Performance modeling of radiant heat recovery exchangers for rotary kilns

Heat loss from rotating kilns may represent a significant percentage of the total energy input especially in highly energy intensive industrial sectors such as cement production. As an alternative to traditional energy recovery methods, the possibility of recovering radiant heat lost through the kiln surface has been proposed in the literature. This may be accomplished by surrounding the kiln with a secondary external shell acting as a heat exchanger for a transfer fluid. In this work a mathematical model for sizing and performance estimation of a heat exchanger recovering waste heat from the external surface of rotating kilns is developed. The equipment has been configured as an array of pressurized water carrying tubes arranged in a longitudinal pattern on the surface of a cylindrical outer shell coaxial with the rotary kiln. An example of model application is utilized to discuss the involved heat transfer process and its impact on equipment design. An economic model has also been developed to determine the optimal size of the equipment.