Document qa1VqroE3DE2wJem0Je439zBq

724 CHAPTER 53 1959 Guide bed is to be loaded with 6.820 lb per sq ft of bone-dry material. Depth of the bed is to be 4 in. Step t: Previous experience indicates that the commercial drying time is 70 percent greater than the test time obtained in the particular laboratory setup used. { Therefore, the commercial drying time = 1.7 X 25 = 42.5 min. Step 3: In order to dry the desired 3000 lb per hr of material the holding capacity of the dryer is 3000 X = 2125 lb at 4 percent bone-dry basis. The required conveyor area is .21_2_5_ = 312 sq ft. Assuming O.Om) that a perforated plate conveyor with an 8 foot effective width is used, the length of the drying sone is -g- " 39 ft. Step 4-' The amount of water entering the dryer is 3^000 X jjjjj -- 2370 lb per hr, while the amount of water leaving the dryer is X " 115 lb per hr. Thus, the evaporation rate, in the dryer is 2370 -- 115 -- 2255 lb per hr. Step 6: Since the air circulation is perpendicular to the per forated plate conveyor, 4he total quantity of air that must be circulated equals the air velocity (based on the face area) multiplied by the conveyor area. Thus, Supply air = 250 X 312 -- 78,000 efm From Fig. 5 the humidity ratio of the supply air at 160 F dry-bulb, 100 F wet-bulb is 0.0285 lb per lb dry air. The specific volume of the supply air is 16.33 cu ft of moist air per lb of dry air, from Table 2 and Equation 27 of Chapter 3. The quantity of dry air circulated is 7--8--,-0j0g0-gXg--60 = _2_8_6_,500 ,,lb per hr. Step 6: The amount of moisture pickup is 2255 = 0.0079 lb per lb of dry air. The humidity ratio of the exhaust air is 0.0079 + 0.0285 - 0.0364 lb per !b dry air. Substitute in Equation 6 and solve for Gt the velocity of dry air, to determine the required quantity of make-up air. The humidity ratio of the make-up air is 0.0086 lb per lb dry air, from Fig. 5. G (0.03M-0.0086) - (^)(^l) Therefore, G = 81,000 lb dry air per hr. Mak. e-up air - 81,000 -- 28.2,, percent. Recirculated air -- 71.8 percent. Step 7: Heat Balance Sensible heat of material " M(tm -- rar (10 - * 03 34,600 Btu per hr Sensible heat of water Mu>i(tw -- Ci)c. - 2370 (100 - 60) 1.0 = 94 300 Btu per hr Latent heat of evaporation " M{vh -- tot)H = 2255 X 1037 = 2,338,400 Btu per hr Sensible heat of vapor = -- to*) (h -- t*)e, - 2255 (160 - 100) 0.45 = 60,900 Btu per hr Required heat for material -- 2,528,700 Btu per hr The temperature drop (U -- f) through the bed is Required heat = 2,528,700 ,,, Supply air, lb per hr X c. " 286,500 X-034 " Therefore, the exhaust air temperature is 160 -- 37 = 123 P Required heat for make-up air =* GtU* -- *i)c - 81,000 (123 - 70) 0.24 ' - 1,030,300 Btu per hr. The total heat required for material and make-up air is 2,528,700 + 1,030,300 = 3,559,000 Btu per hr. Additional heat must be provided for the radiation and con vection losses, which may be calculated from the known con struction of the dryer surfaces. LETTER SYMBOLS USED IN CHAPTER 53 A ** area of beat transfer and evaporation, square feet per pound of material. a -- drying area, square feet per cubic foot of bed volume, c. = specific heat of air, Btu per (pound) (Fahrenheit de gree). e* = specific heat of material, Btu per (pound) (Fahrenheit degree). c, = humid heat, Btu per. (pound of dry air) (Fahrenheit degree). e, = specific heat of water vapor, Btu per (pound) (Fahren heit degree). c -- specific heat of water, Btu per (pound) (Fahrenheit de gree). d = diffusivity of the liquid or vapor, square feet per hour. Dp = average diameter of particle, feet. G -- mass velocity of dry air, pounds per (hour) (square foot). Gt -- dry air supplied