Plume Behaviour and Stack Design Questions and Answers

Q: The distance at which the maximum ground level concentration occurs increases with

Correct answer: Effective height and stabilityA plume released higher must spread further vertically before it touches the ground, so the maximum moves downwind as effective height increases - and further still in stable air, where the dispersion coefficients grow slowly with distance. Emission rate changes the magnitude of the maximum but not where it occurs.

Q: A stack's exit velocity is usually specified to be at least

Correct answer: 1.5 times the design wind speedBelow about 1.5 times the wind speed the plume is drawn into the low-pressure wake behind the stack - stack tip downwash - and the effective height collapses. The rule is a cheap insurance: it costs a little fan power and protects the whole benefit of the stack’s height.

Q: Reducing stack diameter at constant volumetric flow

Correct answer: Raises velocity but also draught lossAt constant volumetric flow, velocity is \(Q/A\), so a smaller bore means a higher exit velocity - more momentum rise and less risk of downwash. But the pressure loss through the stack rises roughly with the square of velocity, so the fan must work harder. The diameter is therefore a compromise between dispersion and running cost.

Q: Aviation constraints affect stack design chiefly by

Correct answer: Limiting height near airfieldsNear an airfield, obstacle limitation surfaces cap how tall a structure may be, and a stack is no exception - often forcing a shorter stack than dispersion would call for, with lighting and marking required as well. The remedy is usually higher exit velocity or a hotter gas to buy back some effective height.

Q: Wind speed generally increases with height according to

Correct answer: A power law in heightWind speed increases with height as\[ u_2 = u_1 \left(\frac{z_2}{z_1}\right)^{p} \]where the exponent \(p\) depends on surface roughness and stability - about 0.1 over open water in unstable air, and 0.4 or more over a city in stable air. It matters because the wind at stack height, not at the 10 m of a standard anemometer, is what governs plume rise and dilution.

Q: A plume that reaches the ground in a fumigation event does so because

Correct answer: The inversion beneath it erodesFumigation happens when the ground warms in the morning and the overnight inversion is eaten away from below. The plume, which had been trapped and concentrated above it, is suddenly mixed down to the surface in a body - giving the highest short-term ground-level concentrations of any plume type, and often lasting only half an hour.

Q: Corrosion in a stack lining is a particular concern when

Correct answer: Flue gas falls below its dew pointSulphur trioxide in the flue gas combines with water vapour, and below the acid dew point - typically 110 to 140°C for a sulphur-bearing fuel - sulphuric acid condenses on the lining and attacks it. Keeping the gas above that temperature, or lining the stack with an acid-resistant material, is what prevents it.

Q: A multi-flue stack is used chiefly to

Correct answer: Carry several sources in one structureA multi-flue stack puts several independent flues inside one windshield, so a plant with several boilers needs only one tall structure. The flues stay separate, so each unit can run at its own temperature and be taken out of service alone, and the plumes merge outside - which, being a larger combined buoyancy source, actually rises rather better.

11.

Tall stacks are best described as

A.A method of destroying pollutant thermally
B.A means of reducing emission rate
C.A dilution measure rather than a control measure
D.A control measure that removes pollutant

Answer: C

Correct answer: A dilution measure rather than a control measure

A tall stack removes nothing: the same mass of pollutant is emitted, and simply released where the wind has more room to dilute it before it reaches anyone. It is dispersion, not control - useful for meeting a local ground-level standard, and useless against a regional problem such as acid deposition, which is precisely the criticism that led to limits on stack height as a compliance strategy.

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12.

The distance at which the maximum ground level concentration occurs increases with

A.Emission rate
B.Stack diameter
C.The pollutant's density
D.Effective height and stability

Answer: D

Correct answer: Effective height and stability

A plume released higher must spread further vertically before it touches the ground, so the maximum moves downwind as effective height increases - and further still in stable air, where the dispersion coefficients grow slowly with distance. Emission rate changes the magnitude of the maximum but not where it occurs.

Discuss in Forum

13.

