Particulate Control Devices Questions and Answers

Answer: C

Correct answer: Efficiency is at its highest immediately after cleaning

This is the false statement and it inverts the mechanism. Cleaning removes the dust cake, which is what does the collecting, so efficiency is at its lowest just afterwards and recovers as the cake rebuilds. The other three are all true: the ratio is the sizing parameter, condensation blinds the fabric irreversibly, and the cake rather than the weave is the filter.

Answer: D

A higher flow shortens the residence time and moves the Deutsch exponent the wrong way. The other three each raise the product of area and migration velocity relative to the flow, which is what the equation depends on.

Answer: A

Correct answer: It is damaged by hot or sticky particulate

Tolerance of hot, sticky and near-saturated gas is a scrubber’s chief advantage, and precisely the condition under which it is chosen over a baghouse. The other three are genuine disadvantages: a high pressure drop, the fan power that follows from it, and the slurry that must be thickened, dewatered and disposed of.

Answer: B

Correct answer: Unworkable: settling velocity is far too low

A 2.5 micrometre particle settles at a fraction of a millimetre per second, so a chamber long enough for it to reach the floor before the gas leaves would be measured in hundreds of metres. Lowering the gas velocity helps only in proportion, and no practical length can close the gap. A settling chamber is a pre-cleaner for material above about fifty micrometres, and nothing else.

Answer: C

Both effects follow from the higher centrifugal acceleration a smaller radius produces at the same tangential velocity. Multiclones exist to obtain the small cut diameter without the pressure drop of a single narrow body at full flow.

Answer: D

Correct answer: An unusually low pressure drop across the unit

A precipitator’s pressure drop is low by design and is not an indicator of its collection performance at all - the gas passes between plates with nothing obstructing it whether the fields are working or not. The other three are all genuine causes of poor performance: resistivity above the window brings back corona, excess gas flow reduces the residence time in the field, and over-vigorous rapping re-entrains the layer that has just been collected.

Answer: A

Adding efficiencies would give figures above one hundred per cent for any reasonable pair. Multiplying the penetrations is the correct treatment and it makes the diminishing returns of further stages immediately clear.

Answer: B

Correct answer: The colour of the collected dust

Colour has no bearing on how a particle is captured. What decides the selection is the particle size distribution, which determines whether an inertial device can work at all; the gas temperature and dew point, which decide whether a fabric filter is possible; the dust resistivity, which decides whether a precipitator is; and whether an aqueous effluent can be handled, which decides whether a wet device is acceptable.

Answer: C

The generality of the result is what makes it useful, since it says that no clever geometry escapes the energy requirement. Fine particle collection in a wet device must be paid for in pressure drop whatever the arrangement.

Answer: D

The pre-cleaner's job is to protect the expensive device rather than to meet the outlet standard itself. Removing the coarse fraction cuts both the mechanical wear and the frequency with which the cake must be dislodged.