Book - School of Science and Technology

Equipment for central stores 139Table 5.12Wet core boilers ± typical particulars (Farnell Instruments)Input rating (kW)Item 6 12 18 36 48 72Performancecharge acceptance (kWh) 23 60 90 135 180 303recommended connected load (kW) 2.5 6.67 10 15 20 33Storagewater at 98 C (litre) 350 910 1360 2036 2720 4540Outputdirect boost capacity (kW) 3 9 12 18 24 36Dimensionsheight (mm) 2205 a 2121 2121 2121 2121 2121width (mm) 750 1016 1219 1473 1372 1956depth (mm) 600 610 711 889 1219 1372Weightin situ (kg) 390 1174 1701 2642 3438 5766Notea Includes height of integral espansion cistern.terms of quality. The principal differences between a store of this type and those which areoutlined under the previous heading are first: the pressure and, thus, the temperature atwhich the hot water is stored; second the manner in which electrical energy is transferredto the water and third, a corollary, the overall size of the system. Figure 5.10 illustrates acylinder arrangement suitable for a smaller installation, where banks of immersionheaters are used, and Figure 5.11 shows a complete installation incorporating cylindersin association with an electrode boiler.Since, for a given heat output, the volume of storage required depends principally uponits temperature, it follows that the design criteria are interactive, i.e. the pressure whichcan be imposed upon the store and the space which can be made available for the vessels.Early systems made use of the height of the building, and the siting there of an expansioncistern to produce the necessary pressure, but more recent practice has been to use someexternal means (see Chapter 9) for this purpose. The temperature at which water is storedis normally about 10±17 K less than that at which steam would be produced. Table 5.13provides data in this context.Safety valveManholeFlow sparge pipeImmersionheatersFixed cradleExpansion pipeRoller cradleReturn sparge pipeFigure 5.10 Immersion heater type storage vessels