Particulate Control Devices Questions and Answers

Q: Assertion (A): Adding plate area to a precipitator raises its pressure drop substantially. Reason (R): Efficiency follows an exponential in plate area divided by gas flow. Select the correct answer from the options below.

Correct answer: A is false but R is trueThe 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.

Q: Consider the following statements about cyclones: 1. Cut diameter is the size collected with fifty per cent efficiency. 2. A narrower body gives a smaller cut diameter. 3. A narrower body also gives a lower pressure drop. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 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.

Q: Consider the following statements about fabric filters: 1. The dust cake does most of the collecting. 2. Air to cloth ratio is the primary sizing parameter. 3. They are well suited to gases near their acid dew point. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 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.

Q: Consider the following statements about electrostatic precipitators: 1. Efficiency follows the Deutsch-Anderson exponential in plate area. 2. Dust resistivity must lie within a workable window. 3. Their pressure drop is among the highest of the common devices. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 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.

Q: Consider the following statements about wet scrubbers: 1. The contacting power principle relates efficiency to power expended. 2. They handle sticky and hot gases that would defeat a baghouse. 3. They avoid producing any effluent requiring treatment. Which of the statements given above is or are correct?

Correct answer: 1 and 2 onlyStatements 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.

Q: Match each device in List I with its principal limitation in List II. List I A. Gravity settling chamber B. Fabric filter C. Electrostatic precipitator D. Wet scrubber List II 1. Sensitive to dust resistivity 2. Useless on fine particulate 3. Produces a slurry effluent 4. Damaged by heat and by condensation Select the correct match.

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

Q: Which of the following is NOT an inertial collection device?

Correct answer: Electrostatic precipitatorThe 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.

81.

Assertion (A): Adding plate area to a precipitator raises its pressure drop substantially.

Reason (R): Efficiency follows an exponential in plate area divided by gas flow.

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: 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.

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82.

Consider the following statements about cyclones:

1. Cut diameter is the size collected with fifty per cent efficiency.

2. A narrower body gives a smaller cut diameter.

3. A narrower body also gives a lower pressure drop.

Which of the statements given above is or are correct?

A.1, 2 and 3
B.1 and 2 only
C.1 and 3 only
D.2 and 3 only

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.

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83.

Consider the following statements about fabric filters:

1. The dust cake does most of the collecting.

2. Air to cloth ratio is the primary sizing parameter.

3. They are well suited to gases near their acid dew point.

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

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.

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84.

Consider the following statements about electrostatic precipitators:

1. Efficiency follows the Deutsch-Anderson exponential in plate area.

2. Dust resistivity must lie within a workable window.

3. Their pressure drop is among the highest of the common devices.

Which of the statements given above is or are correct?

A.1 and 3 only
B.2 and 3 only
C.1, 2 and 3
D.1 and 2 only

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.

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85.

Consider the following statements about wet scrubbers:

1. The contacting power principle relates efficiency to power expended.

2. They handle sticky and hot gases that would defeat a baghouse.

3. They avoid producing any effluent requiring treatment.

Which of the statements given above is or are correct?

A.1 and 2 only
B.1 and 3 only
C.2 and 3 only
D.1, 2 and 3

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.

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86.

Match each device in List I with its characteristic parameter in List II.

List I

A. Cyclone

B. Fabric filter

C. Electrostatic precipitator

D. Venturi scrubber

List II

1. Air to cloth ratio

2. Cut diameter

3. Throat velocity and liquid to gas ratio

4. Migration velocity

Select the correct match.

A.A-3, B-1, C-4, D-2
B.A-2, B-1, C-4, D-3
C.A-1, B-2, C-4, D-3
D.A-2, B-4, C-1, D-3

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.

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87.

Match each device in List I with its principal limitation in List II.

List I

A. Gravity settling chamber

B. Fabric filter

C. Electrostatic precipitator

D. Wet scrubber

List II

1. Sensitive to dust resistivity

2. Useless on fine particulate

3. Produces a slurry effluent

4. Damaged by heat and by condensation

Select the correct match.

A.A-2, B-1, C-4, D-3
B.A-3, B-1, C-4, D-2
C.A-2, B-4, C-1, D-3
D.A-1, B-2, C-4, D-3

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.

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88.

Match each mechanism in List I with the device that uses it in List II.

List I

A. Centrifugal inertia

B. Impaction on atomised droplets

C. Electrical migration

D. Sieving on a dust cake

List II

1. Venturi scrubber

2. Cyclone

3. Fabric filter

4. Electrostatic precipitator

Select the correct match.

A.A-1, B-2, C-4, D-3
B.A-2, B-4, C-1, D-3
C.A-3, B-1, C-4, D-2
D.A-2, B-1, C-4, D-3

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.

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89.

Match each remedy in List I with the problem it addresses in List II.

List I

A. Sulphur trioxide conditioning

B. Raising the cleaning set point

C. Adding a cyclone upstream

D. Selecting a higher temperature fabric

List II

1. Excessive coarse dust loading on a baghouse

2. High dust resistivity causing back corona

3. Bag failure from heat

4. Over-cleaning that strips the dust cake

Select the correct match.

A.A-2, B-4, C-1, D-3
B.A-1, B-2, C-4, D-3
C.A-2, B-1, C-4, D-3
D.A-3, B-1, C-4, D-2

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.

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90.

Which of the following is NOT an inertial collection device?

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

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.

Discuss in Forum

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