e. Steam Power Plant - Lecture

7/29/2019 e. Steam Power Plant - Lecture

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E.STEAM POWER PLANT - LECTURE

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1. Basic Elements of Plant Design1.1Steam Generator is a combination of apparatus for producing, furnishing, or recovering heat, together with

apparatus for transferring to a working fluid the heat thus made available. It indicates the furnace, boiler

waterwalls, water floor, water screen, superheater, reheater, economizer, air preheater, and fuel-burnin

equipment. The term boiler has been used for such a long period of time that the two terms are used

interchangeably.1.2Steam Turbine is the most versatile prime mover capable of an almost endless variety of application. It is apractical power source when built in as small as 5 hp or as large as 100,000. It is relatively quiet and smooth in

operation.

1.3Condenser a heat exchanger where steam enters the top and the condensate is collected in the hot well at thebottom while cooling water flows through the tubes.

1.4Boiler Feed Pump or Feedwater Pumps its function is to increase the pressure existing on a liquid an incremensufficient to the required service.

2. Rankine CycleRankine cycle is the ideal steam power cycle. This ideal plant consist of a steam generator which receives

feedwater under pressure from a pump, a prime mover in which to obtain the working expansion, and a condenser

to reduce the exhaust steam to liquid, ready for pumping.

1-2 isentropic (or reversible adiabatic) expansion

2-3 isobaric (or reversible constant-pressure) heat rejection

3-4 isentropic (or reversible adiabatic) compression

4-5 isobaric (or reversible constant-pressure) heat addition

Turbine Work

( )21

hhmWt =


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Actual turbine work

( ) ( ) tt hhmhhmW 2121 ==

Heat rejected in condenser

( )32

hhmQR =

Actual heat rejected in condenser

( )32 hhmQR = Pump work

( )34

hhmWp =

( )343

ppmvWp

Actual pump work

( )

p

p

hhmW

34

=

( )

p

p

ppmvW

343

Head added to boiler

( )41 hhmQA =

Actual heat added to boiler

( )

b

A

hhmQ

41

=

where:

t= turbine efficiency

p = pump efficiency

b = boiler efficiency

Boiler efficiency is meant the measure of ability of a boiler or steam generator to transfer the heat given it by the

furnace to the water and steam.

Thermal Cycle Efficiency

For Rankine Cycle

( )

( )

( ) ( )

41

3421

31

21

hh

hhhh

Whh

Whh

Q

WWe

p

p

b

pt

cycle

=

=

=

For Rankine engine or turbine (combination with condenser)

31

21

hh

hheengine

=

For plant thermal efficiency

HVm

EP

fuelbypliedsupheat

outputpowerelectrical

ef

p ==

3. Methods used in increasing the thermal efficiency of a Rankine cyclea. For the same throttle pressure and condenser pressure, increase the throttle temperature.b. For the same throttle temperature and condenser pressure, increase the throttle pressure.c. For the same throttle temperature and pressure, decrease the condenser pressure.d. Using reheat cyclee. Using regenerative cyclef. Using reheat-regenerative cycle


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4. Reheat CycleReheat cycle- to increase turbine power, increase thermal efficiency

Turbine work

( ) ( )4321 hhmhhmWt += Heat added in the boiler

( )61

hhmQAb =

Heat added in the reheater

( )23

hhmQArh =

Pump work

( ) ( )56556

ppmvhhmWp =

Heat rejected in the condenser

( )54

hhmQR =

Thermal efficiency of reheat cycle

ArhAb

pt

A

ptcycle

QQWW

QWWe

+==

5. Regenerative CycleRegenerative cycle to improve the cycle efficiency, decrease turbine power, decrease heat addition.

Turbine work

( ) ( )( )32121

hhmmhhmWt +=


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Heat added in the boiler

( )71

hhmQA =

Pump work 1

( )( ) ( ) ( )45414511

ppvmmhhmmWp =

Pump work 2( ) ( )

676672ppmvhhmWp =

Heat rejected in the condenser

( )( )431

hhmmQR =

Heat balance in regenerative heater (feedwater heater or deaerator)

( )65121

mhhmmhm =+

Thermal efficiency of reheat cycle

A

ppt

A

ppt

cycleQ

WWW

Q

WWWe

2121+

=+

=

6. Reheat-Regenerative Cycle

7. Steam Generators (Boilers)Steam generators commonly referred to as boiler is an integrated assembly of several essential components the

function of which is to produce steam at a predetermined pressure and temperature.

