Draught in Boilers

Draught or Draft refers to the pressure difference between the burner and the atmosphere.  This pressure difference or draught causes the air to flow from the burner to the atmosphere.  The residue of combustion such as waste gases, soot, etc are carried away by the flow of air.

Draught also has a great role to play in combustion.  The flow of fresh air into the burners is necessary for proper combustion.  Hence, the draught system should be designed such that the combustion can take place properly. 

The draught of a combustion system can be measured using a manometer when the furnace is in operation.  One end of the manometer is connected to the furnace while the other end is left open to the atmosphere.  The pressure difference indicates the draught of the system. 

There are different types of draught

  • Natural Draught where the draught occurs naturally due to the pressure difference between the furnace and the atmosphere. 
  • Induced Draught where the draught occurs by means of fans which create a negative pressure in the furnace causing fresh air to enter
  • Forced Draught where the draught occurs due to fans which provide combustion air and create a positive draught in the furnace.  This  drives the air through the chimney