Power Plant
Vapor Power Cycles
2-4 Reheat
There are some methods to improve cycle performance. The thermal
efficiency of the cycle can be improved by following methods:
(i) By reheating of steam
(ii) By regenerative feed heating
(iii) By water extraction
(iv) By using binary vapour
Reheating System
The T-s diagram of the ideal reheat Rankine cycle and the schematic of the
power plant operating on this cycle are shown in Fig. 2.9.
In the first stage (the high pressure turbine), steam is expanded
isentropically to an intermediate pressure and sent back to the boiler where it
is reheated at constant pressure, usually to the inlet temperature of the first
turbine stage. Steam then expands isentropically in the second stage (lowpressure turbine) to the condenser pressure. Thus the total heat input and the
total turbine work output for a reheat cycle become:
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Power Plant
Vapor Power Cycles
Figure 2.9: The ideal reheat Rankine cycle
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Power Plant
Vapor Power Cycles
Advantages (or) effects of Re-heating:
(1) Network done increases
(2) Heat supply increases
(3) Thermal efficiency increases
(4) The turbine exit steam dryness fraction increases, thus moisture
decreases, therefore blade erosion becomes minimum, and that lead to
increase the life of the turbine.
(5) Erosion and corrosion problems in the steam turbine are eliminated/or
may be avoided
Disadvantages of Reheating:
1- Reheating requires more maintenance.
2- The increase in thermal efficiency is not appreciable in comparison to the
expenditure incurred in reheating.
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Power Plant
Vapor Power Cycles
EXAMPLE 5
Consider a steam power plant operating on the ideal reheat Rankine cycle.
Steam enters the high-pressure turbine at 15 MPa and 600°C and is
condensed in the condenser at a pressure of 10 kPa. If the moisture content
of the steam at the exit of the low-pressure turbine is not to exceed 10.4
percent, determine (a) the pressure at which the steam should be reheated
and (b) the thermal efficiency of the cycle. Assume the steam is reheated to
the inlet temperature of the high-pressure turbine.
SOLUTION:
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Power Plant
Vapor Power Cycles
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Power Plant
Vapor Power Cycles
H.W
A smaller power plant produces s
The first turbine section expands to 500 kPa, and then flow is reheated
followed by the expansion in the low-pressure turbine. Find the reheat
temperature so that the turbine output is saturated vapor. For this reheat, find
the total turbine power output and the boiler heat transfer.
[Answer. (Reheat temperature= 529ºC , total turbine power output= 6487
kW, the boiler heat transfer= 16 475 kW)]
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Power Plant
Vapor Power Cycles
2.5 The Ideal Regenerative Rankine Cycle
A practical regeneration process in steam power plants is accomplished by
steam, which could have produced more work by expanding further in the
turbine, is used to heat the feedwater instead. The device where the
feedwater is heated by regeneration is called a regenerator, or a feedwater
heater (FWH). Most modern steam power plants use between 5-8 feedwater
heaters.
A feedwater heater is basically a heat exchanger in which heat is transferred
from the steam to the feedwater. There are two types of feedwater as follow:
1.Open or direct contact type.
2.Closed type with:
a- drains cascaded backward
b- drains pumped forward.
feedwater
Closed feedwater heaters
without mixing them
Open feedwater heaters
mixing the two fluid
streams
Backward
cascaded drain
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Forward
pumped drain