DUST CAPTURE PERFORMANCE OF A WATER EXHAUST CONDITIONER 

FOR ROOF BOLTING MACHINES 

Timothy W. Beck, National Institute for Occupational Safety and Health, Pittsburgh, PA 

Abstract 

Roof bolter occupations in underground coal mines 

continue to experience overexposure to respirable dust. One 

potential source of dust in roof bolting operations is the 

exhaust from the roof bolter dust collection system. A wet 

exhaust conditioner (“water box”) has been developed to 

reduce dust emissions from the exhaust of the vacuum dust 

collection system on roof bolting machines. The exhaust 

conditioner consists of a muffler chamber and a water 

chamber with four internal partitions to direct airflow over 

the surface of the water. The National Institute for 

Occupational Safety and Health conducted a series of 

laboratory tests to assess the respirable dust capture ability 

of this device as compared to the standard exhaust muffler. 

To determine the dust collection efficiency, both devices 

were placed in the exhaust stream of a simulated roof bolter 

dust collection system. Gravimetric samplers were operated 

upstream and downstream of the exhaust conditioner and 

muffler to evaluate the ability to capture both respirable 

coal and limestone dust. Researchers observed a 41 percent 

reduction in dust concentrations for the water exhaust 

conditioner in the laboratory configuration. Significant 

levels of dust deposition in the sampling chambers may 

have contributed to this effective capture rate. 

Introduction 

Occupational overexposure to coal mine dusts can 

result in coal workers’ pneumoconiosis (CWP) and 

silicosis, both disabling and possibly fatal lung diseases. 

Although remarkable progress has been made in the United 

States (U.S.) in the years following the enactment of the 

Federal Coal Mine Health and Safety Act in 1969, severe 

cases of CWP continue to occur among coal miners. 

Among miners who have worked 25 or more years 

underground, the disease rate has increased from 4.2 

percent in 1999 to 9.0 percent in 2006 (National Institute 

for Occupational Safety and Health [NIOSH], 2008). 

Similarly, for workers with 20 to 24 years of experience, 

the rate has increased from 2.5 percent to 6.0 percent. Coal 

mining also continues to be the industry most closely 

associated with worker-related silicosis, with the industry 

identified in 7.8 percent of recent silicosis deaths (NIOSH, 

2008). 

In order to ameliorate these occupational diseases, Title 

30 CFR §70.100 limits the respirable dust exposure of coal 

mineworkers to a time-weighted average of 2.0 mg/m3

 for 

an eight hour working shift (Code of Federal Regulations 

[CFR], 2009). If the respirable dust sample contains more 

than 5 percent silica by weight, Title 30 CFR §70.101 

reduces the allowable limit according to the formula 

10/(percent silica) in order to maintain silica dust levels at 

or below 100 μg/m3

 (CFR, 2009). Data from the U.S. Mine 

Safety and Health Administration (MSHA) database reveals 

that inspectors collected nearly 13000 samples for roof 

bolting occupations during the years 1999-2008 (U.S. 

Department of Labor, 2009). Of these samples, over 18 

percent exceeded 100 μg/m3

 silica and resulted in reduced 

dust limits. Further analysis shows that when roof bolter 

operators were subject to reduced dust standards, over 23 

percent of inspector samples exceeded the adjusted 

permissible exposure limits. This indicates that many roof 

bolter operators continue to experience overexposure to 

both coal and silica dusts and suggests the need for further 

investigation into improved measures to control worker 

exposure to these harmful dusts. 

