Plume Behaviour and Stack Design Questions and Answers

Q: Consider the following pairs of quantity and how it varies: Maximum ground level concentration - As the inverse square of effective height Exit velocity - As the inverse square of stack diameter at constant flow Plume rise - Inversely with wind speed Distance to the maximum - Inversely with stack height How many of the pairs given above are correctly matched?

Correct answer: Only threeThree hold: the maximum goes as \(1/H^2\); exit velocity goes as \(1/d^2\) at constant flow; and plume rise goes as \(1/u\). The fourth is the wrong way round - the distance to the maximum increases with stack height, because a plume released higher must spread further before it reaches the ground.

Q: Consider the following pairs of constraint and its origin: Obstacle limitation height limit - Aviation requirements near an airfield Minimum exit velocity - Downwash avoidance Acid-resistant lining - Gas below its dew point Multi-flue construction - Reducing the total mass emitted How many of the pairs given above are correctly matched?

Correct answer: Only threeThree are correctly traced: the height limit comes from aviation obstacle rules, the minimum exit velocity from the need to avoid downwash, and the acid-resistant lining from the gas being below its dew point. The fourth is wrong - a multi-flue stack exists to carry several sources in one structure, and reduces no emissions whatever.

Q: Three of the following raise the effective stack height. Which lowers it? Stack tip downwash Higher gas temperature Higher exit velocity Greater physical stack height

Correct answer: Stack tip downwashDownwash is the only one that lowers effective height: the plume is drawn into the stack’s wake instead of rising, so the effective height falls towards the physical height or below. A hotter gas, a faster jet and a taller stack all raise it.

Q: Three of the following are classical plume types. Which is a mechanism rather than a type? Building downwash Coning Fanning Lofting

Correct answer: Building downwashThe classical plume types - looping, coning, fanning, lofting, fumigation and trapping - are named for the stability profile in which they occur. Building downwash is not one of them: it is an aerodynamic effect of an obstacle in the flow, and it can happen in any stability class.

Q: Three of the following reduce ground level concentration. Which does not? Reducing exit velocity to save fan power Raising the physical stack height Reducing the emission rate Raising the gas exit temperature

Correct answer: Reducing exit velocity to save fan powerThe other three all help: a taller stack and a hotter gas both raise the effective height, and a lower emission rate reduces the concentration in proportion. Reducing exit velocity does the opposite - it cuts momentum rise and invites stack tip downwash, so the fan power saved is paid for in higher ground-level concentrations.

Q: Three of the following are stack design constraints. Which is a meteorological input? Pasquill stability class Aviation height limits Structural wind loading Acid dew point of the flue gas

Correct answer: Pasquill stability classThe Pasquill stability class describes the state of the atmosphere - it is an input from meteorology, not something the designer controls. Aviation limits, wind loading and the acid dew point are all constraints the design must satisfy.

Q: Three of the following make a plume reach the ground close to the source. Which keeps it aloft? Lofting Building downwash Stack tip downwash Looping

Correct answer: LoftingLofting keeps the plume aloft: the stable layer beneath it blocks downward mixing while the unstable air above disperses it. Looping brings intermittent high concentrations to the ground near the stack, and both forms of downwash pull the plume down at the source.

Q: A new stack is to be built beside a 35 m building of 25 m projected width. What height does the good engineering practice rule require?

Correct answer: 72.5 mThe lesser of the two dimensions is the width, 25 m:\[ H_{GEP} = 35 + 1.5 \times 25 = 72.5\ \text{m} \]Using the height as the lesser dimension would have given 87.5 m - the distractor.

51.

Consider the following pairs of quantity and how it varies:

Maximum ground level concentration - As the inverse square of effective height

Exit velocity - As the inverse square of stack diameter at constant flow

Plume rise - Inversely with wind speed

Distance to the maximum - Inversely with stack height

How many of the pairs given above are correctly matched?

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

Answer: C

Correct answer: Only three

Three hold: the maximum goes as \(1/H^2\); exit velocity goes as \(1/d^2\) at constant flow; and plume rise goes as \(1/u\). The fourth is the wrong way round - the distance to the maximum increases with stack height, because a plume released higher must spread further before it reaches the ground.

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52.

Consider the following pairs of constraint and its origin:

Obstacle limitation height limit - Aviation requirements near an airfield

Minimum exit velocity - Downwash avoidance

Acid-resistant lining - Gas below its dew point

Multi-flue construction - Reducing the total mass emitted

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 are correctly traced: the height limit comes from aviation obstacle rules, the minimum exit velocity from the need to avoid downwash, and the acid-resistant lining from the gas being below its dew point. The fourth is wrong - a multi-flue stack exists to carry several sources in one structure, and reduces no emissions whatever.

