Particulate Control Devices Questions and Answers

Q: Wet scrubber is used to treat pollution of :

A wet scrubber treats air pollution.In a wet scrubber the polluted gas stream is brought into contact with a scrubbing liquid, usually water or an alkaline solution, by spraying, bubbling or passing through packing. Particles are captured in the droplets and soluble gases such as sulphur dioxide, hydrogen chloride and ammonia are absorbed, so it controls both particulate and gaseous emissions from stacks.It produces wastewater rather than treating it, and it has nothing to do with soil remediation or noise control.

Q: Electrostatic precipitators work on the principle of :

Electrostatic precipitators work on the principle of charge separation.In an electrostatic precipitator the dust-laden gas passes between discharge electrodes held at a high negative voltage and earthed collecting plates. The corona discharge charges the particles, which are then attracted to and held on the collecting plates, from which they are rapped off into hoppers. Removal efficiencies above 99 per cent are achieved for fine fly ash from power plants.Gravity settling chambers and cyclones (centrifugal separation) are different devices that work on other principles, so 'all of the above' is wrong.

Q: Which of the following is not used for removal of particulate pollutants ?

The device not used for particulate removal is the Incinerator.An incinerator burns waste at high temperature to reduce its volume and destroy organic and infectious matter. It produces flue gas and particulates rather than removing them, and itself needs dust control devices on its stack.The other three are standard particulate control equipment. An electrostatic precipitator charges particles and collects them on plates, a bag filter (baghouse) traps them on fabric, and a cyclone throws heavier particles out of a spinning gas stream by centrifugal force.

Q: Which air pollution control equipment works on the principle of centrifugal separation ?

The cyclone works by centrifugal separation.In a cyclone separator, dusty gas enters tangentially and spirals down the cylinder. The heavier particles are thrown outward by the centrifugal force onto the wall, slide down into a hopper, and the cleaned gas leaves upward through the central outlet. It suits coarse particles above about 10 µm.An electrostatic precipitator charges particles and collects them on plates by electric force. A wet scrubber captures them in liquid droplets, and a bag filter traps them by filtration through fabric.

Q: Flue gas laden with fine particles from a thermal power plant with a volume flow rate of 100 \(\mathrm{m^{3}}\)/second passes through an electrostatic precipitator (ESP) having 5000 \(\mathrm{m^{2}}\) of collector plate area. If the particle collection efficiency of the ESP is 98%, the drift velocity of the flue gas must be

~ 0.078 m/sFor an electrostatic precipitator the Deutsch equation gives the efficiency \[ \eta = 1 - \exp\left(-\frac{wA}{Q}\right) \] \[ 0.98 = 1 - \exp\left(-\frac{5000\,w}{100}\right) \] \[ \ln 50 = 50\,w \] \[ w = \frac{3.91}{50} \approx 0.078\ \mathrm{m/s} \] The other options do not satisfy the equation.The other options, ~ 0.052 m/s; ~ 0.15 m/s; ~ 1.5 m/s, are not the answer to this question. This agrees with the official final answer key for the paper, which gives option B.

101.

Wet scrubber is used to treat pollution of :

A.Water
B.Air
C.Soil
D.Noise

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

A wet scrubber treats air pollution.

In a wet scrubber the polluted gas stream is brought into contact with a scrubbing liquid, usually water or an alkaline solution, by spraying, bubbling or passing through packing. Particles are captured in the droplets and soluble gases such as sulphur dioxide, hydrogen chloride and ammonia are absorbed, so it controls both particulate and gaseous emissions from stacks.

It produces wastewater rather than treating it, and it has nothing to do with soil remediation or noise control.

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102.

Electrostatic precipitators work on the principle of :

A.Gravity settling
B.Centrifugal separation
C.Charge separation
D.All of the above

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

Electrostatic precipitators work on the principle of charge separation.

In an electrostatic precipitator the dust-laden gas passes between discharge electrodes held at a high negative voltage and earthed collecting plates. The corona discharge charges the particles, which are then attracted to and held on the collecting plates, from which they are rapped off into hoppers. Removal efficiencies above 99 per cent are achieved for fine fly ash from power plants.

Gravity settling chambers and cyclones (centrifugal separation) are different devices that work on other principles, so 'all of the above' is wrong.

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103.

Which of the following is not used for removal of particulate pollutants ?

A.Electrostatic precipitator
B.Bag filter
C.Incinerator
D.Cyclone

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The device not used for particulate removal is the Incinerator.

An incinerator burns waste at high temperature to reduce its volume and destroy organic and infectious matter. It produces flue gas and particulates rather than removing them, and itself needs dust control devices on its stack.

The other three are standard particulate control equipment. An electrostatic precipitator charges particles and collects them on plates, a bag filter (baghouse) traps them on fabric, and a cyclone throws heavier particles out of a spinning gas stream by centrifugal force.

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104.

Which air pollution control equipment works on the principle of centrifugal separation ?

A.Electrostatic precipitation
B.Cyclone
C.Wet scrubber
D.Bag filter

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The cyclone works by centrifugal separation.

In a cyclone separator, dusty gas enters tangentially and spirals down the cylinder. The heavier particles are thrown outward by the centrifugal force onto the wall, slide down into a hopper, and the cleaned gas leaves upward through the central outlet. It suits coarse particles above about 10 µm.

