Boiler draught may be defined as the small difference between the pressure of outside air and that of gases within a furnace or chimney at the grate level, which causes flow of air/hot flue gases to take place through boiler.

The draught is necessary to force air through the fuel bed/ grate to aid in proper combustion of fuel and to remove the products of combustion i.e. flue gases to the atmosphere after they have given their heat to water being evaporated in boiler. Draught also provides velocity to flue gases and so increases the heat transfer co-efficient in the boiler. Thus draught is essentially required in a boiler and can be produced by a number of methods.

draught

Boiler draught is the pressure difference between the atmosphere and the pressure inside the boiler. Generally a modern coal fired boiler has balanced draught. Draught is maintained inside boilers using fans.

  1. Induced draught — In this system a fan or blower is located at or near the base of the chimney which creates a partial vacuum in the furnace and flue passage and thus the air and flue gases are drawn through the boiler due to comparatively higher pressure of outside air. It is convenient to produce induced draught and like in forced draught, any type of fan/blower may be used. This is produced by the help of an induced draught fan, it is installed before the chimney and it maintains negative pressure inside the furnace, the disadvantage is the air ingression inside the furnace due to improper sealing, thus leading to losses.
  2. Forced draught — It is produced by the help of a forced draught fan installed before the furnace, air blown inside the furnace, thus producing positive pressure inside furnace, disadvantage is that hot flue gases will tend to leak into atmosphere and flames come out from the inspection holes in the furnace. It is a positive pressure draught in which fan is installed at the base of the boiler before grate which forces the outside air through fuel bed, furnace and air pre-heater and then flue gases through flue passage, economizer etc. The enclosure for the furnace has to be very tightly sealed so that gases from the furnace do not leak out in the boiler house.
  3. Balanced draught — It is a combination of forced and induced draught. In this system, the forced draught fan overcomes the resistance in air pre-heater and grate while the induced draught fan overcomes draught losses through boiler, economizer and connecting flue passages etc. Balanced draught is most effective and used in large boilers. Depending on the type of fuel burnt and type of boiler, the fan or blower used may be of any type as radial or axial etc. It is usually run by an electric motor. In case of oil burners, air blower is fitted in the burner assembly itself. Finally the flue gases pass on to chimney and escape to atmosphere because of pressure created by fan. It has both ID as well as FD fan and the draught or pressure difference produced is balanced or almost equal to atmospheric pressure.

Comparison between forced draught and induced draught

Forced Draught

Induced Draught

1) Fan or blower is placed before grate or situated within the oil burner assembly in case of oil fired boiler. The fan or blower is placed after the grate or after the flue passages and before chimney.
2) Pressure inside the flue passages is slightly more than atmospheric pressure. Pressure inside the flue passages is slightly less than atmospheric pressure.
3) Due to above reason, there is serious danger of fire to come out and injure someone if any leakage in boiler flue passage take place. Due to above reason, there is no danger of fire to come out from boiler flue passage in case of a leakage. Hence Induced draught is more safe.
4) It forces fresh air into the combustion chamber, which helps in burning of fuel and production of hot combustion gases. Due to this force, the hot flue gases are further pushed through the flue passages in boiler. It sucks the hot flue gases from the combustion chamber through flue passages and then passes on these to economizer, air pre heater and chimney. Due to this suction, fresh air is also sucked in to combustion chamber.
5) Forced draught fan require less power because it has to handle cold and dense air, the volume per unit mass of which is less. Induced draught fan requires more power because it has to handle hot flue gases, the volume per unit mass of which is more.
6) Flow of flue gases through boiler is more uniform.

Flow of flue gases is less uniform

 

 

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