SECOND LAW OF THERMODYNAMICS ThermodynamicsAn actual turbine does less work because of friction losses in the blades, leakage past the bladesand, to a lesser extent, mechanical friction. Turbine efficiency , sometimes called isentropich tturbine efficiency because an ideal turbine is defined as one which operates at constant entropy,is defined as the ratio of the actual work done by the turbine Wt, actualto the work that would bedone by the turbine if it were an ideal turbine W_{t,ideal}.(1-26)h tW_{t,actual}W_{t,ideal}(1-27)h(h_{in}h_{out})_{actual}(h_{in}h_{out})_{ideal}where:= turbine efficiency (no units)h tW_{t,actual}= actual work done by the turbine (ft-lbf)W_{t,ideal}= work done by an ideal turbine (ft-lbf)(h_{in}- h_{out})_{actual}= actual enthalpy change of the working fluid (Btu/lbm)(h_{in}- h_{out})_{ideal}= actual enthalpy change of the working fluid in an ideal turbine(Btu/lbm)In many cases, the turbineFigure 27 Comparison of Ideal and Actual Turbine Performancesefficiency has been determinedh tindependently. This permits theactual work done to be calculateddirectly by multiplying the turbineefficiency by the work done byh tan ideal turbine under the sameconditions. For small turbines, theturbine efficiency is generally 60%to 80%; for large turbines, it isgenerally about 90%.The actual and idealizedperformances of a turbine may becompared conveniently using a T-sdiagram. Figure 27 shows such acomparison. The ideal case is aconstant entropy. It is representedby a vertical line on the T-s diagram. The actual turbine involves an increase in entropy. Thesmaller the increase in entropy, the closer the turbine efficiency is to 1.0 or 100%.h tHT-01 Page 80 Rev. 0