Particulate Control Devices Questions and Answers

Q: Assertion (A): A wet scrubber removes the particulate problem entirely. Reason (R): The collected dust leaves the scrubber as a slurry requiring thickening and disposal. Select the correct answer from the options below.

Correct answer: A is false but R is trueThe 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.

71.

Cut diameter and pressure drop for four cyclone configurations.

Configuration	Body diameter (m)	Cut diameter (micrometres)	Pressure drop (kPa)
Large conventional	1.6	18	0.6
Conventional	1.0	12	0.9
High efficiency	0.6	8	1.9
Multiclone tubes	0.25	5	1.5

Why does the multiclone achieve a small cut diameter without the highest pressure drop?

A.It uses a fabric insert
B.It operates at a lower flow
C.Many parallel tubes keep each velocity moderate
D.Its tubes are longer

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.

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72.

Gas conditions and their effect on device selection.

Condition	Fabric filter	Precipitator	Wet scrubber
Gas at 250 degrees	Special fabric needed	Suitable	Suitable
Gas near acid dew point	Unsuitable	Marginal	Suitable
Sticky particulate	Unsuitable	Marginal	Suitable
Aqueous effluent hard to treat	Suitable	Suitable	Unsuitable

Which device is most tolerant of difficult gas conditions, and what does it cost?

A.The fabric filter, at the price of pressure drop
B.The precipitator, at the price of resistivity problems
C.All three are equally tolerant
D.The wet scrubber, at the price of an effluent to treat

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.

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73.

Air to cloth ratio by cleaning method.

Cleaning method	Typical air to cloth ratio (m/min)	Relative bag life
Shaker	0.8	Long
Reverse air	0.6	Longest
Pulse jet	2.0	Shorter
Pulse jet, abrasive dust	1.4	Short

Why does pulse jet cleaning permit the highest air to cloth ratio?

A.Aggressive cleaning sustains a higher velocity
B.Its bags are made of stronger material
C.It operates at a lower pressure drop
D.It collects a coarser dust

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.

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74.

Assertion (A): Gravity settling chambers are used only as pre-cleaners.

Reason (R): Stokes settling velocity varies with the square of the particle diameter.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

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.

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75.

Assertion (A): A fabric filter's efficiency is lowest just after cleaning.

Reason (R): The dust cake, which does most of the collecting, has just been removed.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

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.

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76.

Assertion (A): Precipitators are built with several fields in series.

Reason (R): Efficiency follows an exponential in plate area, so each field removes a similar fraction of what reaches it.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

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.

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77.

Assertion (A): Sulphur trioxide conditioning restores a precipitator's performance on low sulphur coal.

Reason (R): A conductive film on the particle surface brings the dust resistivity back into the workable window.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

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.

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78.

Assertion (A): A venturi scrubber has the highest running cost of the common devices.

Reason (R): Fine particle collection in a wet device is obtained only by expending contacting power.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

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.

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79.

Assertion (A): A wet scrubber removes the particulate problem entirely.

Reason (R): The collected dust leaves the scrubber as a slurry requiring thickening and disposal.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

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.

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80.

Assertion (A): Cyclones cannot meet a modern fine particulate standard alone.

Reason (R): Inertial separation depends on particle mass, which becomes negligible in the fine fraction.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

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.

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