HVAC Training - Superheat

Principles of Superheat

• Superheat is the temperature of the refrigerant vapor above its saturation temperature.
• Superheating is done by the load. If the load is low, the superheat will be low. If the load is high, the superheat will be high.
• Superheat is sensible heat transfer that provides very little useful cooling. This occurs because there is no change of state; only change of temperature.
• Refrigerants should never leave the evaporator coil at saturation temperature because liquids are still present.
• Latent heat transfer gives us our greatest energy transfer and results in massive cooling capacity.

We use the superheat and subcooling levels of an air conditioning system as a measure of the system's operating conditions. As an air conditioning service technician you must ALWAYS measure both the superheat and subcooling levels of a system.

Inside the Evaporator Coil The level of the suction vapor superheat is the primary measurement that all diagnostics procedures begin with. Suction vapor superheat can be defined as the temperature of the suction vapor above its saturation temperature. Since any rise in temperature without change of stat is Sensible Heat, the superheating of refrigerants does not provide a great amount of cooling. It is necessary to prevent liquid from being present in the suction vapor as it enters the compressor.

Superheating is a three step process that happens in the evaporator coil.

1. The refrigerant enters the evaporator coil just after the metering device. The metering device restricts the flow of liquid to the evaporator, creating a pressure drop that lowers the refrigerant saturation temperature.  At this point the temperature of the refrigerant is well below the temperature of the return air that is passing over the evaporator coil. Because the saturation temperature is very low, the warm return air gives up heat to the colder refrigerant, where heat transfer takes place. The saturated refrigerant absorbs the heat. Since any addition of heat at saturation temperature causes a change of state, liquid boils to a vapor. When liquid changes to vapor, latent heat transfer takes place, causing a massive heat transfer into the refrigerant vapor. As the liquid boils to a vapor, more vapor molecules are formed, the molecules move faster and the density of the vapor rises. The pressure in the evaporator reacts to the rise in density and molecular movement starts to increase.
2. At this point, the saturation temperature of the refrigerant is higher due to a higher pressure, even though the refrigerant is in a saturated conditioning. Remember, refrigerants at saturation temperature are a mixture of liquid and vapor.
3.  When all of the liquid has boiled off to a vapor, the coil pressure will have risen to a level that is higher than the original pressure at point 1. The saturated refrigerant will have reached a critical state as it enters the additional tubing in the evaporator's circuit. The evaporator is designed with additional tubing runs to absorb the additional heat. Since the refrigerant leaving the point 2. area of the evaporator has no more liquid changing to vapor, the additional heat absorbed in the tubing raises the temperature of the vapor to a level that is above the saturation temperature. This new heat transfer is sensible heat transfer, that provides minimal cooling effect. The amount of heat contained in the refrigerant above saturation temperature is SUPERHEAT.

Physics – Superheating of Refrigerant Vapor The heat transfer process that causes superheating is the fundamental process of providing mechanical refrigeration. Both latent and sensible heat transfers allow the service technician to determine the operating performance of the system. Heat transfer in the evaporator coil has two effects upon the saturated refrigerant. First, the liquid refrigerant boils off into a vapor, and second, the vapor temperature rises to a level that is higher than the saturation temperature of the refrigerant. Again, the temperature of the vapor above its saturation temperature is called Superheat. The first heat transfer process in the evaporator changed the state of the liquid refrigerant into a vapor. The energy (heat) required to complete latent heat transfer is massive and gives the refrigeration cycle its best energy exchange. When the latent heat transfer process is completed in the evaporator, the refrigerant is still at saturation temperature. At this stage, the refrigerant will still have some vapor molecules condensing into a liquid. The refrigerant is highly unstable. Service technicians should never allow a system to operate in a condition where saturated refrigerant leaves the evaporator outlet. If this condition does occur, liquid slugging of the compressor is sure to happen.The second heat transfer process adds additional heat to the refrigerant vapor in the evaporator. As the refrigerant passes through the last areas of the coil tubing, it gains heat and raises the vapor temperature. This heat transfer process is called sensible heat transfer. Unlike latent heat transfer, sensible heat changes the temperature of the refrigerant, without changing its state. Sensible heating of the vapor Superheats the refrigerant.

by Ron Walker