Book - School of Science and Technology

610 Piping design for central hot water supply systemsTable 21.10 Theoretical heat emission from horizontal steel (BS 1387) and copper (BS 2871) pipesEmission (W/m run)Pipe size (mm) Bare InsulatedSteel Copper Steel CopperThickness ofinsulation(mm)15 15 42 31 8 2520 22 51 43 9 2525 28 62 53 9 3232 35 75 64 10 3240 42 84 75 12 3250 54 102 93 13 3265 67 125 112 16 3280 76 143 125 17 32100 108 179 171 18 38NotesMean temperature difference ˆ 40 K.Insulation conductivity ˆ 0:04 W/m K.from both outflow and return pipework plus the useful output of any towel rails or linencupboard coils connected to them. Piping emission may be calculated using data fromTable 21.10 and the sectional emissions are collected and totalled to produce sectionalloadings in exactly the same way as for a heating system.The sizes of the outflow piping will be known as a result of an earlier calculation but thewater quantities representing heat emission will be small in comparison with thoserequired for draw-off. Thus, the pressure loss arising from circulation in these pipes islikely to be low and, in conventional design, the return pipes will be a size or two smallerthan the equivalent flow pipes. Thus, emission from the returns may be taken either fromassumed smaller sizes or, by way of an approximation, as being two-thirds of that of theassociated flows.If a gravity circulation appears practicable, reference would be made to data such asthat listed in Table 9.3 (p. 249) and, for flow at storage temperature (65 C) and return atsay 55 C, the available circulating head would be 50.25 Pa per m height. Alternatively, ifa pumped circulation were necessary as a result of either system size or a preponderanceof mains pipework below cylinder level, then a unit pressure drop of about 60 Pa per m oftotal travel may be taken as a first approximation. This figure includes a 25% allowancefor bends and other resistances and takes account of the `over-size' flow pipework.ExampleUsing Figure 21.7 as an example, this being the same system as was shown in Figure 21.6but with return pipework added. Each branch circulation serves a towel rail and a linencupboard coil per floor, rated together at 0.5 kW, and a drying coil at 200 W is fitted in thekitchen (6.2 kW in all). The sizes of the outflow pipework have already been calculatedand heat emission may thus be set out as shown in Table 21.11.For an extensive circulation system, it would be necessary to make a full calculation inorder to apportion the mains emission, using the method described in Chapter 9. In thepresent case, however, the towel rails are uniformly disposed and the emission representsonly 0:85/7:05 ˆ 12% of the useful heat output. It would, in consequence, be reasonableto apportion the pipe losses pro rata.