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Air compressors are key components in many manufacturing and process industries. They’re interesting from an engineering standpoint because of the many disciplines involved in their design and application. Also, they’re interesting historically because they are among the earliest machines, and most people have an intuitive understanding of compressor operation. Anyone who has used a bicycle pump or a bellows has operated an air compressor, and they know that work is required to compress air. Anyone knows that compression heats air.
Another type is the rotary positive displacement compressor, in either helical screw or sliding vane varieties. The helical screw compresses air between a meshing rotating rotor and screw assembly. Helical screw compressors are available in sizes from about 3 hp to several thousand horsepower.
The sliding vane compressor uses a set of sliding vanes placed in slots on a rotor eccentrically mounted in a cylindrical casing. As the rotor spins, centrifugal force presses the vanes against the casing wall to compress air between the vanes and the casing.
The second major compressor class is the dynamic type, which compress by converting air velocity into air pressure using blades mounted on a rotating shaft. In centrifugal compressors, air enters near the base of the impeller blades, accelerates along the blade and exits near the ends of the blades at the circumference of the compressor case. Centrifugal compressors range in size from about 100 hp to several thousand horsepower.
In an axial flow compressor, the air enters and exits along the axis of the shaft, usually after passing through several stages of rotor blades. Each set of rotating blades is separated from the next by nonrotating stator blades. Air compressors in aircraft jet engines are a common example of the axial type. Axial-flow compressors are available in sizes from a few hundred horsepower to several thousand horsepower.
Both positive-displacement and dynamic compressors can be single- or multistage. Multiple stage compressors need two or more stages to reach the final output pressure; the output of one stage being the input to the next. Cooling the air between stages improves compressor efficiency.
Each compressor type — reciprocating, screw, rotary vane, centrifugal and axial — has typical operating characteristics. There is, however, overlap and, for a given application, one might have a choice of types. Some important characteristics are flow, pressure, capacity control and lubrication.
Output pressures from positive-displacement units are similar, with reciprocating units developing the greatest pressure. Within the dynamic compressor family, centrifugal compressors provide greater pressure capabilities than the axial type. Figure 1 shows the approximate range of flow and pressure for various compressor types.
Other control schemes for reciprocating compressors include start-stop, variable speed and bypass control (in which compressed air is bypassed to the suction). Vibration and bearing lubrication might limit variable-speed control in reciprocating compressors to about 40% of nominal speed.
Slide valve control is unique to rotary screw compressors. The slide valve varies compressor displacement by returning air back to the suction. Some slide valve applications also vary the discharge port location, which varies the volume ratio. Lift valve unloaders also allow air to return to the suction. The fixed location of the lift valves results in stepped capacity control as opposed to a slide valve’s stepless control.
Start-stop, suction throttling and variable-speed operation also can control rotary screw compressor capacity. Several manufacturers offer VFD-drive screw compressor packages.
Centrifugal compressors use inlet vane control, which pre-rotates the incoming air to alter the compressor’s performance curve. Variable-speed control also is effective for centrifugal compressor capacity reduction. Output pressure, however, is proportional to the square of the rotational speed. Inlet vane capacity control results in less of a reduction in pressure output than capacity control using variable-speed control.
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