10 MCE341 VaporCycles.pptx

8/11/2019 10 MCE341 VaporCycles.pptx

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Vapor and Combined Cycles

Thermodynamics II

MCE341

Dr. Andreas PoullikkasVisiting Associate Professor

College of [email protected]

!


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1-2:water in state 1 is evaporated in a

boiler to form saturated steam in state 2

- isothermal heat addition in a boiler :

Q12=h2-h1

2-3:steam is expanded isentropically to

state 3 while doing work -isentropic

expansion in a turbine : W23=h2-h3

3-4:after expansion steam is condensed

at constant pressure - isothermal heat

rejection in a condenser: Q34=h4-h3

4-1:condensation is stopped at 4 andthe wet steam is compressed

isentro

pically to state 1 -isentropic

compression in a compressor: W41=h4-h1#

Carnot vapor cycle

not a suitable model for

power cycles


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Advantages over Carnot cycle

As feed pump term is small, work

ratio is high and cycle is insensitive

to process inefficiencies

The feed pump is small, as ithandles liquid

The feed pump is easy to control,

e.g., it would not be easy to stop

condensation The specific steam consumption is

less

$

Rankine cycle (saturated)


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Rankine superheat cycle

Advantages over Rankine cycle

(saturated)

Maximum temperature can be taken to

metallurgical limit (about 580oC) so raising

the average temperature at which heat issupplied and increasing efficiency

Net work is much grater, so dropping

specific steam consumption

Steam is dryer at 4, thus making less likely

that low pressure turbine blading would be

eroded


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Rankine superheat cycle

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 heatrejection in a condenser


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Rankine superheat cycle analysis

Thermal efficiency

rbw

=

wpump

wturbine

rw =

wnet

wturbine

Work ratio

Back work ratio

Specific steam consumption(kg/kWh)

SSC =3600

wnet

Heat rate (kJ/kWh)

HR =3600

!


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How to increase the efficiency

of the Rankine cycleBasic idea:

Increase the average

temperature at which heat

is transferred to theworking fluid in the boiler

or

Decrease the average


is rejected from theworking fluid in the

condenser

(


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Lowering condenser pressure

Decrease average


is rejected

Increase in power output

Side effect: moisture

content of the steam at the

final stages of the turbine

increases


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Superheating to higher temperatures

Increase averagetemperature at which heat

is supplied


but also increase in heatsupplied

Moisture content of the

steam at the final stages of

the turbine decreases Metallurgical limitation:

currently at 620oC


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Increasing boiler pressure

Increase average


is supplied


Side effect: moisture

content of the steam at the

final stages of the turbine

increases

moisture content of the steam at the final stages

of the turbine can be corrected by reheating


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Advantages over superheatedRankine cycle

Higher dryness fraction at 6

Specific steam consumption is less

Reheat temperatures very close or

equal to turbine inlet temperature.

Optimum reheat pressure about

one-fourth of the maximum cycle

pressure

Reheating economically viable for

units > 90MW

Rankine reheat cycle


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Rankine reheat cycle analysisHeat input

Work output


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Example A

Calculate the heat and work transfers, cycle

efficiency, work ratio and steam consumption of

a Carnot cycle using steam between pressures of

15 bar and 0.5 bar abs.

#$


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Example B

Calculate the cycle efficiency, work ratio and

steam consumption of a Rankine (saturated)

cycle using steam between pressures of 15 bar

and 0.5 bar abs.

#%


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Example C


steam consumption of a Rankine superheat

cycle using steam between pressures of 15 bar

and 0.5 bar abs and a superheat temperature of

300oC.

#&


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Example D


steam consumption of a Rankine reheat cycle

using steam between pressures of 15 bar and 0.5

bar abs, a superheat temperature of 300oC and a

reheat temperature of 300oC with an

intermediate pressure of 3.5bar.

#'


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Process inefficiencies Actual vapor power cycle differs from the ideal Rankine

cycle due to irreversibilities in various components such

as:

- fluid friction

- heat loss to the surroundings

Isentropic efficiencies:


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Example 10.2A steam power plant operates on the cycle shown in the figure. If

the isentropic efficiency of the turbine is 87% and the isentropic

efficiency of the pump is 85% determine:

(a) The thermal efficiency of the cycle

(b) The net power output of the plant for a mass flow rate of 15kg/s

#)


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Regenerative Rankine cycle

Advantages The first part of the heat-addition process in

the boiler takes place at relatively low

temperatures

cycle efficiency is reduced

Use of regeneration: steam is extracted from the turbine at various

points

heating of feedwater (although could have

produced more work by expanding further in

the turbine)

