Particulate Control Devices Questions and Answers

Answer: C

Correct answer: Only three

The first three are correct. The fourth is wrong: a precipitator is in fact good on submicrometre particulate, since electrical migration does not depend on particle mass in the way inertia does - it is the one to two micrometre range where its efficiency dips. What rules a precipitator out is dust resistivity outside the workable window, not fineness.

Answer: D

The first three follow directly from the governing relations. Raising the throat velocity of a venturi improves collection and raises the pressure drop together, since both come from the same expended contacting power.

Answer: A

Correct answer: Venturi scrubber

The venturi is the only wet device among the four: it injects water into a high velocity throat so that particles impact on the droplets, and the collected dust leaves as a slurry. Cyclone, fabric filter and precipitator all collect the dust dry, which means it can be handled and disposed of, or recovered as product, without any water treatment.

Answer: B

Correct answer: Electrostatic precipitator

A precipitator charges particles and drives them to a plate by an electric field, which is not an inertial mechanism at all - and that is precisely why it works on very fine particles, whose inertia is negligible. The cyclone uses centrifugal inertia, the settling chamber gravitational settling, and the venturi impaction on droplets; all three depend on the particle’s mass and all three fail as the particle gets small.

Answer: C

Correct answer: Increasing precipitator plate area

A precipitator’s pressure drop is very low and adding plate area changes it hardly at all - which is exactly why efficiency is improved that way rather than by driving the gas harder. The other three all raise the resistance the fan must overcome: a narrower cyclone body, a higher filtration velocity through the cake, and a higher venturi throat velocity, whose pressure drop rises with the square of it.

Answer: D

Correct answer: Migration velocity

Migration velocity is the speed at which a charged particle drifts towards the collecting plate, and it appears in the Deutsch-Anderson equation for precipitator efficiency. Air to cloth ratio, pulse jet cleaning and cake formation all belong to the fabric filter, whose whole operation is built around the cake and its periodic removal.

Answer: A

Correct answer: Raising the gas flow rate through it

The Deutsch-Anderson relation is

so raising \( Q \) reduces the exponent and the efficiency: the gas spends less time in the field and fewer particles reach the plates. The other three all help - more plate area and another field both raise \( A \), and sulphur trioxide conditioning brings a high resistivity dust back into the workable window.

Answer: B

Sulphur in the fuel produces the trioxide that conditions the dust surface, and removing it raises resistivity out of the workable window. Sulphur trioxide injection is the standard remedy and was introduced for exactly this reason.

Answer: C

The cake is a far finer filter than the weave beneath it, so removing it entirely leaves the fabric doing all the work for a time. Cleaning less aggressively or at a higher pressure drop set point restores the steadier performance.

Answer: D

Correct answer: A fabric filter, which would blind

A gas near its acid dew point is the one condition a fabric filter cannot tolerate: condensation within the cake produces a paste that no cleaning cycle will remove, and the bags are lost. A precipitator manages, a venturi scrubber is entirely at home with wet and near-saturated gas, and a cyclone is indifferent to it - though useless alone on submicrometre material, which is why the option pairs it with a filter downstream.