Feature
PASSIVATION
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.
In cases where the pH cannot be kept 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.
46
RACA Journal I January 2020
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 any risk and the implementation of protective
measures, a risk analysis by a specialised risk assessor
is recommended. It is also recommended to develop an
operations plan for the cooling system.
Algae are plants, which, like all plants, require sunlight
and nutrients to grow. In evaporative cooling equipment,
algae are aesthetically undesirable and may promote other
microbial growth. However, unless the algae interferes with
the thermal performance of the unit, such as blocking fill or
plugging nozzles, it is of itself relatively benign. A biologically
active system is one with an active slime layer and high
planktonic bacteria count. Such a system is at risk for poor
thermal performance, microbial influenced corrosion and
pathogens.
Algae growth, combined with high total bacteria count,
can be a warning sign of a biologically active system. Algae
growth combined with low total bacteria count is not a
warning sign for a biologically active system. Consistently
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