Document Yr0aaQOdQ54nQXYGOBpZYK7VE

912 CHAPTER 47 1949 Guide. be produced having a final moisture content of 5 per cent wet basis and having initial moisture content of 85 per . cent, the weight of solids in volved is 95 lb . and the initial amount of -water is 538 lb. The water to be removed is therefore 533 lb. If the initial moisture content should be increased to 87 per cent, then the initial amount of water is 636 pounds and 631 pounds must be removed. Hence a 2 per cent variation in initial moisture content results in almost 20 per cent increase in required drying capacity. The use of the dry basis, however, is not misleading. FACTORS INFLUENCING DRYING RATES Temperature The driving force for evaporation is directly proportional to the difference between the absolute humidity of the air being used and the absolute hur HUMID HE*T.0>" PER0^G F DRY -3l ' Drying Systems 913 period of drying, the temperature of the material approximates. .the wet-, bulb temperature of the air. Therefore; high dry-bulb temperatures, mav be employed. The nature of the material and its characteristics often determine the- temperatures that can be employed with safety. For example, chemically combined water may be lost, cells may be steam exploded, charring may take place or'exothermic, reactions initiated if the product temperature, exceeds a critical value. - To illustrate the effect of temperature refer to Fig. 4, a typical vegetable' drying curve.*. Fig. 5 shows the change in drying time as affected by drybulb temperature change-other variables remaining constant.- Humidity As previously pointed out, absolute humidity is the driving force for evaporation, therefore, a low humidity increases the drying rates. How ever, certain materials if dried too rapidly may case harden, i.e., form a TEMPERATURE. DEG F` Fig. 3. Psychrometric Chart midity of saturated air at-'this temperature. It may be seen from the psychrometric chart, Fig. 3, that as the temperature increases the hu midity of saturated air corresponding to that temperature increases im mensely. For example, at 100 F one pound of dry ah' will carry about 0.04 pounds of water, while at 120 F this figure has become 0.08 pounds. .Thus during the constant rate and uniform falling rate periods an increase in temperature will increase the drying rate. During the Varying falling rate period, i.e., sub-surface drying where diffusion is.the coritroiling factor, ah increase in temperature will lower the viscosity permitting a more rapid drying. Temperature and thickness of the material hip the only external variables which affect the drying rate, during the variable falling rate period. The drying rate varies inversely with the sqhare 'of the' thickness. In general, then, it is preferable to use as high a temperature as possible for any drying operation because of the faster rate of drying and the greater capacity of the air to remove water. During the constant rate Russet potato, blanched A~-in. strips, wood-slat tray, load 1.5 lb. per square foot, cross-circulation, air velocity 500 fpm, air temperature 150 F, wet-bulb temperature 90 F.' nearly impermeable skin which greatly reduces the drying rate. Also, certain undesirable physical changes may take place such as the cracking and warping of lumber.' ' In such instances, it may even be necessary to increase artificially the humidity to assure satisfactory, drying;, by the recirculation of moist air or the use of water or steam jets. - " In the cate, of substances that must be dried at relatively low tem peratures (120-140 F) it is often necessary to dehumidify the air to tecure satisfactory drying rates. In such instances, a closed system using the adsorptive methods described in Chapter 38, is frequently applicable. Fig. 6 shows the effect of change in humidity, on'the rate of drying curve for the same material considered in Figs. 4 and' 5. Air Circulation Air- velocity and distribution are important factors only in the first two. periods.of drying when surface evaporation is taking place. .During.the varying falling rate period of gaseous diffusion enough ventilation.must