250+ TOP MCQs on Evaporators – Liquid Characteristics and Answers

Heat Transfer Operations Multiple Choice Questions on “Evaporators – Liquid Characteristics”.

1. What is the advantage of basket type evaporators over short tube evaporators?
a) More evaporation per unit time
b) More heat transfer coefficient
c) Basket can be removed to be cleaned
d) Tubes can be removed to be cleaned

Answer: d
Clarification: Basket type evaporator is a variant of the short-tube type in which the whole tube bank may be easily removed for cleaning and is more economical compared to the former evaporator.

2. If the liquid in use is a fouling liquid, then which one of the following cannot be used as an evaporator?
a) Falling film evaporator
b) Nucleate boiling Evaporator
c) Non- Nucleate Boiling Evaporator
d) Forced Circulation Evaporator

Answer: b
Clarification: Forced circulation is observed when boiling is suppressed at the heating surface due to the hydrostatic heads operating on it, hence falling film evaporators which generally use crystallizers, viscous fluids, waste streams, and other fouling fluids, nucleate boiling would naturally increase their fouling rate and is hence is not suited.

3. How is Evaporator different from dryers?
a) Final product is not a solid in evaporator
b) Evaporator is less efficient
c) Dryers use boiling to remove liquid
d) Dryers use very high temperatures to remove liquid

Answer: a
Clarification: Evaporators are used to concentrate a liquid but not to dry it to a solid, whereas dryers use prolonged evaporation to obtain a solid product.

4. In nucleate boiling evaporators, the evaporation occurs at the outer film of the fluid.
a) True
b) False

Answer: b
Clarification: The statement is wrong because in nucleate boiling the nucleus starts forming at the heating wall that carries vapour, which comes to the surface and releases vapour.

5. Where is the heat supplied to an evaporator actually go?
a) 100% to evaporating liquid
b) 99% evaporating the liquid
c) 95% evaporating it and rest in overcoming surface tension
d) 99% in overcoming surface tension

Answer: c
Clarification: The heat energy supplied to the evaporator is usually distributed as:
i. Breaking the surface tension
ii. Evaporation
iii. Retained by the fluid.

6. Which one of the following is the governing factor for evaporation to take place?
a) Surface area
b) Wall surface area
c) Amount of heat supplied
d) Boiling procedure

Answer: a
Clarification: The more the surface area of the liquid with the surrounding dry air, the more is the rate of evaporation. Hence this is the reason why spray type evaporators have maximum evaporation rate.

7. For highly viscous solutions, which one is best suited evaporator?
a) Falling film evaporator
b) Nucleate boiling Evaporator
c) Non- Nucleate Boiling Evaporator
d) Forced Circulation Evaporator

Answer: a
Clarification: For highly viscous solutions, we cannot use wall heating or any other method of heating as this may cause burning of the fluid, hence the best suited evaporator is falling film evaporator which only depends on exposed surface for evaporation.

8. In case of pharmaceutical industries, which evaporator must be used?
a) Falling film evaporator
b) Nucleate boiling Evaporator
c) Non- Nucleate Boiling Evaporator
d) Forced Circulation Evaporator

Answer: a
Clarification: In pharmaceutical industries handling very sensitive solutions, it is usually avoided to heat the solution for the concentrating process as it may cause unwanted chemical reactions. Hence it is commonly seen that such industries prefer to use Falling film evaporator.

9. In which one of the following evaporator, boiling takes place inside the tubes and evaporation later?
a) Falling film evaporator
b) Rising film Evaporator
c) Non- Nucleate Boiling Evaporator
d) Forced Circulation Evaporator

Answer: b
Clarification: In rising film evaporators, the tubes are heated to boil the fluid contained in it, the film keeps on rising with the bubbles formed by boiling carrying the vapour upward.

10. What is the disadvantage of rising film over falling film?
a) Quick scaling
b) Boiling used
c) Less evaporation
d) Economically inferior

Answer: a
Clarification: In rising film evaporators, the tubes are heated to boil the fluid contained in it, the film keeps on rising with the bubbles formed by boiling carrying the vapour upward. As boiling takes place in the tubes, there is high chance of scaling and fouling on the tube walls.

11. The basic design of the climbing/falling film plate evaporator consists of two phases, which are?
a) First climbing phase then falling phase
b) First falling phase then climbing phase
c) Liquid to vapour phase while climbing
d) Liquid to vapour phase while falling

Answer: a
Clarification: In the climbing/falling film plate evaporator which is a multiple effect evaporator uses two phases of operation, which are climbing phase when the fluid is heated and falling phase when evaporation takes place.

12. The residence time in climbing/falling film plate evaporator is usually tried to keep low.
a) True
b) False

Answer: a
Clarification: The residence time has to be kept low else there would be unnecessary scaling in the tubes; hence residence time is one of the important parameters to be kept in mind while designing these setups.