8. Boiler Types8.1Classification according to the contents of the tubular heating surface.

8.1.1 Fire-tube boilersFire-tube boilers are those in which the products of combustion pass through the tubes and the water

lies around the outside of them.

a. Horizontal or vertical axesb. External or internal furnacesc. Fully cylindrical or partially cylindrical shells


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8.1.2 Water-tube boilersWater-tube boilers are those in which the water is inside the tubes while the products of combustion

surrounds the tubes.

Classification according to:

a. Shape of the tubes1.

Straight tube - have a parallel group of straight equal-length tubes, arranged in a uniformpattern and joined at either end to headers.

Classification of headers

a. Box headersb. Sectional headers

2. Bent-tube - are header less. The drum serve the same function as the headers.b. Drum position

1. Longitudinal

2. Cross

c. Method of Water Circulation

1. Forced

2. Natural

d. Number of Drums1. Drum and-a-half a long upper drum is paralleled by a shorted drum.

2. Two-Drum two parallel horizontal drums of equal length but not necessarily equal diameter

are set on one above the other and joined by multiple rows of bent tubes.

3. Three-Drum two upper drums and one lower drums are arranged so that one upper drum

carries the water level and the other, being lower, really acts as a header.

e. Service

1. Marine

2. Stationary

f. Capacity

g. Thermal Conditions

9. Parts of Steam Generator9.1Pressure parts

9.1.1 Boiler heating surface tubes with attached drums or shells for storage of water and steam.9.1.2 Superheated surface provides more heating surface through which the steam must pass after leaving

the boiler if a final superheated state is desired.

9.1.3 Economizer is a feedwater pre-heating device which utilizes steam mixed with the feedwater.9.2Enclosure or setting

9.2.1 Water walls water tubes installed in the furnace to protect furnace against high temperature.9.2.2 Furnace encloses the combustion equipment to utilize effectively the heat generated.

9.2.2.1 Factors to be considered in furnace designa. Air supplyb. Character of fuel usedc. Degree of pre-heatingd. Draft equipment available

9.2.2.2 Types of furnace wallsa. Air-cooled masonry wallsb. Partially water-cooled wallsc. Solid masonryd. Water-jacketed furnace


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9.2.3 Combustion equipmenta. Burner used in fire-tube boilers for firing liquid and gaseous fuels.b. Stoker used in water-tube boilers for firing solid fuels

9.2.4 Auxiliaries and accessoriesa. Air preheater a heat exchanger utilizing the heat of the flue gases to pre-heat the air needed for

combustion.b. Forced-draft fan forces air inside to support fuel combustionc. Induced-draft fan usually situated at the bottom of the chimney or smokestack, it is responsible in

extracting flue gases out.

d. Soot blower removes soot around steam pipes developed as a result of combustion, employs theuse of extracted steam from the main steam line.

e. Blowdown valve valve through which the impurities that settle in the mud drum are removed; alsocalled blow-off valve.

f. Breeching duct connecting boiler to chimney.g. Baffles direct the flow of the hot gases to effect efficient heat transfer between the hot gases and

the heated water.

h. Fusible plug a metal plug with a definite melting point through which the steam is released in caseof excessive temperature which is usually caused by low water level.

i. Safety valve a safety device which automatically releases the steam in case of over-pressure.10.Definitions from PSME Code 2008

Boiler or Steam Generator a closed vessel intended for use in heating water or for application of heat to generate

steam or other vapor to be used externally to itself.

Coal-Fired Boiler used stoketed water temperature coal or pulverized coal for water-tube.

Condemned Boiler Unfired Pressure Vessel a boiler or unfired pressure vessel that has been inspected an

declared unsafe to operate or disqualified, stamped and marked indicating its rejection by qualified inspectingauthority.

Existing Installations any boiler or unfired pressure vessel constructed, installed, placed in operation but subject to

periodic inspection.

