Electricity + Control December 2017

FLOW MEASUREMENT

BAC only measures dry-bulb temperature, since the relative humidity on the outlet of the BAC is 100% while the BACs are operating. After all the sensors were installed, measure- ments were taken during normal operation to es- tablish a baseline. The test was started and the flow to the BAC was controlled. The water to the BAC during the test was completely stopped dur- ing the Eskom peak period for three consecutive days. The BAC fans were switched off and the ef- fect on the airflow through the mine was observed to be negligible. The BAC fans ensure that the heat transfer within the BAC is optimal. The ventilation fans in the return airway ensure that the mine is still ventilated if the BAC fans are switched off. The BAC fans are therefore also switched off during the Eskom peak period. Test results A baseline BAC water flow was establish and is represented by the solid line in Figure 4 . This flow was measured over a period of three working week days. The water is gravity fed to underground and to the BAC from a common manifold. As a result there is a pressure drop in the water supplied to the BAC whenever water is sent underground. This explains the saw tooth shape of the solid line representing the baseline BAC water flow. The dashed line represents the BAC flow after the flow reduction test. The sections of the line where the flow is zero are the periods between 18:00 and 20:00. This is followed by the night time periods, where the enthalpy is low and as a result the flow to the BAC is reduced. During the day however, the flow is increased, because the enthalpy increases. Next it is neces- sary to investigate the effect this flow reduction has on the BAC outlet temperature, which can be seen in Figure 5 . From Figure 5 it is evident that the temperature on the outlet of the BAC is increased during the test days. Again, the temper- ature is lower during the night-time periods and increases during the day. The surface ventilation air moves down the main shaft to where it is secondarily cooled by the underground BAC on Level 75. Figure 6 shows the temperature and relative humidity measured at the first inlet. The temperature at the inlet of

separate inlets of the Level 75 BAC. A sensor was also installed on the outlet of the surface BAC. The placement of the sensors in mine A can be seen in Figure 3. The sensor on the outlet of the surface

Figure 3: Temperature sensor placement in mine A. The temperature sensor on the outlet of the surface BAC measures dry-bulb temperature. The two temperature sensors on level 75 measures both dry-bulb temperature and relative humidity.a

Figure 4: BAC water flow during baseline and testing period [2]. The solid line shows the flow during the three day baseline period. The dashed line shows the flow during the three day testing period.

Figure 5: BAC air outlet temperature [2]. The solid line shows the BAC outlet temperature during the three day baseline period. The dashed line shows the BAC outlet temperature during the three day testing period.

28 Electricity + Control

DECEMBER 2017