Book - School of Science and Technology

142 Electrical storage heatingExpansionspaceStoragevesselMixingvalveControlsSecondarycircuitPumpImmersionheatersHeatexchangerFigure 5.13 Packaged storage vessel and system (Heatrae Industrial)mixing valve shown, the variable speed circulating pump and all controls etc. are part of thepackage for a range of cylinder sizes from 2000 to 50 000 litre, with loadings of 36±720 kW.The standard operating criteria are 350 kPa and a charged temperature of 138 C.Control of central storesAs with room stores, some form of charge controller for input is provided for centralstores. In view of the increased size and capacity in this case, these are rather morecomplexand sophisticated and will in most instances incorporate some method forautomatic sensing of outside temperature during the charging period, in anticipation thatthis will provide a closer approximation. It is not possible to generalise as to outputcontrol from central stores since this will almost always be related to the connectedindirect systems.Electrical supply to storage equipment and installationsThis present volume does not pretend to deal with matters related to the design ofelectrical systems. The following notes are therefore included only as items for convenientreference.Direct electric heating elements up to 3 kW (13 amp at 250 volt) are served by a twowirecircuit. Heavier duty elements are usually arranged in banks and the load dividedover the phases of a three-phase supply if available.For thermal storage with immersion heaters, the usual arrangement is balanced on athree-phase three- or four-wire supply. Thermal storage with electrode boilers is servedinvariably from a three-phase three-wire supply at medium or high voltage.The current passing in the conductors may be calculated from the load and voltage.Taking as an example a heating load of 500 kW, the current passing through the circuitwould be as follows:for two-wire DC or single-phase AC at 240 volt, 500 1000/240 ˆ 2080 ampere; for threewireAC three-phase at 415 volt between phases, load balanced, 500 1000/(3 415) ˆ700 ampere per phase;