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University of Hail
Faculty of Engineering
DEPARTMENT OF MECHANICAL ENGINEERING
ME 435 – Thermal Power Plants
Lecture notes
Steam power plants; Rankine cycle
Part I
Prepared by : Dr. N. Ait Messaoudene
Based on:
El-Wakil, Power Plant Technology,
McGraw-Hill, 1984.
2nd semester 2012-2013
Shoaiba Steam Power Plant
The power station consists of 11 units with a total capacity of 4,400 MW. After completing
the third stage, the power station will consists of 14 units with total capacity of 5,600
MW, which makes it one of the largest fossil fuel-fired power stations in the world. The
expansion will be built by Alstom and it expected to become operational by 2010.[2] The
oil for power production is supplied from Saudi Aramco by tankers.
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Introduction
Ideal cycle: Carnot cycle; but not suitable for steam (liquid-vapor phases)
Ideal Rankine cycle: accepted as the standard for steam power plants.
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IDEAL RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLES
1-2 Isentropic compression in a pump
2-3 Constant pressure heat addition in a
boiler
3-4 Isentropic expansion in a turbine
4-1 Constant pressure heat rejection in a
condenser
Be careful!
Temperature of
cooling medium
must be lower
State 1:
Saturated
liquid
Energy Analysis of the Ideal Rankine Cycle
All four processes that make up the Rankine cycle can be analyzed as steady-flow processes.
ke and pe changes of the steam are usually small relative to the W and Q terms
The steady-flow energy equation per unit mass of steam reduces to :
Incomp..
Liq.
Pump (q = 0 + rev.
or
isentropic)
where
Boiler (w = 0 + p=cte)
Turbine(q = 0 +rev.
isentropic)
Condenser (w = 0 + p=cte)
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The thermal efficiency of the Rankine cycle is determined from
where
The conversion efficiency of power plants in the United States is often expressed in
terms of heat rate, which is the amount of heat supplied, in Btu’s, to generate 1 kWh of
electricity.
Considering that
1 kWh = 3412 Btu
Superheating the Steam to High Temperatures (Increases Thigh,avg)
Limited by
materials
More heat input
but net gain in
work
Other
advantage:
increases x4
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One way for increasing efficiency is to increase
boiler pressure; but this also has negative effects
Increase in Thigh,avg so
increase in efficiency
Two solutions:
Superheat steam to
very high temperature
(Increase T3)
But
disadvantage:
increases x4
But not viable because
of materials limitations
Expand the steam in
two-stage turbine (HP
and LP) , and reheat it
in between.
Reheating is a practical
solution and is
commonly used in
modern steam power
plants
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Thus the total heat input and the total turbine work output for a reheat cycle become
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( + the fact that T5 = T3)
( = 1.0 – 0.104 ) wet vapor or sat. mixt. state
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THE IDEAL REGENERATIVE RANKINE CYCLE
In a simple Rankine cycle, heat is transferred to the working fluid at a relatively low
temperature as it enters the boiler. This lowers the average heat addition temperature
and thus the cycle efficiency.
To remedy this shortcoming, we look for ways to raise the temperature of the liquid
leaving the pump (called the feedwater) before it enters the boiler by using some of the
heat of the cycle. This is called regeneration.
A practical regeneration process in steam power plants is accomplished by extracting, or
“bleeding,” steam from the turbine at various points. This 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).
A feedwater heater
is basically a heat
exchanger where
heat is transferred
from the steam to
the feedwater
by mixing the two fluid streams (open feedwater heaters)
without mixing the two streams (closed feedwater heaters)
DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES
common sources of irreversibilities: fluid friction and heat loss to the surroundings
A particular attention is given to
losses due to irreversibilities within
the pump and the turbine
Other losses to be considered : subcooling of the liquid in actual condensers to prevent the
onset of cavitation, losses at the bearings between the moving parts, steam leaks (out), air leaks
(in), power consumed by the auxiliary equipment (such as fans that supply air to the furnace)…
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pressure drop
Heat loss +
pressure drop
Friction
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Specific volume=1/r
Note:
No need to use J when
working in SI units
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Mass and energy balance on a unit mass flowrate basis
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Note:
No need to use J when
working in SI units
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