Air conditioning (Receiver/Drier): Description and operation

Receiver/Drier

This liquid then passes through a receiver/drier which fulfils four functions. It acts as a reservoir, moisture extractor, filter and by means of a sight glass, provides a method of determining the state of the refrigerant without breaking into the system. From the receiver/drier, the moisture free refrigerant liquid passes through a thermostatic expansion valve. This valve, converts the liquid refrigerant to a low temperature, low pressure vapour/liquid mixture. To prevent liquid passing through to the compressor, a low temperature thermistor, attached to the outlet pipe of the evaporator and connected to the thermostatic expansion valve by a capillary tube, controls the amount that the valve opens and closes in relation to the temperature of the low pressure high temperature refrigerant at the outlet.

Evaporator

The refrigerant then passes through the evaporator. Blower assisted air passes through the matrix of the evaporator and is cooled by absorption due to the low temperature refrigerant passing through the evaporator. Most of the moisture being held in the air is condensed into water by the evaporator and drains away beneath the vehicle via a drain tube.

A thermistor is fitted in the airflow out of the evaporator to sense the temperature of the exterior fins. Should ice begin to form due to a too cold condition it will signal the switch pack to disengage the clutch on the compressor.

From the evaporator, low pressure slightly superheated refrigerant vapour passes to the compressor to complete the cycle. During this transition between a gas and a liquid, the refrigerant becomes very hot and is passed to the condenser to be cooled.

AUTOMATIC TEMPERATURE CONTROL - ATC

This system has the facility to automatically control blower speed, air distribution, air temperature, fresh/recirculated air selection and the air conditioning compressor to provide a comfortable interior temperature with reference to a driver controlled setpoint.

System description

To enable the ATC unit to control the interior compartment temperature effectively it needs to obtain information from sensors located in and around the vehicle. These sensors are as follows:

Ambient temperature sensor

The sensor consists of an encapsulated thermistor measuring air temperature. The sensor is energised via the main harness from the ATC unit. This sensor allows the control unit to modify the setpoint in order to compensate for extremes in ambient temperature.

Sunlight sensor

Mounted between the defrost outlets on the fascia. The sensor consists of a photodiode sensitive to the brightness of the sun. The sensor is energised from the ATC computer via the sensor harness. When this sensor detects the light radiation is sufficiently high it prompts the control unit to increase the amount of air directed towards the face vents. This helps to prevent the car interior temperature increasing due to the effects of direct sun light.

Interior temperature sensor

Mounted in the instrument binnacle on the fascia. The sensor is screwed to the binnacle surround on the side inboard of the driver. The mounting screws are located behind a clip retained intake grille.

In order to achieve control of interior temperature over a wide range of vent settings/occupant variations, it is necessary to employ an aspirated sensor having a fan used to draw a controlled stream of air over the sensing element. The fan operates when the ignition is switched on even if the heating system is switched off. This allows the ATC unit to receive an accurate reading of the passenger compartment temperature when the system is activated.