Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 127or,(3–2)Both the total enthalpy H and specific enthalpy h are simply referred toas enthalpy since the context clarifies which one is meant. Notice thatthe equations given above are dimensionally homogeneous. That is, the unitof the pressure–volume product may differ from the unit of the internalenergy by only a factor (Fig. 3–29). For example, it can be easily shownthat 1 kPa · m 3 1 kJ. In some tables encountered in practice, the internalenergy u is frequently not listed, but it can always be determined fromu h Pv.The widespread use of the property enthalpy is due to Professor RichardMollier, who recognized the importance of the group u Pv in the analysisof steam turbines and in the representation of the properties of steam in tabularand graphical form (as in the famous Mollier chart). Mollier referred tothe group u Pv as heat content and total heat. These terms were not quiteconsistent with the modern thermodynamic terminology and were replacedin the 1930s by the term enthalpy (from the Greek word enthalpien, whichmeans to heat).1aH U PV1kJ2Saturated Liquid and Saturated Vapor StatesThe properties of saturated liquid and saturated vapor for water are listed inTables A–4 and A–5. Both tables give the same information. The only differenceis that in Table A–4 properties are listed under temperature and inTable A–5 under pressure. Therefore, it is more convenient to use Table A–4when temperature is given and Table A–5 when pressure is given. The useof Table A–4 is illustrated in Fig. 3–30.The subscript f is used to denote properties of a saturated liquid, and thesubscript g to denote the properties of saturated vapor. These symbols arecommonly used in thermodynamics and originated from German. Anothersubscript commonly used is fg, which denotes the difference betweenthe saturated vapor and saturated liquid values of the same property. Forexample,v f specific volume of saturated liquidv g specific volume of saturated vaporv fg difference between v g and v f 1that is, v fg v g v f 2The quantity h fg is called the enthalpy of vaporization (or latent heatof vaporization). It represents the amount of energy needed to vaporize aunit mass of saturated liquid at a given temperature or pressure. It decreasesas the temperature or pressure increases and becomes zero at the criticalpoint.Chapter 3 | 127kPa · m 3 ≡ kJkPa · m 3 /kg ≡ kJ/kgbar · m 3 ≡ 100 kJMPa · m 3 ≡ 1000 kJpsi · ft 3 ≡ 0.18505 BtuFIGURE 3–29The product pressure volume hasenergy units.Temp.°CTSat.press.kPaP sat85 57.86890 70.18395 84.609SpecifictemperatureCorrespondingsaturationpressureSat.liquidv fSpecific volumem 3 /kg0.001032 2.82610.001036 2.35930.001040 1.9808Specificvolume ofsaturatedliquidFIGURE 3–30A partial list of Table A–4.Sat.vaporv gSpecificvolume ofsaturatedvapor