Compressor VI decrease
Oil flows from the SC3 port through the valve ports A and T to the compressor suction side. On SAB
355 models, the SC4 port does not exist. On these models, YY4 is energized, which allows oil to vent
from port A to T with assistance from the unloader spring.
To control the rate of VI change, throttle the needle valve at the SC3 port.
Compressor oil cooling systems
The unit can be equipped with one of several systems for controlling the compressor oil
temperature. They are liquid injection and refrigerant-cooled (thermosyphon) or water-cooled oil
coolers. Each system is automatically controlled, independent of compressor loading or unloading.
Oil cooling systems should maintain oil temperature within the following ranges for R717 and
HCFC/HFC:
Liquid injection oil cooling:
55-77°C
External oil cooling:
Note:
Refrigerant-cooled (Ther-
mosyphon) oil cooling (TSOC) or
water-cooled oil cooling (WCOC).
49-71°C
Liquid injection
The liquid injection system is designed to permit liquid refrigerant injection into one port on the
compressor at any given moment, and it operates as outlined.
The solenoid valve, YY5, is energised by the microprocessor when the temperature sensor, installed
in the compressor discharge, exceeds the LIOC (Liquid Injection Oil Cooling) set point. High-
pressure liquid refrigerant is then supplied to the temperature control valve (TCV). See
Liquid injection adjustment
For discharge temperature set point and PID regulator parameters, refer to the operating manual
for the control system. The discharge temperature set point is set to the required oil temperature,
see
Compressor oil cooling systems
.
Suction check valve by-pass
The unit is equipped with a low-pressure-drop suction check valve bolted directly to the compressor
housing. Units with an 8 in. stop valve or larger are piped, as shown in
. During normal
operation, valve NV-1 is closed. This is a pump-out connection to allow refrigerant removal to the
system suction prior to evacuation for servicing. In most systems, valve NV-2 must be open at
all times. It should normally be cracked open to allow the oil separator to slowly bleed down to
approximately system suction pressure when the unit is stopped, having this valve cracked open
allows the compressor drive motor to have an easier start and the discharge check valve will seat
more tightly. If the drive coupling back spins, start closing the valve until the back spin stops. If the
separator oil level foams excessively on shutdown, NV-2 should be closed slightly. If the separator
takes more than 20 to 30 minutes to equalise to suction pressure after shutdown, you can open
.
SAB 283-355 A-Frame OHU 6042 and OHU 7650 (including ATEX) Screw Compressor Unit
54
