Chapter 43 

LABORATORY TESTING TO QUANTIFY DUST ENTRAINMENT DURING 

SHIELD ADVANCE 

G.J. Chekan 

National Institute for 

J.M. Listak 

J.F. Colinet 

Occupational Safety and Health 

Pittsburgh Research Laboratory 

Dust and Toxic Substance Control 

Branch 

Pittsburgh, PA 15236 USA 

National Institute for 

Occupational Safety and Health 

Pittsburgh Research Laboratory 

Dust and Toxic Substance Control 

Branch 

Pittsburgh, PA 15236 USA 

ABSTRACT 

As longwall shields are lowered in preparation for advance, dust from the canopy falls directly into the air stream 

with the potential of becoming entrained. Historical dust sampling data from the early 1980s shows that shield 

advance contributed an average of 12 percent to the airborne respirable dust levels found on longwalls in the United 

States. Data from the 1990s indicates that shield dust liberation had increased to an average of 23 percent. Increased 

longwall production levels over this time resulted in the need to move a larger number of shields during each shift, 

while an increase in air velocities on the face was also occurring. These factors combine to suggest that greater 

quantities of shield dust are being entrained into the face air. A series of controlled laboratory tests were conducted in 

a test stand simulating shield dust entrainment to evaluate changes in airborne respirable dust levels at different air 

velocities. Significant differences in respirable dust entrainment were observed. The test facility, test methodology, 

and dust sampling results will be reviewed and discussed

KEYWORDS 

Longwall, coal dust, shield dust, dust entrainment, engineering controls 

INTRODUCTION 

Longwall mining operations continue to be an 

increasing contribution of coal production, now 

accounting for approximately 50% of the underground 

coal produced in the U.S. (Energy Information 

Administration, 2000). Although advances have been 

made at mitigating airborne respirable dust along 

longwall faces, many operations still have difficulty 

maintaining compliance with federal dust standards. 

Entrainment of dust along longwall faces contaminates 

the work place for face workers, thus increasing the 

risks of coal workers pneumoconiosis and silicosis. 

Figure 1 compares the contribution of longwall dust 

from the major sources during the 1980's and 1990's 

(Colinet and Jankowski, 1997). During the early 1980's, 

face air velocities ranged from 0.6 to 3.3 m/s (125 to 

650 fpm) (Jankowski and Organiscak, 1983), while 

average production was 810 tons/shift (890 st/shift) in 

1981 (Niewiadomski, 2000). During the early 1990's, 

the range of air velocities increased to 1.0 to 7.6 m/s 

(195 to 1500 fpm) (Colinet and Jankowski, 1997), while 

the average production nearly increased four-fold to 

3180 tons/shift (3500 st/shift) in 1993 (Niewiadomski, 

2000). During this period, dust control technology has 

concentrated on reducing the dust generated by the two 

largest sources: shearer and stageloader. Improved 

water spray application at the shearer and enclosing the 

stageloader while adding water sprays helped control 

dust liberation from these sources despite increased 

production. However, little research addressed dust 

liberation during shield advance as suggested in Figure 

1, which shows that the percent of dust contribution 

attributed to shield movement has almost doubled. The 

primary factors which have lead to this increase are 

most likely a combination of higher production and lack 

of effective control technology. Higher production rates 

have led to increased shearer speeds which requires that 

shields be moved faster and in greater numbers. As 

shield supports are lowered and advanced, broken coal 

and/or rock fall from the top of the shield canopy 

directly into the airstream ventilating the longwall face. 
PROCEEDINGS OF THE 7TH 292 INTERNATIONAL MINE VENTILATION CONGRESS

In addition, it has been noted that the air velocities 

being found on longwall faces have significantly 

increased. These factors combine to increase the 

potential for entraining greater quantities of dust during 

shield advance. 

0

10

20

30

40

50

60

Intake Stageloader Shields Shearer

Percent

1980s 1990s

Figure 1. Comparison of dust source contributions on 

longwalls

Shield spray systems that wet material on top of the 

shield canopy are available but the effectiveness of 

these systems have not been documented. Two factors 

that are of concern with the shield spray systems are 

how uniformly the sprays wet the material across the 

canopy and how long do these sprays operate as 

designed. Several mines have indicated that it is 

difficult to maintain the shield sprays throughout the life 

of a longwall panel. Wetting can be an effective means 

of controlling shield dust, however, to achieve 

maximum benefits from water sprays, it is imperative 

that the sprays be maintained to function as designed. 

