In an isothermal process, the temperature is unchanged during compression. Although it is impossible to build a machine that will compress isothermally, isothermal performance is approached as the number of intercoolers or other cooling devices is increased.
Furthermore, although isothermal compression cannot actually be attained in practice, it is often used as the basis for comparison with other compression processes. The effect of the number of coolers on compression power will be covered under Polytropic Compression.
The following equation applies to an isothermal compression process:
“Head” is a term often used for the work input to the compression process. The units of head are foot-pounds (force) divided by pounds (mass). In general practice, the unit of head is usually taken as “feet.” The theoretical head for an isothermal process is:
Equation 100-18 may be used to evaluate other compression processes with various amounts of cooling.