# Air Pollution Control Questions and Answers – Cyclone Separators

This set of Air Pollution Control Multiple Choice Questions & Answers (MCQs) focuses on “Cyclone Separators”.

1. What is the main working principle of the cyclone separator?
a) Centrifugal force
b) Gravitational force
c) Ionisation

Explanation: The principle on which cyclones work is the centrifugal force. Upon entering the equipment, the entrained gas enters rotational motion. The particles get thrown outwards, thus separating them from the gas.

2. Cyclones are used to separate which kinds of air pollutants?
a) Two types of gases
b) Particulates and gases
c) Two types of particulates
d) Particles from liquid slurry

Explanation: Particulates are separated from gases using cyclones. The centrifugal force is responsible for this separation. The force pushes the gas inwards (towards the centre) and the particles outwards (towards the walls) radially.

3. Which of the following is true in case of a cyclone separator?
a) Fc = g
b) Fc < g
c) Fc > g
d) Fc ≤ g

Explanation: In the case of cyclone separators, the centrifugal force is greater than the gravitational force. This allows the equipment to accomplish the separation of very small particles, and more efficiently than gravity settlers.

4. Why are cyclones more advantageous than gravitational settlers?
a) They are less expensive
b) They occupy more space
c) Low pressure drop
d) Small particles are separated easily

Explanation: In the case of cyclones, they separate smaller particles more efficiently than gravity settlers. This is because the centrifugal force, which is the main driving force for separation, is greater than the force of gravity.

5. Gravitational settlers require less space than cyclone separators to separate the same quantity of gas.
a) True
b) False

Explanation: Cyclone separators require less space than gravity settlers. The gravity settlers require a large rectangular space for slow enough motion to allow for the separation. The high – speed operation of the cyclones and the centrifugal force allow for a smaller space requirement.

6. Which of the following is untrue when comparing cyclones and gravity settlers?
a) For cyclones, Fc > g
b) Gravitational settlers can easily remove particles smaller than 50 microns
c) Cyclones require less space
d) Gravity settlers utilise gravity

Explanation: Gravitational settlers have difficulty removing particles having sizes smaller than 50 microns. Particles of such small sizes, most often released from industries, do not settle under just the influence of gravity. The other listed statements are true.

7. Which type of cyclone does the following illustration indicate?
a) High – efficiency type
b) Impeller collector
c) Reverse flow type
d) Through – flow type

Explanation: The illustrated cyclone is the reverse flow type cyclone. The entrained gas enters tangentially at the top. The particulates (outer vortex) are collected from the bottom of the equipment, while the clean gas exits from the top.

8. In the given reverse – flow cyclone, why is the gas outlet extended into the cylinder?
a) To ensure proper operation
b) To avoid gas leaving from the particulate outlet
c) To avoid re – entrainment
d) To avoid gas going from the inlet to the outlet

Explanation: The gas outlet pipe is extended into the bulk of the cylinder to avoid the entrained gas from the inlet to directly flow out. If it were not extended, the gas coming out from the outlet would be entrained as well.

9. Which of the following is not a commercially used cyclone type?
a) Wet scrubber type
b) Impeller collector type
c) Reverse flow type
d) Through – flow type

Explanation: Among the given options, wet scrubber type cyclone does not exist. Wet scrubbers are another type of equipment used to separate particulates from gases, not a type of cyclone. The rest of the listed options are all commercially used.

10. How many vortexes exist simultaneously in a cyclone?
a) 1
b) 2
c) 3
d) 0

Explanation: The centrifugal force separates the entrained gases into two vortexes. The force sends the particulates outwards. Thus, the outer vortex is for the particulates, and the inner vortex is for the gases.

11. How is the magnitude of the centrifugal force used in the cyclone usually expressed?
a) In terms of radial velocity
b) In terms of separation factor
c) How many times it exceeds g
d) In terms of pressure drop

Explanation: The centrifugal force (Fc) in the cyclone is greater than the gravity. For this reason, it is usually expressed in terms of the number of times its value exceeds that of gravity. The formula to calculate the Fc is:
Fc = m(vθ)2 / r ; where, vθ is the particle’s tangential velocity, and r is the rotation radius.

12. For cyclones, what is the formula used to calculate the centrifugal force (Fc)?
a) Fc = mvθ / r
b) Fc = (vθ)2 / gr
c) Fc = m(vθ)2 / g
d) Fc = m(vθ)2 / r

Explanation: The formula used to numerically estimate the centrifugal force is
Fc = m(vθ)2 / r
where, vθ is the particle’s tangential velocity, and r is the rotation radius, and m is the mass of the particle.
On the other hand, (vθ)2 / gr estimates the separation factor.

13. What is the formula used to numerically estimate the separation factor (S) of cyclone separators?
a) S = (vθ)2 / gr
b) S = mvθ / r
c) S = m(vθ)2 / g
d) S = m(vθ)2 / r

Explanation: The formula used to calculate the separation factor is:
S = (vθ)2 / gr
Where vθ is the particle’s tangential velocity, and r is the radius of rotation. Separation factor is the ration of centrifugal and gravitational forces.

14. What is the range of separation factors from low – velocity to high – velocity units?
a) 10 to 1000
b) 5 to 2500
c) 10 to 2500
d) 5 to 1000

Explanation: The separation factor is the ratio of centrifugal and gravitational forces. Its formula is
S = (vθ)2 / gr
It can range from as low as 5, for a large, low – velocity cyclone, to as high as 2500, for a small, high – velocity cyclone.

15. Which is true about separation factor?
a) Its formula is S = mvθ / gr
b) Low separation factor means better performance
c) High separation factor means better performance
d) It can be as high as 3000

Explanation: Among the given statements, only the statement about high separation factor is true. A high separation factor entails a cyclone with better performance, i.e,. more separation occurs. Its value caps out at 2500.

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