Traditionally, dust control technology has employed 

air and water as a means to limit, direct, and/or, 

eliminate respirable dust. Dilution is another method 

used to control dust and often times, the face airflow is 

increased as a means to further dilute the dust generated 

by all sources. Studies have shown (Tomb et .al., 1990, 

Breuer, 1972) that with adequate moisture on the coal, 

such as that provided by sprays, air flow may be 

increased without significantly increasing the dust 

entrainment on the face. The addition of water will 

increase the surface moisture of the particles and should 

increase their adhesion properties resulting in particle 

agglomeration. Other studies examined velocities from 

0.5 to 4.6 m/s (100 to 900 fpm), and showed that a 

minimum longwall face air velocity of 2.0 to 2.3 m/s 

(400 to 450 fpm) is required for proper dilution of dust 

along the face (Foster-Miller Assc., 1982, Breuer, 

1972). However, due to the presence of methane, higher 

air velocities are often required for dilution of gas. 

When air velocity increases above 2.0 m/s (400 fpm), an 

increase in dust levels can occur if the moisture content 

of the dust is insufficient to prevent entrainment into the 

airstream (Hall, 1956, Hodkinson, 1960). A more recent 

study (Tomb et. al.,1990) shows that as face air 

quantities have increased, even beyond 5.1 m/s (1000 

fpm), dust exposure levels due to dust generated along 

the face decrease when adequate controls are used. 

Currently, some U. S. longwall operations are using 

face velocities in excess of 7.6 m/s (1500 fpm). 

These studies clearly indicate that higher air velocities 

do not increase dust generation from controlled sources 

such as the shearer or stageloader if adequate dust control 

systems are employed. However, this observation may 

not be the case for shield dust. As previously stated, 

water can be an effective means of controlling dust. 

However, sprays on the tops of shields are difficult to 

maintain and as a result soon become inoperable. More 

importantly, controlling shield dust using increased air 

dilution is difficult because increasing the air velocity 

provides greater potential for dust entrainment since the 

dust falls directly into the airstream under the canopy 

edges. This entrainment, in the absence of effective 

control technology, is the most likely source of increased 

dust contribution from the shields. In an effort to better 

understand the effects of high air velocity on the 

entrainment associated with shield dust, a test facility 

capable of entraining dust in a wind tunnel was designed 

and constructed at the NIOSH Pittsburgh Research 

Laboratory. The wind tunnel enabled laboratory studies 

to be conducted under controlled conditions that would 

simulate dust entrainment similar to that created by shield 

movement. This research is being conducted to provide 

fundamental information on the entrainment 

characteristics of dust at high air velocities and eventually 

provide solutions to shield dust generation and control. 

TEST APPARATUS 

The test apparatus consisted of four main components 

as shown in figure 2: 1) Dust entrainment tunnel, 2) 

Variable speed axial vane fan, 3) Vibratory material 

feeder, and 4) Dust sampling station. 

Figure 2. Dust entrainment tunnel schematic 
LABORATORY TESTING TO QUANTIFY DUST ENTRAINMENT DURING 293

Dust Entrainment Tunnel

The tunnel provided an enclosed and controlled area in 

which to entrain the dust. It was constructed of 12.7 mm 

(1⁄2-in) plywood and 50.8 by 101.6 mm (2 x 4 in) wood 

framing. The dimensions of the tunnel were 0.6 × 1.2 m 

(2 × 4 ft) providing an area of 0.7 m2

 (8 ft2). The length of 

the wind tunnel was 12.2 m (40 ft) with a 1.5 m (5 ft) 

long evase at the open end to reduce head loss and 

turbulence as air entered the tunnel. The interior of the 

tunnel was waterproof painted and all seams sealed with 

caulking. A clean-out was provided in the tunnel to wash 

accumulated dust from the interior surface with water 

after each test. Two observation windows were built into 

the tunnel at the material dump point and at the sampling 

station to observe conditions during testing. 

