BOILER DRAUGHT

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BOILER DRAUGHT

  • Draught refers to the pressure difference created for the flow of gases inside the boiler.
  • Boiler unit has a requirement of the expulsion of combustion products and supply of fresh air inside furnace for continuous combustion.
  • The flues gases or combustion products formed during combustion should be discharged at such a height.
  • A chimney or stack is generally used for carrying these combustion products from inside of boiler to outside, i.e. draught is created by use of chimney.

  • Draught may be created naturally or artificially by using some external device. 
  • Draught can be classified as below:
    • In natural draught the pressure difference is created naturally without using any positive displacement device.
    • Artificial draught is created using some external assistance causing forced displacement of gases. It can be created either by using mechanical devices or steam. Artificial draught can be of induced type, forced type or combination of two types.


  • Thus the draught in boiler may be said to be required for, ‘providing and maintaining the supply of sufficient air for combustion’, ‘expulsion of combustion products from furnace region’ and ‘discharge of burnt gases to atmosphere’.
  • The amount of draught required shall depend upon, ‘type of boiler’, ‘rate of fuel burning’, ‘and rate at which combustion products are produced ’and‘the air requirement rate’.
  • As the pressure difference is very small so draught is measured in ‘mm’ of water.
DRAFT CLASSIFICATION


NATURAL DRAUGHT
  • The natural draught is produced by a chimney due to density difference between hot flue gas inside chimney and cold atmospheric air outside chimney.
  • So in a boiler continue flue gas is replaced by fresh air. This fresh air participate in combustion process.
  • Chimney draft is depend on pressure difference causes by density difference 
  • Draft is also depend on the chimney height. So in tall chimney high draft is created as compare to smaller chimney. 
  • Because of the pressure difference in taller chimney is high. 
  • Chimney is a vertical structural duct which made of concrete or metals.

ARTIFICIAL DRAUGHT

When the natural draught becomes insufficient for exhaust of flue gases.
In the modern large power plants this draught produced by chimney  is insufficient and requires some artificial method.
In natural draught the draught value is limited and is depend on the height of chimney. For mor draught the height of chimney is more. As it is a metal or concreat structural so it's height is also limited.
For same steam generation the fuel consumption gets reduced by up to 15% with use of artificial draught in a boiler.

 So to get sufficient pressure difference between two point in natural draught is limited because of chimney height is limited. 
so as boiler size is increases the draught required is more which is not possible with the help of natural draft.
So artificial draft is employed.
Artificial draught may be produced either by mechanical means such as fans, blowers etc. or by using steam jet for producing draught. 

 The draught is also derived in three types according to pressure inside furnace.
Induced draught
Force draught
Balance draught
Thus artificial draught can be classified as,
(i) Mechanical draught (ii) Steam jet draught.

Artificial draught systems do not require tall chimney/stack, but small stack is required for discharge of flue gases to certain height in atmosphere.



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