This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Long Tube Vertical Evaporators”.
1. Long tube evaporators can be classified as rising or climbing film.
a) True
b) False
View Answer
Explanation: Yes, the statement is true, because of the long tubes, the film arrangement can be rising or falling.
2. Long tube evaporators are usually ______ evaporators.
a) Natural convection
b) Forced convection
c) Nucleate boiling
d) Radiation heated
View Answer
Explanation: The long tube evaporators, whether falling or rising are natural convection evaporators.
3. What is the average tube length of long tube evaporators?
a) 4-10ft
b) 4-10m
c) 1-2ft
d) More than 10m
View Answer
Explanation: The average tube size of long tube evaporators are around 4-10m for any industrial process, however it may change but the range remains the same.
4. The heating medium in the Long tube evaporator is usually _______
a) Gas at temperature just above 130℃
b) Steam
c) Gas at temperature just below 100℃
d) Gas at very high temperatures
View Answer
Explanation: Long tube evaporators use steam at 100℃ to boil the solution by avoiding nucleate boiling, because of which we cannot use other given options.
5. Long tube evaporators find their use in which one of the following most commonly?
a) Feedstock concentration
b) Food industries
c) Pharmaceuticals
d) Acid concentration
View Answer
Explanation: Long tube evaporator being used in natural convection cannot afford to use corrosive materials as its fluid, hence feedstock and acids are not used, whereas the most common application is in dairy farming as concentrating milk.
6. While using Long tube evaporators, the desired concentration is usually not achieved in single flow.
a) True
b) False
View Answer
Explanation: Yes, the statement is true as the natural convection procedure does not guarantee complete heating of the fluid at the film which reduces evaporation. To overcome this, we use recirculation.
7. Which one of the following is not a property of long tube evaporators?
a) It uses recirculation frequently
b) It has a down-comer
c) Has very long tubes
d) Uses natural convection
View Answer
Explanation: Down-comer presence is a property of short tube evaporators which increases their surface area, but absent in long tube evaporators.
8. Which one of the following is not an advantage of Long tube evaporators?
a) Good heat transfer coefficients at low temperature gradients
b) Low head space required
c) Thin fluids have very large product economy
d) Relatively inexpensive to manufacture
View Answer
Explanation: In long tube vertical evaporators, corrosive fluids cannot be used as the setup is not easy to dismantle and clean and hence we usually use cleaner fluids whose deposits can be removed by water. It also requires large head space to accommodate the fluid that is falling/climbing.
9. The following are few demerits of using short tube vertical evaporators, select the incorrect one.
a) Long tubes efficiently evaporate and hence recirculation is hardly required
b) Corrosive fluids cannot be used as it increases cleaning cost
c) Large head room requirement
d) Very low heat transfer coefficients with thick fluids
View Answer
Explanation: The long tube evaporators use recirculation as the natural convection procedure does not guarantee complete heating of the fluid at the film which reduces evaporation. To overcome this, we use recirculation.
10. What is the direction of flow in the centre tubes and wall-side tubes of a long tube falling film evaporator?
a) Both downwards
b) Both upwards
c) Downward and upward respectively
d) Upward and downward respectively
View Answer
Explanation: The fluid in long tube evaporators experience free fall in the long tubes and climbing back is not an option, for recirculation, the fluid is carried in a separate tube to the top.
11. Which one of the following is a Long tube evaporator?
a) 
b) 
c) 
d) 
View Answer
Explanation: The given answer is a long tube evaporator recognized from the direction of flow of the fluid in it and the length of the tubes. It has longer tubes and vapour escapes from top.
12. How is recycle carried out in Long tube evaporators?
a) 
b) 
c) 
d)
View Answer
Explanation: The fluid in long tube evaporators experience free fall in the long tubes and climbing back is not an option, for recirculation, the fluid is carried in a separate tube to the top.
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