UBC Theses and Dissertations
Convective heat transfer in a rotary kiln Tscheng, Shong Hsiung
Convective heat transfer in a rotary kiln was studied as a function of operating parameters. The experiments were carried out in a steel kiln of 0.19 m in diameter and 2.44 m in length. The operating parameters covered included gas flow rate, solid throughput, rotational speed, degree of solid holdup, inclination angle, particle size and temperature. To minimize radiation effects, air was used as the heating medium and maximum inlet air temperatures were limited to 650 K. Ottawa sand was used in all the runs except in the study of the effect of particle size where limestone was employed. The experiments were conducted under conditions where the bed height along the kiln was maintained constant and the bed was in the rolling mode. Both the heat transfer coefficients from the gas to the solids bed and the gas to the rotating wall were found to be significantly influenced by gas flow rate. Increasing rotation al speed increases the gas to bed heat transfer, but decreases the gas to wall heat transfer. The former effect is relatively small. The effect of degree of fill was slightly negative in the gas to solids bed heat transfer, and insignificant in the heat transfer from the gas to wall. The effects of inclination angle, solid throughput, particle size and temperature were found negligible over the range tested. One of the major findings in this study is that contrary to suggestions in the literature, the coefficients for gas to bed heat transfer are about an order of magnitude higher than those for gas to wall. The higher coefficients for gas to solids bed are attributed to two factors, the underestimation of the true area by basing coefficients on the plane chord area and the effect on the gas film resistance of the rapid particle velocity on the bed surface. The experimental data were correlated in a form suitable for design purposes, and the results compared with meager data available in the literature. A mathematical model was developed for convective heat transfer from the gas to a rolling solids bed. The model requires the knowledge of the gas to particle heat transfer coefficient and the rolling velocity of the aerated particles. The model gives a reasonable prediction of the gas to bed coefficient in a rotary kiln using values of the gas to particle coefficient taken from the literature. The required data on the surface velocity of particles was obtained in a lucite kiln of the same size. Residence time distribution of particles was also studied briefly to verify that solids were nearly in axial plug flow. A simple mathematical model of a rotary kiln heat exchanger is presented. This model predicts gas, solids and wall-temperatures in a kiln as a function of the kiln design and operating parameters using the heat transfer correlations developed in this work.
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