Centrifugal compressors function

Mixed-Flow Turbine. The turbine as shown in Figure 1-35, is almost identical to a centrifugal compressor—except its components have different functions. The scroll is used to distribute the gas uniformly around the periphery of the turbine. 

Compressor performance can be represented in various ways. The commonly accepted practice is to plot the speed lines as a function of the pressure delivered and the flow. Figure 3-9 is a performance map for a centrifugal compressor. The constant speed lines shown in Figure 3-9 are constant aero-dynamic speed lines, not constant mechanical speed lines. 

The approximate number of stages for a centrifugal compressor, column (I), is a function of the adiabatic (and actually the polytropic) head and varies with the efficiency and physical properties of the gas. 

The polytropic efficiency is a function of the physical properties of the gas and the machine design. Generally, the higher the feed flowrate, the higher the polytropic efficiency of the compressor. A first estimate of the polytropic efficiency of a centrifugal compressor can be obtained from ... 

Figure 7.27. Efficiencies of centrifugal and reciprocating compressors, (a) Polytropic efficiencies of centrifugal compressors as a function of suction volume and compression ratio (Clark Brothers Co.), (b) Relation between isentropic and polytropic efficiencies, Eqs. (7.22) (7.23). (c) Isentropic efficiencies of reciprocating compressors (De Laval Handbook, McGraw-Hill, New York, 1970). Multiply by 0.95 for motor drive. Gas engines require 7000-8000 Btu/HP.

The gas turbine is driven by high-temperature gases from a combustion space, as indicated in Fig. 8.11. The entering air is compressed (supercharged) to a pressure of several bars before combustion. The centrifugal compressor operates on the same shaft as the turbine, and part of the work of the turbine serves to drive the compressor. The unit shown in Fig. 8.11 is a complete power plant, as are Otto and Diesel engines. The gas turbipe is just one part of the assembly and performs the same function as the steam turbine in a steam power plant (Fig. 8.1). 

If the polytropic efficiency of a centrifugal or axial compressor is assumed to be a function of volumetric flowrate, then the efficiency, in principle, will change from stage to stage. This is because the density changes between stages, even if the gas is cooled back to the same temperature as a result of the pressure increase. However, such effects are not likely to have a significant influence on the predicted power. 

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