Cross-circulation drying

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

Values of critical moisture contents for some representative materials are given in Table 12-7 for diying by cross circulation and in Table 12-15 for diying by through circulation. The tabulated values are only approximate, since critic moisture content depends on the drying histoiy. It appears that the constant-rate period ends when the moisture content at the surface reaches a specific value. Since the critical... 

No agitation, e.g., tray and band dryers. This is desirable for friable materials. However, drying rates can be extremely low, particularly for cross-circulation and vacuum drying. 

Gas is blown across the surface of a bed or slab of solids or across one or both faces of a continuous sheet or film. This process is called cross-circulation drying (Fig. 24.1fl). 

Gas is blown through a bed of coarse granular solids that are supported on a screen. This is known as through-circulation drying. As in cross-circulation drying the gas velocity is kept low to avoid any entrainment of solid particles (Fig. 24.16). 

MASS TRANSFER IN DRYERS. In all dryers in which a gas is passed over or through the solids, mass must be transferred from the surface of the solid to the gas and sometimes through interior channels of the solid. The resistance to mass transfer, not heat transfer, may control the drying rate. This is most often true in cross-circulation drying of slabs, sheets, or beds of solids. From the standpoint of the gas, this kind of drying is much like adiabatic humidification from that of the solid it is like evaporation when the solid is very wet and like solvent desorption from an adsorbent when the solid is nearly dry. 

Mass transfer between the solid surface and the gas is covered by the relations discussed in Chap. 21. The drying rate of solids containing internal liquid, however, depends on the way the liquid moves and on the distance it must travel to reach the surface. This is especially important in cross-circulation drying of slabs or beds of solids. Drying by this method is slow, is usually done batchwise, and has been displaced by other faster methods in most large-scale drying operations it remains important, however, in the production of pharmaceuticals and fine chemicals, especially when drying conditions must be carefully controlled. 

For small batches of pharmaceuticals, ovens are still a good choice. They allow for placing material upon several shelves, which can be carried by a truck. During drying, hot air is cross-circulated between the shelves to permit drying of different products at the same time. Internal circulating... 

If a solid is initially very wet, the surface will be covered with a thin film of liquid, which we shall assume is entirely unbound moisture. When it is exposed to relatively dry air, evaporation will take place from the surface. The rate at which moisture evaporates can be described in terms of a gas mass-transfer coefficient ky and the difference in humidity of the gas at the liquid surface 7, and in the main stream Y. Thus, for cross-circulation drying... 

Tunnel driers In drying solids by cross circulation of air over the surface, as in the case of materials on trays or solids in sheet form, the surface temperature in zone II can be estimated through Eq. (12.18). Unless all surfaces are exposed to heat transfer by radiation, this feature of the heat transfer is better ignored, and U in Eq. (12.46) can be taken as This value will also serve in zone I,... 

A sample of a porous, manufactured sheet material of mineral origin was dried from both sides by cross circulation of air in a laboratory drier. The sample was 0.3 m square and 6 mrn thick, and the edges were sealed. The air velocity over the surface was 3 m/s, its diy-bulb temperature was 52 0, and its wet-bulb temperature 2I C. There were no ratUation effects. The solid lost moisture at a constant rate of 7.5 X 10 kg/s until the critical moisture content, 15% (wet basis), was reached. In the falling-rate period, the rate of evaporation feU linearly with moisture content until the sample was dry. The equilibrium moisture was negligible. The dry weight of the sheet was 1.8 kg. 

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