Variable Speed Axial Vane Fan

The fan provided a means to adjust air velocity in the 

tunnel. Air velocity was controlled with a 29.7 kw 

(40 hp) Joy axial vane fan capable of producing air 

velocities in the tunnel in excess of 10.2 m/s (2000 

fpm). This was coupled with an Allen-Bradley variable 

speed controller to adjust fan rpm as needed. A 1.2 m 

(4 ft) long reducer duct between the tunnel and fan was 

used to transition the round fan opening to the 

rectangular tunnel and served to reduce shock loss and 

turbulence on the intake side of the fan. The exhaust 

side of the fan was connected by duct to a bag house 

(not shown) which filtered the dust from the air for 

environmental concerns. 

Vibratory Material Feeder

The feeder provided a controlled means to introduce 

the material (dust and coarser coal) into the tunnel for 

each test. The unit was positioned on top of the tunnel as 

shown in figure 2. The feeder was a Eriez Model FBV513 with a 0.1 m3

 (4 ft3) hopper and dual controls to 

adjust vibration and feed rate. The feeder was retrofitted 

with a 56 cm (22 in) wide feeder pan that distributed the 

material across the entire width of the tunnel. 

Dust Sampling Station

The sampling station provided a means to collect dust 

samples for each test. Dust samples were collected using 

gravimetric sampling pumps and personal impactors 

through three ports located in the sides of tunnel. Sampling 

probes were inserted into each port, which allowed the 

sampling pumps and impactors to be located on the outside 

of the tunnel. The distance between the material dump 

point and the location of the downstream sampling probes 

was 7.6 m (25 ft), as shown in figure 2. 

TEST AND SAMPLING PROTOCOL 

Instrumentation

Marple cascade impactors were chosen as the primary 

sampling instrument as the objective of the tests as it relates 

to air velocity were twofold: 1) to study the entrainment 

characteristics of both total dust (< 50 microns) and 

respirable dust (< 10 microns) and 2) to study the change in 

dust concentration and size distribution of the dust. The 

cascade impactor is an instrument that measures the size 

distribution of airborne particles and is used for particle size 

classification of dust in many industries. In use, the dust 

laden aerosol is drawn into the multi-stage instrument and 

separated into aerodynamic size classes. The aerosol stream 

moves with low velocity over the upper stages and increases 

in velocity at each subsequent stage. The collection process 

is made possible through the use of mylar substrates on each 

of the instrument’s stages. Those particles with higher inertia 

(larger particles) will ‘impact’ the mylar substrate and be 

collected on the upper stages, whereas smaller particles will 

pass and be collected on the subsequent lower stages. The 

mylar substrates were coated with silicone grease to 

minimize particle bounce from one stage to the next. The 

substrates are pre- and post-weighed to determine the mass 

distribution on each stage of the impactor. The dust 

concentration and size distribution of the dust cloud at each 

test velocity were then determined. 

Sampling Method

Isokinetic sampling, a sampling method by which dust 

laden air is drawn into a sampling nozzle at a velocity equal 

to that of the air in the tunnel (Brockmann, 1993, Quilliam, 

1994), was employed. The nozzle diameter required to 

match the velocity in the wind tunnel is a function of the 

sampling pump air volume. Pumps were operated at 

2 liters/min and tests were performed at four velocities, 2.0, 

4.1, 6.1, and 8.1 m/s (400, 800, 1200, and 1600 fpm). To 

match these velocities, nozzle diameters of 4.6, 3.2, 2.6, and 

2.3 mm (0.180, 0.127, 0.104, and 0.090 in) respectively were 

used. The inset of Figure 2 shows a cross sectional view of 

the arrangement of isokinetic probes in the tunnel. Three 

probes were placed on 0.3 m (1 ft) spacings in relation to 

each other and the sides of the tunnel. The use of a threepoint sampling grid to calculate an average concentration 

over the area of the tunnel minimized the variation in dust 

levels that may occur due to dust gradients within the tunnel. 

