Electricity + Control December 2017

FLOW MEASUREMENT

Effect of Surface BAC Water Flow on Deep-level Mine Temperatures

HG Brand, M Kleingeld, P Maré, and AJ Schutte, North-West University

The authors investigate the effect of varying the flow rate of the BAC on underground temperatures.

M ines require fresh, cold air to ensure that the underground inlet working section temperatures do not exceed the maxi- mum allowed temperature. For mines, this max- imum working place temperature is 27,5ºC (wet- bulb) [1]. To achieve this, BACs are used to cool and dehumidify ambient air before entering the mine shaft. The ventilation through the shaft is induced by a large network of booster and extraction fans in the return airways. This ensures that the cool air is circulated through the mine and limits methane and other dangerous gas build-up [2]. BAC towers require large amounts of cold water to cool the air. Hot water is pumped from underground after be- ing utilised by drills and other cooling systems. The water is usually around 30ºC when exiting the mine [3]. It is then stored in the hot water dambefore being pumped through pre-cooling towers. Here it is cooled to approximately 18ºC with atmospheric air [2]. From the pre-cooling sump the water is pumped through a refrigeration machine, where it is cooled and stored in a chill dam. This cold water is then utilised in the BAC. The water is pumped

to the BAC at approximately 3ºC to 6ºC from the chill dam [3]. The cooling process is electrical ener- gy intensive and limiting the water usage reduces the electricity consumption. The BACs are usually forced draft spray towers [4]. With the virgin rock temperatures increasing with depth to 60ºC [5], and adiabatic compres- sion, the temperature of the air rapidly increases after exiting the surface BAC. After the absolute humidity has been reduced in the BAC, the air wet-bulb temperature is lower than it would have been if atmospheric air entered the shaft. The dry- bulb temperature of the air eventually stabilises as it approaches the rock face temperature after prolonged contact. It is suspected that the air reaches this temperature irrespective of the BAC outlet temperature. This will ensure that the dry- bulb temperature will be the same underground, whether the BAC is utilised or not. However, since the absolute humidity has been reduced by the BAC, the wet-bulb temperature underground should be lower when the BAC is utilised. The amount of water pumped through the BAC should be carefully considered.

Take Note!

Mine cooling and ven- tilation systems ensure favourable working con- ditions underground. Mines are hesitant to reduce the Bulk Air Cool- er (BAC) water flow as doing so raises safety concerns. These researchers inves- tigate how the tempera- ture of the underground network is affected if the BAC water supply flow is varied, by comparing the wet-bulb temperature before and after flow control strategies are implemented.

1

2

3

BAC Cooling In the BAC, heat is absorbed from the atmospheric air while the wa- ter in the air condenses. This can be seen on the psychometric chart in Figure 2 , which shows with Arrows 1 and 2 how a BAC cools air

Figure 1: A typical mine cooling system layout showing the hot dam, pre-cool- ing towers and refrigeration machines supplying the BACs and mine with cold water.

26 Electricity + Control

DECEMBER 2017