Docfoc.com-Chapter 10 VAPOR AND COMBINED POWER CYCLES Mehmet Kanoglu University of Gaziantep Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction.

8/17/2019 Docfoc.com-Chapter 10 VAPOR AND COMBINED POWER CYCLES Mehmet Kanoglu University of Gaziantep Copyright © The McGraw-Hill Companies, Inc. Permission required fo…

1/22

Chapter 10

VAPOR AND COMBINED POWERCYCLES

Mehmet Kanogl

!n"#er$"t% o& 'a("antep

Cop%r"ght ) *he M+'ra,-."ll Compan"e$/ In+ Perm"$$"on re"re2 &or repro2+t"on or 2"$pla%

*hermo2%nam"+$3 An Eng"neer"ng Approa+hSe#enth E2"t"on "n SI !n"t$

Yn$ A Cengel/ M"+hael A Bole$

M+'ra,-."ll/ 4011


2/22

2

O56e+t"#e$

• Evaluate the performance of gas power cycles for

which the working fluid remains a gas throughout theentire cycle.

• Analyze vapor power cycles in which the working fluid

is alternately vaporized and condensed.

• Analyze power generation coupled with processheating called cogeneration.

• Investigate ways to modify the basic Rankine vapor

power cycle to increase the cycle thermal efficiency.

• Analyze the reheat and regenerative vapor powercycles.

• Analyze power cycles that consist of two separate

cycles known as combined cycles and binary cycles.


3/22

*.E CARNO* VAPOR CYCLE

T-s diagram of two !arnot vapor cycles.

"he !arnot cycle is the most efficient cycle operating between two specified temperature

limits but it is not a suitable model for power cycles. #ecause$

%rocess &'2 (imiting the heat transfer processes to two'phase systems severely limits thema)imum temperature that can be used in the cycle *+,-! for water

%rocess 2' "he turbine cannot handle steam with a high moisture content because of the

impingement of li/uid droplets on the turbine blades causing erosion and wear.

%rocess ,'& It is not practical to design a compressor that handles two phases.

"he cycle in *b is not suitable since it re/uires isentropic compression to e)tremely high

pressures and isothermal heat transfer at variable pressures.

1-4 isothermal heat

addition in a boiler

4-7 isentropic e)pansion

in a turbine

7-8 isothermal heatre0ection in a condenser

8-1 isentropic

compression in a

compressor


4/22

,

RANKINE CYCLE3 *.E IDEAL CYCLE

9OR VAPOR POWER CYCLES

1any of the impracticalities associated withthe !arnot cycle can be eliminated by

superheating the steam in the boiler and

condensing it completely in the condenser.

"he cycle that results is the Ran:"ne +%+le

which is the ideal cycle for vapor power plants.

"he ideal Rankine cycle does not involve anyinternal irreversibilities.

"he simple ideal

Rankine cycle.


5/22

3

Energ% Anal%$"$ o& the I2eal Ran:"ne C%+le

"he efficiency of power plants in

the 4.5. is often e)pressed interms of heat rate which is the

amount of heat supplied in #tu6s

to generate & k7h of electricity."he thermal efficiency can be interpreted

as the ratio of the area enclosed by the

cycle on a T-s diagram to the area under

the heat'addition process.

5teady'flow energy e/uation


6/22

8

DEVIA*ION O9 AC*!AL VAPOR POWER

CYCLES 9ROM IDEALI;ED ONES

*a 9eviation of actual vapor power cycle from the ideal Rankine cycle.

*b "he effect of pump and turbine irreversibilities on the ideal Rankine cycle.

"he actual vapor power cycle differs from the ideal Rankine cycle as a

result of irreversibilities in various components.

:luid friction and heat loss to the surroundings are the two common sources

of irreversibilities.

I$entrop"+ e&&"+"en+"e$


7/22+

.OW CAN WE INCREASE *.E

E99ICIENCY O9 *.E RANKINE CYCLE<

"he effect of lowering the

condenser pressure on the

ideal Rankine cycle.

