of any moving object be five times greater than they would be in the mine. This results in volumetric airflow rates being only one-fifth of their fullscale values and in the cutting drum rotating at 25 times the revolutions per minute.

9

It is difficult to generate airborne dust at a controlled rate and also to make accurate real-time dust measurements. For this reason, methane was used as a tracer gas to simulate respirable dust. Since respirable dust particles are less than 10 um in diameter, they follow the airflow almost as if the dust were a gas. Methane was introduced into the model through inlets cut in the face just beneath the cutting drum. Methane concentrations were measured continuously with a hydrocarbon analyzer that could measure concentrations in the range 10 to 1,000 parts per million (ppm). The output from the hydrocarbon analyzer was recorded on a strip-chart recorder. Concentration measurements were made at the continuous miner cab, the shuttle car cab, and in the return, as shown in figure 1. The continuous-miner operator is generally the high risk occupation for this type of mining section, and it is at this location that the greatest efforts must be made to reduce dust concenConcentrations measured at the shuttle car operator's position were negligible for the tests discussed in this report, all of which were done with exhausting face ventilation.

Effect of Cutting Speed on Dust Concentration

Rotation of the cutting drum had a significant effect on concentrations at the miner operator location. This had also been observed with the low-coal mine model.10 Table 1 shows concentrations at various cutting drum speeds for three positions of the line brattice. Concentrations can be reduced by lowering the drum speed, which reduces the fanning action of the cutting drum. There was a continuous circulation of air at the face, which was induced by the rotating drum. Smoke was introduced in the model at the top of the continuous miner for flow visualization. The smoke was drawn along the top of the continuous miner toward the face and thrown out below the drum, and then along the floor on the right side of the machine back toward the miner operator's location.

Effect of drum speed on concentrations at continuous-miner
operator's position, ppm

0 cfm.

49 cfm.

52 cfm...

64 cfm... 76 cfm..

94 cfm..

10,

Unless stated otherwise, all numerical values given here, such as linear dimensions, velocities, and rotational speeds, will refer to full-scale

values.

Work cited in footnote 5.

Effect of Line Brattice or Tubing Distance on Dust Concentration

The tests showed that the most significant parameter affecting the concentration at the continuous-miner operator's position was the distance of the end of the line brattice or tubing from the face. This result had also been found by Luxner.11

Table 2 shows the concentrations measured at the continuous-miner operator's position for different line brattice distances and ventilation flow rates. The line brattice was 4 ft from the rib. In a 6-ft-high, 16-ft-wide coal mine entry with the line brattice 4 ft from the rib, 4,300 cfm of ventilation air is needed to meet the legal requirement of 60 fpm of air velocity at the face. The law also requires that the line brattice be kept within 10 ft of the face. Measurements were made at greater distances to quantify the importance of this parameter on dust concentrations at the continuous-miner operator's position. It can be seen from table 2 that concentrations are substantially reduced at the miner operator's position as the ventilation airflow rate is increased. For example, with the end of the brattice 20 ft from the face, an increase of airflow from 4,300 to 5,200 cfm results in almost a 50-pct reduction in concentration. However, much greater reduction can be achieved by keeping the brattice closer to the face. For example, at 4,300 cfm, concentration is reduced by more than 90 pct when the brattice is moved from 20 to 15 ft from the face. Percentage reductions in dust might not be quite as great in a real mine where secondary sources of dust might occur. In the model, only dust generated at the face by the cutting drum was being simulated. In a real mine, dust may also be generated at the loading pan and conveyor of the continuous miner and at the point where coal is dumped onto the shuttle car. In some cases, the air entering the face area may contain quantities of respirable dust substantial enough to cause a problem at the continuous-miner operator's position.

Effect of brattice distance and ventilation flow rate on concentrations at continuous-miner operator's position,

The results of a comparison between line brattice and tubing are shown in table 3. With both the end of the brattice and the tubing at distances of 20 or 25 ft from the face, both were about equally effective in reducing methane concentrations at the continuous-miner operator's position. However, brattice

11 Luxner, J. V. Face Ventilation in Underground Bituminous Coal Mines. flow and Methane Distribution Patterns in Immediate Face Area-Line Brattice. BuMines RI 7223, 1969, 16 pp.

