head pressure (psig)

160 (88°F)

170 (91°F)

180 (95°F)

Time (minutes)

Compressor 1

Compressor 2

Compressor 3

fan on

pump on

fan on fan on

fan offfan off

on

on

on

Floating Head Pressure Control

• When ambient temperature falls below design

conditions, the condenser has excess capacity,

therefore head pressure can be lowered.

• Allow head pressure to float, following ambient

wet bulb temperature.

Average Monthly WB temperaturesfor 2004 (approx.)

[www.weather.uwaterloo.ca]

FJ M A M J J A S O N D

10

20

30

40

50

60

Month

Wet

bul

b te

mpe

ratu

re

(

°F)

0

70

80"ASHRAE summer 1% design " wet bulb (72°F)

75

80

85

90

95

100

0 1000 2000 3000 4000 5000 6000 7000

HEAT REJECTION (MBH)

CO

ND

EN

SIN

G T

EM

PE

RA

TU

RE

(°F

)

1 c

omp

ress

or

2 c

omp

ress

ors

3 c

omp

ress

ors

50º F W

B55º F W

B

60º F W

B

65º F W

B

70º F W

B

75º F W

B

FAN & P

UMP

Floating Head Pressure Sample Evaporative Condenser Capacity Diagram

A

B

C

COMPRESSOR HEAT REJECTIONCONDENSER CAPACITY (RUNNING FAN & PUMP) AT GIVEN WB

95 F CT 75 F WB

85 F CT 65 F WB

75 F CT 50 F WB

Floating Head Pressure Control

Power Consumption Comparison

CT TR HP COP kWh/Day

(°F)

95 130.1 214.4 2.86 3839

85 134.0 190.8 3.31 3416 (-11%)

75 137.9 167.9 3.87 3006 (-22%)

A

B

C

Measured COP, compressor power and fan power for floating and fixed head pressure control

[ASHRAE JOURNAL]

CO

P

Time (minutes)

1.25

1.00

1.5

1.75

2.00

2.25

2.50

5 10 15 20 25 30 35 40

Pow

er

(kW

)

10

20

30

40

50

60

70

80

0

floating head pressure fixed head pressure

Fan PowerCompressor powerCOP

Floating Head PressureDesign Considerations

• There are limitations to how far head pressure can

be dropped.

• System limitations include expansion valve design,

condenser water freeze protection, condenser sizing,

compressor operating parameters and waste heat

equipment performance.