Evaporator pressure regulation helps us achieve 100% runtime between defrost. EPR valves can come in mechanical or electronic varieties. There is a mechanical EPR valve on the right side; you can adjust the stem of a mechanical EPR to set your evaporator saturation where you want. There is an electronic EPR valve on the left side; electronic EPR valves reference a temperature sensor and pressure transducer to maintain a constant temperature.

A mechanical EPR will hold steady at the pressure you set, but the electronic EPR will modulate a little bit.

However, for the EPR valves to work as they should, the suction pressure must be below the coldest evaporator at the main suction header. So, you’ll probably need a bit of wiggle room to allow for some variation in temperature and make sure the valves can operate properly.

The overall goal of the EPR valves is to throttle the suction gas to control the evaporator pressure and maximize runtime by keeping constant, steady pressure on the load.