Plume Behaviour and Stack Design Questions and Answers

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.

Q: Consider the following statements about tall stacks: 1. They reduce the maximum ground level concentration. 2. They reduce the mass of pollutant emitted. 3. Regulators normally limit the dispersion credit given for height. Which of the statements given above is or are correct?

Correct answer: 1 and 3 onlyStatements 1 and 3 are correct: a tall stack does lower the maximum ground-level concentration, and regulators do cap the credit allowed for height. Statement 2 is false - the mass emitted is unchanged, which is exactly why the credit is capped.

Q: Consider the following statements about plume rise: 1. Buoyancy dominates for a hot combustion plume. 2. Rise falls as wind speed increases. 3. Rise is independent of exit velocity. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 1 and 2 hold: buoyancy dominates for a hot combustion plume, and rise falls as \(1/u\) with wind speed. Statement 3 is false - exit velocity supplies the momentum term \(v_s d/u\), and for a cold stack it is the only source of rise there is.

Q: Consider the following statements about downwash: 1. Stack tip downwash follows from low exit velocity relative to wind speed. 2. Building downwash follows from the wake cavity around a structure. 3. Both are prevented by increasing the emission rate. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 1 and 2 correctly describe the two mechanisms. Statement 3 is nonsense: the emission rate has no bearing on downwash, which is an aerodynamic effect - and raising it would simply increase the concentration that the downwash then brings to the ground.

Q: Consider the following statements about stack design: 1. Exit velocity is normally fixed before the diameter is calculated. 2. The acid dew point may govern whether reheat is required. 3. Aviation limits never constrain stack height. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 1 and 2 are correct: the exit velocity is chosen first (from the wind speed and downwash rule) and the diameter follows from \(A = Q/v\); and the acid dew point may well decide whether reheat is needed. Statement 3 is false - aviation obstacle limits frequently cap stack height near an airfield.

Q: Match each plume type in List I with its ground level effect in List II. List I A. Lofting B. Fumigation C. Looping D. Fanning List II 1. Intermittent high peaks close to the stack 2. Almost nothing reaches the ground 3. Very little nearby, but a risk far downwind 4. The whole accumulated plume arrives at once Select the correct match.

Correct answer: A-2, B-4, C-1, D-3Lofting keeps almost everything aloft (2). Fumigation brings the whole accumulated plume down at once as the inversion erodes (4). Looping gives intermittent high peaks close to the stack, as the plume is thrown up and down by large eddies (1). Fanning gives very little nearby but carries an undiluted plume far downwind (3).

Q: Match each design variable in List I with its principal effect in List II. List I A. Exit velocity B. Gas temperature C. Stack diameter D. Physical stack height List II 1. Buoyant plume rise 2. Momentum rise and downwash avoidance 3. Effective height directly 4. Exit velocity at a given flow Select the correct match.

Correct answer: A-2, B-1, C-4, D-3Exit velocity gives momentum rise and avoids downwash (2). Gas temperature drives buoyant rise (1). Diameter fixes the exit velocity at a given flow (4). Physical height contributes to effective height directly (3), metre for metre.

Q: Match each problem in List I with its usual remedy in List II. List I A. Stack tip downwash B. Building downwash C. Acid condensation in the stack D. Lost buoyancy after wet scrubbing List II 1. Good engineering practice stack height 2. Higher exit velocity 3. Flue gas reheat 4. Acid-resistant lining or reheat Select the correct match.

Correct answer: A-2, B-1, C-4, D-3Stack tip downwash is cured by a higher exit velocity (2). Building downwash by applying the GEP height rule (1). Acid condensation by an acid-resistant lining or reheat (4). And the lost buoyancy after wet scrubbing by reheat (3), which restores the temperature difference the rise depends on.

Q: Match each quantity in List I with how it varies in List II. List I A. Maximum ground level concentration B. Exit velocity at constant flow C. Plume rise D. Distance to the maximum List II 1. As the inverse square of stack diameter 2. As the inverse square of effective height 3. Increases with effective height and stability 4. Inversely with wind speed Select the correct match.

Correct answer: A-2, B-1, C-4, D-3Maximum concentration varies as \(1/H_e^2\) (2). Exit velocity as \(1/d^2\) at constant flow (1). Plume rise inversely with wind speed (4). And the distance to the maximum increases with effective height and with stability (3).

91.

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.

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92.

Assertion (A): Multi-flue stacks are used where several units share one structure.

Reason (R): Each flue can then keep a high exit velocity even when its unit is at part load.

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

A multi-flue stack lets each unit keep its own flue at full velocity while others are shut down or at part load - which is precisely the situation that would otherwise bring on stack tip downwash in a single large flue. Sharing one windshield is the structural convenience; keeping the velocity up is the reason it works well.

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93.

