One of the fundamental problems facing the water treatment
industry today is the persistence of "fouled" heat
transfer surfaces, inlet screens, pipes, etc. The term "FOULING"
means the deposition of any undesirable material in the cooling
water system. Fouling may occur on the tower deck and fill,
interfering with water flow and cooling efficiency.
All solid deposits
(Fouling) other than scale and corrosion oxides are liable
to occur in a cooling system. Such deposits not only tend
to complicate operation but, in the same way as scale, they
also cause corrosion phenomena as a result of differential
aeration, and these are sometimes aggravated by a pitting
bacterial corrosion. There are four possible sources of such
fouling:
- the make-up water;
- the air from the atmosphere;
- the manufacturing process: fluids
and appliances which are cooled;
- biological growths in the circuit.
1) Fouling due
to the make-up water
The make-up water
may contain suspended solids which can be removed by suitable
treatment to prevent their being deposited at points on the
circuit where the flow is slowest. Coming between suspended
matter and matter in solution, there is what is commonly referred
to as colloidal matter. This is unstable and may be transformed
into an adhesive and adsorbent gel by a slight rise in temperature
or by concentration. It is very often responsible for the
fouling of condensers or exchangers with a gluey deposit which
varies in color according to the kind of solid matter adsorbed.
Most of the least stable and therefore the most dangerous
part of such matter can be eliminated by a physico-chemical
process.
2) Pollution
by air from the atmosphere
A cooling tower
is an air scrubber; all matter entrained by the cooling air
is transferred to the circulating water. Both suspended solids
and colloidal matter are found. When the concentration ratio
(This is the ratio between the concentration of dissolved
salts in the circulating water and in the make-up water respectively.
This figure is normally determined by measuring the chlorides,
which are easy to analyses and, because of their solubility,
are the most stable in the system.) is less than 3, de-concentration
blow-down is normally enough to entrain such matter.
Immediately upon
the concentration ratio going above 3, a fraction drawn from
the main cooling flow has to be filtered and possibly coagulated;
this fraction can range from 3 to 15%, according to the degree
of pollution, the residence time and the sensitivity to fouling
of the equipment which has to be cooled.
Organic dispersants
can delay the formation of colloidal deposits. They were particularly
valuable when a high proportion was blown down, so that the
water in circulation was renewed fairly quickly. In addition,
the fouling potential remaining in the system was a constant
threat. Coagulation and filtration of a deverted fraction
is more thorough and reliable, and costs no more.
Soluble alkaline
matter (lime, ammonium salts) and acid matter (CO2, SO2, SO3)
can also enter the water from the air. These permanently changes
the chemical composition of the water in circulation, and
the change often differs according to the direction of the
wind. So far as possible, cooling towers should not be sited
the prevailing wind from a chimney or a lime kiln for example.
These circumstances should be known when planning the conditioning
of a circuit, if only to provide the necessary equipment for
stricter control (pH for example).
3) Pollution
caused by manufacturing process
As it passes
through the different production machines at a factory, the
cooling water can collect all kinds of matter, including,
for example, rolling-mill scale, liquid or gaseous hydrocarbons
in the oil industry, and solid, liquid or gaseous chemicals.
Such matter is liable to disturb the operation of the system.
Remedies may be based on the guidance given in previous sections,
but it is better to study each case on its merits.
4) Biological
growths
The water and
wetted areas of open recirculating cooling systems present
an environment in which several types of microbiological organisms
can grow and propagate. Microbiological organisms enter the
system with the make-up water and with airborne particulate
scrubbed in the cooling tower.
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