Document LKYDbEDxNeeQeg6MXBe5gqbbg

* 1146 CHAPTER 47 1957 Guide Industrial Drying Systems 1147 Table 1 gives an approximate classification of materials which are most likely to obey Equations 19 and 20. ' Equations 18 and 20 hold for cross-circulation drying. When through- where _ p.Lx(lV. - IV.) _ 1_ a ~ hi(U - U) K circulation drying is involved, the appropiate constant-rate expression 0, = total drying time, hours. given by Equation 4 must be used to determine K in Equation 16. Thus, for through-circulation drying in the falling-rate period when Equation 15 0. = drying time for constant-rate period, hours. 0f = drying time for falling-rate period, hours. holds, the rate is given by W0 = initial moisture content, pounds per pound of dry solid. _ W = critical moisture content, pounds per pound of dry solid. 0.37c,ag|>'B(Al)c. (W - (W.) pMV uGE> - JFd (21) W. = equilibrium moisture content, pounds per pound of dry solid. W = moisture content at time 0t, pounds per pound of material. hi -- total overall heat transfer coefficient Btu per (hour) (square foot) ;Fahren- where the symbols have been defined for Equations 4, 12, and 16. heit degree). I Critical Moisture Content. In order to use the above equations for estimating the drying time in the falling-rate period, it is necessary to know values of the critical moisture content. Such values are usually difficult t. = air temperature, Fahrenheit. 1, - temperature of surface of material, Fahrenheit. L = depth of material in tray, feet. X = latent heat of evaporation at <,, Btu per pound, p, = density of dry solid; pounds per cubic foot. Table 1. Approximate Classification of Materials Most Likely to Obey Equations 19 and 20 19Materials Obeying Equation 20Materials Obeying Equation 1. Single-phase solid systems such as soap, gelatin, glue. 2. Wood and similar solids below the fiber saturation point. 3. Last stages of drying starches, textiles, paper, clay, hydrophilic Bolids, and other materials when bound water is being removed. 1. Coarse granular solids, such as sand, paint pigments, minerals, etc. 2. Materials in which moisture flow occurs at concentrations above the equi librium moisture content at atmos pheric saturation, or above the fiber saturation point. Equation 22 will apply to those materials satisfying Equation 20 when (hying to very low moisture content is not involved. For through-circulation drying, an expression similar to Equation 22 is obtained. Thus, the total drying time for through-circulation drying is given by where 2.7p.XDp oqy. - IF.) c.aG-(Ai)m to obtain without making actual drying tests which, in themselves, would give the required drying time and thereby obviate the necessity of the calculations. It appears that the constant-rate period ends when the moisture content The drying times estimated from Equations 22 and 23 apply only to cross-circulation drying and through-circulation drying, respectively. Drying times for other methods, such as rotary drying or drum drying, must be estimated by other methods. at the surface reaches some specific value. If the rate of drying is great, Equilibrium Moisture Content i'1 the moisture gradients within the solid will be steep and the average moisture content considerably greater than that at the surface. It is for this reason that the critical moisture content (average through the ma terial) increases with increase in rate of drying, and with an increase in In the (hying of solids it is important to distinguish between hygroscopic and non-hygroscopic materials. A hygroscopic material is one which retains a definite percentage of moisture under definite conditions of air humidity. This bound moisture is in a state of equilibrium with the water thickness of the layer being dried. vapor in the surrounding air, and a decrease in the water vapor content will Approximate Equations for Estimating Drying Time decrease the amount of equilibrium bound water. Water so retained by a solid in equilibrium with the humidity of the surrounding air, is An estimate of the overall drying time for a given drying problem designated as the equilibrium, moisture content. Such moisture may be held usually involves an estimate of the time required for the constant-rate as adsorbed surface films or condensed in fine capillary structures at period, plus an estimate of the time for the falling-rate period. An ap reduced vapor pressure. proximate equation for the overall drying time applicable to the cross circulation drying of materials of the type listed in Table 1 as obeying The equilibrium moisture content varies with the temperature and humidity of the surrounding air. Consequently, any correlation of equili i Equation 20, may be written as follows: or. - WjhLp. , P.LHW, - W,) W.-Wj 0t = 0c H" ht(t, - O + fc,(f.-e.) g" w - W' brium moisture content should take these two factors into account. How ever, at low temperatures, e.g., 60 to 120 F, a plot of equilibrium moisture content vs. percent relative humidity, expressed as 100 (p/p,), is essentially independent of temperature. Such a plot usually results in a curve of =B Wo - w. w, -w: we - w. + log. w - w._ double curvature with a point of inflection (see Fig. 5). (22) The equilibrium moisture content at a given relative humidity is not ii