This set of Dairy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Evaporator – 2”.
1. The evaporator in which the material to be evaporated boils outside the horizontal tube and steam condenses inside the tubes is called ________
a) Horizontal tube evaporator
b) Vertical tube evaporator
c) Vacuum pan evaporator
d) Forced circulation evaporator
View Answer
Explanation: In Horizontal tube evaporator, the material to be evaporated boils outside the horizontal tube and steam condenses inside the tubes. These evaporators are not considered effective as the heat transfer is lower especially if the liquid is viscous.
2. Evaporator which promotes natural circulation is?
a) Horizontal tube evaporator
b) Vertical tube evaporator
c) Vacuum pan evaporator
d) Forced circulation evaporator
View Answer
Explanation: In vertical tube evaporator, heating and boiling of liquid cause it to rise and flow upwards and un-evaporated liquid flows down through the central large diameter tube. Thus natural circulation is promoted in this type of evaporator.
3. Evaporators in which pumps are used to force the evaporating liquid through the tubes are called?
a) Horizontal tube evaporator
b) Vertical tube evaporator
c) Vacuum pan evaporator
d) Forced circulation evaporator
View Answer
Explanation: Forced Circulation Evaporator is a modified form of vertical tube evaporator. In this type a pumps is used to force the evaporating liquid through the tubes. This helps to further improve the evaporating capacity.
4. Name of the evaporator which consists of one pass vertical shell and tube heat exchanger discharging the product to be evaporated into a relatively small vapor head?
a) Horizontal tube evaporator
b) Long tube vertical evaporator
c) Vacuum pan evaporator
d) Forced circulation evaporator
View Answer
Explanation: In Long tube vertical evaporator, the feed enters at the bottom of the tube, starts boiling as it moves up the tube and the mixture of liquid and vapor leaving at the top at high velocity strikes against a deflector placed above the tube.
5. The deflector used in long tube vertical evaporator can be used as?
a) Vapor separator
b) Foam breaker
c) Pasteurizer
d) Vapor separator and foam breaker
View Answer
Explanation: Deflector placed above the tube. This deflector is effective both as a vapor separator and as a foam breaker. When the ratio of feed to evaporation is low, recirculation of liquid is provided.
6. Which of the following is the disadvantage of climbing film evaporator?
a) Severe scale formation
b) Over heating
c) Small concentration
d) Low pressure
View Answer
Explanation: The main disadvantage of climbing film evaporator is that a comparatively large volume of liquid is present in the evaporator at one time and most of it located at the bottom. Because of this, severe scale formation occurs more rapidly in this region.
7. As the film rises further evaporation occurs until they reach the top of the tubes. The mixture of vapor and milk then passes into a separate chamber from which the vapor passes to a condenser. Name the evaporator.
a) Climbing film evaporator
b) Falling film evaporator
c) Plate evaporator
d) Forced circulation evaporator
View Answer
Explanation: Climbing Film Evaporator, in this type the liquid to be evaporated is fed at the lower end of the vertical tubes, heated by the condensing steam on the outside of the tube. The liquid starts boiling at the bottom; the vapor along with the liquid tends to rise further up the tube in the form of a thin film. As the film rises further evaporation occurs until they reach the top of the tubes.
8. Which of the following is not the advantage of falling film evaporator?
a) Short residence time of liquid
b) Good performance with small temperature difference across heating surfaces
c) Operates at lower temperature difference than rising film evaporator
d) Long residence time of liquid
View Answer
Explanation: The advantages claimed for the falling film evaporator are short residence time of liquid, good performance with small temperature difference across the heating surface. More effects can be used with falling film evaporator as it will operate with a lower temperature difference than climbing film evaporator.
9. Great economies of operation may be made in large installations with ___________
a) Multiple effect evaporator
b) Centri-therm evaporator
c) Falling film evaporator
d) Rising film evaporator
View Answer
Explanation: Considerable saving in steam and water can be effected by using multiple effect evaporators. The vapors from one effect have considerable latent heat and may thus be used for heating second effect and vapors from the second effect to heat the third effect and so on. Thus great economies of operation may be made in large installations with multiple- effect evaporator.
10. Device used to improve thermal efficiency of an evaporator is called?
a) Thermo compressor
b) Deareater
c) Dryer
d) Pasteurizer
View Answer
Explanation: The vapor evolved from the product can be compressed and used as a heating medium. This improves the thermal efficiency of the evaporator. A thermo compressor is used for this purpose.
11. Evaporators in the dairy industry don’t concentrate which of the following?
a) Whole and skimmed milk
b) Whey
c) Condensed milk
d) Cheese
View Answer
Explanation: Evaporators are ideal partner for solutions for the concentration of Whole- and skimmed milk, Whey, Condensed milk and Lactose.
12. Ratio of capacity to economy is known as ________
a) Water consumption per hour
b) Steam consumption per hour
c) Sediment consumption per hour
d) Milk consumption per hour
View Answer
Explanation: Capacity of an evaporator is the amount of water vaporized in the evaporator per unit time. Ratio of capacity to economy is called the steam consumption per hour.
13. __________ is the total mass of water vaporized per unit mass of steam input to the evaporator.
a) Efficiency of evaporator
b) Economy of evaporator
c) Rate of evaporator
d) Capacity of evaporator
View Answer
Explanation: Capacity is defined as the number of kilograms of water vaporized per hour. Economy is the number of kilograms vaporized per kilogram of steam fed to the unit. In a single effect evaporator the economy is approximately always less than 1, but in multiple-effect equipment it may be considerably greater.
14. Single effect evaporation is simple and steam effective.
a) True
b) False
View Answer
Explanation: When a single evaporator is used, the vapour from the boiling liquid is condensed and discarded. Simple but does not use steam effectively.
15. ________ Rule is often used to compare a pure liquid and a solution at a given concentration.
a) Duhring’s rule
b) Plank rule
c) Friction rule
d) Doppler rule
View Answer
Explanation: Duhring’s rule states that a linear relationship exists between the temperatures at which two solutions exert the same vapour pressure. The rule is mostly used to compare a pure liquid and a solution at a given concentration.
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