SECOND LAW OF THERMODYNAMICS
A steam turbine is designed to extract energy from the working fluid (steam) and use it to do
work in the form of rotating the turbine shaft. The working fluid does work as it expands
through the turbine. The shaft work is then converted to electrical energy by the generator. In
the application of the first law, general energy equation to a simple turbine under steady flow
conditions, it is found that the decrease in the enthalpy of the working fluid Hin - Hout equals the
work done by the working fluid in the turbine (Wt).
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)
= 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)
These relationships apply when the kinetic and potential energy changes and the heat losses of
the working fluid while in the turbine are negligible. For most practical applications, these are
valid 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 the
working 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 Sin
equals the entropy of the working fluid leaving the turbine.
Sin = Sout
sin = sout
Sin = entropy of the working fluid entering the turbine (Btu/oR)
Sout = entropy of the working fluid leaving the turbine (Btu/oR)
= 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 of
turbines. An ideal turbine performs the maximum amount of work theoretically possible.