1.The product concentration in a backward feed evaporator ___ as we move to successive evaporators.
a) Increases
b) Decreases
c) Remains same
d) Is different for each
Explanation:The backward feed setup has series connection where the product of a particular single effect is the feed to the other in the opposite direction, i.e. from back to front. Hence the product concentration decreases with each subsequent evaporator.
2. For mass balance in the forward pass evaporator, select the correct option.
a) F = E3 + P3
b) F = E1 + E2 + E3 + P3
c) F = P1 + P2 + P3
d) F = E1 + E2 + E3 + P1 + P2 + P3
Explanation: The mass balance is F = E1 + E2 + E3 + P3 because the product of a particular setup is the feed for another and if we join the whole setup as one, we see F entering and E1 + E2 + E3 + P3 leaving.
3. Which evaporator uses the maximum number of pumps for its operation?
a) Forward feed
b) Backward feed
c) Parallel feed
d) Mixed feed
Explanation: The backward feed evaporator requires a pump between each pair of effects to transfer liquid from lower pressure effects to higher pressure effects and this is in maximum effect only in backward feed and to an extent in the mixed feed.
4.In a multiple effect evaporator, the pressure __________ as we move to subsequent effects.
a) Decreases
b) Increases
c) Remains same
d) Doubles
Explanation: Because the boiling temperature of water decreases as pressure decreases, the setups are made to allow us to use the superheat of one of the setup to heat the subsequent ones.
5. What is the reason for using decreasing pressure on every subsequent effects in a multiple effect evaporator?
a) To be able to reuse the vapour from previous effect
b) To decrease the vapour pressure
c) To increase the boiling point
d) To decrease the boiling point
Explanation: The main reason to reduce the pressure is to be able to decrease the boiling point so that the previous effect’s vapour can evaporate the water in this effect. Because the boiling temperature of water decreases as pressure decreases, the setups are so made to allow us to use the superheat of one of the setup to heat the subsequent ones.
6.In a multiple effect evaporator with more than one effects, the size of each effect should __________
a) Decrease if pressure decreases
b) Increase if pressure decreases
c) Remain same
d) Should increase if pressure increases
Explanation:The size of the evaporator does not matter to increase the rate of evaporation whatsoever, but keeping them all the same size helps reduce manufacturing cost.
7. Which one of the following evaporator is usually preferred for operation in a multiple effect for less viscous liquids?
a) Falling film long tube
b) Agitated vessel
c) Rising film
d) Plate type evaporator
Explanation:The falling film is most well suited for operation if we want to use it for low viscous fluids as it is cheap for both construction and operation.
8. Which one of the following evaporator setup is the most versatile of all evaporators?
a) Falling film long tube
b) Agitated vessel
c) Rising film
d) Plate type evaporator
Explanation: The agitated vessel evaporator is the most versatile evaporator as it allows us and give the industry the liberty to use thin or thick fluids, viscous or non-viscous ones, and also the fouling and scaling ones without any extra maintenance.
9. The multiple effect evaporators usually use __________ system of evaporation.
a) Flash
b) Natural convection
c) Forced convection
d) Bubble boiling
Explanation:Multiple effect evaporation usually uses sensible heat in the condensate to preheat liquor to be evaporated. Hence as the pressure drops significantly in subsequent effects, the feed is always flashed into these effects, thus the flash system is predominant.
10. Which one of the following is not an assumption in multiple effect evaporator’s calculations?
a) There is no sub cooling of the condensate from different steam chests
b) Heat transfer surface does not undergo fouling
c) The entering steam is at their boiling point
d) Pressure is same in every effect
Explanation: The assumptions that are usually made while designing evaporators are:
- The vapours entering in to steam shell of respective effects are at their saturation temperature
- There is no sub cooling of the condensate from different steam chests
- Condensation of vapour in steam chest occurs at constant pressure
- There is no carry-over of liquid droplets with vapours leaving the respective effects.
- There is no heat dissipation to surroundings
- Heat transfer surface does not undergo fouling.