COMPRESSION PROCESSESThermodynamicsFluids that are compressible have much more complex equations to deal with, due to densitychanges, and have property relationships that vary more rapidly than incompressible fluids. Inaddition, fixing the state of a liquid can be done easily by knowing its temperature and pressure.Once the substance becomes a gas, the process becomes more difficult.ConstantPressureProcessTo determine the work done in a constant pressure process, the following equation is used:W1-2 = P(DV)(1-44)ConstantVolumeProcessThe solution of Equation 1-45 for a constant volume process is also not difficult. The workdone in a constant volume process is the product of the volume and the change in pressure.W1-2 = V(DP)(1-45)In addition to gases, Equation 1-45 also applies to liquids. The power requirement for pumpsthat move incompressible liquids (such as water) can be determined from Equation 1-44.Replacing the volume (V) with the product of the specific volume and the mass yields Equation1-45.W1-2 = mv(DP)(1-46)Taking the time rate of change of both sides of Equation 1-46 determines the powerrequirements of the pump.(1-47)W1 2mv(DP)EffectsofPressureChangesonFluidPropertiesThe predominant effect of an increase in pressure in a compressible fluid, such as a gas, is anincrease in the density of the fluid. An increase in the pressure of an incompressible fluid willnot have a significant effect on the density. For example, increasing the pressure of 100 °Fwater from 15 psia to 15,000 psia will only increase the density by approximately 6%.Therefore, in engineering calculations, it is assumed that incompressible fluids' density remainconstant.HT-01Page 100Rev. 0
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