closed feed water heaters

7/28/2019 Closed Feed Water Heaters

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A Train of Closed Feed Water Heaters

A Trade off between Irreversibility and Reliability !!!

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi


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Diagram of Large Power Plant Turbine


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Typical Modern Power Plant Turbine


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HP Turbine Rotor


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LP Turbine Rotor


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LP Turbine Rotor


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Condenser

Block Diagram of A Large Steam Turbine

Reheat Steam

HP

Main Steam

Steam for

Reheating

IPIP

LPLP

CFWH 6 CFWH 5

OFWH 4

CFWH 2

CFWH 1

CFWH 3


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Thermodynamic Analysis of A Power Plant


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34.520

0.057

M

7.135

KCAL/KG

34.700

735.8

683.2

15.87

61.067 424.534.700 3

09.4

200.0

639.314

HEAT RATE=1985.05 K CAL/KW

Radiation losses are Ignored

210.3

206.0

639.314

61.067

256.21

0.000M

247.0

6.0

K

0.0K

D

38.54

205.5

170.0

172.0

509.026

124.0

6.414

639.31495.766

T/HR CEL

162.1

160.7

205.5

639.314

6.0

K

0.0K

168.3

164.1

12

0.8

12

1.3

34.520

6.564

26.299

2.269

2.8K

195.5

740.70352.2

2.154

M

0.024

M

0.935

M

2.186

M

740.70

352.2

61.067

40.57

740.70

350.4

40.57

572.218

14.970 M

639.314

816.06

537.0

ABC

150.0

A

1.251

M

0.701

M

0.018

M

0.043

M

0.946

M

537.0

843.89

789.916.70789.9

423.0

572.156

36.52

CB C B

4.352 M

777.2 H

B

A

0.4361 619.864.846 M

D

C

123.8

95.0

95.0

76.2

76.3

58.8

509.026

92.4

509.026 92.2

26.299

43.183

72.7

72.6

63.693

63.693

58.8

3.7K

0.5616

16.883

2.8K

106.8

642.9

0.4143

20.510

619.8

76.

5

509.028

509.028

D

77.96

49.2

49.0

20.510

47.0

0.299M

46.8

99.9

99.9

0.299

46.3

46.7

46.446.1

509.028

D12.0

K

509.0280.1033

19.38748.8H

0382M

0.078

M

16.833

0.9069

26.299

2.389 683.2

195.8

310.0

735.8

642.9

107.1

310.0

735.8

506.53

7.135

C B C

0.382

0.078

577.3

P=210.061 MW

46.45441.114

0.1033

3.068 M

B

0.854 MD

G

B C

2

kg

cm

LAYOUT OF MODERN 210 MW COAL FIRED POWER PLANT

THERMODYNAMIC CYCLE OPTIMIZATION


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THERMODYNAMIC CYCLE OPTIMIZATION

Effect of Higher Steam Conditions on Unit Performance

As the first step in the optimization of cycle steam conditions, thepotential cycle efficiency gain from elevating steam pressures andtemperatures needs to be considered.

Starting with the traditional 165 bar/5380C single-reheat cycle,

dramatic improvements in power plant performance can be achievedby raising inlet steam conditions to levels up to 310 bar andtemperatures to levels in excess of 600 C.

It has become industry practice to refer to such steam conditions, and

in fact any supercritical conditions where the reheat steam temperaturesexceed 566 C, as ultrasupercritical.


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Heater Selection and Final Feedwater Temperature

In order to maximize the heat rate gain possible with ultrasupercritical

steam conditions, the feedwater heater arrangement also needs to be

optimized.

In general, the selection of higher steam conditions will result in additional

feedwater heaters and a economically optimal higher final feedwater

temperature.

In many cases the selection of a heater above the reheat point (HARP) willalso be warranted.

The use of a separate desuperheater ahead of the top heater for units with a

HARP can result in additional gains in unit performance.

Other cycle parameters such as reheater pressure drop, heater terminal

temperature differences, line pressure drops and drain cooler temperature

differences have a lesser impact on turbine design, but should also be

optimized as part of the overall power plant cost/performance trade-off

activity.


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Analysis of Regeneration through CFWH

81 hhmQ SGin

Definey as fraction of mass extraction:

SC

extraction

m

my

SGm

SGmy

SGm

ymSG 1

47431 hhyhhymQ SGout

3221

1 hhyhhmW SGturbine

45 hhmWpump


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Energy Balance for CFWH

5SG&hm

2&hymSG

8&hmSG

6258hhymhmhm SGSGSG

5862hhyhh

62

58

hh

hhy

7&hymSG

6&hymSG


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HP Closed Feed Water Heater


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DS

TTD

Feedwater heater with Drain cooler and Desuperheater

-TTD=Terminal

temperature difference

C=Condenser

DC=Drain cooler

DS=Desuperheater

Bled steam

T

L

DCC

Condensate

CDC

Feed Water in

DS

Bleed Steam

Feed Water out

5SG &hm

2&hymSG

8&hmSG

6&hymSG


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Desuperheating Zone - The integral desuperheating zone

envelopes the final or hotest feed water pass and is thermally

engineered to assure dry wall tube conditions with a minimum

zone pressure loss.

Dry wall conditions in this zone provide maximum heat recovery

per square foot of transfer surface by taking full advantage of

the available temperature differential between the superheated

steam and the feedwater.

Dry wall conditions also prevent flashing, which is detrimental to

proper desuperheating zone operation.

