Getting Technical
With regard to electrolysis to prevent scale formation as
well as removing existing scaling deposits in water flows and
circuits, the main reason why this has not been done until
recently is that scale control has generally been regarded as
an integral part of established chemical dosing programmes.
Actually, the introduction of speciality chemicals for scale
control, particularly to prevent the most common cause of
scaling, dissolved calcium carbonate in supply water, began
not all that long ago during the 1970s. This was when the
established practice of using softened supply water, which is
corrosive to unprotected steel surfaces, became impractical
due to chromium-based chemicals, which were the only cost-
effective anti-corrosion treatment formulations at the time, no
longer being permitted. By far the most commonly occurring
scale occurs from concentrating supply make-up water
containing soluble calcium bicarbonate, which results in micro
crystals of calcium carbonate beginning to form throughout
the whole volume of the water circuit. Chemical treatment
therefore needs sufficient chemical dosing to achieve and
maintain a minimum chemical amount throughout the total
volume of circulating water.
58
RACA Journal I January 2019
SUMMARY OF CHEMICAL TREATMENT
Current scale-control chemicals have a primary effect of
maintaining calcium and magnesium carbonate ions in solution
in water at higher than normal concentrations. However,
when these dissolved carbonate ions are subjected to higher
temperatures close to or in contact with condenser tubes, some
of them will precipitate out of solution and form crystals that
are capable of binding together and growing into sheets of
scale deposits. The treatment chemicals then also distort the
normal regular crystal growth process by an activity knows as
the ‘threshold effect’, which counteracts the tendency of the
crystals to grow into symmetrical shapes that bind together
and form scale. Optimum anti-scaling chemical concentrations
in circulating water are determined by the nature and amounts
of dissolved calcium (and magnesium) carbonates in the supply
make-up water as well as maximum cycles of concentration at
which the evaporative circuit can be operated to reduce water
usage. Cycles of concentration (C/C) are determined by amounts
of bleed-off water, which transports calcium carbonate in both
dissolved and suspended crystal forms, as well as other dissolved
or suspended substances out of the circulating water.
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