page_title: Compressor
section_title: Effect of cooling during the compression process
hierarchical_section_title: Compressor / Thermodynamics of gas compression / Minimizing work required by a compressor / Effect of cooling during the compression process
caption_attribution_description: English: Combined P-v diagram of isentropic compression processes.
context_page_description: A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
  Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible; while some can be compressed, the main action of a pump is to pressurize and transport liquids.
  Many compressors can be staged, that is, the fluid is compressed several times in steps or stages, to increase discharge pressure. Often, the second stage is physically smaller than the primary stage, to accommodate the already compressed gas. Each stage further compresses the gas and increases pressure. Those that are powered by an electric motor can also be controlled using a VFD or power inverter, however many compressors can only work at certain speeds, since they may include built-in oil pumps. The oil pumps are connected to the same shaft that drives the compressor and forces oil into the compressor and motor bearings.
context_section_description: isentropic process: involves no cooling,
  polytropic process: involves some cooling
  isothermal process: involves maximum cooling
  By making the following assumptions the required work for the compressor to compress a gas from  to  is the following for each process:
  
  Assumptions:
   and 
  All processes are internally reversible
  The gas behaves like an ideal gas with constant specific heats
  Isentropic (, where ):
  Polytropic ():
  Isothermal ( or ):
  By comparing the three internally reversible processes compressing an ideal gas from  to , the results show that isentropic compression () requires the most work in and the isothermal compression( or ) requires the least amount of work in. For the polytropic process () work in decreases as the exponent, n, decreases, by increasing the heat rejection during the compression process. One common way of cooling the gas during compression is to use cooling jackets around the casing of the compressor.
caption: P-v (Specific volume vs. Pressure) diagram comparing isentropic, polytropic, and isothermal processes between the same pressure limits.
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