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.

DYNAMIC CENTRIFUGAL COMPRESSORS

Dynamic compressors are high-speed rotating machines in which air is compressed by the

Figure 11-8. - Rotary helical screw compressors.

action of rotating impellers or blades that impart velocity and pressure to the air. Figure 11-9 shows the internal parts of a multistage centrifugal compressor. This type will deliver air at an essentially constant pressure over a wide range of capacities. The direction of airflow is radial with respect to the axis of rotation.

Centrifugal compressors have a lower limit of stable operation called the surge point. Operation below this point results in pumping or surging of the airflow. Prime movers are normally electric motors or steam turbines.

Centrifugal compressors are intended for near continuous industrial air service when the load is reasonably constant. These compressors also work well when oil-free air is required and can be used for breathing air.

Table 11-1 shows typical application recommendations for both positive displacement and dynamic class compressors.

AUXILIARY EQUIPMENT

A system that functions to provide a continuous supply of usable compressed air requires certain auxiliary devices in addition to the air compressor. Most compressed air systems require a minimum of auxiliary equipment that should include air intakes, intake filters, silencers, intercoolers, aftercoolers, air discharge systems, separators, dryers, receivers, and so forth. These auxiliary equipments will be discussed in this section in addition to less common auxiliary equipment.

Air Intakes

Air intakes should be located high enough to eliminate intake of particles of dust, smoke, dirt, water, and snow. Carbon monoxide sources should not be able to discharge into compressor intakes. Special attention should be given to the elimination of flammable fumes into the compressed air system.

Whenever air intakes must be placed through a roof that is surrounded by parapets, they should be 8 to 10 feet above the roof.

Noise may be generated by air intakes and must be considered during installation. Reciprocating compressors are most likely to develop resonance through intake piping. If this possibility exists, the use of intake dampeners or surge chambers will help. High velocities present noise level problems. Intake pipe velocities should be limited to 1,000 fpm in open areas or 350 fpm across filters. Acoustical silencers combined with filters and/or pulse dampeners are available and