Fan Settings

Fan settings at the four selected air velocities were 

established using a pitot tube with manometer through a 

cross-sectional traverse which consisted of averaging the 

velocity over sixteen quadrants within the tunnel. Air 

velocity profile measurements were conducted at the 

sampling station to determine fan settings for the test 

velocities. Velocity measurements should be made at least 

7.5 duct diameters away from any major upstream 

disturbances, in this case the material dump point (Guffey 

and Booth, 1999). The distance between the material dump 

point and the sampling station is 7.6 m (25 ft) which is 

approximately 7.5 duct diameters for a round duct of 

equivalent area of 0.7 m2

 (8 ft2). It is important that 

turbulent flow exits to insure that satisfactory dispersion 

and removal of dust (Hartman, 1961). The critical velocity 

for this tunnel was calculated and the tests velocities 

exceed the velocity necessary for turbulent flow. 
PROCEEDINGS OF THE 7TH 294 INTERNATIONAL MINE VENTILATION CONGRESS

Feed Material

The feed material consisted of crushed coal (coarse) 

and finely ground bituminous coal dust. A previous 

shield dust study (Organiscak, et. al., 1985) showed that 

mines that left coal as immediate roof had higher 

amounts of support generated respirable dust than those 

with non-coal roofs. Therefore, initial studies were 

conducted using coal as the feed. However, most 

longwall operations today mine the entire seam and a 

rock roof is more typical on today’s operations. 

Therefore, subsequent testing in this apparatus will use 

rock as the feed material. The coal dust, Keystone 

Mineral Black 325BA, is a commercially available 

material, manufactured by Keystone Filler and 

Manufacturing Company. The physical properties of 

this material are consistent, having a maximum particle 

size of 50 microns, sixty-five percent of which is <10 

microns, and a moisture content of <1%. The purpose of 

the larger sized coal was to simulate the coarser debris 

that falls into the airstream as face supports are 

advanced and may enhance/hinder entrainment of the 

respirable portion of the coal dust. Air dry loss analysis 

on the total material mixture was <1%. 

Initially, preliminary tests were conducted to 

determine the relationship between two key material 

parameters: 1) material weight and mix proportions, and 

2) feed rate of material. The objective was to achieve a 

balance between the amount of material introduced into 

the tunnel and the time or rate of introduction. Material 

weight introduced into the tunnel varied between 9.1 to 

18.2 kg (20 to 40 lbs), consisting of a mix of 50 percent 

coal dust ( less than 50 microns), 25 percent coal 4.75 to 

1.18 mm (0.18 to 0.05 in), and 25 percent coal 9.5 to 

4.75 mm (0.37 to 0.18 in). The feed rate was varied so 

test runs were between 20 to 60 minutes in length. A 

series of preliminary tests were run and it was 

determined that a material weight of 18.2 kg (40 lbs) 

introduced into the tunnel in 30 minutes time gave 

sufficient weight gains on the impactor mylar filters 

with no filter overloading problems at the different 

velocities. This material weight and time were 

standardized for each test. Six test were run at each air 

velocity for a total of 24 tests. 

DATA ANALYSIS 

As previously stated, a minimum face air velocity of 2.0 

to 2.3 m/s (400 to 450 fpm) is required for improved dust 

control along a longwall face. Therefore, a velocity of 2.0 

m/s (400 fpm) was used as a baseline velocity for data 

collection and to make comparisons with increased air 

velocities. The data collected from the dust entrainment 

tests are shown in Table I. The table shows the total and 

respirable dust concentrations for each of the six tests 

conducted at each air velocity. Respirable fractions of the 

collected dust samples were calculated using methods 

previously defined in earlier research (Potts et.al., 1990). 

The table also shows the average and standard deviation of 

the concentrations, and the 95% confidence interval using a 

t-distribution. Dust measurements collected by the 

impactors were also analyzed for size distribution 

characteristics relative to changes in air velocity. 

Dust Concentrations

The data from Table 1 clearly show that, as velocity 

increases in the entrainment tunnel, average dust 

concentrations rise in both the total and respirable 

ranges. Figures 3 and 4 show the mean and the 95% 

confidence interval for total and respirable dust 

concentrations respectively, as a function of velocity. 

