Every air conditioning system includes two heat exchangers. That is because air conditioning is all about heat transfer. The evaporator is one of those heat exchangers. In order to remove the heat from the air entering the vehicle, the A/C evaporator allows the refrigerant to absorb the heat from the air passing over it.

Located at the firewall or, in some cases, under the dash of the vehicle, the A/C evaporator holds liquid refrigerant entering the bottom of the evaporator. When the hot air passes over the evaporator, the cold refrigerant absorbs the heat, because heat will always transfer from areas of high temperature to areas of low temperature. The air is hot and the refrigerant is cold. Therefore, the heat from the air is transferred to the cold refrigerant.

That heat transfer causes the refrigerant to boil and change state to vapor refrigerant. After changing state to vapor refrigerant, the vapor is removed from the top of the evaporator by compressor suction.

In the automotive A/C system, the evaporator is the largest storage facility for refrigerant oil or A/C compressor oil. Typically, the evaporator will hold between 3 and 5 ounces of oil. Whenever replacing an evaporator, it is important to assure that oil is replaced.

The biggest reason for failure on an evaporator is internal corrosion. When accumulators or receiver driers become contaminated and can no longer hold any moisture in the system, the moisture begins to react with the refrigerant. The resulting chemical reaction causes harmful acids to form. Those acids travel throughout the system and can quickly deteriorate the aluminum components, like the evaporator.

The fact that the evaporator is the single largest source of refrigerant and A/C compressor oil in the A/C system means that it is also the largest storage facility for any acids that form. Those acids corrode the evaporator from the inside causing leaks that are typically not repairable.

In many cases the evaporator inlet also holds the orifice tube. The orifice tube or expansion tube causes a restriction in the A/C system causing the high pressure liquid refrigerant from the condenser to be changed to low pressure liquid refrigerant before entering the bottom of the evaporator. Whenever replacing an evaporator, it is important to assure a replacement orifice tube is installed back into the replacement unit.