Applying the pressure

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.

The major compressor classes are positive displacement and dynamic. An example of the positive displacement class is the bicycle pump or fireplace bellows, both of which change the volume of a chamber to compress air. If a piston inside a cylinder forms the chamber, the compressor is known as a reciprocating type. These are further subdivided into single-acting and double-acting. In a single-acting type, only one piston face compresses the air; double-acting types use both faces alternately. Reciprocating compressor sizes range from fractional horsepower to more than 600 hp.

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.

The higher operating speed and continuous flow through dynamic compressors gives them the greatest flow capacity. Axial units provide the greatest flow capacity, but there’s overlap in flow capacity between centrifugal and axial compressors. A rough ranking of the flow capacity of the positive displacement compressors from highest to lowest would be screw, sliding vane and reciprocating, but there’s a great deal of overlap.

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.

Some methods of capacity control are unique to a compressor type, whereas other methods apply to all types. Cylinder unloading is uniquely applied to reciprocating compressors. It controls capacity by delaying the closing of the suction valves so that air drawn into the cylinder can leak back into the suction plenum before compression starts. Keeping the valve open through the entire compression stroke completely unloads the cylinder. More sophisticated systems that allow the valve to close at any time during the compression stroke achieve 100% to 0% capacity variation.

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.