These figures show graphically how the dust 

concentrations rise with each air velocity increase. At a 

95% confidence level, statistically significant 

differences in mean dust levels at each air velocity were 

observed except for the total dust levels between 6.1 and 

8.1 (1200 and 1600 fpm). Adding a regression line to 

each of the data sets show a positive correlation 

between the two variables, and high (>98%) coefficients 

of determination (R2

), indicating that a strong 

relationship exists between the total and respirable dust 

concentrations and air velocity. 

The average total dust concentration at 2.0 m/s (400 

fpm) is 18.6 mg/m3

, as shown in figure 3. At 8.1 m/s 

(1600 fpm), average total dust concentration was 

measured to be 117.08 mg/m3

, an increase of more than 6 

times the concentration at 2.0 m/s (400 fpm). Since dust 

concentration is a function of mass and air quantity, 

increasing air velocity would increase air quantity in the 

test structure and for the same quantity of introduceddust, should dilute the dust cloud and result in lower 

concentrations. However, sampling results suggest that 

there is substantially less particle deposition at the higher 

velocities allowing significantly more of the total dust to 

reach the sampling station, thus overcoming dilution 

effects. Higher air velocities have the energy necessary to 

entrain larger particles and transport these particles 

greater distances before deposition occurs. In addition, 

moisture can affect the agglomeration of particles and 

impact the entrainment and transport potential of dust 

particles. Elevated levels of moisture increases the bond 

between particles and increases the energy needed to 

separate agglomerated particles (Breuer, 1972). 

Representative samples of the feed material were airdried and found to have an average moisture content of 

<1.0%. Consequently, the increased energy available at 

the high air velocity and relatively low moisture content 

of the feed material combine to allow total dust levels to 

rise significantly as air velocities increase. Unfortunately, 

rock and coal crushed above the shield canopies of 

longwall face supports can often have low moisture 

content and as previously indicated, air velocities on 

longwalls are increasing. The test results show that when 

these factors are combined the potential for greater 

entrainment of total dust from shields along longwall 

faces also significantly increases. 
LABORATORY TESTING TO QUANTIFY DUST ENTRAINMENT DURING 295

Table 1. Total and respirable dust concentrations at each air velocity 

Total Dust 

Average Concentration, mg/m3

Test Velocity 

m/s (fpm) 

T1 T2 T3 T4 T5 T6 

Mean 

Stand

ard 

Deviat

ion

Ratio 

to 

Basel

ine 

Dust 

95% 

Confidence 

Interval 

(+/-)

2.0 (400) 14.43 14.38 15.70 22.94 28.60 15.57 18.60 5.86 ----- 6.15 

4.1 (800) 62.75 64.40 57.34 59.71 77.60 51.06 62.14 8.90 3.30 9.34 

6.1 (1200) 80.77 103.03 92.60 93.40 92.16 75.06 89.50 10.00 4.80 10.49 

8.1 (1600) 82.24 122.76 132.41 118.68 114.70 129.72 117.08 17.40 6.30 18.26 

Respirable Dust 

Average Concentration, mg/m3

Test Velocity 

m/s (fpm)

T1 T2 T3 T4 T5 T6 

Mean 

Standa

rd 

Deviat

ion

Ratio 

to 

Basel

ine 

Dust 

95% 

Confidence 

Interval 

(+/-)

2.0 (400) 1.03 1.42 1.35 1.51 2.30 1.18 1.47 0.44 ----- 0.47 

4.1 (800) 5.14 6.03 5.52 4.78 7.39 5.04 5.65 0.96 3.80 1.00 

6.1 (1200) 8.25 16.43 14.25 13.25 14.03 11.51 12.95 2.80 8.80 2.94 

8.1 (1600) 15.39 19.67 24.89 20.73 16.54 21.84 19.84 3.49 13.50 3.67 

y = 15.92x - 8.98

R2

 = 0.98

0

20

40

60

80

100

120

140

0123456789

VELOCITY, m/s

CONCENTRATION, mg/m3

Mean

I 95% Confidence Interval

 

Figure 3. Relationship between total dust concentration 

and air velocity

The average respirable dust concentration at 2.0 m/s 

(400 fpm) is relatively low at 1.47 mg/m3

. Figure 4 

shows that respirable dust levels rise at each velocity 

increase, with respirable dust levels at 8.1 m/s (1600 

fpm) found to be 13.5 times greater than baseline levels. 

