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 TM 55-1905-219-14-8
4-28. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(e) In operation, compressor oil is pumped through the control oil strainer and orifice to the
unloader power element supply line. As soon as pump oil pressure reaches a power element the piston rises,
the lifting fork pivots and the unloader sleeve is lowered, permitting the suction valve to seat. The system is
governed by suction pressure which actuates the capacity control valve. The capacity control valve controls
the flow of compressor oil to an unloader power element by metering the amount of oil allowed to bypass back
to the crankcase.
(f)
Suction pressure increases or decreases according to increases or decreases in the
refrigeration load requirements of the plant. With a decrease in suction pressure the capacity control valve
moves to increase the oil bleed to the crankcase from the unloader power element supply line. The resulting
decrease in oil pressure in a power element allows the piston to move downwards, the suction valve rises and
that cylinder unloads. With an increase in suction pressure, the above process is reversed and the controlled
cylinders will load in succession.
(3)
Checking Capacity Control.
(a) Before setting the capacity control, check its operation. Compressors are shipped from the
factory with the adjusting stem or screw of the control valve backed out all the way. In this position all cylinders
will be fully loaded as soon as normal oil pressure is established. The control valve adjusting stem is located
under a seal cap.
(b) A quick check on the operation of the capacity control system may be made by connecting
gauges to read oil pump and control oil pressures. First, be sure that correct oil pump pressure is established.
(Oil pressure should be 45 to 55 psi (310.1 to 379.2 kPa) above suction pressure.) Next, with the compressor
running loaded, turn the adjusting stem of the capacity control valve clockwise and listen for the first cylinder to
unload. As each cylinder unloads, there will be a change in sound level and a sudden change in control oil
pressure which will cause the control oil pressure gauge needle to jump. Continue to turn the adjusting stem
slowly until the proper number of steps of unloading have taken place. Refer to the previous table. Then, turn
the stem counter-clockwise slowly until the same number of steps of loading have taken place.
(4)
Setting Capacity Control.
(a) The capacity control valve adjusting stem should be backed out all the way (counter-clockwise)
during the temperature pulldown period. This will insure that all cylinders will remain loaded and the full
capacity of the compressor will be available for fast pulldown.
4-852
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