Solved Problem 6.1 - A Supercritical Steam
Power Plant for Athens, Ohio
Consider the supercritical steam power plant
with reheat for Athens, Ohio, which we evaluated in Solved
Problem 4.1. The system diagram is repeated here for convenience:
In this exercise we wish to evaluate the high
pressure (HP) and low pressure (LP) turbines of this system (circled
in red), both of which are assumed to be adiabatic.
- 1) Plot the two turbine processes (Stations
(1)-(2) and (3)-(4)) on the enthalpy-entropy h-s "Mollier" diagram. Plot also the equivalent isentropic turbine processes
on the diagram, and indicate the actual turbine specific work
as well as the isentropic turbine specific work for both turbines
on the h-s diagram.
- 2) Using steam
tables, determine the turbine adiabatic efficiency ηT
of both turbines.
- 3) Discuss your results as well as the feasibility
of the turbine set.
Justify all values
used and derive all equations used starting from the basic
energy equation for a flow system, the basic definition of turbine
adiabatic efficiency ηT.
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Solution Approach:
- 1) Plot the two turbine processes (Stations
(1)-(2) and (3)-(4)) on the enthalpy-entropy h-s "Mollier"
diagram below. Plot also the equivalent isentropic turbine processes
on the diagram, and indicate the actual turbine specific work
as well as the isentropic turbine specific work for both turbines
on the h-s diagram. [refer
h-s diagram below]
- 2) Using steam
tables, determine the turbine adiabatic efficiency ηT
of both turbines. [enthalpy
and entropy data derived from steam tables shown in red on schematic
diagram below. State (2s) required linear interpolation of the
superheat
table values and
both states (4a) and (4s) required the saturation
properties (pressure) table and use of the quality relation X indicated below.
These values were then indicated on the above h-s diagram
plot in order to validate their feasibility.]
- 3) Discuss your results as well as the feasibility
of the turbine set. [Both
adiabatic efficiencies seem to be feasible and do not violate
the second law. We need to question why the LP turbine efficiency
(90%) is so much higher than the HP turbine efficiency (77%)
- it may be due to the ability of higher relative accuracy in
the manufacture of a much larger turbine however this large difference
is suspect and needs more investigation.]
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Engineering Thermodynamics by Israel Urieli is licensed under a
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