Though refrigeration and A/C share the same basic cycle and many of the same components, they also have many differences, including the meaning of the existence of frost. Normally an A/C coil temperature stays above freezing. Frost on an A/C coil tells you there is a problem, but not so in refrigeration.
A refrigeration coil is normally operating at an SST of 25 degrees or less, so it won't just sweat but will also frost. The fin spacing is much larger on a refrigeration coil to allow for proper air flow through the coil, even if it has frost on it. As the coil builds frost some type of a defrost cycle is needed. On a walk-in cooler, especially with an outdoor condensing unit, it is a good idea to install a defrost timer to cycle the refrigeration off two-four times per day to allow accumulated frost to melt. The evaporator fans continue to run circulating air across the coil melting the frost. On a freezer, defrost heaters may be added to melt the ice during a defrost cycle. When heaters are used the fans will shut off during the defrost cycle. Many indoor condensing units run on a low-pressure control instead of a thermostat. Adjusted properly, the low-pressure control will not allow the compressor to turn back on until the coil has warmed up enough to ensure that the frost is gone from the coil.
In some instances, frost may also indicate a problem in the refrigeration system.
When the coil only frosts on the inlet, a starveling evaporator is indicated. Low charge, restriction or a TXV that's out of adjustment are some common causes.
Frost on a suction line may indicate a flooded evaporator. A flooding evaporator is being fed more refrigerant than it can boil off and this may slug the compressor.
Liquid leaving the evaporator may be indicated by frost on the suction line or compressor. A flooding evaporator may be caused by the low load on the evaporator (fans off) or a TXV that's out of adjustment. Frosting evenly across the evaporator is what should happen on a properly feeding coil. Check the coil superheat at the evaporator outlet. Normally the superheat should be between 8 and 12 degrees. Too high and the coil is starving. Too low and the coil may be overfed. Proper superheat will indicate the problem is elsewhere.
Frost on the suction line or on the compressor may not be a problem at all! On freezers, the coil temperature could be -10 or even lower. With 10 degrees of superheat, the suction line would be 0 degrees. 0 degree suction gas may cause part of the compressor to frost as well. As long as we have proper or any superheat, even with a frosted compressor there is no liquid getting to the compressor. In this case, the frosted suction line and even compressor are ok!
Leaking door gaskets especially inside a building that has high humidity may increase the rate at which frost builds up on a coil. Replacing worn non-sealing door gaskets will help keep frost accumulation to a minimum. Bad door gaskets may also prevent the cooler or freezer from holding proper temperature, so be sure to check your gaskets.