The Refrigerant Cycle: Notes

During stabilized conditions (A/C  system shutdown), the refrigerant pressures are equal throughout the system. When the A/C  compressor is in operation, it uses a piston pump to compress the cool vapor, causing it to become high-temperature/high-pressure vapor. The high-temperature/high-pressure vapor is then released into the top of the A/C  condenser core.

The A/C  condenser, being close to ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubing. The now liquid refrigerant, still at high pressure, exits from the bottom of the A/C  condenser and enters the inlet side of the A/C  receiver/drier (integral to the condenser). The receiver/drier is designed to remove moisture and contaminants from the refrigerant system.

The outlet of the receiver/drier is connected to the TXV. The TXV provides the orifice which is the restriction in the refrigerant system and separates the high and low pressure sides of the A/C  system. As the liquid refrigerant passes across this restriction, its pressure and boiling point are reduced. An internal temperature sensing bulb senses the temperature of the refrigerant flowing out of the evaporator core and adjusts an internal pin-type valve to meter the refrigerant flow into the evaporator core. The internal pin-type valve decreases the amount of refrigerant entering the evaporator core at lower temperatures and increases the amount of refrigerant entering the evaporator core at higher temperatures.

The liquid refrigerant is now at its lowest pressure and temperature. As it passes through the A/C  evaporator, it absorbs heat from the airflow passing over the plate/fin sections of the A/C  evaporator. This addition of heat causes the refrigerant to boil (convert to gas). The now cooler air can no longer support the same humidity level of the warmer air and this excess moisture condenses on the exterior of the evaporator coils and fins and drains outside the vehicle.

The refrigerant cycle is now repeated with the A/C  compressor again increasing the pressure and temperature of the refrigerant.

A thermistor monitors the temperature of the evaporator core and controls A/C  clutch cycling. If the temperature of the evaporator core is low enough to cause the condensed water vapor to freeze, the A/C  clutch is disengaged by the PCM.

The high-side line pressure is also monitored so that A/C  compressor operation will be interrupted if the system pressure becomes too high or is determined to be too low (low charge condition).

The A/C  compressor relief valve will open and vent refrigerant to relieve unusually high system pressure.