External Inspection an inspection made on the external parts, accessories and/or component even when a boile

or unfired pressure vessel is in operation.

Fire Tube Boiler a boiler where heat is applied inside the tube.

Fusion Welding a process of welding metals in a molten and vaporous state, without the application of mechanica

pressure or blows.

Gas-Fired Boiler uses natural gas or liquefied petroleum gas (LPG) for heating boiler, fire tube or water-tube.

Heat-Recovery Steam Generator unfired pressure vessel that uses flue gas heat.

Internal Inspection an inspection made when a boiler or unfired pressure vessel is shut-down and handholes

manholes, or other inspection openings are opened or removed for inspection of the interior.


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Locomotive Boiler a boiler mounted on a self-propelled track locomotive and used to furnish motivating power fo

traveling on rails.

Low Pressure Heating Boiler a boiler operated at a pressure not exceeding 1.055 kg/cm2 gage steam wate

temperature not exceeding 121 C.

Medium Pressure Heating Boiler a boiler operated at a pressure not exceeding 103.5 MPa gage steam, or wate

temperature not exceeding 130 C.

Miniature Boiler as used in this Code herein mean any boiler which does not exceed any of the following limits

405 mm inside diameter, 1065 mm overall length of outside of heads at center, 1.85 m2

of water heating surface

7.03 kg/cm2

maximum allowable working pressure.

New Boiler or Unfired Pressure Vessel Installation include all boilers and unfired pressure vessels constructed

installed, placed in operation or constructed for.

Oil-fired Boiler uses Bunker C as fuel for heating boiler and power boiler.

Portable Boiler an internally fired boiler which is self-contained and primarily intended for temporary location and

the construction and usage is obviously portable.

Power Boiler a closed vessel in which steam or other vapor (to be used externally to itself) is generated at a

pressure of more than 1.055 kg/cm2 gage by the direct application of heat.

ASME Boiler Construction Code the term, ASME Boiler Construction Code of the American Society of Mechanica

Engineers with amendments and interpretations thereto made and approved by the Council of the Society.

Reinstalled Boiler or Unfired Pressure Vessel a boiler or unfired pressure vessel removed from its original settinand re-erected at the same location or erected at a location without change of ownership.

Second Hand Boiler or Unfired Pressure Vessel as used herein shall mean a boiler or unfired pressure vessel o

which both the location and ownership have been changed after primary use.

Unfired Pressure Vessel a vessel in which pressure is obtained from an external source, or from an indirect

application of heat.

Waste-Heat Boiler unfired pressure vessel that uses flue gas heat from waste incinerator.

Waste Tube Boiler a boiler where heat is applied outside the tube.


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11.Performance of Boilers

11.1 Factor of Evaporation, FEfg

fws

h

hhFE

=

where:

hfg = latent heat of vaporization or evaporation at standard atmospheric conditions.

hfg = 970.3 Btu/lb or

hfg = 2257 Btu/lb or

hfg = 539 kcal/kg

11.2 Equivalent Evaporation, EEFEmEE s=

where:

ms = amount of steam generated.

11.3 Equivalent Specific Evaporation, ESEff

s

m

EEFE

m

mESE ==

where:

mf= amount of fuel burned in the furnace.

11.4 ASME Evaporation unit,ASME EUfwss hhmEUASME =

11.5 Rated Boiler Horsepower (Rated Bo Hp)Rated Bo Hp = Total Heating Surface / kwhere:

k = 12 sq ft = 1.1 sq m for fire-tube boilers

k = 10 sq ft = 0.91 sq m for water-tube boilers

Also Package Fire-Tube Boiler have a heating surface of 5 sq ft per boiler horsepower.


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11.6 Developed Boiler Horsepower (Dev Bo Hp)c

EUASME

c

hhmHpBoDev

fwss=

=

where:

c = 33,475 Btu/hr = 35,316 kJ/hr = 8,433 kcal/hr

11.7 Percent Rating Developed (% Rating Dev)100=

HpBoRated

HpBoDevDevRating%

11.8 Over-all Boiler Efficiency or Steam Generator Efficiency, eo.( )

HHVm

hhmhhmhhme

f

fwboborirorsfwss

o

++=

where:

mrs = amount of steam reheated

hro = enthalpy of steam leaving reheater

hri= enthalpy of steam entering reheater

mbo = amount of water blowdown at boiler pressure

hbo = enthalpy of saturated liquid at boiler pressure

if there is no reheater and no boiler blowdown.