as make-up air to the dryer, pounds per hour. H " enthalpy of evaporation of water at wet-bulb tempera ture, Btu per pound. h -- film heat transfer coefficient, Btu per (hour) (square foot) (Fahrenheit degree). he " coefficient of heat transfer by convection, Btu per (hour) (square foot) (Fahrenheit degree), hr = coefficient of heat transfer by radiation, Btu per (hour) (square foot) (Fahrenheit degree). h( = total gas film beat transfer coefficient, Btu per (hour) (square foot) (Fahrenheit degree). .. It/ gas film thermal conductivity, Btu per (hour) (square foot) (Fahrenheit degree per foot), fc, > mass transfer coefficient, pounds per (hour) - (square foot) (atmosphere). L <= one-half material thickness, feet. M * weight of stock dried in a continuous dryer, pounds per hour. Mi * weight of stock charged in a discontinuous dryer, pounds per bateh. Ap - p, -- p. -- vapor-pressure difference, atmospheres, p. = partial pressure of water vapor in air, atmospheres, p, = vapor pressure of water at t,, atmospheres. U ** air or gas temperature, Fahrenheit. I* -- temperature of particle, solid, or surface of evaporation, Fahrenheit. tp = wet-bulb temperature of drying air, Fahrenheit. <1 *= make-up air temperature, Fahrenheit. = temperature of air entering stock, Fahrenheit. t, =* exhaust air temperature, Fahrenheit. U,i ~ entering stock temperature, Fahrenheit. tm* = leaving stock temperature, Fahrenheit. 61 = (t* -- t.) = temperature difference between air and sur face of evaporation, Fahrenheit. 6tm = logarithmic mean between temperature entering and Industrial Drying Systems leaving the bed, and the wet-bulb temperature, Fahren heit. Wi " humidity ratio of entering air, pounds of water vapor per pound of dry air. Wt " humidity ratio of leaving air, pounds of water vapor per pound of dry air. to TM moisture content on dry basis at any time 0, pounds of water per pound. toc = critical moisture content, pounds water per pound dry material. tcu ~ water content, dry basis, of the drop as it enters the drying chamber, pounds per pound of dry solid. to* -- moisture content at equilibrium with external condi tions, pounds per pound dry material. to* = initial moisture or moisture content at start of diffusional period, pounds per pound dry material. ^ = drying rate, pounds of water per (hour) (pound dry material). / ^ = constant drying rate, pounds per (hour) (pound dry \de dry material). (dlD ^ = falling rate, pounds water per (hour) (pound of \-dd dry stock). 0 TM time, hours. p. = bulk density of dry granular bed, density of dry particle, pounds per cubic foot. REFERENCES 1 W. R. Marshall, Jr. and S. J. Friedman: Drying (Perry's Chemical Engineer*' Handbook, McGrawTHilI Co., New York, 1950, 3rd ed.). Indicated material supplied by. W. R. Marshall, Jr. and S. J. Friedman, authors of the Section on Drying in tho Third Edition of the Chemical Engineers' Handbook. Permis sion to use this material has been kindly granted to The Guide by the Editor, John H. Perry, and by McGraw-Hill Book Company, publishers of the Chemical Engineers' Hand book. * W. R. Marshall, Jr: The drying of foods (Heating, Piping and Air Conditioning, September to December 1942, also No vember and December 1943). * C. B. Shepherd, C. Hadlock, and R. C. Brewer: Drying materials in trays (Industrial and Engineering Chemistry, April 1938). 4 B. W. Gamson, G. Thodos, and O. A. Hougen: Heat, mass and momentum transfer in the flow of gases through granular solids (American Institute of Chemical Engineers Transactions, 1943). 