Book - School of Science and Technology

Secondary outflow and return pipework 613Table 21.12 Pressure loss in secondary circulation: worked example1 2 3 4 5 6 7 8 9 10 11Flow pipingReturn pipingSectionLoading(kW/K)Length(m)Pipesize(mm)Pressure lossUnit(Pa/m)Section(Pa)Total(Pa)Pipesize(mm)Pressure lossUnit(Pa/m)Section(Pa)Total(Pa)OG 0.69 5 54 1.9 9.5 9.5 42 6.5 32.5 32.5OF 0.67 5 54 1.7 8.5 18.0 42 6.2 31.0 63.5OE 0.45 5 42 3.2 16.0 34.0 35 7.5 37.5 101.0OD 0.22 7 42 1.0 7.0 41.0 28 6.5 45.5 146.5OC 0.17 4 35 1.5 6.0 47.0 22 13.7 54.8 201.3OB 0.11 4 35 0.3 1.2 48.2 15 42.5 170.0 371.3OA 0.06 4 28 0.7 2.8 51.0 15 15.0 60.0 431.3Floor 1 0.06 5 28 0.7 3.5 15 15.0 75.0OG±OD ± 22 ± ± 41.0 44.5 ± ± 146.5 221.5Floor 2 0.06 5 28 0.7 3.5 15 15.0 75.0OG±OC ± 26 ± ± 47.0 50.5 ± ± 201.3 276.3Floor 3 0.06 5 28 0.7 3.5 15 15.0 75.0OG±OB ± 30 ± ± 48.2 51.7 ± ± 371.3 446.3Floor 4 0.06 5 28 0.7 3.5 15 15.0 75.0OG±OA ± 34 ± ± 51.0 54.5 ± ± 431.3 506.3Notes1. Pressure loss as read from Figure 21.8 (p. 612).2. Equivalent lengths for pipe fittings are assessed on a per cent basis.small proportion (Table 21.12) absorbed in the flow pipework must be deducted, e.g. thebalance available at floor 1 is 251 45 ˆ 206 Pa. Hence, the unit pressure available forthe return pipework from floor 1 is 206/27 ˆ 7:6 Pa/m run.This unit availability may be used as for the flow piping and the pipe sizes noted incolumn 8 of Table 21.12 selected. As a result, again by reference to Figure 21.8, the listingsin columns 9±11 in Table 21.12 are produced, those in column 11 being the proportion ofthe available pressure which is absorbed in the return mains to each floor.It will be noted that an exact balance is not possible between available pressure (206 Pa)and pressure loss (222 Pa) for either floor 1 or any other floor. It is rarely practicable toachieve a precise match using commercial sizes of pipe but the consequences, in thisapplication, are not very significant when of the magnitude illustrated. The result wouldbe that the system came into hydraulic balance at a temperature difference varying by adegree or less from the arbitrary 10 K chosen.If it had been decided for some reason that a pumped circulation could be provided to asystem similar to that considered here, then a temperature drop of 5 K might have beenchosen, thus doubling the water quantities previously calculated. The index circuit in thiscase would then be the most distant, that at floor 4 of riser Z, and the pressure lossthrough the flow pipes to that extremity would then be about 180 Pa. Assuming that thereturn pipes were one size smaller than those previously selected, with a minimum of15 mm, the pressure loss through them would be 3650 Pa giving a total of 3.7 kPa.It might be thought that the disparity between the pressure loss totals for flow andreturn is too great and that larger pipes should be chosen for the latter. However, a