Compressor Types

Air compressors can produce pressures ranging from slightly above atmospheric to more than 60,000 psi (4,000 bar), although most industrial applications use pressures of around 100 psig (7.9 bar). There are two general methods used to compress gaseous matter:

1. Positive displacement compressors compress air (or other gases) by admitting successive volumes of air into a closed space and then decreasing the volume. Reciprocating and rotary screw compressors operate on this principle.

2. Dynamic compressors are machines in which air or gas is compressed by the mechanical action of rotating vanes or impellers imparting velocity and pressure to the air or gas. Dynamic compressors include axial and centrifugal types.

The basic concept behind the dynamic or centrifugal compressor principle can be easily explained with an analogy of a person whirling a ball attached to the end of a string.

The ball pulls outward on the string due to the action of centrifugal force, and the pull is increased by a heavier ball, a longer string, or a faster rotation. As shown in Figure 30-1, the effect is the same if the ball is replaced with a molecule of gas, the string is replaced with an impeller, and the person is replaced with a mechanical driver. The centrifugal force imparted to the gas molecule will fling it outward, compressing it into the narrower impeller passageway. The larger the diameter of the impeller, the heavier the molecular weight of the gas, or the greater the speed rotation, the greater the pressure produced.

Fig. 30-1 Centrifugal Principle Illustration. Source: Carrier Corporation

In contrast, in positive displacement machines, a finite amount of the working fluid (e.g., air) is positively moved through the machine. Compression occurs as the machine encloses a finite volume of gas and reduces the internal volume of the compression chamber. The batch process characteristic allows positive displacement compressors to achieve higher pressures and respond to load change more effectively than centrifugal compressors. In particular, positive displacement compressors are more compatible with speed control as a means of controlling capacity under part-load conditions.

Rotary screw and reciprocating (piston) compressors are both positive displacement machines, which trap a finite volume of gas, compress it, and transports it to discharge. Dynamic machines, such as axial and centrifugal compressors, use rotary action to exert a torque on the gas to transport it and change its kinetic energy, without positive displacement.

Whereas positive displacement compressors are essentially constant volume, variable pressure machines, centrifugal compressors are essentially variable flow, constant pressure machines. Figure 30-2 provides a comparison of performance characteristics of centrifugal versus reciprocating compressors. Line JK represents the constant flow, variable pressure performance of the reciprocating compressor. Due to the decrease in volumetric efficiency at increasing pressures, the compressor will actually have a sloping characteristic, as shown by Line JL. Line FM represents the variable flow, constant pressure performance of the centrifugal compressor. Due to internal losses, the compressor characteristic is not a straight line, but is similar to Line FG.

Generally, positive displacement units are selected for smaller volumes of gas and higher pressure ratios. Dynamic machines are selected for higher volumes of gas and smaller pressure ratios. As such, centrifugal compressors are most common in large capacities, except when high pressure ratios are required.

Figure 30-3 shows the basic types of compressors discussed in this chapter.

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