1 

LABORATORY EVALUATION OF SHIELD DUST ENTRAINMENT IN HIGH VELOCITY AIRSTREAMS 

J. M. Listak, G. J. Chekan, and J. F. Colinet 

Natl Inst for Occuptnl Sfty and Health 

Pittsburgh, PA, USA

ABSTRACT 

In conjunction with steady increases in production 

levels, longwall operators have applied greater 

quantities of ventilating air and spray water in an 

effort to control respirable dust and/or methane. Air 

velocities greater than 7.6 m/s (1500 fpm) have been 

measured on longwall faces. Operators have 

expressed concern over the potential entrainment of 

respirable dust at these high velocities. The greatest 

likelihood for entraining dust into the high velocity 

airstream may occur as shields are advanced 

allowing relatively dry particles to fall from the shield 

canopy directly into the airstream. Laboratory tests 

to simulate dust liberation during shield movement 

were conducted at the Pittsburgh Research 

Laboratory of NIOSH at velocities ranging from 2.0 

to 8.2 m/s (400 to1600 fpm). Airborne dust samples 

were obtained with personal impactors to quantify 

the size distribution and concentration of the dust 

entrained at the various air velocities. Statistically 

significant increases in both total and respirable dust 

concentrations were measured as test air velocities 

were increased. 

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. Comparisons of dust source contributions on 

longwalls from the: A) 1980s and B) 1990s. 

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
2 

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

reduce 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. 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. 

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

shielddust, 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 thus 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 

eventuallyprovide 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. 

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 
3 

and 50.8 by 101.6 mm (2 x 4 in) wood framing. The 

dimensions of the tunnel were 0.6 x 1.2 m (2 x 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 FBV-513 

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. 

Figure 2. Dust entrainment tunnel schematic. 
4 

 

 

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

drawninto 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 fourvelocities, 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. Figure 2 inset 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 three-point

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. 

Feed Material 

The feed material consisted of crushed coal 

(course) 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, sixtyfive 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 

(namely the respirable dust) 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 
5 

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. 

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

Total dust 

. 

Velocity m/s, 

(fpm) 

Average Concentration, mg/m3 

Mean Standard 

Deviation 

Ratio to 

Baseline Dust 

95% Confidence 

Interval (+/-) Test 

T1 T2 T3 T4 T5 T6

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) 84.24 122.76 132.41 118.68 114.70 129.72 117.08 17.40 6.30 18.26

Respirable dust 

Velocity m/s, 

(fpm) 

Average Concentration, mg/m3 

Mean Standard 

Deviation 

Ratio to 

Baseline Dust 

95% Confidence 

Interval (+/-) Test 

T1 T2 T3 T4 T5 T6

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

Figure 3. Relationship between total dust concentration

and air velocity.

y = 15.92x - 8.98

R2

 = 0.98

0

20

40

60

80

100

120

140

02468

Velocity, m/s

Concentration, mg/m3

Mean

I 95% Confidence Interval

10

Dust concentrations 

The data from Table I 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. 
6 

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 (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 introduced-dust, 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 increase the bond 

between particles and increase the energy needed 

to separate agglomerated particles (Breuer, 1972). 

Representative samples of the feed material were 

air-dried 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. 

The average respirable dust concentration at 2.0 

ms (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. 

Figure 4. Relationship between respirable dust

concentration and air velocity. 

y = 3.07x - 5.60

R2

 = 0.98

0

5

10

15

20

25

02468

Velocity, m/s

Concentration, mg/m3

I 95% Confidence Interval

Mean

10

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 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). 

Figure 5. Percent of respirable dust in impactor samples at 

each air velocity. 

 









0

20

40

60

80

100

120

2.0 4.1 6.1 8.1

Dust Concentration, mg/m3



 Respirable

Total

7.9% 9.1%

14.5%

16.9%

Air Velocity, m/s
7 

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 1200 fpm), 

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. 

Figure 6. Rise in dust concentrations for each air velocity 

increase 











 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

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 II 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. 

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 

non-respirable range was small compared to the 

respirable range. 

Figure 7. Average dust distribution by impactor cut points. 