In underground U.S. coal mines, Title 30 CFR §72.630 

requires that any dust resulting from drilling be controlled 

by the use of permissible dust collectors, water, water with 

a wetting agent, or ventilation (CFR, 2009). A majority of 

roof bolting machines use an MSHA-approved vacuum dust 

collection system to capture drill cuttings and prevent the 

release of dust into the mine atmosphere. The collection 

systems are comprised of the following major components: 

pre-collector, dust tank (or dust collector box), vacuum 

blower, and exhaust muffler. As the drill bit advances, air 

transports the cuttings from the drill steel and base through 

a section of dust collection hose to the pre-collector. The 

pre-collector removes the large particles from the air stream 

and deposits them on the mine floor when discharged. The 

pre-cleaned dust-laden air is then routed to the fourchamber dust collector box where particulates are classified 

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and removed from the air stream. Under typical operating 

conditions, the dust enters the collector at the top of the box 

and the larger, heavier particles collect in the main filter 

bag. The smaller, lighter dust particles pass through the 

woven bag surface and are carried to two cyclones that 

further classify the aerosol. The air stream then continues to 

the final chamber where it collects on a single paper 

canister filter. The resulting air passing through the final 

filter is approximately 99 percent cleaner (Thaxton, 1984). 

The cleaned air continues to the vacuum blower, through 

the muffler, and into the mine atmosphere. 

Though the primary dust source for roof bolter 

operators comes from upwind activities (Colinet et al., 

1985), the exhaust from the roof bolter dust collection 

system is a potential source of dust. Past research has 

shown that failure to properly operate or maintain the dust 

collection system can significantly contribute to operator 

respirable dust exposure. Divers (1984) observed dust 

concentrations as high as 20 mg/m3

 in the exhaust of 

machines with leaks around or through the paper canister 

filter. The same study showed that maintenance of the dust 

collection system and replacement of worn or damaged 

filters results in much reduced dust emissions. A recent 

NIOSH field study monitored the bolter exhaust and 

reported average exhaust respirable dust concentrations 

ranging from 0.31 mg/m3

 and 1.19 mg/m3 (Listak and Beck, 

2008). This suggests that dust emissions may be controlled 

with currently available technology. 

A water exhaust conditioner (or water box) has been 

developed to further reduce these small dust liberations and 

minimize the possibility of occupational overexposure from 

the roof bolter exhaust. The exhaust air enters the waterfilled metal container, passes through an internal muffler, 

and is deflected toward the surface of the water. Four 

internal plates act as baffles and cause the air to change 

direction before exiting. When filled to a depth of 13 cm (5 

in.), the water box contains approximately 23.6 liters (6.2 

U.S. gallons) of liquid. The water box contains an internal 

noise-reducing muffler and replaces the standard muffler in 

the exhaust stream. A schematic of the exhaust conditioner 

assembly showing the internal features and typical air 

movement is provided in Figure 1. 

Figure 1. Schematic of Internal Exhaust Conditioner 

Assembly Illustrating Typical Air Flow 

Inlet 

Exhaust 

Internal muffler 

Water level 

plug 

Not to scale Water level 

Airflow

A prior MSHA study at an underground coal mine in 

West Virginia, though limited in duration, determined that 

there was no significant change in area dust samples or 

operator exposures when using a water exhaust conditioner 

(Fields, 1999). To address potential confounding factors 

and the limited scope of the previous underground study, a 

laboratory study was planned to isolate the water exhaust 

conditioner from other system components and determine 

any improvement in dust collection efficiency when placed 

in the roof bolter’s exhaust. This paper details the findings 

of such experiments and discusses the potential impact on 

respirable dust exposures for roof bolting personnel in 

underground coal mines. 

Experimental Design 

Laboratory experiments were performed in a surface 

NIOSH facility at the Pittsburgh Research Laboratory. The 

test platform consisted of dust collection system 

components representative of the type and size found on 

J.H. Fletcher and Company roof bolting machines used in 

coal mines. The pre-collector, dust collector bag and 

canister filter were removed from this laboratory test 

system to maintain a consistent throughput of dust for all 

tests. Doing so eliminated potential sources of variation 

between individual tests caused by filter loading and 

corresponding changes in airflow. 

