Evaporator Core: Description and Operation

EVAPORATOR







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."

As air passes over the evaporator fins, the moisture condenses on the fins as the air cools.

Water collects in the bottom pant 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.)

EVAPORATOR TEMPERATURE SENSOR







If the evaporator temperature is allowed to cool below 1°C, condensation can freeze on the evaporator. The ice then insulates it from the air passing over it, and it works much less efficiently. A temperature sensor is used to protect the evaporator from freezing, by signaling the control module to turn the compressor off, so that condensation cannot freeze on the evaporator. The compressor is typically disengaged at 34°-37°F (1°-2°C). When the compressor is turned off, refrigerant flow is reduced and the evaporator temperature rises.

The temperature sensor is a Negative Temperature Coefficient (NTC) thermistor whose resistance varies according to the temperature of the evaporator core. Resistance is higher at lower temperatures, and decreases as the temperature rises.