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24 TRG-TRC012-EN
notes
period three
Compressor Capacity Control
The slide valve is operated by a piston and cylinder assembly. Extension and
retraction of the operator shaft positions the slide valve along the compressor
rotors.
High-pressure refrigerant vapor, controlled by 2 solenoid valves, is fed to or
bled from this assembly in order to position the valve. To close the slide valve
and load the compressor, the load valve is opened and the unload valve is
closed. This allows high-pressure refrigerant vapor to enter the cylinder from
the discharge of the compressor, extending the operator shaft and moving the
slide valve so that it covers more of the rotor length.
Conversely, to open the slide valve and unload the compressor, the load valve
is closed and the unload valve is opened. This allows the high-pressure
refrigerant vapor to bleed out of the cylinder into the low-pressure area within
the compressor housing. A spring helps return the piston, retracting the
operator shaft and moving the slide valve such that the rotors are more
uncovered. This reduces the effective length of the rotors, which are
compressing vapor.
On shutdown the unload solenoid valve is energized and the spring drives the
slide valve to the fully-unloaded position. This ensures that the compressor
always starts fully unloaded.
When both solenoid valves are closed, the operator shaft holds the slide valve
at its current position.
Slide Valve Operation
to low-
to low-
pressure
pressure
compressor
compressor
housing
housing
high-pressure
high-pressure
refrigerant vapor
refrigerant vapor
slide valve
slide valve
operator
operator
shaft
shaft
spring
spring
piston
piston
cylinder
cylinder
load
load
valve
valve
unload
unload
valve
valve
Figure 36