TM 9-4120-398-14
(4) EXPANSION. After following through the falter-drier and sight glass, the liquid refrigerant enters the
therrnostatic expansion valve. The pressure difference across the valve produces refrigerant flow through the valve orifice into
the lower-pressure space in the evaporator. The rate of flow through the thermostatic expansion valve (TXV) determines the
degree of refrigerating action that takes place. Extending from the valve is a sensing element that is affixed to the compressor
suction line. The sensed ternperature affects the throttling action in the valve, allowing more refrigerant to flow through at high
loads to maintain 10F superheat, and throttling the flow at low loads to prevent refrigerant from entering the compressor in the
liquid state.
(5) HEAT ABSORPTION. Reduced pressure inside the evaporator causes the refrigerant to vaporize. In
changing into a vapor, the refrigerant absorbs heat initially from the evaporator coils and fins, and subsequently from the air
blown across the evaporator. A venturi-type distributor at the evaporator inlet breaks up the refrigerant flow into six circuits
for equal passage through the various parallel coils in the evaporator.
(6) ACCUMULATOR. Suction produced by the compressor draws refrigerant into the compressor intake.
Refrigerant is compressible as a vapor but not as a liquid. Liquid refrigerant entering the compressor results in "slugging" and
can cause mechanical damage. An accumulator between the evaporator and compressor collects any liquid refrigerant in the
system at that point and in an inverted Ushaped outlet tube in the accumulator ensures that only vapor is drawn into the
compressor.
(7) QUENCH VALVE. Refrigerant pumped through the compressor carries away the heat produced by current
flow through the sealed-in motor windings. The temperature of the refrigerant gas depends upon the superheat and rate of
refrigerant flow. A quench valve, with sensor at the compressor suctions, opens when the gas superheat temperature reaches 20
degrees above the normal boiling (vaporizing) temperature of the refrigerant. Liquid refrigerant from the condenser vaporizes
in flowing through the quench valve, mixing cool gas with the hot gas to maintain the temperature of the refrigerant entering
the compressor below 90 F.
(8) COMPRESSOR CRANKCASE HEATER. A tubular strip heater clamped around the external shell of the
compressor warms the oil and evaporates the refrigerant prior to air conditioner startup. Turning on the compressor motor
starter automatically disconnects the crankcase heater.
g. HEATING SYSTEM. The heating element grid is located in the evaporator compartment (figure 1-2). When the
heating system is operating, return air, after passing through the evaporator coil, is heated and delivered to the discharge duct
h. THERMOSTATIC CONTROL. The thermostat bulb fastened to the frame near the inner rim of the air return duct
adapter senses the return air temperature and actuates the thermostat. A three-stage thermostat is used. A high temperature
stage turns on the air conditioner two low temperature stages switch on the heating element relays. At the first low temperature
stage, a set of three element rods is energized and thermostatically cycles on and off with return air temperature variations. At
the second low temperature stage, the other set of three rods is energized and the first set remains energized while the second set
therrnostatically cycles on and off. The operating points for the three stages are adjustable by means of the temperature selector
on the control panel
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