Book - School of Science and Technology

Equipment for central stores 135As to the first of these developments, it might seem odd at first sight that any applicationof the storage principle to a hot water heating system should be via three-stage heatexchange, first ± energy to core, second ± core to air; and third ± air to water. The reasonsare not hard to find. As to core capacity, Table 5.7 has shown that water at atmosphericpressure compares unfavourably as a storage medium, in terms of both mass and volume,with the other materials now used. As to heat transfer, it is possible to conceive anarrangement which would have water pipes embedded in a refractory core but, withtemperatures there of 750 C, the prospect of any sort of control failure is uninvitingquite apart from construction problems.Warm air unitsExtensively publicised under the `Electricaire' label, such units are no more than largeversions of storage fan heaters, arranged to serve as a heat source either to an extendedspace or to multiple rooms via an indirect system of air-duct distribution. Figure 5.7illustrates one pattern where the heat store is mounted above the fan, mixing box andoutlet plenum chamber. The reverse arrangement, with the heat store at the bottom, ispreferred by some.Typical equipment of this type will have the dimensions, etc., listed in Table 5.10 for arange of input ratings between 6 and 100 kW. The not inconsiderable floor loadingimposed by the larger sizes should be noted as should the fact that a three-phase powersupply may sometimes be required. Output, usually arranged by admixture with recirculatedair from the heated space in a similar manner to that described for the smallerCasingInsulationCast iron coreMixing damper(core output andreturn air)FilterReturn airFanSupplySupplyFigure 5.7 Warm air unit