In modern boilers, stacks alone cannot create enough mechanical draught because

Explanation:

Mechanical Draught in Modern Boilers

• Mechanical draught in boilers refers to the use of fans, blowers, or other mechanical devices to control the flow of air into the combustion chamber and the removal of flue gases. This is a critical aspect in modern boilers to ensure efficient combustion and proper evacuation of exhaust gases.

This option correctly identifies the reasons why stacks alone cannot create enough mechanical draught in modern boilers. Let’s analyze the factors:

(A) The rate of air supply as well as the rate of flue gas removal is high:

• Modern boilers are designed to operate at high capacities, requiring a significant amount of air for combustion. Likewise, the removal of flue gases must match the rate of combustion to prevent backpressure and ensure uninterrupted operation. The natural draught created by stacks is insufficient to handle these high rates, necessitating mechanical aids.

(B) Various heat exchangers are used on the way cause large pressure drop:

• Modern boilers incorporate various heat exchangers, such as economizers, air preheaters, and superheaters, to recover heat from flue gases and improve thermal efficiency. These components create resistance to the flow of gases, resulting in a substantial pressure drop. Natural draught alone cannot overcome this resistance, making mechanical draught essential.