Thermal effectiveness and NTU of horizontal plate drain water heat recovery unit - experimental study

Abstract

The operating characteristics of plate heat exchanger in horizontal drain water heat recovery unit for domestic shower system were investigated in an experimental study. Based on test data the thermal effectiveness (ε) and number of heat transfer units (NTU) were calculated. The tests were carried out in simulated real-world conditions in three shower water flow rates (water consumption levels) for 4, 8 and 12 l/min, and two primary and secondary flows configurations (balanced and unbalanced), as a reflection of the three shower intensity standards and water temperature range of 30–50 °C covering the usual user-specific shower conditions. The least favorable test results (ε = 18.6% and NTU = 0.24) were obtained for the drain water temperature of 26.6 °C and cold water / drain flow rate at 12/12 l/min. The most beneficial results (ε = 50.0% and NTU = 0.99) were obtained for the sewage temperature of 50.8 °C and 4/12 l/min. It has been shown that the higher the flow on the drain water side and lower on the cold water side, the better the effectiveness. For a system with equal and low flows on both sides, the drain temperature change has a negligible effect on the effectiveness (4% growth with a drain temperature increase of 25.4 °C for 4/4 l/min). The paper presents developed easy-to-use charts of the variability of the effectiveness and NTU with the drain water temperature relates to a different set of primary and secondary flows in horizontal, plate, compact drain water heat recovery exchanger. The results of effectiveness obtained in the research were compared to the values presented in the literature for other drain water heat recovery units, both in vertical and horizontal applications.