Book - School of Science and Technology

580 Hot water supply systemsFor use with a low pressure primary medium, and where the storage capacity requiredis below about 500 litre, indirect cylinders are economical in first cost. Standard sizes arelisted in Table 20.2.Single-feed indirect cylindersThe requirement that a separate feed and expansion cistern be provided to serve theprimary circuit of a conventional indirect cylinder has led to the development of specialtypes, as illustrated in Figure 20.18. Units of this pattern have no moving parts and relyupon air cushions to separate the primary water content from the secondary store, usingnaturally balancingpressures. Once filled, the air cushions expand or contract in tandemas the primary circuit heats or cools: any excess air is vented via the secondary watercontent.Water-to-water calorifiersFor larger units, as covered within this heading, a tubular hairpin pattern heat exchangeror heater battery of the type shown in Figure 20.19 is used. The calorifier shell is providedwith a flanged neck which supports a tube plate and a domed cover, the latter havingapartition within it to route the primary water flow in and out of the tubes. Means forwithdrawal of the heater battery on a runway are incorporated so that the tubes may becleaned. The primary pipework connections should be arranged so that withdrawal maybe achieved with minimum disturbance to insulation, etc.For application to medium temperature and high temperature hot water systems, acalorifier will frequently take the form shown in Figure 20.20 in which the primary waterfollows a single pass in order to maintain a high velocity with the comparatively smallColdwaterfeedSecondaryflowVentAirAirAirCold(filled)PrimariesFigure 20.18 Indirect cylinder, single feed type (Primatic)Hot(expanded)