Head-Pressure Control Valves

Design of air-conditioning and refrigeration systems using air-cooled condensing units involves two main problems that must be solved if the system is to be operated reliably and economically. These problems are high-ambient and low-ambient operation. If the condensing unit is properly sized, it will operate satisfactorily during extreme-ambient temperatures. However, most units will be required to operate at ambient temperatures below their design dry-bulb temperature during most of the year. Thus, the solution to low-ambient operation is more complex.

Without good head-pressure control during low-ambient operation, the system can have running-cycle and off-cycle problems. Two runningcycle problems are of prime concern:

? The pressure differential across the thermostatic-expansion valve port affects the rate of refrigerant flow. Thus, low head pressure generally causes insufficient refrigerant to be fed to the evaporator.
? Any system using hot gas for defrost or compressor-capacity control must have a normal head pressure to operate properly. In either case, failure to have sufficient head pressure will result in low suction pressure and/or iced evaporator coils.

The primary off-cycle problem is the possible inability to get the system on-the-line if the refrigerant has migrated to the condenser. The evaporator pressure may not build up to the cut-in point of the low pressure control. The compressor cannot start, even though refrigeration is required. Even if the evaporator pressure builds up to the cut-in setting, insufficient flow through the TEV will cause a low suction pressure, which results in compressor cycling.

There are nonadjustable and adjustable methods of head-pressure control by valves. Each method uses two valves designed specifically for this type of application. Low-ambient conditions are encountered during fall winter-spring operation on air-cooled systems, with the resultant drop in condensing pressure. Then, the valve’s purpose is to hold back enough of the condensed liquid refrigerant to make part of the condenser surface inactive. This reduction of active condensing surface raises condensing pressure and sufficient liquid-line pressure for normal system operation.