An investigation of the heat transfer behavior of longitudinal finned membrane water wall tubes in circulating fluidized bed boilers

Experiments were conducted in a cold model circulating fluidized bed (CFB) having riser cross sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75°C and superficial velocity of 8 ms− 1. Local sand having average diameter of 231 μm, particle density of 2774 kg m− 3 and bulk density of 1515 kg m− 3 was used as bed material. The experiments were conducted for three tube configurations: a membrane tube, a membrane tube with a longitudinal fin at the tube crest and a membrane tube with two longitudinal fins at 45° on both sides of the tube crest. Heat transfer, heat transfer coefficient ratio and heat transfer ratio of the three tubes are presented as a function of cross sectional average suspension density. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes although they have lower heat transfer coefficient. In addition, numerical simulations were conducted to investigate the heat transfer behavior of the three tubes under the normal operating conditions of CFB boilers. It was found that the membrane tube with two longitudinal fins at 45° on both sides of the tube crest had the flattest inner wall tube heat flux profile. Moreover, in terms of temperature distribution in the tube material, it has the lowest profile.

Heat transfer rates of a membrane tube, a membrane tube with a longitudinal fin at the tube crest and one with two longitudinal fins at 45° on both sides of the tube crest were investigated in a cold circulating fluidized bed (CFB) model. The results were used to simulate the heat transfer behaviors of the three tube types in commercial CFB boilers. It was found that the membrane tube had the highest heat transfer coefficient. Heat transfer can be improved by adding a longitudinal fin on tube surfaces. The tube with two fins has the flattest temperature distribution profile in the tube material and heat flux on inner tube wall.Figure optionsDownload as PowerPoint slide