Air was pulled through the Fletcher dust collection box 

by a Roots Frame 2504 DVJ Whispair Dry Vacuum 

Exhauster (Dresser Roots, Houston, TX) rated for a 

flowrate of 0.03 m3

/s (60 ft3/min) at a static pressure of 68 

kPa (20 in. Hg) at 3540 RPM. Utilizing an adjustable 

frequency drive (Dayton AC Inverter Model 3HX79, 

Dayton Electric Mfg. Co., Niles, IL) with an inverter duty 

motor (Dayton Industrial Motor Model 3KV79, Dayton 

Electric Mfg. Co., Niles, IL), the speed of the Roots blower 

was varied to maintain an airflow of 0.03 m3

/s (60 ft3/min) 

for all tests. The air velocity was monitored before and after 

each test using a TSI thermal anemometer (VelociCalc 

Model 8346, Shoreview, MN) in a straight length of pipe 

between the collection box and the Roots blower. After 

traveling through the Roots blower, the exhaust air then 

traveled to a large sampling chamber, through the target 

exhaust device (muffler or water box), into another large 

sampling chamber, and then into the building’s exhaust. 

2
 

 

3 

Connecting all components of the system was rubber dust 

collection hose with an internal diameter of 3.2 cm (1.25 

in). Two 1.2 m (4 ft) lengths of hose linked each sampling 

chamber with the selected exhaust device. A schematic of 

the laboratory test layout is presented in Figure 2. 

In order to test the water exhaust conditioner under 

ordinary operating conditions, a respirable dust

concentration of 1.19 mg/m3

 was targeted (Listak and Beck, 

2008). Two TSI Model 3400 fluidized bed aerosol 

generators (Shoreview, MN) operated simultaneously to 

deliver respirable dust into the test system. To avoid the 

potential health risks of handling quartz dust, bituminous 

coal dust (Keystone Filler & Mfg. Co., Muncy, PA) and 

limestone dust (Allegheny Mineral Corporation, Kittanning, 

PA) were selected as feed materials. The coal dust, 

Keystone Mineral Black 325A, has a relative density of 

1.31 with 65% smaller in diameter than 10 μm. The 

selected grind of limestone dust was both denser and 

coarser with a relative density of 2.75 and 44.5% smaller 

than 10 μm. Because the interaction between dust and water 

may differ for limestone and coal, a separate series of tests 

were conducted using each material. 

Painted steel drums with a nominal capacity of 208 L 

(55 U.S. gallons) were used as sampling chambers upstream 

and downstream of the target exhaust device. At 0.03 m3

/s 

airflow the average velocity through each chamber was 

slowed to approximately 0.1 m/s (20 fpm) with a residence 

time of 7 seconds. Two gravimetric sampling assemblies 

were suspended inside each chamber. Dust samples were 

collected by pulling dust-laden air through 10-mm nylon 

cyclone separators with the respirable dust fraction being 

deposited onto preweighed 37-mm PVC filters. A vacuum 

pump (Model DOA-P104-AA, GAST Manufacturing Corp., 

Benton Harbor, MI) drew air through a multi-port manifold 

(Part Number MCM20-250-10B, Polyconn Inc., Plymouth, 

MN), providing air flow for each of the four sampling 

assemblies. Critical orifices (Part Number SO0, BGI Inc., 

Waltham, MA) placed on each port maintained sample flow 

rates at 2.0 L/min for each gravimetric assembly. The 

vacuum pump maintained critical (or choked) flow by 

drawing a minimum downstream vacuum of 61 kPa (18 in. 

Hg) for the duration of each sampling period. After each 

test, the 37-mm PVC filters were subsequently desiccated 

and weighed with dust levels calculated. In order to ensure 

a sufficient mass on the sample filters, tests were conducted 

for durations of 180 minutes. 

To measure the dust capture for the chosen feed 

materials and both exhaust devices, a 2x2 factorial 

experimental design was conducted as shown in Table 1. 

Each experimental factor combination was replicated for 

six tests with a total of twenty-four tests. Experimental tests 

were blocked for the feed material due to the difficulty in 

changing materials in the fluidized bed aerosol generators. 

Twelve tests were first conducted using coal dust, followed 

by a series of twelve tests for limestone dust. The target 

exhaust device was randomly selected prior to each test. 

