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2- REASONS FOR DRYING:

!a" To reduce the cost of transport.

!b" To make a material more suitable for handling as, for e#ample, with soap

powders, dyestuffs and fertilizers.

!c" To provide definite properties, such as, for e#ample, maintaining the free$

flowing nature of salt.

!d" To remove moisture which may otherwise lead to corrosion. %ne

e#ample is the drying of gaseous fuels or benzene prior to chlorination.

3- GENERAL PRINCIPLES:

The moisture content of a material is usually e#pressed in terms of its

water content as a percentage of the mass of the dry material, thoughmoisture content is sometimes e#pressed on a wet basis.

&f a material is e#posed to air at a given temperature and humidity, the

material will either lose or gain water until an euilibrium condition is

established.

This euilibrium moisture content varies widely with the moisture

content and the temperature of the air.

A non$porous insoluble solid, such as sand or china clay, has an

euilibrium moisture content approaching zero for all humidities and

temperatures, although many organic materials, such as wood,

te#tiles, and leather, show wide variations of euilibrium moisture

content.

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Moisture may be present in two forms:

Bound moisture' This is water retained so that it e#erts a vapourpressure less than that of free water at the same temperature. Such

water may be retained in small capillaries, adsorbed on surfaces, or as

a solution in cell walls.

Free moisture' This is water which is in e#cess of the euilibrium

moisture content.

4- HUMIDIFICATION TERMS:

&n design of dryers using air, &t is reuired to know the properties of air$

water system !humidity chart", and humidity terms'

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Wet !" te#$e%&t!%e: T'e te#$e%&t!%e *+&te* , & t'e%##ete% .'e .&te%

&* &% %e&+' e/!"%!#0

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5- TYPES OF DRY!"'

During the drying process, (ot air enters with a humidity !()", and a relative

humidity (r), and e#its with a (umidity !(*", and relative humidity (r*. The

path from (r)to (r*may be carried out at a constant Td.b, in this case the

process is called isothermal drying. &f the path is taken at constant Tw.b, in

this case, the process is called adiabatic drying.

Types of dryin#:

$-sot%erma& dryin#

'-(diabati) dryin#

E*amp&e +$,:

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Air containing +.++ kg water vapour per kg of dry air is heated to -* in

a dryer and passed to the lower shelves. &t leaves these shelves at /+ per cent

humidity and is reheated to -* and passed over another set of shelves,

again leaving at /+ per cent humidity. This is again repeated for the third and

fourth sets of shelves, after which the air leaves the dryer. %n the assumption

that the material on each shelf has reached the wet$bulb temperature and that

heat losses from the dryer may be neglected, determine'

!a" the temperature of the material on each tray0

!b" the amount of water removed in kg1s, if m-1s moist air leaves the dryer0

!c" the temperature to which the inlet air would have to be raised to carry out

the drying in a single stage.

So&ution:2or each of the four sets of shelves, the condition of the air is

changed to /+ per cent humidity along an adiabatic cooling line. &nitialcondition of air' T = -* , ( 3 +.++ kg1kg

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- Rate of Dryin#:

&n drying, it is necessary to remove free moisture from the surface and also

moisture from the interior of the material. &f the change in moisture content

for a material is determined as a function of time, a smooth curve is obtained

from which the rate of drying at any given moisture content may be

evaluated. The form of the drying rate curve varies with the structure and

type of material, and two typical curves are shown'

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&n curve ), there are two well$defined zones' A4, where the rate of drying is

constant and 45, where there is a steady fall in the rate of drying as the

moisture content is reduced. The moisture content at the end of the constant

rate period is represented by point 4, and this is known as the critical

moisture content.

5urve * shows three stages, D6, 62 and 25. The stage D6 represents a

constant rate period, and 62 and 25 are falling rate periods. &n this case, the

Section 62 is a straight line, however, and only the portion 25 is curved.

Section 62 is known as the first falling rate period and the final stage, shown

as 25, as the second falling rate period. The drying of soap gives rise to a

curve of type ), and sand to a curve of type *.

- TIME FOR DRYING:

&f a material is dried by passing hot air over a surface which is initially wet,the rate of drying curve in its simplest form is represented by 456, shown in

2igure'

where' w is the total moisture,

weis the euilibrium moisture content !point 6",

w 7 we is the free moisture content, and

wcis the critical moisture content !point 5".

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!ote: f . w-we

f$.w$-we

f). w)-we

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