What is Draft
                     What is Draft?
Anthea Johnson
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In thermal electric energy generation, boilers play an 
important role in the production of the electricity. The air and 
gases flow rate inside the boilers is maintained by the set of 
fans called draft fans. There are mainly two kinds of draft 
fans – forced draft fans and induced draft fans. The main 
purpose of the draft fans is to supply the required amount of 
air inside the boiler and to remove the gases that are formed 
after combustion of the fuel. The draft fans used in the 
industry may vary depending upon the type of the industry, 
size and shape of the boiler.
Meaning of draft
The difference between atmospheric pressure and the pressure inside a boiler 
is called draft. It can also be defined as the pressure difference between 
motion of fuel gases and the air flow. Draft in the boiler is produced by the rise 
of the combustion gases in the stack, fuel or by any mechanical means.
Types of Drafts
Natural Draft    
In this kind of draft air and fuel gases flow due to differences in the density of 
the hot gases and ambient air. When the density difference creates pressure 
difference it moves the hotter fuel gases into the cooler environment. The 
chimney must be kept warm enough to exhaust the burnt fuel gases. The 
pressure inside the system is negative with this kind of draft. Natural draft 
system do not have any kind of fan arrangement in it.
Centrifugal blower
When air flow and fuel gases are maintained at higher pressure it is called 
as forced draft. This phenomenon is seen at air inlet of the combustion 
chamber. All oil burners and few gas burners use forced draft. We cannot 
use a single forced draft system, but two forced draft systems can be 
coupled. Some forced draft systems produce negative vent pressure as in 
case of conventional oil furnace and some produce positive vent pressure 
as in case of gas furnaces.
Induced Draft
In the induced draft, the air flow and fuel gases flow is maintained at low 
pressure.     This phenomenon is seen at outlet of the fuel gases after 
combustion. Induced draft is produced in order to move the exhaust gases 
faster. This helps in the quick burning of the next set of fuel. If the 
induced draft system is not available in the  process there can be     traces 
of burnt gases which reduce the efficiency of the system.
Balanced Draft
In balanced draft the static pressure inside the combustion chamber becomes 
equal to the atmospheric pressure. In this kind of draft there is no action of fan, 
as the system is balanced.The induced and forced draft process involves the 
action of the fan in order to increase the efficiency of the system. Draft fans 
can be either centrifugal or axial.
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