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.
Superheat is the amount of heat added to a dry vapor, in the absence of liquid, to raise the vapor temperature above its boiling point (saturation point) corresponding to the pressure at which it is operating. If there is liquid refrigerant present, there can be no superheat.
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.
Superheat can also be found in the discharge line of the compressor. There is a greater amount of superheat in this location due to the heat of compression, friction created inside of the compressor, and the compressor motor.
In addition to checking the refrigerant state, knowing a system’s superheat level can be useful in diagnostic procedures and verifying proper refrigerant levels. Measure suction pressure at compressor inlet. Convert this pressure to its equivalent saturation temperature using a pressure-temperature chart. Measure the temperature of the suction line at compressor inlet. The difference between these two temperatures is the superheat.
SUBCOOLING Subcooling is the amount of heat removed from a liquid below its boiling point (saturation point). If there is refrigerant vapor present, there can be no subcooling.
Refrigerant leaves the compressor in a hot, superheated vapor state. As it passes through the upper portions of the condenser, its heat is rejected. Next it turns into a saturated liquid. After reaching its boiling point near the lower portion of the condenser, any additional heat that is rejected is subcooling. Upon leaving the condenser, the desired state of the refrigerant is a subcooled liquid. It continues in this state until it reaches the metering device. After passing through the metering device, its partial liquid/vapor state moves into the evaporator to start the cycle over again.
MEASURING SUBCOOLING As with superheat, knowing a system’s subcooling will reveal its refrigerant state. In addition, it will be of use in diagnostic procedures and verifying proper refrigerant levels. Measure high-side pressure before the evaporator. Convert this pressure to its equivalent saturation temperature using a pressure-temperature chart. Measure the temperature of the liquid line before the evaporator. The difference between these two temperatures is the superheat.
A WINDOW Measurements of superheat and subcooling are the window to the inside of a refrigerant circuit. Remember, if there is superheat, there can be no liquid. If there is subcooling, there can be no vapor.
Rothacker is a director of area51hvac.com. For questions or comments on the Tech Page, contact Rothacker at email@example.com (e-mail).
Publication date: 07/15/2002