EFFECTS OF BED PARTICLE-SIZE ON BEAT TRANSFER IN CIRCULATING FLUIDIZED-BED BOILERS

Local values of heat transfer to membrane walls of a circulating fluid ized bed (CFB) boiler were measured for three sizes of the same silica sand, with mean diameters of 0.22, 0.34 and 0.44 mm. A change from 0. 44 to 0.22 mm sand at constant fluidization velocity led to a consider able increase in particle concentration and, hence, heat transfer. How ever, at a given cross-sectional average bulk density, the average hea t transfer coefficient across the membrane wall was insensitive to the changes of bed particle size. Also, the lateral distribution of heat flow, to the crest and side of the tube and to the fin, was independen t of particle size when the bulk densities were similar. This was achi eved by keeping constant the ratio of fluidization velocity to termina l velocity of a single average size particle. It was possible to estim ate the vertical distribution of the heat transfer coefficient in the CFB furnace with an accuracy of +/-20% by a simple semi-empirical meth od.