HHVm

hhme

f

fwss

o

=

11.9 Boiler and Furnace Efficiency, ebfrrf

fwss

bf

HVmHHVm

hhme

=

where:

mf= amount of ash refired

HVr= heating value of ash

11.10 Net Efficiency of Steam Generating Unit, enet( )

HHVm

hhmme

f

fwsauxs

net

=

where:

maux= amount of steam used for SGU auxiliaries.

11.11 Gross Station (Power Plant) Heat Rate, GSHR- Defined as the amount of heat required per unit power developed .

outputworkGross

fuelbypliedsupheatGrossGSHR =

11.12 Net Station (Power Plant) Heat Rate, NSHR( ) ( )sauxiliariebyusedhrkWgeneratedhrkW

HHVmfuelbypliedsupHeatNSHR

f

=

,


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11.13 Over-all (Gross) Station Efficiency, ofuelbypliedsupHeat

alsmintergeneratoratoutputkWo

=

11.14 Grate Efficiency, egrHHVm

HVmef

rcgr =1

where:

mc = amount of carbon in refuse or ash

HVc = heating value of combustible in refuse or ash

12.Steam TurbinesThe operation of the steam turbine generator involves the expansion of steam through numerous stages in the

turbine, causing the turbine rotor to turn the generator rotor. The generator rotor is magnetized, and its rotation

generates the electrical power in the generator stator.

12.1 Principal Partsa. Rotor is the main moving element of a turbine.b. Casing is the principal stationary element, often called the cylinder. It surrounds the rotor and

holds, internally, any nozzles, blades, and diaphragms that may be necessary to control the path and

physical state of the expanding steam.

c. Bearings this the main bearings of a single-cylinder turbine which are two in number and areplaced outboard of the shaft seal.

d. Shaft seals to prevent outflow at the high-pressure end and air inflow at the vacuum end.e. Steam control regulate the flow of steam through a stationary turbine to produce constant

rotative speed in the presence of variable power demand.

f. Oil system is required for lubricating the bearings.12.2 Classification of Steam Turbine

12.2.1 Types of Bladesa. Impulse Stages - consists of a stationary nozzle with rotating buckets or blades. The steamexpands through the nozzle, increasing in velocity as a result of the decrease in pressure

The steam then strikes the rotating buckets and performs work on the rotating buckets

which in turn decreases the steam velocity.

1. Velocity compound stage involves a stationary nozzle followed by several rotating andstationary buckets. The nozzle has a large pressure drop with a resulting increase in

velocity. The velocity compound stage is also called a Curtis stage.

2. Pressure compound stages involve several sets of nozzles with small pressure dropsthrough each set of nozzles and complete velocity dissipation in each row of rotating

buckets. The pressure compound stages are also called Rateau impulse stages.

b. Reaction Stages are composed of one stationary row of blades and one rotating row ofblades with a pressure drop occurring in each stationary and rotating row.

12.2.2 Cylinder Arrangementa. Single cylinder

- With all rotating blades attached to one shaft and the steam flow all in one direction.b. Double flow units

- Single cylinder units with steam entering in the center and flowing in two equaquantities, but in opposite directions along the shaft.

c. Tandem-compound units


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d. Cross-compound units- Differ from tandem-compound units only in that the high- and low-pressure ends are

not on the same shaft.

e. Steeple- or vertical-compound units12.2.3 Back Pressure12.2.4

Initial Temperature and PressureHigh Pressure 1800 to 2400 psig range.

Supercritical Pressure Above 3206 psig.

Low Pressure 200 to 400 psig range.

High Temperature Inlet temperature above 900 F.

12.2.5 ReheatReheat turbine when steam is extracted from the turbine and its temperature increased

(usually in the steam generator) before being returned to the turbine.

