There are frequent situations where refrigerant flow controls must regulate pressure; several are illustrated in Fig. 11.8. Controlling the pressure in the evaporator, as in Fig. 11.8a, may be required in two different situations. It may be necessary to prevent the evaporating temperature from dropping too low in order to protect the product being refrigerated. Another application of a control that regulates the pressure in the evaporator is used during hot-gas defrost, as explained in Sec. 6.22. A different technique to prevent evaporating pressure from dropping too low, shown in Fig. 11.8b, is called hot-gas bypass. The valve controls its downstream pressure and opens if the evaporator pressure drops below the outlet pressure setpoint. Another pressure-regulating valve that controls its downstream pressure is shown in Fig. 11.8c. This valve might be used to prevent the suction pressure to the compressor from rising so high that the compressor motor overloads.
A cutaway view of a small valve that regulates the upstream pressure is shown in Fig. 11.9a. The valve remains closed until the upstream pressure rises to the setpoint whereupon the diaphragm lifts off the port. For the valve to open wider and pass a greater flow rate, the upstream pressure rises somewhat higher than the setpoint as the diaphragm works against the spring force. The setpoint is fixed by adjusting the screw that regulates the spring precompression.
Pilot-operated valves are available in larger sizes, as in Fig. 11.9b, where in an operation similar to pilot-operated solenoids5 the upstream pressure controls a flow of pilot fluid to the power piston.
While both the valves shown in Fig. 11.9 are manually adjustable, in some installations it would be convenient or necessary to provide automatic adjustment of the settings. Additional capabilities are available6 that allow adjustment of the setting through a pneumatic signal, a signal based on temperature, or an electric signal. In addition, pressure-regulating valves are available that will control one of two different pressures, depending on whether a small pilot solenoid is open or closed.