Document wKgDpG4MrvL8Vw16v88rJXVyJ

1162 CHAPTER 47 1958 Guide X1 where the constant rate ends and the drying rate begins to decrease, is termed the critical moisture content. The portion of the curve designated by CB in Figs. 1 and 2 represents a warming-up period, and it may, or may not, be a significant item depending on the total time involved. Constant-Rale Period. Drying during the constant-rate period is equiv alent to evaporation from a free-water surface on the surface of the solid. The rate of drying in this period is determined by the rate of diffusion of water vapor through an air film at the wet surface of the solid. A con stant rate of evaporation on the surface of the solid maintains the surface at a constant temperature, which, in the absence of other heat effects, is very nearly the wet-bulb temperature of the air. If heat flows to the surface of evaporation by radiation and conduction, or both, in addition Industrial; Drying Systems 1163 .. i At = total heat transfer coefficient, Btu per (hour)..(square foot), (Fahrenheit degree). ' '.a,'s A = area of heat transfer and evaporation,'square feet per'pound of hone dry material. H =;enthalpy of evaporation at t., Btu per pound. , ks = ,mass transfer coefficient, pounds per (hour) (square foot) (atmosphere). | A( =. (*-- !) = temperature difference between air and surface of evaporation) Fahrenheit degrees. :'. s!`> U = air temperature, Fahrenheit. . U -- .temperature of surface of evaporation, Fahrenheit.. ., Ap = (p. -- p.) = vapor pressure difference, atmospheres, p, = vapor pressure of water at t., atmospheres, p, = partial pressure of water vapor in air, atmospheres. When h, -- hc, the coefficient of heat transfer by convection only, then t, under equilibrium conditions becomes U>, the wet-bulb temperature of the air, and p, is the vapor pressure at this temperature. If heat is also Fio. 1. Moisture Content w vs. Drying Time 8 (From Reference 2) to convection, the surface temperature will be constant at some value between the air temperature and the wet-bulb temperature. This higher temperature in turn produces a higher constant rate of evaporation. In those dryers in which heat is transferred to a wet solid by conduction through hot surfaces, and heat transfer by convection is not a factor, the wet surfaces approach the boiling point temperature rather than a wet- bulb temperature. When all the heat for evaporation in the constant-rate period is supplied by a hot gas, a dynamic equilibrium is established between the rate of heat transfer to the material and the rate of vapor removal from the surface. This equilibrium between heat and mass transfer rates can be expressed as follows: dw de hiAM H = k,AAp (1) where dw da drying rate, pounds of water per (hour) (pound of bone dry material). Fig. 2. Rate of Drying --drB,, vs. Moisture Content to8 (From Reference 2)' supplied by radiation, then h, is the sum (hc -f hr) where hT is the radia- tbn ?ec`ent and K is the convection coefficient, and t, becomes higher than the wet-bulb temperature. A similar result occurs when heat reaches tne surface of evaporation by convection and conduction. Effect of Air Velocity. The principal effect of air velocity is on ficand s, since the rate of transfer of heat and mass in the constant-rate, period depends mainly on the rate of diffusion of heat and vapor through the air .i the surface of the solid, and air velocity is the chief factor affecting be thickness of this film. The influence of direction of air flow on the heat transfer coefficient h0 and on the corresponding drying rate ~ is shown in Table 1. " temperature or Humidity. Temperature or humidity enters hnth aTM6 rate equation as a driving force across the air film. The wet- and t<Tepression- which is the difference between the dry-bulb temperature :n ... wet-bulb temperature, is directly proportional to the drying rate nis period. When dealing with heat transfer coefficients the wet-bulb