ME 416 CAD of Thermal Systems
1
ME 416
Computer Assisted Design of
Thermal Systems
Steam Turbine Calculations
In an ideal thermodynamic sense steam turbines are treated as isentropic devices.
Though actual steam turbines deviate from this ideal performance, the treatment of a
steam turbine as an isentropic device still forms the foundation for actual turbine
calculations. Calculations for steam turbines are demonstrated from three
perspectives in this document: using an isentropic assumption, using a specified
adiabatic or isentropic efficiency, using actual manufacturers operating data. Also a
second law analysis for steam turbines is provided.
Ideal or Isentropic Steam Turbine Calculations
We consider the schematic shown in Figure 1 where state 1 represents the inlet
conditions for the turbine and state 2s represent the ideal/isentropic outlet state for the
turbine.
Figure 1. Schematic of Steam Turbine
Normally the inlet state for the turbine is fixed by specifying the temperature and
pressure of the incoming steam. For the outlet state only the pressure is specified,
which mean that state 2s is not fixed. Often the mass flow rate of steam passing
through the turbine is given, however, since the calculations for the ideal turbine will
be linear they may be done on a per mass basis and scaled up according to the mass
flow rate. We begin by going to the steam tables and obtaining the entropy and
enthalpy at state 1 with the given temperature and pressure. Next the outlet state may
be fixed by using the isentropic condition or
State 1
State 2s
Turbine
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2
s
2s
= s
1
At this point we know both our pressure and entropy at state 2s, which fixes the state.
This allows us to go to the steam table and identify the fluid phase, followed by an
evaluation of the enthalpy and temperature of the outlet state. We can now calculate
the specific work (work per mass) and power produced by
w = h
1
 h
2
( )
W = m h
h
1
2

Adiabatic Turbine Calculation
For our purposes an adiabatic turbine will have an adiabatic or isentropic efficiency
that has been specified. The specific work is calculated with
w =
w
s
ideal
η ⋅
where
η
s
is the isentropic efficiency of the turbine and w
ideal
is the work that would be
produced if the turbine behaved isentropically. Hence, an adiabatic turbine
calculation first requires an ideal turbine calculation. Once the ideal work is
calculated the actual work is calculated with the equation provide above. The final
step of the calculation is to determine the actual outlet conditions, since up to this
point in time only the isentropic outlet conditions have been found. Using the fist law
we can write
w = h  h
1
2a
where the subscript 2a denotes the actual outlet state, state 2a. To demonstrate this
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 Fall '07
 SOMERTON
 Thermodynamics, Thermal Systems

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