Document ByqJnVvZageaX0RnBynxgxqL4

1164 CHAPTER 47 '1958 Guide depression is the driving force, whereas with mass transfer coefficients the driving force is expressed in terms. of humidity or vapor pressure differential. Fig. 3 permits a ready estimate of the constant drying rate for various air temperatures and humidities. The chart is based on the difference between'.the dry-bulb' and wet-bulb temperatures of the entering stream of air' and on an air velocity of 300 fpm. It may be assumed satisfactory for tray drying of any material in the constant-rate drying period. It does not apply to rotary or through-circulation drying. A' curve for correcting Table 1. Convection Heat Transfer (hc) and Rates of Drying ... . ,Coefficients:for Constant Rate, Period. ,, _ , Direction of Aib Flow .,, = ., 1! . dw d$ Parallel to plane-surfaces* i, Perpendicular to plane surfaces1 0.0128G08 , 0.37G" 0.0128G98A jj (>a t-w) 0.37G0 J7A , - g (f. - f.) Through circulation4' (for Reynolds number > 300) 0.37G0-59c, Dp 0.37c,oG'Afm Letters Symbols for Table I (not previously defined) hc -- convection heat transfer coefficient, Btu per (hour) (square foot) (Fahren heit degree). G = mass velocity of dry air, pounds per (hour) (square foot). t., -- wet-bulb temperature of drying air, Fahrenheit. a = drying area, square feet per cubic foot of bed volume. p, = bulk density of dry granular bed, pounds per cubic foot. Dp = average diameter of particle, feet: Atm = logarithmic mean difference between air temperature entering and leaving the bed and the wet-bulb temperature, Fahrenheit. c,, = humid heat, Btu per (pound of dry air) (Fahrenheit degree). the air velocity is incorporated in Fig; 3. This1 curve is based on the vari ation of drying'rate'with the 0.8 power'of the velocity: Evaporation'from'Liquid, Drops. ' For the important problem ofspray drying,' evaporation rates of liquid drops must be estimated, ' Below a value: of-Reynolds'number (D,,G/p) of 10 for spherical particles, the heat transfer"coefficient across the gas film surrounding the drop is givenjb? ...........h . ' where '' ' ' h = --' " : '- ,'i'1'(2) ,,.-r ;lj . A = film heat transfer coefficient, Btu per (hour) (square foot) (Fahrenbeit-.de- . gree)-, , ... . ki = thermal conductivity of gas film, Btu per (hour) (square foot) (Fahrenh?A ' "degree per'foot). ' . foflfi 'Equation'2'is applicable when the Reynolds number for liquid'drops Industrial'Drying Systems llOfir is less than 10. Drop diameters are almost always less,than 500imicrons,-, and usually;in the range-of 20 to 150 microns. . -. The rate of evaporation of drops may be expressed in terms'of- heat1 transfer or mass transfer. In terms of heat transfer; the evaporation'rate is given by the equation: i- dw 2rrkfPt de H (fa - f.) (3) where - = evaporation rate, pounds per hour: tb ^>T^ln^ Air Temperatures above the Boiling Point of the Liquid. When the temperature of the drying air is maintained above the boiling point of he liquid being evaporated, or when superheated vapors are used for arying, the usual equations for mass transfer, expressing rate of evaporation as a function of the vapor pressure difference, lose significance, since large rrors are introduced in the expression for 'vapor, pressure driving force ie tl ^ apparently small value. Such cases can be treated, conven- I ly ona basis of heat transfer, since a temperature difference must ways existnn order that drying may proceed. :- RadiPeriod When Heat Transfer Depends on Conduction and deDpH*771 in(hrect drying, where heat transfer and drying do not pend on the flow of heated gases, the drying rate depends either on heat