250+ TOP MCQs on Evaporators – Effect of Liquid Head and Boiling Point Elevation

Heat Transfer Operations Assessment Questions and Answers on “Evaporators – Effect of Liquid Head and Boiling Point Elevation”.

1. The boiling point of the solution ________ as we move to subsequent effects in a forward feed evaporator.
a) Decreases
b) Increases
c) Remains same
d) Oscillates

Answer: b
Clarification: The concentration of the feed increases as we move to subsequent effects in the evaporator setup. We know that due to the colligative properties of water, the boiling point increases with increasing concentration.

2. The boiling point of the solution ________ as we move to subsequent effects in a backward feed evaporator.
a) Decreases
b) Increases
c) Remains same
d) Oscillates

Answer: a
Clarification: The concentration of the feed decreases as we move to subsequent effects in the evaporator setup because the feed supply is opposite to that of forward flow, hence the lowest concentration is in the last effect and highest in the first one. We know that due to the colligative properties of water, the boiling point increases with increasing concentration.

3. The boiling point of the solution ________ as we move to subsequent effects in a Mixed feed evaporator.
a) Decreases
b) Increases
c) Remains same
d) Oscillates

Answer: d
Clarification: The concentration of the feed as we move to subsequent effects in the evaporator setup cannot be sequentially represented because the feed supply is mixed/ haphazard as compared to that of forward/backward flow. We know that due to the colligative properties of water, the boiling point increases with increasing concentration and hence we can follow the feed to determine the trend of concentration.

4. The boiling point of the solution ________ as we move to subsequent effects in a parallel feed evaporator.
a) Decreases
b) Increases
c) Remains same
d) Is independent

Answer: d
Clarification: The parallel feed operation has independent feed supplies to each effect which has no connection to the prior evaporators, and hence it has no such trends.

5. What is the possible solution to increase the evaporation rate in subsequent effects in forward feed evaporators?
a) Increasing pressure
b) Decreasing pressure
c) Increasing temperature
d) Decreasing temperature

Answer: b
Clarification: The most economic and fruitful way to evaporate water after its concentration increases (BP also hiked) is to lower the pressure as lowering the pressure decreases its boiling point and hence the solution can now boil by the steam from the previous effect.

6. Forward feed evaporator use minimum energy of operation as the there is no requirement of pumps to carry the feed from effect to effect.
a) True
b) False

Answer: a
Clarification: Yes, the statement is correct. Forward feed evaporator in order to evaporate water after its concentration increases is to lower the pressure as lowering the pressure decreases its boiling point and hence the solution can now boil by the steam from the previous effect. This in a way creates pressure difference in the effects which pull the feed from former effects to the subsequent effects without requiring pumps.

7. Which one of the following evaporator uses maximum mechanical energy to operate?
a) Forward feed
b) Backward feed
c) Parallel feed
d) Mixed feed

Answer: b
Clarification: The backward feed does have more running cost than others because it pumps backwards from low pressure to high pressure and makes sure that the highest concentration product receives the maximum heat (directly from steam) which increases the evaporate yield considerably but at a cost.

8. The pressure in backward feed evaporator also decreases along the feed direction to avoid excessive use of pumps.
a) True
b) False

Answer: b
Clarification: This operation is not possible as in such cases the vapour released in the 1st effect will not be able to effectively heat the solution in subsequent effect.

9. The concentration of the feed in parallel feed evaporator ____________ with subsequent effects if pressure decreases.
a) Should increase
b) Should decrease
c) Should be same
d) Cannot be inferred

Answer: a
Clarification: The concentration should increase only then the evaporation would be effective for the setup, else if we have same concentration for all the feed, we should opt for forward feed.

10. Which one of the following evaporator can work without pump?
a) Forward feed
b) Backward feed
c) Parallel feed
d) Mixed feed

Answer: a
Clarification: Forward feed evaporator in order to evaporate water after its concentration increases is to lower the pressure as it decreases its boiling point and hence the solution can now boil by the steam obtained from the previous effect. This in a way creates pressure difference in the effects which pull the feed from former effects to the subsequent effects without requiring pumps.

11. Backward feed evaporators are superior to forward feed in concentrating solutions to a higher extent! Why?
a) Because the highest concentration effect gets the highest pressure
b) Because the highest concentration gets the highest temperature
c) Because the highest concentration effect gets the lowest pressure
d) Because the highest concentration gets the lowest temperature

Answer: b
Clarification: The backward feed has more running cost than others as it pumps backwards from low pressure to high pressure making sure that the highest concentration product receives the maximum heat (directly from steam) which increases the evaporate yield considerably but at the cost of electrical pumps.

12. How is the pressure application in different effects in a multiple effect evaporator decided?
a) Evaluating boiling point elevation by Duhring’s rule
b) Boiling point fall due to pressure drop by Arrhenius rule
c) Evaluating boiling point elevation by Kozeny Carman rule
d) Boiling point fall due to pressure drop by Sieder tate rule

Answer: a
Clarification: The Duhrings’ rule is a rule that measures BPE of a solution. The boiling point of a solution is a linear function of the boiling point of the pure water at the same pressure.