Table 1. Experimental conditions for tests of two exhaust 

devices with two feed materials. 

Exhaust Device Feed Material

Muffler Water Box 

Coal Coal - Muffler Coal - Water Box

Limestone Limestone - 

Muffler 

Limestone - 

Water Box 

Four-chamber 

dust box Controller, 

motor, Roots 

blower 

Aerosol Upstream 

sampling 

chamber 

Downstream 

sampling 

chamber

generators 

Water 

exhaust 

conditioner To building 

exhaust 

Figure 2. Schematic of Laboratory Dust Collection System and Test Apparatus
Experimental Results 

After completion of these laboratory experiments, the 

collected data were analyzed to determine the effectiveness 

of the water exhaust conditioner to reduce respirable dust 

emissions from the exhaust of a simulated roof bolter dust 

collection system. The respirable dust capture efficiency 

was calculated for each separate test using the formula: 

Preliminary tests of the exhaust system with no device 

installed showed that a large proportion of the dust was lost 

between the two sampling chambers. Dust concentrations 

were reduced 16 and 18 percent for coal and limestone, 

respectively. This indicates that significant dust particle 

deposition occurred on the inside surface of the sampling 

chambers and dust collection hose. The slow air speed 

through the sampling chambers may have contributed to 

this effect, which presumably acted consistently for all 

subsequent tests. 

The individual test averages for upstream and 

downstream sampling locations for muffler tests are 

presented in Table 2. These values indicate that 21 percent 

of the respirable coal dust measured in the first (upstream) 

chamber was not present in the second (downstream) 

sampling chamber. Similarly, the data show that 25 percent 

of the respirable limestone dust was lost between the two 

sampling locations. The observed efficiencies were fairly 

consistent regardless of feed material, with a range of 19 to 

23 percent for coal and 24 to 27 percent for limestone. 

These efficiency values represent the system losses in the 

transport of particles through the sampling chambers, 

lengths of dust collection hose and muffler. Because all 

components are present in both test conditions, these 

muffler efficiencies establish the baselines to which the 

water box tests are compared. 

Table 2. Respirable dust concentrations and efficiencies for the muffler in the exhaust stream. 

Test Coal Limestone 

Number In (mg/m3

) Out (mg/m3) η In (mg/m3) Out (mg/m3) η

1 1.55 1.21 0.22 0.40 0.30 0.25 

2 1.54 1.25 0.19 1.13 0.83 0.26 

3 1.31 1.01 0.23 1.50 1.11 0.26 

4 1.08 0.83 0.23 1.03 0.78 0.24 

5 0.89 0.70 0.22 1.40 1.03 0.27 

6 0.58 0.47 0.19 0.95 0.73 0.24 

Average 1.16 0.91 0.21 1.07 0.80 0.25 

The samples from the water box tests were analyzed 

and reported in a similar manner in Table 3. The average 

collection efficiency for these tests was found to be 64 

percent, with 63 and 66 percent dust capture for coal and 

limestone dusts, respectively. Considering that an average 

of 23 percent of the dust loss can be attributed to the 

muffler and dust hose for both materials, the exhaust

conditioner device was responsible for collecting 41 percent 

of the airborne respirable dust. As was the case for muffler 

tests, the efficiencies observed for limestone test were fairly 

consistent, with baseline adjusted values of 39 to 43 

percent. The efficiencies observed for individual coal tests 

were less consistent, ranging from 38 to 45 percent after 

accounting for baseline dust losses. 

Table 3. Respirable dust concentrations and efficiencies for the water box in the exhaust stream. 

