Book - School of Science and Technology

Chapter 21Piping design for central hot water supply systemsPiping arrangements for hot water supply systems fall naturally into four categories andare best considered within these, as follows:. The primary pipework which provides for circulation of water between the energysource and the storage vessel or vessels. This circulation is related to the maintenance ofthe required temperature in the store and, except in that sense, is independent of therate of outflow at draw-off points.. The secondary outflow pipework which is required to pass hot water in the quantitydemanded by the draw-off points when taps, etc., are opened.. The cold water feed pipe to the hot water store which is required to pass make-up waterequivalent to the hot water quantity drawn off. This pipe, originating from either anelevated cistern or a mains pressure service pipe, conveys the motive force which causeshot water to flow at draw-off points.. The secondary circulating pipework which, in conjunction with the outflow pipework,provides means whereby hot water is constantly available near to the draw-off points.Primary circulationsPrimary pipework (direct system)To ensure that circulation between energy source and storage is brisk, taking account ofthe probability that the available pressure difference will be small, the primary flow andreturn pipework should be sized generously. In addition, it is sensible in hard water areasto make allowance for the inevitable reduction in diameter due to scale deposition andthus, even for the smallest system, piping should not be less than 25 mm galvanised steelor 28 mm copper.The pipe size required may be determined using exactly the same methods of calculationas those described in Chapter 7 for a heating system operating with gravity circulation,taking the circulating head as the difference in pressure between the hot rising andthe cold falling columns. The effective height is from the centre of the boiler to the centreof the cylinder, as shown in Figure 21.1, and a temperature difference, flow to return, of20 K may be assumed for purposes of calculation. As a first approximation, the rates ofenergy flow through given sizes of pipe, as listed in Table 21.1, may be used. These arebased upon a boiler flow temperature of 60 C and a height of 3 m, the travel being 15 mand incorporating eight bends. The tabulated figures for energy flow are 20% less thanthe theoretical in order to allow for scaling in the pipes.