OM 135
PFS 155C - 315C
9
Crankcase Heaters
The compressors are equipped with crankcase heaters. Crankcase heaters keep the temperature in the
crankcase high enough to prevent refrigerant from migrating to the crankcase and condensing in the
oil during the off-cycle.
Turn on power to the heaters for at least 12 hours before the compressors are started. The
crankcase temperature should be at least 80°F (26.7°C) before the system is started minimizing
lubrication problems of liquid slugging .
Service
Disconnect all power before doing any service inside the unit.
CAUTION
Service on this equipment is to be performed by qualified refrigeration personnel familiar with
equipment operation, maintenance, correct servicing procedures, and the safety hazard
inherent to this work. Causes for repeated tripping of safety controls must be investigated and
corrected.
Compressor Solenoids
The PFS unit screw compressors are equipped with three solenoids to control compressor capacity.
The solenoids are controlled by MicroTech outputs (see unit wiring diagrams). The solenoids are
energized at various compressor load conditions as indicated in Table 1.
Table 1, Solenoid status
Compressor Unloading Solenoid Staus
Percent Compressor
Top
Bottom Front
Bottom Rear
Loading
Solenoid
Solenoid
Solenoid
100
Energized
Off
Energized
75
Energized
Energized
Off
50
Off
Off
Energized
25
Off
Energized
Off
Location of the solenoids is as follows:
•
The top solenoid is on top of the compressor near the discharge end.
•
The bottom solenoids are on the lower side of the compressor on the opposite end from the
terminal box. The bottom front solenoid is the one closest to the discharge end of the
compressor. The bottom rear solenoid is the one closest to the motor end of the compressor.
If the compressor is not loading properly check the solenoids to see if they are energized. A complete
check will include a check of the MicroTech output, the wiring to the solenoid and the solenoid coil
itself.
