In some large capacity Circulating Fluidized Bed (CFB) boilers with water walls, additional heat transfer surfaces are necessary to control the furnace temperature while restricting the boiler height to about 40 m due to commercial considerations. These surfaces could be in the form of (1) External Heat Exchanger (EHE) in the solids recycle loop, (2) wing wall and omega panels at the very top of the furnace, or (3) a division or a curtain wall running through the core region of the upper dilute zone of the boiler. It is necessary to know the heat transfer characteristics for these core surfaces from design and operating considerations. In this context, experimental heat transfer studies have been conducted on a 0.6-m high single vertical tube, 0.5- and 1.0-m high U-tubes, and a 0.5-m high finned U tube, located in the top 10% height of a 100 mm×100 mm cross-section and 5.5-m tall CFB riser using silica sand of three mean sizes in the range of 143 to 363 μm as the bed material. The fluidizing air velocity and solid circulation flux were varied in the range of 4-8 m/s and 13-80 kg/m2/s, respectively. The resulting cross-sectional average suspension density and measured total (gas convective plus particle convective) surface-average convective heat transfer coefficients for U-tubes were in the range of 9 to 25 kg/m3 and 30 to 105 W/m2/K, respectively. For single vertical tube at axis, they were in the range of 10 to 20 kg/m3 and 60 to 128 W/m2/K, and at off-axis positions, they were in the range of 10 to 20 kg/m3 and 70 to 140 W/m2/K, respectively. The results confirmed the increasing trend of the heat transfer coefficient from the axis towards the riser wall, thus reflecting the core-annulus flow structure of the CFB riser. It was observed that the length of the tube influences the heat transfer coefficient and fins augmented the heat transfer rate. Gas convective contribution was found to be substantial especially at high gas velocities and for large particles. Experimental data were compared with published heat transfer results under somewhat similar conditions. Using the experimental data points for the U-tubes, two empirical heat transfer correlations were developed in terms of the important design and operating parameters. Copyright © 2002 Elsevier Science B.V.