Electricity + Control December 2017

FLOW MEASUREMENT

This research was sponsored by HVAC International. This article has been extracted from a paper presented at the 15 th Industrial and Commercial Use of Energy (ICUE) Conference held in 2017 at the Cape Peninsula University of Technology.

from 25ºC (dry-bulb) and 50% relative humidity, to 10ºC (dry-bulb) and 100% relative humidity. This means that the air is effectively cooled from 17,7ºC (wet-bulb) to 10ºC (wet-bulb). The air starts to cool down and the relative hu- midity increases. The air reaches 100% relative humidity (see Arrow 1) and is cooled further. The water starts to condense from the air, while the relative humidity remains 100%. The condensed water ends up in the BAC sump. The absolute humidity decreases (see Arrow 2). To reduce the underground temperature, more water is pumped through the BAC.

tilation air will absorb more sweat and water than what the atmospheric air would have absorbed.

Methodology A peak clip and flow control test was conducted on a deep level gold mine during the evening peak con- sumption period. This was done to determine if the BAC flow could be reduced to save energy during the Eskom evening peak period and throughout the day. The test was conducted on Mine A, which is a gold mine situated near Carletonville. Mine A is approximately 3,3 km deep with surface and under- ground BACs on level 75, as shown in Figure 3 .

No miners are underground during the Eskom evening peak period, as this is the blasting period. For this reason the water flow through the BAC can be reduced during this period. The load on the BAC depends on the atmospheric conditions. In addition to stopping the flow through the BAC in the Eskom peak period, the water flow is also con- trolled throughout the day. This is done using the enthalpy of the atmospheric air, as it is directly proportional to the wet-bulb temperature. The enthalpy control was done between 25 and 70 KJ/kg. When the enthalpy is below 25 KJ/kg, the flow is stopped com- pletely. Between 25 and 70 KJ/kg the flow is regulated linearly between zero

Mines require fresh, cold air to ensure that the underground inlet working section temperatures do not exceed the maximum allowed temperature.

Figure 2: A psychometric chart showing air being cooled by a BAC and heated by the rock surface while going underground. Arrows 1 and 2 show how the BAC cools the air before entering the shaft. Arrow 3 shows how the air is heated while going down the shaft. The air now goes underground where it starts to heat up again, as illustrated by Arrow 3. The air is heated to 25ºC (dry-bulb), and the ab- solute humidity of the air remains the same. The relative humidity of the ventilation air is now 39%, down from the original 50% before it entered the surface BAC. Air with a lower relative humidity allows for more evaporation and for this reason the BAC ven-

flow and the maximum flow that can be supplied to the BAC. Above 70 KJ/kg the flow remains at the maximum flow. Two dry-bulb temperature and relative humidity sensors were installed on Level 75, 2.3 km underground, since the BAC on this level has two inlets. The wet-bulb temperature was calculated to see how reducing the chill water flow through the surface BAC will affect the under- ground conditions. Mining occurs below Level 75 and this is the highest temperature point the surface BAC tem- perature reaches before the air is cooled down again. As a result, this is where the highest risk to exceed the maximum temperature exists. The two sensors on Level 75 were placed in the two

Electricity + Control

DECEMBER 2017

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