Centrifugal compressors are second only to reciprocating compressors in numbers of machines in service. In the process plant arena, the leader in numbers is too close to call with any degree of certainty. Where capacity or horsepower rather than numbers is considered as a measure, the centrifugal, without a doubt, heads the compressor field. During the past 30 years, the centrifugal compressor, because of its simplicity and larger capacity/size ratio, compared to the reciprocating machine, became much more popular for use in process plants that were growing in size. The centrifugal compressor does not exhibit the inertially induced shaking forces of the reciprocator and, therefore, does not need the same massive foundation. Initially, the efficiency of the centrifugal was not as high as that of a well-maintained reciprocating compressor. However, the centrifugal established its hold on the market in an era of cheap energy, when power cost was rarely, if ever, evaluated.

The centrifugal compressor has been around for quite a long time. Originally, it was used in process applications at relatively low-pressure, high-volume service. In the early 1930s, the main application was in the steel industry, where it was used chiefly as an oxidation air compressor for blast furnaces. The centrifugal displaced the reciprocating blowing engines that were being used at the time. The centrifugal was employed in the coal-to -coke conversion process, where it was used to draw off the gas from the coke ovens. In the late 1930s, the beginning of air conditioning for movie theaters, department stores, and later office buildings, gave birth to a generation of small centrifugals, which gained the advantage because of smaller size and absence of shaking forces. These forces were difficult to contain when a comparable capacity reciprocating compressor was used in a populated environment. It was the smaller compressor design that was able to penetrate the general process plant market, which had historically belonged to the reciprocating compressor. As stated previously, the growth of plant size and low-cost energy helped bring the centrifugal compressor into prominence in the 1950s. As the compressor grew in popularity, developments were begun to improve reliability, performance, and efficiency. With the increase in energy cost in the mid 1970s, efficiency improvements moved from last to first priority in the allocation of development funds. Prior to this turn of events, most development had concentrated on making the machine reliable, a goal which was reasonably well achieved. Run time between overhauls currently is three years or more with six-year run times not unusual. As plant size increased, the pressure to maintain or improve reliability was very high because of the large economic impact of a nonscheduled shutdown. This being the case, even with an increase in the efficiency emphasis, there is no sympathy for an energy versus reliability trade-off. The operating groups tend to evaluate reliability first, with the energy cost as secondary.

The centrifugal compressor has been applied in an approximate range of 1,000 cfm to 150,000 cfm. Plant air package centrifugals are available somewhat lower in capacity but have problems competing because of other more efficient compressors that are available in the lower ranges. Pressure ratios and pressure levels are difficult to describe in general terms because of the wide range of applications. Pressure ratio is probably the best parameter for comparing the centrifugal compressor to other types of compressors. Polytropic head, as defined in Chapter 2, is much more definitive to the dynamic machine but does not mean much numerically to a user. Pressure ratios of up to 3 and higher are available for single-stage compressors, operating on air or nitrogen. Multistage machines, of the process type, generally operate at a pressure ratio of less than 2 per impeller.

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