12.2.6 Other Methodsa. Single-stage or multistage unitsb. Mixed-pressure unitsc. High or low speed turbinesd. Nonextraction or extraction turbinese. Uses stationary, marine, or mechanical-drive turbines.

13.Power RatingMechanical drive turbines are rated in horsepower; turbine-generator units, in kilowatts.

Internal power is the product of torque and rotor speed.

Nominal rating is a declared power capacity expected to be the maximum load.

Capability is the manufacturers guaranteed maximum continuous output for a clean turbine, operating unde

specific throttle and exhaust conditions, with full extraction at any openings, if provided.

Overload capacity is the difference between capability and rating.

14.Willans LineWillans line is a straighlt line which shows the relation between the steam consumption in lb per hr and the load

in kW of a steam turbine generator unit.


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Note that the Willans line for throttle governing and for an infinite number of governor valves is a straight line

and will conform to the general equation

y= a + bx

Where y= throttle steam flow, lb per hr

a = no-load steam consumption, lb per hrb = slope of the curve, lb per kwhr

x= load, kw

15.Performance of Steam Turbines

15.1 Ideal Turbine Work( )

21hhmW st =

where:

h1= enthalpy of steam entering

h2 = enthalpy of steam after ideal (isentropic) expansion

15.2 Actual Turbine Work( ) ( ) stsast hhmhhmW 2121 ==

where:

h2a= enthalpy of steam after actual expansion

hst= stage efficiency

15.3 Turbine Power Output15.4

( ) ( ) mststst hhmhhmW 2121 ==

where:t = turbine efficiency = stx m

m = mechanical efficiency

15.4 Electrical or Generator Efficiency

outputTurbine

outputGeneratore =

Generator output = Turbine Output x e= ms(h1 h2)te


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15.5 Thermal Efficiency15.5.1 Brake thermal efficiency

( )21 fs

bhhm

outputTurbinee

=

15.5.2 Combined or overall thermal efficiency( )

21 fs

chhm

outputGeneratore

=

15.5.3 Ideal Rankine thermal efficiency21

21

f

rhh

hhe

=

15.6 Engine Efficiency of Turbine15.6.1 Brake engine efficiency

( )21

hhm

powerBrake

s

eb

=

15.6.2 Combined or Overall engine efficiency( )

21hhm

outputGenerator

s

ec

=

16.Steam CondensersSteam condenser a heat exchanger where steam enters at the top and the condensate is collected in the hot well

at the bottom while cooling water flows through the tubes.

17.Functions of Steam Condensera. To convert steam to liquid before entering the steam-generating unit.b. To create a vacuum at turbine exhaust thereby increasing turbine power.

18.Classification of steam condensersa. Surface condenser where steam and cooling water are not allowed to mix; commonly shell and tube design.


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b. Direct-contact condenser (mixing) also called jet condensers , where steam and cooling water are allowed tomix.

19.Heat Balance in Condenser

( ) Ehhmttcm fsspw = 12

where:

cp = 4.187 kJ/kg-C or 1.0 Btu/lb-F

E= heat extraction factor

20.Vacuum Efficiency, hvacsatatm

condatmvac

pp

pp

=

where:

patm atmospheric pressure

pcond absolute condenser pressurepsat saturation pressure

21.Feedwater HeaterTerminal difference is the difference between the saturation temperature of the steam in the heater and th

temperature of the water leaving the heater.

Subcooling the reduction below saturation temperature.


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Open heaters or Contact heaters are feedwater heaters that function by mixing steam with the feedwater.

Deaerator a contact heater especially designed to remove the noncondensable gases.

22.Feedwater Pumps and Boiler Feed PumpBoiler feed pump whose function is to increase the pressure existing on a liquid an increment sufficient to therequired service.

( )12

hhmWorkPump =

( )121 ppmvWorkPump mgHWorkPump =

where:

m = mass flow rate, kg/s

v1 = specific volume, m3/kg

p1 = entrance pressure, kPa

p2= exit pressure, kPa

H = head, m

Pump input power (Brake power of the pump)EfficiencyPump

WorkPump=

23.Steam EnginesSteam engines where steam is admitted to the engine cylinder at throttle pressure during the first part of the

working stroke, then cut off by closure of the steam valve. The steam so trapped in the cylinder expands

adiabatically to the release pressure, then is exhausted from the cylinder during part of the return stroke. Steam

engines are double-acting and the process is isentropic.


