TM 9-4120-357-14
Section III. TECHNICAL PRINCIPLES OF OPERATION
1-8. Functional Description.
a. Cooling. When the selector switch is set at COOL, and the temperature control thermostat is set at DECREASE,
the air conditioner is in the cooling mode of operation. Liquid refrigerant is metered into the evaporator coil at the front
face of the air conditioner through a thermostatic expansion valve (V5, Fig. 1-5). Warm room air forced around the
finned tubs of the evaporator coil (E) causes the liquid refrigerant to vaporize and absorb heat from the air, thereby cool-
ing the air. The vaporized refrigerant is then piped to the compressor (B1) where the gas increases in both temperature
and density as it is compressed. The hot compressed gas is then piped to the condenser coil (C) where a flow of outside
air is forced around finned tubs to remove the heat that has been absorbed during evaporation and compression. Re-
moval of the heat causes the compressed gas to liquefy to a high temperature liquid. Excess refrigerant, not required by
the evaporator, is stored in receiver tank (R1). The liquid refrigerant is returned to the expansion valve (V5) evaporator
coil (E) through the check valve (V6) dehydrator (D), liquid line solenoid valve (K3) and liquid indicator (G). Here the
cycle is repeated. Any liquid that is not. immediately required for cooling is stored indefinitely in a pair of accumulator
tanks (R2-R3) between the condenser and evaporator coils for use when the refrigerant pressure is too low for low tem-
perature operation. Since the compressor runs continuously, it is necessary to prevent evaporator coil freeze up during
the low heat load operation. The equivalent pressure for refrigerant 22 at 32F (0C) is approximately 58 psig (4.08
kg/cm2). With the low heat load on the evaporator (suction) pressure will reduce and as it approaches 58 psig (4.08
kg/cm2) the pressure regulating valve (V2) begins to open allowing hot gas from the discharge line of the compressor to
bypass to the suction line thereby maintaining suction pressure and evaporator coil temperature above freezing point. To
prevent the development of excessive heat in suction line which could overheat compressor motor, a second thermo-
static expansion valve injects liquid refrigerant into suction line to cool gas to a safe operating temperature.
b. Bypass-Cooling. When the temperature of the air returning to the air conditioner is below set point of tempera-
ture control thermostat, the cooling is stopped even though the compressor continues to operate. To accomplish this, the
liquid line solenoid valve (K3) which is a normally open valve, is energized by the temperature control and closes the
valve stopping refrigerant flow to the evaporator coil (E). As the compressor continues to operate the suction pressure
reduces to the point where the pressure regulating valve (V2) opens to create a bypass condition of refrigerant being
pumped. As in the "low load" condition described in the cooling operation above, the repeated compression caused by
this bypass increases temperature of the refrigerant vapor. To prevent the development of excessive heat in the suction
line which would overheat and damage the compressor motor, the thermostatic expansion valve (V4) injects liquid refrig-
erant from the liquid line, ahead of liquid line solenoid valve (K3), into the suction line to cool the refrigerant vapor to a
safe operating temperature for the compressor motor.
c
Cooling Control. The refrigeration compressor operates continuously when the mode selector switch is set at
COOL. When the room or return air temperature is higher than the setting of the temperature control thermostat (S1,
Fig. 4-16), solenoid valve (K3) is open to permit liquid refrigerant to flow to the evaporator coil, through expansion valve
(V5). Suction pressure to the compressor is maintained above a minimum level by the pressure regulating valve,(V2)
which bypasses compressed gas to the suction side of the compressor on demand. When the room temperature falls to
the setting of the temperature control thermostat (S1, Fig. 4-16), solenoid valve (K3) closes to shut off refrigerant to
evaporator coil, and refrigerant is by-passed through the pressure regulating valve (V2). This bypass circuit prevents
build-up of excessive pressure differentials, but by bypassing the evaporator coil, it permits heat to build-up in refrigerant
gas. To reduce heat build-up, a second expansion valve (V4) opens when its sensor bulb detects excessive heat,
thereby injecting liquid-refrigerant into the compressor suction line to act as coolant. If failure of control or loss of refrig-
erant results in pressures beyond normal limits, the high-pressure cutout switch (S5) or the low-pressure cutout switch
(S6) will open to stop operation of compressor. The solenoid valve (K4), a normally open valve, is open only when the
compressor is not operating for system pressure equalization.
d. Heating Mode. Placing the mode selector switch in HI HEAT or LO HEAT provides two levels of heat. The tem-
perature control thermostat only controls half of the heating elements. With the selector switch in HI HEAT position,
three of the six heating elements are constantly energized and the remaining three heating elements are controlled by
the temperature control thermostat. While it is possible to select HI HEAT whenever heating is desired it is not practical
for use in only moderately cool weather since three of the six heating element
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