WD Catalog 2018

Chapter 2

– System Operation

2-6

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Table of Contents

Index

4. Expansion Valve (Refrigerant Metering Device)

When refrigerant moves from the receiver-drier, it travels through an-

other high pressure hose to a metering device at the inlet of the evapora-

tor 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.

HIGH PRESSURE LIQUID

METERING ORIFICE

VALVE SPRING

INLET FROM

RECEIVER DRIER

OUTLET TO

COMPRESSOR

OUTLET TO

EVAPORATOR

DISCHARGE FROM

EVAPORATOR

WHEN OPERATING,

PIN PUSHES BALL

AWAY FROM BALL

SEAT, REFRIGERANT

BLEEDS INTO

EVAPORATOR

OPERATING

SEALED SENSING BULB

VALVE DIAPHRAGM

INTERNAL

EQUALIZATION

PASSAGE

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 refrig-

erant 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 evapora-

tor 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 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 pres-

sure holds the valve closed.