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53.

Three of the following raise the effective stack height. Which lowers it?

Stack tip downwash

Higher gas temperature

Higher exit velocity

Greater physical stack height

A.Stack tip downwash
B.Higher gas temperature
C.Higher exit velocity
D.Greater physical stack height

Answer: A

Correct answer: Stack tip downwash

Downwash is the only one that lowers effective height: the plume is drawn into the stack’s wake instead of rising, so the effective height falls towards the physical height or below. A hotter gas, a faster jet and a taller stack all raise it.

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54.

Three of the following are classical plume types. Which is a mechanism rather than a type?

Building downwash

Coning

Fanning

Lofting

A.Lofting
B.Building downwash
C.Coning
D.Fanning

Answer: B

Correct answer: Building downwash

The classical plume types - looping, coning, fanning, lofting, fumigation and trapping - are named for the stability profile in which they occur. Building downwash is not one of them: it is an aerodynamic effect of an obstacle in the flow, and it can happen in any stability class.

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55.

Three of the following reduce ground level concentration. Which does not?

Reducing exit velocity to save fan power

Raising the physical stack height

Reducing the emission rate

Raising the gas exit temperature

A.Reducing the emission rate
B.Raising the gas exit temperature
C.Reducing exit velocity to save fan power
D.Raising the physical stack height

Answer: C

Correct answer: Reducing exit velocity to save fan power

The other three all help: a taller stack and a hotter gas both raise the effective height, and a lower emission rate reduces the concentration in proportion. Reducing exit velocity does the opposite - it cuts momentum rise and invites stack tip downwash, so the fan power saved is paid for in higher ground-level concentrations.

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56.

Three of the following are stack design constraints. Which is a meteorological input?

Pasquill stability class

Aviation height limits

Structural wind loading

Acid dew point of the flue gas

A.Aviation height limits
B.Structural wind loading
C.Acid dew point of the flue gas
D.Pasquill stability class

Answer: D

Correct answer: Pasquill stability class

The Pasquill stability class describes the state of the atmosphere - it is an input from meteorology, not something the designer controls. Aviation limits, wind loading and the acid dew point are all constraints the design must satisfy.

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57.

Three of the following make a plume reach the ground close to the source. Which keeps it aloft?

Lofting

Building downwash

Stack tip downwash

Looping

A.Lofting
B.Building downwash
C.Stack tip downwash
D.Looping

Answer: A

Correct answer: Lofting

Lofting keeps the plume aloft: the stable layer beneath it blocks downward mixing while the unstable air above disperses it. Looping brings intermittent high concentrations to the ground near the stack, and both forms of downwash pull the plume down at the source.

Discuss in Forum

58.

A boiler runs at part load and monitoring shows a sharp rise in nearby ground level concentrations, although the emission rate has fallen.

What is the most likely explanation?

A.The pollutant has become denser
B.Reduced flow has brought on stack tip downwash
C.The stack has become shorter
D.The wind has dropped

Answer: B

Correct answer: Reduced flow has brought on stack tip downwash

At part load the volumetric flow falls, and with it the exit velocity. Once it drops below about 1.5 times the wind speed the plume is drawn into the stack’s wake and the effective height collapses - so ground-level concentrations rise even though less pollutant is being emitted. The usual remedies are to run fewer flues at full flow, or to fit a variable-area exit cone.

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59.

A new stack is to be built beside a 35 m building of 25 m projected width.

What height does the good engineering practice rule require?

A.87.5 m
B.35 m
C.72.5 m
D.52.5 m

Answer: C

Correct answer: 72.5 m

The lesser of the two dimensions is the width, 25 m:

\[ H_{GEP} = 35 + 1.5 \times 25 = 72.5\ \text{m} \]

Using the height as the lesser dimension would have given 87.5 m - the distractor.

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60.

A plant installs a wet scrubber and afterwards finds both corrosion in the stack and higher ground level concentrations.

What has happened?

A.The scrubber has increased the emission rate
B.The stack has become too tall
C.The wind pattern has changed
D.Cooled gas has lost buoyancy and condenses acid

Answer: D

Correct answer: Cooled gas has lost buoyancy and condenses acid

A wet scrubber cools the gas sharply, and two things follow at once. The gas falls below its acid dew point, so sulphuric acid condenses on the lining and corrodes it; and it has lost almost all its buoyancy, so plume rise collapses and ground-level concentrations rise. Both are cured by the same measure - reheating the gas after the scrubber.

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