Abstract
In this paper a process of cold water withdrawing from hot water supply pipe systems without recirculation is considered. System of partial differential equations was used to describe the pipe and water temperature changes. An exact solution of a simplified form of the equations was obtained and validated experimentally. The exact solution was applied to calculate the hot water temperature changes at the pipe outflow. Calculations were done for typical pipe materials (PP, PE, Cu), different pipe diameters and lengths as well as for various water flow rates. It was shown that in order to obtain the required hot water temperature in the tap, there is necessary to withdrawn much more (even two times) water from the pipe in comparison to the pipe volume. The reason of such significant water wastes is a heat exchange between hot water flowing inside the pipe and the colder pipe walls. The results can be useful for optimal selection of hot water supply pipes as well as for making decision about applying of hot water recirculating systems.
Highlights
Domestic hot water in supply pipes will cool due to heat loss when it will be stagnant for quite some time
The aim of this paper is to investigate the effect of pipe type as well as an influence of flow rate on hot water temperature changes at the pipe outflow
The theoretical model was experimentally verified and introducing a time correction it was found useful for investigations of hot water temperature changes at the pipe outflow
Summary
Domestic hot water in supply pipes will cool due to heat loss when it will be stagnant for quite some time. In large domestic hot water systems, where the distribution piping is extensive, it will take a relatively long time to remove this cold water from the pipes before the hot water can be available at the tap. This wastes both water and energy are often not acceptable. To overcome this problem, a water recirculation system can be used. According to the regulation [1] the maximum internal volume of the hot water supply pipe for which recirculation system is not required is 3 dm. We are interested in time needed to obtain a required outflow water temperature and mass of water withdrawn from the pipe before the accepted temperature is reached
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