Maximum diying time

Effect of increased final moisture. Because of the marked increase in diying time required to dry to low moisture contents, the permissible maximum final moisture should always be estabhshed. 

Employing wood chips, Cowan s drying studies indicated that the volumetric heat-transfer coefficient obtainable in a spouted bed is at least twice that in a direct-heat rotaiy diyer. By using 20- to 30-mesh Ottawa sand, fluidized and spouted beds were compared. The volumetric coefficients in the fluid bed were 4 times those obtained in a spouted bed. Mathur dried wheat continuously in a 12-in-diameter spouted bed, followed by a 9-in-diameter spouted-bed cooler. A diy-ing rate of roughly 100 Ib/h of water was obtained by using 450 K inlet air. Six hundred pounds per hour of wheat was reduced from 16 to 26 percent to 4 percent moisture. Evaporation occurred also in the cooler by using sensible heat present in the wheat. The maximum diy-ing-bed temperature was 118°F, and the overall thermal efficiency of the system was roughly 65 percent. Some aspec ts of the spouted-bed technique are covered by patent (U.S. Patent 2,786,280). 

DisK Presses Figure 18-157 shows a disk press. The two disks, or press wheels, converge to a veiy narrow space at the bottom. This is the point of maximum compression, which can be more than 14 times the feed pressure. The press wheels have channels to cany the hquid from the dewatered product, and they are covered with a screen plate. Wheels 1.5 m (5 ft) in diameter are used on a large press that requires about 80 connected horsepower and produces just under 1 tonne/h (0.9 ton/h) of sohds (diy basis). Typical apphcations are fibrous materials such as coffee grounds, pineapple and citrus peels and wastes, alfalfa, and brewers spent grain. 

The rate of diying of unbound moisture [1], Consider a bed of uniform cross section as in Fig. 12.17, fed with a gas of humidity 7j at the rate of mass dry gas/(area bed cross section) (time). The maximum rate of drying will occur if the gas leaving the bed is saturated at the adiabatic-saturation temperature, with humidity... 

---