These dust level increases are contrary to a study by 

Tomb (Tomb et. al., 1990) which found that as face air 

velocities increase above 5.1 m/s (1000 fpm), respirable 

dust exposure levels decrease. However, in the study by 

Tomb, water spray systems were being utilized at 

primary dust sources (shearer and stageloader), which 

indicates that there was moisture added into the material 

to promote particle agglomeration. In addition, the dust 

generated at these sources was being shielded from the 

face airflow by physical barriers and/or water sprays in 

an effort to minimize entrainment. 

y = 3.07x - 5.60

R2

 = 0.98

0

5

10

15

20

25

0123456789

VELOCITY, m/s

CONCENTRATION, mg/m3

I 95% Confidence Interval

Mean

Figure 4. Relationship between respirable dust 

concentration and air velocity 

Respirable sampling results from the laboratory tests 

suggest that some agglomeration was occurring within 

the feed coal but moisture levels were not high enough 

to keep all of this material agglomerated as the higher 

air velocities were encountered. Therefore, at the low 

end of the test velocity, 2.0 m/s (400 fpm), these 

particles remained agglomerated and were deposited in 

the tunnel before they reached the sampling station or 

were deposited on the upper stages of the impactors. At 

higher velocities, the adhesion and auto-adhesion forces 

become less dominant allowing more of the particles to 
PROCEEDINGS OF THE 7TH 296 INTERNATIONAL MINE VENTILATION CONGRESS

be entrained into the airstream as respirable-sized dust 

particles. Figure 5 supports this conclusion by showing 

the respirable and total dust concentrations and the 

percent of respirable dust observed at each velocity. As 

shown, the percent of respirable dust in the collected 

dust sample increases as the velocity increases. The 

largest increase in respirable dust content occurred 

when the air velocity increased from 4.1 to 6.1 m/s (800 

to 1200 fpm). 

 

0

20

40

60

80

100

120

2.0 4.1 6.1 8.1

AIR VELOCITY, m/s

DUST CONCENTRATION, mg/m3

Respirable

Total

7.9 % 9.1 %

14.5 %

16.9 %

 

Figure 5. Percent of respirable dust in impactor 

samples at each air velocity 

Figure 6 shows the increase in dust concentration 

from one velocity to the next for both total and 

respirable dust. As evident from the graph, the largest 

increase in total and respirable dust concentrations 

occurs from the 2.0 to 4.1 m/s (400 to 800 fpm) and 4.1 

to 6.1 m/s (800 to 1200fpm), respectively. After 6.1 m/s 

(1200 fpm), the increase in dust concentrations level off. 

Previous studies (Foster-Miller Assc., 1982, Breuer, 

1972) have shown that, increases in dust levels begin to 

occur at about the 2.0 to 2.3 m/s (400 to 450 fpm) 

velocity range for dust having moisture content of about 

3%. Data collected during this study are consistent with 

the findings from these studies. 

0

5

10

15

20

25

30

35

40

45

50

2.0 to 4.1 4.1 to 6.1 6.1 to 8.1

AIR VELOCITY INCREASE, m/s

DUST CONCENTRATION, mg/m3

Respirable

Total

Figure 6 Rise in dust concentration for each air 

velocity increase 

Size Distribution

Particle size classification is important to examine 

how dust is transported and to determine its potential 

health effects on workers. To characterize the dust, 

multiple stage impactors were used to classify dust 

particle size in the entrainment tunnel. Table 2 shows 

the impactor stages and associated cut points, and the 

average particle weight for each stage at each velocity. 

Mass median diameters (MMD) of the dust sample at 

each velocity were determined and are also shown in 

Table 2. The MMD gives an overall measure of the size 

distribution of the particles and specifically is the 

particle size at which 50% of the particles are greater 

than the MMD and 50% of the particles are smaller than 

the MMD. 