Test Coal Limestone 

Number In (mg/m3

) Out (mg/m3) η In (mg/m3) Out (mg/m3) η 

1 1.38 0.57 0.59 0.91 0.31 0.66 

2 1.36 0.51 0.63 1.36 0.48 0.65 

3 1.39 0.47 0.66 1.59 0.57 0.64 

4 1.01 0.35 0.65 1.68 0.58 0.66 

5 0.62 0.25 0.60 1.59 0.51 0.68 

6 0.95 0.36 0.63 2.07 0.68 0.67 

Average 1.12 0.42 0.63 1.53 0.52 0.66 

4
A minor difference in system throughput was observed 

between the selected feed materials. Coal dust tended to be 

better transported through the system than limestone dust in 

muffler tests, resulting in lower capture efficiencies. In 

contrast, slightly less limestone dust was captured in water 

box tests, when adjusted for baseline system losses. It is 

possible that a difference in properties between the 

materials may have had an impact on the dust loss and 

capture mechanisms present in the system. In other NIOSH 

research regarding dust control with wet collection systems, 

it has been suggested that particle density, particle size 

distribution, and wettability may contribute to differences in 

collection efficiencies (Colinet et al, 1990). 

Prior to each water box test, the water level in the box 

was checked and replenished as needed. During this series 

of tests, very little water addition was needed to maintain 

the full level. In practice, increased loads on the Roots 

blower from drawing air through the many vacuum 

collection system components will result in increased air 

temperatures. These elevated temperatures would result in 

higher rates of water depletion through evaporation, 

requiring more frequent maintenance. The researchers also 

observed that the water box tests required a higher input 

frequency to maintain airflow at 0.03 m3

/sec (60 ft3/min). A 

frequency of 37 Hz supplied to the motor for muffler tests 

while 42 Hz maintained airflow for water box tests. This 

represents an increase of over ten percent due to restrictions 

to airflow through the water box. The resulting increased 

load on the Roots blower presumably increased the air 

temperature for water box tests, though this was not 

measured. Lower ambient humidity would also increase 

water depletion. For this series of water box tests, the 

relative humidity in the laboratory averaged 52 percent. 

Discussion 

Though these experimental results indicate a potential 

for dust reduction in the roof bolter exhaust, it is necessary 

to evaluate the capacity to reduce occupational exposures in 

the mine environment. Title 30 CFR §75.325 requires that a 

minimum air quantity of 1.42 m3

/s (3000 cfm) be supplied 

to each working face (CFR, 2009). Using this minimum 

quantity and an exhaust dust concentration of 1.19 mg/m3

for a twin-boom bolter with total exhaust airflow of 0.06 

m3

/s (120 cfm), this fugitive dust would contribute 0.05 

mg/m3

 to the ambient dust concentration. Even with 

significant reductions in respirable dust as found in this 

series of tests, dilution in the mine atmosphere will result in 

minimal impact on operator dust exposures. 

The following key considerations and limitations of 

this series of laboratory tests should also be noted: 

• These laboratory tests evaluated the performance 

of the water exhaust conditioner on the full range 

of respirable dust of size less than 10 μm 

Aerodynamic Equivalent Diameter (AED). 

Previous NIOSH research has shown that more 

than eighty percent of the dust particles bypassing 

the final paper filter and entering the bolter 

exhaust are smaller than 2 μm AED (Listak and 

Beck, 2008). It is not clear what effect variations 

in particle size has on the performance of the 

exhaust water conditioner, but it is possible that 

different size fractions will not perform in a 

similar manner. 

• These laboratory tests considered only coal and 

limestone dusts. Cuttings captured during roof 

drilling in a coal mine would contain a dust with 

different characteristics (homogeneity, density, 

hydrophobicity, cohesion, etc). These differences 

in dust size and characteristics would affect and 

possibly decrease the efficiency of the water 

exhaust conditioner in practice. 

• These tests maintained the water in the box at the 

full level for all tests. NIOSH field observations 

have documented instances of water boxes 

operated in a low water or even empty condition. 

These practices have not been evaluated for dust 

capture performance, but it is expected that the 

system would not function optimally. 

• Accumulations of material in the water box were 

not observed during this series of laboratory tests. 

In field operation, depending on the size 

distribution and concentration of dusts in the roof 

bolter exhaust, build-up of material could vary and 

influence the operation of the water box. 

