Thermostatic Expansion Valve System
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| Illustration 1 | g01077014 |
Many earlier model machines are equipped with the thermostatic expansion valve system. The following information explains the purpose of the thermostatic expansion valve:
- Restrict refrigerant flow and allow the compressor to increase the pressure on the high side of the air conditioning system.
- Control the amount of refrigerant that enters the evaporator.
The part of the air conditioning system from the compressor outlet to the expansion valve inlet is called the high side. The thermostatic expansion valve causes a restriction to refrigerant flow that increases the pressure between the expansion valve (restriction) and the compressor. The increase in pressure allows the refrigerant to change from a gas to a liquid.
The compressor increases the temperature of the refrigerant by concentrating the refrigerant into a smaller space. The expansion valve decreases the temperature, by allowing the refrigerant to spread out as the refrigerant leaves the orifice in the expansion valve. Because the pressure is greatly decreased, the refrigerant is coldest as the refrigerant leaves the expansion valve and enters the evaporator. The part of the air conditioning system from the expansion valve outlet to the compressor inlet is called the low side.
The thermostatic expansion valve system is equipped with a receiver-dryer.
Thermostatic Expansion Valve
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| Illustration 2 | g01077017 |
Two types of expansion valves are used on machines: internally equalized and externally equalized. Both the internally equalized expansion valve and the externally equalized expansion valve have a thermal bulb that is connected to a diaphragm by a small tube. The thermal bulb contains a refrigerant. A clamp holds the thermal bulb securely to the evaporator exhaust line. The thermal bulb is sensitive to exhaust temperature. If the exhaust temperature increases, the refrigerant inside the bulb expands. The expanding refrigerant exerts pressure against the diaphragm in the top of the valve. The diaphragm is connected through a pin to the valve seat. Pressure that is exerted against the diaphragm causes the diaphragm pin and valve seat to move. As the valve seat moves away from the orifice, more refrigerant flows into the evaporator. An increase in the flow of refrigerant causes the evaporator exhaust to become cooler. The cooler exhaust temperature causes the refrigerant to condense in the thermal bulb. This reduces the pressure that is against the diaphragm, the pin, and the valve seat. The valve seat moves in order to reduce flow through the orifice.
In the internally equalized valve, the pressure of the refrigerant that enters the evaporator is fed to the bottom of the diaphragm through the internal equalizing passage. Gas expansion in the thermal bulb must overcome the internal balancing pressure and the spring before the valve will open in order to increase refrigerant flow.
On the external equalizer valve, the pressure that is fed to the bottom of the diaphragm is taken from the evaporator exhaust line by an equalizer tube. The equalizer tube balances the evaporator exhaust pressure against the pressure that is caused by the expansion of the gas in the thermal bulb.
The superheat spring prevents surges of excessive liquid from entering the evaporator. Superheat is an increase in temperature of the refrigerant gas above the temperature at which the refrigerant evaporated. The superheat spring is installed against the valve and adjusted to a predetermined setting at the time of manufacture. The expansion valve is designed so that the temperature of the refrigerant at the evaporator exhaust line must have 3 °C (5 °F) of superheat before more refrigerant is allowed to enter the evaporator. The spring tension is the determining factor in the opening and closing of the expansion valve. When the expansion valve opens and closes, the spring tension retards or assists valve operation, as required.
Receiver-Dryer
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| Illustration 3 | g01077018 |
The receiver-dryer has three functions: Dry, store, and filter liquid refrigerant. As the high pressure liquid refrigerant flows into the receiver-dryer, the refrigerant is filtered through a desiccant that removes any moisture that may have entered the refrigerant. The refrigerant is stored until the refrigerant is needed by the system. When the system calls for refrigerant, high pressure liquid flows through a fine mesh screen that is fitted on the pickup tube. The screen prevents any debris from circulating through the air conditioning system. High pressure liquid flows from the receiver-dryer to the thermostatic expansion valve.