Document QryYprw2p2yGOZ51rn0Vn42k

720 Chapter 41 1945 Guide Ventilation Phase The technique of. attack of the ventilation phase of a drying problem is best made clear by an illustration. Assume that a material containing 40 per- cent moisture is to be dried until this quantity of moisture is reduced to 5 per cent by weight. The material will stand an air-tempera ture of 150 F and it is possible to provide sufficiently good contact' between the material and the drying air so that the effluent air can be brought up to 50 per cent humidity at 150 F. The dryer is to use room air, the temperature and humidity of which may be assumed to average 70 F and .50 per cent. A counter-current dryer will be employed and the air in this dryer will be kept at a substantially constant temperature of 150 F by heaters thermostatically controlled. The stock enters at 70 .F, rises quickly to the wet-bulb temperature of the air, with which it is in contact, and is found experimentally to maintain wet-bulb temperature until the moisture content has fallen to 20 per cent. From this point its temperature rises progressively as it dries. In this range the difference in 150F air temp. Air f travel ^136^12 0.105 H2 -0.0787 r!29F t2 vt-117 t.-123 0.008 Hi dr W-0.150 W 0.250 ~ Stock travel W-0.500 Wy0-6687 W2-0.0527' W-POUNDS OF WATER PER POUND DRY STOCK Fig. 7. Temperature Humidity Relations in a Dryer temperature between stock and air, divided by the wet-bulb depression, may be assumed proportional to the moisture content. The moisture content of the entering stock, in the units here employed, is: Wl 40 per cent water 60 per cent dry stock = 0 6667- T = 5 per cent water ' * 95 per cent dry stock = 0.0527 v>i -- tvt = A w = 0.614 lb water evaporated per pound of dry stock. Since the air leaving the dryer is 50 per cent saturated at 150 F from Fig. 6, Ht = 0.105. Similarly, Hi = 0.008, corresponding to 50 per cent humidity at 70 F. Consequently Hi -- Hi = iff = 0.097 lb water evaporated per pound dry air. . An analysis of Equation 4 shows that (H) is linear in w. Hence, one can construct on Fig. 7, the line marked (H) being drawn connecting the initial and final points just computed. Since the air leaving the dryer has a temperature of 150 F and a humidity of 0.105, Fig. 6 shows that its wet-bulb temperature is 129 F. This is plotted at the right hand side of Fig. 7. Since the stock maintains a wet-bulb temperature down to 20 per cent moisture, where w = 0.25, the corresponding humidity can be computed by the use' of Equation 4 or by reading directly from the diagram, the value being 0.0392. Fig. 6 shows that the corresponding wet-bulb temperature is 105 F. Any --vDrying- Systems~ 721}* intermediate point on the wet-bulb temperature curve can be calculated similarly. The points for w = 0.5 are shown in Fig. 7. Below the point, w = 0.25, the temperature of the stock begins to rise appreciably above the wet-bulb temperature. Its temperature at any given point in this range, for example at w = 0.15, may be computed as follows: At this point, H = 0.0234 (from Equation 4) and from Fig. 6, tv = 95 F. Hence the wet-bulb depression, < - *, = 150 -- 95 = 55 F. Fig. 8. Core Drying Time Temperature Relations The assumption made regarding the relation between stock temperature and moisture content in this range may be formulated: ''- A t' = w t - tv 0.25 At the point w = 0.15, A/' = 33 F, /' = 117 F. The temperature of the stock leaving the dryer, similarly computed, is 136 F. Fig. .7 thus computed gives in graphical form the information as to the temperature humidity relationships in the dryer. The air requirements can be computed by Equation 4. Thus, per 100 lb of dry stock, it is necessary to supply 633 lb of dry air. Furthermore, since from Fig. 6 it is seen that the volume of 50 per cent saturated air at 70 F, is 13.55 cu ft per pound; 8580 cu ft of room air must be supplied per 100 lb dry stock. Similarly, since the volume of 50 per cent saturated air at 150 F is 18.0 cu ft per pound, the volume of hot wet air discharged from the dryer is Fig! 9. Vent 33$ per cent at 422 F / Recirculation 66$ per cent f at 422 F*Y lb 15 tb product of perfect ~hcombustion, per pound fuel Oven T-D Excess air for combustion X lb at 70 F Core Drying Diagram of Combustion Products and Air 11,400 cu ft per 100 lb of dry stock. Finally, the heat necessary to supply to the dryer, as a whole, or to any section of it, may be computed from Equation 5.. '' High temperature Dryer In the design of a high temperature dryer unit a method of approach to the necessary calculations involved is outlined as follows: Example 2. Cores 4 and 5 in. thick are to be dried by heating to a temperature at 400 F. An intermittent type box oven is to be used, size 12 x 14 x 10 ft with 856 sq ft surface having an average heat transfer of 0.3 Btu per square foot per degree per hour