Air-preheater for Conservation of Flue Gas Energy
P M V Subbarao
Professor
Mechanical Engineering Department
Minimize Final Exhaust Gas Temperature….
Properly Utilize Enthalpy of Flue Gas….
Steam Temperatures
Gas Temperatures
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Platen Super Heater:
Inlet Temperature: 1236.4 0C
Outlet Temperature: 1077 0C
Final Super Heater:
Inlet Temperature: 1077 0C
Outlet Temperature: 962.4 0C
Reheater:
Inlet Temperature: 962.4 0C
Outlet Temperature: 724.3 0C
Low Temperature Super Heater:
Inlet Temperature: 724.30C
Outlet Temperature: 481.3 0C
Economizer:
Inlet Temperature: 481.3 0C
Outlet Temperature: 328.5 0C
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Platen Super Heater:
Inlet Temperature: 404 0C
Outlet Temperature: 475 0C
Final Super Heater:
Inlet Temperature: 475 0C
Outlet Temperature: 540 0C
Reheater:
Inlet Temperature: 345 0C
Outlet Temperature: 5400C
Low Temperature Super Heater:
Inlet Temperature: 3590C
Outlet Temperature: 404 0C
Economizer:
Inlet Temperature: 254 0C
Outlet Temperature: 302 0C
Combustion Losses
C & R losses
CSH
Pendent SH
Platen SH
Furnace absorption
Reheater
Hot Exhaust Gas
losses
~3280C
Economizer
Design 1: 500 MW
Design 1: 500 MW
LMTD for various Devices: Model 1
Design 1: 500 MW
Thermal Structure of A Modern Fuel Fired SG
DPNL
SH
Platen SHTR
drum
R
H
T
R
screen
tubes
LTSH
Economiser
stack
BCW
pump
Furnace
APH
Bottom ash
ESP
ID Fan
Thermal Balance in Air Pre-Heater.

• The energy rate gained by air
• The energy rate lost by flue gas

Q air  mair (hair,out  hair,in )
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Q gas  m gas (hgas,in  hgas,out )
• Overall Convective rate of Heat Exchange
• Overall Coefficient of Heat Transfer, U
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Q air  Q gas  UAaphTLMTD
U
1
1  as  me  dust 1




hg as me dust ha
The Creative Design……
• Heat exchanger is complete only if there are Donor,
Receiver and Mediator in Equilibrium.
• Presence of all the three necessary?
• Time sharing & Space Sharing
• Time Sharing : Donor And Mediator for sometime and
Mediator and Receiver for sometime : Repeat!
• Space sharing: All present always.
• Central Limit Theorem : It is impossible to have time and
space sharing in one system.
• Time Sharing : Regenerators
• Space Sharing : Recuperators
The concept of Time Sharing
• At any time:
• The overall heat transfer coefficient, U
U gas 
1
1  as  me


hg as me
OR
U air 
1
 me  dust 1


me dust ha
• At stead operation:
U gas 
1
1
 Rcond
hg
OR
U air 
1
1
Rcond 
ha
Stockholm 1920
The Ljungström Air Preheater
Historical Significance of Landmark
• Throughout the history of boilers there have been many
advancements in order to obtain a better performance and
lower fuel consumption.
• Ljungström Air Preheater invented by Fredrik Ljungström,
then Technical Director at Aktiebolaget Ljungström Ångturbin
(ALÅ) is the most successful invention.
• The first installation in a commercial boiler saved as much as
25% of the fuel consumption.
• In a modern Steam generator the Ljungström Air Preheater
provides up to 20% of the total heat transfer in the boiler
process, but the Ljungström Air Preheater only represents 2%
of the investment.
Economic Impact of the Landmark
• The use of a Ljungström Air Preheater in a modern power
plant saves a considerable quantity of fuel.
• The cost of the preheater is generally recovered after only
a few months.
• The total world-wide fuel savings resulting from all
Ljungström Air Preheaters which have been in service is
equivalent to 4,500,000,000 tons of oil (1994).
• An estimate shows that the Ljungström Air Preheaters in
operation annually saves about $30 Billion US.
• The distribution of thermal power capacity in which
Ljungström Air Preheaters are installed over the world is
shown in the table below.
Landmark Contribution to Development
of New World Wide Industry
• The use of the Ljungström Air Preheater started in the
1920s throughout the whole world.
• In the beginning the marketing of the Ljungström Air
Preheater was made in close connection with ALÅ.
• but most of the deliveries have actually been made through
a network of licensees throughout the world.
Rotary or Regenerative Air Pre-Heater
Stationary-Plate Type Air Pre-Heater
Regenerative Pre-Heaters
• Rotates with a low speed
• Rotor is divided into 12 or 24 sections and 12 or 24 radial divisions.
• Each sector is divided into several trapezoidal sections with transverse
division plates.
• Heat storage pales are placed in these sections.
• Weight : 500 tons.
• This consists of : rotor, sealing apparatus, shell etc.
Schematic diagram of the rotary air preheater in thermal power plant.
Thermodynamic process and flow diagram
Leakage in APH
Multiple Channel RAPH
Fluid flows in rotary regenerator