Even though cluster renewal model appears to hold the ground for calculating heat transfer coefficients on water walls of a Circulating Fluidized Bed (CFB) boiler, there are certain parameters of this model including thickness of the gas-gap between the wall and the particle suspension, wall coverage, thermal boundary layer, cluster concentration, cluster velocity, dispersed phase convection are yet to be determined. Fractional wall coverage is the most important parameter among them. This paper presents a correlation of fractional wall coverage for commercial boilers based on the data from large CFB boilers where average solids concentration, size and height of the combustor were taken as variable. Data were deduced by applying cluster renewal model with recent findings on gas gap, thermal boundary layer, cluster concentration, cluster velocity to the reported heat transfer coefficients on four commercial CFB boilers. The improved cluster renewal model is validated against available data.

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