Book - School of Science and Technology

Water systems - footnotes 247Water systems - footnotes`Hydraulic' designThis ill-named concept, which may have originated from study of the self-balancingcharacteristics of the embedded panel system, is worth noting as an oddity if nothingelse. The basis of the routine is that the system pipework of the index circuit alone isselected in the normal way and the pressure loss through it established for the requiredwater quantity. Balance is achieved for other circuits not by adjustment of pipe size but byselection of a water quantity which will produce equal pressure loss. With this approach,the temperature drop across the various circuits will differ but the argument is that this, interms of emission (i.e. the difference between mean water temperature and air temperaturein the space to be heated) will be so small as to be irrelevant. Application of theconcept means, of course, that the volume duty of the circulating pump will be greaterthan necessary, as will be the energy which it consumes. This approach is not recommendedfor normal usage.Mechanised designA totally refined solution to routine calculations directed at selection of pipe sizes wouldbe a lengthy task using long-hand methods, having regard to the large number ofinterdependent variables. Traditional methods have followed principles and limitationswhich, in the main, have worked in practice. With the advent of the computer, newtechniques have been developed to assimilate all the variables and to produce iterativesolutions which would be too time consuming to deal with by any other means.A number of computer programs are now available, some written by engineers, andexperience suggests that a project of suitable size may be processed in a fraction of thetime which would apply to an equivalent manual exercise. In this context, however, thelimitations imposed by commercial pipe sizes and other imponderables apply equally toboth computer and manual solutions. In the end, despite the means, circuits must bebalanced one with the other by Fred's skill in adjustment of regulating valves!System pressure conditionThe point at which the feed and expansion pipe (or the equivalent influence from apressurisation unit) is connected to a piping system is of some significance as far as boilerand pump operation are concerned. This point of connection is the only position at whichany external pressure is applied and is known as the neutral point of the system. Withrespect to the boiler or other energy source, this neutral point could be connected either tothe flow or to the return but common sense dictates that it should be at the position wherethe water of expansion will be at the lower of the two temperatures, if this be practicable.As to position vis aÁ vis the circulating pump, the alternatives are only that it should be oneither the suction side or the delivery side.Since, as will be obvious, the pressure at the neutral point can be no greater or less thanthat imposed by the external influence, it follows that connection to pump delivery willresult in the whole of the piping system being under less than external pressure whereasconnection to pump suction will place the system under external pressure plus whateverresidual pump pressure is available at any given point.