Figure 1: ‘Hold back’ valve at the outlet of the condenser.
This column deals with head pressure control valves that
work in conjunction with one another in cold ambient conditions. They keep the
proper condensing pressure at the metering device for proper feeding of the
evaporator. My column in the June 4 issue of The NEWS
cover two other low ambient head pressure control valves that have charged
domes but more or less serve the same purpose.
The solution to the low head pressure problem is to install
a pressure actuated “hold back” valve at the outlet of the condenser (Figures 1
and 2). This valve will throttle shut when the condenser pressure reaches a
preset minimum for low ambient conditions. This will allow liquid refrigerant
in the condenser to be held back and actually flood portions of the condenser.
This partial flooding will cause some of the condenser to
become inactive and to have a smaller internal volume.
Now desuperheating and condensation must take place in a
smaller volume condenser. Condensing pressures will rise, thus giving
sufficient liquid line pressures and pressure differences across the
thermostatic expansion valve (TXV) for normal system operation in the colder
The valve (Figure 3) is referred to as an ORI (Open on Rise
of Inlet) valve. It is an inlet pressure-regulating valve and responds to
changes to inlet pressure (condensing pressure) only. A decrease in condensing
pressure causes less pressure to act on the bottom of the seat disc. This
action throttles the valve more in the closed position and starts to back up
liquid refrigerant in the bottom of the condenser.
Soon the head pressure will start to rise from the reduced
condenser internal volume. Any increase in inlet (condensing pressure) above
the valve setting will tend to open the valve. The condensing pressure is
opposed by adjustable spring force acting on top of the seat disc.
Figure 2: Schematic showing positions of head pressure
Either increasing or decreasing the tension of the spring
with a screwdriver or Allen wrench can change the valve setting. Increasing the
spring pressure by turning the fitting clockwise will increase the minimum
opening pressure of the valve. The outlet pressure of the valve is cancelled
out and has no bearing on valve movement.
This is because the outlet pressure is exerted on top of the
bellows and on top of the seat disc simultaneously. Since the effective area of
the bellows is equal to the area of the top of the seat disc, the pressures
cancel one another and do not affect the valve movement (Figure 3). Only
changes in condensing pressure can throttle the valve either opened or closed.
An ORI valve is usually used in conjunction with the ORD
(Open on Rise of Differential) valve (Figures 4 and 5). The ORD valve is
located between the discharge line and the receiver inlet (Figure 2). It
responds to changes in pressure differences across the valve. The ORD valve is
thus dependent on the ORI valve for its operation. The ORD valve will bypass
hot compressor discharge gas from the compressor to the receiver inlet when it senses
a preset determined pressure difference across the valve.
Figure 3: Cutaway of ORI valve.
As the ORI valve senses a drop in condenser pressure and
starts to throttle shut, a pressure difference is created across the ORD valve.
The pressure difference is created from the reduced flow to the receiver
because of the throttling action of the ORI valve.
Once the ORI valve starts to throttle shut, the receiver is
still supplying refrigerant to the TXV and its pressure will eventually drop.
If the receiver pressure drops too low, its ability to keep feeding the liquid
line and TXV’s liquid will diminish. Something has to keep receiver pressure up
while the ORI valve is throttling liquid from the condenser. This is when the
function of the ORD valve comes into play.
Figure 4: ORD valve.
When the ORD valve senses a factory preset pressure
difference of 20 psid, it will start to open and bypass hot compressor
discharge gas to the receiver’s inlet. If the pressure difference across the
ORD valve ever reaches 30 psi, the valve will be fully open. The hot gas from
the compressor that also flows through the ORD valve serves to heat up any cold
liquid being throttled through the ORI valve at the receiver’s inlet. This hot
gas entering the receiver will also increase the pressure of the receiver and
allow it to deliver liquid to the liquid line and TXV when the ORI valve is
throttling shut on the outlet of the condenser.
Figure 5: An ORI valve is usually used in conjunction with
the ORD valve.
Both the ORI and ORD valve will automatically work in
conjunction with one another to maintain proper receiver pressure, regardless
of the outside ambient conditions. The ORD valve also acts as a check valve to
prevent reverse flow from the receiver to the compressor’s discharge line
during the off cycle.