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Continued from page 43
The satisfactory performance of cooling equipment is
dependent on correct selection and proper attention to overall
system design, installation, water care and maintenance.
Our water-treatment programmes include automatically
controlled dosing of chemicals and bleed-off systems. The
cycles of concentration are optimised to minimise water loss
through bleed-off while controlling corrosion and scaling.
The selection of a model is based on a thermal duty and
the wet-bulb temperature. Thermal ratings are based on the
wet-bulb temperature of the air entering the equipment and do
not consider any recirculation of warm and humid discharge air
which may occur under certain weather and wind conditions.
Verification of ratings assumes a test according to a recognised
test standard and the application of tolerances as recorded
during the test and applied to the test results.
When the evaporative cooling units are closed circuit
coolers (or evaporative condensers if the fluid in the closed
circuit is a refrigerant), the fluid to be cooled must be
compatible with the coil material. Fluids not compatible with
coil materials can lead to corrosion and tube failure. Certain
fluids may require occasional pressure cleaning or mechanical
cleaning of the inside of coil tubes. In such cases the coil must
be designed to provide this capability.
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.
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.
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
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OPTIMISED PERFORMANCE
make-up and recirculating water should be maintained within the
limitations listed below.
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
on 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 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.
We specifically discourage acid dosing as means of scale
controls unless under certain strict circumstances for open circuit
cooling towers with very large volumes and remote sumps or
constructed from stainless steel.
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.
RACA Journal I January 2020
Piping should
be sized and
installed in
accordance with
rules of good
practice.
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