Book - School of Science and Technology

594 Hot water supply systemsA complete discussion of this subject would be beyond the scope of this book and thebrief notes which follow are intended only as an introduction to the methods commonlyused to treat cold water supplies to hot water systems. Further information may be foundin the CIBSE Guide G6 but specialist advice should be sought where particular problemsare known to exist.Hardness of waterThe calcium and magnesium salts present in raw water exist in two forms, as bicarbonateswhich form what is known as temporary hardness and as sulphates, chlorides and nitrateswhich form what is known as permanent hardness. The salts within the temporarycategory fall out of solution when the water temperature is raised to about 70 C whichsituation occurs adjacent to heat exchange surfaces even when the storage temperature isbelow that level. The salts of permanent hardness remain in solution at the temperaturesgenerally encountered in indirect systems but calcium sulphate may cause problems indirect systems at boiler surfaces or where the primary medium of an indirect system is athigh temperature.Commercial installationsHere, dependent upon circumstances, it is probable that a full scale water treatment plantwill be installed. This is most likely to be a base- or ion-exchange process which makes useof beds of either natural or synthetic zeolites (sodium aluminium silicates). These mineralscontain sodium in combination and have the property of exchanging this with the calciumand magnesium salts in a raw water to form sodium bicarbonate which is not hard-scaleforming. Natural zeolites (greensands) are impermeable and have a lower capacity forexchange, per unit volume, than the synthetic type. The latter, as a result of their porousstructure, must not be used with an unfiltered raw water supply and have a shorter lifethan the natural material.The exchange process does not continue indefinitely since the capacity of the zeolite issaid to be exhausted when all the sodium content has been exchanged: it is then necessaryto regenerate the mineral by slow flushing with a strong solution of common salt (sodiumchloride) followed by brisk backwash to remove the brine residual and dispose of thecalcium and magnesium. It must be emphasised that the process of base exchange, whilstreducingscale deposition, does not reduce the total content of dissolved solids in the water.DosingInternal water treatment by dosingboiler make-up water has been a familiar process inthe case of industrial plant where the facilities exist to allow the effect to be monitored andthe quantities of additive adjusted accordingly. For use at domestic level, it has beennecessary to develop simple methods for automatic meteringof dosage which require noskill to operate and will not permit overfeed. A number of devices which allow suitablequantities of phosphates and polyphosphates to be injected into an incomingmains watersupply is now available for use at domestic level. This injection modifies the hardness saltssuch that, followingtreatment, they remain suspended in the water, instead of bondingone to another and formingscale when temperature is increased. The process requiresonly that the bulk supply of chemicals introduced be renewed from time to time, in theform of powder or spheroid concentrates.