All desuperheating zones are analyzed to make sure they are

free of destructive vibration.


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HP Closed Feed Water Heater

Desuperheater

Condensing Shell Drain Cooler

HP Turbine

TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi


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Desuperheater


TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi

Tube length

Tf


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LP Closed Feed Water Heater


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LP Closed Feed Water Heater


LP Turbine

TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi


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Drain Subcooling Zone - When the heater drains temperature

is required to be lower than the heater saturation temperature, a

drain subcooling zone is employed.

The drain subcooling zone may be either integral or external,

and as a general rule, it is integral.

The integral drain subcooling zone perates as a heat exchanger

within a heat exchanger, since it is isolated from the condensing

zone by the drain subcooling zone end plate, shrouding, and

sealing plate.

This zone is designed with generous free area for condensate

entrance through the drains inlet to minimize friction losses

which would be detrimental to proper operation.

The condensate is subcooled in this zone, flowing up and overhorizontally cut baffles.


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Condensing Shell

Drain Cooler

TRAP

Tbi, pbi, Tbsi

Tfin

Tfout

Tube length

Tf


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Work done by Bleed Steam

1h

2h

5h

8h

Work done by bleed (extracted) steam: 21hhywbleed

216258 hhhh

hhwbleed

6h

Closed Feed Water Heaters (Throttled


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Closed Feed Water Heaters (ThrottledCondensate) 1

2

3 4

56

78

9

10

11

12


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Analysis of Regeneration through Two CFWH

T

s

1

2

3

45

67

8

9

10

11

12

121hhmQ SGin

Definey as fraction of mass extraction:

SG

b

SG

b

m

my

m

my

2,

2

1,

1&

54215821 1 hhyyhhyymSGout

432132121

11 hhyyhhyhhmSGturbine

56 hhmWpump

E B l f LP CFWH


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Energy Balance for LP-CFWH

3211168219 hymhymhmhyymhm

6&hm

32&hym

9

&hm

821

&hyym

721

&hyym

111&hym

83

1181

83

692

hhhhy

hhhhy

E B l f HP CFWH


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Energy Balance for HP-CFWH

9&hm

21&hym

12& hm

111&hym

101&hym

1021912hhymhmhm

9121102hhyhh

102

912

1hh

hhy


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Work done by Bleed Steam

21102912

2111, hhhh

hh

hhymwbleed

31

83

118

1

83

69

3122,hh

hh

hhy

hh

hhhhymwbleed

31

83

118

102

912

83

69

3122,hh

hh

hh

hh

hh

hh

hhhhymwbleed

31

83

118

102

912

83

69

21

102

912

,hh

hh

hh

hh

hh

hh

hhhh

hh

hhw totbleed


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31

83

118

102

912

83

69

21

102

912

,hh

hh

hh

hh

hh

hh

hhhh

hh

hhw totbleed

T

s

1

2

3

45

67

8

9

10

11

12

21, unitext

HPbleed

wastebleed

HPbleed

HPfeed

LPbleed

LPfeed

unitext

HPbleed

HPfeed

totbleed wh

h

h

h

h

hw

h

hw


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34/40

Thermodynamic Analysis of A Power Plant

Train of Shell & Tube HXs


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Train of Shell & Tube HXs.


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0.0

K

D0.0

K

6.0

K

6.0

K

C

Variation of Feedwater Temperature and Enthalpy along the

FeedWater Heater

0

50

100

150

200

250

300

Temperature(C)

0

200

400

600

800

1000

1200

Enthalpy(kJ/kg)

Temperature

Enthalpy 6

5

4

3

21

DCGSC

6 5 43 2 1

DC

GSC


37/40

The Mechanical Deaerator

The removal of dissolved gases from boiler feedwater is an

essential process in a steam system.

Carbon dioxide will dissolve in water, resulting in low pH

levels and the production of corrosive carbonic acid.

Low pH levels in feedwater causes severe acid attack

throughout the boiler system.

While dissolved gases and low pH levels in the feedwater

can be controlled or removed by the addition of chemicals.

It is more economical and thermally efficient to remove

these gases mechanically.

This mechanical process is known as deaeration and will

increase the life of a steam system dramatically.


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Deaeration is based on two scientific principles.

The first principle can be described by Henry's Law.

Henry's Law asserts that gas solubility in a solution decreases as

the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the

relationship between gas solubility and temperature.

Easily explained, gas solubility in a solution decreases as thetemperature of the solution rises and approaches saturationtemperature.

A deaerator utilizes both of these natural processes to removedissolved oxygen, carbon dioxide, and other non-condensablegases from boiler feedwater.

The feedwater is sprayed in thin films into a steam atmosphereallowing it to become quickly heated to saturation.

Spraying feedwater in thin films increases the surface area of theliquid in contact with the steam, which, in turn, provides morerapid oxygen removal and lower gas concentrations.


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This process reduces the solubility of all dissolved gases and

removes it from the feedwater.

The liberated gases are then vented from the deaerator.

Correct deaerator operation requires a vessel pressure of about

2030 kPa above atmospheric, and

a water temperature measured at the storage section of 50C

above the boiling point of water at the altitude of the

installation.

There should be an 4560 cm steam plume from the deaerator

vent, this contains the unwanted oxygen and carbon dioxide.

The following parameters should be continuously monitored to

ensure the correct operation of the deaerator.

Deaerator operating pressure.

Water temperature in the storage section.


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Principle of Operation of A Dearator

closed feed water heaters

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