Heat transfer between a vertical tube bundle and fine particles in a CFB downcomer with and without circulation of solids

Abstract

This paper reports experimental data on the heat transfer between an electrically heated vertical finned tube bundle and cracking catalyst, fluidized with air in the downcomer of a circulating fluidized bed system. The effects of the air fluidizing velocity, bed temperature and external recirculation of solids were investigated. We were particularly interested in low gas velocities in the downcomer. Up to a 40% increase in the heat transfer coefficient at low fluidizing velocity in the downcomer was obtained with the external recirculation of solids. This may be the result of increasing particle movement and mixing in the downcomer due to the external recirculation of solids. However, at higher fluidizing velocity in the downcomer, an increase in the external recirculation of solids had a smaller effect on the heat transfer coefficient. 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.