At times, combining compressors may be worth considering. For example, in compressing to a very high pressure, it may be possible to use a centrifugal machine or a rotary-screw machine for a lower pressure and then pipe the gas to a reciprocating unit. In some instances, axial and centrifugal impellers may be placed on the same shaft. In addition, one might also resort to placing axial- and centrifugal-compressor cases in a common drive train.
As an alternative to an axial compressor, three or four single-stage centrifugal compressors may be connected by a gear train to a single drive. With the gas cooled after each stage of compression—and gears designed to permit each stage to be run at its optimum speed—the efficiency of these centrifugals is comparable to that of an axial compressor, while their operating characteristics are those of a centrifugal machine. Extensive gearing, however, is a distinct disadvantage.