Factors affecting heat transfer between gas-fluidized beds and immersed surfaces

Abstract

Heat transfer coefficients were measured between gas-fluidized beds and spherical calorimetric probes of different sizes and to a water-cooled horizontal tube. The bed materials used were alumina and sand of narrow size ranges. Operating temperatures extended up to 980 °C and some estimate of the magnitude of the radiant heat transfer component was obtained by comparing the results for surfaces of oxidized copper (high emissivity) and polished fine gold (low emissivity). Tests with small spherical calorimetric probes immersed in beds of large mean particle diameter showed that there is an effect of relative heat capacity leading to enhanced bed-to-surface heat transfer coefficients if the heat capacity of the ‘heat transfer surface’ is less than an order of magnitude greater than that of the bed particles. Relative bed/surface temperature affects the heat transfer coefficient in two ways: through the temperature influence on gas thermal conductivity affecting the particle convective component of the coefficient, and through the rate of cooling of material directly adjacent to the transfer surface affecting the radiant component of heat transfer. Bed-to-tube coefficients are lower than those to a small calorimetric probe because of the sustained low temperature of a coolant tube and the obstruction that it presents to the circulation of the fluidized solids. The predictive capabilities of the published correlations are poor over the range of operating temperatures tested.