Investigating the Performance of a Falling-Film Drain Water Heat Recovery System

Abstract

Drain Water Heat Recovery device (DWHR) is a counter-flow heat exchanger which is designed to recover heat from warm greywater in buildings. In this study, the thermal performance of a DWHR device was experimentally investigated under transient and steady conditions using the effectiveness-number of transfer units, thermal resistances, and thermal response test methods. An empirical correlation of the falling film Nusselt number as a function of Reynolds number was obtained for Reynolds range of 310 – 1285. An actual operational effectiveness was introduced to consider the effect of the transient response of the DWHR device. Performance degradation of the DWHR device due to fouling was simulated. It was found that the effectiveness decreases by about 27% of its value at the clean condition if the mains-water is soft (hardness < 3.5 grains/gal), and the effectiveness decreases by 46% if the mains-water is very hard (hardness > 15 grains/gal).