Thermal conductance of pneumatic conveying preheater for air–gypsum and air–sand heat transfer

The use of pneumatic conveying duct as gas–solid heat exchanger is in vogue in the form of preheater and dryer in cement and pharmaceutical industries, among several other industries. Experiments were conducted to study the effect of solids feed rate, particle size and air velocity on thermal conductance of a vertical pneumatic conveying heat exchanger for preheating of dry solids. Sand and gypsum were used as cold medium while air was used as hot medium. Thermal conductance (defined as the ratio of heat transfer rate to driving force) was found to increase with solids feed rate and air velocity. A dimensionless correlation has been proposed for thermal conductance that predicts the present experimental data for air–sand and air–gypsum heat transfer within an error of ±18%. The relevant properties of solids are incorporated in the form of a dimensionless number, Fedorov number (Fe). The proposed correlation may be used to analyze pneumatic conveying heat exchanger of similar geometry.