Plume Behaviour and Stack Design Questions and Answers

Answer: C

Correct answer: They reduce the mass of pollutant emitted

That is the false statement. A tall stack changes where the pollutant arrives, not how much leaves. The other three are true: it lowers the maximum ground-level concentration, moves that maximum further downwind, and is for exactly this reason given only limited credit by regulators.

Answer: D

Correct answer: Widening the stack at constant volumetric flow

Widening the stack lowers the exit velocity, since \(v = Q/A\) - which makes downwash more likely, not less. The other three all raise the velocity relative to the wind: increasing it directly, narrowing the bore, or concentrating the flow into fewer flues.

Answer: A

Correct answer: To reduce the mass of pollutant emitted

Reheat does nothing whatever to the mass emitted - it only warms the gas. Its three genuine purposes are all listed: to lift the gas above its acid dew point, to protect the lining from the acid that would otherwise condense, and to restore the buoyancy the scrubber removed.

Answer: B

Correct answer: Risky, since the lower exit velocity invites downwash

At constant flow a wider stack means a lower exit velocity. Once that falls below about 1.5 times the wind speed the plume is drawn into the stack’s wake, effective height collapses, and ground-level concentrations rise - a poor exchange for the fan power saved.

Answer: C

Correct answer: It places the release above the wake cavity

The GEP height is the height at which the release clears the recirculating wake generated by nearby buildings. It guarantees nothing about ambient standards, it is calculated from building dimensions rather than emissions, and it is not a maximum permitted height - it is a ceiling on the dispersion credit that may be claimed, which is a different thing.

Answer: D

Correct answer: Cooling the flue gas before it leaves

Plume rise comes from two sources: momentum, from the exit velocity, and buoyancy, from the temperature difference between the gas and the surrounding air. Cooling the gas removes the buoyant component and so reduces the rise - which is why heat recovery from a stack has to be weighed against the dispersion it costs. A lower wind speed helps because a strong wind bends the plume over before it can climb.

Answer: A

Correct answer: It decreases as the stack gets taller

That is the false one. A taller stack - or a greater effective height once plume rise is added - moves the point of maximum ground level concentration *further* downwind, because the plume must spread through a greater depth before it reaches the ground. The other three are correct: stability slows the spreading and pushes the maximum further out, so in stable air it may lie several kilometres away.

Answer: B

Correct answer: The pollutant's molecular formula

Dispersion is a physical process, and the equations contain the emission rate, the effective stack height, the wind speed and the atmospheric stability - not the identity of the molecule. The chemical formula matters for the concentration limit that must be met and for any reaction in the plume, but not for the sizing of the stack itself. Building heights, gas volume and design wind speed all enter directly.

Answer: C

Correct answer: A plume in the wake cavity reaches the ground

A building disturbs the flow around it and creates a recirculating cavity in its lee. A plume drawn into that cavity is mixed to the ground immediately, so the whole benefit of the stack is lost. The remedies work the other way from the option offered: *raising* the exit velocity and building the stack to about two and a half times the height of the nearby structure keep the plume clear of the wake.

Answer: D

Correct answer: The maximum goes as height to the minus two

The maximum ground level concentration varies inversely with the square of the effective stack height, so anything that adds to that height pays back twice over - doubling it quarters the peak concentration. Plume rise is often a substantial fraction of the total effective height, which is why exit velocity and gas temperature are designed as carefully as the physical stack itself.