Air conditioning (3): Description and operation

The thermostatic expansion valve (block valve) incorporates a restrictor which converts the liquid refrigerant into a low temperature, low pressure liquid vapour mixture (fine spray). The valve has an internally located temperature sensing bulb for precise control of superheat. If high temperatures are sensed, the valve opens wider and vice versa.

The thermostatic expansion valve is attached to the inlet and outlet ports of the evaporator, in the cooling unit behind the passenger's side of the fascia. The valve is the parallel charge type, consisting of an aluminium housing containing inlet and outlet passages. A ball and spring metering valve is installed in the inlet passage and a temperature sensor is installed in the outlet passage. The temperature sensor consists of a push rod connected to a diaphragm. The bottom end of the push rod acts on the ball of the metering valve. Pressure on top of the diaphragm is controlled by evaporator outlet temperature conducted through the push rod. The bottom of the diaphragm senses evaporator outlet pressure via internal passages.

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M82 0720

1 Diaphragm 2 Housing 3 Metering valve

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4 Inlet passage to evaporator 5 Outlet passage from evaporator 6 Temperature sensitive tube

Liquid refrigerant flows through the metering valve into the evaporator. The restriction across the metering valve reduces the pressure and temperature of the refrigerant. The restriction also changes the solid stream of refrigerant into a fine spray, to improve the evaporation process. As the refrigerant passes through the evaporator, it absorbs heat from the air flowing through the evaporator matrix. The increase in temperature causes the refrigerant to vapourise and increase in pressure.

The temperature and pressure of the refrigerant leaving the evaporator act on the diaphragm and temperature sensitive tube, which move to regulate the metering valve opening and so control the volume of refrigerant flowing through the evaporator. The warmer the air flowing through the evaporator matrix, the more heat available to evaporate refrigerant and thus the greater the volume of refrigerant allowed through the metering valve.