Draught economics comparison: The ratio of the energy required to produce artificial draught (expressed as head or J per kg of flue gas) to the mechanical equivalent of the extra heat carried away per kg of flue gases when natural draught is used is termed the:

Difficulty: Medium

Correct Answer: efficiency of the fan

Explanation:


Introduction / Context:
Plant designers often compare natural draught (chimney) with artificial draught (fan or steam-jet) on an energy basis. The key question is whether the mechanical or electrical energy spent on artificial draught is justified by the heat saved that would otherwise be lost with a high-temperature chimney plume.


Given Data / Assumptions:

  • Artificial draught requires input energy (fan power per kg of flue gas), expressed as J/kg or equivalent head.
  • Natural draught generally entails higher stack temperatures, increasing heat loss in flue gases.
  • Comparison is made per unit mass of flue gas for a fair basis.


Concept / Approach:
The defined ratio contrasts the input energy for artificial draught with the saved heat that would have been carried away under natural draught. In boiler engineering shorthand, this ratio is referred to as the efficiency of the fan system with respect to draught economics. A value well below 1.0 indicates that spending a little mechanical energy saves a lot of chimney heat loss, making artificial draught an efficient choice.


Step-by-Step Solution:
Let E_mech = energy required by the fan per kg of flue gas.Let E_loss = extra heat carried away per kg in natural draught operation.Define ratio R = E_mech / E_loss.Interpretation: smaller R → more attractive fan draught.


Verification / Alternative check:
Typical fan powers are modest compared to heat rates, so artificial draught commonly improves overall plant efficiency and control.


Why Other Options Are Wrong:
Efficiency of the chimney: pertains to performance of a stack, not a comparison between fan input and chimney heat loss.

Boiler efficiency and power of the boiler: relate to fuel-to-steam conversion or output, not draught production comparison.

Specific draught coefficient: not a standard term for this ratio.



Common Pitfalls:
Confusing thermodynamic boiler efficiency with draught-system economics; ignoring that the ratio is per unit mass of flue gas.


Final Answer:
efficiency of the fan

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