Book - School of Science and Technology

248 Piping design for indirect heating systems(A)h(B)hbPbPPv(C)h(D)hbbPFigure 9.10 Alternative arrangements for cold water feed and expansion pipeFour situations are shown in Figure 9.10:. In case A, the vent pipe must be carried above the water level in the cistern, to a heightexceeding the pressure exerted there by the pump, if water discharge is to be avoided innormal operation. With a high pump pressure this may not be possible.. In case B, the vent pipe is not subject to pump pressure and no additional height isnecessary but there is some tendency to draw air into the system at any air vent or otheropen high point.. In case C, with the pump in the flow pipe, the vent and the feed and expansion pipe arein balance. Since virtually the whole system is under pump pressure, air release willpresent no problems.. In case D, the feed and expansion pipe is combined with the vent. This is a solutionshown to be very dangerous more than 50 years ago but still sometimes used.Water systems - gravity circulationA simple single-pipe system is shown in Figure 9.11 and, with a gravity circulation, theforce creating water movement will be that due to the difference in mass between thepositive column P 1 at temperature t 2 and the negative column N 1 at temperature t 1 . Theforce available will be that of unit mass, subject to acceleration due to gravity, which maybe taken as 9:81 m/s 2 , and thus:CP ˆ 9:81( p 2 p 1 )