Book - School of Science and Technology

Types of boiler 275To load control motorInsulatorsInsulating shield (up)ElectrodeInsulating andneutral shields(down)Boiler pumpsuctionBoiler pumpdeliveryFlow tostorageElectrodeNeutral shield (up)Return fromstorageGuide tubesFigure 10.12 High voltage electrode water heater(up to 650 V) or to high voltage (3.3, 6.6 or 11 kV), the latter being the more common forinstallations of any significant size. Such boilers, as illustrated in Figure 10.12, areavailable in ratings up to about 2.5 MW, the principal difference between the variousdesigns being in the detail of the method adopted for load regulation.Current is passed from electrode to electrode, the resistance of the water in which theyare immersed acting as the heating element. The load is varied by increasing or decreasingthe length of the path which the current has to take between and around the nonconductingneutral shields which shroud the electrodes. The material from which these shields aremade and the actual mechanism used to raise and lower them varies from maker tomaker. A brisk water movement is required around the electrodes and most manufacturersmount a centrifugal pump to the boiler shell for this purpose. This pump must notbe confused with that which must be provided to circulate water, during the chargingperiod, between the boiler and the storage cylinder or cylinders.Where electrode boilers are used for steam generation, at ratings between 20 kW and2.5 MW, it is most probable that they will operate `on-peak' although some form ofthermal storage may still be used to meet short fluctuations in demand. The arrangementInterleavedelementsNeutralshieldFigure 10.13 Interleaved elements for electrode steam boiler (plan view)