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A theoretical model is proposed, and experimental data presented and analyzed, for the overall convective heat transfer coefficient at ambient conditions between horizontally immersed tubes and a gas fluidized bed of particle sizes usually applicable in fluidized bed combustion of coal. The overall heat transfer coefficient is a function of "gas" and "particle" heat transfer coefficients. The bubbling flow assumption, together with the two-phase theory of fluidization, is used to obtain a simple analytical solution for the particle convective heat transfer coefficient. The equation for gas convective heat transfer coefficient is obtained through an analogy with heat transfer in packed beds. To a first order approximation the particle and gas convective components are additive. For the accurate prediction of the overall heat transfer coefficient some important parameters are carefully studied: the effective thermal conductivity of the emulsion phase in the bed, the gas film thickness around the tube, the emulsion phase contact time and replacement frequency and the bubble voidage. These parameters are utilized in explaining differences between the heat transfer characteristics of beds of large and small particle sizes. Experimental results for the local and overall heat transfer coefficients are presented for single tubes in beds of narrow and wide particle size distribution and for tube bundles in a bed of narrow particle size distribution. A comparison of experimental data of the present study and other available data with the model calculations has been made. An agreement between the model and the experimentally determined local and overall heat transfer coefficients confirms the validity of the proposed model.