For producing the same overall draught in a furnace–stack system, the power required by a forced-draught fan compared with an induced-draught fan is generally:

Introduction / Context:
Draught systems move air and flue gases through furnaces and heat-recovery equipment. Choosing forced vs. induced draught impacts fan duty, energy consumption, and control strategy.

Given Data / Assumptions:

• Same draught (pressure difference) must be achieved.
• Forced-draught fan handles cold, dense air; induced-draught fan handles hot, expanded flue gases.

Concept / Approach:
Fan power ~ (flow * pressure rise) / efficiency. For equal draught, the induced-draught fan often moves a much larger volumetric flow because hot flue gases are less dense, increasing power requirement relative to a forced-draught fan that moves denser, lower-volume cold air.

Step-by-Step Solution:
Recognize density effect: V_hot > V_cold for the same mass flow.At equal pressure rise, higher volumetric flow raises fan power on the induced side.Therefore, forced-draught power is generally less than induced-draught power for the same draught.

Verification / Alternative check:
Compare typical fan curves and system curves; induced fans frequently have larger casings/motors due to hot-gas volume and higher losses downstream.

Why Other Options Are Wrong:
Same or more: contradicts usual density/volume effect.
Depends solely on flue-gas density: while density matters, under comparable systems forced draught typically needs less power.

Common Pitfalls:
Ignoring temperature’s impact on volumetric flow; neglecting additional pressure losses on the gas-side path.

Final Answer:
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