Thermodynamics COMPRESSION PROCESSESThe ideal gas law is utilized by engineers working with gases because it is simple to use andapproximates real gas behavior. Most physical conditions of gases used by man fit the abovedescription. Perhaps the most common use of gas behavior studied by engineers is that of thecompression process using ideal gas approximations. Such a compression process may occurat constant temperature (pV = constant), constant volume, or adiabatic (no heat transfer).Whatever the process, the amount of work that results from it depends upon the process, asbrought out in the discussion on the First Law of Thermodynamics. The compression processusing ideal gas considerations results in work performed on the system and is essentially the areaunder a P-V curve. As can be seen in Figure 40, different amounts of work result from differentideal gas processes such as constant temperature and constant pressure.Figure 40 Pressure-Volume DiagramFluidA fluid is any substance that conforms to the shape of its container. It may be either a liquidor a gas.CompressibilityofFluidsUsually a fluid may be considered incompressible when the velocity of the fluid is greater thanone-third of the speed of sound for the fluid, or if the fluid is a liquid. The treatment of a fluidthat is considered incompressible is easy because the density is assumed to be constant, givinga simple relationship for the state of the substance. The variation of density of the fluid withchanges in pressure is the primary factor considered in deciding whether a fluid isincompressible.Rev. 0Page 99HT-01
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