The deeper and farther from the shaft, the warmer
Today in mines, the most advanced air-cooling system in operation is central air conditioning. In simple terms, it cools the water above ground (to about 1–2°C) and transfers it via a vertical pipeline underground, where it travels through horizontal pipelines to various areas of the mine. Then, the water fuels the air coolers that cool down the air in the parts of the mine in which the extraction is carried out. In the early days of any mine, this solution can be quite efficient; however, the longer the mining lasts, the longer the horizontal pipelines get, and therefore, the cooled water has a longer distance to travel. This causes trouble because the rest of the mine gets really hot and, despite the good thermal insulation of the pipes, the water that passes through those hot spots gets warmer with every kilometre. If it starts at the top with 2°C, before it reaches the cooler located in the extraction spot, after travelling several kilometres underground, it can heat up to even 8°C, sometimes even 12°C. This significantly reduces the efficiency of air coolers, and therefore, in mining areas, the temperature increases.
We cannot further cool down the water above the ground, first of all because it would simply freeze, making it impossible to transfer it down the pipeline. This could be prevented by creating a special mixture – adding alcohol or salt to the water, the temperature of the solution could drop below 0°C, simultaneously staying liquid. However, the application of such a solution is prohibited by regulations – it is forbidden to introduce liquids below 0°C into mines.
‘What is the solution then? We can build a new shaft, that is, we can drill a hole, build new above-ground infrastructure, and new vertical and horizontal pipelines. We can replace all currently used pipes and pumps for bigger ones, we can replace the entire infrastructure above and underground, so that we can transfer more water to the mine, so it won’t have time to warm up so much on the way. However, this requires us to stop the extraction, which is quite expensive. We can also apply this type of solution, which was developed for the ‘Bogdanka’ mine', explains Dr hab. Eng. Łukasz Mika, Associate Professor at the AGH University, the author of the innovative cooling system project implemented in the aforementioned mine.
Melting ice can cool it down
While working on the new cooling system, the scientist from the AGH University was aware of the limitations placed on such solutions by human and physical laws. However, with the system he had developed, he managed to harness the phenomenon of freezing and melting of ice, turning it into an advantage, and, observing all the regulations, smuggle some ice particles down below. The system designed by the scientist has been operational in the ‘Bogdanka’ mine for more than a year, helping to maintain an optimal temperature for the miners to work. In his project, Prof. Mika used the knowledge which he acquired during the work on his doctoral dissertation and the postdoctoral qualifications procedure at the Cracow University of Technology.
‘It should be noted that the basis of the modernised system is the existing central air conditioning system of a mine. Then it is expanded with ice slurry generators. Subsequently, the ice, in the form of ice flecks or wet snow, travels to a mixer, where it is combined with the cooled water already present in the mine’s infrastructure, and in the form of a slurry flows down through the pipelines’, says Łukasz Mika about his cooling system. ‘Hence, we use the existing pipeline to pump down not only water, but water with ice particles; thus, the temperature at the top is zero degrees Celsius. The slurry is delivered to the mine via pipelines, and, as a result of the high temperature of the surroundings, the pipe is hit by heat (transfer losses). But, the temperature of the water doesn’t rise because it’s the ice particles that melt in the first place, and only when they’re gone, the water in the pipe starts to heat up. When you design and use the system appropriately, you can pump water with a temperature of 1 to 2°C, or even near 0°C to distant parts of the mine. And this is a dream come true for all those responsible for cooling mines: with the water systems, we had 8–12°C water reaching the coolers; with the ice slurry, we’re close to 0°C. When we fuelled the coolers with 8–12°C water, their efficiency is reduced by several dozen per cent compared to that declared by the producer, which makes it impossible to effectively cool the air. However, the effectiveness of coolers powered by the ice slurry surpasses the catalogue efficiencies, as they are calculated with 4°C water in mind. With shorter distances, coolers might receive unthawed ice slurry, but this won’t do them any harm; in contrast, it might even be beneficial, as they get more “chill”, which they can transfer to the air in the region where the miners work’.