Effect of mechanical draught on plant efficiency Consider a steam-plant firing system. As mechanical draught (fan-assisted air and flue-gas movement) is implemented and properly controlled, the overall plant efficiency generally ________.

Introduction:
Draught provides the pressure difference necessary to move air into the furnace and carry flue gases through the boiler passes and chimney. Natural draught relies on chimney buoyancy, while mechanical draught uses fans. This question probes understanding of how mechanical draught influences energy efficiency.

Given Data / Assumptions:

• Appropriate fan selection (forced/induced draught) with correct control.
• Adequate combustion tuning (excess air control, air–fuel ratio).
• Reasonable maintenance of leakage and fouling.

Concept / Approach:
Mechanical draught provides better control of air supply and gas flow than natural draught, enabling optimal excess air, higher furnace temperatures, and improved heat transfer. It reduces stack losses via precise air control and supports air preheaters and economisers at design conditions. Although fans consume auxiliary power, the net effect is typically an increase in overall plant efficiency due to improved combustion and heat recovery.

Step-by-Step Solution:

Verification / Alternative check:
Energy balance: auxiliary fan power is small compared to fuel energy saved by better combustion and lower flue-gas exit temperatures, producing a higher net plant efficiency.

Why Other Options Are Wrong:

• Decreases: contradicts typical outcomes when properly designed.
• Remains constant: ignores the tangible benefits of control and recovery.
• Oscillates or drops to zero: unrealistic and not aligned with engineering practice.

Common Pitfalls:
Running with excessive excess air due to poor control (which can negate gains); neglecting leakage in ducts; ignoring fan power in net efficiency calculations.

Final Answer: