Chapter 4: Design Characteristics

OVERVIEW

Centrifugal compressors are used extensively because of their smooth operation, large tolerance of process fluctuations, and their higher reliability compared to other types of compressors. Centrifugal compressors range in size from pressure ratios of 1:3 per stage to as high as 12:1 on experimental models. The selection of this type of compressor is prevalent in the petrochemical industry. The proper selection of a compressor is a complex and important decision. The successful operation of many plants depends on smooth and efficient compressor operations. To ensure the best selection and proper maintenance of a centrifugal compressor, the engineer must have knowledge of many engineering disciplines.

Centrifugal compressors in general are used for higher-pressure ratios and lower-flow rates compared to lower-stage pressure ratios and higher-flow rates in axial compressors.

Figure 4-1 is a map for centrifugal compressors that shows the effect of specific speed (N _s) and specific diameter (D _s) on their efficiency. The most efficient region for centrifugal compressor operation is in a specific speed range between 60 s> 1,500. Specific speeds of more than 3,000 usually require an axial-flow-type compressor. In a centrifugal compressor the angular momentum of the gas flowing through the impeller is increased partly because the impeller's outlet diameter is significantly greater than its inlet diameter. The major difference between axial and centrifugal compressors is the variance in the diameters of the inlet and the outlet. The flow leaving the centrifugal compressor is usually perpendicular to the axis of rotation.

Figure 4-1: