Plume Behaviour and Stack Design Questions and Answers

Q: Classical plume types and the stability profile that produces each. TypeBelow stackAbove stackGround level effect LoopingUnstableUnstableIntermittent high peaks near the stackConingNeutralNeutralModerate and predictableFanningStableStableVery little near the sourceLoftingStableUnstableAlmost none Which type carries the greatest risk of a sudden high concentration later, far from the stack?

Correct answer: FanningFanning 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.

Q: Good engineering practice height for four building geometries. Building height (m)Projected width (m)Lesser dimension (m)GEP height (m) 302020602460246040404010015501537.5 Why does the rule use the lesser of height and width?

Correct answer: The wake scales with the smaller dimensionThe 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.

Q: Wind speed with height for two power law exponents. Height (m)Exponent 0.10Exponent 0.25 105.05.0405.77.1806.28.41206.49.3 What does the comparison imply for a plume rise calculation?

Correct answer: Using the 10 m wind would overestimate risePlume 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.

Q: Assertion (A): Reducing stack diameter always improves dispersion. Reason (R): Exit velocity varies as the inverse square of diameter at constant volumetric flow. Select the correct answer from the options below.

Correct answer: A is false but R is trueThe 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.

71.

Classical plume types and the stability profile that produces each.

Type	Below stack	Above stack	Ground level effect
Looping	Unstable	Unstable	Intermittent high peaks near the stack
Coning	Neutral	Neutral	Moderate and predictable
Fanning	Stable	Stable	Very little near the source
Lofting	Stable	Unstable	Almost none

Which type carries the greatest risk of a sudden high concentration later, far from the stack?

A.Coning
B.Lofting
C.Fanning
D.Looping

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.

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72.

Good engineering practice height for four building geometries.

Building height (m)	Projected width (m)	Lesser dimension (m)	GEP height (m)
30	20	20	60
24	60	24	60
40	40	40	100
15	50	15	37.5

Why does the rule use the lesser of height and width?

A.It gives the tallest stack
B.It is easier to measure
C.It gives the cheapest stack in every case
D.The wake scales with the smaller dimension

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.

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73.

Wind speed with height for two power law exponents.

Height (m)	Exponent 0.10	Exponent 0.25
10	5.0	5.0
40	5.7	7.1
80	6.2	8.4
120	6.4	9.3

What does the comparison imply for a plume rise calculation?

A.Using the 10 m wind would overestimate rise
B.The 10 m wind may be used at any height
C.Wind speed falls with height
D.The exponent has no practical effect

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.

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74.

Assertion (A): Exit velocity is kept above about one and a half times the design wind speed.

Reason (R): Below that ratio the plume is drawn into the low pressure wake behind the stack.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

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.

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75.

Assertion (A): Plume rise deserves careful calculation.

Reason (R): Maximum ground level concentration varies roughly as the inverse square of effective height.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

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.

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76.

Assertion (A): The good engineering practice height rule takes the lesser of building height and width.

Reason (R): The wake cavity scales with the smaller dimension of the obstacle.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

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.

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77.

Assertion (A): Reheat is used after a wet scrubber.

Reason (R): The cooled gas has fallen below its acid dew point and has lost almost all its buoyancy.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

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.

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78.

Assertion (A): A fanning plume can produce a sudden high concentration far from its source.

Reason (R): It travels for great distances almost undiluted within a stable layer.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

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.

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79.

Assertion (A): Regulators limit the dispersion credit given for stack height.

Reason (R): A tall stack releases the same mass of pollutant, merely where it is diluted more.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

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.

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80.

Assertion (A): Reducing stack diameter always improves dispersion.

Reason (R): Exit velocity varies as the inverse square of diameter at constant volumetric flow.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

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.

Discuss in Forum

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