cycle begins as the air is directed to the opposite
in a casing. Air inlet and outlet ports are located
(discharge) end of the compressor. The rotors
on opposite sides of the casing. Atmospheric air
mesh, and the normal free volume is reduced. The
is drawn into the compressor through the filter-
reduction in volume (compression) continues, with
silencer. The air passes through the air cylinder-
a resulting increase in pressure, until the closing
operated unloader (butterfly) valve and into the
pocket reaches the discharge port.
inlet part of the compressor when the valve is in
The entrained water is removed from the
the open (load) position. Fresh water is injected
discharged air by a combined separator and water
into the airstream as it passes through the inlet
holding tank. The water in the tank passes
port of the compressor casing.
through a seawater-cooled heat exchanger. The
The injected fresh water serves two purposes:
cooled water then recirculates to the compressor
(1) it reduces the air discharge temperature caused
for reinjection.
by compression, and (2) it seals the running
During rotation and throughout the meshing
clearances to minimize air leakage. Most of the
cycle, the timing gears maintain the correct
injected water is entrained into the airstream as
clearances between the rotors. Since no contact
it moves through the compressor.
occurs between the rotor lobes and grooves,
The compression cycle starts as the rotors
between the rotor lobes and casing, or between
unmesh at the inlet port. As rotation continues,
the rotor faces and end walls, no internal oil
air is drawn into the cavity between the male rotor
lubrication is required. This design allows the
lobes and into the grooves of the female rotor.
compressor to discharge oil-free air.
The air is trapped in these grooves, or pockets,
and follows the rotative direction of each rotor.
For gear and bearing lubrication, lube oil from
a force-feed system is supplied to each end of the
As soon as the inlet port is closed, the compression
Figure 7-23.--LPAC (reciprocating type).
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