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862 CHAPTER 47 1946. Guide 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, The assumption made regarding the relation between stock temperature and moisture.content in this.range may.be formulated: i Aw < -- fa, = 0.25 At the point w = 0.15, At' = 33 F, t' = 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 Vent 331 Ptf cent St 422 F / Recirculetion 66$ per cent Fig. 9. j_ at 422 F-V lb 2ir IS 4b product of perfect combustion per pound fuel Excess eir for combustion Xlb*t70F Core Drying Diagram of Combustion Products and Air 11,400 cu ft per 100 lb of dry stock. Finally, the.heat required for 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 14x10 ft with 856 sq ft surface, Having an average heat transfer of 0.3 Btu per (square foot) (degree) (hour). Drying Systems 863,. Drying time as determined by test is 2 hr (Fig. 8). Cores weighing 6 tons, and 15-ton , steel plates, trucks etc. are delivered to the dryer at 70F. 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. The fuel oil used has 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 for perfect combustion. Cores consist of 91 per cent saind, 3 per cent oil binder, and 6 per cent water. Solution. Heat required per ton of cores: Lb Material X Temp. Rise XSp. Ht. = Sand______________________ 0.91 X 2,000 X (400 - 70) X 0.2 = Binder.,, 0.03 X 2,000 X (400 - 70) X 0.4 = Water heatingTM::_________ 0.06. X 2,000 X (212 - 70) X 1.0 Water evaporation__ . 0.06 X 2,000 X 970 (Fig. 6) = Water superheating (approx. 50 per cent reaches 575 F) . = 0.5 X 0.06 X 2,000 X (575 - 212) X 0.45 = Total Heat.______________ ________________ ___ _" 271,400 Btu Btu 120,120 7,920 17,040 116,520 9,800 Heat in 1 lb fuel oil Heater Loss (10 per cent) Duct Loss (5 per cent) = =1883 = 942 - : 18,830 Btu 2,825 Btu Btu available to heat oven 16,005 Btu Heat content of gases in 1 lb fuel oil at 825 F is 205 Btu Sensible heat in.products of perfect combustion = (151b +205) = 3,075 Btu Btu to heat air X and Y (Fig. 9). . = 12,930 Btu Y {Sm - Sm) + X (Sm - S,,) = 12,930 ' Y = 2 {X. + 15)Tor 66.7 per cent recirculation (6) where .............. S = heat content of air at temperature noted taken from Fig. 6'. (Recirculation and exhaust contain water vapor, products of combustion, and a greater portion of air. Heat capacities of all-vary so little that-they have all been, assumed to be air). . ' Sets -- Sta = 190 -- 91 = 99Sm - Sn =,190 ^ 8.6 = 181.4.\ .. ^ / , Substituting values of Y, H, etc. in Equation 6, (2 X + 30) 99 + 181.4 X `= 12,930 ; X = 26.3 lb excess air. . Y = 82.6 lb.recirculating air. . V * - > ' - >: - .. '. ' Total' = 26.3 + 82.6 + 15 = 123.9 Ib.air and products of combustion circulated per pound fuel burned. : ;j . . Heat in air exhausted from oven. at< 422 F per pound fuel burned= 0.333 X123.9 X (54a --,.Sn) = 41.3 (91 '-- 8.6) =*'3,400, Btu. s. # Btii available for heating,material =.16,005 -- 3,400.*. 12,605 Btu per pound fuel. Fuel used in-first hour ='2,180,470 '+'12,605 '.= ,173 lb == 25.6 gal .. .. During the second8 hour the heater capacity will be much greater than required. If an automatic oven temperature control operates on the oil supply, the delivery tem perature of the air entering the oven and the quantity of oil Burned will 'decrease, the air supply being constant. Heat in air exhausted = 41.3 (S6 " 'S7c).: = .41".3C(127 -- 8.6) = 4,880 Btu per pound fuel. ' Heat available fbr'heatihg material = 16,OOo'--"4;880 = 11,125 Btu: ;; Fuel used.in second hour'.=. 1,072,008.+ 11,125.= 96.51b.oil ==;14;3 gal. . ' ;v Total oil. used per load = 25.6 + 14.3 = 39.9 gal. (.