evaporator coil The evaporator coil is the component in an air conditioner or heat pump that is responsible for the cooling of a home. The evaporator reduces the temperature but does not actually make anything cold. Cold is really defined as there being no heat. Things are made colder when there is heat being removed. So when cooling your home, your AC is just removing the heat from your home. Technically your evaporator is there to absorb the heat and remove it.

The direction of heat transfer is normally from a warmer substance to a cooler one. Your evaporator needs to be cooler than the air being blown over it so that way it picks up the heat in the air and transfers it through the refrigerant. The constant flow of refrigerant results in constant heat transferring. Dehumidification also occurs because the evaporator operates at a temperature less than the dew point, if the temperature is above the dew point then there will be moisture in the air, and if it’s less than it, then it will condense and there won’t be moisture in the air.

The combined effect of the compressor and metering device of the air conditioner or heat pump allow the refrigerant in the evaporator to stay at a low temperature because they make the pressure of the system really low. The lower the pressure of the coil the lower the temperature and the higher the pressure the higher the temperature. For the evaporator we want low pressure so that its at a low temperature that will absorb heat. The compressor makes the refrigerant denser and the metering device releases pressure. Refrigerant will enter the evaporator coil as an 80% liquid and a 20% vapor from the compressor and metering device.

As the refrigerant flows in the evaporator heat is absorbed by the refrigerant, the liquid refrigerant will boil or evaporate into vapor. Heat that cannot be measured by a thermometer, known as latent heat or hidden heat causes the refrigerant to boil. The temperature of the refrigerant does not increase but its physical state changes instead (changing from a liquid to a vapor). The refrigerant will continue through this process making all of it become vapor and then it goes through super heat.

Super heat means the “vapor refrigerant” is still receiving more heat that is increasing temperature. The heat transfer that causes the super heating is a sensible heat transfer, sensible heat can be measured by a thermometer, and it’s called sensible heat because it can be “sensed”. The refrigerant super heating at 100% vapor will still continue to absorb heat as it enters the compressor. Evaporators on residential homes are dry-type evaporators because they boil off all refrigerant into 100% vapor making the coil “dry”.

Summary Of Your AC’s Evaporator Process

Your evaporator coil will have refrigerant flowing through it from the rest of the air conditioning system. The refrigerant will enter the evaporator as a cold 80% liquid and 20% vapor. The refrigerant picks up heat from the air blowing over the evaporator coil as the refrigerant flows through it. Once the refrigerant picks up enough heat it goes through latent heat transfer, physically changing it completely into a vapor but not increasing temperature, because latent heat cant be measured with a thermometer. Once the refrigerant becomes 100% vapor, it superheats, superheat is just the amount of heat absorbed by the “vapor refrigerant” that increased the temperature. Then the “vapor refrigerant” goes through high pressure in the compressor, condenser, txv and metering device, where it comes back into the evaporator as low pressure cold refrigerant that is 80% liquid and 20% vapor.


Silberstein, Eugene. “Section 1 Evaporator.” Residential Construction Academy: HVAC, Second Edition. Clifton Park, NY: Thomson Delmar Learning, 2012. 27-28. Print.

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