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Particulate Control Devices Questions and Answers

51.

Statement I: A venturi scrubber can collect fine particulate at low pressure drop.

Statement II: The contacting power principle relates wet scrubber efficiency to the power expended in contact.

In the light of the above statements, choose the correct answer.

Answer: C

Correct answer: Statement I is incorrect but Statement II is correct

Statement II is correct and it contradicts statement I. The contacting power principle holds that the efficiency of a wet scrubber depends chiefly on the power expended in bringing gas and liquid into contact, almost regardless of how that power is applied. Fine particle collection therefore requires a high pressure drop - a venturi runs at 5 to 20 kPa - and cannot be had cheaply.

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52.

Statement I: A cyclone with a smaller body achieves a smaller cut diameter.

Statement II: A smaller radius gives a greater centrifugal acceleration at the same tangential velocity.

In the light of the above statements, choose the correct answer.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Centrifugal acceleration is \( v^2/r \), so reducing the radius at the same tangential velocity increases the force driving particles to the wall, and smaller particles can be captured. Hence the multiclone: many small cyclones in parallel achieve a far smaller cut diameter than one large one. The penalty is a higher pressure drop.

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53.

Statement I: A wet scrubber solves the particulate problem outright.

Statement II: Collected dust leaves a scrubber as a slurry requiring thickening, dewatering and disposal.

In the light of the above statements, choose the correct answer.

Answer: A

Correct answer: Statement I is incorrect but Statement II is correct

Statement II is correct and it is precisely why statement I is wrong. A scrubber transfers the dust from the gas to the water; it does not destroy it. The resulting slurry must be thickened, dewatered and the solids disposed of, and the water treated or recirculated. The air problem has become a water and solid waste problem, which is the standing objection to wet collection.

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54.

Statement I: Fabric filters must be kept above the acid dew point of the gas.

Statement II: Condensation blinds the fabric with a paste that cleaning cannot remove.

In the light of the above statements, choose the correct answer.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second gives the consequence. If the gas falls below its acid dew point, sulphuric acid condenses within the cake and turns it into a paste that cleaning cannot dislodge - the bags are blinded, the pressure drop climbs and they must be replaced. The remedy is to keep the whole filter, including the hopper and casing, above the dew point by insulation and by heating during start-up and shutdown.

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55.

Arrange these devices in order of increasing ability to collect fine particulate:

1. Gravity settling chamber

2. Conventional cyclone

3. High efficiency cyclone

4. Fabric filter

Select the correct sequence.

Answer: C

The three inertial devices improve as the acceleration acting on the particle rises, and the filter then works by an entirely different mechanism. Capital cost and pressure drop generally rise along the same sequence, which is what makes the choice a real one.

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56.

Arrange the events in an electrostatic precipitator in order:

1. A corona discharge forms at the discharge electrode

2. Particles passing through the corona acquire charge

3. The electric field drives charged particles to the collecting plate

4. Rapping dislodges the collected layer into the hopper

Select the correct sequence.

Answer: D

Charging and collection are separate steps performed by the same field, and removal of the collected dust is a third. Rapping too gently leaves an insulating layer and too hard re-entrains the dust, so its timing is a matter of some care.

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57.

Arrange the steps of a fabric filter's operating cycle in order:

1. Clean fabric begins to collect dust

2. A cake forms and collection efficiency rises

3. Pressure drop climbs to the cleaning set point

4. Cleaning dislodges the cake and the cycle repeats

Select the correct sequence.

Answer: A

Efficiency is at its lowest immediately after cleaning and at its highest just before, which is the opposite of what intuition suggests. Setting the cleaning trigger too low strips the cake and keeps the filter in its least effective state.

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58.

Arrange these steps in selecting a particulate control device:

1. Characterise the gas stream and the particle size distribution

2. Establish the outlet concentration the standard requires

3. Identify the device types capable of that efficiency on that size range

4. Compare capital cost, pressure drop and effluent handling for each

Select the correct sequence.

Answer: B

Size distribution and gas condition together rule most options in or out before any cost is considered. Choosing a device before characterising the stream is how baghouses end up on gases above their fabric's temperature limit.

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59.

Arrange the causal chain of back corona in a precipitator:

1. A high resistivity dust layer builds on the collecting plate

2. The layer holds its charge rather than conducting it away

3. A reverse discharge forms within the layer

4. The effective field collapses and collection efficiency falls

Select the correct sequence.

Answer: C

Each step follows from the one before, and the whole chain is set off by a dust property rather than by any fault in the equipment. Sulphur trioxide conditioning attacks it at the second step by making the layer conductive.

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60.

Consider the following pairs of device and its collection mechanism:

Cyclone - Centrifugal inertia

Fabric filter - Sieving and interception on a dust cake

Electrostatic precipitator - Electrical migration to a plate

Venturi scrubber - Gravity settling of particles

How many of the pairs given above are correctly matched?

Answer: D

Correct answer: Only three

Cyclone, fabric filter and precipitator are correctly described. The fourth is wrong: a venturi scrubber collects by impaction, accelerating the gas through a throat so that particles cannot follow the streamlines around the injected water droplets and strike them instead. Gravity settling is the mechanism of the settling chamber, and it is far too weak to catch what a venturi is built for.

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