Document q3mbNzBZGodeybV8Qnmk4yOmK

716 Chapter 41 1945 Guide increases the drying time very much or causes a change of the physical properties of the material. It is often necessary to add humidity to the air in the initial stage of drying. Lumber case-hardens, cracks, and warps if the outside is dried too fast. Ceramics crack if not heated through before drying commences. Elastic materials warp or crack if not even ly dried, Many paints case-harden if not dried under high humidity. On the other hand, in the case of those materials whose physical or chemical properties require that they be dried at relatively low tem peratures, high humidity tends to retard drying in the first stage and may even stop it altogether in the final stage. Where drying temperatures below 120 to 140 F are used, the drying rate may be highly dependent on atmospheric humidity conditions. In such instances it is often desirable to dehumidify the air entering the dryer during periods of high atmos pheric humidity; where a high degree of uniformity is required, it is pften possible to secure complete independence of atmospheric conditions by recirculating the air in a closed system which includes a suitable dehu midifier. For this purpose absorptive dehumidifying systems have the advantage of accomplishing the desired reduction of humidity without appreciably elevating or lowering the dry-bulb temperature of the air; for this reason after-cooling is not required, and reheating is reduced to a minimum.' Dehumidifying systems are described in Chapter 23. Air Circulation As noted under Mechanism of Drying, air velocity is more important :in the first two stages of drying than in the last, arid for this reason zone drying in continuous dryers is frequently considered. It permits accurate regulation of temperature, huiriidity, and velocity in the different zones. High velocity results in more rapid drying, more even distribution of temperature and consequently more even drying in the first period. Too. high a velocity may be detrimental because of excessive power needed for creating it, or because the material may blow away if it is light and fluffy. In the drying of paints, varnishes, and enamels, high velocity or improper distribution of the air even with the use of filters, may cause dust already in the dryer to be blown against the material, ruining the finish. DRYER CALCULATIONS The fundamental calculations for the design and performance of dryers are based on the thermodynamics of air and water mixtures treated in Chapter 1, and the fundamentals of heat transfer treated in Chapter 3. For the humidity calculations a high-temperature psychrometric chart is given in Fig. 6. In addition to the fundamental heat transfer calculations . of radiation, conduction and convection, the heat losses through the walls of the dryer will be computed by the methods illustrated in Chapter 4 and Chapter 31. Data on radiation calculations are given in Chapter 45. Where products of combustion are used directly in a dryer, a knowledge of the properties of fuels and combustion products is important. Data on fuels and combustion are given in Chapter 8. For determining the heat available in products of combustion, a specific heat of 0.25 Btu per pound per degree Fahrenheit may be used. The calculations for drying during the constant-rate period are different from those applying to the falling-rate period. Constant-Rate Period The rate of drying by air passing over, a wet surface is directly pro portional to the vapor pressure difference, and also proportional to the -Drying-Systems-------------------------- 7V7 0.8 power of the air velocity. For practical calculations the wet surface is assumed to attain the wet-bulb temperature of the air passing over it, and evaporation takes place at constant rate under equilibrium condi tions. The equation may then be expressed in three forms: where R = CA V (AP) ' R = C'A V>-` (AH) R = O' A V ' (AT) (i) (2) (3) R -- rate of drying during constant-rate period, pounds of moisture per hour. A = area of bed or material in contact with air, square feet. V = air velocity over material, feet per minute. AP = difference between vapor pressure at wet-bulb (surface) temperature and at dew-point of air. SH = difference between humidity ratio of saturated air, at the surface temperature, and the actual humidity ratio of the air stream, pounds of water per pound of dry air. A T = difference between dry-bulb and wet-bulb temperatures of air, i.e., the . wet-bulb depression. C, O, O' = proportionality constants (for numerical values consult references). These equations are useful mainly for computing the effects of changes in operating conditions, such as changes in air velocity, air temperature, humidity and surface area. The equations assume that the material, is in .equilibrium at the wet-bulb temperature of the air. If equilibrium has not been reached, or if heat is being added to the charge by radiation or conduction, such conditions must be taken into account. For large tray dryers or continuous surfaces, the logarithmic mean difference should be substituted for the simple difference in AP, AH and AT. When the constant of proportionality is known for a given set of con ditions, Equations 1, 2 or 3 may be applied for basic design, as illustrated in Example 1. Example 1. Compute the rate of drying of a granular material initially 35 pet cent moisture (dry basis), if the material is spread in trays and is to" be dried by blowing air horizontally over the surface at 1000 fpm. The air is 140 F dry-bulb, 90 F wet-bulb. Density of the dry material is 85 lb per cubic foot, and the drying constant O', in Equation 3, has been found to be about 1/25,000. Find.the size of dryer for a capacity of one ton per hour (dry basis), and the time required.for drying each batch from 35 .to. 10 per cent moisture content, if the material is spread in trays, in a layer one inch thick. (The critical moisture content of the material is below 10 per cent, hence the drying is at corfstant rate.) . Solution.* Assume that the surface of the material attains the wet-bulb temperature of the air, then AT -- 140 -- 90 = 50 F. The rate of drying by Equation 3 is:- R = O' A V -8 AT = ^^25 000 ^ = 0.50 lb of water per hour per square foot of surface. The total water evaporated per square foot of surface is: 85 fF - -jg- (0.35 -- 0.10) = 1.77 lb (per batch). Then the time required per batch is: 1.77 0.50 3.54 hr. The size of dryer required to dry the material at the rate of one ton of dried material per operating hour will be: 2000 X 3.54 85/12 = 1000 sq ft, total area of trays.