Book - School of Science and Technology

Magnetic/electromagnetic conditioning etc.Materials, etc. 595As an alternative to dosing, use may be made of magnetic/electromagnetic conditioningwhich have no chemical effect upon the various hardness salts and do not change thecomposition of the water. The process involves introduction of a pipeline unit whichincorporates a magnetic circuit through which the water passes. As a result of thisexposure, the particles of hardness salts do not agglomerate but remain freely suspendedin the water and thus do not form scale on heated surfaces.An alternative but not dissimilar process results from exposure of the incomingwater toan electrochemical device within which water flow past a zinc±copper combination acts toproduce an electrolytic cell. This, in sequence, provides an electrical potential whichdiscourages the particles of scaling salts from attracting one another and thus precludesformation of a layer of scale.Materials, etc.Vessels and pipeworkThe two materials most commonly used in construction of hot water systems have beengalvanised steel and copper. Selection of one or the other has often been made followingstudy of data such as that set out in Table 20.7, the results representingthe limits withinwhich steel could be used. As a result of the ease with which it is manipulated and thesimpler fabrication techniques which are associated with it, copper pipingis now usedalmost universally. Hence, since a mixture of metals is particularly undesirable where theassociated water is subject to continuous change, with a consequent continuous release ofdissolved oxygen, the use of galvanised steel for storage vessels is no longer usual. Coppervessels havingthin shells are now very often provided with sacrificial aluminium anodesto protect them against corrosion.Over recent years the use of other materials has become available to the designer.Materials for pipework such as CPVC. This pipework material remove the problemsassociated with corrosion. However, the greater expansion rates of this material must betaken into consideration by the system designer.For hot water storage vessels the use of stainless steel or glass lined vessels have beenadopted by manufacturers.Table 20.7 Limits for use of galvanised steelTemporary hardness greater than:pH valuemg/litre Parts per 100 000 Grains per imp. gallon a Grains per US gallon7.3 210 21 15.0 12.37.4 150 15 10.5 8.87.5 140 14 9.8 8.27.6 110 10 7.7 6.47.7 90 9 6.3 5.37.8 80 8 5.6 4.77.9 and over 70 7 4.9 4.1a 1 grain per Imperial Gallon ˆ 1 degree Clark.