HEAT-TRANSFER BETWEEN A VERTICAL TUBE BUNDLE AND FINE PARTICLES IN A CFB DOWNCOMER WITH AND WITHOUT CIRCULATION OF SOLIDS

This paper reports experimental data on the heat transfer between an e lectrically heated vertical finned tube bundle and cracking catalyst, fluidized with air in the downcomer of a circulating fluidized bed sys tem. The effects of the air fluidizing velocity, bed temperature and e xternal recirculation of solids were investigated. We were particularl y interested in low gas velocities in the downcomer. Up to a 40% incre ase in the heat transfer coefficient at low fluidizing velocity in the downcomer was obtained with the external recirculation of solids. Thi s may be the result of increasing particle movement and mixing in the downcomer due to the external recirculation of solids. However, at hig her fluidizing velocity in the downcomer, an increase in the external recirculation of solids had a smaller effect on the heat transfer coef ficient. The measurements are not as complete as would be desirable as the equipment was fabricated simply to heat the recirculating solids. Nevertheless, features of the results may be of interest to designers .