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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