Discussion
A designer must halve the maximum ground level concentration from an existing stack.
Which change is most effective?
Answer: D
Correct answer: Raise effective stack height by 40 per cent
Since the maximum ground level concentration varies roughly as \( 1/H^2 \), raising the effective height by 40 per cent reduces it by a factor of \( 1.4^2 = 1.96 \) - very nearly a halving. That is the change to make.
Halving the exit velocity does the opposite: it reduces the momentum contribution to plume rise and so lowers the effective height. Doubling the stack diameter at fixed volumetric flow also lowers the exit velocity, with the same effect. Moving the stack \( 100 \) m sideways relocates the footprint without changing the peak concentration at all.
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