Document o9NzLzgeBdxYqeVO5pjZDOaYE

HEATING VENTILATING AIR CONDITIONING CUIDE 1940 for perfect combustion. Cores consist of 91 per cent sand, 3 per cent oil binder, and 6 per cent water. Solution. Heat required per ton of cores: Sand................. Binder._______ Lb Material X Temp. Rise X Sp. Ht. == Btu .. 0.91 X 2,000 X (400 - 70) X 0,2 = 120,120 .. 0.03 X 2,000 X (400 - 70) X 0.4 = 7,920 .. 0.06 X 2,000 X (212 - 70) X 1.0- = 17,040 .. 0.06 X 2,000 X 970 (Fig.4) . = 116,520 Water superheating (approx. SO per cent reaches 575 F) = 0.5 X 0.06 X 2,000 X (575 - 212) X 0.45 9,800 271,400 Btu Heating Load First Hour 4 Heated to 212 F 212 F 212 F 66.7% 66.7% m x 120'120 igx 7,920 0.667 X 116,520 0.667 X 9,800 Btu = 51,688 = 3;408 = 17,040 = 77,680 = 6,530 390 F 422 F Avg. 6 X 156,346 320 X 30,000 X 0.12 352 X 856 X 0.30 = 938,076 -- 1,152,UUU = 90,394 2,180,470 Heating Load Second Hour 400 F 400 F 33.3% 33.3% H'X 120'120 || X 7,920 0.333 X 116,520 0.333 X 9,800 = 68,432 = 4,512 = 38,840 = . 3,270 6 X 115,054 = 690,324 460 575 70 X 30,000 X 0.12 505 X 856 X 0.30 = 129,684 TotaL.. 1,072,008 oBinder oxidizes and liberates heat, which is neglected in this calculation. bAverage value of coefficient is less than 0.3 because oven is not ut> to 575 F. This is neglected. 422 t is arrived at by taking area under curve as compared to area under 57o 1' ordinate. 636 CHAPTER 36. DRYINC SYSTEMS Heat in 1 lb fuel oil = Heater Loss (10 per cent) = 1883 . Duct Loss (5 per cent) = 942 18,830 Btu 2,825 Btu 16,005 Btu available to heat oven. Heat content of gases in1 lb fuel oil at 825 F is 205 Btu (Fig. 8) 15 lb X 205 = 3,075 Btu sensible heat in products of perfect combustion. 12,930 Btu to heat air X and Y (Fig. 11). Y (Sm - Sm) + X (Sm - Sn) = 12930 (4) Y = 2 (X + 15) for 66.7 per cent recirculation where S = heat content of air at temperature noted taken from Fig. 8. (Recirculation and exhaust contains 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). - 5c = 190 - 91 =99 5ms -- 5m = 190 - 8.6 = 181.4 Substituting values of Y, H, etc. in Equation 4, (2 X + 30) 99 + 181.4 X = 12,930 X = 26.3 lb excess air. Y = 82.6 lb recirculating air. Total = 26.3 + 82.6 + 15 = 123-9 lb air and products of combustion circulated per pound fuel burned. Heat in air exhausted from oven at 422 F per pound fuel burned = 0.333 X 123.9 X (5m - 5jo) = 41.3 (91 - 8.6) = 3,400 Btu. Btu available for heating material = 16,005 -- 3,400 = 12,605 Btu per pound fuel. Fuel used in first hour = 2,180,470 -5- 12,605 = 173 lb = 25.6 gal. During the second hour the heater capacity will be much greater than required. If an automatic oven temperature control operates on the oil supply, the delivery tem spuepraptluyrebeoinf gthceoanisrtaenntt.ering the oven and the quantity of oil burned will decrease, the air fueHl. eat in air exhausted = 41.3 (5ms -- Sn) -- 41.3 (127 -- 8.6) = 4,880 Btu per pound Heat available for heating material = 16,005 -- 4,880 =.11,125 Btu. Fuel used in second hour = 1,072,008 -s- 11,125 = 96.5 lb oil = 14.3 gal. Total oil used per load = 25.6 + 14.3 = 39.9 gal. ESTIMATING METHODS Values based on practical experience are available for rough estimating of drying problems. The temperature will drop approximately 8.5 F per grain of water evaporated per cubic foot of air (measured at 70 F) or approximately 0.62 F per pound of air at any temperature. Air will drop 55 F per cubic foot for each Btu extracted. Generally air will absorb from 2 grains to 5 grains per cubic foot of air in one passage through an air drier, depending on the temperature and the degree of contact with the material. The amount of steam required to evaporate a pound of water will vary from 1.5 lb to a more usual figure of from 2.5 to 3 lb of steam per pound of water evaporated. 637