Experimental study on enhancing wastewater energy recovery: Effect of heat exchanger tilt angle and fin design

Wastewater heat recovery using heat exchangers offers significant potential to enhance energy efficiency in the residential and industrial sectors by capturing otherwise wasted thermal energy. Despite extensive prior research in this field, the influence of heat exchanger tilt angle (i.e., the angle of the heat exchanger relative to the horizontal plane) on heat transfer performance and wastewater residence time has received limited attention. This study addresses this gap by conducting an experimental investigation of various tilt angles. A copper heat exchanger, 1250 mm in length, with a spiral fin having a surface area of ² 28,509.27 mm² , is designed to facilitate wastewater flow around it. To simulate wastewater, water is preheated to three temperatures ( 41.5 ° C , 50 ° C , and 60 ° C ) and passed through the exchanger as the hot flow. Cold water flows in the same direction as the hot flow, circulating through small copper tubes attached to the exchanger's periphery. Heat transfer between the hot and cold flows is tested at tilt angles ( θ ) of 30°, 45°, 60°, and 90°. The results indicate that the spiral fin effectively enhances heat transfer. Furthermore, the tilt angle significantly affects the heat recovery effectiveness ( ε ) and the overall heat transfer coefficient ( U ). The highest values are achieved at θ = 45 ° , with an ε of 40.2 % and a U of ⁻ ² ⁻ ¹ 628.47 W m ⁻ ² K ⁻ ¹ . In contrast, the lowest values are observed at θ = 90 ° , corresponding to an ε of 24.1 % and a U of ⁻ ² ⁻ ¹ 369.44 W m ⁻ ² K ⁻ ¹ . Following the 45° tilt angle, the next highest values are observed at θ = 60 ° ( ε = 34.4 % , ⁻ ² ⁻ ¹ U = 539.86 W m ⁻ ² K ⁻ ¹ ), followed by θ = 30 ° ( ε = 31.5 % , ⁻ ² ⁻ ¹ U = 423.28 W m ⁻ ² K ⁻ ¹ ).