Book - School of Science and Technology

136 Electrical storage heatingTable 5.10 Warm air units ± typical particularsInput rating (kW)Item 6.79 9.86 14.79 30 72 100Performancecharge acceptance (kWh) 47 73 104 210 504 485Dimensionsheight (mm) 1300 1300 1750 2500 2178 2588width (mm) 630 630 630 860 2065 2228depth (mm) 610 610 610 775 1265 1156Weightin situ (kg) 370 484 754 1490 4400 4375Fanduty (litre/s) 116 175 244 ± 1415 940Manufacturer a A A A B C BNotea A, Unidare Engineering; B, Chidlow and Co.; C, Powermatic.equivalents, is normally at about 60 C and, as would be expected, is almost entirelyconvective.Dry core boilersIn production model equipment of this type the heat store is not dissimilar in principlefrom that provided in a warm air unit having a similar rating, although greatly refined indetail as to configuration.The Feolite blocks which make up the storage core are disposed about a central hotdraught tube and have vertical passages formed in them to accommodate the heatingelements: they are contained within a casing which is lined with two layers of `Microtherm'insulation. The core assembly stands upon a substantial layer of insulating blockswhich isolate it thermally from the plenum chamber forming the base of the unit.The plenum chamber is a boxconstruction containing a fan, an air to water heatexchanger, a pump and the associated piping connections. A further insulated enclosuresurrounds the complete unit and mounts the control equipment, as shown in Figure 5.8.The elements are arranged so that separate switching of one or more is possible in order toprovide a facility for use as a daytime boost on occasions when the store is exhausted.Example particulars of three models are listed in Table 5.11, the input ratings beingrelated to a 5 1 ¤ 2 hour charging period. The larger units may require a three-phase supply.An alternative design operates on a different principle, with energy transfer fromstorage to the external space heating circuit via a steam/water heat exchanger. The steamis produced by a controlled `boiling circuit' within the core, piped in stainless steel andtopped up from a small de-ionised water tank. This alternative, rather less bulky than anair/water arrangement, allows the height of the boiler casing to be reduced to permitinstallation below a kitchen worktop.It should be noted that the characteristics of this equipment are such that watertemperatures not very different from the traditional can be produced and that, in consequence,it may be used to replace a boiler connected to an existing hot water heatingsystem. The proportions of radiant and convective output are, of course, determined bythe type of indirect system connected.