Natural draught system: - When the fuel is burnt (combustion) in the furnace due to natural circulation of air, the draught is called natural draught.
Mechanical draught system: - In a mechanical draught device (fan/blower) is used to create the pressure is called mechanical draught.
Forced draught system: - If the air is circulated in combustion chamber with the help of fan/blower, the draught used is called forced (artificial) draught. OR Its function is to provide forced air (oxygen) for combustion process in furnace. OR If fan/blower is placed before the furnace then it is called as forced draught
Induced Draught system:- Its function is to remove rapidly flue gases (smoke) from the furnace chamber produced during combustion. OR If fan/blower is placed after the furnace then it is called as induced draught