Book - School of Science and Technology

574 Hot water supply systemsVentDraw-offto tapsSupplycisternPrimary flowand returnClean-outcoversDirectcylinderCold waterfeed andexpansionClean-outcoverBoilerFigure 20.12 Direct hot water systembowerbarifing or, where made in mild steel, were treated internally with some form ofvitreous enamel coating.Modern systems utilize either direct fired storage water heaters similar to the unitshown in Figure 20.6 or where larger demands are required direct fired non-storagemulti-tube hot water boilers as shown in Figure 20.13. This type of boiler is constructedof a horizontal bank of water carryingfinned tubes mounted above the gas burnersproducinga highly efficient instantaneous water boiler. In hard water areas scalebuild-up in the tubes is inhibited by the scouringaction of the water through the tubes.This type of unit can be married to a hot water storage vessel to increase the hot watervolume where infrequent peak demands exceed the instantaneous capability of the waterboiler.Indirect systemsThe total separation of boiler water from that drawn off at taps, etc., and a solution to theassociated problem of cleaningthe internal surfaces of the boiler, are both found when anindirect system is adopted. Figure 20.14 illustrates such an arrangement in which any typeof heatingboiler may be used. The storage cylinder becomes indirect as a result of theprovision of heatingsurface within it to contain the (primary) water circulated from theboiler. The (secondary) water to be heated is outside the heatingsurface and is thusisolated from the boiler. It will be noted that this separation introduces the need for anadditional feed and expansion cistern.The primary water within the heatingsurface is recirculated and thus will remainunchanged with the result that scale deposition internally is likely to be negligible.Further, when this primary water is kept at a temperature below about 90 C then, exceptin areas where the supply has an extremely high temporary hardness, there will be littledeposit on the external secondary surfaces.