工程热力学与传热学英文第6章动力装置循环

Thermodynamics---

Chapter

6 Gas

and

VaporPower

Cycles

---Chapter

6 Gas

and

Vapor

PowerCyclesContentsRankine

cycle:

The

ideal

cycle

for

vapor

powercyclesThe

ideal

reheat

Rankine

cycleThe

ideal

regenerative

Rankine

cycleGas

power

cyclesHeat

engine：Devices

that

convert

heat

into

work.Power

cycle：Vapor

power

cycle（蒸汽动力循环）Gas

power

cycle（气体动力循环）ContrastSubstance（工质）Device（设备）Fuel（燃料）Processes

and

cycles（热力过程和循环）水蒸气锅炉廉价燃料燃料燃烧后的燃气汽缸汽油，柴油，煤气方法和步骤：首先:把实际问题抽象为可逆理论循环；分析找出热效 率及提高该循环热效率的可能措施，以指导实际 循环的改善。然后:分析实际循环与理论循环的偏离程度，找出实际 损失的部位，大小，原因，及改进措施。分析动力循环的目的：在热力学基本定律的基础上分析动力循环能量转换的经济性，寻求提高经济性的方法和途径。6-1 Vapor

power

cycles（蒸汽动力装置循环）6-1-1

Rankine

cycle:

The

ideal

cycle

for

vapor

power

cycles（蒸汽动力装置的理想循环—朗肯循环）1.

Componentspump,

boiler,

turbine,

and

condenser.（水泵，锅炉，汽轮机和冷凝器等四个主要设备）134BoilerTurbineCondenserPumpElectricgenerator22.

ProcessesSuperheater6Coolingmediumstate

2

—

state

3saturated

liquid-vapor

mixture

—saturated

liquid（4）Isentropic

compression

in

a

pumpstate

3

—

state

4saturated

liquid

—

subcooled

liquidstate

4

—

state

5

—

state

6

—

state

1subcooled

liquid

—

saturated

liquid

—

saturated

vapor

—superheated

vaporIsentropic

expansion

in

a

turbinestate

1

—

state

2superheated

vapor

—

saturated

liquid-vapor

mixtureConstant

pressure

heat

rejection

in

a

condenser（1）Constant

pressure

heat

addition

in

a

boiler过热器134锅炉给水泵汽轮机发电机冷凝器26冷却水3.

P-v

and

T-s

diagramsvp06

14

532过热器134锅炉给水泵汽轮机发电机冷凝器26冷却水TS31456206-1-2 Energy

analysis

of

the

ideal

Rankine

cycle（朗肯循环的净功和热效率）w

s

,1-2

=h

1

-

h

2（2）Condenser：Steam

is

condensed

at

constant

pressureand

reject

heat

to

a

coolingmedium.q

2

=

h

2

-

h

3（3）Pump：Water

is

compressed

isentropically

to

the

operatingpressure

of

the

boiler.w

p,3-4

=

h

4

-

h

31.

Net

work

of

Rankine

cyclePer

unit

massof

steamTS31（1）Turbine：Superheated

vapor

expandsisentropically4and

produce

work5620Net

work：w

net

=w

s

,1-2-w

p,3-4=(h1

-h

2)-(h

4-h

3)Net

heat：q

net

=q

1-2

-

q

3-4=

(h1

-

h

4)

-(h

2-

h

3)312

11

2

11qqt

net

h

-

hw

q

-

q

h

-

h=

=h

=（4）boiler：Heat

is

transferred

to

the

water

at

constant

pressure.water

enters

the

boiler

as

a

compressed

liquid

andleaves

as

a

superheated

vaporq

1=h

1-

h

42.

The

thermal

efficiency

of

the

Rankin

cycleTS3145620h4=h3wp,3-4=h4-h3=06-1-3 How

can

we

increase

the

efficiencyof

the

Rankine

cycle？1

3th

-

h

h1

-

h

2

h

=11

3Tth

-

h=

h

1-

h

2

=

1

-

T2hh

1

=

f

(

p1

,

t

1

)h

2

=

f

(

p1

,

t

1

,

p

2

)h

3

=

f

(

p

2

)T12TDsHeat

addition

average

temperatureHeat

rejection

average

temperature放热平均温度吸热平均温度T1

high

,

av=

TT2

=

Tlow,avTS3145620p1p21.Superheating

the

steam

to

high

temperature(Increase

Thigh,av)（蒸汽初温T1的影响）Fixed

p1

,

p2

,

superheating

the

steam

to

higher temperature

T1—T1’

.Thigh,av

increase,

Tlow,avwithout

increasing,the

overall

effect

is

an

increase

in

thermal

efficiency.（3）The

quality

increases,

it

is

desirableto

the

turbine

operating

safety.（4）The

highest

temperature

allowedat

the

turbine

inlet

is

about620ºC,however,by

metallurgical

considerations.1’ST134562

2‘0p1p2121

31

2TTt=1-h

-hh

-hh

=av)2. Increasing

the

boiler

pressure

(Increase

Thigh,（蒸汽初压p1的影响）increases.

