Book - School of Science and Technology

Secondary outflow and return pipework 605Cold water feed pipeworkThe cold water feed to a hot water store is an integral part of the secondary outflowpipework. However, it is convenient to deal with it separately here, since it may originateeither from an elevated storage cistern or, as permitted by the Water Regulations from amains pressure service pipe. In either case, the head of water (or the mains pressure)provides the motive force which produces outflow at the points of draw-off. The relationshipbetween these alternatives is given in Table 21.5.Piping design for outflowTaking the system illustrated in Figure 21.5 as an example of feed from an elevatedcistern, the hot water flow rate through the cold water feed and the secondary outflowpipework would be determined as shown in Table 21.6. Many designers mark up the`demand units' and flow rates on a diagram of the system but the sequence of workingis more easily illustrated by the tabular approach used here.If the cold water feed were from a mains pressure service pipe, the procedure followedwould be just the same. In all probability, the cold water supply arrangements would beTable 21.6 Application of demand unit calculation: worked exampleFlow (litre/s)Floor Fittings Demand units (public)FromFigure 21.4Totals4 5 basins 5 ˆ 252 baths 25 ˆ 50 75 0.4 0.43 6 basins 5 ˆ 303 baths 25 ˆ 75 105 0.511 shower 0.15 litre/s 0:15 0.662 6 basins 5 ˆ 304 baths 25 ˆ 100 130 0.561 shower 0.15 litre/s 0:15 0.711 8 basins 5 ˆ 405 baths 25 ˆ 125 165 0.662 showers 0.15 litre/s 0:30 0.96Kitchen 2 sinks 0.3 litre/s 0.6 0.6Flow (litre/s)ItemSectionDemandunits(cumulative)From Figure21.4 Showers and sinks TotalsRiser OA 75 0.4 ± 0.4OB 180 0.74 ‡ 1 0:15 ˆ 0.89OC 310 1.0 ‡ 2 0:15 ˆ 1.3Mains OD 475 1.45 ‡ 4 0:15 ˆ 2.05OE 950 2.5 ‡ 8 0:15 ˆ 3.7OF 1425 3.45 ‡ 12 0:15 ˆ 5.25Total OG 5.25 ‡ 2 0:3 ˆ 5.85