Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 18

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.Constant Pressure ProcessTo determine the work done in a constant pressure process, the following equation is used:W1-2 = P(∆V)(1-44)Constant Volume ProcessThe 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(∆P)(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 (∆P)(1-46)Taking the time rate of change of both sides of Equation 1-46 determines the powerrequirements of the pump.W1 2mv(∆P)(1-47)Effects of Pressure Changes on Fluid PropertiesThe 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. 0ThermodynamicsCOMPRESSION PROCESSESEffects of Temperature Changes on Fluid PropertiesAn increase in temperature will tend to decrease the density of any fluid. If the fluid is confinedin a container of fixed volume, the effect of a temperature change will depend on whether thefluid is compressible.If the fluid is a gas, it will respond to a temperature change in a manner predicted by the idealgas laws.

A 5% increase in absolute temperature will result in a 5% increase in the absolutepressure.If the fluid is an incompressible liquid in a closed container, an increase in the temperature willhave a tremendously greater and potentially catastrophic effect. As the fluid temperatureincreases, it tries to expand, but expansion is prevented by the walls of the container. Becausethe fluid is incompressible, this results in a tremendous increase in pressure for a relativelyminor temperature change.

The change in specific volume for a given change in temperatureis not the same at various beginning temperatures. Resultant pressure changes will vary. Auseful thumb rule for water is that pressure in a water-solid system will increase about 100 psifor every 1 °F increase in temperature.Rev. 0Page 101HT-01COMPRESSION PROCESSESThermodynamicsSummaryThe important information from this chapter is summarized below.Compression Processes Summary•The ideal gas law can be used to determine how the properties of pressure,temperature, and volume will be related during compression processes.Pv = R T•A fluid may be considered incompressible if one of two conditions is true:The fluid is a liquid.The fluid is a gas with a velocity greater than one-third of the speed ofsound in the gas.•The work for certain types of processes can be determined as follows:Constant pressure processConstant volume processHT-01Page 102W1-2 = P(∆V)W1-2 = V(∆P)Rev.

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