Transient behavior of a falling-film drain water heat recovery device, thermal and economic performance assessments

Abstract

The drain water heat recovery device (DWHR) is a vertical, counter-flow heat exchanger, which is designed to recover heat from warm grey water in buildings. In this study, the transient characteristics of a DWHR device is experimentally and analytically investigated. A transient model is developed to predict the response time of the DWHR device under different flow conditions. The thermal performance of the system is evaluated with the transient and steady state effectiveness. The effect of system operating conditions and geometry on the response time was investigated experimentally and well predicted analytically. It was found that the response time increases with the volume of water in the system and decreases with the water flow rate. In addition, a large decrease in the effectiveness is observed when short and frequent use of hot water occurs. An economic analysis concluded that there is a 37% overall reduction in annual savings when the device transient performance is taken into consideration, where individually the sink and shower usage annual saving decreases by 56% and 13% respectively.

Keywords

• Transient analysis
• Drain water heat recovery
• Greywater
• Heat exchanger
• Experimental