The chemistry of water has a direct impact on the four main problems of
cooling water systems.
SCALE
Water impurities such as calcium and magnesium hardness can precipitate
and deposit depending on their concentrations, water temperature, pH,
alkalinity, and other water characteristics. The deposit forms a film inside
the surfaces, technically known as scale that in addition to its high
insulating value; progressively narrows pipe internal diameters, roughens tube
surfaces and thereby impeding proper flow.
While scale formation proceeds more rapidly in open re-circulating
systems owing to the concentration effect of evaporation, once-through systems
are not exempt from scaling if high temperatures are combined with silt and
iron.
1. What is scale?
Scale is a dense coating of predominantly inorganic
material formed from the precipitation of water-soluble constituents. Some
common scales are
§
Calcium Carbonate
§
Calcium phosphate
§
Magnesium salts
§
Silica
2.
Principle Factors Responsible for Scale Formation
§
Calcium content of water
§
Alkalinity or pH of water
§
Temperature of re-circulation water
§
Higher concentration of solids (TDS)
§
Insufficient bleed off from cooling towers
As any of above factors changes, scaling tendencies
also change. Most salts become more soluble as temperature increases. However,
some salts, such as calcium carbonate, become less soluble as temperature
increases. Therefore they often cause deposits at higher temperatures.
A change in pH or alkalinity can greatly affect scale
formation. As alkalinity increases, calcium carbonate- the most common scale
constituent in cooling systems-decreases in solubility and deposits. Some
materials, such as silica (SiO2) are less soluble at lower
alkalinities.
Hardness
levels are associated with the tendencies of cooling waters to be scale forming
or not. Higher the level of scale forming solids, the greater the chances of
scale formation
4. How can scale formation be controlled?
There are four basic means to control scale.
§ Limit
the concentration of scale forming materials by controlling cycles of
concentration or by removing the minerals before they enter the system. A part
of water is purposely drained off (blow down) to prevent minerals built up. A
cycle of concentration is the ratio of the make-up rate to the blow down rate.
§ Feed
acid to keep the common scale forming materials dissolved form.
§ Make
the mechanical changes in the system to reduce the chances for scale formation.
Increased water flow and exchangers with larger surface areas are examples.
§ Treat
with chemicals designed to prevent scale.
5. How do chemical scale inhibitors work?
ü
Scale inhibitor chemicals keep the scale
forming materials in soluble form and do not allow deposit to form.
ü
Scale conditioners modify the crystal structure of scale, creating a bulky
transportable sludge instead of hard deposit.
6. What are common scale-control chemicals?
ü
Scale inhibitors:
Organic phosphates, polyphosphates, polymer compounds
ü
Scale conditioning compounds: Lignin, tannins, polymeric compounds
7. What are the effects of Scale Deposits?
The build up of scale leads directly to
§
Loss of heat transfer efficiency
§
Loss of production
§
Increased downtime and maintenance costs
§
High-energy costs
8. What is the most important factor in scale control?
To prevent formation of scale, water is treated prior
to using it for coolant purposes. The water treatment methods are classified in
three broad categories:
§ Water
Treatment (Softening, Dealkalization, Demineralization, Reverse Osmosis)
§ Chemical
dosing
A chemical program in addition to the cooling water
treatment is the only way to insure that scale formation does not become a
problem.
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