Figure 19: Female/male rotor
A shaft seal is mounted at the shaft end (driving shaft) of the male rotor. The shaft seal ensures
complete tightness between the compressor housing and the compressor shaft which prevents
atmospheric air from entering the compressor.
No pump oil system
The screw compressor unit is designed to be self-lubricating. Oil supplied to the compressor from
the oil separator is at system head pressure. In the compressor, oil from all lubricating points
is returned to a point in the compressor body where the pressure is lower than the compressor
discharge pressure. During normal compressor operation, the compressor unit essentially operates
as its own oil pump. All oil entering the compressor is moved by the compressor rotors out the
compressor outlet and back to the oil separator.
For normal high-stage operation an oil pump is not required.
Demand pump oil system
This system is designed to provide adequate compressor lubrication when the oil differential
pressure across the compressor suction and discharge side is low. It applies to high-stage and
booster applications.
On start-up, the control system calculates the differential pressure between the compressor
discharge and the main oil injection port. If this differential pressure is less than 2.5 bar [35 psi], the
demand pump will turn on and continue to run until obtaining a 3 bar [45 psi] differential pressure.
Note:
For alarm descriptions and shutdown or cut-out parameters, refer to the separate
control system manual.
Compressor oil separation system
The compressor is an oil flooded screw compressor. Most of the oil discharged by the compressor
separates from the gas flow in the oil charge reservoir. Some oil, however, is discharged as a mist
which does not separate readily from the gas flow and is carried past the oil charge reservoir. One
or more coalescer elements then coalesce the oil mist into droplets which fall to the bottom of the
coalescer section of the oil separator. The return of this oil to the compressor is controlled by a
throttling valve on both high stage and booster applications.
Note:
Original: Open needle valves only enough to keep coalescer end of separator free from
oil.
The sight glass located near the bottom of the coalescer section of the oil separator should remain
empty during normal operation. If the oil level rises and remains in the sight glass, there is a
problem in the oil return separation system or compressor operation. For information on how to
47
SAB 283-355 A-Frame OHU 6042 and OHU 7650 (including ATEX) Screw Compressor Unit
