Particulate Control Devices Questions and Answers

Q: Consider the following pairs of gas condition and the device it rules out: Temperature above the fabric limit - Fabric filter Gas near its acid dew point - Fabric filter Difficulty in handling aqueous effluent - Wet scrubber Fine particulate below one micrometre - Electrostatic precipitator How many of the pairs given above are correctly matched?

Correct answer: Only threeThe 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.

Q: Three of the following are dry collection devices. Which is wet? Venturi scrubber Cyclone Fabric filter Electrostatic precipitator

Correct answer: Venturi scrubberThe 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.

Q: Three of the following collect by inertia. Which does not? Electrostatic precipitator Cyclone Gravity settling chamber Venturi scrubber

Correct answer: Electrostatic precipitatorA 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.

Q: Three of the following raise fan power. Which does not? Increasing precipitator plate area Narrowing a cyclone body Raising the air to cloth ratio Raising the venturi throat velocity

Correct answer: Increasing precipitator plate areaA 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.

Q: Three of the following describe a fabric filter. Which describes a precipitator? Migration velocity Air to cloth ratio Pulse jet cleaning Cake formation

Correct answer: Migration velocityMigration 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.

Q: Three of the following would improve a struggling precipitator. Which would not? Raising the gas flow rate through it Adding another field in series Conditioning the gas with sulphur trioxide Increasing the plate area

Correct answer: Raising the gas flow rate through itThe Deutsch-Anderson relation is\[ \eta = 1 - e^{-wA/Q} \]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.

Q: A plant must remove submicrometre particulate from a hot gas near its acid dew point. Which device is least suitable?

Correct answer: A fabric filter, which would blindA 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.

51.

Consider the following pairs of gas condition and the device it rules out:

Temperature above the fabric limit - Fabric filter

Gas near its acid dew point - Fabric filter

Difficulty in handling aqueous effluent - Wet scrubber

Fine particulate below one micrometre - Electrostatic precipitator

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

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.

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52.

Consider the following pairs of design change and its effect:

Smaller cyclone body - Smaller cut diameter and greater pressure drop

Greater precipitator plate area - Higher efficiency at the same flow

Higher air to cloth ratio - Higher pressure drop and shorter bag life

Higher venturi throat velocity - Lower pressure drop and better collection

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

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.

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53.

Three of the following are dry collection devices. Which is wet?

Venturi scrubber

Cyclone

Fabric filter

Electrostatic precipitator

A.Venturi scrubber
B.Cyclone
C.Fabric filter
D.Electrostatic precipitator

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.

Discuss in Forum

54.

Three of the following collect by inertia. Which does not?

Electrostatic precipitator

Cyclone

Gravity settling chamber

Venturi scrubber

A.Venturi scrubber
B.Electrostatic precipitator
C.Cyclone
D.Gravity settling chamber

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.

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55.

Three of the following raise fan power. Which does not?

Increasing precipitator plate area

Narrowing a cyclone body

Raising the air to cloth ratio

Raising the venturi throat velocity

A.Raising the air to cloth ratio
B.Raising the venturi throat velocity
C.Increasing precipitator plate area
D.Narrowing a cyclone body

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.

Discuss in Forum

56.

Three of the following describe a fabric filter. Which describes a precipitator?

Migration velocity

Air to cloth ratio

Pulse jet cleaning

Cake formation

A.Air to cloth ratio
B.Pulse jet cleaning
C.Cake formation
D.Migration velocity

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.

Discuss in Forum

57.

Three of the following would improve a struggling precipitator. Which would not?

Raising the gas flow rate through it

Adding another field in series

Conditioning the gas with sulphur trioxide

Increasing the plate area

A.Raising the gas flow rate through it
B.Adding another field in series
C.Conditioning the gas with sulphur trioxide
D.Increasing the plate area

Answer: A

Correct answer: Raising the gas flow rate through it

The Deutsch-Anderson relation is

\[ \eta = 1 - e^{-wA/Q} \]

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.

Discuss in Forum

58.

A power station switches to a low sulphur coal and its precipitator's efficiency falls sharply although nothing else has changed.

What is the most likely cause?

A.The rapping frequency is too high
B.The dust resistivity has risen
C.The plate area has become inadequate
D.The gas flow has increased

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.

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59.

A baghouse shows a sudden drop in efficiency immediately after each cleaning pulse, recovering within minutes.

What does this indicate?

A.The air to cloth ratio is too low
B.The gas is below its dew point
C.Cleaning is stripping the dust cake
D.The bags are torn

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.

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60.

A plant must remove submicrometre particulate from a hot gas near its acid dew point.

Which device is least suitable?

A.An electrostatic precipitator
B.A venturi scrubber
C.A high efficiency cyclone with a filter downstream
D.A fabric filter, which would blind

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.

Discuss in Forum

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