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12.6 A louver-type continuous rotary drier (Fig. 12.22) was used to dry wood chips from 40 to 15% moisture (Horgan, Trams, inst. Chem. Eng., 6, 131 (1928)]. The wood was at 33F (0.56C), while the dried product was discharged at 100F (37.8C) at a rate of 3162 lb/h (0.398 kg/s). The drying medium was the gas resulting from the combustion of fuel, but for the present calculation it may be assumed to have the characteristics of air. It entered the drier at 715F (380C), with a humidity 0.038 kg water vapor/kg dry gas, at a rate of 275 lb/min (2.079 kg/s) wei. The gas was discharged at 175F (77). The heat capacity of the dry wood can be taken as 0.42 Btu/1b F (1758 J/kg .K), and the heat of wetting can be ignored. Estimate the rate of heat loss. 12.7 A direct-heat, cocurrent-flow rotary drier, 8 ft diameter, 60 ft tong (2.44 by 18.3 m), was used to dry chopped alfalfa (see Gutzeit and Spraul, Chem. Eng. Prog., 49, 380 (1953)]. Over a Sab test period, the drier delivered an average of 2200 lb/h (0.28 kg/s) of dried product at 11% moisture and 145*F (63) when fed with alfalfa containing 79% moisture at 80F (26.7C). The drying medium was the combustion products resulting from the burning of 13 074 ft /h (80F, 4 oz/ina gauge pressure) (0.1029 m/s (26.7C, 862 N/m2 gauge pressure)] of natural gas (85% methane, 10% ethane, 5% nitrogen by volume) with air at 80F (26.7C), 50% humidity, The gas analyzed 2.9% CO2, 15.8% 0, 81.3% N, by volume on a dry basis; it entered the drier at 1500F (816C) and left at 195F (91C). The heat capacity of dry alfalfa is estimated to be 0.37 Btu/lb F (1549 J/kg .K), and the heat of wetting can be neglected. Compute the volumetric rate of gas flow through the exhaust fan and the heat losses. Ans.: 6.4 m/s (13 550 ft/min), 1500 kW (5.12 X 10Blu/b). 1