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Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Moving Air to the Mining Face: Extensible Systems – Jet Fans

The introduction of conventional mining methods, which increased the rate of mining, was an important step in the mechanization of mining. The intermittent nature of the conventional mining process halted the extraction process for coal-loading and usually allowed time for methane gas to be dispersed. However, the introduction of continuous mining machines in the 1940s produced a constant flow of coal from the working face of the mine and resulted in an increase in methane levels.

The number of face ignitions increased as more continuous mining machines were placed underground. Methane levels were found to be dangerously high. In some cases, methane concentrations measured 20 ft from the mining face exceeded the lower explosive limit (5% by volume) [USBM 1958]. The need for better face area ventilation was recognized to reduce the potential for explosions.
Excessive levels of methane gas can affect the safety of the underground work force. Available methane control systems have been challenged in recent years by mining developments which include the use of continuous mining machines.

In the past 10 years, explosions have led to 65 fatalities and 18 injuries with major explosions occurring at the Sago Mine in West Virginia in 2006 (12 fatalities and 1 injury), the Darby No. 1 Mine in Kentucky in 2006 (5 fatalities and 1 injury) and, most recently, at the Upper Big Branch Mine in West Virginia in 2010 (29 fatalities) [NIOSH 2011]. The occurrence of a methane gas explosion puts the lives of the entire underground workforce at risk.
The U.S. Bureau of Mines (USBM) was formed in 1910 following a series of underground explosions that resulted in many fatalities and injuries [Kirk 1996]. The agency was responsible for conducting scientific research and disseminating information on the extraction, processing, use, and conservation of mineral resources. The USBM research program for mining health and safety was transferred to NIOSH in 1996. Since that time, NIOSH has established a ventilation test gallery where techniques for methane control and monitoring are evaluated under a variety of conditions that simulate airflow near the working face of a continuous mining section. Airflow patterns and methane concentrations are studied in a detailed manner that is not possible in a working underground mine.

Moving Air to the Mining Face

Effective face ventilation requires that a sufficient quantity of intake air be delivered to the mining face in order to dilute liberated methane to a safe level. Federal regulations include the following requirements:
• Face ventilation control devices shall be used to provide ventilation to dilute, render harmless, and to carry away flammable, explosive, noxious, and harmful gases, dusts, smoke, and fumes [30 CFR 75.330].
• A minimum quantity of air (3,000 ft3/min) is required at each face area [30 CFR 75.325].

A mine operation must specify in its ventilation plan the minimum quantity of air required to maintain methane levels below 1% at their working faces. Early USBM research examined ways to deliver air to the end of the curtain or tubing with minimal losses. Recent NIOSH research has examined more effective ways to move air from the end of the curtain to the face. New monitoring instruments and sampling techniques made it possible to examine how operating conditions affect airflow inby the curtain or tubing.

Extensible systems – Jet Fans

A jet fan is a free-standing fan with no duct work and is usually placed on the upstream corner of the last open cross cut. It projects a high-velocity jet of air, which expands as it entrains the surrounding air into its stream. The jet expands until fresh air is flowing to the face in half the entry and returning as contaminated air through the other half (Figure 1).
Figure 1 - Expansion of jet in mine opening.

Figure 1 - Expansion of jet in mine opening.

Underground tests have demonstrated the value of using jet fans for ventilating mines with large openings [USBM 1978; Engineers International, Inc. 1983]. Tests were conducted in the ventilation test gallery to determine if jet fans could be used to effectively ventilate mining entries having smaller cross-sectional areas, such as found in coal mines.

Tests were conducted in an area of the test gallery approximately 90 ft deep, 16½ ft wide and 7 ft high (Figure 2) [USBM 1992]. Exhaust airflow was 13,000 ft3/min. An 18-in-diameter vane-axial fan with a maximum flow of 6,800 ft3/min was used as the jet fan. The fan was positioned adjacent to the left (intake air) side of the entry, with the centerline of the fan exhaust 2 ft from the floor. The fan exhaust was directed straight toward the face, parallel to the sides of the entry.
Figure 2 - Jet fan test area in ventilation test gallery.

