dryers ppt

7/27/2019 Dryers PPT.

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Drying Equipment

1.Requirements for commercial dryers

(1)Desired quality of dried products;

(2)Rapid drying rates, and short drying time;

(3)Reliable, safe, and economical operation. Pollution

controlled; energy consumption minimized.

2.Types of drying equipment (Table 5-2, p.276)

(1)Convective/Direct dryers;

(2)Conductive /Indirect dryers;(3)Dryers by radiant energy;

(4)Dryers by dielectric/microwave energy.


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5.4 Drying Equipment

The first and larger group comprises dryers

for rigid or granular solids and semisolidpastes;

the second group consists ofdryers that can

accept slurry or liquid feeds.3.Introduction to common used dryers


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(1).Tray dryersFig.5-16 or Fig.24.8Advantages: Simple structure; Small

fixed charges on equipment; Drying

almost anything, especially for

materials that can not be agitated.

Disadvantages: Expensive to operatebecause of the labor required for

loading and unloading; Great heat

losses; Different qualities of dried

products. 1=air inlet; 2=exhaust duct;3=fan; 4=motor; 5=heater;6=baffles; 7=support racks;

8=truck wheels.


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(2)Screen-conveyor dryers(Fig.5-19 or Fig.24.9)

Coarse granular, flaky, or fibrous materials can

be dried. Capacity of production is large; but

thermal efficiency is low.


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(3)Pneumatic conveying dryers(Fig.5-20)

Exhaustair

Cleaned

air

Feed

SectionFirst

SecondSection

forRoom

solidsseparatingCharacteristics: 1)Greatcontacting area between

air and solids;Higher

heat transfer and mass

transfer rates; Shortdrying time(0.5~2s).

2)Stable operation and

fine product quality.

3)Applying to thermally sensitive and easily

oxidized materials.

Defects: Materials easily broken; great flow

resistance; high drying duct. (about 30m)


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Pneumatic conveying duct maybe as high as 30

meters, but the section of maximum drying rate(maximum heat transfer coefficient ) happens only

near the feed inlet.

1~3m above the feed inlet of the pneumatic

conveying duct is the speeding section, with 1/2~3/4of the total heat transfer quantity.

In order to use the pneumatic conveying and drying

duct effectively, the pulsed structure is applied, and

the relative velocity between air and solids increases,hence heat and mass transfer enhanced.


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(4)Fluid-bed dryers (Fluidized-bed dryers)/Boiling-bed dryers(pp.280~282 or800~802, Figs.5-23 or Fig.24.12, 5-24, 5-25 )

Concept of fluidization

Gas velocity realizing the change from stationary(fixed)

bed into fluidized bed is called the critical fluidized

velocity ucritical.

When gas velocity is increased to the particle free

setting velocity u0 , the particle will be carried over, u0

is called the carried over velocity ucarried.

ucritical


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Characteristics Of fluid-bed dryers:

Advantages:High heat and mass

transfer rates; simple structures; low

fabricating costs; convenient

maintenance; higher thermal efficiencythan pneumatic conveying dryers;

drying time can be changed; applied to

drying of granular materials.

Disadvantages:Strict operation and

control requirements; multi-separately

fluidizedcompartments(Plug flow

dryers) have complex structures and

great flow resista

CollectorDustto

Feed

AirHotrDistributo

Plate

product

discharge

Horizontally separately

fluidized compartments

(Fig. 5-25) can get evendrying and relatively

low flow resistance.


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*(5)Other dryers: Rotary dryers;Spray dryers; etc.


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4.Design of drying equipment(1)Determining the drying conditions

1)Selection of drying media

Air[Applications: temperature not too high, with

or without oxidation]Inert gases[Applications: Easily oxidized and

explosive]Superheated steam[Applications: Clean and high

temperature]Flue gas [Applications: High temperature;

Pollution may occur].


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2)Selection of flow patterns

Cocurrent flow? Countercurrent flow? Cross flow?

Characteristics Of cocurrent flow: Drying rate is high at

the beginning, but drying rate becomes lower at the later

period. Applications: a)materials with high water content;

b)materials without map cracking and coking under highdrying rates; c)materials easily oxidized, color-changed and

decomposed.

Characteristics Of countercurrent flow: Moderate drying

rate. Applications: a)materials with high water content;b)high drying rate not permitted; c)products endurable of

high temperature, water content of product may be very

low.


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Characteristics Of cross flow: Greater driving force of

drying, and high drying rate. Applications: rapid drying ofmaterials and high temperature permitted;

3)t1(temperature of drying medium at the inlet of the dryer)

If t1 is as high as possible, drying process can be enhanced

and economics increased.

4)Determining t2 and H2

;2 dryingofefficiencyThermalt

][ forvcedrivingtransferheatcertainensure30~10(2 Ct

][ againhumidifiednotproductensure

50~20(12 ttor as


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Second, choose two of Lt1t2H2or 2the left two variables are determined by material

balances and heat balances.

max=Maximum permissible temperature of materials

Usually known:1121 HXX

max2

2

)10~5(:

min)5

CncorrelatioEmpirical

ingDeter


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(2) Material balances and heat balances

(3)Selection of drying equipmentConsider:

1)Thermally sensitive materials2)Structure, quality and prices of products;

3)Drying rate curve and critical water content XC4)Adhesion strength of materials.

Generally, choose two or three possibilities first; the

final choice is then made on the basis of capital and

operating costs. Attention must be paid to the costs ofthe entire isolation system, not just the drying unit

alone.


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(4)Calculation of sizes of dryers

sizes of different dryers have different methods of

calculation; refer to examples 5-1, 5-12.

Diameter of pneumatic conveying duct:

)605.(4

2 EqDvLVuD HSg

Height of pneumatic conveying duct:

)615()(0

uuZ g

dryers ppt

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