Climate Control: Evaporator: Notes

Fig 1: Identifying Evaporator

Courtesy of BMW OF NORTH AMERICA, INC.

From the expansion valve, the liquid refrigerant passes into the evaporator. Once the refrigerant passes the orifice in the expansion valve, its pressure drops. The liquid refrigerant, now in the evaporator, immediately begins to boil. As it boils, it absorbs heat from the air that passes over the fins and tubes of the evaporator. This cools the air and heats the refrigerant. The refrigerant, now a vapor again, is then drawn back into the low side of the system by the compressor.

The evaporator on current BMW cars is mounted crosswise in the housing. It is similar in construction to a radiator (a copper or aluminum coil with fins). The fins provide a large surface area to transfer heat from the air to the colder refrigerant inside the coil. The evaporator (like the condenser) is a "heat exchanger."

Fig 2: Identifying Evaporator (Like Condenser) Heat Exchanger Function

Courtesy of BMW OF NORTH AMERICA, INC.

As air passes over the evaporator fins, the moisture condenses on the fins as the air cools. Water collects in the bottom part of the housing and exits through one or two drains.

Evaporator service:

• Check water drains.
• Straighten bent fins (a tool is available locally)
• A frost ring on a tube indicates a restriction. (On some vehicles, the evaporator and expansion valve can be accessed by removing the glove box.)