40
Centrifugal Chillers
D-EIMWC00804-14EN
Capacity Control System
The opening or closing of the inlet vanes controls the quantity of refrigerant entering the impeller
thereby controlling the compressor capacity. The vane movement occurs in response to oil flow
from the SA or SB 4-way solenoid valves, which in turn, respond to instructions from the unit
microprocessor as it senses leaving chilled water temperature. This oil flow activates a sliding
piston that rotates the vanes.
Vane Operation
The hydraulic system for the inlet guide vane capacity control operation consists of a 4-way
normally open solenoid valve located in the oil management control panel or on the compressor
close to the suction connection. Oil under pressure from the oil filter is directed by the 4-way
valve to either or both sides of the piston, depending on whether the control signal is to load,
unload, or hold.
To open the vanes (loading compressor), solenoid SA is de-energized and SB is energized,
allowing oil flow from port SA to one side of the piston. The other side drains through port SB.
To close the vanes (unload compressor), valve SB is de-energized and valve SA is energized to
move the piston and vanes toward the unload position.
When both solenoid valves SA and SB are de-energized, full oil pressure is directed to both sides
of the piston through ports SA and SB, and the vanes are held in that position. Refer to Figure 22
and Figure 23 for solenoid action. Note that both solenoids cannot be energized simultaneously.
Vane Speed Metering Valves
The speed at which the capacity control vanes are opened or closed can be adjusted to suit system
operating requirements. Adjustable needle valves in the oil drain lines are used to control the rate
of bleed-off and consequently the “vane speed”. These needle valves are part of the 4-way
solenoid valve assembly located in the compressor lube box (Figure 21).
The valves are normally factory set so that the vanes will move from fully closed to fully opened
in the time periods shown in Table 12 on page 41.
Figure 20, Needle Valve Location
The speed must be slow enough to prevent
over-controlling and hunting.
The left adjusting screw is the SB needle valve
for adjusting the vane OPENING speed for
loading the compressor. Turn this screw
clockwise to decrease the vane opening speed
and counterclockwise to increase the opening
speed.
The right adjusting screw is the SA needle
valve for adjusting the CLOSING speed to
unload the compressor. The same adjustment
method applies; clockwise to decrease closing,
counterclockwise to increase vane closing.
These adjustments are sensitive. Turn the
adjusting screws a few degrees at a time.
The vane speed is factory set and varies by
compressor size.
The start-up technician may readjust the vane speed at initial start-up to meet job
conditions.
Open (Load)
Close (Unload)
