ENGINEMAN 1 & C
Capacity Control Valve
The capacity control valve (figure 6-8) is
located in the compressor crankcase cover. The
valve is actuated by oil pressure from the main
oil pump, and its function is to admit or relieve
oil to or from the individual unloader power
elements, depending on suction or crankcase
pressure. When the compressor is at rest the two
cylinders equipped with the unloader element are
unloaded and remain unloaded until the com-
pressor is started and the oil pressure reaches nor-
mal operating pressure.
As the high pressure oil from the pump enters
chamber A of the capacity control valve and
passes through an orifice in the top of the piston
to chamber B, it forces the piston to the end of
its stroke against spring A. When the piston of
the valve is forced against spring A, the circular
grooves which form chamber A are put in con-
tact with the unloader connections. This admits
high pressure oil to the unloader cylinder,
actuating the unloader mechanism.
To control the oil pressure from the capacity
control valve, a capacity control regulating
needle valve is installed. It is connected to the
crankcase and has an oil connecting line to
chamber B of the capacity control valve. As the
or suction pressure pulls down
slightly below the setting of the regulating valve,
the regulator needle valve opens and relieves the
oil pressure from chamber B of the capacity con-
trol 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 action
relieves the 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 needle valve relieves more oil pressure
and more cylinders become unloaded. On the
other hand, if the heat load increases, the suction
pressure increases, causing the regulating needle
valve to close and more cylinders to become
If a compressor cannot be pumped down and
is damaged to the extent that it has to be opened
for repairs, it is necessary first to close the suc-
tion and discharge valves and then following all
safety precautions, to allow all the refrigerant in
the compressor to vent to the atmosphere through
a drain plug.
When it becomes necessary to remove, replace,
or repair any internal parts of the compressor,
observe the following precautions:
1. Carefully disassemble and inspect while
removing all parts, noting their correct relative
position so that errors will not be made when
2. Inspect all parts that become accessible
after the removal of those parts requiring repair
3. Make certain that all parts and surfaces are
free of dirt and moisture.
4. Apply clean compressor oil freely to all
bearing and rubbing surfaces of the parts being
replaced or reinstalled.
5. If the compressor is not equipped with an
oil pump, make certain that the oil dipper on the
lower connecting rod is in the correct position for
dipping oil when the unit is in operation.
6. Position the ends of the piston rings so that
alternate joints come on the opposite side of the
7. Take care not to score the gasket surfaces.
8. Renew all gaskets.
9. Clean the crankcase and renew the oil
following correct procedures.
EVACUATING THE COMPRESSOR
In all but emergency situations it is desirable
to evacuate the compressor with a vacuum pump
rather than with the compressor itself. However,
if you do not have a vacuum pump available, use
the following procedure:
1. Disconnect the connection in the com-
pressor discharge gage line, between the discharge
line stop valve and the compressor.
2. Start the compressor and let it run until the
greatest possible vacuum is obtained.
3. Stop the compressor and immediately open
the suction stop valve slightly in order to blow
refrigerant through the compressor valves and
purge the air above the discharge valves through
the open gage line.