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.