Modelling heat transfer through a novel design of rotary kiln

A novel form of rotary kiln has been developed which confers advantages over conventional designs. Details are given of the main features of the kiln, along with an approach used to study its heat transfer characteristics when hot processing waste products into a lightweight synthetic aggregate for recycling in building materials. Computer aided ®nite element modelling was used to predict temperature pro®les and heat ¯uxes involving non-linear properties of the exterior insulation materials and internal radiation effects. Observations are given comparing predicted temperatures for two different cross sectional shapes and with those measured in practice on a prototype novel kiln. Observations are also given on the methods of approach to the modelling. (2002) `Modelling heat transfer through a novel design of rotary Kiln', Int. A area of cross section of heat ¯ow A A area of cross section of processing aggregate c speci®c heat capacity h heat transfer (®lm) coef®cient k thermal conductivity k rl thermal conductivity, lower density k rh thermal conductivity, higher density q heat ¯ow, radiation Q heat ¯ow, conduction, convection t temperature t a temperature, external ambient t w temperature, external wall x direction of heat ¯ow E emissivity r density s Stefan±Boltzmann constant 2 INTRODUCTION The rotary kiln has been used since the late nineteenth Century. Conventional designs comprise essentially a circular cross section steel tube mounted on riding rings and rotated slowly about a longitudinal axis inclined a few degrees from horizontal. The inside of the tube is lined with a heavy refractory material such as ®re bricks. A burner positioned at the lower end establishes a temperature gradient inside along the axis. Material to be processed feeds in at the cooler upper end and slowly makes its way under the in¯uence of gravity towards the lower end, rising in temperature. The bed of process material within the tube is mixed as the tube rotates. The object of the rotary kiln was to overcome the problem of uneven burning in the thermally more ef®cient shaft kiln. Despite higher fuel costs, the design