This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Forced Circulation Evaporator”.
1. What do we mean by forced convection evaporator?
a) This is nucleate boiling
b) This has a fan to remove vapour from the surface
c) There is an agitator to mix the heated fluid
d) There is an agitator to mix the heating medium such as steam
View Answer
Explanation: By forced convection we mean to say that the fluid being heated does not face wall/surface boiling by providing proper mixing conditions, hence in such evaporators, there is an agitator to mix the heated fluid.
2. With forced convection boiling, nucleate boiling is absent which makes wall temperature very close to interface temperature.
a) True
b) False
View Answer
Explanation: In forced convection boiling, heat transfer occurs directly from the wall to the interface and this process becomes so efficient that the wall temperature is unable to sustain the nucleate boiling.
3. The presence of forced convection makes the complete mixed fluid boiling pattern known as __________
a) Pool boiling
b) Surface boiling
c) Bubble boiling
d) Film boiling
View Answer
Explanation: The forced convection induced boiling is called pool boiling because the fluid in contact with the heating wall is continuously renewed and the bulk of the liquid reaches that temperature, hence its name- Pool boiling.
4. The boiling that takes place in forced convection boiling is popularly known as surface boiling.
a) True
b) False
View Answer
Explanation: The forced convection induced boiling is called pool boiling because the fluid in contact with the heating wall is continuously renewed and the bulk of the liquid reaches that temperature, hence its name- Pool boiling.
5. Which one of the following evaporator does not use forced boiling as its boiling operation?
a) Plate type evaporators
b) Horizontal tube evaporators
c) Vertical tube evaporators
d) Falling tube evaporators
View Answer
Explanation: The falling film evaporator belongs to the class of natural convection evaporators and not to forced convection.
6. The Residence tie in forced convection evaporators is ________ than natural convection evaporator.
a) More
b) Less
c) Same
d) Very less
View Answer
Explanation: The residence time in forced convection evaporators is more because the time taken to heat the bulk of liquid rather than the surface is more.
7. Forced circulation is sometimes more suitable because of which one of the following statements?
a) It has Low pressure operations
b) The temperature can be maintained as circulation is present
c) The flow velocity in the tubes and the liquid temperature can be controlled to suit the product requirements
d) Cheaper than others
View Answer
Explanation: Liquid circulation is maintained with the flow velocity in the tubes and the liquid temperature can be controlled with the product requirements. Forced circulation is sometimes more suitable with flow velocity.
8. Forced convection boiling is also known as __________
a) Flash boiling
b) Nucleate boiling
c) Partial nucleate boiling
d) Bubble boiling
View Answer
Explanation: The liquid used in forced convection boiling is superheated with high pressure, more than its normal pressure of boiling and hence when it enters the setup, the pressure in the liquid falls quickly leading to the liquid being rapidly boiled off (flashed). This is a reason why it is known as flash boiling.
9. Which one of the following is not an advantage of forced convection boiling?
a) High heat transfer coefficient
b) Positive circulation
c) Very cheap to operate
d) Low fouling or scaling
View Answer
Explanation: The advantages of forced circulation evaporators can be summarized as –
i. High heat-transfer coefficients
ii. Positive circulation
iii. Reduced fouling or scaling.
Because of these high boiling ratio and extra circulation, the operation cost increases due to the electrical cost of running the agitator and pump, hence it is not usually cheap.
10. Which one of the following is not a demerit of forced circulation evaporator?
a) High operation cost
b) Power consumption for pumps
c) High residence time
d) High temperatures of operation
View Answer
Explanation: The demerits of forced convection evaporator can be summarized as –
i. Possibly higher cost
ii. Power consumption for the circulating pump
iii. A longer holdup of the product within the heating zone
Whereas the high temperature of operation is a merit as it leads to flash boiling.
11. Forced circulation evaporators are highly suited for ________
a) High corrosive fluids
b) Fouling fluids that usually scale
c) Thin fluids
d) Highly volatile fluids
View Answer
Explanation: Liquids with a high tendency for fouling, highly viscous liquids, are generally preferred to be used in these evaporators so as to reduce maintenance cost in other types of evaporators.
12. Why is forced convection evaporation also known as flash evaporation?
a) Because heat is supplied in a flash
b) Boiling the fluid in high pressure operation then exposing to low pressure
c) Total evaporation takes place in a flash
d) Very large temperature created
View Answer
Explanation: The liquid used in forced convection boiling is superheated with high pressure, more than its normal pressure of boiling and hence when it enters the setup, the pressure in the liquid falls quickly which leads to rapid boiling off (flashed) of the liquid. This is the reason why it is known as flash boiling.
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