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23.1 Ideal p-V Diagram

23.2 Piston Volume DisplacementPiston rod neglected:

LNDVD2

4

2

=

Piston rod considered:

( )LNdDLNDVD 22244

+

=

23.3 Indicated PowerDmiVpIP =

pmi= indicated mean effective pressure

ScaleSpringDiagramofLength

DiagramofAreapmi =

23.4 Brake PowerTNBP 2=

where:

T= torque, kN-m

N = speed, rev/s

Using brake mean effective pressure,pmb

DmbVpBP =

23.5 Friction PowerFriction Power = Indicated Power Brake Power

FP = IP BP

23.6 Mechanical EfficiencyPowerIndicated

PowerBrakem =


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23.7 Thermal Efficiencya. Indicated thermal efficiency

( )21 fs

ihhm

PowerIndicatede

=

b. Brake thermal efficiency( )

21 fs

bhhm

PowerBrakee

=

23.8 Engine Efficiencya. Indicated engine efficiency

( )21

hhm

PowerIndicated

s

i

=

b. Brake engine efficiency( )

21hhm

PowerBrake

s

b

=

23.9 Efficiency of Equivalent Rankine Cycle21

21

f

rhh

hhe

=

24.Combined Cycle Power PlantCombined gas turbine-steam cycle is employed to transfer heat carried by the flue gas in the gas turbine cycle to

the feedwater in the steam cycle; the heat exchanger performs the function of a boiler.

Schematic Diagram


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Gas Turbine Cycle:

Net Work of the Cycle,

( ) ( )[ ] ( ) ( )[ ]abdcpaabdcanet TTTTcmhhhhmW ==

Heat Added in the Combustion Chamber,( ) ( )bcpabcaA TTcmhhmQ ==

Heat Loss in the Heat Exchanger,

( ) ( )cdpacdaL TTcmhhmQ ==

Steam Cycle:

Net Work of the Cycle,

( ) ( )[ ]34321

ppvhhmW snet =

Heat Gained in the Heat Exchanger,

( ) ( )4141

hhmhhmQ sfwG ==

Thermal Efficiency of the Combined Cycle,

A

netSnetG

A

netk

Q

WW

Q

We

+==

Energy balance in the heat exchanger,

Heat lost by exhaust gases = heat gained by feedwater

( ) ( )41

hhmTTcm scdpa =

( )

41hh

TTcmm

cdpa

s

=

where:

ms = steam mass flow rate

ma= air mass flow rate

25.Binary Mercury-Steam Cycle Power PlantBinary mercury-steam cycle - is employed to transfer heat carried by the mercury in the mercury vapor cycle to the

feedwater in the steam cycle; the heat exchanger performs the function of a boiler.


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Schematic Diagram

Overall Turbine Work,( ) ( )

21hhmhhmWWW sbahgsthgtt +=+=

Overall Pump Work,

( ) ( )34

hhmhhmWWW scdhgsphgpp +=+=

( ) ( )343

ppvmppvmW scdchgp +=

Heat Added in the Mercury Boiler,

( )dahgA hhmQ =

Thermal Efficiency of Binary Cycle,

A

pt

A

netb

Q

WW

Q

We

==

Energy Balance in the Heat Exchanger,

Heat lost by the mercury heat gained by water

( ) ( )41

hhmhhm fwcbhg =

sfw mm =

Thus,

( )

cb

fw

hg hh

hhmm

=

41

where:

ms = steam mass flow rate

mfw= feedwater flow rate

mhg = mercury flow rate


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26.Cogeneration Steam Power PlantThe terms cogeneration and CHP are used interchangeably paper and are defined as the combined simultaneou

generation of heat and electrical energy with a common source of fuel. Common examples of cogeneration

applications include pulp and paper mills, steel mills, food and chemical processing plants, and District Heating (DH)

applications.

Schematic Diagram

- End -

e. Steam Power Plant - Lecture

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