As refrigerant courses through your typical residential air conditioning system (one consisting of a direct expansion evaporator coil and an air-to-air condenser), its state changes from a vapor to a liquid and then back to a vapor. For the system to produce years of proper cooling, it is imperative that the refrigerant be in the right state at the right time in the right location. As we cannot see inside of the system piping and components, how can we tell just what state the refrigerant is in? By taking superheat and subcooling measurements.
Refrigerant enters the evaporator in a partial liquid/vapor mixture. Part of the liquid turns to vapor as it passes through the expansion device by way of pressure drop. The remaining mixture absorbs heat (from the area to be cooled) and turns into a saturated vapor. This occurs in the top two-thirds of the coil. Any additional heat that the refrigerant picks up toward the end of the coil is superheat. Upon leaving the evaporator, the desired refrigerant state in the suction line is a superheated vapor. For the compressor to operate correctly, it must see the manufacturer’s recommended level of superheat. Too much superheat and the compressor will overheat. No superheat and the compressor will be subjected to liquid refrigerant.