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Heavy-Duty Air Compressors | Up
Construction Mechanic Basic Volume 02 - Construction methods and practices | Next
Air Compressor Maintenance |
view of the vanes in the slots. The rotor revolves about the
center of the shaft that is offset from the center of the
pumping casing. Centrifugal force acting on the rotating
vanes maintains contact between the edge of the vanes
and the pump casing. This feature causes the vanes to
slide in and out of the slots, as the rotor turns.
Notice in figure 3-52 the variation in the clearance
between the vanes and the bottom of the slots, as the rotor
revolves. The vanes divide the crescent-shaped space
between the offset rotor and the pump casing into
compartments that increase in size, and then decrease in
size, as the rotor rotates. Free air enters each
compartment as successive vanes pass across the air
intake. This air is carried around in each compartment
and is discharged at a higher pressure due to the
decreasing compartment size (volume) of the moving
compartments as they progress from one end to the other
of the crescent-shaped space.
The compressor is lubricated by oil circulating
throughout the unit. All oil is removed from the air by an
oil separator before the compressed air leaves the service
valves.
The screw compressors used in the NCF are direct-
drive, two-stage machines with two precisely matched
spiral-grooved rotors (fig. 3-53). The rotors provide
positive-displacement internal compression smoothly
and without surging. Oil is injected into the compressor
unit and mixes directly with the air, as the rotors turn
compressing the air. The oil has three primary functions:
1. As a coolant, it controls the rise in air
temperature normally associated with the heat
of compression.
2. It seals the leakage paths between the rotors and
the stator and also between the rotors themselves.
3. It acts as lubricating film between the rotors
allowing one rotor to directly drive the other,
which is an idler.
After the air/oil mixture is discharged from the
compressor unit, the oil is separated from the air. The oil
that mixes with the air during compression passes into the
receiver-separator where it is removed and returned to the
oil cooler in preparation for re-injection.
All large volume compressors have protection
devices that shut them down automatically when any
of the following conditions develop:
1. The engine oil pressure drops below a certain
point.
Figure 3-53.—Compression cycle in a screw compressor.
2. The engine coolant rises above a predetermined
temperature.
3. The compressor discharge rises above a certain
temperature.
4. Any of the protective safety circuits develop a
malfunction.
Other features that may be observed in the
operation of the air compressors is a governor system
whereby the engine speed is reduced when less than
full air delivery is used. An engine and compression
control system prevents excessive buildup in the
receiver.
3-32
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