Book - School of Science and Technology

Primary circulations 599Direct cylinderIndirect cylinderHHFigure 21.1 Effective heights for gravity circulation in primariesTable 21.1 Pipe sizes and energy flow (kW) for primary circuits in copper (BS 2871: Table X) and galvanisedsteel (BS 1387: heavy) pipesCopper pipesSteel pipesPipesizeSystem typeSystem typeDirect IndirectPipesizeDirect Indirect28 5 7 25 4 735 8 10 32 8 1342 12 18 40 11 1954 22 31 50 19 3167 35 51 65 35 5976 47 66 80 53 82It will be appreciated that, although the flow temperature from the boiler may becontrolled at a relatively constant level, the temperature of water returning to it from thestore will vary according to the rate of draw-off and, in consequence, the circulating headwill change also. The most active circulation will occur during and after a period of heavydraw off, i.e. when the lower part of the store is certain to be cold, and the least activeafter a period of no draw-off when the contents of the store are nearing boiler flowtemperature.This situation suggests that supply and demand are in equilibrium at all times, which isgenerally the case, and that the variation in circulating head will offer a form of temperaturecontrol. The exception arises when, following a period of `stagnation' at near to therequired storage temperature, a quick response to sudden heavy draw-off is required andthe necessary rapid circulation is delayed by system inertia.Where a direct fired type of water heater (Chapter 20, p. 573) is used no primarypipework is needed and the response rate to demand is always extremely rapid.