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7/28/2019 251 RankineCycle MAR
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Vapour Powered Cycles
Prepared by
PM Muhammad Abd Razak
FKM, UiTMPP
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VAPOUR POWERED CYCLE
From the previous chapter
The Carnot cycle is the most efficient cycle operating betweentwo constant temperature limits (TH & TL).
But it is not practical for power cycles.
Why??? Process 1-2 : the heat transfer
process in the saturation region isseverely limited. (eg 374C for waterand as P , hfg )Process 2-3: The turbine cannot
handle steam with a high moisturecontent which will cause erosion andwear on the blades.Process 4-1: not practical to design
a compressor that handles twophases(L+V)
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http://prevpage/7/28/2019 251 RankineCycle MAR
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The impracticalities of the Carnot cycle are removed by modifyingthe cycle to the Rankine Cycle:
State 4 is allowed to becomes saturated liquid fully.Compression is in the liquid state, therefore less powerneeded.
Heat addition is now at constant pressure (instead of constanttemperature & pressure)
Heat addition into the cycle can be increased by superheatingat constant pressure.
4
1
2
3qout
qin
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The Ideal Rankine Cycle now consist of :1 2 Isentropic compression in liquid state (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)
Ideal Rankine Cycle
Rearranging the state points.
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From SFEE
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Then
Net work, wnet = w = w34 + w12
= (h3 h4) - (h2 h1)
Heat supplied q23 = (h3 h2)
The Ideal Rankine Cycle thermal efficiency
)(
)()(
23
1243
hh
hhhh
q
w
in
netRankine
==
And .this is a criteria of performance
Note:The term (h2 h1) is the pump work which is very small
compared to the turbine work and sometimes is ignored.
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Other criteria of performances are
the work ratio
turbine
pumpturbine
w
ww
outputworkgross
outputworknetratiowork
==
)(
)()(
43
1243
hh
hhhhratiowork
=
the specific steam consumption (ssc) which is the amount ofsteam required to produce a unit power output
)(
1
)( 4334hhwm
mssc
==
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DEVIATION OF ACTUAL VAPOUR POWER CYCLES FROMIDEALIZED ONES
The actual vapour power cycle differs from the ideal Rankinecycle due to irreversibilities in various components. 2 mainsources are :
Fluid friction (normally ignored) heat loss to the surroundings (deviation from adiabatic &isentropic process)
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Isentropic efficiency of pump isentropic efficiency of turbine
HOW TO INCREASE THE EFFICIENCY OF THE RANKINECYCLE
The thermal efficiency can be improved by: increase of the average temperature of heat addition to the
working fluid in the boiler
decrease the average temperature of heat rejection from theworking fluid in the condenser
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1. Superheating the Steam to Higher Temperatures
Through superheating the steam: Increase in heat input Increase of the net work
decreases the moisture content ofthe steam at the turbine exit Most importantly, increase in
thermal efficiency
The superheat temperature is limitedby metallurgical considerations.
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2. Increasing the Boiler Pressure
For a fixed turbine inlet temperature: the cycle shifts to the left the moisture content of steam at the
turbine exit increases.
This side effect can be corrected byreheating the steam.
3. Lowering the Condenser Pressure
The condenser usually operatesbelow the atmospheric pressure.
Side effect: increases the moisture
content of the steam at the turbine.
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A single reheat in a modern power plant improves the cycleefficiency by 4 to 5%.
The reheat temperatures are very close or equal to the turbineinlet temperature.
Heat transfer to the feedwaterfrom 2-2 is at a relatively low
temperature.
This lowers the average heat-addition temperature and thus
the cycle efficiency.
THE IDEAL REGENERATIVERANKINE CYCLE
Tavg
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Some steam is extracted from the turbine at various points(eg at 6) and is used to heat the feedwater.
The device where the feedwater is heated by regeneration iscalled a regenerator, or a feedwater heater (FWH).
Heat is transferred from steam to the feedwater either by
mixing the two fluid streams directly (open feedwaterheaters) or without mixing them (closed feedwater heaters).
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An open (or direct-contact) FWH is basically a mixingchamber
the steam extracted from the turbine mixes with theincoming feedwater, releasing its latent heat to thefeedwater.
the mixture leaves the heater as saturated liquid at theheater pressure.
For a unit mass of steam supply to the boiler, the amountextracted at 6 is y kg.
SFEE for the FWH
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For the regenerative cycle 1 kg
1-y kg
y kg
1-y kg
1 kg