Performance Optimisation of Hot Water Cylinders

The systematic depletion of non-renewable energy sources and the current high capital costs of installing renewable energy systems place significant focus on energy efficiency in engineering. The requirement for energy efficiency covers a wide set of disciplines from industrial power generation to home heating and hot water. This study focuses on the operation of domestic hot water cylinders and how to better understand the factors relating to improving the output of its immersed tube coil heat exchangers. It is shown that by improving the output capability of the immersed coil, a domestic hot water cylinder can transfer the energy provided to it by both renewable and non-renewable sources faster to the stored water.
This was an experimental research project, and all the work was completed to BS EN 12897:2006, which ensured that the results were repeatable, transferable and comparable to other cylinders both from the sponsoring company and to others on the market.
The study looked at varying the parameters of height, diameter and pitch of a coil within a test cylinder and measuring the U values obtained under operation. In each case the coil tested was placed offset to the cylinder axis with the intention of creating a convective current to improve heat transfer. Special tests separate from the parametric investigation were also carried out to evaluate the impact of adding a chimney to the coil, painting the coil surface black and adding a taper angle to the coil. The test coils were also compared to current commercial heating coils in cylinders.
Certain combinations of parameter changes showed slight improvements in the U values of the coil. The taper angle added to the coil showed no significant improvements and painting the coil surface black was proven to be detrimental to the coils performance. The chimney addition showed signs of improved convection within the cylinder.
The offset test coils when compared with current central commercial coils showed a significant improvement in output per square meter of coil surface area, providing same heat up times with 39% less coil length. The offset design has now been presented to the host organisation of this project and is under review for implementation in a future range of large capacity cylinders.