Book - School of Science and Technology

14m12m9m6m3mSecondary outflow and return pipework 609Minimum water levelCold waterfeed(5.85)(0.4) (0.4) OA (0.4)20mtokitchen(0.89) (0.89)OB (0.89)(1.3)(1.3)OC (1.3)(0.6)(0.6)OF(5.25)(2.05) (2.05)OE(3-7)OD(2-05)OG(5.85)2m 3m1m 3m 3m3m3mFigures in bracketsare flows in litre/s(Table 21.6)Figure 21.6 Pipe sizing example: dimensionsPiping design for secondary returnsNote has already been made of the limitations which should be placed upon the length ofdead-legs and thus, where hot water draw-off points are dispersed widely, it is necessaryto consider whether to provide:. A local instantaneous heater to some or all draw-off points.. A number of dispersed storage vessels with a dead-leg distribution from each. (Thesemight be served either by local energy sources or from a primary distribution system asshown in Figure 21.2.). A secondary circulation system.It is not possible to generalise as to either the most practical or the most economicsolution since each application must be considered on its merits. A secondary circulation,with which we are concerned here, offers facilities for heating towel rails and linencupboard coils, etc., at times when a heating system may be shut down but, since thereare continuous heat losses from the circulating pipes even when well insulated, is thereforewasteful of energy.The water quantity to be circulated through secondary pipework is a function of theheat emission from the system and of a chosen temperature drop, normally 10±12 K for agravity circulation and about 5 K where a pump is to be used. The heat emission derives