ROTARY AIR COMPRESSORS

Rotary sliding vane compressors are machines in which longitudinal vanes slide radially in a slotted rotor that is mounted eccentrically in a cylinder. The rotor is fitted with blades or vanes that are free to slide in and out of longitudinal slots and maintain contact with the cylinder walls by centrifugal force. In operation, as the blades are forced outward by centrifugal force, compartments are formed in which air is compressed (fig. 11-6). Each compartment varies from a maximum volume on the suction side of the revolution to a minimum volume on the compression half of the revolution. This gives a positive displacementtype suction and pressure effect.

Another type of rotary compressor is the twinlobe unit sometimes referred to as a blower

Figure 11-3.-A typical two-stage reciprocating low-pressure air compressor.

(fig. 11-7). This unit consists of two impellers mounted This action takes place twice each revolution of an on parallel shafts that rotate in opposite directions impeller and four times per revolution of both within a housing. As the impellers rotate, they trap a impellers. The impellers are positioned in relation quantity of air themselves and the blower housing and between to each other by timing gears, located at the move the air around the casing to the discharge port. end of each shaft and external to the blower housing.

Figure 11-4.-W-type, two-stage, three-cylinder arrangement.

Figure 11-7.-Twin-lobe rotary compressor.

You should always use maintenance and service literature provided by manufacturers when you are working with rotary compressors. Maintenance information is given in Operation and Maintenance of Compressed Air Plants,

Figure 11-5.-Differential piston with a two-stage, vertical arrangement.

Figure 11-6.-Compression cycle of rotary compressor.

HELICAL SCREW COMPRESSORS

Helical screw compressors contain two mating rotating screws, one locked and one grooved, which provide the driving force. The unit's screws take in air, decreasing its volume as it progresses in a forward-moving cavity toward the discharge end of the compressor. Figure 11-8 shows a typical single-stage compressor and a double-stage helical screw compressor. These compressors are best used in booster or near constant-load conditions at low-pressure, oil-free application. Helical screw compressors may also be found in aircraft start facilities.

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