It

is

highly

undesirable

inturbines

becauseit

decreasestheturbine

efficiency

and

erodestheturbine

blades.The

value

of

quality

x:

0.86—0.88.2’1’5’6’p1’S134’45620content

of

steam

at

the

turbineexitT1p1p2Fixed

T1,

p2,

increasingthe

operatingpressure of

the

boiler

p1

—

p1’

.Thigh,av

increase,

Tlow,avwithout

increasing,the

overall

effect

is

an

increase

in

thermal

efficiency.TThe

quality

decreases,

the

moisture121

31

2TTt=1-h

-hh

-hh

=3. Lowering

the

condenser

pressure

(Lowers

Tlow,av)（乏汽压力p

的影响）2Fixed

T1,

p1

,

lowering

theoperating

pressure of

the

condenser

p2

—

p2’

.The

change

of

Thigh,av

is

very

small,

Tlow,av

decreases, the

overall

effect

is

an

increase

in

thermal

efficiency（3）Thereis

a

lower

limit

on

the

condenserpressure

that

can

beused.It

cannotbe

lower

than

the

saturationpressure

correspondingtothetemperature

of

the

cooling

medium.4’3’2’p2’ST

1

345

620T1p2p1（4）It

creates

the

possibility

of

air

leakageinto

the

condenser.

It

increases

themoisture

content

of

the

steam

atthefinal

stages

of

theturbine.121

31

2TTt=1-h

-hh

-hh

=The

thermal

efficiency

of

Rankine

cycle（在不同初，终参数下朗肯循环的热效率）When

P2=0.05bar,

t1=435ºCP1152535507590ηt33.735.937.038.539.940.6When

P1=35bar,

P2=0.05bart1300350400435450500ηt35.436.036.637.037.438.2When

P1=35bar,

t1=435ºCP20.040.050.10.20.51ηt37.537.035.133.129.626.9How

can

we

increase

the

efficiency

ofthe

Rankine

cycle?ConclusionChange

the

properties

of

steam（改变蒸汽参数）：Lowering

the

condenserpressureSuperheating

the

steam

to

high

temperatureIncreasing

the

boiler

pressureImproving

Rankine

cycle（改进朗肯循环）Reheat

Rankine

cycle（再热循环）Regenerative

Rankine

cycle（回热循环）Combined

thermal-electrical

power

cycles（热电联供循环等）6-1-4 The

ideal

reheat

Rankine

cycle（再热循环）ST1345602

2‘5’4’’

1’p1’

6ST134562’

20T1p1How

can

we

take

advantageof

the

increasedefficiencies

at

higher

boiler

pressures

withoutfacing

the

problem

of

excessive

moisture

atthefinal

stages

of

the

turbine?1’Question在初温不允许继续提高的情况下，为了提高初压，以提高循环热效率，且不使汽轮机排汽干度过低，在朗肯循环基础上引入蒸汽中间再过热方法。T435671

1’2

2‘0sT–s

Diagram1.

The

Ideal

Reheat

Rankine

Cycle（一次再热循环装置的系统图及T-s图）The

single

reheat

cycle（一次再热循环）给水泵锅炉过热器再热器高压汽轮机低压汽轮机134发电机冷凝器26冷却水1‘72.

T-s

diagram

of

Rankine

cycle

and

single

reheat

cycle0ssingle

reheat

cycle4（朗肯循环和一次再热循环T–s图比较）T

T

1

1

1’35

6

5

6

742

3

2

2‘0sRankinecycle3.

Thermal

efficiency

and

Intermediate

pressureTotal

heat

addition:q

1

=(

h

1-

h

4

)

+(

h

1¢-

h

7

)Heat

rejection:q

2

=

h

2¢

-

h

3Thermal

efficiency：t(

h

1

-

h

4

)

+

(

h

1¢-

h

7

)=

(

h

1

-

h

4

)

+

(

h

1¢-

h

7

)

