1. Which one of the following is not an advantage of forced convection boiling?
a) High heat transfer coefficient
b) Positive circulation
c) Very cheap to operate
d) Low fouling or scaling
Explanation: The advantages of forced circulation evaporators can be summarized as –
i. High heat-transfer coefficients
ii. Positive circulation
iii. Reduced fouling or scaling.
Because of these high boiling ratio and extra circulation, the operation cost increases due to the electrical cost of running the agitator and pump, hence it is not usually cheap.
2. Which one of the following is not a demerit of forced circulation evaporator?
a) High operation cost
b) Power consumption for pumps
c) High residence time
d) High temperatures of operation
Explanation: The demerits of forced convection evaporator can be summarized as –
i. Possibly higher cost
ii. Power consumption for the circulating pump
iii. A longer holdup of the product within the heating zone
Whereas the high temperature of operation is a merit as it leads to flash boiling.
3. Forced circulation evaporators are highly suited for ________
a) High corrosive fluids
b) Fouling fluids that usually scale
c) Thin fluids
d) Highly volatile fluids
Explanation: Liquids with a high tendency for fouling, highly viscous liquids, are generally preferred to be used in these evaporators so as to reduce maintenance cost in other types of evaporators.
4. Why is forced convection evaporation also known as flash evaporation?
a) Because heat is supplied in a flash
b) Boiling the fluid in high pressure operation then exposing to low pressure
c) Total evaporation takes place in a flash
d) Very large temperature created
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 which leads to rapid boiling off (flashed) of the liquid. This is the reason why it is known as flash boiling.
5. Agitated film evaporators are best suited for_________
a) High corrosive fluids
b) Fouling fluids that usually scale
c) Highly viscous fluids
d) Highly volatile fluids
Explanation: Highly viscous liquids are generally preferred to be used in these evaporators so as to increase their heat transfer coefficients which otherwise is not possible in other evaporators.
6.How do the Agitated film evaporator increase heat transfer coefficient?
a) It has a set of blades that keeps renewing the film
b) It has mixers that mix the fluid
c) It is a type of climbing film evaporator with agitators
d) It is a horizontal tube evaporator with agitated outer fluid
Explanation: The agitated film evaporators have set of blades rotating around the film with a very small gap that keeps renewing the film and hence exposing the hot fluid to the open surface for larger evaporation.
7. Which one of the following is a merit of agitated film evaporator?
a) Nucleate boiling
b) Falling film
c) Self cleaning equipment
d) High temperature management
Explanation:The agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced, hence it can be regarded as a self cleaning equipment. Self cleaning equipment is a merit of agitated film evaporator.
8. The blades of the agitated film evaporator have a self cleaning effect on the evaporator and hence give us the freedom to use fouling fluids.
a) True
b) False
Explanation: The agitated film evaporator has blades that move at a very close vicinity to the tube with the falling film renewing the film as well as removing the scaling produced, hence it can be regarded as a self cleaning equipment. Self cleaning equipment is a merit of agitated film evaporator and gives us the freedom to use fouling fluids
9. Which one of the following is not a property of agitated film evaporator?
a) Prevents the formation of hot spots due to agitation
b) Absence of static pressure on the boiling liquid
c) Staging or vapour recompression for energy recovery
d) Ability to work in continuous or batch mode
Explanation: In agitated film evaporators, energy cannot be recovered by vapour recompression because it is not practical to conduct compression on the vapour which was obtaining from fouling fluids as it is more or less a waste and the energy expenditure is not a sane step
10. The residence time of agitated film evaporators is ______ than rising film evaporators.
a) More
b) Less
c) Very much higher
d) Negligible
Explanation: The rising film evaporators are the few evaporators which have high residence time compared to others. Agitated film evaporators usually have residence time of 1-5secs.