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CHAPTER 6
REPAIR INSTRUCTIONS
Section I.
REFRIGERATION SYSTEM
6-1.
General.
The refrigerant system, illustrated
by the refrigerant flow diagram (figure
5-1), is a mechanical, vapor cycle type
circuit consisting of the evaporator,
thermal expansion valve, motor-compres-
sor, condenser, and the necessary valves
and cutout devices for automatic control
during operation.
The thermal expansion
valve releases high-pressure liquid re-
frigerant into the evaporator at reduced
pressure.
The liquid refrigerant begins
to vaporize by absorbing heat from the
air passing over the external surface of
the evaporator coil.
The heated vapor
is sucked out of the evaporator section
by the motor-compressor and forced into
the condenser section under high pres-
sure where it is cooled and condensed
back into a liquid.
The heat released
during condensation is carried off by
the condensing air stream.
The liquid
refrigerant flows from the condenser to
a receiver, to a subcooler, and then to
the thermal expansion valve.
If the
temperature control switch (evaporator
return air thermostat) becomes satis-
fied, or the evaporator return air tem-
perature is lower than the control
switch set point, the refrigerant system
will switch to a bypass condition.
The
temperature control switch will activate
the normally-open liquid bypass solenoid
valve, closing the valve, and therefore
shutting off the evaporator section of
the unit.
The motor-compressor will
continue to pump as usual and the suc-
tion pressure will begin to drop.
When
it reaches approximately 58 psig (400
kPa), the pressure regulating valve will
start to open in an effort to maintain
the suction pressure above 55 psig (380
kPa) (approximately).
As the suction
temperature increases, due to the pres-
sure regulating valve opening, the
TM 5-4120-361-14
quench expansion valve will start to
meter liquid refrigerant into the suc-
tion line in an effort to maintain the
suction temperature below 75°F (24°C)
(approximately), or 30°F (-1°C) super-
heat (approximately).
This action (the
pressure regulator and quench valve ac-
tions) is totally automatic and also may
occur at extreme conditions in an at-
tempt to maintain the suction pressures
(even during the cooling mode) at a
condition above 55 psig (380 kPa) and
the suction temperatures (measured at
the quench bulb well) below 75°F (24°C).
The condenser louvers are operated by a
refrigerant powered piston located in
the high pressure part of the system.
This piston should be fully extended
(louvers open at 80°F (27°C) -
proximately) at 250 psig (1140 kPa) head
pressure and fully closed at 165 psig
(1140 kPa).
Failure to perform this
function could result in icing of the
evaporator coil and/or cutout on the low
pressure cutout.
6-2.
Pressure Testing the Refrigerant
System.
a.
General. A pressure test Will
indicate whether the air conditioner is
operating at normal or at abnormal pres-
sures.
When the air conditioner is not
operating at normal pressures, the cause
should be ascertained and corrected.
Refer to table 5-1 for troubleshooting
chart.
b.
System Pressure Test. Remove
caps from high and low pressure service
valves (fig. 5-6), connect suction and
discharge pressure gages to their re-
spective service valves.
Compare the
gage reading with the normal range of
system pressure shown in table 6-1.
6-1
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