-

(

h

2¢-

h

3

)h（再热循环的热效率及中间压力）（1）Thermal

efficiency

of

single

reheat

cycle（一次再热循环的热效率）T42356

71

1’2‘0s再热循环T–s图The

optimum

reheat

pressure

can

bothincrease

quality

and

thermal

efficiency.（中间最佳压力应能够同时提高排汽干度和循环热效率）（2）Intermediate

pressure（中间再热压力）T42356

71

1’2‘0s再热循环T–s图How

Intermediate

pressure

effect

on

the

cycle?（分析再热压力过高或过低对循环的影响）The

optimum

reheat

pressure

is

aboutone-fourth

of

the

maximum

cycle

pressure.（pm一般为初压p1的20%—30%，通常一次再热可使热效率提高2%—3.5%）4. Analysis

of

reheat

Rankine

cycle（再热循环热经济性分析）1. For

reheat

Rankine

cycle:

quality

of

turbine

exit

increase,it

is

desirable

to

the

turbine

operatingsafety.At

the

same

conditions, wrehe>

wrank

,

ηrehe>ηrank

.（在相同参数范围内，再热循环的有用功和热效率均高于

朗肯循环的结果，即再热循环的热经济性高于朗肯循环）Reheat

stages（再热级数）If

pressure

p1: p1

<10MPa，noneedreheat

；If

pressure

p1:

13MPa

<p1<

supercritical-pressure, single

reheat

cycle.If

pressure

p1:

p1

>

supercritical-pressure

(>22.09MPa), double

reheat

cycle.T4356

71

1’2

2‘0s再热循环T–s图6-1-5 The

ideal

regenerative

Rankine

cycle（回热循环）1.Purpose（回热循环的目的）Decrease

the

heat

rejection

tocooling

medium,

increase

thetemperature

of

feedwater,

increasethe

heat

addition

average

temperature,therefore

increase

thermal

efficiency.（减少冷源损失，提高给水温度，提高朗肯循环的吸热平均温度，从而提高热效率）回热：将在汽轮机中做了部分功的蒸汽从汽轮机中抽出来，用以加热进入锅炉前的给水，这样不仅避免抽汽的冷源损失，锅炉的给水温度也同时提高。TS15620p11T2T4p23Ds2.

The

ideal

regenerative

Rankine

cycle（回热装置系统图及T-s图）sT1810942735

１kg

6αkg0T-sdiagram(1-α)

kg冷凝器1

kg17给水泵凝结水泵锅炉过热器10

3

冷却水6a

kg(1-a)kg

机29

4回热加热器Single

–stage

regenerative

cycle发电3.

Quantity

of

steam

extracted

and

thermal

efficiency（抽汽量及热效率）Quantity

of

steam

extractedNeglect

the

heat

loss

in

theregeneratorHeat

rejection

of

a

Kg

steamextracted=

heat

additionof

（1-

a）kg

water（aKg抽汽放出的热量=(1-a)kg凝结水吸收的热量）7αkg1

kg9(1-α)

kg4a

(h

7

-h

9)

=

(1-a

)(

h

9

-h

4

)Thermal

equilibrium

equation：a

=

h

9

-

h4h

7

-

h

4Quantity

of

steam

extracted（抽汽量）a

=

h

9

-

h

3h

7

-

h

3h3

=

h4sT1810942735

１kg

6αkg0T-s

图(1-α)

kgsT185104273１kg

6αkg9(1-α)

kg0T-sdiagram（2）Thermal

efficiency（热效率）Heat

addition（吸收热量）：q

1

=

h1

-

h10Heat

rejection（放出热量）：q

2

=

(1

-

a

)

(

h

2

-

h3

)1

91qth

-h=1-

(1-a

)(

h

2

-h

3

)h

=1-

q

2Thermalefficiency（热效率）：Work

done

in

the

turbine：w

=

a

(h1

-

h7

)

+

(1

-

a

)

(h1

-

h2

)4. Analysis

of

regenerative

Rankine

cycle（回热循环的分析）（1）At

the

same

conditions,

ηrege>

ηrank.（回热循环热效率高于同参数朗肯循环热效率）Work

donein

the

turbine

decrease

for

per

unit

mass

steam at

the

condition

of

regenerative.（采用回热后，每kg蒸汽在汽轮机中作功量减少）The

optimum

intermediate

pressure（最佳的抽汽压力：最有利的给水温度为锅炉压力下饱和温度的0.65-0.75倍）Theoptimum

number

of

feedwater

heaters

is

determined from

economical

considerations.Small

plants:

1-3

feedwater

heatersMiddle

and

large

plants:

4-8

feedwater

heaterss/p>

１kg

6αkgT-s

图(1-α)

kg1. Consider

a

steam

power

plant

operating

on

theideal

Rankine

cycle.

The

steam

enters

the

turbineat

3

MPa

and

350ºC

and

is

condensed

in

thecondenser

at

a

pressure

of

10kPa.

Determine(a)

the

thermal

efficiency

of

this

power

plant.

the

thermal

efficiency

if

steamis

superheated

to

600ºCinstead

of

350ºC

the

thermal

efficiency

if

theboiler

pressure

is

raised

to15MPa

while

the

turbine

inlettemperature

is

m