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been in operation for a longer period of time. In older mines, not only do stoppings deteriorate over time, creating leakage, but more mined-out areas inherently exist. When dealing with previously pillared or longwalled areas, legally speaking, a mine must either ventilate these areas with bleeder entries or seal them. Since air dedicated to ventilating old workings and seals does not count as usefully employed air; mines with large amount of bleeder entries or seals tend to have a lower ventilation efficiency value
Data Analysis
Ventilation data was gathered from 23 mines from all regions of the country: Central and Northern Appalachia, Western Kentucky, Illinois Basin, Western Mountains and the San Juan Basin. Of the 23 mines, 11 are longwall operations and 12 are Room and Pillar operations. Not only are the studied mines diverse in location, but also in size, as the total mine airflow values range from 139,700 cfm for a small room and pillar mine to 1,150,000 cfm for a large longwall operation. These data are tabulated below in Table 1.
Table 1. Total Mine Intake Air Statistical Data by Mine
Mine Type Total Number of Mines 11 12 23 Total Mine Intake Air (kcfm) Min Mean Max 270.0 139.7 139.7 686.6 332.1 551.2 1150.0 525.0 1150.0 Longwall Room and Pillar Total/Combined The following information was gathered from all mines; including the necessary data for calculating ventilation efficiency: ? mine type
? main mine fan airflows
? airflows at last open crosscuts or headgates
? airflows at battery charging stations, electrical equipment and pump stations, etc.
Noticeable differences were discovered between ventilation efficiency values for
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longwall mines versus room and pillar mines. The data analysis shows that, on average, room and pillar mining operations tend to have higher ventilation efficiencies. The average room and pillar operation usefully employs 44.4% of its total air, as opposed to 33.9% for longwall operations. These results are shown below in Table 2.
Table 2. Volumetric Efficiency Statistical Data by Mine
Total Number of Mines Longwall 11 Room and Pillar 12 Total/Combined 23
The following graph (Figure 1) represents the distribution of mines in the various ranges of volumetric efficiency. For example, the chart shows that there are a large number (8) of mining operations between the range of 20% and 30%.
987654321010%-20 %-300%-40@%-50P%-60`%-70p%-80%Volumetic Effiency RangeMine Type Volumetric Efficiency (%) Min Mean Max 14.5% 33.9% 65.0% 23.1% 44.4% 71.6% 14.5% 38.3% 71.6% Figure 1.Statistical Distribution of Number of Mines in Each Volumetric Efficienc
Value Range
Based on average values for all mines, both longwall and room and pillar, the following chart (Figure 2) was created to illustrate the distribution of ventilation air in mines. As one would expect, the majority of usefully employed air is used to ventilate
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the working areas, while ventilating supporting equipment is notably less. The following is a list of supporting mine data that was gathered: ? number of longwall and continuous miner production units ? average seam height
? main mine fan operating pressures
? airflows sweeping mine seals (intake to seal to return configuration) ? approximate number of seals in mine
All additional supporting data was gathered with the intention of investigating any possible correlations between volumetric efficiency and other mine performance parameters. The most obvious correlation occurs when plotting volumetric efficiency versus total fan pressure. One would expect that higher fan pressure would create higher volumes of leakage throughout the mine, and reducing volumetric efficiency. The following graph in Figure 3 illustrates this correlation with a power function trend-line.
Figure 3. Volumetric Efficiency vs. Fan Pressure
However, one must keep in mind the complexity of the factors behind a low ventilation efficiency value. As just stated, it is well known that higher fan pressure facilitates more leakage and lower ventilation efficiency.But keep in mind the assumption could be made that the mines having the highest fan pressures are most likely the largest mines. It could also be assumed that the largest mines have the largest number of stoppings (creating potential leakage) as well as a potentially high
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amount of previously mined-out areas, which are required either to be ventilated or sealed. Therefore, it is highly possible that these other factors may have a role in the relationship between fan pressure and volumetric efficiency.No obvious correlations were discovered between ventilation volumetric efficiency and seam height,number of production units, mine production or number of mine seals. This is not particularly surprising as the main factors that affect ventilation efficiency, leakage and air ventilating old workings, are not highly affected by the previously stated parameters
Conclusions
This study illustrates that the average coal mine ventilation system in the United States is operating with reasonable volumetric efficiency, at approximately 38%, but has room for improvement as one mine shows that it is possible to reach over 70% efficiency. Room and pillar mines are operating with slightly higher efficiencies of approximately 44% while longwall mines are operating with an average efficiency of 34%. The lower efficiency value for longwall operations, in combination with the correlation of high main mine fan pressure to lower efficiency, illustrates that larger mines have the greatest challenges to increase their ventilation efficiency.
While safety is the primary concern behind maintaining an efficient ventilation system, it is also important to keep in mind that a good ventilation system and proper ventilation planning is essential to efficient production. Additionally,increasing the efficiency of a ventilation system can often significantly reduce power cost. It is therefore beneficial to a mining operation to keep in mind the factors and practices which can foster an efficient ventilation system.
The factors behind what helps and hinders the efficiency of a ventilation system are numerous and complicated. Upon examination of the data acquired for this study, it was apparent that it is not possible to single out any single factor that solely affects ventilation efficiency.The most common factors include: leakage caused by unmaintained or deteriorating stoppings; increased leakage due to high fan pressures; amount/area of mined-out/gob areas requiring ventilation; and the number of seals requiring ventilation. Ventilation system inefficiencies at every mine are influenced by
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