Book - School of Science and Technology

242 Piping design for indirect heating systemsG 22.14 10.7H 22.76 13.8J 23.79 18.9The effect of the mains losses upon the heat emitters is that the full system temperaturedrop of 12 K will not be available at the individual flow and return connections to thoseheaters. Taking the two extreme examples:Heater O, temperature drops*in flow main ˆ (12 0:35)/20:71 ˆ 0:25 Kacross heater ˆ (12 20)/20:71 ˆ 11:5Kin return main ˆ (12 0:35)/20:71 ˆ 0:25 KHeater J, temperature drops*in flow main ˆ (12 1:89)/23:79 ˆ 0:95 Kacross heater ˆ (12 20)/23:79 ˆ 10:1Kin return main ˆ (12 1:89)/23:79 ˆ 0:95 KThe object of this calculation is to produce energy loadings (and hence water quantities)which will lead to the mean temperature at each heat emitter being the same throughoutthe system. Naturally, this small simple example does not show up the relative importanceof mains loss as would be the case in an extensive system, but no doubt it will serve toshow the principle.It will be seen that, for a large installation involving a great many branches of differentlengths and sub-circuits of varying size, the apportionment of mains losses, if pursued toan ultimate refinement, can be a very laborious process. Various methods have beendevised to simplify this task, such as have appeared in previous editions of this book. Byway of rough compromise, if the total mains loss of one circuit from the boiler becalculated and divided by the number of branches on that circuit, even if not of uniformload, some attempt at apportionment can generally be made by sight to achieve apercentage basis which is probably not far from reality.However arrived at, the mains loss for each section is added to the emitter load of thebranch, and these are added progressively back to the boiler, or to the headers if there areseveral main circuits.. The loads which each section of main must carry are now available and the size can bejudged from a starting basis of say 100 Pa/m unit pressure drop. The length of eachsection, flow plus return, plus single resistances can be set down and a table prepared,thus, for heater J in the example.. Next come the other branches and sub-circuits. At each off-take from the index circuitthere will be some surplus pressure available: this must be dissipated in the branchconnections, otherwise short circuiting will occur.Each branch taken in turn then becomes a fresh exercise to be retabulated as above,sizes being adjusted to absorb surplus pressure. There is of course a limit to the practicabilityof such adjustments since the range of commercial pipe sizes is not infinite and,because pressure loss varies with the square of the velocity, `steps' between pipe sizes arequite large. For example, take an energy flow rate of 3 kW/K and note from Figure 9.1* These are not strictly correct since the flow pipe is hotter than the return, and will thus have a slightlyhigher heat loss.