Summary 

NIOSH researchers conducted laboratory evaluations 

of the respirable dust capture ability of a water exhaust 

conditioner in the exhaust airstream of a roof bolter. The 

tests isolated the device from other components of the 

bolter vacuum collection system. The results indicate that 

the exhaust water conditioner captured 41 percent of the 

airborne respirable dust. It is unclear what mechanism was 

responsible for this significant reduction: impingement on 

the water surface or deposition on the inside metal surfaces 

of the apparatus. Considering the system losses of 17 

percent, it is possible that a similar metal surface/dust 

particle interaction acted to reduce dust transmission 

through the water box. 

Given these experimental observations, it appears that 

the exhaust water conditioner may function as a supplement 

to a properly maintained and functioning roof bolter 

vacuum collection system in an underground setting. While 

the use of the water box may provide a reduction in 

respirable dust exposure to roof bolting personnel, this 

effect would be minimal. As was observed by MSHA 

5
6 

personnel, diluted in the mine atmosphere, the reduction in 

dust concentrations in a roof bolter exhaust is unlikely to be 

significant. In addition, the previously listed factors may 

influence the practical performance of the water exhaust 

conditioner in underground coal mines. The use of the 

water box also introduces additional maintenance measures, 

including periodic replenishment of the reservoir and 

removal of potential material accumulations. The device 

would not function as a substitute for regular replacement 

of the dust collector bag or replacement of the final paper 

filter.

Disclaimer 

Mention of any company or product does not constitute 

endorsement by the National Institute for Occupational 

Safety and Health. The findings and conclusions of this 

report are those of the author and do not necessarily 

represent the views of the National Institute for 

Occupational Safety and Health. 

References 

1. CFR (2009), “Code of Federal Regulations, Title 30, 

Mineral Resources,” Office of the Federal Register, 

National Archives and Records Service, General 

Services Administration, Washington, DC. Parts 70 

and 72. 

2. Colinet, J.F., McClelland, J.J., Erhard, L.A., 

Jankowski, R.A. (1990), “Laboratory Evaluation of 

Quartz Dust Capture of Irrigated-Filter Collection 

Systems for Continuous Miners,” Report of 

Investigations 9313, U.S. Department of the Interior, 

Bureau of Mines, Pittsburgh, PA. 

3. Colinet, J.F., Shirey, G.A., Kost, J.A. (1985), “Control 

of Respirable Quartz on Continuous Mining Sections,” 

Contract Report Number J0338033, U.S. Department 

of the Interior, Bureau of Mines, Pittsburgh, PA. 

4. Divers, E.F. (1984), “Better Roof Bolter Dust Collector 

Maintenance Reduces Silica Dust Levels,” Technology 

News No. 198, U.S. Department of the Interior, Bureau 

of Mines, Pittsburgh, PA. 

5. Fields, K.G. (1999), “Environmental Dust Survey, 

PS&HTC-DD-00-506, Number 5 Mine, Coastal Coal 

Company, Mine I.D. 46 08218,” U.S. Department of 

Labor, Mine Safety and Health Administration, 

Pittsburgh Safety and Health Technology Center, 

Pittsburgh, PA. 

6. Listak, J.M., Beck, T.W. (2008), “Laboratory and Field 

Evaluation of Dust Collector Bags for Reducing Dust 

Exposure of Roof Bolter Operators,” Mining 

Engineering, Vol. 60, Number 7, pp. 57-63. 

7. NIOSH (2008), “Work-Related Lung Disease 

Surveillance Report 2007, Volume 1,” Department of 

Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for 

Occupational Safety and Health, Division of 

Respiratory Disease Studies, DHHS (NIOSH) 

Publication No. 2008-143a. 

8. Thaxton, R.A. (1984), “Maintenance of a Roof Bolter 

Dust Collector as a Means to Control Quartz,” 

Proceedings of the Coal Mine Dust Conference, 

Morgantown, WV, pp. 137-143. 

9. U.S. Department of Labor (2009), Mine Safety and 

Health Administration, Program Evaluation and 

Information Resources, MSHA Standardized 

Information Systems, Arlington, Virginia.