Plume Behaviour and Stack Design Questions and Answers

Q: Arrange the causal chain of stack tip downwash: 1. Exit velocity falls below about one and a half times the wind speed 2. A low pressure region forms immediately downwind of the stack 3. The plume is drawn into that region rather than rising 4. Effective stack height falls and ground level concentration rises Select the correct sequence.

Correct answer: 1, 2, 3, 4The chain runs: exit velocity falls below about 1.5 times the wind speed → the wind separating round the stack leaves a low-pressure region immediately downwind → the slow plume is drawn into it rather than rising → effective stack height falls and ground-level concentration rises. The pressure region forms because of the flow round the stack, so it follows the velocity condition rather than preceding it.

Q: Arrange the components of effective stack height in the order they are established: 1. The physical stack height is fixed by the design 2. Momentum rise occurs as the gas leaves the stack 3. Buoyancy carries the plume further as it cools towards ambient 4. The plume levels off at the effective height Select the correct sequence.

Correct answer: 1, 2, 3, 4Effective height is built up in that order: the physical height is fixed by the design; momentum rise occurs first, as the jet leaves the stack; buoyancy then carries the plume further while it is still warmer than the air; and the plume levels off at the effective height once its temperature and vertical velocity have come into balance with the surroundings.

Q: Arrange the consequences of cooling flue gas in a wet scrubber: 1. Gas temperature falls sharply as the gas is washed 2. The gas falls below its acid dew point 3. Sulphuric acid condenses on the stack lining 4. Reheat or an acid-resistant lining becomes necessary Select the correct sequence.

Correct answer: 1, 2, 3, 4The sequence is causal: the wet scrubber cools the gas sharply → it falls below the acid dew point → sulphuric acid condenses on the lining → reheat or an acid-resistant lining becomes necessary. Each step follows from the one before, and the last is the design response to the first three.

Q: Consider the following pairs of plume type and the profile that produces it: Lofting - Stable below, unstable above Fumigation - Inversion eroding from below Trapping - Stable layers both above and below Coning - Strongly unstable throughout How many of the pairs given above are correctly matched?

Correct answer: Only threeThree pairs are right: lofting with stable air below and unstable above; fumigation with an inversion eroding from below; trapping with stable layers both above and below. The fourth is wrong: coning occurs in neutral conditions, with moderate turbulence roughly equal in the vertical and crosswind directions. Strongly unstable air gives looping instead.

Q: Consider the following pairs of design variable and its effect: Higher exit velocity - More momentum rise and less downwash Higher gas temperature - More buoyant rise Smaller stack diameter - Higher exit velocity at constant flow Taller stack - Lower total mass emitted How many of the pairs given above are correctly matched?

Correct answer: Only threeThree are correct: a higher exit velocity gives more momentum rise and less downwash; a hotter gas is more buoyant and rises further; and a smaller diameter raises exit velocity at constant flow. The fourth is false - a taller stack does not reduce the mass emitted at all. It disperses the same mass more widely, which is a quite different thing.

51.

Statement I: The good engineering practice height rule is a licence to disperse by height alone.

Statement II: The rule exists to place the release above the wake cavity generated by nearby buildings.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are correct
B.Both Statement I and Statement II are incorrect
C.Statement I is correct but Statement II is incorrect
D.Statement I is incorrect but Statement II is correct

Answer: D

Correct answer: Statement I is incorrect but Statement II is correct

The good engineering practice rule is the opposite of a licence: it sets the height needed to escape the building wake, and it is used as a ceiling on the credit a plant may claim for stack height in demonstrating compliance. The intention is to stop dispersion being used as a substitute for control.

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52.

Statement I: Reducing stack diameter at constant flow raises exit velocity.

Statement II: Velocity is the volumetric flow divided by the cross-sectional area.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are correct
B.Both Statement I and Statement II are incorrect
C.Statement I is correct but Statement II is incorrect
D.Statement I is incorrect but Statement II is correct

Answer: A

Correct answer: Both Statement I and Statement II are correct

Statement II is simply the arithmetic behind statement I. With \(v = Q/A\) and \(A = \pi d^2/4\), reducing the diameter at constant flow raises velocity as \(1/d^2\). The cost is a draught loss rising roughly with \(v^2\).

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53.

Statement I: The distance to the maximum ground level concentration is independent of stack height.

Statement II: A plume released higher must grow further before it reaches the ground.

In the light of the above statements, choose the correct answer.

A.Statement I is correct but Statement II is incorrect
B.Statement I is incorrect but Statement II is correct
C.Both Statement I and Statement II are correct
D.Both Statement I and Statement II are incorrect

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

Statement II contradicts statement I and is the correct one. Because a plume released higher must grow further vertically before touching the ground, the distance to the maximum ground-level concentration increases with effective stack height - it is anything but independent of it.