"he basic idea behind all the modifications to increase the thermal efficiencyof a power cycle is the same$ Increase the average temperature at which heat is

transferred to the working fluid in the boiler, or decrease the average

temperature at which heat is rejected from the working fluid in the condenser.

Lo,er"ng the Con2en$er Pre$$re =Lowers T lo,/a#g>

"o take advantage of the increasedefficiencies at low pressures the condensers

of steam power plants usually operate well

below the atmospheric pressure. "here is a

lower limit to this pressure depending on the

temperature of the cooling medium

S"2e e&&e+t3 (owering the condenserpressure increases the moisture content of

the steam at the final stages of the turbine.


8/22;

"he effect of superheating the

steam to higher temperatures

on the ideal Rankine cycle.

Sperheat"ng the Steam to ."gh *emperatre$

=Increases T h"gh/a#g>

#oth the net work and heat input

increase as a result ofsuperheating the steam to a higher

temperature. "he overall effect is

an increase in thermal efficiency

since the average temperature at

which heat is added increases.5uperheating to higher

temperatures decreases the

moisture content of the steam at

the turbine e)it which is desirable.

"he temperature is limited bymetallurgical considerations.

%resently the highest steam

temperature allowed at the turbine

inlet is about 82


9/22=

In+rea$"ng the Bo"ler Pre$$re =Increases T h"gh/a#g>

"he effect of increasing the boiler

pressure on the ideal Rankine cycle.

:or a fi)ed turbine inlet temperature

the cycle shifts to the left and the

moisture content of steam at the

turbine e)it increases. "his sideeffect can be corrected by reheating

the steam.

A supercritical Rankine cycle.

"oday many modern steam power

plants operate at supercritical

pressures *P > 22.


10/22&<

*.E IDEAL RE.EA* RANKINE CYCLEHow can we take advantage of the increased efficiencies at higher boiler pressures

without facing the problem of ecessive moisture at the final stages of the turbine!

&. 5uperheat the steam to very high temperatures. It is limited metallurgically.2. E)pand the steam in the turbine in two stages and reheat it in between *reheat

"he ideal reheat Rankine cycle.


11/22

&&

"he average temperature at whichheat is transferred during reheating

increases as the number of reheat

stages is increased.

"he single reheat in a modern power

plant improves the cycle efficiency by , to

3> by increasing the average

temperature at which heat is transferred

to the steam."he average temperature during the

reheat process can be increased by

increasing the number of e)pansion and

reheat stages. As the number of stages is

increased the e)pansion and reheat

processes approach an isothermalprocess at the ma)imum temperature.

"he use of more than two reheat stages

is not practical. "he theoretical

improvement in efficiency from the

second reheat is about half of that which

results from a single reheat.

"he reheat temperatures are very close

or e/ual to the turbine inlet temperature.

"he optimum reheat pressure is about

one'fourth of the ma)imum cycle

pressure.


12/22

&2

*.E IDEAL RE'ENERA*IVE RANKINE CYCLE

"he first part of the heat'addition

process in the boiler takes place at

relatively low temperatures.

?eat is transferred to the working fluid

during process 2'2 at a relatively lowtemperature. "his lowers the average

heat'addition temperature and thus the

cycle efficiency.

In steam power plants steam is e)tracted

from the turbine at various points. "his

steam which could have produced morework by e)panding further in the turbine is

used to heat the feedwater instead. "he

device where the feedwater is heated by

regeneration is called a regenerator or a

&ee2,ater heater =9W.>.

A feedwater heater is basically a heat

e)changer where heat is transferred from

the steam to the feedwater either by

mi)ing the two fluid streams *open

feedwater heaters or without mi)ing them

*closed feedwater heaters.


13/22

&

Open 9ee2,ater .eater$

An open *or 2"re+t-+onta+t &ee2,ater

heater is basically a miing chamber,

where the steam e)tracted from the

turbine mi)es with the feedwater e)itingthe pump. Ideally the mi)ture leaves

the heater as a saturated li/uid at the

heater pressure.

"he ideal regenerativeRankine cycle with an open

feedwater heater.


