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

Diagram of Large Power Plant Turbine

Typical Modern Power Plant Turbine

HP Turbine Rotor

LP Turbine Rotor

LP Turbine Rotor

Condenser

Block Diagram of A Large Steam Turbine Reheat Steam

HP

Main Steam

Steam for Reheating

IP

IP

LPLP

CFWH 6 CFWH 5

OFWH 4

CFWH 2

CFWH 1

CFWH 3

Thermodynamic Analysis of A Power Plant

THERMODYNAMIC CYCLE OPTIMIZATION

Effect of Higher Steam Conditions on Unit Performance

• As the first step in the optimization of cycle steam conditions, the potential cycle efficiency gain from elevating steam pressures and temperatures needs to be considered.

• Starting with the traditional 165 bar/5380C single-reheat cycle, dramatic improvements in power plant performance can be achieved by raising inlet steam conditions to levels up to 310 bar and temperatures 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

temperatures exceed 566 C, as “ultrasupercritical”.

• 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) will also 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.

Analysis of Regeneration through CFWH

81 hhmQ SGin

Define y as fraction of mass extraction:

SC

extraction

m

my

SGm

SGmy

SGm

ymSG 1 47431 hhyhhymQ SGout

3221 1 hhyhhmW SGturbine

45 hhmW pump

Energy Balance for CFWH

5SG & hm

2& hymSG

8& hmSG

6258 hhymhmhm SGSGSG

5862 hhyhh

62

58

hh

hhy

7& hymSG

6& hymSG

HP Closed Feed Water Heater

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

• 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.

HP Closed Feed Water Heater

Desuperheater

Condensing Shell Drain Cooler

HP Turbine

TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi

Desuperheater

Condensing Shell Drain Cooler

TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi

Tube length

Tf

LP Closed Feed Water Heater

•

LP Closed Feed Water Heater

Condensing Shell Drain Cooler

LP Turbine

TRAP

Tbi, pbi, Tbsi

Tfi+1Tfi

• 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 over horizontally cut baffles.

Condensing Shell

Drain Cooler

TRAP

Tbi, pbi, Tbsi

Tfin

Tfout

Tube length

Tf

Work done by Bleed Steam

1h

2h

5h8h

Work done by bleed (extracted) steam: 21 hhywbleed

2162

58 hhhh

hhwbleed

6h

Closed Feed Water Heaters (Throttled Condensate)

Analysis of Regeneration through Two CFWH

s

121 hhmQ SGin

Define y as fraction of mass extraction:

SG

b

SG

b

m

my

m

my

2,

2

1,

1 &

54215821 1 hhyyhhyymQ SGout

432132121 11 hhyyhhyhhmW SGturbine

56 hhmW pump

Energy Balance for LP-CFWH

3211168219 hymhymhmhyymhm

6& hm

32 & hym

9& hm

821 & hyym

721 & hyym

111 & hym

83

1181

83

692 hh

hhy

hh

hhy

Energy Balance for HP-CFWH

9& hm

21 & hym

12& hm

111 & hym

101 & hym

1021912 hhymhmhm

9121102 hhyhh

102

9121 hh

hhy

Work done by Bleed Steam

21102

9122111, hh

hh

hhhhymwbleed

3183

1181

83

693122, hh

hh

hhy

hh

hhhhymwbleed

3183

118

102

912

83

693122, hh

hh

hh

hh

hh

hh

hhhhymwbleed

3183

118

102

912

83

6921

102

912, hh

hh

hh

hh

hh

hh

hhhh

hh

hhw totbleed

3183

118

102

912

83

6921

102

912, hh

hh

hh

hh

hh

hh

hhhh

hh

hhw totbleed

s

21, unitextHPbleed

wastebleed

HPbleed

HPfeed

LPbleed

LPfeedunitext

HPbleed

HPfeedtotbleed w

h

h

h

h

h

hw

h

hw

Thermodynamic Analysis of A Power Plant

Train of Shell & Tube HXs.

6

5

4

3

21

DCGSC

6 5 4 3 2 1

DC

GSC

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.

• 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 the

temperature of the solution rises and approaches saturation temperature.

• A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater.

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

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

• 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 20 – 30 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 45 – 60 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.

Principle of Operation of A Dearator