Book - School of Science and Technology

606 Piping design for central hot water supply systemsserved from the same intake into the building and the duty of the pressure-reducing valvewould thus be based upon the total combined demand from all draw-off points, hot andcold.Once the water quantities to be carried by each section of outflow pipework havebeen established, the next step is to consider either the head of water available from thecistern or the residual of the working pressure selected where feed is from a mainsservice pipe.In either event, the worst case is that presented by the highest draw-off point in thebuilding. Progressing down a multi-storey building, a greater head or pressure will beavailable on lower floors which may lead to splashing problems there unless steps aretaken to throttle the supply.Tables 21.7 and 21.8 have been compiled from data included in the Guide Section C4for water flowing at 75 C in copper pipes to BS 2871: Table X and in galvanised steelpipes to BS 1387: heavy respectively. Bearing in mind the other potential variables inthis application, errors arising from applying these tables to designs where water atother temperatures or other grades of copper or galvanised pipe are to be used arenegligible. It will be noted that flow rates may be read from these tables against eitherhead loss (m water per m run) or pressure loss (kPa per m run), whichever is moreconvenient for the application. Table 21.9 gives the equivalent data for water flow at65 C in PVC pipes.ExampleTaking the system shown in Figure 21.5 as an example, physical data and the flow ratespreviously calculated may be added as shown in Figure 21.6.Table 21.7 Flow of water at 75 C in copper pipes (BS 2871: Table X)Head loss(m of waterper m run)Water flow (litre/s) in pipes of stated outside diameter (mm)15 22 28 35 42 54 67 76 108Pressure loss(kPa per mrun)0.01 ± 0.13 0.25 0.46 0.77 1.56 2.80 3.97 10.4 0.10.02 ± 0.19 0.37 0.67 1.13 2.29 4.11 5.81 15.2 0.20.03 ± 0.23 0.47 0.84 1.42 2.87 5.14 7.26 18.9 0.30.04 ± 0.27 0.55 0.99 1.66 3.36 6.02 8.50 22.1 0.40.05 0.11 0.31 0.62 1.12 1.88 3.80 6.80 9.60 25.0 0.50.06 0.12 0.34 0.69 1.24 2.08 4.20 7.51 10.6 27.6 0.60.07 0.13 0.38 0.75 1.35 2.26 4.57 8.17 11.6 30.0 0.70.08 0.14 0.40 0.81 1.45 2.44 4.92 8.78 12.4 32.2 0.80.09 0.15 0.43 0.86 1.55 2.60 5.24 9.37 13.2 34.3 0.90.10 0.16 0.46 0.92 1.64 2.75 5.56 9.92 14.0 36.3 1.00.12 0.18 0.51 1.01 1.82 3.04 6.14 11.0 15.5 40.1 1.20.14 0.19 0.55 1.10 1.98 3.31 6.67 11.9 16.8 43.6 1.40.16 0.21 0.59 1.19 2.13 3.56 7.18 12.8 18.1 46.8 1.60.18 0.22 0.63 1.27 2.27 3.80 7.65 13.6 19.2 49.8 1.80.20 0.23 0.67 1.34 2.40 4.02 8.10 14.4 20.4 ± 2.0Equivalentlengths for ˆ 1:0 0.6 1.0 1.5 2.0 2.5 3.6 4.7 5.6 8.9Equivalentlengths for ˆ 1:0NotesValues of for fittings: bends, tees, reducers, enlargements ˆ 1:0.Screw down valve or tap ˆ 10:0; connections to cistern and cylinder ˆ 1:5.