Particulate Control Devices Questions and Answers

Answer: A

Correct answer: A is false but R is true

The reason is a correct statement of the Deutsch-Anderson relation, but the assertion is false. A precipitator’s pressure drop is very low, a few hundred pascals, because the gas simply passes slowly between the plates with nothing obstructing it, and adding plate area changes that hardly at all. This is precisely why efficiency is improved by enlarging the unit rather than by forcing the gas harder, and why precipitators are so cheap to run.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the cut diameter is the fifty per cent point, and a narrower body raises the centrifugal acceleration \( v^2/r \) and so lowers it. Statement 3 is the reverse - a narrower body means higher velocities and a higher pressure drop. That is the trade a multiclone makes: many small cyclones in parallel give a fine cut at a substantial cost in fan power.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the cake does the collecting and the air to cloth ratio is the sizing parameter. Statement 3 is the fabric filter’s worst case - condensation at the acid dew point blinds the bags irreversibly, and the filter and its casing must be kept above that temperature throughout, including during start-up and shutdown.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct. Statement 3 is the reverse of the truth: a precipitator has among the lowest pressure drops of any device, a few hundred pascals against several kilopascals for a fabric filter and up to twenty for a venturi. Low running cost is its principal attraction, and the resistivity window is its principal limitation.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: efficiency follows the contacting power expended, and water handles hot and sticky gases that would destroy a baghouse. Statement 3 is exactly wrong - the scrubber produces a slurry that must be thickened, dewatered and disposed of, and that effluent is the price paid for its tolerance of difficult gas.

Answer: B

Each device is sized and compared on its own parameter, and none of the four transfers to another device. Knowing which parameter belongs where is the quickest way to recognise a question that has muddled two devices together.

Answer: C

Correct answer: A-2, B-4, C-1, D-3

Each device has one characteristic weakness. A settling chamber is useless on fine particulate, since Stokes velocity falls with the square of diameter. A fabric filter is damaged by heat above the fabric limit and blinded by condensation. A precipitator depends on the dust resistivity lying within a workable window. A wet scrubber transfers the problem to a slurry that must be treated.

Answer: D

Three of these mechanisms depend on the particle's mass and one does not, which is why the precipitator handles the fine fraction so much better. Understanding the mechanism predicts the efficiency curve against particle size.

Answer: A

Each fault has a specific and well established remedy, and applying the wrong one wastes money without improving anything. Diagnosing which problem is present is the whole of the work in most cases of poor performance.

Answer: B

Correct answer: Electrostatic precipitator

The precipitator charges the particles and moves them to a plate by an electric field, which does not depend on particle inertia - and that is why it works on very fine material. The cyclone, the settling chamber and the venturi scrubber all rely on the particle’s failure to follow a change of direction in the gas, whether around a bend, downward under gravity, or past a droplet.