There are several aspects that are very important when it comes to top performance of any HVAC or refrigeration unit. One of those is known as Superheat. The first thing that came to my mind when I heard this term is this question, “What does heat have to do with the performance of an air conditioner or refrigeration unit?” You are probably asking the same question. In this article we will try to take a look at what Superheat actually is and how it determines the performance of any HVAC unit.
Superheat is the only truly accurate way to measure if the AC or refrigeration unit is correctly and fully charged. If the unit is charged with the exact correct amount of coolant it will operate at peak efficiency. Superheat measurements is required in every type of system. The constant superheat temperature value can only be determined mainly be the refrigerant charge in the system. Here are a few principles of Superheat:
· Superheat is the actual temperature of the refrigerant above the its’ saturation temperature.
· Superheating is also 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 cooling. This occurs because the state stays the same only the temperature changes.
· Refrigerants should never leave the evaporator coil at saturation temperature because liquids are present.
Superheating usually occurs in the evaporator coil in a three step process.
In the first step the refrigerant will enter the coil right behind the metering device. This metering device will restrict the flow of liquid to the evaporator. This will create a pressure drop that allows refrigerate saturation temperature to lower. At this point of the process the refrigerant Is below the temperature that is flowing through the return air and passing over the evaporator coil. Because of this low temperature the warm return air gives up heat to the colder refrigerant. This is where heat transfer takes place. At any addition of heat at saturation temperature a change of state will occur. Liquid boils to vapor. When this liquid turns to vapor, latent heat transfer takes place causing a massive heat transfer into the refrigerant vapor. The pressure in the evaporator reacts to the rise in density and molecular movement starts to increase.
In the second step at the saturation temperature is higher due to higher pressure. Refrigerants at saturation temperature are a mixture of liquid and vapor.
In the last process when all of the liquid has boiled off into a vapor, the pressure at the coil will have risen to a point that is higher than the original pressure point 1. The saturated refrigerant will reach a critical state while it enters the additional tubing in the evaporator.
The evaporator is designed with additional tubing that will absorb the heat. While the refrigerant leaving the point 2 area of the evaporator will have no more liquid changing to vapor. At this point the additional heat absorbed in the tubing raises the temperature of the vapor to a level that is above the saturation temperature. This heat transfer provides a minimal cooling effect. The amount of heat contained in the refrigerant above saturation temperature is Superheat.