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INTRODUCTION :

Drying of solids is considered to occur in two stages , a constant rate period followed by a falling rate period. In the constant rate period the rate of drying corresponds to the removal of water from the surface of solid. The falling rate period corresponds to the removal of water from the interior of the solid.The rate in either case is dependent on :

1. Flow rate of air

2. Solid characteristics

3. Tray material.

 

 

Drying can be described in terms of gas mass transfer and heat mass transfer co-effecients.

Rate of drying is given by :

.................................................1

Where Q is the total heat supplied by the gas stream to the solid and is given by :

.................................................2

Total heat supplied = Convective heat + Radiation heat + Conduction heat

Where hc , hr , hk are the heat transfer co-effecient for convection , radiation , and conduction respectively.

These co -effecients are given by :

.................................................3

 

.................................................4

 

.................................................5

 

Putting the value gives

.................................................6

Ratio of (hc/ky) = Lewis number (Le) and is given by:

.................................................7

 

C_s is the specific heat of saturated gas at T_s

For an air water system Le is approximately equal to 1

Equqtions (3) and (7) yields h_c and k_y respectively.

Simultaneously iterative solutions of eqn. (6) with saturation humidity curve provides the the solid surface temperature ,T_s and the corresponding value of humidity Y_s.

Knowing Y_s eqn. (1) is used to find out the drying rate Nc.

NOMENCLATURE :

A : Drying surface area, m²

A_U : Non drying surface area of drying solid , m²

A_m : Average solid surface area , m²

C_S : Saturated specific heat of air, J/kg⁰C

G : Mass velcity of gas, kg/m²-sec

N : Drying rate

N_C : Constant drying rate , kg/m^{^}2-sec

S : Mass of dry solid

T_g : Absolute temperature of gas (dry bulb),K

Z_m : Metal thickness

T_r : Absolute temperature of radiating surface ,K

T_s : Absolute temperature of solid surface ,K