Originally Posted by upallnight

Automotive refrigeration system are designed so that the evaporator core does not get below 32 F. Why, when our refrigerator at home can get to a temp below 32F. Because unlike a refrigerator that has a frost free heating element design into it to eliminate ice build up on the evaporator, the evaporator in an automobile does not have this feature. And with the hot humid air that is drawn into the car, a lot of humidity is condensing on the evaporator. If the evaporator core was allowed to get below 32F, the water would freeze and block the incoming air from passing through the evaporator. This will prevent the air from cooling down so now you have warm air going around the evaporator instead of through it. When this happens you don't have cold air coming out of the vent.

A low side pressure also determine the coldness of the evaporator core. The lower the low side pressure the colder the evaporator can get. This is why the design low side pressure is usually around 34 - 40 PSI.