Basic Boiler In Draught (Draft)


                The difference in pressure of the combustion products within a boiler furnace and the cold air outside is known as (draft).

             Draught is also the differential pressure between the air column (chimney height) outside and the hot flue-gas column (chimney height) inside the chimney. See Fig 1.0

Necessity of Draft

  1. To force adequate air through the combustion chamber to assist in the proper combustion of fuel.
  2. To draw out the resulting hot flue gas from the combustion chamber.
  3. To vent the products of combustion to atmosphere after necessary heat recovery in superheter, economizer and air heater.

Draft can be broadly classified into two classes - Artificial draft and Natural draft.

Artificial Draft : The draft produced by steam jet or mechanical means like fans and blowers. 

        Artificial draft is of two types
  • Steam jet draft
  • Mechanical draft.

Steam jet draft can be of induced as well as forced draft type.

Mechanical draft can be of induced, forced and balanced draft types.

Natural Draft :When air or flue gases flow due to difference in density of the hot flue gases and cooler ambient gases. The difference in density creates a pressure differential that moves the hotter flue gases into the cooler surroundings.
               When the draft is generated with the help of the chimney only.

Forced Draft : When air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan.

Induced Draft : When air or flue gases flow under the effect of a gradually decreasing pressure below atmospheric pressure. In this case the system is said to operate under induced draft. The stacks provide sufficient natural draft to meet the low draft loss needs. In order to meet higher pressure differentials, the stacks must simultaneously operate with draft fans.

Balanced Draft : When the Static Pressure is equal to the atmospheric pressure the system is referred to as balanced draft

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Basic Boiler In Draught (Draft) Basic Boiler In Draught (Draft) Reviewed by Thermal Power Tech on February 29, 2020 Rating: 5


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