Document ZBQKZq4wKMzNv2zOq81yR8zpd

American Society of Heating and Ventilating Engineers Guide, 1937 brought up to 50 per cent humidity at 150 F. The drier is to use room airf the temperature and humidity of which may be assumed to average 70 F and 50 per cent. A counter-current drier will be employed and the air in this drier 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 I1 H-Oj 0.008 Hi W2-0.6687 W2*0.0527 W-POUNDS OF WATER PER POUND DRY STOCK Fig. 9. Temperature-Humidity Relations in a Drier Oven temp oven temp f rst hour Temp of iron'' 'v~Tem > in center of core 3\ v. Ir -j I i 0 20 40 60 80 100 120 TIME. MINUTES Fig. 10. Core Drying Time Temperature Relations Vent33^percentat422 F Recirculation 66$ per cent I at 422 F"Y lb 1 15 lb product of perfect -combustion per pound fuel Fig. 11. gircm air for combustion X lb at 70 f Core Drying Diagram of Combustion Products and Air 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 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: Wi = 40 per cent water 60 per cent dry stock = 0.6667: w, = ,,-.-5-Pel-ge-n-.t,water-^ 95 per cent dry stock = 0.0527 754 Chapter 41--Drying Systems Wl - , = A i = 0.614 lb water evaporated per pound of dry stock. Since the air leaving the drier is 50 per cent saturated at 150 F from Fig. 4, = 0.105. Similarly, W = 0.008, corresponding to 50 per cent humidity at 70 F. Consequently Ht -- Hi = = 0.097 lb water evaporated per pound dry air. Inspection of equation (2) shows that (H) is linear in w. Hence, one can construct on Fig. 9, the line marked (H) being drawn connecting the initial and final points just computed. Since the air leaving the drier has a temperature of 150 F and a humidity of 0.105, Fig. 4 shows that its wet-bulb temperature is 129 F. This is plotted at the right hand side of Fig. 9. 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 (2) or by reading directly from the diagram, the value being 0.0392. Fig. 4 shows that the corresponding wet-bulb temperature is 105 F. Any intermediate point on the wet-bulb temperature curve can be calculated similarly. The points for w = 0.5 are shown in Fig. 9. 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 = 0.1$, may be computed as follows: At this point, H = 0.0234 (from equation (2)) and from Fig. 4, tv = 95 F. Hence the wet-bulb depression, t -- = 150 -- 95 = 55 F. The assumption made regarding the relation between stock temperature and moisture content in this range may be formulated: A I1 _ w t -- tw 0.25 At the point w = 0.15, Af' = 33 F, t' = 117 F. The temperature of the stock leaving the drier, similarly computed, is 136 F. Fig. 9 thus computed gives in graphical form the information as to the temperature humidity relationships in the drier. The air requirements can be computed by equation (2). Thus, per 100 lb of dry stock, it is necessary to supply 633 lb of dry air. Furthermore, since from Fig. 4 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 drier is 11,400 cu ft per 100 lb of dry stock. Finally, the heat necessary to supply to the drier, as a whole, or to any section of it, may be computed from equation (3). High Temperature Drier - In the design of a high temperature drier unit a method of approach to the necessary calculations involved are outlined as follows: Example 1. 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. Drying time as determined by test is 2 hr (Fig. 10). Cores weighing 6 tons, and 15-ton steel plates, trucks etc. are delivered to the drier at 70 F. The oven is heated by an external heater; the products-of combustion and 66% per cent recirculated air will be delivered to the oven at 825 F. Fuel oil of 19,980 Btu gross and. 18,830 Btu per pound net heating value, weighing 6.75 lb per gallon and having 15 lb product per pound fuel 755 A