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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.

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Gas Temperatures

Platen Super Heater:

Inlet Temperature: 1236.40

C 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

Steam Temperatures

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

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Furnace absorption

Platen SH

Pendent SHCSH

Reheater

Economizer

Combustion Losses C & R losses Hot Exhaust Gaslosses

~3280C

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Design 1: 500 MW

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Design 1: 500 MW

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LMTD for various Devices: Model 1

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Design 1: 500 MW

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DPNL SH

Pla

tenSHTR

R

HTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCW

pump

Bottom ash

stack

screen

tubes

Thermal Structure of A Modern Fuel Fired SG

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Thermal Balance in Air Pre-Heater.

The energy rate gained by air )( ,, inairoutairairair hhmQ

LMTDaphgasair TUAQQ

adust

dust

me

me

as

as

g hhU 11

1

The energy rate lost by flue gas

Overall Coefficient of Heat Transfer, U

)( ,, outgasingasgasgas hhmQ

Overall Convective rate of Heat Exchange

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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 andspace sharing in one system.

Time Sharing : Regenerators Space Sharing : Recuperators

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The concept of Time Sharing

At any time:

The overall heat transfer coefficient, U

me

me

as

as

g

gas

h

U

1

1

adust

dust

me

meair

h

U1

1

OR

At stead operation:

cond

g

gas

Rh

U

1

1

a

cond

air

hR

U1

1

OR

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Stockholm 1920

The Ljungstrm Air Preheater

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Historical Significance of Landmark

Throughout the history of boilers there have been manyadvancements in order to obtain a better performance andlower fuel consumption.

Ljungstrm Air Preheater invented by Fredrik Ljungstrm,then Technical Director at Aktiebolaget Ljungstrm ngturbin(AL) is the most successful invention.

The first installation in a commercial boiler saved as much as25% of the fuel consumption.

In a modern Steam generator the Ljungstrm Air Preheaterprovides up to 20% of the total heat transfer in the boilerprocess, but the Ljungstrm Air Preheater only represents 2%of the investment.

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Economic Impact of the Landmark

The use of a Ljungstrm Air Preheater in a modern powerplant saves a considerable quantity of fuel.

The cost of the preheater is generally recovered after onlya few months.

The total world-wide fuel savings resulting from allLjungstrm Air Preheaters which have been in service isequivalent to 4,500,000,000 tons of oil (1994).

An estimate shows that the Ljungstrm Air Preheaters in

operation annually saves about $30 Billion US. The distribution of thermal power capacity in whichLjungstrm Air Preheaters are installed over the world isshown in the table below.

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Landmark Contribution to Development

of New World Wide Industry

The use of the Ljungstrm Air Preheater started in the1920sthroughout the whole world.

In the beginning the marketing of the Ljungstrm Air

Preheater was made in close connection with AL. but most of the deliveries have actually been made through

a network of licensees throughout the world.

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Rotary or Regenerative Air Pre-Heater

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Stationary-Plate Type Air Pre-Heater

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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 transversedivision plates.

Heat storage pales are placed in these sections.

Weight : 500 tons. This consists of : rotor, sealing apparatus, shell etc.

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Schematic diagram of the rotary air preheater in thermal power plant.

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Thermodynamic process and flow diagram

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Leakage in APH

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Multiple Channel RAPH

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Fluid flows in rotary regenerator

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