The stratifying solar heat exchanger of COAX 390 consists of a tightly wound smooth coil which is surrounded on the inside and outside by a synthetic pipe (figure 1). The heat transfer fluid is carried in a spiral from top to bottom against the up-flow of the domestic hot water being heated. Thus the heated water can be carried into the middle of the tank or into the heating zone at the top, where the water is then released through openings. Sufficient radiation provided, the heated domestic hot water is immediately ready for con­sumption, which leads to fewer boiler start ups and the water remaining cool at the bottom of the tank for longer time periods, which increases the energy yields of solar applications.

A specific construction of these pipes, that optimises the channel widths for low-flow rates of the solar circuit as well as that for the domestic hot water leads to a more intensive im­mersion of the heat transferring surfaces, even at low-flow rates. The surface-specific heat-transfer-coefficient is therefore two to three times that of conventional immersed smooth coils. Figure 2 illustrates the heat-transfer-coefficient of an enamelled smooth coil heat exchanger (reference tank of ITW) in comparison to measurements of the COAX heat exchanger.

The result of this extremely efficient heat transfer is that the temperature at the entrance of the solar collector is only 1 to 5 degrees higher than the temperature at the bottom of the

Figure 2: Comparison of the COAX heat exchanger with a conventional enamelled heat exchanger (1" pipe, area 1.7 m2). Measurement: ITW, University Stuttgart

tank, which leads to minimised heat losses of the collector whilst achieving maximum yield. The heat exchanger has been developed especially with view to scaling problems of filigree heat exchanger structures, as they are used by other suppliers for low-flow: here the smooth stainless steel areas combined with the high current and the exposure to con­stantly changing temperatures show a significantly reduced tendency to scaling than con­ventional immersed heat exchangers. When the heat exchanger is hot the upward current starts up thus scale precipitates primarily in the water. It is then rinsed from the tank or sinks down to the bottom of the tank where it can be removed via the cold-water connec­tion.