Particulate Control Devices Questions and Answers

Answer: C

A single tube of that diameter would have to run at a very high velocity to pass the whole flow, but sharing the flow between many keeps each one moderate. Uneven distribution between the tubes is the usual reason such a unit underperforms.

Answer: D

Correct answer: The wet scrubber, at the price of an effluent to treat

Reading across the table, the scrubber is the only device suitable in every difficult condition - high temperature, near the acid dew point, and sticky particulate - because water handles all three. Its single unsuitable entry is the one where an aqueous effluent is hard to treat, which is precisely the cost of that tolerance: the air problem has been converted into a water and solids problem.

Answer: A

The more effectively the cake is removed, the higher the velocity the fabric can sustain before the pressure drop becomes unacceptable. The price is paid in mechanical wear, which shows up directly as shorter bag life.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Since Stokes velocity varies as \( d^2 \), a particle of a tenth the diameter settles a hundred times more slowly, so a chamber sized to catch fine material would be impossibly long. The chamber therefore takes out the coarse fraction cheaply and at almost no pressure drop, and something else - a filter, a precipitator, a scrubber - deals with the rest.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the effect. The cake, not the fabric, is the filter, so removing it removes most of the collecting mechanism and the emission rises briefly until a new cake has formed. This is why cleaning is done compartment by compartment rather than all at once, why the cleaning is deliberately incomplete, and why a filter’s emission trace shows a spike at each pulse.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason accounts for the arrangement. Because efficiency follows an exponential in plate area, each field removes a similar proportion of what reaches it - so if the first takes 90 per cent, the second takes 90 per cent of the remaining tenth, and so on. Three or four fields in series therefore reach 99.5 per cent and above, and the fields are separately energised so that one can be taken out for maintenance.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Low sulphur coal produces a dust of high resistivity, above the workable window, and back corona follows. Injecting sulphur trioxide upstream lays down a thin conductive film of adsorbed acid on the particle surfaces, which lowers the apparent resistivity into the workable range and restores collection. It is a standard retrofit where a plant has switched to a lower sulphur fuel.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. The contacting power principle holds that fine particle collection in a wet device is bought with the power expended in contacting gas and liquid, and in a venturi that power appears as a pressure drop of 5 to 20 kPa. The fan bill that follows - hundreds of kilowatts on a large unit - is what makes the venturi the most expensive of the common devices to run.

Answer: C

Correct answer: A is false but R is true

The reason is true and is exactly why the assertion fails. A scrubber transfers the dust from the gas into the water rather than destroying it, and what leaves is a slurry that must be thickened, dewatered and disposed of, with the water treated or returned. The particulate problem has been moved, not removed - a real consideration wherever effluent treatment is difficult or expensive.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Centrifugal separation depends on the particle’s inertia, which falls with the cube of its diameter, so below a few micrometres the force available is negligible and the particle follows the gas round the bend. A cyclone’s cut diameter is five to ten micrometres, well above the fine fraction a PM2.5 standard concerns, so it serves as a pre-cleaner rather than a final device.