Project and depth of mining , divergent rock temperatures and other geotechnical factors as well as the mining method and plant mine production rates ; the infrastructure for conveying the cooling , whether it is packed , ducted and insulated and whether it is returned .
He says in this particular application , there is not much opportunity to implement unorthodox energy efficiency measures , given the need for a high cooling delivery . “ There are instances where one can use reject heat or geothermal heat for low-power absorption chillers – but this is not one of those cases .
“ Service water that ' s used in a mine is typically supplied underground as cold as possible , and then is returned to surface as hot as possible to extract all the cooling you can out of the water . That results in a need to bring that water temperature down quite a long way , for a large delta T between return and supply . Because the return water is hotter than ambient air , precooling towers are employed before the water goes into the refrigeration plant , which is an inexpensive cooling process . This evaporative cooling cost just consists of the price of fan power on the cooling tower .”
Pre-cooling provides another 14MW of almost-free cooling on top of the 70MW of installed refrigeration capacity . “ We do a cooling design to be as efficient as possible by arranging the chillers to handle a large temperature difference and get the best result . Apart from that this plant has been designed to be robust and reliable , which is key for mining operations .”
TECHNICAL DETAILS The medium voltage switchgear is Actom , the programmable logic controller ( PLC ) and programmable automation controller ( PAC ) are ABB Freelance . The refrigeration system is based on York centrifugal packaged chillers using R-134a . This type of equipment was selected based on its proven performance and reliability and has become somewhat of an industry standard among South African mines .
He explains that Northam mine has an existing refrigeration plant built in the 90s to early 2000s that has these machines on them , so staff are familiar with the equipment and have a long-lasting relationship with the service providers .
Ultimately , there will be 10 of these refrigeration machines , which are being phased in – the first six in this current phase to be commissioned by the end of 2024 , and the remaining four to be installed and commissioned by April 2027 .
“ The primary product is chilled water that ' s used underground for hydropower equipment such as drills ; as general service water ; and as underground air coolers , be it bulk air coolers , open spray chambers , cooling coils or spot coolers . Some of the water is diverted to bulk air coolers , which will be a portion of about 9MW of the 70MW used for air cooling on surface . Heat rejection is through evaporative cooling towers , the large cell concrete structures which are designed for longevity and maintainability . Splash type fill , also something of a standard in the mining industry , is used because of its good resistance to fouling . The rest of the system is the standard steel piping , galvanised corrosion protected , and centrifugal pumps .
“ The centrifugal modules are well-established equipment with a long service history in South Africa and are well supported with spares readily available . They ' ve got standard open-drive motors , for which service and spares are again accessible .”
The switchgear and other equipment are in line with the project specifications . “ We wouldn ’ t put something totally foreign down on the site , which complicates spares holdings , training and operations . We stick with the mine ’ s preferred vendor list for equipment ,” he adds .
ELEMENTS OF DIFFICULTY IN MEETING THE DESIGN As a greenfield project there are fewer constraints in terms of design and construction , Branch says . However , the main focus
As a greenfield project , there are fewer constraints in terms of design and construction , Branch says . However , the main focus here is on longevity .
www . refrigerationandaircon . co . za RACA Journal I May 2023 39