Improved waste heat recovery through surface of kiln using phase change material

The heat losses that occur from the surface of the rotary kilns during calcination process are a major source of waste heat in cement production industry. In order to recover this heat, a multi-shell heat exchanger that forms an annular duct over the high temperature zone of the kiln is used. The phase change material (PCM) paraffin wax with a melting point of 68 °C is filled in between the gap of the two concentric annular steel shells which are thermally insulated from the outside. In order to draw a comparison and to establish that phase change material improves the waste heat recovery, the heat exchanger model made up of mild steel, which extracts waste heat from a kiln, is experimentally investigated with and without the tertiary shell that contains the phase change material. The outer surface of the heat exchanger is insulated by glass wool, and to facilitate the passage of air between the shells for heat transfer, a variable speed centrifugal fan (for variable flow rate) is installed. The results show that the waste heat recovery rate is increased by 3% to 8% (depending on different air-flow rate) with the use of phase change material. This implies that phase change materials such as paraffin wax can be used in heat exchangers to obtain an improved waste heat recovery rate.