Intercooling

Cooling between compressor stages limits the value of the discharge temperature and reduces energy demands. Normally there would be no argument against intercooling because of the obvious operating cost saving. However, in some process applications, the higher temperature of the gas leaving the compressor can have additional uses such as driving a reboiler. Since heat would have to be added to the gas anyway, it is more economical to use the heat in the gas at the compressor discharge and forego the benefit of the intercooling. However, each application must be evaluated if there is a temperature limit for the gas, or the power savings from cooling overshadows the alternate heat sources available to drive the reboiler.

The capital cost of the coolers, the piping, and the installation must become a part of any evaluation. Figure 2-1 shows a two-intercooler compressor. Cooling water must be added as an operating expense. Air cooling is an alternative, subject, however, to higher outlet temperature and higher capital cost. The extremes in ambient temperature and their effect on operation should not be ignored. An additional consideration is to observe the gas stream for possible condensation of components during cooling. If there is an objection to these components coming out of the stream, then some form of temperature control must be provided. If the condensation is acceptable, provision must still be made to remove this liquid fraction from the coolers before it enters the compressor. Most compressors are quite sensitive to liquid with some more so than others. In the sizing procedure, the loss of the fraction must also be considered in the resulting gas properties for the succeeding stages.

Figure 2-1. Two-water cooled intercoolers on a three-stage air compressor. (Courtesy of Elliott Company)
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