Consider the following statements about tall stacks:

1. They reduce the maximum ground level concentration.

2. They reduce the mass of pollutant emitted.

3. Regulators normally limit the dispersion credit given for height.

Which of the statements given above is or are correct?

A.1, 2 and 3
B.1 and 3 only
C.1 and 2 only
D.2 and 3 only

Answer: B

Correct answer: 1 and 3 only

Statements 1 and 3 are correct: a tall stack does lower the maximum ground-level concentration, and regulators do cap the credit allowed for height. Statement 2 is false - the mass emitted is unchanged, which is exactly why the credit is capped.

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94.

Consider the following statements about plume rise:

1. Buoyancy dominates for a hot combustion plume.

2. Rise falls as wind speed increases.

3. Rise is independent of exit velocity.

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 hold: buoyancy dominates for a hot combustion plume, and rise falls as \(1/u\) with wind speed. Statement 3 is false - exit velocity supplies the momentum term \(v_s d/u\), and for a cold stack it is the only source of rise there is.

Discuss in Forum

95.

Consider the following statements about downwash:

1. Stack tip downwash follows from low exit velocity relative to wind speed.

2. Building downwash follows from the wake cavity around a structure.

3. Both are prevented by increasing the emission rate.

Which of the statements given above is or are correct?

A.1 and 3 only
B.2 and 3 only
C.1, 2 and 3
D.1 and 2 only

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 correctly describe the two mechanisms. Statement 3 is nonsense: the emission rate has no bearing on downwash, which is an aerodynamic effect - and raising it would simply increase the concentration that the downwash then brings to the ground.

Discuss in Forum

96.

Consider the following statements about stack design:

1. Exit velocity is normally fixed before the diameter is calculated.

2. The acid dew point may govern whether reheat is required.

3. Aviation limits never constrain stack height.

Which of the statements given above is or are correct?

A.1 and 2 only
B.1 and 3 only
C.2 and 3 only
D.1, 2 and 3

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the exit velocity is chosen first (from the wind speed and downwash rule) and the diameter follows from \(A = Q/v\); and the acid dew point may well decide whether reheat is needed. Statement 3 is false - aviation obstacle limits frequently cap stack height near an airfield.

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97.

Match each plume type in List I with its ground level effect in List II.

List I

A. Lofting

B. Fumigation

C. Looping

D. Fanning

List II

1. Intermittent high peaks close to the stack

2. Almost nothing reaches the ground

3. Very little nearby, but a risk far downwind

4. The whole accumulated plume arrives at once

Select the correct match.

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

Answer: B

Correct answer: A-2, B-4, C-1, D-3

Lofting keeps almost everything aloft (2). Fumigation brings the whole accumulated plume down at once as the inversion erodes (4). Looping gives intermittent high peaks close to the stack, as the plume is thrown up and down by large eddies (1). Fanning gives very little nearby but carries an undiluted plume far downwind (3).

Discuss in Forum

98.

Match each design variable in List I with its principal effect in List II.

List I

A. Exit velocity

B. Gas temperature

C. Stack diameter

D. Physical stack height

List II

1. Buoyant plume rise

2. Momentum rise and downwash avoidance

3. Effective height directly

4. Exit velocity at a given flow

Select the correct match.

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

Answer: C

Correct answer: A-2, B-1, C-4, D-3

Exit velocity gives momentum rise and avoids downwash (2). Gas temperature drives buoyant rise (1). Diameter fixes the exit velocity at a given flow (4). Physical height contributes to effective height directly (3), metre for metre.

Discuss in Forum

99.

Match each problem in List I with its usual remedy in List II.

List I

A. Stack tip downwash

B. Building downwash

C. Acid condensation in the stack

D. Lost buoyancy after wet scrubbing

List II

1. Good engineering practice stack height

2. Higher exit velocity

3. Flue gas reheat

4. Acid-resistant lining or reheat

Select the correct match.

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

Answer: D

Correct answer: A-2, B-1, C-4, D-3

Stack tip downwash is cured by a higher exit velocity (2). Building downwash by applying the GEP height rule (1). Acid condensation by an acid-resistant lining or reheat (4). And the lost buoyancy after wet scrubbing by reheat (3), which restores the temperature difference the rise depends on.

Discuss in Forum

100.

Match each quantity in List I with how it varies in List II.

List I

A. Maximum ground level concentration

B. Exit velocity at constant flow

C. Plume rise

D. Distance to the maximum

List II

1. As the inverse square of stack diameter

2. As the inverse square of effective height

3. Increases with effective height and stability

4. Inversely with wind speed

Select the correct match.

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

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Maximum concentration varies as \(1/H_e^2\) (2). Exit velocity as \(1/d^2\) at constant flow (1). Plume rise inversely with wind speed (4). And the distance to the maximum increases with effective height and with stability (3).

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