The buildup of scale deposits is a common and costly problem in many
industrial processes using natural water supplies. In Britain alone
the formation of scales in industrial process plant where water is
heated or used as a coolant is estimated to cost £1 billion per year.
Such costs can be attributed to cleaning (i.e. decaling) or the poor
thermal conductivity of scaled surfaces; heat transfer is decreased by
95% by a CaCO3 scale layer 25 mm thick whereas a SiO2 scale layer 0.5
mm thick results in a 90 % decrease in heat transfer. The principle
scale-forming ingredient in cooling water is calcium carbonate, which
has a solubility of about 15 PPM and is formed by the decomposition of
calcium bicarbonate. The maximum amount of calcium bicarbonate that
can be held in a solution depends upon the temperature and the free
carbon dioxide content of the water. Raising the temperature or
decreasing the free carbon dioxide, at the point of equilibrium, will
result in the deposition of scale. If agents (such as sulfuric acid)
are added to convert a portion of the calcium bicarbonate to calcium
sulfate, the resultant concentration of calcium sulfate should not
exceed 1200 PPM
(expressed as CaCO3). Otherwise, sulphate
scale may begin to form, which is very dense and quite difficult to
remove. Treatment companies may also advise the use of selected
compounds.
Since the solubility
of calcium carbonate is dependent on temperature, the water in a
cooling system will have a different index for each temperature
encountered. In practice, the Indices are used to arrive at a
calculated method of treatment Removable lengths of pipe or metal
coupons, should be inspected periodically to confirm that the
treatment is in balance. If objectionable scaling is occurring, an
increase in acid feed may be required. If corrosion is evident, a
reduction of acid feed or the introduction of a lime or soda ash
solution may be needed. For a system in which a considerable
temperature spread occurs and treatment is set to control scale lay
down, it
is frequently necessary to use an inhibitor to prevent corrosion in
the low temperature areas. It is desirable to calculate both the
Saturation Index and the Stability Index in order to most accurately
predict the scaling or corrosive tendencies of water. |