III-63
Air Conditioner—System Operation
4. Expansion Valve (Refrigerant Metering Device)
When refrigerant moves from the receiver-drier, it travels through another high pres-
sure hose to a metering device at the inlet of the evaporator coil. The metering device
can be an expansion valve, an expansion tube or a combination (multiple function)
valve. Between the compressor and this point inside the system, the pressure is high
and can range from 150 to 250 pounds per square inch. The expansion valve (TXV) is
closely connected to the evaporator. A diaphragm opens the valve by exerting pressure
on the spring. Pressure comes from gas inside the diaphragm housing on top of the
valve and in the sealed sensing bulb. The sensing tube is located in the outlet of the
evaporator and picks up heat from warm refrigerant leaving the evaporator. The gas in
the valve diaphragm housing and sensing tube expands when it gets warmer and forces
the expansion valve open at the metering orifice.
5. Evaporator Coil
The expansion valve or other type of metering device bleeds high pressure refrigerant
into the evaporator coil, where the pressure is low. The refrigerant expands rapidly in
this low pressure environment. When it expands it “changes state”. The sudden drop
in pressure brings the refrigerant temperature down quickly inside the evaporator coil.
Figure 2-8 shows an evaporator coil and thermostat. Refrigerant is sprayed into the
evaporator by the high side pressure when the expansion valve opens. The refrigerant
absorbs heat from the air when the blower forces the air through the fins. When the
thermostat probe senses the upper limit of the thermostat heat setting, a circuit closes.
The compressor clutch engages and the compressor operates and moves more refriger-
ant to the high side of the system.
Figure 2-7
This block type expansion
valve cutaway view will
give you a better idea how
these valves work. Spring
pressure holds the valve
closed.
To Next Page
Loading...
Need help?
General
