Takeuchi TB175W, Руководство по ремонту

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III-53 TB175W
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 pres-
sure 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 refrig-
erant 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
refrigerant 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.