Steam Boilers, Engines, Nozzles and Turbines
The object of producing draught in a boiler is

To provide an adequate supply of air for the fuel combustion
To exhaust the gases of combustion from the combustion chamber
All of the listed here
To discharge the gases of combustion to the atmosphere through the chimney

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Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
All the fuel burns instantaneously producing high energy release
Explosion in furnace
Fuel burns with less air

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Steam Boilers, Engines, Nozzles and Turbines
In forced circulation type boiler

Heating takes place at bottom and the water supplied at bottom gets converted into the mixture of steam bubbles and hot water which rise to drum
Water is converted into steam in one pass without any recirculation
Feed pump is employed to supplement natural circulation in water wall type furnace
Water is supplied in drum and through down comers located in atmospheric condition it passes to the water wall and rises to drum in the form of mixture of water and steam

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