SECOND LAW OF THERMODYNAMICS Thermodynamics An actual turbine does less work because of friction losses in the blades, leakage past the blades and, to a lesser extent, mechanical friction.   Turbine efficiency , sometimes called isentropic h t turbine 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 W t, actual to the work that would be done by the turbine if it were an ideal turbine W_t,ideal. (1-26) h t W_t,actual W_t,ideal (1-27) h (h_in h_out)_actual (h_in h_out)_ideal where: =   turbine efficiency (no units) h t W_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 turbine Figure 27    Comparison of Ideal and Actual Turbine Performances efficiency has been determined h t independently.    This  permits  the actual work done to be calculated directly by multiplying the turbine efficiency by the work done by h t an  ideal  turbine  under  the  same conditions.  For small turbines, the turbine efficiency is generally 60% to  80%;  for  large  turbines,  it  is generally about 90%. The actual and idealized performances of a turbine may be compared conveniently using a T-s diagram.   Figure 27 shows such a comparison.    The  ideal  case  is  a constant entropy.  It is represented by a vertical line on the T-s diagram.   The actual turbine involves an increase in entropy.   The smaller the increase in entropy, the closer the turbine efficiency is to 1.0 or 100%. h t HT-01 Page 80 Rev. 0