Book - School of Science and Technology

608 Piping design for central hot water supply systemsClearly, the worst case is that of the fourth-floor taps on riser Z which are both themost distant from the cistern and only 3 m below minimum water level, i.e. they have only3 m head of water available to cover frictional resistance and provide outflow. The travelto this point is:Cold water feed ˆ 24 mallow for fittings ˆ 3 m 27 mOutflow main ˆ 12 mallow for fittings ˆ 2 m 14 mRiser Z ˆ 11 mallow for fittings ˆ 2 m 13 mTotal ˆ 54 mHence, the available unit head ˆ 3/54 ˆ 0:055 m/m run and, interpolating from Tables21.7 and 21.8, the following listing may be made:Pipe size (mm)Riser ZCopperSteelBranch 28 25OA 28 25OB 35 32OC 35 40OD 42 40OE 42 50OF 54 65OG 54 65CW feed 67 65The example could be continued by considering the lower branches of riser Z since agreater head of water will be available progressively. The head lost due to friction on thislongest run, as far as the junction to each branch, would thus be deducted from the totalavailable and the surplus used to size the pipes of the branch. The same process wouldthen be applied to the other risers X and Y and to the branches from them, the run to eachbeing shorter than that to the furthest branch on riser Z.Re-examination is particularly appropriate for the long branch to sinks in the kitchensince the head available there is 9 m and the loss through the cold water feed pipe andmain section OG is only about 2 m (33 0:055 ˆ 1:82 m). Thus a head of 7 m is availableat the branch which, allowing for 5 m equivalent length there for fittings, represents a unitavailability of 7/(29 ‡ 5) ˆ 0:2 m/m run. The pipe size required, in consequence, for a flowof 0.6 litre/s, would be 22 mm in copper or 25 mm in galvanised steel, one size less in eachcase than if the original unit figure of 0.055 m/m run had been used.For a system of modest size, such refinement in calculation may not be worthwhilebearing in mind the many variables. An experienced designer may be content to use asingle value for unit head loss except where this would lead to obviously uneconomicdesign. However, in the case of large systems or those in buildings having eitherinadequate cistern height or a variety of floor levels, exhaustive examination is necessary.