Getting Technical
VINDEXES DEVELOPED FROM PH PREDICT SCALING AND CORROSION IN WATER CIRCUITS
By Charles Nicolson
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to good use by applying speciality chemicals in mining and industrial processes where water is a major factor . This created an enduring interest in water technology , a passion that expanded to the HVAC industry in 1984 when he joined BHT Water Treatment . Since then , water technology in HVAC water circuits has continued to be an abiding interest .
pH is the most widely-used measure of what has been described as the basic technical ‘ character ’ of water .
pH itself was defined and formalised only in 1923 , less than one hundred years ago , which is surprisingly late considering the mass of research work previously concentrated on it . pH is not a measure which has any units like mass , volume , or viscosity for example .
It is a ratio of the amounts of positive and negative ionic charges present and therefore simply a number which varies from 0.00 ( zero ) up to fourteen ( 14.00 ). pH is a fundamental value which occurs wherever water is present , even in tiny trace amounts . pH , therefore , is present in all heating and cooling water circuits used in HVAC installations which are becoming more numerous as more heat pumps are used and various solar developments are increasingly included .
The technical formalisation of pH in 1923 enabled indexes to be developed which predict whether the nature and quality of different types of supply waters can be expected to result in corrosion and / or scale deposition problems in water circuits .
Indexes calculated from water analyses which predict not only whether corrosion and / or scaling can be expected to occur but also the severity of these potential problems are very useful in determining optimum water treatment programmes at lowest practical costs over the projected lifetime of an HVAC plant installation .
Water supplied to closed re-circulating heating and cooling loops in which water flow transfers heat from one or more heat exchangers to other heat exchangers does not , in theory , change in character when it becomes circulating water .
Water fed to open re-circulating loops such as evaporative cooling and adiabatic humidifying plants will almost always change in character , often very markedly , when the feedwater becomes circulating water . However , these water changes can be calculated accurately enabling cost-effective water treatment programmes to be implemented .
In 1936 , Dr Langelier introduced the entirely new idea of a ‘ theoretical pH ’ calculated for when water held as much calcium carbonate in solution as it could . Dr Langelier called this the ‘ Saturated pH ’ and designated his new concept as pHs . The actual method of calculating pHs shows how Dr Langelier maintained pHs within the same chemical character as pH .
pHs = [ 9.3 ( a constant ) + A + B ] – [ C + D ] Where : A = ( Log10 [ TDS ] - 1 ) / 10 B = -13.12 x Log10 ( o C + 273 ) + 34.55 C = Log10 [ Ca2 + as CaCO3 ] - 0.4 D = Log10 [ alkalinity as CaCO3 ]
• Factor A represents the effect of varying TDS [ total dissolved solids ].
• Factor B is the effect of temperature and is a reminder that the character of circulating water varies as it flows through different temperature zones .
• Factors C and D calculated from theoretical amounts of calcium carbonate needed for saturation and the total alkalinity created by these amounts are the major factors in determining pHs values .
To complete his major advance in deriving a useful predictive index which is still possibly the most widely used one today , as well as being the basis for all the other recognised and generally used indexes , Dr Langelier proposed his Langelier Saturation Index [ LSI ] as – LSI = pH - pHs - Where pH is the actual ‘ as measured ’ pH .
LSI values above 0 indicate scaling potential which increases according to the increase of the positive LSI values .
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