The evaporator pressure regulator (Figs 2.43 and 2.44) is mounted between the outlet of the evaporator and the compressor inlet (suction side).The valve regulates the pressure inside the evaporator to prevent icing. If the pressure drops below a certain threshold (196 kPa (28.4 psi)) then the valve closes to restrict the flow of refrigerant and increase the pressure inside the evaporator. This is to stop the evaporator temperature from reaching 0°C due to the relationship between temperature and pressure.
When the cooling load is high the vapour pressure of the refrigerant in the evaporator is high. The valve fully opens and the refrigerant flows unobstructed to the compressor.The valve operation is based on a spring bellows which expands and contracts with changes in refrigerant pressure.This device virtually eliminates the need for the compressor to cycle on and off to regulate the temperature of the evaporator.
De-ice switch (TXV system)
A de-ice switch (Fig. 2.45) is a temperature sensor and relay built as one unit. The temperature sensor is fitted to the evaporator’s fins and measures the temperature of the evaporator surface using an NTC type temperature sensitive resistor.This sensor sends the information to the relay in the form of a voltage drop and when approaching the freezing point of water (0°C) the current to the compressor clutch is interrupted to increase the pressure in the evaporator and avoid the surface water freezing. With a system threshold of 1°C at the surface, the relay will turn the compressor off. Once the surface increases to 2.5°C, the compressor, via the relay, will be switched
on again.
Evaporator sensor
An evaporator sensor is used on a system which generally uses electronic control as opposed to electrical control. These systems use the temperature sensor to feed a voltage reading to a control module which is programmed to use the data and compare it to stored data in its memory. The sensor is generally an NTC type sensor which means it has a negative temperature coefficient.This means that with an increase in temperature the resistance of the sensor will reduce. This will affect the current flowing through the sensor and the voltage across the sensor. The module can apply this data and when the corresponding voltage is sensed, associated with 1°C, the module will disengage the compressor clutch via a relay to stop the evaporator from freezing.The compressor clutch will be re-engaged when the temperature of the evaporator reaches 2.5°C. The sensor is generally fitted to the evaporator fins for direct measurement. Advanced vehicle electronic systems may use a multiplex wiring system to transfer the information digitally (see Chapter 3, section 3.2).
Compressor cycling switch (FOV system)
If the temperature in the evaporator approaches 0°C then icing will occur on the surface of the evaporator due to the water droplets which form. This will reduce the volume of air flowing through the evaporator and reduce the efficiency of the system. To prevent this a compressor cycling switch (Fig. 2.46) is fitted to the accumulator to deactivate the compressor clutch when a specific pressure is reached. For example, when 1.5 bar is reached on the low pressure side of
the system the switch contacts will open and directly or indirectly interrupt the current flowing to the compressor clutch. Once the pressure rises to 2.9 bar then the switch contacts will close again and the compressor will re-engage. Often the cycling switch is also used as a low pressure switch in case there is a system leak and the refrigerant escapes. In this case the switch will open at 1.5 bar and disengage the compressor clutch ensuring the compressor is not damaged due to no refrigerant flow (refrigerant carries the lubricant to lubricate the compressor).
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