Regenerative Air Preheater

CONTENTS 

INTRODUCTION 

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OBJECTIVE 

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CIRCUIT DIAGRAM 

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OPERATION/WORKING PRINCIPLE 

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CALCULATIONS 

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 ADVANTAGES  DISADVANTAGES   APPLICATIONS  REFERENCES 

INTRODUCTION  

 A Regenerative air preheater is a type of air preheater used in thermal power plants to preheat the steam generator's combustion air with the hot flue gas from the steam generator.

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The types of Regenerative Air Preheaters are 1. Bisector 2. Trisector 3. Qudrasector

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Here the discussion is carried on the Trisector Air Preheater.

OBJECTIVE  

Mainly there are two types of Air Preheaters 1. Recuperative Air Preheater 2. Regenerative Air Preheater

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The main objective of this project is to prove that the efficiency of  regenerative air pre heater is more, when compared to the recuperative air pre heater.

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It is to say that the Trisector type of Regenerative air pre heater is more advantageous than Bisector type.

CIRCUIT DIAGRAM OF BOILER Steam generator 

Burner 

  r    i   a   y   r   a    d   n   o   c   e    S

  r    i   a   y   r   a   m    i   r    P

Air preheater 

Electric precipitator 

Coal bunker     d   e   z    i   r   e   v   l    l   e   u   u    P   f

Intermed. Flyash bunker  Fly ash

Coal mill

Excess Air 

Flue gas

CIRCUIT DIAGRAM OF AIR PREHEATER

WORKING PRINCIPLE  

 Air Preheater is a device which is designed to heat the air before the combustion process.

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The purpose of Air Preheater is to recover the heat from the flue gases  which are obtained from the boiler thus increases the thermal efficiency of the boiler.

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Due to the discontinuous action taking place in the air pre heater, it is so called as regenerative air pre heater.

TRISECTOR AIR PREHEATER 

There are three sections in the Trisector Air Preheater. 

Flue Gases

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Primary Air

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Secondary Air

PRIMARY   AIR  FLUE GASES SECONDARY   AIR 

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Flue gases are sent through the chimney through Induced Draft(ID) Fan.

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Primary Air (PA) is used to dry the fine particles of  pulverized coal, which is then forced into the furnace with the help of PA fan, from the gravimetric coal feeder. Secondary Air (SA) is used to supply sufficient oxygen for complete combustion of fuel and this is done with the help of Forced Draft (FD)fan.

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Primary Air occupies 1/6th part of the Air Preheater.

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Flue gases occupies 1/2nd part of the Air Preheater.

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Secondary Air occupies 1/3rd part of the Air Preheater.

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Temperature at inlet conditions: Flue gas  Air temperature

– 3600 c – 35o c

Exit Temperature: Flue gas  Air temperature

– 1400 c – 180o c

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CALCULATIONS

1) 2) 3)

air in 4) Weighted temperature

[(Fraction of primary air x Inlet air temperature of primary air sector) + = (Fraction of secondary air x Inlet air temperature of secondary air sector)]

5) Convective heat transfer takes place in air preheater i.e. q = hA (t2 – t1)

6)  Area of Cross section occupied of flue gas, PA and SA systems: i) Gas flow area

= 25.94 m2

ii) Primary air flow area iii) Secondary air flow area

= 8.96 m2 = 14.62 m2

 ADVANTAGES 

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Trisector Air Preheater provides both Primary and Secondary Air sections in a single plate. Due to the presence of Primary section immediate to the Flue Gas path, complete drying up of coal particles takes place. It has quick response with variation of loads.

DISADVANTAGES 

Leakages are more, but this can be avoided up to some extent by perfect sealing system.

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Constructional cost for regenerative air preheater is more when compared to recuperative type.

 APPLICATIONS 

It is used where in waste heat recovery is possible for increase in efficiency of particular device.

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Hence it is installed in every thermal power station.

REFERENCES 

Material provided by KTPS.

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Thermal Engineering By R.K.Rajput, 6th Edition, published by Laxmi Publications.

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Heat And Mass Transfer By C.P.Kothandaraman and S.Subramanyan, 6th Edition.

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Heat And Mass Transfer By R.K.Rajput.