Maximizing The Ventilation Of Large-Opening Mines

The National Institute for Occupational Safety and Health (NIOSH) has conducted research to improve the ventilation of large-opening mines. Large-opening mine ventilation is unique for the following reasons: (1) it is challenging to keep airflow velocities high enough to effectively remove or dilute airborne contaminants, (2) large air volumes can be moved through the mines with little static pressure drop, and (3) stoppings to direct ventilation airflows are costly to construct and maintain. The research results suggest that by incorporating ventilation planning into the mine planning process, using propeller fans, developing new stopping materials and construction methods, and using long pillars to eliminate crosscuts where possible, the ventilation of large-opening mines can be significantly improved. The ventilation improvements created by incorporating these various techniques into the ventilation plan will help reduce the exposure of mine workers to airborne contaminants in underground large-opening mines. 2.2 Controlling and directing the airflow The size of large-opening underground stone mines creates problems for controlling and directing ventilation airflows. Stopping designs and techniques suitable for use in coal mines generally cannot be applied in stone mines because of the entry sizes and the associated cost of construction. Ventilation curtain materials, such as mine brattice, can be problematic because of significant air leakage due to its deterioration caused by repetitive flapping (especially near the main mine fan) and blast pressures. 2.3 Planning ventilation systems Stone production from most large-opening, roomand-pillar underground stone mines comes from multiple working faces located on the perimeter of the mine. In the initial mine development stages when few operating faces are present, it is relatively simple to provide adequate ventilation airflow across the production faces. Natural ventilation and an auxiliary fan may be sufficient to reduce airborne contaminants to acceptable levels at this stage of mine development. However, as a mine expands, ventilating all the working areas becomes more difficult as the number of working faces and the distances between the main mine fan and the working faces increase. 3 NIOSH LARGE-OPENING MINE VENTILATION RESEARCH NIOSH is conducting a research effort to improve the ventilation airflow quantity and quality in largeopening underground mines. Investigations have focused on a four-point approach: − Developing techniques to estimate the required air quantity to dilute airborne contaminants. − Identifying alternative fans capable of efficiently moving the required air quantity to dilute airborne contaminants to statutory levels − Developing improved stoppings to direct and control the ventilation airflow to the production faces − Evaluating improved mine designs to deliver and distribute the required ventilation airflows without interfering with production requirements 3.1 The Air Quantity Estimator Mine ventilation planners must determine the air quantity needed to meet the statutory DPM concentration limits by taking into account the emissions characteristics of the site-specific fleet of engines at their operation. To meet this challenge, NIOSH developed the Air Quantity Estimator (AQE) to assist mine operators with ventilation planning (Robertson et al. 2004). The AQE provides an initial estimate of the required air quantity needed to dilute DPM contaminates to statutory levels in the main air stream of the mine. The AQE is a user-friendly, stand-alone computer program that uses diesel engine performance test data from both the Environmental Protection Agency (EPA 2002) and the Mine Safety and Health Administration (MSHA 2002). The AQE is available upon request to NIOSH at: http://www.cdc.gov/ niosh/mining/products/analysissoftware.htm#AQE. 3.2 Improving ventilation airflow volumes with propeller fans Due to the opening size, the resistance to flow in large-opening drift mines is minimal. Low-pressure propeller fans can efficiently move large airflow volumes in such mines as demonstrated by NIOSH (Grau et al. 2002, 2004, Krog et al. 2004). Grau (2002, 2004) found that historically, large-opening drift mines almost exclusively used vane-axial fans for main mine fans, where in most cases, a better choice would have been propeller fans. The difference in the air quantity-pressure loss relationship between a large-opening mine and a typical coal mine is significant, as shown in Figure 1. In both cases, the mine resistance pressure increases as the square of the ventilation airflow volume. The required ventilation pressure for an underground stone drift mine rarely exceeds 249 Pa (1 in w.g.) (Grau et al. 2004a) since the resistance is so small. Figure 1 also shows the fan curves for a typical high-pressure, vane-axial fan commonly used to ventilate a coal mine and the fan curve for a typical 3.66-m (12-ft) propeller fan. The operating point for the coal mine (Point A, Fig. 1), occurs at a flow volume of 118 m/s (250,000 ft/min) and a static pressure of 1.77 kPa (7.1 in w.g.). The same coal mine vane-axial fan used in a large-opening mine would operate at 153 m/s (325,000 ft/min) and 52 Pa (0.21 in w.g.) (Point B, Fig. 1). However, at that operating point, the static fan efficiency is less than 6%, well outside the manufacturer’s normal operating envelope. The operating point for the 3.66-m (12-ft) propeller fan (Point C, Fig. 1) occurs at 175 m/s (370,000 ft/min) and 67 Pa (0.27 in w.g.) however, the static fan efficiency is much higher at 14%. Although this efficiency is low for the propeller fan, it is 2.3 times more effective than the axial-axial fan used in the same mine. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 20 40 60 80 100 120 140 160 180 200 Fan Flowrate (m/s) Fa n St at ic P re ss ur e (k Pa ) 0 2 4 6 8 10 12 0 50 100 150 200 250 300 350 400 Fan Flowrate (x 1000 cfm) Fa n St at ic P re ss ur e (in . w .g .) 3.66 m Propeller Large Mine Resistance