Document 6bazvg9jnN7D89KL3OKjjJeBg

1176 CHAPTER 47 1958 Guide tobacco and other farm crops, may be found in current issues of the Agri cultural Index}0 i SOLUTION OF TYPICAL DRYING PROBLEM Since there are so many types of dryers which mayi be used, and so many special conditions surrounding each particular problem, it is usually recommended that those having experience with the dryer to be used be consulted. The following example, however, will serve as a guide for typical dryer calculations. Example 8: Magnesium Hydroxide is to be dried from 82 percent .moisture on bone dry basis to 4 percent moisture content on bone dry basis. The production rate is to be 3000 lb per hr on 4 percent bone dry basis. Previous experience indicates that a continuous single conveyor through-circulation dryer with a fin drum feedas illus trated in Fig. 9 should be used. The dryer is to be heated with steam at 50 psig. The optimum circulating air temperature is 160 F which is not limited by the exist ing steam pressure. ~ Step l: Laboratory tests or previous experience indicate that the material enters the dryer at a temperature of 60 F, with an initial moisture content, of 82 percent bone dry basis.. The test drying time is 25 min and the final moisture content is 4 percent bone dry basis. The temperature of make-up air is 70 F dry-bulb and 60 F wet-bulb. The temperature of Circulating air is 160 F dry-bulb and 100 F wet-bulb. The ait velocity down through the preformed bed is 250 fpm, and the static pressure drop through the bed is 0.4 in of water. The.dryer 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 2: 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 S: In order to dry the desired 3000 lb per hr of material the holding capacity of the dryer is 3000 X-gg- = 2125 lb at 4 percent bone dry basis. The required conveyor area is 3= 312 sq ft. Assuming that a perforated plate conveyor with an 8 foot effective width is used, the length of the drying zone is-3g12- = 3O9Q ff*t. -- Step 4: The amount of water entering the dryer isy30^0j-0 Xy8g2g = 23701b per hr, while the amount of water leaving the dryer X-yjjj = 115 lb per hr.. Thus, the evaporation rate in the dryer is 2370 -- 115 = 2255 lb per hr. > Step 5: Since the air circulation is perpendicular to the perforated plate conveyor, the total quantity of air that must be circulated equals the air velocity (based, on the face area) multiplied by the conveyor area. Thus, 1 Supply air = 250 X 312 = 78,000 cfm 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 oi _ moist air per lb of dry air, from Table 2 and Equation 27 of Chapter 3. The quantity of dry air circulated is 78,0 * 60 = 286,500 lb per hr. . lo.oo Step 8: The amount of moisture pickup is-^^~ = 0.0079 lb per lb of dry air. The humidity ratio of the exhaust air is 0.0079 + 0.0285 = 0.0364 lb per lb dry air. Substitute in Equation 6 and solve for G, the mass velocity of dry air, to deter:mine the required quantity of make-up air. The humidity ratio of the make-up . is 0.0086 lb per lb dry air, from Fig. 5. G (0.0364 - 0.0086) = (SXtot) G = 81,000 lb dry air per hr. ,1.4. Industrial Drying Systems -1177 Therefore, ,, , . 81,000 ,, _ , Make-up air =. ggg 5Q0= 28-2 percent. Recirculated air = 71.8 percent. Step 7: Heat Balance Sensible heat of material = M(tmt -- f,,,)cm = W (10 " 60) 0 3 34,600 Btu per hr Sensible heat of water (f,, fml)Cw = 2370 ( 100 - 60) 1.0 94,800 Latent heat of evaporation = Af(wt -- to,) H = 2255 X 1037 2,338,400 Sensible heat of vapor = M (wi -- wj)(fj -- 1,,) cv = 22551.(160 - 100) 0.45 60,900 Required heat for material 2,528,700 The temperature drop (li -- tj) through the bed is Required heat_______ Supply air, lb per hr X c 2,528,700 286,500 X 0.24 = 37 deg Therefore, the exhaust air temperature is 160 -- 37 = 123 F Btu per hr Btu per hr Btu per hr Btu per hr Required heat for make-up air = Gr (f, -- (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 convection losses, which may be calculated from the known construction of the dryer surfaces. LETTER SYMBOLS USED IN CHAPTER 47 A = area of heat 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 degree). Cm = specific heat of material, Btu per (pound) (Fahrenheit degree), c, = humid heat, Btu per (pound of dry air) (Fahrenheit degree). c* = specific heat of water vapor, Btu per (pound) (Fahrenheit degree). c = specific heat of water, Btu per (pound) (Fahrenheit degree), d = diffusivity of the liquid or vapor, square feet per hour. = 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 temperature, Btu per pound. = film heat transfer coefficient, Btu per (hour) (square foot) (Fahrenheit degree). c = coefficient of heat transfer by convection, Btu per (hour) (square foot) (Fah renheit degree). = coefficient of heat transfer by radiation, Btu per (hour) (square foot) (Fahren heit degree). ht -- total gas film heat transfer coefficient, Btu per (hour) (square foot) (Fahren heit degree). ` -- gas film thermal conductivity, Btu per (hour) (square foot) (Fahrenheit degree per foot). -- mass transfer coefficient, pounds per (hour) (square foot) (atmosphere). -- one-half material thickness, feet.