This is a discharge air temperature graph of a medium temp produce case with an electronic evaporator pressure regulator valve. The peaks are the defrost cycles. Notice how the graph dips down while pulling back down after defrost. The set point on this circuit is 34 degrees.
Here is an illustration of a discharge air temperature on a circuit that uses a mechanical evaporator pressure control. The difference is quite remarkable. The integrity of the refrigerated product should also be apparent.
Posted by Jayson Goff, CM of the Refrigeration Service Engineers Society, the HVACR Training Authority. For more information visit RSES.ORG.