Book - School of Science and Technology

Water systems ± piping arrangements 157RadiatorsReturnRadiators(a)RadiatorsFlowRadiatorsRadiators(b)(c)BoilerBoilerFigure 6.13 Reversed return circuits:(a) basic principle; (b) logical layout; (c) contrived layoutThe reversed return circuitIt is almost always possible to arrange two-pipe circuits in a manner which will amelioratethe hydraulic problems mentioned above. This is achieved, in principle, by laying out thesystem so that the distance water has to travel, from the boiler to each individual terminalitem and then back again to the boiler, is the same, as illustrated in Figure 6.13(a).Whether this is a practical proposition in terms of capital cost depends upon the buildingconfiguration and the ingenuity of the designer. The alternative layouts in Figure 6.13(b)and (c) show that it may in some cases be possible to produce a reversed return at minimaladditional cost, whereas in others there are complications.For a given heat output, the water quantity in circulation around a system is a functionof the temperature difference, flow to return. Table 6.2 demonstrates that a low temperaturehot water system is likely to have over four times more water flowing in each circuitand sub-circuit than would be the case for a high temperature hot water system having thesame output. This comparison serves to show that the use of reversed return arrangements,always desirable, is much easier to justify on technical grounds for medium or hightemperature systems.Hybrid circuitsThe descriptions given of the three principal circuit configurations should not be read assuggesting that they are mutually exclusive. Provided that a clear design logic is followed,no harm is done by mixtures if they go to make up a total arrangement which suits the useand style of a building. Examples, not necessarily recommended for use, follow:Ladder circuits. For high-rise buildings, a reversed return two-pipe system as Figure6.14(a), serves single-pipe circuits at each floor. In contrast, a simple two-pipe systemserves reversed return circuits at each floor, as in Figure 6.14(b).Series circuits. Again for high-rise developments, a system much used in Europe has beenapplied to some buildings in the British Isles. It consists, as shown in Figure 6.15, of amedium temperature hot water circuit which, on a single-pipe basis, serves casedconvectors (having `hairpin' elements) in series:the return water therefrom passes to