A, the circular grooves that form chamber A are put in
communication with the unloader connections. This
admits high-pressure oil to the unloader cylinder and
actuates the unloader mechanism.
A capacity control regulating valve controls oil
pressure from the capacity control valve. It is connected
to the crankcase and has an oil-connecting line to
chamber B of the capacity control valve. As the
crankcase or suction pressure pulls down slightly below
the setting of the regulating valve, the regulator opens
and relieves oil pressure from chamber B of the capacity
control valve. This permits spring A to push the capacity
control piston one step toward chamber B, uncovering
the unloader connection nearest the end of the capacity
control valve. This relieves oil pressure from the power
element and allows the power element spring to rotate
the cam rings and unload the cylinder.
If the suction pressure continues to drop, the
regulator will relieve more oil pressure and unload more
cylinders. If the heat load increases, the suction pressure
will increase, causing the regulating valve to close and
load more cylinders.
MAINTENANCE
As an Engineman, maintaining the refrigeration and
air-conditioning plants may be one of your
responsibilities. To do this, you must understand the
maintenance procedures. In most instances, personnel
who are assigned to maintain refrigeration plants are
graduates of the Navys air-conditioning and
refrigeration school. This school teaches most operating
and maintenance procedures. However, you should
refer to the manufacturers technical manuals for the
details of the plants on your ship.
Testing Suction and Discharge Valves
Faulty compressor valves may be indicated by
either a gradual or a sudden decrease in the normal
compressor capacity. Either the compressor will fail to
pump, or the suction pressure cannot be pumped down
to the designed value, and the compressor will run for
abnormally long intervals or continuously. You may get
a rapid buildup of suction (crankcase) pressure during
an off cycle. This causes the compressor to start after a
very short off-period and indicates leaking discharge
valves.
If the refrigeration plant is not operating
satisfactory, you should first shift the compressors and
then check the operation of the plant. If the operation of
the plant is satisfactory when the compressors have been
shifted, the trouble is in the compressor and not in the
system.
To test the compressor discharge valves, pump
down the compressor to 2 psig. Then stop the
compressor and quickly close the suction and discharge
line valves. If the discharge pressure drops at a rate in
excess of 3 pounds in a minute and the crankcase suction
pressure rises, this is evidence of compressor discharge
valve leakage. If you must remove the discharge valves
with the compressor pumped down, open the connection
to the discharge pressure gauge to release discharge
pressure on the head. Then remove the compressor top
head and discharge valve plate. Be careful not to damage
the gaskets.
If the discharge valves are defective, replace the
entire discharge valve assembly. Any attempt to repair
them would probably involve relapping and would
require highly specialized equipment. Except in an
emergency, such repair should never be undertaken
aboard ship.
You can check the compressor internal suction
valves for leakage by following these steps:
1. Start the compressor by using the manual control
switch on the motor controller.
2. Close the suction line stop valve gradually to
prevent violent foaming of the compressor crankcase
lubricating oil charge.
3. With this stop valve closed, pump a vacuum of
approximately 20 in.Hg. If this vacuum can be readily
obtained, the compressor suction valves are satisfactory.
Do not expect the vacuum to be maintained after the
compressor stops, because the refrigerant is being
released from the crankcase oil. Do not check the
compressor suction valve efficiency of operation for at
least 3 days. It may be necessary for the valves to wear
in.
However, if any of the compressor suction valves
are defective, you can pump down the compressor, open
it, and inspect the valves. Replace defective valves or
pistons with spare assemblies.
Crankcase Seal Repairs
There are several types of crankcase seals,
depending on the manufacturer. On reciprocating
compressors, the crankshaft extends through the
compressor housing to provide a mount for the pulley
wheel or flexible coupling. Now the shaft must be sealed
to prevent leakage of lubricating oil and refrigerant. The
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