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ARMY TM 9-4120-381-14
NAVY EE-000-CA-MMA-010/7053-AC
AIR FORCE TO 35E9-136-31
1-14. OPERATING CYCL.ES Continued.
b.
Bypass Cycle. The air conditioner has a bypass cycle which allows cooling operation during low cooling loads
without cycling the compressor (B2) on and off. In bypass cycle, the refrigerant is routed from the compressor
discharge (high) side to the suction (low) side of the compressor, bypassing the evaporator coil (E).
(1)
During low load cooling conditions, excess cooling capacity causes a reduction in suction line pressure.
This low pressure condition is caused by closing the pressure regulator valve (V3) during redundant unit,
remote control operation or closing of the solenoid valve (L1) during redundant unit, remote control
operation.
(2)
The low cooling loads produce decreasing return air temperature, causing the cool air thermostat to open
solenoid valve (L2). As the compressor suction pressure starts to drop, the hot gas bypass valve (V5)
opens to allow flow of hot gas to the suction line between the evaporator coil (E) and pressure regulator
valve (V3).
(3)
The liquid quench expansion valve (V13) senses the temperature of the hot gas being bypassed through
the hot gas valve (V5) to the suction line. To prevent excessively hot gas from reaching the compressor,
the liquid quench expansion valve (V13) opens when necessary to allow liquid refrigerant to mix with the
hot gas.
c.
Heating Cycle. In redundant unit, remote control configuration, the heating cycle is designed to warm the
conditioned area to approximately 80°F (27°C) and to provide a load before the cooling cycle is started. In this
configuration, the heating cycle is not used for personnel comfort. Duct heaters in the conditioned area external
to this air conditioning unit provide for personnel space heating.
In the single unit, local control configuration, two levels of heating control can be provided. In all modes of
operation, heating is controlled by return air temperature only.
(1)
In redundant unit, remote control configuration, the air conditioner MODE SELECT SWITCH is normally
set to REMOTE. Selection of heating function is made at the evaporator coil (E) to be energized. Heating
is then controlled to a nominal 80°F (27°C) by the heat control thermostat located in the return air section
of the air conditioner.
(2)
In single unit, local control configuration, the heating cycle can be controlled by two methods. When
MODE SELECT SWITCH is positioned to either AUTO or REMOTE, the electric heating element is
automatically capable of being energized. The electric heating element is then controlled to a nominal
70°F (21°C) return air temperature by the cool control thermostat located in the return air section of the air
conditioner.
When MODE SELECT SWITCH is positioned to HEAT in this configuration, the electric heating element is
controlled to a nominal 80°F (27°C) return air temperature by the heat control thermostat located adjacent
to the cool control thermostat.
1-12
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