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54.

Arrange the steps of designing a stack in order:

1. Fix the exit velocity from wind speed and downwash considerations

2. Establish the volumetric flow and gas temperature from the process

3. Check the resulting ground level concentrations against the standard

4. Compute the diameter and the plume rise, then the physical height

Select the correct sequence.

A.2, 4, 1, 3
B.1, 4, 2, 3
C.2, 1, 4, 3
D.1, 2, 4, 3

Answer: C

The process fixes the flow, the wind fixes the velocity, geometry and rise follow from both, and the whole is finally tested against the air quality standard. Reversing any of the first three steps leaves a quantity undetermined.

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55.

Arrange these plume types in order from most favourable to least favourable for ground level concentration:

1. Lofting

2. Coning

3. Looping

4. Fumigation

Select the correct sequence.

A.2, 1, 3, 4
B.1, 3, 2, 4
C.2, 3, 1, 4
D.1, 2, 3, 4

Answer: D

Lofting keeps the plume entirely aloft, coning disperses it evenly, looping brings intermittent peaks to the surface, and fumigation delivers the whole accumulated plume at once. The order follows the stability profile directly.

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56.

Arrange the causal chain of stack tip downwash:

1. Exit velocity falls below about one and a half times the wind speed

2. A low pressure region forms immediately downwind of the stack

3. The plume is drawn into that region rather than rising

4. Effective stack height falls and ground level concentration rises

Select the correct sequence.

A.1, 2, 3, 4
B.2, 1, 3, 4
C.1, 3, 2, 4
D.2, 3, 1, 4

Answer: A

Correct answer: 1, 2, 3, 4

The chain runs: exit velocity falls below about 1.5 times the wind speed → the wind separating round the stack leaves a low-pressure region immediately downwind → the slow plume is drawn into it rather than rising → effective stack height falls and ground-level concentration rises. The pressure region forms because of the flow round the stack, so it follows the velocity condition rather than preceding it.

Discuss in Forum

57.

Arrange the components of effective stack height in the order they are established:

1. The physical stack height is fixed by the design

2. Momentum rise occurs as the gas leaves the stack

3. Buoyancy carries the plume further as it cools towards ambient

4. The plume levels off at the effective height

Select the correct sequence.

A.2, 3, 1, 4
B.1, 2, 3, 4
C.2, 1, 3, 4
D.1, 3, 2, 4

Answer: B

Correct answer: 1, 2, 3, 4

Effective height is built up in that order: the physical height is fixed by the design; momentum rise occurs first, as the jet leaves the stack; buoyancy then carries the plume further while it is still warmer than the air; and the plume levels off at the effective height once its temperature and vertical velocity have come into balance with the surroundings.

Discuss in Forum

58.

Arrange the consequences of cooling flue gas in a wet scrubber:

1. Gas temperature falls sharply as the gas is washed

2. The gas falls below its acid dew point

3. Sulphuric acid condenses on the stack lining

4. Reheat or an acid-resistant lining becomes necessary

Select the correct sequence.

A.1, 3, 2, 4
B.2, 3, 1, 4
C.1, 2, 3, 4
D.2, 1, 3, 4

Answer: C

Correct answer: 1, 2, 3, 4

The sequence is causal: the wet scrubber cools the gas sharply → it falls below the acid dew point → sulphuric acid condenses on the lining → reheat or an acid-resistant lining becomes necessary. Each step follows from the one before, and the last is the design response to the first three.

Discuss in Forum

59.

Consider the following pairs of plume type and the profile that produces it:

Lofting - Stable below, unstable above

Fumigation - Inversion eroding from below

Trapping - Stable layers both above and below

Coning - Strongly unstable throughout

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

Correct answer: Only three

Three pairs are right: lofting with stable air below and unstable above; fumigation with an inversion eroding from below; trapping with stable layers both above and below. The fourth is wrong: coning occurs in neutral conditions, with moderate turbulence roughly equal in the vertical and crosswind directions. Strongly unstable air gives looping instead.

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60.

Consider the following pairs of design variable and its effect:

Higher exit velocity - More momentum rise and less downwash

Higher gas temperature - More buoyant rise

Smaller stack diameter - Higher exit velocity at constant flow

Taller stack - Lower total mass emitted

How many of the pairs given above are correctly matched?

A.Only three
B.Only one
C.Only two
D.All four

Answer: A

Correct answer: Only three

Three are correct: a higher exit velocity gives more momentum rise and less downwash; a hotter gas is more buoyant and rises further; and a smaller diameter raises exit velocity at constant flow. The fourth is false - a taller stack does not reduce the mass emitted at all. It disperses the same mass more widely, which is a quite different thing.

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