Book - School of Science and Technology

158 Indirect heating systemsRadiatorsRadiatorsFlow(a)ReturnFlow(b)ReturnFigure 6.14 Hybrid reversed return circuits with:(a) simple sub-circuits; (b) simple supply mainsTo other circuitsHairpincoilRadiatorsNaturalconvectorspiped inseriesRadiatorsBoilerDampercontrolRadiatorsFigure 6.15 A hybrid series circuitFrom other circuitsa two-pipe system serving radiators. By this means, the potential of the mediumtemperature water is exploited to the full.From time to time the concept of three-pipe arrangements emerges from hiding. It isthen proposed that two circuits should be served, either by separate flows and a commonreturn or by a common flow and separate returns. This arrangement is often considered,as an economy measure, particularly for group and small district heating schemes, asshown in Figure 6.16. On paper, such a piping system will allow each of the two circuits tobe controlled separately:one, perhaps, being live in summer only and the other, or both,in winter. There is no published record that any such arrangement has been successful butample evidence of failure:three-pipe circuits must be ranked with three-card tricks.Steam systems ± characteristicsSteam as a medium for heating radiators and the like is a thing of the past. The flexibilityof hot water systems, with their ability to adjust temperature of output to suit weather