1. The fluids in falling tube evaporator usually form a film inside the tubes but there are applications when the fluid forms a film outside the tube and falls subsequently.
a) True
b) False
Explanation: The falling film evaporator can be used in several possible ways, few of which can be noted when the fluid flows outside the tube and inside the tube, in both the cases there is fully developed film for evaporation to take place.
2. . In order to maintain conservation of mass as this fluid fall proceeds in a falling film evaporator, the downward vapour velocity ___________
a) Decreases
b) Increases
c) Remains Same
d) Oscillates
Explanation: Maintaining the mass conservation falling process proceeds with the downward vapour velocity increases and also increasing the shear force acting on the liquid film as well as the velocity. This result in the turbulent flow increase which allows very high heat transfer coefficients
3. What do we mean by forced convection evaporator?
a) This is nucleate boiling
b) This has a fan to remove vapour from the surface
c) There is an agitator to mix the heated fluid
d) There is an agitator to mix the heating medium such as steam
Explanation: By forced convection we mean to say that the fluid being heated does not face wall/surface boiling by providing proper mixing conditions, hence in such evaporators, there is an agitator to mix the heated fluid.
4. With forced convection boiling, nucleate boiling is absent which makes wall temperature very close to interface temperature.
a) True
b) False
Explanation: In forced convection boiling, heat transfer occurs directly from the wall to the interface and this process becomes so efficient that the wall temperature is unable to sustain the nucleate boiling.
5. The presence of forced convection makes the complete mixed fluid boiling pattern known as __________
a) Pool boiling
b) Surface boiling
c) Bubble boiling
d) Film boiling
Explanation: The forced convection induced boiling is called pool boiling because the fluid in contact with the heating wall is continuously renewed and the bulk of the liquid reaches that temperature, hence its name- Pool boiling.
6.The boiling that takes place in forced convection boiling is popularly known as surface boiling.
a) True
b) False
Explanation: The forced convection induced boiling is called pool boiling because the fluid in contact with the heating wall is continuously renewed and the bulk of the liquid reaches that temperature, hence its name- Pool boiling.
7. Which one of the following evaporator does not use forced boiling as its boiling operation?
a) Plate type evaporators
b) Horizontal tube evaporators
c) Vertical tube evaporators
d) Falling tube evaporators
Explanation: The falling film evaporator belongs to the class of natural convection evaporators and not to forced convection.
8. The Residence tie in forced convection evaporators is ________ than natural convection evaporator.
a) More
b) Less
c) Same
d) Very less
Explanation:The residence time in forced convection evaporators is more because the time taken to heat the bulk of liquid rather than the surface is more.
9. Forced circulation is sometimes more suitable because of which one of the following statements?
a) It has Low pressure operations
b) The temperature can be maintained as circulation is present
c) The flow velocity in the tubes and the liquid temperature can be controlled to suit the product requirements
d) Cheaper than others
Explanation: Liquid circulation is maintained with the flow velocity in the tubes and the liquid temperature can be controlled with the product requirements. Forced circulation is sometimes more suitable with flow velocity.
10. Forced convection boiling is also known as __________
a) Flash boiling
b) Nucleate boiling
c) Partial nucleate boiling
d) Bubble boiling
Explanation: The liquid used in forced convection boiling is superheated with high pressure, more than its normal pressure of boiling and hence when it enters the setup, the pressure in the liquid falls quickly leading to the liquid being rapidly boiled off (flashed). This is a reason why it is known as flash boiling.