Document NGeX7Rgg9wKOZen33orDQyoyD

Table 1. Dkvers for Evaporation of Water Com partment Agitated Vacuum Tunnel Rotary lii.- .Drum Cylinder Festoon Tower or Column Spray Induction Hygroscopic materials dried with vacuum, and packed immediately ,712 _________________ ________ Chapter 41 Where material will stand rough handling and is not subject to balling up . Cost of operation high, for expensive materials 100 Water, Steam to Jacketed, may have Where dust must be saved 330 Vacuum on top When production does not war rant continuous drier i Pi 1 6 00 ** III o SgB S3d* 80 to Steam Coils, Air, 180 Electricity 80 to Water, 300 Steam Steam Coils, Air, Electricity, Products For high production of Combustion Air, Steam, Products of Combustion O P0oo9 32 O**00 ` to Steam inside Steam, may have Vacuum on Top Continuous Sheets, Flowed on Drum, Dry Material Scraped off Cascades through Truck, Tray, Belt Chemicals, Explosives, Phar Tray, Basket, maceuticals, Food Products Tumbling Drum Shoveled into Drum or Pan Suspended, Truck, Ceramics, Chemicals, Lumber, Food Products Chemicals too Sticky for Rotary Drier Yarns, Lumber, Foodstuffs Tray Paper, Leather, ,aa s a 4cf4-t-)1 . o, 4-1 co3 J5 Bulk Liquids, Slurries CD 3 O 3 . co U Paper, Textiles, Chemicals Endless Chain Belt. 350 of Drum Where material comes in sheets or rolls, and will stand,' direct contact with heating surface Paperj Chemicals Continuous Sheets, Suspended on Metal Screens to Air, 200 Steam Coils Where one side cannot come in contact with supports until dry Grains, Sand Falls through by Gravity 125 to Air, 260 Steam Coils Where headroom is available Solutions over 30% Solids . Sprayed into Chamber 120 to Air, Products of 350 Combustion Where heating of metal from in side out is important Drying is almost instantaneous 1945 Guide, Drying Systems. r 723 surface so rapidly that the surface remains thoroughly wet, and evapora tion proceeds at a constant rate, precisely as from a free water surface. The material tends to assume a temperature corresponding to the wet-: bulb temperature of the surrounding air. But the actual temperature is often slightly higher, due to radiation and conduction from dry surfaces adjoining the material. The constant rate period continues until a time . is reached when the moisture no longer comes to the surface as fast as.it is evaporated. This point is called the critical moisture content in the drying process. . As the drying proceeds, a period of uniform falling rate is entered. During this period, the surface of the material is gradually drying out, and Electricity to 400 Placed-in High Frequency Field Metals, for removal of traces of Water the rate of drying falls as the remaining wet surface decreases in area. This period is also known as unsaturated surface drying. As drying continues, the surface is completely dry and the water from the interior evaporates and comes through the surface as vapor. As the plane of water recedes, the diffusion of the vapor becomes more difficult and hence the period is known as varying falling rate period, or sub-surface drying. Drying ceases when the equilibrium moisture content has been reached. The final moisture content of the product depends on the relative humidity of the air in contact with it. Equilibrium is established when the vapor pressure of the moisture in the air and the vapor pressure of the moisture in the material are equal. The equilibrium moisture content varies with the hygroscopic properties of the material, (see table of Regain of Hygro scopic Materials, Chapter 39). The drying of a slab of whiting is shown in Fig. 5 and illustrates the principles referred to previously. The factors affecting the variations of drying rates during the periods mentioned are outlined in Table 2. Omissions in the Cycle . Many solids, such as lumber, are so dry at the beginning of the drying operation that the constant rate period of free surface evaporation does not occur. Frequently the surface of the material is dry enough so that no surface drying can take place, in which case only the final stage of sub surface drying is involved. In other instances, the critical moisture con-