Heat Transfer Operations Multiple Choice Questions on “Dryers – Properties”.
1. Dryers can generate internal heating in the feed by ______________
a) Dielectric or inductive heating
b) Convection
c) Conduction
d) Combined three means of heating
Answer: a
Clarification: Dryers usually generate het by internal generation such as dielectric or inductive heating or also by radiation such as microwaves.
2. Dehydration and drying are more or less synonyms and have the same operation and purpose.
a) True
b) False
Answer: b
Clarification: Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water or the loss of water as a constituent.
3. Which one of the following method removes water molecule by changing its chemical composition?
a) Drying
b) Dehydration
c) Dewatering
d) De-moisturising
Answer: b
Clarification: Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water or the loss of water as a constituent. And the process removing moisture by the application of low pressure is called dewatering.
4. Dehydration removes moisture _________ efficiency as Desiccation.
a) With less
b) With more
c) With same
d) With very much larger
Answer: a
Clarification: Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water. The process removing moisture by the application of low pressure is called dewatering, and hence the process of dehydration maintains much more aromatic properties than any other.
5. Which one of the following processes try to remove moisture but retain other volatile components in the feed material, and is responsible for retaining important aromatic and flavouring properties of that solution/material?
a) Desiccation
b) Dehydration
c) Drying
d) Dewatering
Answer: b
Clarification: Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water or the loss of water as a constituent.
6. Which one of the following is used to completely remove water and helps in preservation of foods?
a) Desiccation
b) Dehydration
c) Drying
d) Dewatering
Answer: a
Clarification: Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water. The process removing moisture by the application of low pressure is called dewatering, and hence the process of Desiccation is best suited for preservation of food and similar processes.
7. Which one of the following methods uses pressure drop/ lowering of applied pressure to remove the moisture content?
a) Desiccation
b) Dehydration
c) Drying
d) Dewatering
Answer: d
Clarification: The process removing moisture by the application of low pressure is called dewatering. Drying is the process of removing moisture from a wet surface whereas dehydration is the process of complete molecular removal of water or the loss of water as a constituent.
8. Dewatering uses __________ to remove the moisture content of the feed.
a) Pressure gradient
b) Temperature gradient
c) Dry air blow
d) Superheated steam blow
Answer: a
Clarification: Removing the feed moisture from its host material by the action of pressure gradients is called dewatering.
9. ____________ is used as a precursor/ primary process to the drying of heavily moist materials when the moisture-solid bonding is quite weak.
a) Desiccation
b) Dehydration
c) Drying
d) Dewatering
Answer: d
Clarification: The process removing moisture by the application of low pressure is called dewatering and hence is an efficient and cheap method to dry a substance when the moisture-solid bonding is not strong.
10. Sub-cooled steam is not suited steam for drying process as it would ________
a) Condense back
b) Not condense back
c) Not heat much
d) Heat it way to far
Answer: a
Clarification: The statement is wrong because the sub cooled steam is at a temperature below the saturation value whereas we require it above this value, hence the correct answer is superheated steam.
11. Above the _________________ drying in steam is faster than drying in perfectly dry air at the same temperature.
a) Superheated steam temperature
b) Inversion-point temperature
c) Critical point temperature
d) Sub –critical point temperature
Answer: b
Clarification: At the temperature above the Inversion-point temperature, the steam does not condense at all, hence drying can be easily carried out.
12. We cannot use steam as a drying gas to remove moisture as it would rather condense and reverse the results.
a) True
b) False
Answer: b
Clarification: The statement is wrong, we can use steam at temperature above Inversion-point temperature. At the temperature above the Inversion-point temperature, the steam does not condense at all, hence drying can be easily carried out.