Book - School of Science and Technology

238 Piping design for indirect heating systems60m 10m 10m 10m 10mEDCBAFigure 9.4 Example of a two-pipe circuitagainst a pressure of 4.2 kPa and a pump rated at perhaps 0.7 litre/s against 5 kPa wouldbe chosen. The piping connections to individual radiators would be sized separately aswill be shown later.For reasons which will become apparent, parallel single-pipe circuits are dealt with later.A two-pipe circuitA simple example of a two-pipe circuit is shown in Figure 9.4 and this will serve toillustrate how the pipe sizes might be selected in an approximate way, perhaps to producea preliminary cost.Assume that each of the heat emittersis a 20 kW fan convector, 5 20 kWˆ 100 kWand that pipe emission is say 10% ˆ 10 kWTotal loadˆ 110 kWIf the temperature drop, flow to return,t ˆ 12 K, then energy flow ˆ 110/12 ˆ 9:1 kW/KMeasured length of piping, from Figure 9.4 ˆ 200 massume that equivalent length ˆ 20 mTotal lengthˆ 220 mIf we assume that a pump pressure of say 20 kPa would be appropriate, then the unitpressure drop for the longest (index) pipe run, from the boiler to the most distant fanconvector, would be 20/220 ˆ 90 Pa/m. From Figure 9.1, using this value, pipe sizes forenergy loads between 1 and 10 kW/K may be read off and the following scheduleconstructed:Pipe size kW/K kW for 12 K t25 mm 1.1 13.232 mm 2.2 26.440 mm 3.1 37.250 mm 6.3 75.665 mm 10.2 122.4Assuming that the pipe emission is constant at 10% of the net energy load, a secondschedule may be constructed: