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Heat Transfer Operations Multiple Choice Questions on “Evaporators – Multiple Effect Evaporators”.
1. In a multiple effect evaporator, the pressure __________ as we move to subsequent effects.
a) Decreases
b) Increases
c) Remains same
d) Doubles
Answer: a
Clarification: 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.
2. 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
Answer: a
Clarification: 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.
3. 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
Answer: d
Clarification: 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.
4. 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
Answer: c
Clarification: 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.
5. 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
Answer: a
Clarification: 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.
6. 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
Answer: b
Clarification: 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.
7. The multiple effect evaporators usually use __________ system of evaporation.
a) Flash
b) Natural convection
c) Forced convection
d) Bubble boiling
Answer: a
Clarification: 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.
8. The vapours entering into the steam chest of respective effects of a multiple effect evaporator are at their saturation temperature.
a) True
b) False
Answer: a
Clarification: The basic assumption that is made that the entering steam is at their boiling point just to reduce the amount of calculations.