Convective heat transfer in a rotary kiln

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Abstract
An experimental study of convective heat transfer from hot air to the solid charge and walls in a non‐fired rotary kiln is reported. Ottawa sand was heated by passing it counter‐current to a flow of preheated air in a 2.5 m × 0.19 m I.D. rotary kiln. Axial temperature profiles of gas, wall and solids were measured. Local and average convective heat transfer coefficients from gas to solids and from gas to wall were determined assuming plug flow of gas and solids. Solid feed rates to 1750 kg/m2 h and air rates to 3300 kg/m2 h were investigated at rotational speeds to 6 r/min, holdup ratios to 17% and gas temperatures from 350–590 K. The gas/solids convective coefficient was found to depend on the gas through‐put and to a lesser extent on solids holdup and rotational speed. Over the range tested, the angle of kiln inclination, solids throughput and particle size showed no significant effect on heat transfer. Gas/wall coefficients were about a factor of ten below gas/solid coefficients. Heat transfer results are compared to the limited data available in the literature, and to commonly used equations. Correlations of the experimental data on gas/solids, and gas/wall coefficients are presented; data from the literature on the wall/solids heat transfer coefficient are summarized.

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