Evaporator (Automotive Air Conditioning)

An evaporator is a device that facilitates the expansion and evaporation of refrigerant. Typically, it is incorporated into a closed refrigerant loop with a condenser, compressor, and metering device (such as an expansion valve). It is often referred to as the ‘cold’ coil or ‘evaporator coil’ in your vehicle’s air conditioning system.

Car air conditioner evaporator
An automotive evaporator core.
Image credit: nopponpat via Bigstock.

The distinction between the evaporator and evaporator core is important. Both of them are part of the ‘low pressure side’ or ‘cold side’ of your air conditioning system. The evaporator is actually the entire low-pressure portion of your refrigerant system, as the evaporation process occurs all throughout that side.

It extends from the expansion valve to the compressor inlet. The evaporator core on the other hand is a finned device that refrigerant passes through on the low pressure side of the system.

Receiver/drier and sight glass excluded for simplicity. The blue parts are cold, and the red parts are hot. The arrows denote refrigerant flow.

Compressor Inlet <<< Tubing <<< Evaporator Core <<< Tubing <<< Expansion Valve <<< Condenser Coil <<< Tubing <<< Compressor Outlet.

The evaporator’s low pressure (maintained with the help of the compressor’s suction and the expansion valve) causes the refrigerant entering it to vapourize rapidly. That rapid vapourization causes the evaporator and its fins to become cold.

What’s really happening is the refrigerant absorbs heat from the those same fins (and the cabin air passing through them) as it expands and turns into a gas. This is how your air conditioner cools down your car.

The flow of heat in your car’s air conditioning system looks something like this:

Heat from the hot air in your car >>> Evap. core fins >>> Evap. core >>> Refrigerant In Evap. >>> Condenser coil >>> Condenser fins >>> Outside air.

The refrigerant (which is now a gas, and is warmer after having absorbed heat from the surrounding air) enters the compressor and is pumped into the condenser. It is crucial that the refrigerant enters the compressor only in gas form. The expansion valve maintains a pressure difference between the high and low pressure side of the system and is designed to optimise performance.

Learn how the condenser works

Learn more about the vapour compression cycle that these air conditioning systems use.