Contact heat transfer analysis in flighted rotary drums

Abstract

Flights are used to improve the mixing of the material and the heat and mass transfer within rotary drums. They lift particles out of the bulk bed and throw them as curtains into the passing gas phase of the drum. This also affects the contact area and contact heat transfer between the rotating drum and the particles. In this study, the contact heat transfer is experimentally investigated in an indirectly heated rotary drum with flights. To determine the heat transfer coefficient, the drum is heated to 350 °C in batch mode with both ends of the rotary drum insulated to avoid axial gas flow. During heating, the temperature profiles of the gas, the particles and the flight and drum walls are measured. Based on the energy balances of the particles, the contact heat transfer coefficient is determined. Glass beads with particle diameters of 0.7 to 4 mm, expanded clay and steel beads (4 mm each) are used as test materials. Operational (rotational speed, filling degree) and flight design parameters (number of flights, flight length ratio) are varied. Models for calculating the contact heat transfer coefficient in flighted rotary drums cannot be found in the literature. In order to show the effect of installations, the measured values are compared with an existing contact heat transfer model for unflighted rotary kilns. This shows good qualitative agreement for most of the parameters investigated. However, most measured values exceed the model values, which indicates a positive effect of flights on the contact heat transfer.