Table 2. Mass on each impactor stage for test velocities 

Average Mass per Stage, mg 

Stage 

Cut 

Points 

microns 

2.0 m/s 4.1 m/s 6.1 m/s 8.1 m/s

1 21 0.159 0.461 0.512 0.599 

2 15 0.153 0.400 0.433 0.570 

3 10 0.330 0.990 1.171 1.421 

4 6 0.232 0.969 1.449 1.691 

5 3.5 0.113 0.505 1.041 1.488 

6 2 0.048 0.195 0.514 0.770 

Final 0.25 0.012 0.022 0.062 0.132 

MMD, microns 10.84 9.82 8.22 7.70 

Figure 7 shows the dust weights that were collected 

on each stage for the four different air velocities during 

the test. The x-axis shows the particle size of each of the 

stages ranging from 21 to 0.25 microns. 

The four curves represent the weight per stage for 

each of the four test velocities. As is evident in figure 7, 

an overall trend can be seen for each velocity and at 

each stage. In general, as the velocity increases from 

one speed to the next, the weights on each of the 

individual stages also increase. Therefore, as can be 

expected, as the velocity increases, less deposition is 

occurring and more of the dust is being transported 

further down the airstream. However, the stage weights 

at 2.0 m/s (400 fpm) remain consistently distributed 

over the entire range of stages relative to the other 

weights at the other velocities. The curves representing 

the three higher velocities show sharp increases in stage 

weights from 15 to 10 micron cut points and continues 

to increase into the respirable dust range (<10 microns). 

The differential in weight for all stages is the greatest at 

the 6 micron cut point. 

Comparing the 3.5 and 6 micron cut points to cut 

points 21 and 15 shows that dust collected at the nonrespirable range was small compared to the respirable 

range. 
LABORATORY TESTING TO QUANTIFY DUST ENTRAINMENT DURING 297

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

21 15 10 6 3.5 2 0.25

CUT POINTS, microns

MASS, mg

2.0 m/s

4.1 m/s

6.1 m/s

8.1 m/s

Figure 7 Average dust distribution by impactor cut 

points 

A graph of the mass median diameter at each air 

velocity is shown in figure 8, and identifies the decrease 

in particle size as velocities increase. The fact that finer 

dust is being collected at the higher velocities further 

suggests that the increased energy of the higher velocity 

air promotes separation of loosely agglomerated 

particles. This observation is in agreement with other 

studies (Shankar and Ramani, 1995) that showed similar 

findings from reentrainment wind tunnel tests. 

6

7

8

9

10

11

12

123456789

VELOCITY, m/s

MASS MEDIAN DIAMETER, microns

Figure 8. Mass median diameter for each test velocit 

Increased Moisture

Past research has indicated that the addition of water 

to a mined product will reduce the quantity of respirable 

dust that becomes airborne. Consequently, mine 

operators utilize water sprays to wet run-of-mine coal 

(e.g., underboom sprays on continuous miners, 

stageloader sprays on longwalls, sprays at belt transfer 

points) in an effort to reduce dust liberation during coal 

transport. Similarly, shield spray systems have been 

installed to wet the unconsolidated coal/rock on the tops 

of shields before it becomes exposed to ventilating air 

during shield advance. The objective of the next series 

of tests was to simulate the effect of these sprays by 

adding moisture to the feed product and determine the 

impact on dust liberation at elevated moisture levels. 

The original feed product was quite dry and only 

contained approximately 1% surface moisture. The goal 

was to add sufficient surface moisture to the feed 

material so that it contained approximately 7% to 9% 

moisture. No information could be found to identify a 

suitable means of wetting the feed coal. Initially, an 

attempt was made to operate humidifiers in a sealed 

chamber to raise moisture levels. After 72 hours of 

continuous operation, the moisture content had only 

increased to approximately 3%. Subsequently, to 

expedite the process a water spray manifold was 

constructed that contained 5 atomizing nozzles. These 

nozzles were operated at 689.5 kPa (100 psi), resulting 

in a flow rate of 0.038 lpm (0.01 gpm) per nozzle. 