14/22

&,

Clo$e2 9ee2,ater .eater$

"he ideal regenerative Rankine cycle with a closed feedwater heater.

Another type of feedwater heater fre/uently used in steam power plants is

the +lo$e2 &ee2,ater heater in which heat is transferred from the

e)tracted steam to the feedwater without any mi)ing taking place. "he twostreams now can be at different pressures since they do not mi).


15/22

&3

"he closed feedwater heaters are more comple) because of the internal tubing

network and thus they are more e)pensive. ?eat transfer in closed feedwater

heaters is less effective since the two streams are not allowed to be in direct contact.

?owever closed feedwater heaters do not re/uire a separate pump for each heater

since the e)tracted steam and the feedwater can be at different pressures.

@pen feedwater

heaters are simple

and ine)pensive and

have good heat

transfer

characteristics. :or

each heater however

a pump is re/uired to

handle the feedwater.

1ost steam powerplants use a

combination of open

and closed feedwater

heaters.

A steam power plant with one open and three closed feedwater heaters.


16/22

&8

SECOND-LAW ANALYSIS O9 VAPOR

POWER CYCLES

E)ergy destruction for a steady'flow system

5teady'flow one'

inlet one'e)it

E)ergy destruction of a cycle

:or a cycle with heat transfer

only with a source and a sink

5tream e)ergy

A second'law analysis of vapor power cycles reveals where the

largest irreversibilities occur and where to start improvements.


17/22

&+

CO'ENERA*ION

A simple process'heating plant.

1any industries re/uire energy input in the form of heat called process

heat . %rocess heat in these industries is usually supplied by steam at 3 to

+ atm and &3< to 2


18/22

&; An ideal cogeneration plant.

4tilization

factor

• "he utilization factor of the

ideal steam'turbine

cogeneration plant is

&


19/22

&=

A cogeneration plant with

ad0ustable loads.

At times of high demand for process heat all

the steam is routed to the process'heating units

and none to the condenser *m+


20/22

2<

COMBINED 'AS?VAPOR POWER CYCLES

• "he continued /uest for higher thermal efficiencies has resulted in rather

innovative modifications to conventional power plants.

• A popular modification involves a gas power cycle topping a vapor power cycle

which is called the +om5"ne2 ga$?#apor +%+le or 0ust the +om5"ne2 +%+le.

• "he combined cycle of greatest interest is the gas'turbine *#rayton cycle topping

a steam'turbine *Rankine cycle which has a higher thermal efficiency than

either of the cycles e)ecuted individually.

• It makes engineering sense to take advantage of the very desirablecharacteristics of the gas'turbine cycle at high temperatures and to use the high'

temperature e)haust gases as the energy source for the bottoming cycle such as

a steam power cycle. "he result is a combined gasDsteam cycle.

• Recent developments in gas'turbine technology have made the combined gasD

steam cycle economically very attractive.

• "he combined cycle increases the efficiency without increasing the initial cost

greatly. !onse/uently many new power plants operate on combined cycles and

many more e)isting steam' or gas'turbine plants are being converted to

combined'cycle power plants.

• "hermal efficiencies over 3 are reported.


21/22

2&!ombined gasDsteam power plant.


22/22

22

Smmar%

• "he !arnot vapor cycle

• Rankine cycle$ "he ideal cycle for vapor power cycles Energy analysis of the ideal Rankine cycle

• 9eviation of actual vapor power cycles from idealized ones

• ?ow can we increase the efficiency of the Rankine cycle

(owering the condenser pressure *"owers T lowavg

5uperheating the steam to high temperatures *Increases T highavg

Increasing the boiler pressure *Increases T highavg

• "he ideal reheat Rankine cycle

• "he ideal regenerative Rankine cycle

@pen feedwater heaters

!losed feedwater heaters

• 5econd'law analysis of vapor power cycles

• !ogeneration

• !ombined gasDvapor power cycles

Docfoc.com-Chapter 10 VAPOR AND COMBINED POWER CYCLES Mehmet Kanoglu University of Gaziantep Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction.

Documents