The device where the feedwater is heated by

regeneration is called a regenerator, or a

feedwater heater (FWH)7

5

6

1

3

4

2

s

T


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Types of FWHs

FWH: heat exchangerwhere heat is

transferred from the

steam to the feedwater

Two types of FWHs

Open FWHs: mixing the

two fluid streams

Closed FHWs: without

mixing of the two fluid

streams

Pump I

TurbineBoiler

CondenserPump II

5

2

76

OpenFWH

4

1

3

y 1 y

Boiler

Condenser

5

Pump II

Turbine

1

9

2

8Mixingchamber Closed

FWH

6

34

7

Pump I


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Analysis of open FWHs

Open FWH (or direct-contact FWH)

a mixing chamber

steam extracted from the

turbine mixes with thefeedwater exiting the

pump

ideally, the mixture leaves

the heater as a saturatedliquid at the heater

pressure

Pump I

TurbineBoiler

CondenserPump II

5

2

76

OpenFWH

4

1

3

y 1 y

7

5

6

1

3

4

2

s

T


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Closed feedwater heaters

Closed FWH Heat transfer from extracted

steam to feedwater without any

mixing of the two streams

the two streams can be at

different pressures

closed FWHs do not require a

separate pump for each heater

Most steam power plants use acombination of open and closed

feedwater heaters

Boiler

Condenser

5

Pump II

Turbine

1

9

2

8Mixingchamber Closed

FWH

6

34

7

Pump I

s

T

6

81

2

7

3

45

9


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

Regenerative Rankine cycle


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Modern regenerative cycle

$%


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Problem 10.44

Turbine bleed steam enters an open feedwater

heater of a regenerative Rankine cycle at

200kPa and 150oC while the cold feedwater

enters at 40oC. Determine the ratio of the bleed

steam mass flow rate to the inlet feedwater mass

flow rate required to heat the feedwater at

100oC.

$&


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Problem 10.46A steam power plant operates on an ideal regenerativeRankine cycle with two open feedwater heaters. Steam

enters the turbine at 10MPa and 600C and exhausts to

the condenser at 5kPa. Steam is extracted from the

turbine at 0.6MPa and 0.2MPa. Water leaves bothfeedwater heaters as a saturated liquid. The mass flow

rate of steam through the boiler is 22kg/s. Show the

cycle on a T-sdiagram, and determine

(a)

the net power output of the power plant

(b) the thermal efficiency of the cycle

$'


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Second-law analysis of vapor cycles

Exergy destruction for a steady-flow system, one-inlet, one-exit

Exergy destruction of a cycle

For a cycle with heat transfer only with a source and a sink

Stream exergy

Second-law analysis reveals where the largest irreversibilities

occur and where to start improvements


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Example 10.7

Consider a steam power plant operating on a simpleideal Rankine cycle. Steam enters the turbine at 3MPa

and 350oC and is condensed in the condenser at a

pressure of 75kPa. Heat is supplied to the steam in a

furnace maintained at 800K and waste heat is rejectedto the surroundings at 300K. Determine:

(a) the exergy destruction associated wit each of the

four processes and the whole cycle

(b) the second-law efficiency of this cycle

$)


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air

inlet

combustion

chamber

turbine

compressor

exhaust

generator

fuel

~

generator

~

heat

recovery

steam

generator

steamturbine

condenser

pump

steam

feed water

Combined cycle (Brayton - Rankine cycle)

Max efficiency ~ 58%-60%

(above 300MWe)

Widely used

$*


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Combined cycle (Brayton - Rankine cycle)


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combinedCycle[1].swf

Combine cycle simulation

%!


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Combine cycle advantages

High thermal efficiency

Low emissions

Low capital costs

Short construction times

Less space requirements

Flexibility in plant size Fast start-up

%#


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Example 10.8Consider the combined cycle shown in

the Figure. The topping cycle is a gas-turbine cycle that has a pressure ratio of

8. Air enters the compressor at 300K

and the turbine at 1300K. The isentropic

efficiency of the compressor is 80% and

that of the gas turbine 85%. The

bottoming cycle is a simple ideal

Rankine cycle operating between the

pressure limits of 7MPa and 5kPa.

Steam is heated in a heat exchanger by

the exhaust gases to a temperature of

500o

C. The exhaust gases leave the heatexchanger at 450K. Determine the ratio

of the mass flow rates of the steam and

the combustion gases.%$


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Problem 10.82Consider a combined cycle plant that has a net power output

of 280MW. The pressure ratio of the gas turbine cycle is 11.Air enters the compressor at 300K and the turbine at 1100K.The combustion gases leaving the gas turbine are used to heatthe steam at 5MPa to 350oC in a heat exchanger. Thecombustion gases leave the heat exchanger at 420K. An open

feedwater heater incorporated with the stean cycle operatesat a pressure of 0.8MPa. The condenser pressure is 10kPa.Assuming isentropic efficiencies of 100% for the pump, 82%for the compressor, and 86% for the gas and steam turbinesdetermine:

(a)

the mass flow rate ratio of air to steam(b) the required rate heat input in the combustion chamber

(c) the thermal efficiency of the combined cycle

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10 MCE341 VaporCycles.pptx

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