Thermodynamics SECOND LAW OF THERMODYNAMICSA steam turbine is designed to extract energy from the working fluid (steam) and use it to dowork in the form of rotating the turbine shaft. The working fluid does work as it expandsthrough the turbine. The shaft work is then converted to electrical energy by the generator. Inthe application of the first law, general energy equation to a simple turbine under steady flowconditions, it is found that the decrease in the enthalpy of the working fluid Hin - Hout equals thework done by the working fluid in the turbine (Wt).(1-24)HinHoutWt(1-25)m(hinhout)wtwhere: Hin = enthalpy of the working fluid entering the turbine (Btu)Hout = enthalpy of the working fluid leaving the turbine (Btu)Wt = work done by the turbine (ft-lbf)= mass flow rate of the working fluid (lbm/hr)mhin= specific enthalpy of the working fluid entering the turbine (Btu/lbm)hout = specific enthalpy of the working fluid leaving the turbine (Btu/lbm)= power of turbine (Btu/hr)wtThese relationships apply when the kinetic and potential energy changes and the heat losses ofthe working fluid while in the turbine are negligible. For most practical applications, these arevalid assumptions. However, to apply these relationships, one additional definition is necessary.The steady flow performance of a turbine is idealized by assuming that in an ideal case theworking fluid does work reversibly by expanding at a constant entropy. This defines the so-called ideal turbine. In an ideal turbine, the entropy of the working fluid entering the turbine Sinequals the entropy of the working fluid leaving the turbine.Sin = Soutsin = soutwhere: Sin = entropy of the working fluid entering the turbine (Btu/oR)Sout = entropy of the working fluid leaving the turbine (Btu/oR)sin= specific entropy of the working fluid entering the turbine (Btu/lbm -oR)sout = specific entropy of the working fluid leaving the turbine (Btu/lbm -oR)The reason for defining an ideal turbine is to provide a basis for analyzing the performance ofturbines. An ideal turbine performs the maximum amount of work theoretically possible.Rev. 0 Page 79 HT-01
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