Experimental investigation of the performance of a falling-film drain water heat recovery system

Drain water heat recovery (DWHR) devices are utilized to reduce the energy consumption of domestic water heating by recovering waste heat from warm greywater. In this study, the thermal performance of a gravity film DWHR device type is experimentally investigated under steady operating conditions. The thermal performance is measured at different flow rates and we surface conditions using the effectiveness - number of transfer units and the logarithmic mean temperature difference models. A thermal resistance model is developed to determine the convection heat transfer coefficient of the falling film at the fully we surface condition. The thermal resistance model showed that the largest thermal resistance in the device is the falling film convection resistance. A new correlation is developed for the falling film dimensionless convection heat transfer coefficient in the Reynolds number range of 1000-5000. The obtained correlation has good agreement with the experimental values with a correlation coefficient of 85%. In addition, it is found that at partially we surface condition; which is expected in actual device operation, there is a considerable reduction in the device effectiveness that could reach 28.5% of its value at the fully we condition.