Discussion
An engineer proposes reducing stack exit velocity to save fan power. This is
Answer: B
Correct answer: Unwise: it reduces plume rise
The proposal is unwise. Exit velocity supplies the momentum contribution to plume rise, so reducing it lowers the effective stack height, and because the maximum ground level concentration varies roughly as \( 1/H^2 \) the penalty is more than proportional. The fan power saved is bought at the cost of dispersion.
There is a second hazard: below roughly \( 1.5 \) times the wind speed, the plume can be caught in the low pressure wake behind the stack and dragged down its lee side, so that a tall stack behaves like a much shorter one. The problem is not confined to stable conditions, and a tall stack does not exempt it.
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