OPERATIONS
Passivation
The standard galvanised steel serpentine coils (prime surface)
are carbon steel, hot-dip galvanised on the outside only, and
are intended for application on closed, pressurised systems
which are not open to the atmosphere. Stainless steel coils
or cleanable coil units (with tubes hot-dip galvanized inside
and out) are available to cool corrosive fluids or water and
ethylene or propylene glycol solutions in systems open to the
atmosphere.
When new systems are first commissioned, special measures
should be taken to ensure that galvanised-steel surfaces
are properly passivated to provide maximum protection
from corrosion. Passivation is the formation of a protective,
passive, oxide layer on galvanised-steel surfaces. To ensure the
galvanised-steel surfaces are passivated, the pH of circulating
water should be kept between 7.0 and 8.2 and calcium hardness
between 100 and 300 ppm (as CaCO 3 ) for four to eight weeks
after start-up, or until new zinc surfaces turn dull-grey in colour.
If white deposits form on galvanised-steel surfaces after the
pH is returned to normal service levels, it may be necessary to
repeat the passivation process.
All evaporative condenser coils supplied from Europe, including
desuperheater coils, are certified according to the European
Pressure Equipment Directive.
Evaporative cooling is accomplished by the evaporation of a
small portion of circulating water. As water evaporates, the
dissolved solids originally present in the water remain in the
system. The concentration of dissolved solids increases rapidly
and can reach unacceptable levels. In addition, airborne
impurities and biological contaminants are often introduced
into the recirculating water, since the evaporative cooler is, in
effect, washing the air.
In case you cannot keep the pH below 8.2, a secondary
approach is to conduct a chemical passivation using inorganic
phosphate or film-forming passivation agents.
Control of biological contamination and water treatment
The growth of algae, slimes and other micro-organisms, if
uncontrolled, will reduce heat transfer efficiency and may
contribute to the growth of potentially harmful micro-
organisms, such as legionella, in the recirculating water.
If impurities and contaminants are not effectively controlled,
they can cause scaling, corrosion, sludge or biological fouling,
which reduce heat transfer efficiency and increase system
operating costs. For optimal heat transfer efficiency and
maximum equipment life, the quality of the make-up and
recirculating water should be maintained within the limitations
listed below.
It is important to monitor the bacteriological contamination of
the recirculating water regularly.
In addition to the control of biological contamination, it may
be necessary to install a water-treatment regime to prevent
the formation of scale or corrosion. To ensure recognition of
To prevent an excessive build-up of impurities in the
recirculating water, a small amount of water must be bled
from the recirculating water. In many localities, this constant
bleed and replacement with fresh make-up water will
keep the concentration of impurities in the system at an
acceptable level.
The water treatment programme must be able to achieve
control of corrosion, scaling, microbiological growth and fouling
with the equipment and the cooling system it serves. The water
treatment regime must also comply with the specific local
requirement in terms of legionella control. The final choice of
the water treatment programme and its follow-up remain the
sole and entire responsibility of the water treatment company or
the equipment owner.
Water treatment chemicals or non-chemical systems need to be
compatible with the materials of construction used in the cooling
system including the evaporative cooling equipment itself.
In case of chemical water treatment, chemicals should be added
to the recirculating water by an automatic feed system. This
will prevent localised high concentrations of chemicals, which
may cause corrosion. Preferably the water treatment chemicals
should be fed into the cooling system at the discharge of
the recirculation pump. The chemicals should not be fed in
concentrated form, nor batch-fed directly into the cold-water
sump of the evaporative cooling equipment.
“A competent water treatment company should be consulted
for the specific water treatment programme to be applied. Next
to the supply of dosing and control equipment and chemicals,
the programme should include regular monthly monitoring of
the circulating and make up water quality,” she says.
28
African Mining April 2020
“We specifically discourage acid dosing as means of scale
controls (unless under certain strict circumstances for open
circuit cooling towers with very large volume and remote sump
or constructed from stainless steel.
Proper and thorough research is imperative when designing HVAC
for a mine.
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