Book - School of Science and Technology

Water systems - gravity circulation 249whereCP ˆ circulating pressure per m height (Pa/m)p 2 ˆ specific mass of water at temperature t 2 (kg/m 3 )p 1 ˆ specific mass of water at temperature t 1 (kg/m 3 )As a matter of convenience, values of circulating pressure (CP) have been pre-calculatedand are listed in Table 9.3 for a range of flow temperatures and temperature drops, flowto return.It will be obvious that the water temperature at any point around the piping circuitmay be determined knowing the ratio between the heat emitted between the boiler andthe point in question and the heat emission overall. For example, if the output ofradiator `1' and the upper horizontal pipework were 25% of the overall emission, andthe temperatures at the boiler were 80 C flow and 60 C return (t ˆ 20 K), thenthe temperature of the water at the top of the drop P would be 80 (0:25 20) ˆ 75 C. The circulating pressure available may be determined by use of this simplerelationship.Equilibrium conditionsFor a given quantity of heat to be transmitted from the boiler to radiators `1' and `2',water must flow through the pipework, the mass depending upon the temperature drop.N 1 21P 1Figure 9.11 Simple example of single-pipe gravity circulationTable 9.3 Circulating pressures for gravity hot water systemsFlowtemperature( C)Circulating pressure (Pa) per metre height, for the following temperaturedifferences (K) flow to return10 11 12 14 16 18 2060 47.4 56.1 64.6 72.8 80.7 88.4 95.765 50.3 59.7 68.8 77.6 86.2 94.6 10370 53.1 63.1 72.8 82.3 91.5 101 10975 55.9 66.4 76.7 86.8 96.6 106 11680 58.5 69.6 80.5 91.1 102 112 12285 61.1 72.7 84.1 95.3 106 117 128