1 W. M. Grosvenor: Calculations for dryer design (American Institute of Chemical Engineers Transactions, Vol. 1, 1908, p. 184). * S. J. Friedman: Steps in the selection of drying equipment (Heating and Ventilating, February 1951, p. 95). T What the air conditioning engineer should know about drying (Heating and Ventilating, December 1942). *B. B. Fogler and R. V. Kleinschmidt: Spray drying (In dustrial and Engineering Chemistry, December 1938). * Tobacco Curing (Virginia Agricultural Experiment Station Technical Bulletin 116, Januray 1951). 14 Agricultural Index (H. B. Wilson Co., New York). BIBLIOGRAPHY W. H. Carrier: The temperature of evaporation (ASHVE Transactions, Vol. 24,1918, p. 25). 725 J. A. Goff and S. Gratch: Thermodynamic properties of moist air (ASHVE Transactions, Vol. 51, 1945, p. 125). C. F. Prutton and C. O. Miller: Factors influencing the per formance of rotary dryers (American Institute of Chemical Engineers Transactions, February, August 1942). W. R. Marshall and 0. A. Hougen: Drying of solids by through circulation (American Institute of Chemical Engineers Transactions, 1942). A. Wcisselberg: Factors that influence dryer performance (Chemical and Metallurgical Engineering, August 1932). 0. A. Hougen: Typical dryer calculations (Chemical and Metallurgical Engineering, January, March 1940). W. K. Lewis, W. H. Carrier, A. E. Stacey, Jr., R. S. Fleming, R. G. Mets, G. B. Ridley, C. O. Lavett, and D. J. Van Marie: Symposium on drying (industrial and Engineering Chemistry, May 1921, p. 427). S. J. Friedman and W. R. Marshall, Jr.: Studies in rotary drying, I and II (Chemical Engineering Progress, 1949). ' Symposium on drying (Industrial and Engineering Chemistry, 1938). H. D. Tiemann: Principles of Drying Lumber and Humidity Diagram (Forest Service Bulletin 104,1912). T. K. Sherwood: The Drying of Solids (Massachusetts Insti tute of Technology Bulletin Nos. 237,247, 258). T. P. Brown: Radiant energy drying with heat lamps (Metal Industry, December 1939, p. 607). E. W. Flosdorf: Drying by sublimation (Pood Industry, January 1945, p. 607). F. H. Slade: Evaporative drying system (Food Manufactur ing, March 1943, p-70). 1. R. Berkness: High frequency methods in gluing and drying wood (Wood Products, Vol. 45, 1940, p. 12). T. K. Sherwood: Development of the unit operations of chemical engineering: drying (Chemical and Metallurgical Engineering, vol. 42, p. 214); D. W. Biocheno: The spray dryer--its possibilities in indus try (Food Manufacturing, June 1944, p. 195). L. E. Stout, K. J. Captan, and W. G. Baird: Mechanism and rate of drying by near-infra-red radiation (American Institute of Chemical Engineers Transactions, Vol. 41, 1945, p. 283). W. B. Van Arsdel: Some engineering problems of the new vegetable dehydration industry (ASHVE Transactions, Vol. 49, 1943, p. 49). V W. Sherman: Electronic dehydration of foods (Electronics, Vol. 17, 1944, p. 94). Air Conditioning and Engineering (American Blower. Co., 1935). Drying in Industrial Plants (J. O. Ross Company). Badger and McCabe: Elements of Chemical Engineering (McGraw-Hill Co.,-New York, 1931). Fan Engineering (Buffalo Forge Co.). M. Hirsch: Die Trockentechnik (Julius Springer, Berlin, 1932). Drying (Kent's Mechanical Engineers Handbook). W. H. Carrier: Drying (Mark's Mechanical Engineers Hand book). A Koehler and R. Thelen: Kiln Drying of Lumber (McGrawHill Co., New York, 1926). H. D. Tiemann: Kiln Drying of Lumber (J. B. Lippincott, New York, 1920). O. A. Hougen, H. J.'McCauley, and W. R. Marshall: limita tions of diffusion equations in drying (American Institute of Chemical Engineers Transactions, Vol. 1, 1940, p. 183). V. P. Victor: An introduction to convection drying and dry ing calculations (Heating and Ventilating, December 1944, p. 67).