A stack passes \( 62 \) m\(^3\)/s at \( 18 \) m/s. Its diameter is
Answer: C
Correct answer: 2.09 m
\[ A = \frac{Q}{v} = \frac{62}{18} = 3.44\ \text{m}^2 \]\[ d = \sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4 \times 3.44}{\pi}} = \sqrt{4.38} = 2.09\ \text{m} \]
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A plume rise of \( 45 \) m is calculated at a wind speed of \( 4 \) m/s. If rise varies inversely with wind speed, at \( 8 \) m/s it becomes
Answer: D
Correct answer: 22.5 m
Plume rise varies inversely with wind speed, so doubling the wind halves the rise:
\[ \Delta h_2 = 45 \times \frac{4}{8} = 22.5\ \text{m} \]Physically, a stronger wind bends the plume over and carries it downwind before buoyancy can lift it.
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The maximum ground level concentration occurs at \( 1.2 \) km with an effective height of \( 90 \) m. Raising the effective height to \( 135 \) m moves it, other things equal, to roughly
Answer: A
Correct answer: A greater distance downwind
A plume released higher must spread further vertically before its edge reaches the ground, and the vertical dispersion coefficient grows with distance - so the point of maximum ground-level concentration moves further downwind, and the maximum itself is lower.
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A building \( 24 \) m high and \( 60 \) m wide requires a good engineering practice stack height of
Answer: B
Correct answer: 60 m
Here the lesser dimension is the height, 24 m, not the 60 m width:
\[ H_{GEP} = 24 + 1.5 \times 24 = 60\ \text{m} \]The 60 m width is the distractor - it is the larger dimension and plays no part.
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Statement I: A tall stack reduces the total mass of pollutant released.
Statement II: A tall stack releases the same mass where it will be diluted more before reaching anyone.
In the light of the above statements, choose the correct answer.
Answer: C
Correct answer: Statement I is incorrect but Statement II is correct
A tall stack changes where the pollutant goes, not how much is released - the mass emitted is exactly the same. What it buys is more atmosphere between the release and the nearest receptor, so the material is diluted further before anyone breathes it. That is why tall stacks relieve local concentrations while doing nothing for regional problems such as acid deposition.
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Statement I: Exit velocity is specified above about one and a half times the wind speed.
Statement II: Below that ratio the plume is drawn into the low pressure wake immediately behind the stack.
In the light of the above statements, choose the correct answer.
Answer: D
Correct answer: Both Statement I and Statement II are correct
Statement II explains statement I. The wind separates round the stack and leaves a low-pressure wake immediately downwind; a plume leaving too slowly is drawn into it instead of rising, and the effective height collapses. Requiring an exit velocity of about 1.5 times the wind speed keeps the jet clear of that wake.
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Statement I: Lofting is the most favourable of the classical plume types.
Statement II: A stable layer beneath the plume prevents downward mixing while unstable air above disperses it.
In the light of the above statements, choose the correct answer.
Answer: A
Correct answer: Both Statement I and Statement II are correct
Statement II is the reason for statement I. In lofting the stable layer beneath the plume blocks downward mixing while the unstable air above disperses it upward, so very little ever reaches the ground - which makes it the most favourable of the classical plume types.
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Statement I: Reheating flue gas after a wet scrubber is done purely to raise plume rise.
Statement II: Cooled flue gas may fall below its acid dew point and condense sulphuric acid in the stack.
In the light of the above statements, choose the correct answer.
Answer: B
Correct answer: Statement I is incorrect but Statement II is correct
Reheat is done chiefly to keep the gas above its acid dew point, so that sulphuric acid does not condense in the stack and corrode it. Restoring some buoyant plume rise is a welcome by-product, not the purpose - and it would rarely justify the fuel cost on its own.
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Statement I: Plume rise falls as wind speed increases.
Statement II: A stronger wind bends the plume over and carries it away before it can rise.
In the light of the above statements, choose the correct answer.
Answer: C
Correct answer: Both Statement I and Statement II are correct
Statement II gives the mechanism for statement I. A stronger wind bends the plume over and carries it downwind before buoyancy has time to lift it, so rise falls roughly as \(1/u\). This is why the worst ground-level concentrations often occur in moderate rather than light winds: the loss of plume rise outweighs the extra dilution.
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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.
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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