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An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change will occur slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 and therefore ΔU = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.  In reality, an isothermal compressor is an old dream of the engineer, but successful implementation could improve the efficiency of various industrial and power generation plants significantly, and it can save power in many compression jobs in process plants, especially in cases of high pressure ratio.

The ideal isothermal compressor has not yet been built, with one historic exception. But approximations with adiabatic compressor stages alternating with intercoolers are widely used.  The intermediate pressure level for the intercooling steps has to be chosen such that the adiabatic discharge  temperatures in each compression step are all equal for the given pressure ratio.  Typically, one intercooling step provides more than half the power saving ideally attainable. It is also useful to look at power saving as a function of the number of intercooling steps.

A novel design is needed to reduce the power consumption of compressors throughout the industry and result in significant benefits to the society.