An electrostatic precipitator charges particles and collects them on plates by electric force. A wet scrubber captures them in liquid droplets, and a bag filter traps them by filtration through fabric.

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105.

Five particles (\(\lt\) 2-3 µm) can be effectively collected from industrial stacks by

A.Fabric filters
B.Cyclone separators
C.Venturi scrubbers
D.Settling chambers

Exam Year(s)

UGC NET - 2012,

Answer: C

The keyed answer is the Venturi scrubber. It gives a high-energy wet collection: the gas is accelerated through a narrow throat, where water injected there is atomised into fine droplets that capture small particles by impaction and diffusion, so particles down to about one micrometre or less are removed. Cyclone separators and settling chambers rely on inertia and gravity, so they are effective only for coarse particles, well above those of 2-3 micrometres. Fabric filters can also remove fine dust, but they are limited by gas temperature and moisture, so the key chooses the scrubber.

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106.

For aerosol particles having sizes \(\gt\) 1 μm, the terminal settling velocity (\(V_{T}\)) of an aerosol particle varies with its diameter (D) as

A.\(V_{T} \propto D\)
B.\(V_{T} \propto D^{2}\)
C.\(V_{T} \propto D^{3}\)
D.\(V_{T} \propto D^{3/2}\)

Exam Year(s)

UGC NET - 2012,

Answer: B

For small particles in the laminar regime Stokes' law gives the terminal settling velocity\[ V_T = \frac{g\, D^{2}\,(\rho_p - \rho)}{18\,\mu} \]which means that V is proportional to the square of the diameter. For particles larger than about 1 micrometre gravitational settling dominates, and this relation applies until the particle becomes large enough for turbulent drag. A linear, cubic or three-halves power of D does not follow from the balance between gravity and viscous drag.

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107.

Given below are two statements. One labelled as Assertion (A) and the other labelled as Reason (R).

Assertion (A) : Electrostatic precipitators (ESPs) can be harmful if not operated properly.
Reason (R) : Corona discharge in ESPs produces ozone.
Choose the correct answer :

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

Exam Year(s)

UGC NET - 2012,

Answer: A

Both statements are true and the reason explains the assertion. Electrostatic precipitators ionise the gas by corona discharge, and the discharge produces ozone, which is harmful, so an ESP that is not operated properly can be harmful. The answer is that both are correct and R is the correct explanation of A.

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108.

An Electrostatic Precipitator (ESP) with collector plate area = 5000 m\(^{2}\) treats a flue gas with drift velocity = 0.12 m/s with 98% efficiency. The volumetric flow rate (m\(^{3}\)/s) of the flue gas is

A.~ 175.2
B.~ 213.5
C.~ 153.4
D.~ 198.9

Exam Year(s)

UGC NET - 2013,

Answer: C

For an electrostatic precipitator the Deutsch equation gives the efficiency as \(1 - e^{-wA/Q}\), where w is the drift velocity, A the plate area and Q the flow.

\[ 0.98 = 1 - e^{-0.12 \times 5000 / Q} \]

\[ Q = \frac{600}{\ln 50} = \frac{600}{3.912} \]

\[ Q = 153.4 \text{ m}^3/\text{s} \]

So option C is correct. The other values do not satisfy the equation.

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109.

Particles of sizes \(\lt\) 1 μm are most efficiently removed by

A.Cyclones
B.Scrubbers
C.Bag filter
D.Electrostatic Precipitator

Exam Year(s)

UGC NET - 2013,

Answer: D

Particles below 1 micrometre are most efficiently removed by an electrostatic precipitator.

An electrostatic precipitator charges the particles and pulls them to a plate, and it keeps a high efficiency, above 99 per cent, down to the submicron range because the electrical force acts on each particle however small it is.

Cyclones are poor at removing particles below about 10 micrometre, and scrubbers lose efficiency below 1 micrometre unless the pressure drop is made very high. Bag filters collect fine particles well but the key picks the precipitator as the most efficient.

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110.

Flue gas laden with fine particles from a thermal power plant with a volume flow rate of 100 \(\mathrm{m^{3}}\)/second passes through an electrostatic precipitator (ESP) having 5000 \(\mathrm{m^{2}}\) of collector plate area. If the particle collection efficiency of the ESP is 98%, the drift velocity of the flue gas must be

A.~ 0.052 m/s
B.~ 0.078 m/s
C.~ 0.15 m/s
D.~ 1.5 m/s

Exam Year(s)

UGC NET - 2013,

Answer: B

~ 0.078 m/s

For an electrostatic precipitator the Deutsch equation gives the efficiency \[ \eta = 1 - \exp\left(-\frac{wA}{Q}\right) \] \[ 0.98 = 1 - \exp\left(-\frac{5000\,w}{100}\right) \] \[ \ln 50 = 50\,w \] \[ w = \frac{3.91}{50} \approx 0.078\ \mathrm{m/s} \] The other options do not satisfy the equation.

The other options, ~ 0.052 m/s; ~ 0.15 m/s; ~ 1.5 m/s, are not the answer to this question. This agrees with the official final answer key for the paper, which gives option B.

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