Heat transfer to a bayonet heat exchanger immersed in a gas-fluidized bed

Bayonet tubes are frequently used for heat removal in fluidized bed coal combustors and gasifiers. They consist of two coaxial tubes, through which the cooling fluid flows; first through the inner tube, and then back through the annulus, or vice versa. Such a flow arrangement creates obvious difficulties in defining a characteristic temperature difference to which the heat transfer rate can be related, and on the basis of which the heat transfer coefficient between the bed and the tube can be evaluated. A bayonet tube-fluidized bed system is mathematically modelled here, and an analytical scheme is presented for the calculation of the bed-tube heat transfer coefficient. A characteristic temperature difference (CTD) is defined which reduces to the familiar logarithmic mean temperature difference (LMTD) for a particular case. A set of curves is presented relating the CTD with the LMTD.