Heat recovery from a hot-water store: Effect of parts of the immersed heat-exchanger's pipe being thermally insulated

A coiled finned-pipe heat exchanger was employed to extract heat rapidly, from a 90-litre hot-water charged tank, the water being initially at approximately 80°C. Free-convective buoyancy movements of the water around the outside of this coiled pipe (which was totally immersed in the store) occurred as a result of mains supply cold water (at 20°C) being forced internally through the pipe. The axis of the heat-exchanger's coil is oriented horizontally, and the heat-exchanger's inlet is arranged to be at its lowest level. The effect of insulating short lengths of the heat-exchanger's pipe upon the rate of heat recovery was investigated. The purpose of this [`]insulation' strategy for the heat exchanger was to prevent some of the heat gained by the water as it passes through the heat-exchanger's pipe being unintentionally returned to the store. Judicious positioning of the insulation has the effect of increasing the heat transfer rate along the exposed lengths of pipe, thereby enhancing the heat-exchanger's effectiveness. Irrespective of the flow rate of water forced through the heat-exchanger's pipe, the best performance of the heat exchanger was achieved when using a 150-mm length of rubber-foam pipe insulation (of 40 mm o.d. including the heat-exchanger's external diameter) at the lowest level of the heat exchanger and near the vertical centre-line of the tank, i.e. in the path of the descending cooled stream of water in the store. The improvement then achieved in the heat-exchanger's effectiveness is approximately 2%.