This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Dryers – Microwave Drying”.
1. Which one of the following dryers match this description – ‘The solid to be dried are fed on to endless, perforated, conveyor belt through which hot air is forced’?
a) Drum drying
b) Microwave Drying
c) Screen Conveyor Dryer
d) Rotary drying
View Answer
Explanation: Screen Conveyor Dryer is setup with a screen that carries the wet feed above and a stream of dry hot air is passed onto it from below which dries it on the way to the end. This dryer is quiet efficient at its work as the screen size can be adjusted according to the particle size. Hence the given description correctly matches it.
2. Any kind of material possible such as coarse ones or granular or even powdery substance can be dried by circulating in a screen conveyor dryer without the requirement of any prior-treatment or loss in feed material.
a) True
b) False
View Answer
Explanation: Yes, the statement is true. The Screen Conveyor Dryer is a setup with a screen that carries the wet materials of any parent shape on a screen and a stream of dry hot air is passed onto it from below which dries it to the end. This dryer is very efficient at its work as the screen size can be adjusted according to the particle size.
3. What is the name of the following drying equipment?
a) Drum drying
b) Microwave Drying
c) Screen Conveyor Dryer
d) Rotary drying
View Answer
Explanation: The Screen Conveyor Dryer is a setup with a screen that carries the wet materials of any parent shape on a screen and a stream of dry hot air is passed onto it from below which dries it to the end to a collector.
4. The thermal efficiency of a Screen Conveyor Dryer is usually ________
a) High at around 50-70%
b) 100%
c) 20%
d) Very Low < 20%
View Answer
Explanation: The Screen Conveyor Dryer is a setup with a screen that carries the wet materials of any parent shape on a screen and a stream of dry hot air is passed onto it from below which dries it to the end to a collector. The thermal efficiency is high and in case of steam heating, the steam consumption rate can be as low as 1.5-1.2 kg per kg of evaporated water.
5. Which one of the following dryers matches this description – ‘A high-frequency generated waves is guided by a wave channel into an oven that is designed to remove the water from the feed to be dried’?
a) Drum drying
b) Microwave Drying
c) Furnace drying
d) Rotary drying
View Answer
Explanation: High frequency waves or low wavelength waves carry the maximum amount of energy in them which can reach the deepest of the capillaries of the substance and heat it to be dried. This is the mechanism of operation of a microwave dryer.
6. Microwave drying setup of heating uses low wavelength waves that carry the maximum amount of energy in them which can reach the deepest of the capillaries of the substance and heat it to be dried, which is a great advantage for the processing in _________________ industries.
a) Cement
b) Pharmaceutical
c) Limestone
d) Petrochemical
View Answer
Explanation: High frequency waves or low wavelength waves carry the maximum amount of energy in them which can reach the deepest of the capillaries of the substance and heat it to be dried. It also takes care of the frequency to be applied on the feed and hence is also suited for sensitive feeds such as in the pharmaceutical industries.
7. Radiation dryers usually use ___________waves.
a) High frequency
b) Low frequency
c) Yellow light
d) Bright red visible frequency
View Answer
Explanation: High frequency waves or low wavelength waves carry the maximum amount of energy in them which can reach the deepest of the capillaries of the substance and heat it to be dried.
8. Why is microwave drying so efficient? Select the most relevant answer.
a) High temperature
b) High heat transfer rate
c) Heat is generated deep within the material to be dried
d) Low temperature operation ability
View Answer
Explanation: Heat is generated by directly transforming the electromagnetic energy into kinetic molecular energy. High frequency waves or low wavelength waves carry the maximum amount of energy in them which can reach the deepest of the capillaries of the substance and heat it to be dried and hence it can remove the unbound as well as the bound moisture from the substance lattice.
9. The efficiency of microwave dryers is usually in the range of __________
a) > 70%
b) < 70%
c) > 90%
d) < 50%
View Answer
Explanation: Microwave dryers heat the substance to the very core and hence can remove the unbound as well as the bound moisture from the substance lattice. As the unbound moisture is about 60% of the feed material, removing bound moisture here increases the efficiency to above 70%.
10. _____________ method allows the food product to be dehydrated without undergoing a phase change from liquid water to water vapour.
a) Superficial fluid Extraction
b) Supercritical Fluid Extraction
c) Superheated fluid Extraction
d) Sub-cooled fluid Extraction
View Answer
Explanation: A supercritical fluid is an extraordinary state of a system when the fluid can behave like a gas as well as a liquid at the same time. The possibilities that arise are that it can dehydrate the food product without undergoing a phase change from liquid water to water vapour.
11. ______________ can move through solids like a gas, and dissolve materials in itself like a liquid and has the ability to possess unique properties that makes it capable to extract components selectively from a mixture which grabs a place for it in drying processes.
a) Superficial fluid
b) Supercritical Fluid
c) Superheated fluid
d) Sub-cooled fluid
View Answer
Explanation: A supercritical fluid is an extraordinary state of a system when the fluid can behave like a gas as well as a liquid at the same time. The possibilities that arise are that it can effuse through solids like a gas, and dissolve materials like a liquid and possess unique properties that enable them to extract components selectively from a mixture which makes its application in drying processes.
12. If the drying operation is concerned as a setup, adiabatic dryers are __________ non-adiabatic dryers.
a) Less expensive than
b) More expensive
c) More efficient
d) Almost similar in cost and efficiency
View Answer
Explanation: Adiabatic dryers are less expensive although they have low efficiency because there is an insulating cover that does not allow heat to escape, which saves the industry of heating cost and increases thermal efficiency in the cost of drying efficiency.
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