Classification

A better definition of a compressor stage can be made here to prevent confusion later on. Up to this point, in the positive displacement compressors, a compressing entity and a stage were one and the same; for example, a cylinder is a stage in the reciprocating compressor. The cen trifugal and the other dynamic compressors to be discussed have the problem of a dual vernacular, one used by the machine design engineer and the one used by the process engineer. To the machine builder, a stage is an impeller-diffuser pair, whereas the process designer tends to think of a stage as a process block that equates to an uncooled section of one or more impeller and diffuser sets. There is no problem with the single impeller machine as the two are synonymous. The confusion comes with the use of the multiple impeller machine. To make everyone equally happy or unhappy, as the case may be, hereinafter, a process compression stage will be referred to as an uncooled section. Whenever the term stage must be used in the process connotation, it will be called a process stage. The multiwheeled machine will retain the name of multistage, and the individual impeller and diffuser pairs will be called a stage.

With the foregoing discussion as an entreƩ to the types of centrifugal compressors, it seems redundant to classify them as single and multistage. A cross classification can be established by the manner in which the machine casing is constructed, whether it has an axial or radial joint. More commonly, this type of construction is referred to as horizontal and vertical split. For simplicity, the second terminology will be used. The overhung style of single stage is an example of the vertical split type of compressor (see Figure 5-1). An example of the horizontally split compressor is the common multistage. Maintenance of the horizontally split compressor is very simple and straightforward, as the rotor may be removed without disturbing the impellers. When the pressure is too high to maintain a proper joint seal or for low molecular weight service, another style commonly used is referred to as a barrel compressor (see Figure 5-2). The barrel uses a vertical split construction. In the multistage configuration, it is constructed with a removable, horizontally split, inner barrel that permits the removal of the rotor without removing the impellers. Many overhung compressors do not permit the removal of the rotor without first removing the impeller.

Another common type of compressor is manufactured in an integrally geared configuration. It is basically an overhung style machine mounted on a gear box and uses the gear pinion shaft extension to mount an

Figure 5-1. Single-stage, vertically split, overhung style centrifugal compressor. (Courtesy of Elliott Company
Figure 5-2. Multistage barrel type compressor. (Courtesy of Nuovo Pignone)

impeller (see Figure 5-3). The casing is also attached to the gear box. This style is built in both the single and multistage configuration. The most common form of multistage is the plant air compressor, which also has intercoolers included as part of the machine package.

Figure 5-3. Overhung, gearbox mounted centrifugal compressor. [Courtesy of Atlas Copco Comptec Inc.)

Figure 5-3. Overhung, gearbox mounted centrifugal compressor. [Courtesy of Atlas Copco Comptec Inc.)

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