This set of Air Pollution Control Multiple Choice Questions & Answers (MCQs) focuses on “Cyclone Separators – Set 2”.
1. What is the relative velocity between the gas and particulates in the cyclone equal to?
a) Radial velocity
b) Gravitational force
c) Separation factor
d) Tangential velocity
View Answer
Explanation: The radial velocity is equal to the relative velocity of the gas and the particulates. This is because the tangential velocities of the gas and the particulate are equal. Tangential velocity and radial velocity are separate terms.
2. What is the default effective number of turns a gas makes in the cyclone?
a) 3
b) 6
c) 18
d) Cannot be determined
View Answer
Explanation: Once the gas enters the cyclone, it undergoes radial motion because of the centrifugal force it experiences. The gas then turns in the cyclone until it exits. The effective number of turns a gas makes is usually 6.
3. For a cyclone, which of the following factors increases with increasing particle size and a small area of cross-section?
a) Tangential velocity
b) Cut diameter
c) Collection efficiency
d) Effective number of terms
View Answer
Explanation: Among the given options, only collection efficiency increases with increased particle diameter and area of cross-section of the inlet. This variation is also accompanied by a high pressure drop, which means high operating costs.
4. What is meant by ‘cut size diameter’?
a) The size of the particle collected with 25% efficiency
b) The size of the particle collected with 75% efficiency
c) The size of the particle collected with 100% efficiency
d) The size of the particle collected with 50% efficiency
View Answer
Explanation: Cut size diameter is the diameter of the particle whose collection efficiency in cyclones is 50%. The particles of sizes greater than the cut size have a higher collection efficiency than 50%, and vice versa. It is denoted by dpc.
5. Which of the following is false about cut size diameter, dpc?
a) Particles larger dpc have a collection efficiency lower than 50%
b) Particles larger dpc have a higher collection efficiency greater than dpc
c) dpc is the particle size collected with 50%
d) Particles larger dpc have a collection efficiency greater than 50%
View Answer
Explanation: Cut size diameter is the diameter for which the collection efficiency is equal to 50%. Particles greater than the cut size diameter have a collection efficiency greater than 50%, the same is true for the opposite.
6. The following formula is used to estimate the cut size diameter. Which term does Ne represent?
dpc = √(9μgb/(2pi*Ne*vi(ρp-ρg)))
a) Reynold’s number
b) Effective number of turns made by the gas
c) Number of gaseous participants
d) Number of cyclones in parallel
View Answer
Explanation: Ne represents the effective number of turns made by the gas while in the cyclone. The value is taken as default to be 6 turns. Cut size diameter is the particle size which has an efficiency of collection to be 50%.
7. Why does the pressure drop need to be calculated for the cyclone along with its efficiency?
a) To determine the load
b) To determine cut size diameter
c) To estimate costs
d) To avoid explosions
View Answer
Explanation: The pressure drop is estimated to estimate costs. A high pressure drop places a huge burden on the equipment. Additionally, the resource requirement is also quite high. This estimation can predict the cost-effectiveness of the equipment for a particular separation.
8. What is the efficiency of a conventional cyclone for a particle size smaller than 5 microns?
a) 50-80%
b) Greater than 50%
c) 50%
d) Less than 50%
View Answer
Explanation: For a conventional cyclone, the efficiency of collection of a particle smaller than 5 microns is less than 50%. This value is generalised from the generalised design configurations of a conventional cyclone.
9. Which of the following is the efficiency range with which a high-efficiency cyclone captures a particle size of 15 microns? (Cyclone has generalised design)
a) 30-50
b) 50-80
c) 80-95
d) 95-99
View Answer
Explanation: The efficiency of collection for particles sizes ranging from 5 to 20 microns is 80% to 95%. This is for a high-efficiency cyclone with a standard design. Particles smaller than 5 microns have a collection efficiency ranging from 50% to 80%.
10. Which of the following factors is the pressure drop in a cyclone proportional to?
a) Inlet velocity
b) Square of inlet velocity
c) Cyclone dimensions
d) Square of the cyclone dimensions
View Answer
Explanation: The pressure drop in the cyclone is directly proportional to the square of the inlet velocity of the entrained gas. This means that the pressure drop will be extremely high for high inlet gas velocities. High pressure drops imply high operating costs.
11. How does the through-flow type cyclone vary from the reverse flow type cyclone?
a) In the former, the pressure drop is high
b) In the latter, the gas leaves from the bottom
c) In the former, the gas leaves from the bottom
d) In the former, the flow rates are higher
View Answer
Explanation: In the case of the through-flow type cyclone, the air outlet is at the bottom. This is precisely the opposite of the reverse flow type cyclone in which the gas outlet is at the top. The gas outlet at the bottom allows for a lower pressure drop.
12. What is a huge advantage of the through-flow cyclone?
a) Lower maintenance
b) Lower space requirement
c) Fc < g
d) Low pressure drop
View Answer
Explanation: Low pressure drop is an advantage of the through-flow cyclone. Low pressure drops are a result of constructing the gas outlet at the bottom of the system, rather than the top. It can also withstand high volumetric flow rates.
13. What is an advantage of the impeller collector cyclone?
a) Low space requirement
b) Low pressure drop
c) Low maintenance
d) Low resource requirement
View Answer
Explanation: In the case of the impeller-collector type cyclone, its main advantage is its space requirement, which is low. The system uses an impeller, which throws the particles outwards; these particles are then collected.
14. What is the most common use of through-flow type cyclones?
a) To remove fumes
b) To remove fly ash
c) To remove PM2.5
d) To remove mist
View Answer
Explanation: Through-flow type cyclones are ideal for the removal of fly ash and large particulates. They are used frequently as pre-cleaners, to remove coarse particles. Impeller-collecter type cyclones have similar applications.
15. What is the best way to obtain a high efficiency of removal from cyclones?
a) Have a high pressure drop
b) Use one huge cyclone
c) Use n number of small cyclones in parallel
d) Have a high centrifugal force
View Answer
Explanation: The usage of n number of small cyclones placed in parallel allows for a high collection efficiency with moderate operating capacities. In addition, such parallel systems require less space than using one large unit.
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