Book - School of Science and Technology

Water systems ± applications 237Except for the smallest heating systems and for primary circuits to some indirect hotwater supply systems (p. 600), water circulation by gravity has now been supplanted byuse of centrifugal pumps. The origins of this change were the sponsorship of domesticsmall bore systems in the mid-1950s by a solid fuel research association and thecoincident introduction of the small relatively inexpensive submerged rotor pumpsnoted in the preceding chapter. In these circumstances, it is logical to give precedence hereto consideration of pumped systems and to refer to gravity circulation subsequently.A single-pipe circuitTo consider the simplest of circuits first, Figure 9.3(a) shows a single-pipe system servingsix radiators. The pipe must carry sufficient energy, via the water flowing in it, to cater forthe emission from all the radiators plus that of the piping. This total will determine the netrequirement imposed upon the boiler.Assume that the radiators emit 64kW ˆ 24 kWand that pipe emission (taken as 32 mm) ˆ 3kWTotal load ˆ 27 kWIf the temperature drop, flow to return,t ˆ 10 K, then energy flow ˆ 27/10 ˆ 2.7 kW/KAssume that the piping measured length ˆ 30 mand that the equivalent length (for fittings etc.) ˆ 5mTotal length ˆ 35 mReferring now to Figure 9.1, a choice is available:40 mm pipe with a pressure loss of 65 Pa/m, thus 65 35 ˆ 2:3 kPa32 mm pipe with a pressure loss of 120 Pa/m, thus 120 35 ˆ 4:2 kPa25 mm pipe with a pressure loss of 550 Pa/m, thus 550 35 ˆ 19:3 kPaBoth pressure loss and velocity (0.45 m/s) are low in the 40 mm pipe and pressure loss isa little on the high side in a 25 mm pipe: thus, it would seem that the original assumptionof 32 mm was correct. The net water duty required would be (2:7/4:2) ˆ 0:65 litre/s(a)(b)Figure 9.3 Single-pipe circuits: (a) simple, (b) multiple