# Heat Transfer Operations Questions and Answers – Continuous Flow Dryers

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This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Continuous Flow Dryers”.

1. Which one of the following is not a continuous feed dryer?
a) Agitated
b) Rotary
c) Drum
d) Flash

Explanation: The agitated dryer is a commonly used batch dryer. Agitated dryer is the most versatile dryer as it has the ability to handle large feed, fouling/scaling feeds, crystallising feed and many others by its advanced methods of blades. It also has very low residence times as the scraping process of these dryers speeds up the process.

2. The moist feed is fed in at the _________ end of a rotary dryer and the material progresses to the ________ end, where it gets discharged.
a) Upper and Lower
b) Lower and Upper
c) Lower and Lower
d) Upper and Upper

Explanation: The setup is made in a gravitation pull setup basis in which the feed comes from above and then leaves from the bottom. Here the feed is subjected to a rotating large drum/vessel that keeps rotating slowly to thoroughly mix the feed with the hot dry air to effectively dry it. The setup is similar to the cement mixers in general building operations.

3. The number of possible heat transfer units in a rotary dryer is approximately ___________
a) 2.0 – 3.0
b) 1.5 – 2.0
c) 2.5 – 6.5
d) 6.5 – 9.5

Explanation: The Number of transfer units defined as the maximum heat transfer that can occur in a Heat exchanger.
To determine NTU or Effectiveness Number there are some assumptions

1. The Heat exchanger has infinite length
2. The rate of Heat transfer is Maximum

4. Which dryer has the following description – ‘a liquid containing dissolved solids or slurry carrying suspended solids forms a thin layer on the outside surface of a large rotating vessel’?
a) Agitated dryer
b) Drum dryer
c) Rotary dryer
d) Furnace dryer

Explanation: The drum dryer can be represented as –

5. In a drum dryer, the thickness of the slurry layer cannot be controlled.
a) True
b) False

Explanation: A drum dryer as given in the diagram above, a mixture of feed having quality solid suspension which is to be dried, is fed to it from the top the rotating drums which there after forms a thin layer on the surface of the rotating drum, which then falls as a powdered product at the bottom. If we use a double drum, the product layer thickness can be controlled by the gap between the two drums.

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6. For a single drum, the thickness of the film can be controlled by ___________ and in case of a double drum setup the thickness of product here can be controlled by ______________
b) The drums, the fins
d) The fins, the drums

Explanation: The drum dryer can be represented as –

A drum dryer as given in the diagram above, a mixture of feed having quality solid suspension which is to be dried, is fed to it from the top the rotating drums which there after forms a thin layer on the surface of the rotating drum, which then falls as a powdered product at the bottom. If we use a double drum, the product layer thickness can be controlled by the gap between the two drums which can be readily adjusted.

7. Drum dryer mainly handles the materials that are too thick for a spray dryer and too thin for a rotary dryer.
a) True
b) False

Explanation: The distance between the drums/ the drum gap can be adjusted according to our requirements hence a drum dryer has the ability to manage thick material unsuitable for spray dryer and quite thin materials unsuitable for a rotary dryer.

8. Which one of the following is the most flexible continuous flow dryer?
a) Agitated dryer
b) Drum dryer
c) Rotary dryer
d) Furnace dryer

Explanation: The distance between the drums/ the drum gap can be adjusted according to our requirement, hence a drum dryer is versatile as it can handle the materials that are too thick for a spray dryer and too thin for a rotary dryer.

9. A flash dryer is similar in its operation to a _________ as the feed is dispersed finely in an upward flowing hot air.
a) Agitated dryer
b) Drum dryer
c) Rotary dryer
d) Spray dryer

Explanation: Flash dryers are used to dry materials by breaking them into fine particles and spread them over a region to pass very hot dry air above it. These types of dryers are usually used for drying materials sensitive to heat as it has low residence time.

10. Which one of the following dryers are best suited for drying of heat sensitive or easily oxidizable materials?
a) Furnace
b) Drum
c) Flash
d) Rotary

Explanation: Flash dryers are used to dry materials by breaking them into fine particles and spread them over a region to pass very hot dry air above it. These types of dryers are usually used for drying materials sensitive to heat as it has low residence time.

11. Which one of the following dryers have high drying rate but low residence time?
a) Furnace Dryer
b) Drum Dryer
c) Flash Dryer
d) Rotary Dryer

Explanation: The drying rate is very high for a flash dryer and hence low residence time. Flash dryers are used to dry materials by breaking them into fine particles and spread them over a region to pass very hot dry air above it. These types of dryers are usually used for drying materials sensitive to heat as it has low residence time. The other options necessary don’t use very high temperature feed gases ant their residence time is greater than a flash dryer.

12. Which one of the following is not an advantage of flash dryers?
a) Low residence time
b) High drying rate
c) The thermal efficiency this type of dryer is high
d) No separate arrangement is required for transporting the dried product

Explanation: The drying rate is very high for a flash dryer and hence has its low residence time, these driers operate by using wet materials (mainly heat sensitive materials) that are to be dried, fed in finely divided form in a stream of upward flowing hot air.

13. For a fluidised bed dryer, which one of the following is correct?
a) Dry hot air is passed as the fluid on a bed of wet material
b) Hot fluid outside the tube containing particles
c) Wet hot air is passed as the fluid on a bed of dry material
d) Dry hot air is passed as the fluid on a packed bed of wet material

Explanation: The fluidised bed can be represented as the following –

The dry hot air is passed as the fluid on a bed of wet material.

14. Fluidised bed dryers work best with which of the following bed material?
a) Crystalline granular solids
b) Finely divided solids
c) Powdered substance
d) Large pellet shaped

Explanation: A Fluidized bed dryer is best suited for granules or crystals because if there are by any chance fine particles present in the bed, there may be solid fly-out with the outgoing gas because of which filters are required for its recovery.

15. In fluidised-bed dryers, a separate component is provided in which the solids move sequentially from the inlet to outlet, such setup is known as ______________ where the residence time is more or less the same for all particles in that compartment.
a) Drum dryers
b) Plug flow dryers
c) Fluidised bed dryers
d) Rotary dryers

Explanation: The plug flow reactor is the setup in which the feed materials spend equal amount of time or have same residence times in a continuously flowing system.

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