ACHRNEWS

Low Ambient Head Pressure Controls

May 7, 2007
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 will 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 ambient.

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 control valves.

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.

Publication date: 05/07/2007