The effect of gas velocity on the average and local heat transfer coefficients between a submerged horizontal tube, (25.4 mm-OD) and a fluidized bed has been determined in a fluidized-bed-heat-exchanger (0.34 x 0.50 x 0.6 m-high) of silica sand particles. The heat transfer coefficients and the properties of bubble and emulsion phases were simultaneously measured at the same location around the tube circumference by thermocouples and an optical probe. The average heat transfer coefficient (h(avg)) exhibits a maximum value with variation of gas velocity (U-g). The local heat transfer coefficient (h(i)) exhibits maximum values at the side of the tube (0degrees). Bubble frequency (f(b)) increases and the emulsion contacting time (t(e)) decreases with increasing U-g. The h(i) increases with increasing f(b) and decreasing t(e). The f(b) exhibits higher values and t(e) is shorter at the bottom (under each side) than those at the top section of the tube. The t(e) and bubble fraction (delta(b)) have been correlated with Froude number. The predicted h(avg) values of small particles based on the packet renewal model and the emulsion contacting characteristics around the tube well accord to the experimental data. (C) 2002 Elsevier Science Ltd. All rights reserved.