For a given emission, the maximum ground level concentration varies approximately as

A.The inverse square of effective stack height
B.The inverse of effective stack height
C.The square of effective stack height
D.Independently of effective stack height

Answer: A

Correct answer: The inverse square of effective stack height

From the Gaussian model the ground-level maximum goes as

\[ C_{max} \propto \frac{Q}{u H^2} \]

so doubling the effective height reduces the maximum by a factor of four. That steep dependence is what makes plume rise so valuable: every metre of rise counts the same as a metre of physical stack.

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14.

A stack's exit velocity is usually specified to be at least

A.Ten times the wind speed
B.1.5 times the design wind speed
C.Equal to the wind speed
D.Half the wind speed

Answer: B

Correct answer: 1.5 times the design wind speed

Below about 1.5 times the wind speed the plume is drawn into the low-pressure wake behind the stack - stack tip downwash - and the effective height collapses. The rule is a cheap insurance: it costs a little fan power and protects the whole benefit of the stack’s height.

Discuss in Forum

15.

Reducing stack diameter at constant volumetric flow

A.Has no effect on plume rise
B.Eliminates buoyancy rise
C.Raises velocity but also draught loss
D.Lowers exit velocity

Answer: C

Correct answer: Raises velocity but also draught loss

At constant volumetric flow, velocity is \(Q/A\), so a smaller bore means a higher exit velocity - more momentum rise and less risk of downwash. But the pressure loss through the stack rises roughly with the square of velocity, so the fan must work harder. The diameter is therefore a compromise between dispersion and running cost.

Discuss in Forum

16.

Aviation constraints affect stack design chiefly by

A.Requiring a particular exit velocity
B.Fixing the gas temperature
C.Determining the dispersion coefficients
D.Limiting height near airfields

Answer: D

Correct answer: Limiting height near airfields

Near an airfield, obstacle limitation surfaces cap how tall a structure may be, and a stack is no exception - often forcing a shorter stack than dispersion would call for, with lighting and marking required as well. The remedy is usually higher exit velocity or a hotter gas to buy back some effective height.

Discuss in Forum

17.

Wind speed generally increases with height according to

A.A power law in height
B.A linear relationship independent of terrain
C.An inverse square law
D.No systematic relationship

Answer: A

Correct answer: A power law in height

Wind speed increases with height as

\[ u_2 = u_1 \left(\frac{z_2}{z_1}\right)^{p} \]

where the exponent \(p\) depends on surface roughness and stability - about 0.1 over open water in unstable air, and 0.4 or more over a city in stable air. It matters because the wind at stack height, not at the 10 m of a standard anemometer, is what governs plume rise and dilution.

Discuss in Forum

18.

A plume that reaches the ground in a fumigation event does so because

A.Rain washes it out
B.The inversion beneath it erodes
C.The plume cools and sinks
D.The wind reverses direction

Answer: B

Correct answer: The inversion beneath it erodes

Fumigation happens when the ground warms in the morning and the overnight inversion is eaten away from below. The plume, which had been trapped and concentrated above it, is suddenly mixed down to the surface in a body - giving the highest short-term ground-level concentrations of any plume type, and often lasting only half an hour.

Discuss in Forum

19.

Corrosion in a stack lining is a particular concern when

A.The stack is very tall
B.The plume is buoyant
C.Flue gas falls below its dew point
D.Exit velocity is very high

Answer: C

Correct answer: Flue gas falls below its dew point

Sulphur trioxide in the flue gas combines with water vapour, and below the acid dew point - typically 110 to 140°C for a sulphur-bearing fuel - sulphuric acid condenses on the lining and attacks it. Keeping the gas above that temperature, or lining the stack with an acid-resistant material, is what prevents it.

Discuss in Forum

20.

A multi-flue stack is used chiefly to

A.Reduce the total emission
B.Eliminate the need for plume rise
C.Avoid aviation marking requirements
D.Carry several sources in one structure

Answer: D

Correct answer: Carry several sources in one structure

A multi-flue stack puts several independent flues inside one windshield, so a plant with several boilers needs only one tall structure. The flues stay separate, so each unit can run at its own temperature and be taken out of service alone, and the plumes merge outside - which, being a larger combined buoyancy source, actually rises rather better.

Discuss in Forum

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