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. 

Types of Draught in Boilers

  • 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

Steam Jet Draught

Steam Jet Draught refers to the Draught created by using a jet of steam.  The steam generated by the boiler can be used for this purpose.  The jet of steam is used to create an airflow which will cause the flue gases to exit through the chimney.

If the steam jet is applied near the stack of the chimney, the negative pressure it creates draws the flue gases from the furnace into the Chimney.  This is known as induced draught.

If the jet is applied below the grate, the steam pushes the flue gases in the direction of the chimney.  This is a forced draught.

The Steam Jet draught is a simple mechanism.  No external equipment such as compressors or blowers are required.  The steam when used below the grate cools the firebars and prevents the clinkers from sticking to the bars.