Correct option is C
In modern boilers, mechanical draught (provided by fans) is essential to overcome the limitations of natural draught (stacks). Stacks alone cannot create sufficient draught due to:
High rates of air supply and flue gas removal:
Modern boilers require large quantities of air for combustion and efficient removal of flue gases. Natural draught (stack effect) is inadequate to handle these high flow rates.
Pressure drops across heat exchangers:
Modern boilers include various heat recovery devices (e.g., economizers, air preheaters, superheaters) that cause significant pressure drops in the flue gas path. The stack effect cannot overcome these losses.
