BBE Consulting has successfully completed a R3-million EPCM contract calling for the design of an underground cooling circuit at Rio Tinto’s Palabora Mining Co. (PMC) in Phalaborwa, Limpopo Province, South Africa. BBE’s design makes provision for two refrigeration machines to provide cooling to current workings and mine development.
The underground cooling circuit comprises two 1.5-MW bulk air coolers (BACs), ± 3 MW of cooling coils, chilled and warm water dams, two 3.5-MW refrigeration machines and a 9-MW condenser spray chamber. The refrigeration duty not used for the BACs will be used for cooling coils.
The cooling circuit will be located underground on the production Level, approximately 1,200 m below surface. BBE’s lead engineer on this project, Marle Hooman, said refrigeration systems are not often located underground at this depth, which is quite shallow compared with other mines.
“However, at PMC high ambient conditions are experienced throughout the year and the surface virgin rock temperature as well as the geothermal gradient of the rock is high,” said Hooman. “Trade-off comparisons were conducted prior to this work to arrive at the most favorable refrigeration system. We made our decision based on achieving the best efficiencies and lowest operating costs, as well as a variety of practical advantages.”
The first BAC has been installed, and construction of the second BAC, condenser spray chamber, dams and the first refrigeration machine is expected to be completed by November 2011.
BBE said it has worked with PMC on ventilation and cooling studies since 2004, including various concept, pre-feasibility and feasibility studies on ventilation layout and mine planning using the BBE Group’s proprietary VUMA-network (a simulation of steady-state environmental conditions encountered in underground mines). More recently, BEE has been involved in an EPCM project for PMC’s headgear cooling system.
BBE also noted that it recently designed and commissioned a refrigeration and air-cooling plant on a mountain ledge at AngloGold Ashanti’s Cuiabá gold mine near Belo Horizonte in Minas Gerais State, southeastern Brazil. The plant comprises a horizontal spray chamber-type BAC with two axial force fans, two refrigeration machines and condenser cooling towers plus associated pumps and pipework.
BBE was responsible for the detail design of the entire plant, including mechanical, civil, structural engineering and, in conjunction with Logichem, the electrical and automation engineering design.
The company said civil construction of the plant was completed in six months by optimizing the design for high-speed construction techniques, including quick-drying concrete, and by working two shifts per day.
The refrigeration plant comprises two 5-MWR York R134a centrifugal compressor machines, cooling 370 kg/s of ambient air which feeds into a dedicated downcast ventilation shaft for use in the mechanized underground mining operation.
Raymond Funnell, a director at BBE Consulting, said initially only one machine will be required to cool the mine but, as the heat load increases, both machines will come into operation. Space provision has been made for a third machine and cooling tower on the site.
“One of the features of this project was that for the first time we made use of 3-D high-resolution renderings of the refrigeration plant design to provide our client with an architectural-quality ‘virtual walk-through’ of the proposed structure,” said Funnell.
“We believe we are one of the first companies in our field to do this in a mining application. The 3-D images were generated directly from our engineering drawings and proved very valuable in ensuring that all parties had a clear understanding of the designs. These images will also prove useful for training and maintenance purposes.”
Space on the refrigeration plant site was tight and BBE was forced to limit the cooling tower size to internal dimensions of 9.6 x 9.6 m. Also as a result of space constraints, the spray chamber operates at higher-than-normal water loading, the BAC fans are equipped with square inlet silencers and two separate substations were required, one in the machine room area and one in the cooling tower area.
A Siemens S7 PLC is used to control the plant. All instrumentation operates on a Profibus system, requiring only network cables for instrumentation reticulation. Funnell said although this approach was more costly, it greatly improves instrument installation and maintenance. A fiber-optic link allows the plant to be controlled remotely from the mine’s central control station located about 1 km away.