What is Boiler Draught, its function, classification

Boiler Draught / Draft

In a boiler the combustion of the fuel requires to supply sufficient amount of air and removal of exhaust gases and this is taken place by draught system.

Boiler Draught is defined as β€œthe pressure difference (between absolute gas pressure at any point in a gas flow passage and the atmospheric air outside the boiler) required to maintain flow of air and exhaust gases through the boiler setting.”

Function of draught system in power plant:

(i) To supply required quantity of air to the boiler furnace for the combustion of fuel.

(ii) To remove exhaust gases from the boiler passage to maintain flow of air and gases.

(iii) To create pressure difference against pressure losses in the flow passage.

(iv) To discharge the exhaust gases such that they will not objectionable or injurious to the surroundings.

Losses in the air-gas circuit of boiler

As we know that draught is nothing but pressure difference required to necessary to accelerate the burnt gases to their final and to overcome the pressure losses in the boiler passage.

Draught = [head required to accelerate the gases] + [head losses in the path of flue gases]

         ht  = [hv] + [ha + hg + hd]


ht= total draught required or total loss

hv= pressure loss due to gas exit velocity from the chimney

ha= pressure loss on air side = sum of losses on air side as air preheater, damper, air ducts, and furnace bed

hg= pressure loss on gas side = sum of losses in gases path as superheater, reheater, economizer, gas side air preheater, and dust collector

hd= pressure losses in chimney and gas ducts due to friction.

Classification of draught / draft

classification of boiler draught draft in thermal power plant
Classification of boiler draught/draft in thermal power plant

For detailed information of each type refer below articles

  1. What is Natural Draught – Click here
  2. What is Forced Draught – Click here

Comparison/Difference (Advantages and Disadvantages) between Mechanical draught and Natural draught in boiler

Mechanical draughtNatural draught
Advantages of mechanical draught:
(1) It does not depend on the atmospheric temperature.
(2) Chimney height is reduced since the function of chimney is only to discharge flue gases at a convenient height in the atmosphere.
(3) It improves the rate of combustion as draught is more. The better distribution and mixing of air with fuel is possible hence quantity of air required per kg of fuel is less. This reduces mass of flue gases formed and heat carried by exhaust gases
(4) Better control of combustion and evaporation rate since the air flow can be regulated as per requirement for combustion.
(5) The heat of flue gases can be recovered using economizer, air preheater since the flue gases temperature can be reduced. Hence thermal efficiency of plant is high.
(6) Less fuel consumption for given power
(7) Low grade fuel can be used due to high intensity of draught and combustion.
(8) Efficiency of draught about 7.

Disadvantages of mechanical draught:
(1) High investment cost.
(2) External power is required to drive the fans.
(3) High operating and maintenance costs
Disadvantages of natural draught
(1) It depends on atmospheric temperature.
(2) In order to create required draught the chimney height is more.
(3) Poor combustion due to less draught. Hence more quantity of air required. this increases mass of flue gases formed and heat caried by exhaust gases
(4) Combustion rate can not be controlled since air flow can not be regulated.
(5) The heat of flue gases can not be recovered since the flue gases have to be discharged at high temperature in order to create appropriate draught Hence thermal efficiency of plant is low.
(6) More fuel consumption forgiven power
(7) High grade fuel are used.
(8) Efficiency of draught hardly about 1%

Advantages of natural draught:
(1) Less investment cost.
(2) No external power is required.
(3) Operating and maintenance costs is practically nil.

Q1. Give advantages of artificial draught over natural draught in boiler.

Q2. What is draught? What are the functions of draught system in boiler?

Q3. Define boiler draught and what are the losses in air-gas circuit of boiler?

Q4. State classification of boiler draught.

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