Discussion
Assertion (A): Reducing a stack's exit velocity lowers ground level concentrations.
Reason (R): Momentum from the exit velocity contributes to plume rise.
Select the correct answer from the options below.
Answer: C
Correct answer: A is false but R is true
The assertion is false, and the reason is exactly why. Momentum from the exit velocity contributes to plume rise, so reducing the exit velocity reduces the rise, lowers the effective stack height, and therefore raises ground level concentrations.
Since the maximum varies roughly as \( 1/H^2 \), the penalty can be significant. The relation is often misread when fan power is being economised: slowing the exhaust saves energy at the stack but costs dispersion, and a very low exit velocity can also allow the plume to be caught in the wake downwash behind the stack itself.
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