Radial-Flow Centrifugal Compressors

The best compressor type for most current applications is the single-stage radial-flow centrifugal compressor. It is a simple and dependable machine that has proven itself for heavy duty use over a broad range of industrial applications. 

Centrifugal Compressors The flow in a centrifugal compressor enters the impeller in an axial direction and exits in a radial direction. 

The radial-flow, or centrifugal compressor is a widely used comj sor and is probably second only to the reciprocating compressor in usage in the process industries. A typical multistage centrifugal compressor can he seen in Figure 1-11. The compressor uses an impeller consisting of... 

Figure 1-11. Radial-flow horizontally split multistage centrifugal compressor. Courtesy of Nuovo Pignone)...

In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 MPa in some cases. Capacities were limited to around 300 - 400 MTPD due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 MTPD with ammonia synthesis loop pressures typically around 15 MPa. Since the 1960 s new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. 

Compressors applied to evaporator systems include positive-displacement, and radial-flow and axial-flow centrifugals. Each has advantages for certain applications. 

Centrifugal compressors are dynamic compressors and are essentially constant pressure, variable volume machines and must be regulated in almost all applications. Before control can be discussed, the operating characteristics must be understood. The discussion to follow will be concerned with radial-flow compressors. Axial-flow compressors have similar characteristics curves, although the change in pressure rise is much greater with equivalent changes in compressor rotational speed. 

Centrifugal compressor 1) radial flow covered wheel 8000 -12000 13-1.5... 

A cross-section of a Delaval turbocharger is shown in Fig. T-94. A mixed-flow centrifugal-compressor stage and a mixed-flow radial-turbine stage are arranged back to back on one side of the bearing case. 

Radial-Inflow Turbine The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed-flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. Radial-inflow turbines are only now beginning to be used because little was know about them heretofore. Axial turbines have enjoyed tremendous interest due to their low frontal area, making them suited to the aircraft industiy. However, the axial machine is much longer than the radial machine, making it unsuited for certain vehicular and helicopter applications. Radial turbines are used in turbochargers and in some types of expanders. 

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. 

The inward-flow radial turbine has many components similar to a centrifugal compressor. There are two types of inward-flow radial turbines the cantilever and the mixed-flow. The cantilever type in Figure 1-34 is similar to an axial-flow turbine, but it has radial blading. However, the cantilever turbine is not popular because of design and production difficulties. 

Centrifugal compressor cases are classified as horizontally split or vertically split (Figures 12-40A-C). The 5th Edition, April, 1988, API Standard 617 refers to the horizontally split style as Axial Split and vertically split units as Radial Split The first terminology is more standard than that of the API-617. Figure 12-40D illustrates one flow arrangement for back-to-back internal flow, with the following advantages ... 

Centrifugal compressors increase gas pressure by accelerating the gas as it flows radially out from a rotating impeller. The increase in velocity is then converted to increase in pressure as the gas leaves the compression stage. Unlike reciprocating compressors, centrifugal compressors involve a constant flow through the compressor. 

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