OPERATIONS
would consider the comfort of the occupants. A mine-ventilation
specialist would consider the maximum allowable temperature
(WB and DB) to enable people to work. The reason for this caveat
is that providing cooling for a mine is an expensive business
due to the rather large energy requirements of these systems.
When designing for a mine, ventilation specialists would often
try to maximise uncooled air from surface. When this is no longer
possible, cooling methods (such as huge refrigeration plants)
must be employed to allow the workforce to move and work in
the underground environment,” says Hattingh.
He says there are several new innovations in the mining HVAC
environment. Chiller technology is evolving, and equipment is
getting more efficient. Reducing operating costs by maximising
the equipment efficiencies would warm any mine owner or
investor’s heart.
The most important decision when it comes to efficiency is the
selection of equipment. Equipment arrangement, however,
is where operating costs can be leveraged. Chillers are often
arranged in series-parallel or series-counterflow configurations
to reduce operating costs. There are of course many savings
possible when you start to consider pumping, and given the
high cost of pumping, this is a high focus area. Some mines
employ thermal storage to maximise time of use savings. This
can take the shape of surface ice dams or large underground
dams to reduce pumping during peak tariff times. The latter is
complex however and requires careful design and management.
While there are local experts on a mine that are well versed in
ventilation design, consultancies are often employed to design
HVAC systems for mines to consider the ‘what ifs’. And this part
is important as a mine is complex and changes constantly. The
ore body in one area may be bigger or smaller than expected,
the grades may be lower than predicted, the ore body may
be depleted or may be affected by various intrusions or faults
and the location of the development changes as a result, or
the mining method may change. There are many variables
and upsets to consider and the designer must juggle these
variables and come up with short, medium- and long-term
plans to provide cooling. Working with the mine personnel
to establish and understand their needs, development and
production rates is crucial.
Water quality and treatment
Water used in cooling towers needs good treatment to protect
the components and heat-transfer surfaces making up the
cooling system. Water treatment is also vital to avoid the
proliferation of harmful bacteria, including legionella, in the
recirculating water.
According to Lizzie Myburgh, water-treatment manager of
Baltimore Aircoil, a company that develops, manufactures
and distributes evaporative or hybrid cooling, one of the most
common mistakes made when designing and installing a
mine HVAC system is not taking water quality
into consideration.
The Chamber of Mines developed sound guidelines for mining
companies and design engineers to build, maintain and inspect
ventilation systems. Indeed, South Africa is often considered a
centre of excellence when it comes to keeping mines cool and
optimising energy use. The mining industry is heavily regulated
and policed as the work force is large.
“I cannot even begin to mention which regulations are more
important than others. Suffice to say that it is so tightly controlled
that the mines have their own version of the Occupational Health
and Safety Act, which is the Mines Health and Safety Act, that
governs every aspect of mining,” says Hattingh.
There are several options available to the mine HVAC designer:
surface plants (for bulk-air cooling), underground plants (for
cooling generation closer to the production areas to reduce
pumping costs), surface ice plants (to reduce pumping costs). The
three different strategies are employed as they become viable at
different mining-production depths. Surface plants become less
viable as the depth of the mine increases and chilled water needs
to be sent underground and pumping cost become prohibitive.
Underground plants can reduce pumping costs but given the high
ambient in which their cooling towers must work, the efficiencies
of these plants are low. At greater depths still, ice generation by
surface plants has proved to be cost effective (lower efficiency
of the refrigeration plant vs greatly reduced pumping cost). As a
mine evolves over a period, the mine would often start with one
technology and change to another as operating costs of another,
more favourable technology becomes beneficial.
“It really depends on the requirements. They may need
surface cooling (in other words, generation of chilled water on
surface), underground cooling (generation of chilled water in
the underground working environment) or something more
exotic like ice. Correct equipment selection is key. Single-stage
centrifugal chillers deliver optimal performance for bulk air
cooling. Screw chillers deliver the best performance for sending
water underground. Understanding the conditions that your
chiller will work under allows equipment suppliers, like AHI
Carrier, to optimise the equipment that we would provide for
these installations,” Hattingh says.
26
African Mining April 2020
How does a client choose the best product and system?
Temperature and humidity are essential factors to ensure optimal
productivity in an underground environment.
www. africanmining.co.za