OPERATIONS
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 interfere 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 low total bacteria
counts are a sign of a biologically inactive system regardless of
the presence or absence of algae.
Algae growth may be particularly noticeable during the spring
and summer. As previously noted, algae require sunlight to
grow; therefore, open cooling systems and systems that receive
direct sunlight are more prone to algae growth. Blown-in dirt
and nutrients also promote algae growth.
The use of filtration systems to relieve the system of blown-in
and precipitated solids can reduce the area for algae to thrive.
Effective filtration will not remove existing algae but should
prevent future blooms.
Location
Each cooling tower, evaporative cooler or condenser should
be located and positioned to prevent the introduction of
the discharge air and the associated drift which may contain
contaminants, such as legionella.
Piping
Piping should be sized and installed in accordance with rules of
good practice. Dead legs and stagnant water conditions in the
piping should be avoided. If more than one inlet connection
is required, balancing valves should be installed to properly
balance the flow to each inlet. Depending on the design of the
hydraulic circuit, it may also be necessary to install balancing
valves at the suction connections of the towers. The use of
shut-off valves is dictated by the necessity to (automatically or
manually) isolate cells or towers for capacity control or servicing.
If the equipment is installed on vibration rails, compensators
must be installed in the connecting piping.
Winter safety
When a unit is shut down in freezing weather, the basin water
must be protected by draining to an indoor auxiliary remote
sump tank or by providing supplementary heat to the cold-
water basin. Supplementary heat can be provided by electric
immersion heaters or in some cases, hot water, steam coils,
or steam injectors. All exposed water piping, make-up lines,
and spray pumps (if applicable) that do not drain at shutdown
should be strapped with electric heater tape and insulated.
When dry operation is planned for low ambient conditions,
centrifugal fan units should be supplied with oversized fan
motors to prevent motor overload when the spray water is
not operating. For remote sump applications, the spray water
pump must be selected for the required flow at a total head
which includes the vertical lift, pipe friction (in supply and
suction lines) plus the required pressure at the inlet header of
the water distribution system (14 kPa). A valve should always
be installed in the discharge line from the pump to permit
adjusting flow to the unit requirement. Inlet water pressure
should be measured by a pressure gauge installed in the
water supply riser at the spray water inlet and adjusted to the
specified inlet pressure.
Unless the system is shut down and drained during
winter, measures must be taken to protect the
system from freezing during the winter,
during operation and standstill. Freeze
protection during operation is achieved
by selecting an adequate method
of capacity control. For reasonably
constant loads and cooling water
or condensing temperatures
above 15°C step control is usually
adequate. For variable loads, in
particular when combined with
free cooling, modulating controls
are recommended.
"For reasonably constant
loads and cooling water
or condensing temperatures
above 15°C step control is
usually adequate.
When the equipment is shut down
in freezing weather the sump
water must be protected. This can
be accomplished by the installation
of electrical sump heaters. The standard
electric heaters are sized to maintain +4°C
sump water when the ambient temperature
drops to - 8°C.
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African Mining Publication
African Mining
African Mining April 2020
29