|
|
Scale is probably the most widely associated problem in cooling
water systems. This term has become so generic that it is often
applied incorrectly to all solid accumulations in the system. Scale,
as originally defined, forms by crystallization of a dissolved salt
when its concentration exceeds its solubility. Most scale forming
compounds have the unusual property of becoming less soluble as the
water temperature increases. Because of this, scale will generally
deposit at the heat of the exchange surface and water interface-the
very place which should be kept scale-free for efficient heat
transfer to take place.
|
|
The most common scale will usually consist of calcium carbonate.
Its ingredients, calcium and alkalinity, are present in all natural
water. For any given water composition and temperature, the tendency
to form calcium carbonate scale can be predicted from the
temperature, pH, calcium hardness, total alkalinity, and the total
dissolved solids (TDS). This prediction calculates when a water of
known composition would be in equilibrium with calcium carbonate at
the indicated temperature. The calculation is further correlated
with field experience data to give a better picture of what might be
expected of a given water under the specified conditions. The final
answer is referred to as the Stability Index. A number below 6.0
indicates scale will form, and become more pronounced the lower the
index number, while as the number rises above 7.0, the water is said
to be increasingly corrosive.
|
|
| |
|
|
