Characteristic effectiveness curves for falling-film drain water heat recovery systems

Abstract

Falling-film drain water heat recovery systems have proven to be a cost-effective and reliable class of heat exchanger for reducing primary energy consumption in residential and commercial buildings and in industrial buildings and processes. It is fitting, therefore, that regulatory bodies are preparing standards by which various products can be characterized, both for rating purposes and to provide data for building energy simulation. Unfortunately, standards development is progressing in the absence of measured performance data that characterize how these heat exchangers perform. The intention of the current work is to examine drain water heat recovery performance at various equal-flow conditions. The effectiveness of three drain water heat recovery systems was examined in a counter flow as a function of volumetric flow rate. The drain water heat recovery systems represented products from two manufacturers and two lengths. One of the drain water heat recovery systems was also tested in parallel flow. While the performance characteristics generally mirrored theoretical performance, there were some key differences. For the units tested, there was a clear transition region occurring between flow rates of 5 and 10 L/min (1.3 and 2.6 gpm). While the presence of this region did not impact the proposed rating process, it could significantly impact the applicability of the data analysis to building simulation. It was also shown that the number of transfer units for the collector changed significantly with flow rate, but in a predictable manner. By fitting the number of transfer units versus flow rate data, and using this correlation in conjunction with theoretical ϵ–number of transfer units equations, the drain water heat recovery performance could be well predicted over the entire range of operation.