A Train of Closed Feed Water Heaters
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
A Trade off between Irreversibility and Reliability !!!
Diagram of Large Power Plant Turbine
Typical Modern Power Plant Turbine
HP Turbine Rotor
LP Turbine Rotor
LP Turbine Rotor
Block Diagram of A Large Steam Turbine
Main Steam
Reheat Steam
HP
IP
IP
LP
Steam for
Reheating
OFWH 4
CFWH 6
CFWH 5
LP
CFWH 3
CFWH 2
CFWH 1
Condenser
Thermodynamic Analysis of A Power Plant
Radiation losses are Ignored
HEAT RATE=1985.05 K CAL/KW
121.3
6.414
162.1
639.314
160.7
kg
cm 2
T/HR
KCAL/KG
CEL
92.4
509.026
92.2
509.026
72.7
72.6
B
0.9069 642.9
16.833 107.1
619.8
0.4361
20.510 77.96
D
0.078
0.382
B C
0.299 99.9
748.8 H
C
99.9
509.028 46.8
0.299 M
47.0
310.0
63.693 58.8
12.0K
2.389 683.2
26.299 195.8
0382 M
0.078 M
506.53
49.2
509.028 49.0
310.0
20.510 76.5
3.7 K
735.8
0.4143 619.8
735.8
58.8
424.5
7.135
63.693 76.3
34.700
C
76.2
789.9
7.135
34.520
43.183 95.0
B
95.0
537.0
A
16.883 106.8
2.8 K
572.156
0.057 M
B
0.5616 642.9
1.251 M
4.352 M
26.299 123.8
740.70
572.218 352.2
40.57
2.8 K
843.89
2.269 683.2
26.299 195.5
36.52
34.520 309.4
777.2 H
124.0
16.70
6.564 735.8
205.5 168.3
639.314 164.1
816.06
509.026 120.8
95.766 170.0
0.946 M
0.043 M
0.701 M
0.018 M
2.154 M
0.024 M
0.935 M
2.186 M
B C C
6.0 K
0.0 K
789.9
352.2
740.70
A
34.700 423.0
639.314 200.0 15.87
350.4
740.70
61.067
40.57
537.0
172.0
61.067
38.54
0.000 M
C B A
205.5
0.0 K
D
150.0
639.314
210.3 6.0 K
61.067 206.0
639.314 247.0
256.21
LAYOUT OF MODERN 210 MW COAL FIRED POWER PLANT
C
P=210.061 MW
G
14.970 M
0.854 M
D
0.1033
577.3
441.114 46.45
3.068 M
D
B
64.846 M
0.1033
509.028
D
46.1
46.4
19.38
509.028 46.3
46.7
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
Define y as fraction of mass extraction:

y
mextraction

m SC


Q in  m SG h1  h8 

m SG
ym SG
m SG 1  y 

Q out  m SG 1  y h3  h4   y h7  h4 


W turbine  m SG h1  h2   1  y h2  h3 


W pump  mh5  h4 
m SG
Energy Balance for CFWH

m SG  y & h2



m SG  h8  m SG h5  m SG  y  h2  h6 
h2  h6 y  h8  h5

m SG & h8

mSG & h5

h8  h5
y
h2  h6
m SG  y & h6

m SG  y & h7
HP Closed Feed Water Heater
Bleed Steam
Feed Water in
C=Condenser
DC
C
DS
Feed Water out
Feedwater heater with Drain cooler and Desuperheater
DC=Drain cooler
DS=Desuperheater

m SG  y & h2
Condensate
Bled steam

TTD

m SG & h8
m SG  y & h6
T
-TTD=Terminal
temperature difference

mSG & h5
C
DC
DS
L
• 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
HP Turbine
Tbi, pbi, Tbsi
Condensing Shell
Drain Cooler
Desuperheater
Tfi
Tfi+1
TRAP
Tbi, pbi, Tbsi
Condensing Shell
Drain Cooler
Desuperheater
Tfi
Tfi+1
TRAP
Tf
Tube length
LP Closed Feed Water Heater
•
LP Closed Feed Water Heater
LP Turbine
Tbi, pbi, Tbsi
Condensing Shell
Tfi
Drain Cooler
Tfi+1
TRAP
• 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.
Tbi, pbi, Tbsi
Drain Cooler
Tfout
Condensing Shell
TRAP
Tf
Tube length
Tfin
Work done by Bleed Steam
h1
h2
h5
h8
h6
Work done by bleed (extracted) steam:
wbleed
wbleed  yh1  h2 
 h8  h5 
  h1  h2 
 
 h2  h6 
Closed Feed Water Heaters (Throttled
Condensate) 1
2
3
12
9
10
11
5
6
7
4
8
Analysis of Regeneration through Two CFWH
1
Define y as fraction of mass extraction:

y1 
mb,1


& y2 
m SG

mb , 2

m SG

Q in  m SG h1  h12 


T
10
2
9
11
7
6
8
5
s
3
12
4
Q out  m SG  y1  y2 h8  h5   1  y1  y2 h4  h5 


W turbine  m SG h1  h2   1  y1 h2  h3   1  y1  y2 h3  h4 


W pump  mh6  h5 
Energy Balance for LP-CFWH

m y2 & h3

m y1 & h11

 h9  h6 
 h8  h11 
y2  
  y1 

 h3  h8 
 h3  h8 
m& h9

m  y1  y2  & h7

m & h6

m  y1  y2  & h8





m h9  m y1  y2 h8  m h6  m y1  h11  m y2  h3
Energy Balance for HP-CFWH

m y1 & h2



m h12  m h9  m y1  h2  h10 

m& h12

m y1 & h10

h2  h10 y1  h12  h9
m & h9

h12  h9
y1 
h2  h10
m y1 & h11
Work done by Bleed Steam
 h12  h9 
  h1  h2 
wbleed ,1  m y1 h1  h2   
 h2  h10 
wbleed , 2
wbleed , 2
wbleed ,tot
 h9  h6 
 h8  h11 
 m y2 h1  h3   
  y1 
  h1  h3 
 h3  h8 
 h3  h8 
 h9  h6   h12  h9  h8  h11 

 m y2 h1  h3   
  
  h1  h3 
 h3  h8   h2  h10  h3  h8 
 h9  h6   h12  h9  h8  h11 
 h12  h9 
  h1  h2   

 
  
  h1  h3 
 h2  h10 
 h3  h8   h2  h10  h3  h8 
wbleed ,tot
 h9  h6   h12  h9  h8  h11 
 h12  h9 
  h1  h2   

 
  
  h1  h3 
 h2  h10 
 h3  h8   h2  h10  h3  h8 
1
T
wbleed ,tot
10
2
9
11
7
6
8
5
s
3
12
4
 hLPfeed   hHPfeed  hwastebleed 
 hHPfeed 
  wunitext1  

  wunitext2
 
  
 hHPbleed 
 hLPbleed   hHPbleed  hHPbleed 
Thermodynamic Analysis of A Power Plant
Train of Shell & Tube HXs.
Variation of Feedwater T emperature and Enthalpy along the
FeedWater Heater
6
Enthalpy
250
1000
5
200
800
4
150
600
3
2
100
400
1
GSC
DC
50
200
0
0
5
6
4
3
2
1
DC
6.0 K
6.0 K
0.0 K
0.0 K
GSC
D
Temperature ( C)
1200
Temperature
Enthalpy (kJ/kg)
300
C
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