An operator proposes to double stack height rather than install control equipment.
How should a regulator respond?
Answer: A
The maximum ground level concentration would indeed fall roughly fourfold, but the same mass is still released and simply spread more widely. Regulators therefore credit height only up to the good engineering practice value.
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A plume is observed rising strongly and dispersing upward, with monitoring showing almost nothing at ground level for several kilometres.
What condition is this?
Answer: B
Correct answer: Lofting
A plume that rises and disperses upward while almost nothing reaches the ground is lofting: a stable layer below blocks downward mixing, and unstable air above disperses it. It is the most favourable of the classical types, and typically occurs in the early evening.
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A designer must decide between a 2.4 m and a 1.8 m stack diameter for the same volumetric flow.
What is the trade-off?
Answer: C
Correct answer: Higher velocity, but a greater draught loss
At the same volumetric flow the narrower stack gives the higher exit velocity, since \(v = Q/A\) - which means more momentum rise and less risk of downwash. The cost is a pressure drop rising roughly with \(v^2\), so the fan works harder for the whole life of the plant. The diameter is a balance between the two.
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The plume being drawn into the low pressure region immediately behind a stack is called stack tip _____.
Answer: D
Correct answer: downwash
Stack tip downwash. The wind separates round the stack and leaves a low-pressure region immediately downwind; a plume leaving too slowly is drawn into it instead of rising. Keeping the exit velocity above about 1.5 times the wind speed is what avoids it.
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The rule setting stack height at the building height plus one and a half times the lesser dimension is the _____ practice height.
Answer: A
Correct answer: good engineering
The good engineering practice (GEP) stack height is \(H_b + 1.5L\), with \(L\) the lesser of the building’s height and projected width. It sets the height needed to clear the building wake - and is used as a ceiling on the dispersion credit a plant may claim.
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The plume behaviour in which a stable layer lies below the stack and unstable air above is called _____.
Answer: B
Correct answer: lofting
Lofting - stable below and unstable above. The plume disperses upward while the stable layer beneath prevents anything from mixing down, so ground-level concentrations are very low.
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The sum of the physical stack height and the plume rise is the _____ stack height.
Answer: C
Correct answer: effective
Effective stack height, \(H_e = h_s + \Delta h\). It is what actually governs dispersion, and it is why plume rise is worth calculating carefully: a metre of rise is worth exactly as much as a metre of steelwork.
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Condensation of sulphuric acid on a stack lining occurs when the flue gas falls below its acid _____ point.
Answer: D
Correct answer: dew
The acid dew point - typically 110 to 140°C for a sulphur-bearing fuel - is the temperature below which sulphuric acid condenses out of the flue gas. Below it, acid forms on the lining and attacks it, which is why the gas is either kept above that temperature or the stack is lined with an acid-resistant material.
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Effect of exit velocity on downwash and plume rise at a wind speed of 9 m/s.
| Exit velocity (m/s) | Ratio to wind speed | Downwash risk | Momentum rise |
|---|---|---|---|
| 6 | 0.67 | Severe | Very small |
| 9 | 1.00 | Likely | Small |
| 13.5 | 1.50 | Acceptable | Moderate |
| 20 | 2.22 | None | Good |
What minimum exit velocity should be specified for this site?
Answer: A
Correct answer: 13.5 m/s
The table shows the risk falling as the ratio rises, and it becomes acceptable at a ratio of 1.5 - that is \(1.5 \times 9 = 13.5\) m/s. Anything lower invites downwash; 20 m/s would be safer still but costs fan power for a benefit already obtained.
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Effective stack height and the resulting maximum ground level concentration.
| Physical height (m) | Plume rise (m) | Effective height (m) | Relative maximum |
|---|---|---|---|
| 80 | 0 | 80 | 1.00 |
| 80 | 30 | 110 | 0.53 |
| 80 | 55 | 135 | 0.35 |
| 120 | 55 | 175 | 0.21 |
What do the middle two rows show about plume rise?
Answer: B
Correct answer: 55 m of rise cuts the maximum by two thirds
Compare the first and third rows, both at 80 m physical height: the relative maximum falls from 1.00 to 0.35 - almost exactly the \((80/135)^2 = 0.35\) the inverse-square rule predicts. Fifty-five metres of plume rise is worth as much as fifty-five metres of stack.
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