Heat transfer and flow of bulk solids in a moving bed

Abstract

The indirect heat transfer of steam-heated tube bundles in a moving bed has been examined in an experimental apparatus. Heat transfer in single tubes is typified by a characteristic flow of the bulk solids along the outer tube wall surface. A cuneiform rest zone is created at the upper tube wall (stagnation point), in which the particles remain for a long time. An ‘insulating’ effect is exhibited by the dammed bulk zone and is responsible for the poor heat transfer in this area. Near the sides of the lateral tubes heat transfer is good and icreases with increasing mass flux and bulk solids velocity. Bubbling occurs at the lower tube wall and the heat transfer again decreases due to the small number of wall-particle contacts. The experimentally confirmed ‘trace theory’ describes the temperature profile at the outlet of a moving bed heat exchanger, being characterized by very good cross-mixing of the bulk solids which allows the intergral heat transition to be calculated. A modelling approach to the heat transfer and bulk solids movement in the moving bed provides a physical model which describes the dependence of the heat transfer at a single tube on the flow profile between two neighbouring tubes. In order to determine the flow profile, the continuity equation is solved vectorially, allowing an analytical relationship of the velocity profile between two tubes to be obtained via the coaxiality of stress and deformation. To allow such a calculation, the heat-transfer model makes use of the residence and contact time behaviour resulting from the velocity profile, with the different components of heat transfer at a tube being determined from the friction properties of the specific bulk material. Calculation of the integral heat transfer in the moving bed may be achieved via heat transfer at a single tube. By using the theory of ‘extended contact time’, the total residence time of the bulk at the first tubes may be considered as a case history for the other tubes. The integral overall heat-transfer coefficients of moving bed heat exchangers thereby determined have been verified experimentally.