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

21 15 10 6 3.5 2 0.3

Cut points, microns

Mass, mg

2.0 m/s

4.1m/s

6.1 m/s

8.1 m/s

Table II. Mass on each impactor stage for test velocities 

 Average Mass per Stage, mg

Stage

Cut 

Points 

microns

2.0 m/s 

(400 

fpm)

4.1 m/s 

(800 fpm)

6.1 m/s 

(1200 

fpm)

8.1 m/s 

(1600 

fpm)

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 1.172 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

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. 
8 

10

Figure 8. Mass median diameter for each test velocity. 

6.00

7.00

8.00

9.00

10.00

11.00

12.00

02468

Velocity, m/s

Mass median diameter, microns

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 1980s. 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, 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 total 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 will be conducted and results

compared to the current study to quantify the effects

of increasing moisture content. 

REFERENCES 

Breuer, H., 1972,“Progress in Dust and Silicosis

Control”, Gluckauf, 108(18), pp. 806-814. 

Brockmann J.E., 1993, “Sampling and Transport of

Aerosols”, “Aerosol Measurement - Principles

Techniques and Applications”, Weilke K., and Baron

P.A., eds, Van Nostrand Reinhold, pp. 77-112. 

Colinet, J.F., and Jankowski, R.A., 1997,

“Ventilation Practices on Longwall Faces in the

l

United States,” Proceedings of Ventilation `97,

Ottawa, Ontario, Canada, September 14-17, 1997,

Session 12-3. 

Energy Information Administration, 2000, “Coa

Industry Annual 1998", DOE/EIA-0584(98), June

2000, 308 pp. 

Foster-Miller Associates Inc., 1982, “Evaluating

Longwall Dust Sources and Control Technology”,

Interim Technical Progress Report, BuMines

Contract Report HO202016, 85 pp. 

Guffey, S.E., Booth, D.W., 1999, “Comparison of

Pitot Traverses Taken at Varying Distances

Downstream of Obstructions”, American Industrial
9 

Hygiene Association Journal, Vol 60, No. 4, pp. 165-

174. 

Hall, D.A., 1956, “Factors Affecting Airborne Dust

Concentrations with Special Reference to the Effects

of Ventilation”, Transactions of the Institution of

Mining Engineers, Vol. 115, pp. 245-275. 

Hartman, H.L., 1961. “Mine Ventilation and Air

Conditioning,” John Wiley and Sons, New York, p.

85. 

Hodkinson, J.R. 1960, “The Relation Between

Ventilation Airspeed and Respirable Airborne-dust

Concentration in Coal Mines”, Colliery Engineering,

Vol. 37, No.6, pp. 236-239. 

Jankowski, R.A., and Organiscak, J.A., 1983, “Dust

Sources and Controls on the Six U.S. Longwalls

Faces Having the Most Difficulty Complying With

Dust Standards”, Bureau of Mines Information

Circular (IC 8957), 19 pp. 

Niewadomski, G.E., 2000, Mine Safety and Health

Administration, personal correspondence. 

Organiscak, J.A, Listak, J.M., and Jankowski, R.A. 

1985. “Factors Affecting Respirable Dust

Generation from Longwall Roof Supports.” Bureau

of Mines IC 9019, 16 pp. 

Potts, D.J., McCawley, M.A., and Jankowski, R.A.

1990, “Thoracic Dust Exposures on Longwall and

Continuous Mining Sections”, Appl. Occup. Environ.

Hyg. Vol. 5, No. 7, pp. 440-447. 

Quilliam, J.H. 1994 “Isokinetic Sampling”, Journal

of the Mine Ventilation Society of South Africa, Vol.

10, No. 7, pp. 153. 

Shankar S., and Ramani, R.V., 1995, “Reentrainment of Coal Dust particles: Wind Tunnel and

In-Mine Studies. SME Preprint 95-93, SME Annual

Meeting, Denver Colorado, March 6-9, 1995, 8 pp. 

Tomb, T.F., Ondrey, R.S., Stoltz, R.T., Haney,

R.A., Chiz Novakowski D.L., Atchison, D.L. and

Gerbee, E.J., 1991 “Evaluation of Respirable Dust

Control on Longwall Mining”, SME Transactions, Vol.

288, pp. 1884-1878.