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.
- Forced Draft
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.