floating head pressure
TRANSCRIPT
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.