Book - School of Science and Technology

138 Electrical storage heatingVentInsulatedtank orcylinderDayelementsFlowReturnColdfeedNightelementsPumpMixingvalveOutercasingFigure 5.9 Wet core boilerPackaged equipment typically consists of an insulated cylinder fitted with two banks ofimmersion heater elements, one near the bottom to produce the over-night charge andanother near the top for use as a daytime boost on occasions when the store is exhausted.Water is stored at a temperature which is varied to suit the static head available from aroof mounted expansion cistern, 95 C being a not uncommon level.The cylinder is piped to a simple three-way mixing valve set to produce a water outlettemperature of about 75 C. A pump is provided to circulate water round the heatingsystem, the volume being restricted to ensure that water returns to the cylinder at not morethan 40 C in order to take maximum advantage of the stored volume. Figure 5.9 illustratesthe usual form of such equipment and Table 5.12 provides leading particulars of one make.A unit of this type will have some advantages over the dry core equivalent, first since noheat exchangers are needed, the same water being used for both storage and distributionto the indirect system, and second because 24 hour standing losses are lower, the storagemedium being held at a comparatively low temperature.Conversely, and probably much more importantly, it would seem that a wet core unitcould not be used to replace a boiler without considerable alterations to the existingsystem connected to it. This situation arises since, with an outlet water temperature of say75 C and the required return water temperature of 40 C, as mentioned above, the meantemperature at the existing radiators would be about 57.5 C. This is some 15±20 K lowerthan that at which they were, in all probability, designed to operate.Thermal storage cylindersThese, of course, represent the traditional method of storing heat which has been madeavailable at a cheaper overnight rate. Such vessels were used in systems designed andinstalled 60 or more years ago and their modern equivalents are very similar except in