Dryers, high-temperature radiation

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

Tunnel Dryers In tunnel equipment, the solids are usually heated by direct contact with hot gases. In high-temperature operations, radiation from walls and refractoiy lining may be significant also. The air in a direct heat unit may be heated directly or indirectly by combustion or, at temperature below 475 K, by finned steam coils. 

High- and medium-temperature radiation dryer, Briehl radiator, A < 3 pm). 

A model of particular importance for the present analysis is concerned with the heatup and possible melting of the upper in-vessd structures (upper shroud head, standpipes, steam separators, and steam dryers). The shroud-head/steam-separator assembly consists of a domed base on top of whidi is welded an array of standpipes with a multi-stage steam separator located on the top of each standpipe. The entire assembly, made of stainless steel, rests on the top-guide grid and forms a cover for the core outlet plenum region. The steam dryer assanbly is mounted in the reactor vessel above the shroud-head/steam-separator assembly. Since, in tihe case of an accident, the upper shroud head may be directiy exposed to a high-temperature core, the combined effects of radiation from the core and convective/radiative heat transfer from the hot steam/gas mixture in the upper plenum, may increase the shroud temperature to failure point. When the weakened shroud head cannot support the mass above it, the upper structures may coUapse onto the core (except for the steam dryer which has a separate support system). The molten steel from these structures may penetrate the hot and partially molten core and flow into the lower plenum and, following lower head failure, into the containment. 

During the constant rate period, evaporation is taking place from the fabric surface. The rate of diying is essentially that of the evaporation of the liquid component under the conditions of temperature and airflow during the process. High air velocities will reduce the thickness of the stationary gas film on the surface of textile material and hence increase the heat and mass transfer coefficients. In commercial forced convection dryers the effects of heat transfer by conduction and radiation may be appreciable, due to the fact that the material surface temperatures are higher than the wet-bulb temperature of the diying air. 

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