Evaporator pressure regulation (EPR) valves (Figure 9.7) can be used in the suc­tion line, and their function is to prevent the evaporator pressure falling below a predetermined or controlled value, although the compressor suction pressure may be lower.

The application of a EPR valve is to:

1. Prevent damage to a liquid chilling evaporator which might result from freezing of the liquid.

2. Prevent frost forming on an air cooling evaporator, where this is close to freezing point, or where a temporary malfunction cannot be permitted to interrupt operation.

3. Permit two or more evaporators, working at different load temperatures, to work with the same compressor.

4. Modulate the evaporator pressure according to a varying load, controlled by the load temperature.

5. Act as a solenoid valve, controlled by a pilot solenoid valve.

The simplest EPR valve is spring-loaded, balancing the thrust of the spring, plus atmospheric pressure, on one side of a diaphragm or piston, against the inlet or evaporator pressure. For working pressures below atmospheric, a helper spring is fitted below the diaphragm. Slight variations will result from changes in atmospheric pressure, but these are too small to materially affect a refrigera­tion control system.

A service gauge is usually fitted adjacent to the valve or as part of the valve assembly, to facilitate setting or readjustment. Above about 40 mm pipe size, the basic EPR valve is used as a pilot to operate a main servo valve. Other pilot signals can be used on the same servo and many control functions are possible.

Figure 9.7 shows a main servo controlled by an electric pilot which responds to a signal from a controller to maintain a constant load temperature to a high level of accuracy. The constant pressure pilot prevents the evaporator tempera­ture becoming too low and the solenoid pilot is for on/off.

Posted in Refrigeration and Air Conditioning