Plume Behaviour and Stack Design Questions and Answers

Answer: C

Correct answer: Fanning

Fanning is the dangerous one. In a stable layer the plume spreads horizontally but hardly at all vertically, so it travels for great distances almost undiluted. Nothing appears near the source - and when the layer later breaks up, that concentrated plume can be brought to the ground in one body, far downwind.

Answer: D

Correct answer: The wake scales with the smaller dimension

The recirculating cavity behind a building is set by the smaller of its dimensions - a tall narrow building and a low wide one generate similar wakes. Using the lesser dimension therefore matches the rule to the physics; using the greater would give a needlessly tall stack for a broad low building.

Answer: A

Correct answer: Using the 10 m wind would overestimate rise

Plume rise varies inversely with wind speed, and the wind at stack height is appreciably greater than at 10 m - at 120 m and an exponent of 0.25 it is nearly twice as fast. Using the 10 m value would therefore overestimate the rise, and with it the effective height, giving a falsely reassuring ground-level concentration.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

The reason states exactly why the assertion holds. Below a ratio of about 1.5 the plume is captured by the low-pressure wake behind the stack and the effective height collapses, so the minimum velocity is specified precisely to keep the jet clear of it.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Because the maximum goes as \(1/H_e^2\), an error in plume rise is squared in the predicted concentration - overestimating rise by 30% understates the maximum by more than a third. That sensitivity is exactly why the calculation deserves care.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

The wake cavity is governed by the smaller of the building’s dimensions, so taking the lesser of height and width matches the rule to the aerodynamics it is meant to describe.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Reheat answers both consequences of wet scrubbing at once: it lifts the gas back above its acid dew point, protecting the lining, and restores some of the buoyancy the cooling destroyed. The reason therefore explains the assertion completely.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

In a stable layer the plume spreads sideways but not vertically, so it travels a long way almost undiluted. If the layer later breaks up - as it does when the ground warms - that concentrated ribbon is mixed down in one body, which is exactly how a fanning plume produces a sudden high concentration far from its source.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

The reason is the whole basis of the limit. Because a tall stack releases the same mass and merely disperses it more widely, allowing unlimited credit for height would let dispersion substitute for control - solving a local problem by making a regional one.

Answer: D

Correct answer: A is false but R is true

The reason is correct arithmetic: \(v = Q/A\) and \(A \propto d^2\), so velocity does vary as \(1/d^2\). But the assertion is too strong: a narrower stack also raises the draught loss, roughly as \(v^2\), and there is a point beyond which the fan power and the noise outweigh any gain in dispersion. “Always” is what makes the assertion false.