Engineering & Mining Journal

FEB 2018

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Page 47 of 59

46 E&MJ • FEBRUARY 2018 OPERATING STRATEGIES With technology rapidly advancing to meet evolving operational challenges, engineers do not often look backward for solutions. However, in examining options for supplying cooling and compressed air for ultra-deep mines, engineers may have found the answer in a technology that is more than 100 years old. Soon, test results will reveal if resur- recting and refi ning Hydraulic Air Com- pressor (HAC) technology holds the key for affordable, resource-conscious cooling. HAC History In these systems, leets and penstocks delivered water from a water course to air-water eductors. The velocity of the water relative to that of air bubbles pro- duced "drag" on the bubbles, pulling them downward. The potential energy of the water was converted to pressure ener- gy as the water descended into a vertical "downcomer" duct and this pressure was transmitted to the air bubbles, compress- ing them. At the base of the downcomer, gravity gas-liquid separators produced a stream of dry, cool, pressurized air that was piped to the demand center, and a second stream of air-bubble-free water that ascended a "riser" duct returned to the water to the water course via a tail race section. Other than gate valves to shut off in- coming water fl ow, these devices had no moving mechanical parts, required no lubricants and were extremely reliable. These HAC installations operated main- tenance-free for decades and in many cases outlived the design lives of their demand centers. Historical performance test results in- dicate the hydropower-to-pneumatic pow- er conversion effi ciency was high, ranging between 40%-78%. A century ago, there was not much interest in the conversion effi ciency because the input energy was renewable hydropower, which had close to zero marginal input cost. In addition to utilizing renewable en- ergy, the systems were also extremely effi cient. In a HAC downcomer, the high interfacial area between the liquid and gas phases, the high heat capacity of water and the ~1,000 times greater mass fl ow of water relative to the gas result in a single-stage, continuous compres- sion process that involves simultaneous work transfer to the gas and heat transfer from the gas to the water and practically leads to isothermal conditions. This ac- complishment makes the achievements of historical HAC installations even more remarkable, even if it was underappreci- ated at the time. Not only did they use renewable hydropower resources, they did so using an extremely energy-effi cient hydropower-to-pneumatic power conver- sion process. An additional advantage of these HACs was that the compressed air produced by them was drier than compressed air pro- duced by mechanical compressors used at the time. The reason is that the satu- rated vapor pressure of water in air reduc- es as pressure increases. The water vapor condensed and joined the bulk circulat- ing water fl ow in the downcomer process and was removed in the HAC's air-water separator device. Applying History Lessons Cooling ultra-deep mines is a challenge. In some cases, like the Mponeng gold mine in Johannesburg, South Africa, where temperatures regularly reach 60°C (140°F), an ice-slurry mixed with salt is pumped down from the surface. Fans blow air over the slurry to create an envi- ronment in which miners can work. While this method of cooling is effec- tive, refrigeration to create the ice slurry requires a lot of power, which not only adds to the cost of system operations, but consumes resources. With the objective of fi nding a pro- cess that was more energy conscious, a group of companies organized to test HAC technology. In executing the Hydraulic Air Compressor Demonstrator Project, partic- ipating companies agreed to set up a way to measure and verify the electricity sav- ings potential of modernized HAC tech- nology for deep mining applications. Project partners include the Centre for Excellence in Mining Innovation's Ul- tra Deep Mining Network (a business-led Resurrected Technology Could Resolve Cooling Challenges in Ultra-deep Mines This test setup is located at a research facility in Sudbury, Ontario, Canada. Project sponsors are using the 30-m-high rig to evaluate possible use of HAC as an energy-effi cient process for providing cooling air to ultra-deep mines.

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