Document 3JJgEdNGqmEb0BNJQOomdnyQO

1048 , CHAPTER 47 1954 Guide 205.5 Step S: The pick-up of moisture per pound for the total air circulated is' 60 X 675 0.0051 lb. 0.0280 -- 0.0051 = 0.0229 lb moisture per lb of air for the supply air. Assuming an existing wet-bulb of 100 F, the supply air dry-bulb will be 182 F. The mixture of recirculated air at 160 F dry-bulb and 100 F wet-bulb, with outside air at 80 F dry-bulb and 72 F dew-point, will be at approximately 120 F dry-bulb and 89 F wet-bulb. Step 6: The heat required may be determined from Equation 26 by substitution of the following values: Ab = 307 X 60 = 18420 lb of air per hr; S = 900 lb; c, = 24 -f 0.45('028 '-2^ = 0.251; t, = 80 F; t, = 160 F; h' = 100 F;X = 1100 (approx.); W = 0.005 lb; si = 0.22. Q = 18420 (0.251) (160 - 80) ^ 18420 (110 + 160 - 100) (0.028 - 0.0168) + 900 (100 - 80) (0.22 + 0.005) + = 609,000 Btu per hr + The heat input requirement is therefore 609,000 Btu per hr plus radiation and con vection losses (Q,c) which may be computed from the known construction of the dryer surfaces and the heat transfer coefficients. Summer conditions were used in Example 1 in order to obtain the maximum heat requirement which would be the case, except under the unusual condition where radi ation and conduction losses are a large percentage of the total. In winter it is usually possible to take advantage of drier makeup air, and either speed up the process or operate at a lower dry-bulb temperature. Controls for the system selected for Example 1 would consist of a thermo stat in the main return air duct controlling the heat input to maintain constant dry-bulb temperature. A wet-bulb controller in the return cir culating duct would maintain constant desired wetrbulb temperature by simultaneous positioning of three sets of dampers in the makeup air, the exhaust air and the recirculated air ducts. LETTER SYMBOLS USED IN CHAPTER 47 A .=. area of heat transfer and evaporation, square feet. a = drying area, square feet per cubic foot of bed volume. B -- a constant (for use in Equation 22). B' -- a constant (for use in Equation 23). c = humid heat, Btu per (pound of dry air) (Fahrenheit degree), d = diffusivity of the liquid or vapor, square feet per hour. Dp = average diameter of particle, feet. Dpi = drop diameter at start of evaporation, feet. Dps = drop diameter of dry particle, feet. e =.Naperian base of logarithms = 2.718. AHGa ---- mloagsasrivthemloiccitmy eoafndroyf ainirle, tpoaunnddosuptelert(hhuoumr)id(istyqudarriveinfogofto).rce across the air film adjacent to the particle through which the water vapor diffuses, pound per pound. (The surface humidity is taken as the humidity cor Hi = hreusmpoidnidtyingrattoiothoef ewnette-rbinuglbateirm, ppoeurnadtusreofowf athteerdvraypinogr paeirr).pound of dry Hi = ahiur.midity ratio of leaving air, pounds of water vapor per pound of dry auA = film heat transfer coefficient, Btu per (hour) (square foot) (Fahrenheit A = dcoeegrffeicei)e. nt of heat transfer by convection, Btu per (hour) (square toon. Ar -- c(Foaefhfirceiennhteiot fdheegaretet)r.ansfer by radiation, Btu per (hour) (square foot), At = t(oFtaahlregnashe.fiitlmdehgereaet).transfer coefficient, Btu per (hour) (square foot), Fahrenheit degree). K = a constant (a function of the constant drying rate)! Industrial Drying Systems 1049 fa = gas film thermal conductivity, Btu per (hour) (square foot) (Fahren heit degree per foot). I, = mass transfer coefficient, pounds per (hour) (square foot) (atmosphere). /, = material thickness, feet, if = molecular weight of the diffusing vapor. A. = dry air supplied to the dryer, pounds per hour. Ap = Pi -- P = vapor pressure difference, atmospheres, p, = vapor pressure of water at ta, atmospheres. = vapor pressure at the particle surface corresponding to the liquid tem perature, atmospheres. pj => partial pressure of water vapor in air, atmospheres: ` p = vapor pressure of liquid in the drying medium, atmospheres. Qre = radiation and conduction loss, Btu per hour. Q = total heat supplied to dryer, Btu. q = rate of heat transfer, Btu per hour. R = gas constant (cubicfeet) (atmospheres) per (Fahrenheit degree). S = weight of stock dried in a continuous dryer, pounds per hour. S' -- weight.of stock charged in a discontinuous dryer, pounds per batch, s = specific heat of stock, Btu per pound. T = absolute temperature of the gas, Fahrenheit. t, = air or gas temperature, Fahrenheit. (h = temperature of the heating medium, Fahrenheit. t, = temperature of particle, solid or surface of evaporation, Fahrenheit, t, = wet-bulb temperature of drying air, Fahrenheit. t\ = entering air temperature, Fahrenheit. k -- leaving air temperature, Fahrenheit. ii = entering stock temperature, Fahrenheit. W = leaving stock temperature, Fahrenheit. t" - average stock temperature over short interval of time, in batch dryer (<i" = entering, h" = leaving) Fahrenheit. Ai = (l - t,) -- temperature difference between air and surface of evapora tion, Fahrenheit. Aim = logarithmic mean between temperature entering and leaving the bed, and the wet-bulb temperature, Fahrenheit. U -- overall heat transfer coefficient, Btu per (hour) (square foot) (Fahren heit degrees temperature difference between heating medium and prod uct). W = moisture content on dry basis at any time 9, pounds of water per pound. Wc = critical moisture content, pounds water per pound dry material. H'd = water content, dry basis, of the drop as it enters the drying chamber, pounds per pound of dry solid. IF, = moisture content at equilibrium with external conditions, pounds per pound dry material. IFo = initial moisture or moisture content at start of diffusional period, pounds per pound. u> = pounds of water. dW = drying rate, pounds of water per (hour) (pound dry material). (dW\ \daj = constant drying rate, pounds per (hour) (pound dry material). V do )t ~ falling rate, pounds water per (hour) (pound of dry stock). dio -fig = drying rate or rate of evaporation, pounds of water per hour (Eq. 1). 9 = time, hours. 9 = drying time for constant rate period, hours. 9( = drying time during falling rate period, hours. 9t = total drying time, hours. * = latent heat of evaporation at t,, Btu per pound, a = viscosity of the air stream, pounds per (hour) (square foot), at = density of evaporating liquid, pounds per cubic foot. <> = bulk density of dry granular bed, density of dry particle, pounds per cubic foot.