Specifications

2 – 20
Section 2 • Operational Descriptions
Single Screw Compressor PLC Operation Manual • Vilter/Emerson • 35391CM
LIQUID INJECTION VALVE
Reference Figure 2-15
If installed, liquid injection is a method of oil cooling
where liquid refrigerant is injected into a port on the
compressor. A modulating valve regulates the amount
of liquid allowed into the compressor to control the
amount of cooling done by liquid injection.
A PID controller adjusts the opening degree of the mod-
ulating valve to control compressor discharge tempera-
ture (temperature of the gas and oil mixture leaving the
compressor). When the discharge temperature exceeds
the desired temperature plus a deadband, the valve will
open further to add more cooling. When discharge tem-
perature drops below the desired temperature minus a
deadband, the valve will begin to close.
Figure 2-15. Operational Diagram - Liquid Injection Valve
• When the compressor is stopped, the valve will be
commanded to 0% open (full closed).
• When the compressor is running, the discharge tem-
perature rises above the “Open Valve Above” set-
point and Separator Oil Temperature rises above “Oil
Separator Override Temp” in the “Liquid Injection
Valve Setpoints” screen, the valve will open to a set-
table minimum position.
• When the compressor is running and the discharge
temperature is above the deadband, the PID control-
ler will increase the valve opening degree.
• When the compressor is running and the discharge
temperature is below the deadband, the PID control-
ler will decrease valve opening degree.
M
LIQUID
REFRIGERANT
S
TE
SUCTION
COMPRESSOR
DISCHARGE
LIQUID INJECTION
VALVES
TARGET
OUTPUT
PRESSURE
START
PRESSURE
MINIMUM VALVE
POSITION
DISCHARGE TEMPERATURE
VALVE POSITION