Book - School of Science and Technology

Requirements for storage capacity and boiler power 585only empirically and is, at best, intermittent. For example, the daily usage of hot water ina school, an hotel and an office block with a canteen might be identical for given sizes butthe peak demands would differ as to both magnitude and timing. Furthermore, usage ineach would not be the same per head, per unit of floor area or per any other criterion.A natural relationship exists for any particular type of demand between the volume ofhot water stored and the boiler power provided to heat it. In general terms, the moregenerous the storage the smaller the boiler capacity needed since it will have a long time torestore the temperature followingdraw-off. Nevertheless, such an arrangement carried toexcess would lead to an unreasonable delay in recovery of storage temperature if this wereto have fallen below normal, due to night-time shut down for example. Conversely, thecombination of a small storage volume and a comparatively large boiler would providequick recovery of temperature after draw-off but probably would be inadequate to meet asustained heavy demand. A compromise between the extremes must be chosen.Approximate methodsIn many cases a storage capacity equal to the maximum draw-off in any one hour at peakload conditions will be an adequate provision. The associated boiler power may then besized on the basis that this volume of water will be heated from cold over some longerperiod such as two or three hours. A rule of thumb basis of this sort is, however,acceptable only in circumstances where no parallel experience with a similar load existsor where no general statistical data are available. A digest of collected data is given inTable 20.3 for the capacities of certain fittings and in Table 20.4 of daily and hourlyconsumptions of hot water in various types of building. These latter values are necessarilyaverages but may be used to settle a first approximation for capacity.Recent investigationsThe approximate method probably overstates the requirement and an approach based ondata established from field studies are included in the Guide Section B4. There, a series offigures represents the relationship between storage capacity and boiler power for anumber of buildingtypes. This information may be presented in a rather simpler manner,without any loss of accuracy, by the listings of Table 20.5. The values given do not includeany allowance for the loss of effective storage capacity which results as incoming coldwater mixes with the hot water held in the vessel. To provide for this situation, an additionof 25% should be made to the total of the volume calculated but not, of course, to theequivalent total representingthe output required of the associated boiler or electricalimmersion heater.Table 20.3 Capacity of various standard fittingsFitting Capacity (litre) Required temperature ( C)Bath, average 80±120 40±45Sink 12±18 50±60Basin, normal fill 5±10 40±50Shower rose, 150 mm 0.6 litre/s 40Shower spray 0.15 litre/s 40