Even though manufactured by different companies, most compressors are quite similar. They are governed by a pressure control system that can be adjusted to compress air to the maximum pressure.

Compressor Design
The compressor unit may be of the reciprocating, rotary, or screw design.

The reciprocating compressor is similar to that of an automotive engine. They may be air-or liquid-cooled. As the pistons move up and down, air flows into the cylinder through the intake valve. As the piston moves upward, the intake valve closes and traps air in the cylinder. The trapped air is compressed until it exceeds the pressure within the collecting manifold, at which time the discharge valve opens and the compressed air is forced into the air manifold (fig. 3- 51). The reciprocating compressor is normally connected to the engine through a direct coupling or a clutch. The engine and compressor are separate units.

The rotary compressor has a number of vanes held in captive in slots in the rotor. These vanes slide in and out of the slots, as the rotor rotates. Figure 3-52 shows an end

Figure 3-52.- Compression cycle in a rotary compressor.

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.

Intercoolers
When air is compressed, heat is generated. This heat causes the air to expand, thus requiring an increase in power for further compression. If this heat is successfully removed between stages of compression, the total power required for additional compression may be reduced by as much as 15 percent. In multistage reciprocating compressors, this heat is removed by means of intercoolers that are heat exchangers placed between each compression stage. Rotary air compressors are cooled by oil and do not use intercoolers.

Aftercoolers
It is obvious that the presence of water or moisture in an air line is not desirable. The water is carried along through the line into the tool where the water washes away the lubricating oil, causing the tool to run sluggishly and increases maintenance. The effect is particularly pronounced in the case of high-speed tools where the wearing surfaces are limited in size and excessive wear reduces efficiency by creating internal air leakage.

Further problems may result from the decrease in temperature caused by the sudden expansion of air at the tool. This low temperature creates condensation that freezes around the valves, ports, and outlets. This condition obviously impairs the operational efficiency of the tool and cannot be allowed.

The most satisfactory means of minimizing these conditions is the removal of the moisture from the air immediately after compression and before the air enters the distribution system. This may be accomplished in reciprocating compressors through the use of an aftercooler that is an air radiator that transfers heat from the compressed air to the atmosphere. The aftercooler reduces the temperature of the compressed air to the condensation point where most of the moisture is removed. Cooling the air not only eliminates the difficulties which moisture causes at points where air is used but also ensures better distribution.