High negative draft in a solid fuel fired boiler carries away the unburnt fuel along with the flue gas. This leads to wastage of fuel.

A draft is necessary in a boiler to ensure the flue gases are led to the chimney and to ensure a smooth exhaust. On the basis of the mechanism of generating the draft a boiler may be classified as a forced draft, induced draft or a balanced draft boiler.

A negative draft is maintained on the air side of the boiler to ensure smooth flow of hot flue gas from the boiler to the chimney. Additionally a negative draft ensures that the hot gases at high temperature do not come out from any leaks in the system, thus leading to safe working conditions.

For most process boilers this negative draft is only a few mm of Hg (between -8 to -15mm). If there is excessive negative draft the lighter fuel particles will get carried away with the flue gas. This will have a twofold impact,

  • It will reduce the fuel available for combustion, leading to excess fuel consumption.
  • The fuel particles will add to the load of the dust handling systems (Bag Filters, Mechanical Dust collectors (MDC) and may eventually lead to higher emissions.

It is thus recommended to monitor and maintain the negative pressure in the boiler to ensure there is no fuel carryover.