CFD analysis of triple-pipes for a district heating system with two simultaneous supply temperatures

Employing triple-pipes for simultaneous yet separate supply of space heating and domestic hot water demands of district heating network is an interesting solution for decreasing the rate of losses and increasing the cost-effectiveness of district heating systems. Although the thermodynamic performance of triple-pipes has been studied before, there is not any detailed performance analysis of such pipes, especially for the specific two-supply-temperature district heating design. This study employs computational fluid dynamic methods to investigate the feasibility of using triple-pipes for this district heating scheme and presents a detailed report of the thermal behavior of such pipes under different operational conditions including different pipe arrangements within the casing. For better understanding the effect of using triple-pipes in such a district heating system, the results obtained for this type of pipe is compared to those given for a twin-pipe in a regular district heating system. The results show that a triple-pipe with any arrangement is better than a twin-pipe supply method while this enhancement can be much better with a triangular arrangement of the three lines within the casing. With certain supply and return temperatures for a 10 km long pipeline, the rate of heat loss from the most efficient triple-pipe design is only 80 kW while it is about 145 kW for the twin-pipe system. (C) 2019 Elsevier Ltd. All rights reserved.