Food Processing Unit Operations Multiple Choice Questions on “Evaporation Equipment”.
1. Which is the suitable source for heating the evaporators?
a) Steam
b) Direct heating
c) Electric resistance
d) Heaters
Answer: a
Clarification: Steam is the best source as it is inexpensive where as in direct heating, it is difficult to control the heating temperature and high cost in case of electricity.
2. Which of the following is called wiped film evaporator?
a) Falling film evaporator
b) Agitated thin film evaporator
c) Shell and tube evaporator
d) Climbing film evaporator
Answer: b
Clarification: Agitated thin film evaporator is also known as wiped film evaporator. It is the ideal apparatus for continuous processing of concentrated material to dry solids.
3. Which of the following is called calandria evaporator?
a) Falling film evaporator
b) Agitated thin film evaporator
c) Short tube vertical
d) Climbing film evaporator
Answer: c
Clarification: The short tube vertical evaporators are the widely used especially in sugar industry for evaporation of the juice from the cane sugar. It contains short tube bundle which is walled by cylindrical shell. The feed is given to upper tube sheet whereas the steam is fed to the shell.
4. Are these statements about the falling film evaporator true?
Statement 1: Minimize the chances of fouling on tube surfaces.
Statement 2: Fast operation and short contact time for working fluid.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: b
Clarification: Falling film washes away the deposition on tubes that minimize the chances of fouling on surface of the tube. It responds fast to heat source and feed quality supplied. It is efficient to operate across small temperature differences that results in maximum efficiency.
5. How is steam economy calculated?
a) Mass of evaporated liquid/Mass of heating steam
b) Mass of evaporated liquid+Mass of heating steam
c) Mass of evaporated liquidxMass of heating steam
d) Mass of heating steam/Mass of evaporated liquid
Answer: a
Clarification: Steam economy is the number kilogram of water vaporized from all the effects per kilogram of steam used. For single effect evaporator, the steam economy is about <1. It is calculated as the ratio of mass of evaporated liquid to mass of heating steam.
6. What is the principle behind rising film evaporators?
a) Thermo-siphon
b) Thermostatic
c) Thermodynamic
d) Pareto
Answer: a
Clarification: Thermo-siphon is a technique of passive heat exchange which is based on natural convection. It circulates a fluid without the requirement of a mechanical pump. Feed enters the bottom of the heating tubes and the steam begins to form. The mounting force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow.
7. Which of the following evaporator is used to avoid crystallization?
a) Falling film evaporator
b) Agitated thin film evaporator
c) Shell and tube evaporator
d) Forced circulation evaporator
Answer: d
Clarification: Forced circulation evaporators are used to avoid due to the fouling characteristics of the product and also to avoid crystallization. The liquid is heated for each pass through heat exchanger. This is also used in crystallizing applications because there is no evaporation and therefore no increase in the concentration.
8. Which of the following evaporator is mainly used when the feed is almost saturated?
a) Forward feed
b) Backward feed
c) Parallel feed
d) Antiparallel feed
Answer: c
Clarification: Parallel feed in multiple-effect evaporators involves the adding of fresh feed and the withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product. For example, this can be used in evaporation of brine to produce salt.
9. Which of the following evaporator has low compression ratio?
a) Vacuum evaporator
b) Agitated thin film evaporator
c) Shell and tube evaporator
d) Forced circulation evaporator
Answer: a
Clarification: The vapor recompression evaporators are has low compression ratios which are considered by low temperature driving forces across heat transfer surfaces, this leads to larger heat transfer areas.
10. Are these statements about the evaporators true?
Statement 1: Product foaming during vaporization is common.
Statement 2: Foaming can often be minimized by special designs for the feed outlet.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: a
Clarification: Product foaming during vaporization is common. Foaming can often be minimized by special designs for the feed inlet. By reducing the boiling intensity of the liquid on the heat transfer surface and reducing the vapor velocity in the tubes considerably reduce foaming.