Air Pollution Control Questions and Answers – Electrostatic Precipitators – Set 2

This set of Air Pollution Control Multiple Choice Questions & Answers (MCQs) focuses on “Electrostatic Precipitators – Set 2”.

1. Which of the following equations represents the collection efficiency of the electrostatic precipitator?
a) η = 1 – exp(- vpm * Ap / Q)
b) η = 1 – exp(- Ap / Q)
c) η = 1 – exp(vpm * Ap / Q)
d) η = 1 – exp(- \Ap / Q)

Explanation: The following expressions represent the collection efficiency of the electrostatic precipitator:
η = 1 – exp(- vpm * Ap / Q)
Where Aprepresents the precipitator area, vpm represents the velocity with which the particulates get deposited, and Q represents the volumetric flow rate of the entrained gas.

2. A parallel plate precipitator is used for removing dust from paper mills. It has 8 channels, with each plate being 3 m long and 3 m high. What is its efficiency, if it handles 15000 cubic metres per hour of gases?
a) 64%
b) 74%
c) 84%
d) 94%

Explanation: Given,
Q = 15000 m3 / h
Ap = 2 * S = 2 * L * H = 2(3 * 3) = 18 m2
vpm = 0.08 m / s for paper mills
Volumetric flow rate in m3 / s = 15000 / 3600 = 4.166 m3 / s
Volumetric flow rate per channel = 4.166 / 8 = 0.521 m3 / s
We know that,
η = 1 – exp(- vpm * Ap / Q) = 1 – exp(- 18 * 0.08 / 0.521) = 0.934 = 94%

3. What is the standard value of the migration velocity of coal fly ash?
a) 0.04 m / s
b) 0.08 m / s
c) 0.12 m / s
d) 0.16 m / s

Explanation: The standard value of the migration velocity of coal fly ash is in the rage of 0.1 to 0.13 m / s. Among the options listed, only 0.12 m / s falls in this range. These values are estimated using air as the gaseous medium.

4. What is the standard value of the migration velocity for cement dust?
a) 0.04 m / s
b) 0.08 m / s
c) 0.10 m / s
d) 0.14 m / s

Explanation: For the wet process, the standard value of migration velocity ranges between 0.1 m / s and 0.11 m / s for cement dust. This value is between 0.6 m / s and 0.7 m / s for the dry process. The air is the gaseous medium from which the cement dust is being removed.

5. What is a unique characteristic of the electrostatic precipitator?
a) It uses an electric field
b) There is no pressure drop
c) The driving force is applied directly to the particles
d) The speed of separation

Explanation: A unique trait of the electrostatic precipitator is that the driving force of the separation is applied directly to the particles. This feature makes accelerating the gas obsolete, which is a common feature in other particle separation processes.

6. How does the pressure drop in the electrostatic precipitator compare with other particle collectors?
a) It is much lower
b) It is much higher
c) It is comparable
d) Nothing can be said

Explanation: The pressure drop during the operation of the electrostatic precipitator is relatively lesser when compared to other particulate collection devices. It is so because the driving force is applied directly to the particulates themselves.

7. What happens when the resistivity of the particles to be collected by the electrostatic precipitator is too low?
a) Particles become airborne again
b) Field strength drops
c) It does not affect the operation
d) Pressure drop is high

Explanation: When the resistivity of the particles to the electric field is too low, the particles get airborne again. This is because they easily lose the charge imparted to them, and are mobilised by the gas flowing past the collector.

8. What happens when the resistivity of the particles to be collected by the electrostatic precipitator is too high?
a) Particles become airborne again
b) Field strength drops
c) It does not affect the operation
d) Pressure drop is high

Explanation: When the particulate resistivity is too high, the efficiency of the equipment drops. This is because the particulates do not give up their charge easily, causing a reduction in the strength of the electric field.

9. When is the operation of the electrostatic precipitator most economical?
a) The gas flow rate is low
b) Particle resistivity is low
c) Particle resistivity is high
d) Particle resistivity is between certain limits

Explanation: The operation is most economical when the particle resistivity to the electrical charge is between certain limits. When it is too high, the efficiency drops; when it is too low the particles become airborne again.

10. How can the particulate resistivity to the electric field in electrostatic precipitators be controlled?
a) Increasing area of plates
b) Reducing electric current
c) By injecting water
d) Reducing particle velocity

Explanation: The particulate resistivity indirectly controls the efficiency of operation, as well as the cost of operation. This makes its control necessary. Operating temperatures need to be controlled, or water is injected to control and maintain the resistivity.

11. What are irrigated electrostatic precipitators used?
a) Paper mills
b) Cement dust
c) Coal fly ash
d) Fumes from electric arc furnaces

Explanation: An electrostatic precipitator is considered irrigated when water is injected to control the particulate resistivity. Controlling the resistivity ensures economical operation. Such a device is used to collect fumes from electric arc furnaces.

12. Which other device is the electrostatic precipitator commonly used with?
a) Cyclones
b) Gravity settlers
c) Wet scrubbers
d) Fabric filters

Explanation: Electrostatic precipitators are commonly used in series with cyclones. The cyclones act as a pre – cleaner, where the coarser particulates are first removed. The gas then enters the precipitator for further removal.

13. Which of the following is an advantage of using electrostatic precipitators?
a) Low installation cost
b) Low pressure drop
c) Only for dry collection
d) Low efficiency

Explanation: Electrostatic precipitators have high operating efficiencies, as high as 99%. The installation costs are high. It offers a low pressure drop, meaning operating costs are low. This makes the equipment economical to use.

14. Which operating conditions can the electrostatic precipitator with stand?
a) Wet only
b) Dry only
c) Wet and dry both
d) It depends on the separation itself

Explanation: Electrostatic precipitators can withstand wet and dry operating conditions. This makes them viable for collection of dust, gases, and mists. The collection efficiencies remain high for either condition.

15. Which of the following is not a disadvantage of electrostatic precipitators?
a) Their efficiency falls with time
b) High installation costs
c) Sensitive to varying flow rates
d) High pressure drops

Explanation: Electrostatic precipitators have high installation costs, but their operating costs are very low because of low pressure drops. Even though their efficiency is as high as 99%, it falls over time due to various factors. They are sensitive to flow rates so the loading has to remain constant over time.

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