The coal feed material was spread onto a flat platform 

that could be pulled under the spray manifold at a 

controlled rate of speed of 0.08 m/s (16 fpm) It was 

determined that the feed material needed to pass under 

the spray manifold 10 times in order to disperse the 

desired quantity of water onto the feed product to 

achieve the 7% to 9% moisture range. After each pass, 

the feed material was combed to evenly distrubute the 

water through the product. Five separate tests were 

conducted and samples were collected and analyzed for 

surface moisture at an independent laboratory. Results 

of this analysis indicate that this wetting technique 

increased the surface moisture to the desired range and 

could be applied in a reproducible manner. This method 

has been adopted for preparation of feed material for 

subsequent tests. The tests to evaluate the impact of 

added moisture have been initiated. It is anticipated that 

the preliminary results of these tests will be available 

for discussion during the conference presentation. 

SUMMARY 

Substantial gains in longwall production levels have 

occurred and longwall operators have increased the 

levels of air and water applied in an effort to control 

dust generation and worker exposure. Research and 

application of control technologies have focused on the 

shearer and stageloader/crusher, which were the two 

highest sources of respirable dust as characterized by a 

U.S. Bureau of Mines study in the 1980 s. A similar 

study by the Bureau in the 1990s indicated that dust 

liberation during shield movement has emerged as a 

significant contributor to dust generation on longwalls. 

As shields are advanced, material drops off of the shield 

canopy directly into the airstream, which would 

facilitate entrainment of dust into the airstream. Higher 

production levels have resulted in faster shield advance, 

greater numbers of shields being moved, and air 

velocity increases. These factors combine to elevate the 

dust problems associated with shield advance. Past 

studies (Hall, 1956, Hodkinson, 1960) have shown that, 
PROCEEDINGS OF THE 7TH 298 INTERNATIONAL MINE VENTILATION CONGRESS

for low moisture dust, velocities in excess of about 

2.3 m/s (450 fpm), can cause dust concentrations to rise. 

To better understand the effects of dust entrainment at 

air velocities being observed on today’s longwall faces, 

research was conducted in a wind tunnel at test 

velocities of 2.0, 4.1, 6.1, and 8.1 m/s (400, 800, 1200, 

and 1600 fpm). This study examined dust entrainment 

characteristics in the absence of any dust control 

technologies. 

Higher air velocities result in higher air quantities 

which can serve to dilute dust and should therefore, 

lower concentrations in the wind tunnel. However, both 

total and respirable dust concentrations rose at each 

successive higher air velocity indicating that particle 

entrainment was greater than dilution effects for these 

tests. Total dust concentration increased from 

18.6 mg/m3

 at 2.0 m/s (400 fpm) to 117.1 mg/m3 at 8.1 

m/s (1600 fpm). Similarly, respirable dust 

concentrations increased from 1.47 mg/m3

 at 2.0 m/s 

(400 fpm) to 19.84 mg/m3 at 8.1 m/s (1600 fpm). 

Statistical analysis of the concentrations measured at 

each velocity resulted in significant differences at a 95% 

confidence interval. 

Size distribution of the sampled dust shows that as 

velocities rise, a higher percentage of the dust particles in 

the airstream are finer (< 10 microns) than what was 

collected at 2.0 m/s (400 fpm). At 2.0 m/s (400 fpm), 

7.9% of the collected dust was in the respirable size 

range, while at 8.1 m/s (1600 fpm), 16.9% of the 

collected dust was respirable. The mass median diameter 

was found to be 10.8 microns at 2.0 m/s (400 fpm) and 

decreased to 7.7 microns at 8.1 m/s (1600 fpm). Higher 

concentrations and finer particle size distributions suggest 

that at a moisture content of approximately 1%, a portion 

of the dust particles were loosely agglomerated and 

remained agglomerated at the 2.0 m/s (400 fpm) velocity. 

As the velocity increased, the adhesion forces are 

overcome by the increased energy supplied to the system 

resulting in higher concentrations and smaller particle 

sizes in the airstream. 

Past research has indicated that higher surface 

moisture in the mined product may serve to reduce 

airborne dust concentrations at higher air velocities. An 

entrainment study using test material with higher 

moisture content has been initiated and the results will 

be compared to the current study to quantify the effects 

of increasing moisture content. Results of these tests 

will be provided at the conference. 

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