Air

was more effective than tubing when they both were located closer than 20 ft to the face. This could be because the tubing was located near the roof and tended to draw in pure intake air from over the top of the machine, whereas the brattice extended to the flow and thus could draw in dusty air from near the floor more effectively.

Respirable dust is often a severe problem in coal mine sections using twin-borer-type continuous miners because of the difficulty of ventilating the face. The borer machine occupies most of the cross-sectional area of the entry. Most borer sections have blowing face ventilation using auxiliary fans and tubing, but this method of ventilation is not very effective in control

ling respirable dust.12 It is difficult to mount a dust collector directly on

the twin-borer machine because of the limited space available. Under a Bureau of Mines contract, a high-efficiency dust collector and fan would be located on a self-propelled transfer car, which would be located behind the borer and have a conveyor to transfer coal from the borer to shuttle cars. The continuous-miner operator will control the borer from a cab on the transfer car instead of from the normal position at the rear of the borer. This should also help reduce his exposure to respirable dust.

12Mundell, R. L., and R. S. Ondrey. Blowing Vs. Exhausting Face Ventilation for Respirable Dust Control on Continuous Mining Sections. First Symp. on Underground Mining, NCA/BCR Coal Conf., Louisville, Ky., Oct. 21-23, 1975. V. 1, 1975, pp. 82-92; available from National Coal Association, Washington, D.C.

Description of Model

A one-sixth-scale model of a mine entry with a Goodman model 405 twin borer was constructed to determine the airflow rate required for the dust collector to be effective for reducing the workers' exposure to respirable dust. The model was 1.17 ft high, 2.33 ft wide, and 14 ft long. Figure 3 is a diagram of this model with the borer and transfer car; figure 4 shows the twin borer and the front of the transfer car in the model entry. The dust collector and fan on the full-scale transfer car is connected by flexible tubing to ducting on the top of the borer so that dusty air can be drawn from near the cutting wheels. The inlet to the dust collector in the model is an opening on the top of the borer covering the full width of the machine and located 4.5 ft from the face. Air is drawn through this inlet and then out of the model via a tube through the floor behind the borer. Thus, the model simulates a secondary ventilation system in which the effluent from the fan and dust collector on the transfer car is ducted out of the entry to the return airway. Methane, which was used to simulate respirable dust, was introduced into the model through the face near the cutting wheels. Methane concentration measurements were made (1) at the rear of the borer, which is the current position of the miner operator, (2) in the transfer car cab, where the operator will be located in the new system, (3) in the middle of the entry behind the transfer car, and (4) in the tube carrying the air from the secondary ventilation system.

Effect of Main and Secondary Face Ventilation on Dust Concentration

Figure 5 shows concentration at the three measurement locations versus the airflow rate through the secondary face ventilation system. For these measurements, the main ventilation was 3,000 cfm blowing through the tubing, which ended 6 ft from the rear of the transfer car canopy or 41 ft from the face. Measurements were also made with the end of the tubing moved back to 16 and 26 ft behind the transfer car canopy; the results were very similar to those shown in figure 5. With no secondary ventilation, the concentration was highest in the center of the entry, which is the approximate position of the shuttle cars while loading coal. The concentration was somewhat lower at the continuous miner and transfer car locations because there was some pure air flowing from the tubing passing near these measurement points. Thus, the shuttle car operator may be exposed to more dust than the continuous-miner operator even though he works further from the face. The fact that the concentration is slightly higher at the transfer car than at the rear of the continuous miner shows that moving the continuous-miner operator back a few feet from the face does not result in reducing his exposure to respirable dust.

Figure 5 shows that as the secondary face ventilation flow rate increases, there is no significant reduction in concentration until the secondary face ventilation exceeds the 3,000 cfm of the main blowing face ventilation. When the secondary face ventilation flow rate is below 3,000 cfm, contaminated air is carried down the entry away from the face. As the secondary face