Dynamic compressors are based on the principle of imparting velocity to a gas stream andthen converting this velocity energy into pressure energy. These compressors are frequentlycalled turbocompressors, and centrifugal machines comprise perhaps 80% or more ofdynamic compressors. The remaining 20% or less are axial flow machines intended forhigher-flow, lower-pressure applications, as illustrated in Fig. I.1.

CENTRIFUGAL COMPRESSOR OVERVIEW

Centrifugal compressors are relatively troublefree, dependable gas movers. Almost any gascan be compressed by these machines, and their extensive size and pressure ranges mademodern process plants and efficient production of bulk chemicals possible in manyinstances.

Thousands of centrifugal compressors are single-stage machines, either direct-driven orgeared (see Fig. II.1), and thousands are executed in multistage configuration (Fig. II.2).Both single- and multistage machines are generally made up of standardized components.There are two principal casing types: (1) horizontally split casing (Fig. II.3) and (2) verticallysplit casing (barrel-type compressors) (Fig. II.4). The nozzle configurations can be selectedover a wide range.

To date, many machines have been built for intake volumes between 500 and 200,000 m3/h(294 to 117,000 cfm) at discharge pressures up to 160 bar (2352 psi). Barrel-type compres-sors for higher pressures have been designed and are operating very successfully. Dependingon the volume flow and the compression ratio, two, three, or more casings can be arrangedin line even with intervening gears. Drive is usually provided directly from a steam turbine,

PART II

DYNAMIC COMPRESSOR TECHNOLOGY

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FIGURE II.1 Single-stage centrifugal compressor with integral step-up gearing. (Dresser-RandCompany, Olean, N.Y.)

FIGURE II.2 Multistage centrifugal compressor in a petrochemical plant. (Elliott Company,Jeannette, Pa.)

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FIGURE II.3 Centrifugal compressor with horizontally split casing construction. (Mannesmann-Demag, Duisburg, Germany)

FIGURE II.4 Centrifugal compressor with vertically split (also called radially split) design.(Mannesmann-Demag, Duisburg, Germany)

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gas turbine, or expander turbine, as well as by an electric motor with gears or with variable-speed drivers.

With regard to the volume and compression ratio, and in the selection of the materialsfor the casing, impellers, and other components, the design is extremely flexible. As will beseen later, labyrinth seals, mechanical contact seals, floating seals, or dry gas seals can beprovided for shaft sealing. Consequently, centrifugal compressors can be used for practi-cally every gas compression requirement.

AXIAL COMPRESSOR OVERVIEW

Axial-flow compressors can handle large flow volumes in relatively small casings and withfavorable power requirements. They are available in sizes producing pressures in excess of7 bar (about 100 psi) at intake volumes between 40,000 and 1,000,000 m3/h (23,500 and588,500 cfm).

200 DYNAMIC COMPRESSOR TECHNOLOGY

FIGURE II.5 Axial-flow compressor set for anaircraft test bed in France. These machines can beused to generate compressed air or vacuum. Theinstallation comprises six identical axial compres-sors. Capacity: 244,000 N �m3/h in compression;38,000 N �m3/h in vacuum mode. (Sulzer, Ltd.,Winterthur, Switzerland)

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Axial-flow compressors (Figs. II.5 and II.6) are most often used for blast furnace air andair separation services but can also be used for nitric acid plants, natural gas liquefaction,and so on. Typical performance maps for these compressors are depicted in Figs. II.7 and II.8.

Drive is provided by steam turbines or electric motors. In the case of direct electricmotor drive, low speeds are unavoidable unless sophisticated variable-frequency motorsare employed.

AXIAL COMPRESSOR OVERVIEW 201

FIGURE II.6 Typical axial-flow compressors. (Sulzer, Ltd., Winterthur, Switzerland)

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202 DYNAMIC COMPRESSOR TECHNOLOGY

FIGURE II.7 Performance maps for axial compressors with speed variation. (Sulzer, Ltd.,Winterthur, Switzerland)

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AXIAL COMPRESSOR OVERVIEW 203

FIGURE II.8 Performance maps for axial compressors with adjustable stator blades. (Sulzer, Ltd.,Winterthur, Switzerland)

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