Figure 2 - Jet fan test area in ventilation test gallery.

Three different jet fan configurations were tested (Figure 3). The nozzle was 16.5 in long with an outlet diameter that reduced from 18 in to 14.5 in. The flow extender had an 18-in diameter and was 48 in long. It had a set of internally mounted helical fins, which imparted a slight spinning action to the airflow.
Figure 3 - Jet fan configurations.

Figure 3 - Jet fan configurations.

Using digital anemometers, velocity readings were taken at locations 10, 30, 50, and 70 ft from the fan outlet and at 2-ft intervals across the width of the test gallery. The data were used to determine the expansion angle and penetration depth of the air jet (Figure 4).
• With the fan alone, airflow was erratic, the expansion angle was approximately 26 degrees, and total penetration distance was less than 50 ft.
• The nozzle and flow extender increased the cohesion of the jet flow, resulting in higher velocities closer to the face. The angles ranged from 10 to 15 degrees, and the air jet flow penetrated to the face 70 ft away.
Figure 4 - Airflow profiles for three jet fan configurations.

Figure 4 - Airflow profiles for three jet fan configurations.

When using jet fans, recirculation is a concern. If the contaminated air from the face enters the fan, it can cause methane and dust concentrations at the face to increase [Campbell 1987]. Jet fan recirculation can be reduced or eliminated by placing the fan on the intake side of the last open cross cut and extending a short length of tubing from the fan exhaust to the mouth of the entry (Figure 5).
Figure 5 - Rigid ducting on jet fan used to reduce recirculation.

Figure 5 - Rigid ducting on jet fan used to reduce recirculation.

A check curtain can also be used to direct air away from the fan inlet and reduce recirculation. In one series of tests, a check curtain was draped across the fan housing and was extended from the left rib to a point 8 ft across the heading. The effect of adding the curtain on fan recirculation is illustrated in Figure 6. In addition, the amount of air reaching the face doubled when the curtain was used.
Figure 6 -  Use of curtain to reduce recirculation.

Figure 6 - Use of curtain to reduce recirculation.

NOTE: The above control information is taken directly from the following publication:
NIOSH [2010]. Information circular 9523. Guidelines for the control and monitoring of methane gas in continuous mining operations. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-141.
Campbell JAL [1987]. The recirculation hoax. In: Proceedings of the 3rd U.S. Mine Ventilation Symposium. University Park, PA: Pennsylvania State Univ, pp. 24–29.

Engineers International, Inc. [1983]. Testing jet fans in metal/nonmetal mines with large cross-sectional airways. By Dunn MF, Kendorski FS, Rahim MO, Mukherjee A. Engineer's International, Inc. USBM contract no. J0318015, Open File Report, No. 106-84, NTIS PB 84-196393, 132 pp.

Kirk WS [1996]. The history of the Bureau of Mines. In: U.S. Bureau of Mines Minerals Yearbook, 1994. Washington, DC: U.S. Bureau of Mines.

NIOSH [2011]. Ventilation and explosion prevention highlights. []

USBM [1958]. Auxiliary ventilation of continuous miner places. By Stahl RW. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 5414.

USBM [1978]. Jet fan effectiveness as measured with SF6 tracer gas. By Matta JE, Thimons ED, Kissell FN. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 8310.

USBM [1992]. Jet fan ventilation in very deep cuts—a preliminary analysis. By Goodman GVR, Taylor CD, Thimons ED. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 9399.
coal mining
continuous mining operations
deep-cut mining
• Adding a flow nozzle or extender improved penetration of air jet toward the face.
• Recirculation can be a concern when using jet fans – erecting a curtain across the fan can reduce this concern.