Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Catalog CIC-2003-1/US
Thermostatic and Constant Pressure (Automatic) Expansion Valves
178
Constant Pressure (Automatic) and EPR ValvesTechnical Information
The constant pressure valve is a vital component of
many refrigeration and A/C systems. It automatically
meters refrigerant to the evaporator at a rate equal to
compressor pumping capacity.
The constant pressure valve contains a diaphragm,
control spring (FS1), seat and valve needle or ball.
The control spring, above the diaphragm, moves the
diaphragm down. This moves the valve open.
The opposing force is provided by low side evaporator
pressure (FE) and a constant body spring force (FS2).
This moves the valve closed. During the off cycle,
evaporator pressure builds and overcomes spring
pressure. This keeps the valve closed until the next on
cycle. At the start of the on cycle, the compressor
quickly reduces evaporator pressure. When this
pressure equals the control spring pressure, the valve
begins to open.
The valve opens when evaporator pressure is just
below the control spring pressure setting. This is the
valve’s opening point, or setting.
Bleeds
Bleed type valves permit pressures in the system to
reach a balance point during the off cycle. At the next
running cycle, the motor starts under practically no
load. This allows the use of low starting torque com-
pressor motors.
The bleed type (or slotted orifice) valve has a small slot
in the valve seat. This prevents complete close off at
the end of the machine’s-on cycle, permitting refriger-
ant flow at a reduced rate.
Proper selection should result in a bleed and orifice
which will always have control over the refrigerant flow
at all standard operating conditions. Application of a
larger bleed will speed equalization time, but may
cause the valve to lose control at high head pressure
operating conditions. Loss of control means all the flow
will be through the bleed and the valve will be closed
because the bleed capacity matches the compressor
capacity.
How to Select Constant Pressure Expansion Valves
1. Load on the system in Btu’s per hour or in tons
(12,000 Btu per hour equals 1 ton)
2. System refrigerant
3. Evaporator temperature or pressure
4. Condensing temperature or pressure
5. Pressure drop across the valve
6. Off-cycle unloading, if required
Elevation Change and Valve Setting
The control spring in a constant pressure valve works
with atmospheric pressure to move the valve in an
opening direction. Any substantial change in altitude
after a valve has been adjusted will alter the low side
flow rate maintained by the valve.
If a low side gauge is available, adjust the valve to
increase the system pressure above the sea level
reading by the amount shown in the gauge pressure
correction column of the table below.
Understanding the Constant Pressure Valve
Altitutde Gage Pressure
Feet Inches Hg. psia correction (psia)
0 29.92 14.70 -
500 29.38 14.70 -0.30
1000 28.86 14.19 -0.51
1500 28.33 13.91 -0.79
2000 27.82 13.58 -1.12
2500 27.32 13.41 -1.29
3000 26.82 13.20 -1.50
3500 26.33 12.92 -1.78
4000 25.84 12.70 -2.00
4500 25.37 12.44 -2.26
5000 24.90 12.23 -2.57
5500 24.43 12.01 -2.69
6000 23.98 11.78 -2.92
6500 23.53 11.55 -3.15
7000 23.09 11.33 -3.37
7500 22.65 11.10 -3.60
8000 22.22 10.92 -3.78
8500 21.80 10.70 -4.00
9000 21.39 10.50 -4.20
9500 20.98 10.30 -4.40
10000 120.58 10.10 -4.60
Barametric Pressure