An investigation of drain-side wetting on the performance of falling film drain water heat recovery systems

Falling film drain water heat recovery (DWHR) systems are single pass, vented heat exchangers. The heat from the drain water is transferred through the DWHR system to the incoming cold mains water, recovering otherwise wasted energy. This article discusses the theory behind DWHR systems, and examines how drain-side wetting affects performance. It also describes the apparatus built by the Solar Thermal Research Laboratory (STRL), at the University of Waterloo, for testing the effectiveness of DWHR systems. The results show that as the flow rate increases, the effectiveness of the DWHR pipe decreases. The data also indicates that there is a Critical Flow Rate (CFR) below which the performance of a DWHR pipe cannot be extrapolated. The inability to extrapolate is due to the incoming drain water not repeatedly wetting the same amount of area on the pipe's interior wall. This CFR is dependent on the diameter and interior surface characteristics of the pipe.