Experimental measurements of bed-to-tube surface heat transfer coefficient were conducted in a 12 MWth circulating fluidized bed boiler/gasifier where a vertical U-tube was immersed into the bed. The aim was to study the influence of superficial gas velocity, solid flux of bed material, direction of the tube relative to the circulating particle flow, choice of fluidization gas, bed temperature and to compare the results with predicted values based on heat transfer correlations. The experiments displayed high heat transfer coefficients of around 550 W/(m(2)K) with sand and with ilmenite approximately 850 W/(m(2)K) when operating with air at a superficial gas velocity of 0.25 m/s. Replacing air with steam and recirculated flue gas as fluidization gas increased the heat transfer by more than 40% with both steam and flue gas. Having the U-tube oriented parallel to the circulating particle flow reduced the heat transfer by around 8% compared to a perpendicular orientation. (C) 2020 Elsevier B.V. All rights reserved.