This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Agitated Film Evaporator”.
1. Agitated film evaporators are best suited for_________
a) High corrosive fluids
b) Fouling fluids that usually scale
c) Highly viscous fluids
d) Highly volatile fluids
View Answer
Explanation: Highly viscous liquids are generally preferred to be used in these evaporators so as to increase their heat transfer coefficients which otherwise is not possible in other evaporators.
2. How do the Agitated film evaporator increase heat transfer coefficient?
a) It has a set of blades that keeps renewing the film
b) It has mixers that mix the fluid
c) It is a type of climbing film evaporator with agitators
d) It is a horizontal tube evaporator with agitated outer fluid
View Answer
Explanation: The agitated film evaporators have set of blades rotating around the film with a very small gap that keeps renewing the film and hence exposing the hot fluid to the open surface for larger evaporation.
3. Which one of the following is a merit of agitated film evaporator?
a) Nucleate boiling
b) Falling film
c) Self cleaning equipment
d) High temperature management
View Answer
Explanation: The agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced, hence it can be regarded as a self cleaning equipment. Self cleaning equipment is a merit of agitated film evaporator.
4. The blades of the agitated film evaporator have a self cleaning effect on the evaporator and hence give us the freedom to use fouling fluids.
a) True
b) False
View Answer
Explanation: The agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced, hence it can be regarded as a self cleaning equipment. Self cleaning equipment is a merit of agitated film evaporator and gives us the freedom to use fouling fluids.
5. Which one of the following is not a property of agitated film evaporator?
a) Prevents the formation of hot spots due to agitation
b) Absence of static pressure on the boiling liquid
c) Staging or vapour recompression for energy recovery
d) Ability to work in continuous or batch mode
View Answer
Explanation: In agitated film evaporators, energy cannot be recovered by vapour recompression because it is not practical to conduct compression on the vapour which was obtaining from fouling fluids as it is more or less a waste and the energy expenditure is not a sane step.
6. The residence time of agitated film evaporators is ______ than rising film evaporators.
a) More
b) Less
c) Very much higher
d) Negligible
View Answer
Explanation: The rising film evaporators are the few evaporators which have high residence time compared to others. Agitated film evaporators usually have residence time of 1-5secs.
7. Agitated film evaporator is sometimes more suitable because of which one of the following statement?
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) Self cleaning because of presence of blades
View Answer
Explanation: The agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced; hence it can be regarded as self cleaning equipment.
8. Agitated film evaporator is also known as _______ film evaporator.
a) Wiped
b) Swiped
c) Flushed
d) Flash
View Answer
Explanation: The agitated film evaporators are commonly known as Wiped surface evaporators because agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced, hence it can be regarded as a self cleaning equipment.
9. Where does the agitation take place in an agitated film evaporator?
a) Before the liquid falls
b) At the film on the tube surface
c) At the upper end as fluid enters rest is turbulent
d) At the bottom-most point
View Answer
Explanation: The agitation is carried out on the total length of the evaporator and not at parts, hence the option of agitation on the film is correct and others are incorrect (at the upper end as fluid enters rest is turbulent).
10. Which one of the following is not an advantage of agitated film evaporator?
a) Elevated heat transfer coefficients due to agitation.
b) Very low operation cost
c) Ability of working at high vacuum (down to 0.001 mbar)
d) Very low residence times (to avoid product degradation)
View Answer
Explanation: The operation cost of agitated film is relatively high owing to the presence of pumps and motors for the agitators, hence this option (Very low operation cost) is wrong.
11. Which one of the following is not a disadvantage of agitated film evaporator?
a) Cost of motors for agitators
b) Higher cost compared to standard evaporation equipment
c) Absence of static pressure on the boiling liquid
d) Staging or vapour recompression for energy recovery are not possible
View Answer
Explanation: The demerits of agitated film evaporator can be summarised as
i. Higher cost compared to standard evaporation equipment
ii. Staging or vapour recompression for energy recovery are not possible
Otherwise agitators are very useful and mostly preferred over others.
12. Recognize among the following which is an agitated film evaporator.
a) 
b) 
c) 
d) 
View Answer
Explanation: The given answer is an agitated film evaporator recognized from the blades that obstruct the flow of the fluid in it. It has longer tubes and vapour escapes from top as well as the direction of fluid flow is downward.
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