Evaluation of Dust Collector Bags for Reducing Dust Exposure 

of Roof Bolter Operators

Objective 

To evaluate the effectiveness of dust collector bags for 

reducing dust liberation from a roof bolter dust collection 

system. 

Background 

Respirable dust exposure in underground coal mines during 

the roof bolting process continues to be a problem for roof bolter 

operators. During 2000–2004, Mine Safety and Health 

Administration (MSHA) inspectors collected nearly 5,000 

respirable dust samples for roof bolting occupations. Of these 

samples, 20% exceeded a respirable silica dust concentration of 

100 μg/m3

, a level that MSHA considers excessive. From these 

data, it is clear that roof bolter occupations exhibit a continued 

risk for overexposure to respirable silica dust. 

 Most roof bolting machines use an MSHA-accepted (30 CFR 

33) vacuum dust collection system to capture dust as holes are 

drilled. A vacuum pump on the machine draws the dust through 

the bit and drill steel into an enclosed dustbox. The box has 

several compartments and functions as a rough size classifier, 

allowing the coarser dust sizes to settle out of the airstream first 

in the large compartment (about 95% of all the dust entering the 

box). The dust that passes through the large compartment is 

routed through cyclones and then into the filter chamber for 

deposition on a paper canister filter. The filtered air flows 

through the vacuum pump, a noise-reducing muffler, and then is 

exhausted into the mine environment. The dustbox and filter are 

usually cleaned on a preselected schedule to avoid overfilling the 

box and/or overloading the filter. Normally the dustbox is 

emptied at the end of every cut. As the filter accumulates fine 

particles of dust, resistance increases and flow through the system decreases, requiring removal and cleaning of the filter, 

usually after several cuts. 

 The majority of the dust is collected in the main (large) 

chamber of the collector. This dust is removed from the collector 

box by opening the door and pulling a “rake” toward the opening 

to drag the dust out, allowing it to dump onto the mine floor. 

Operator exposure occurs when this dust is entrained into the air 

as it falls from the box to the floor. Another source of exposure 

is from the canister filter. Dust that is too fine to be captured in 

the main and subsequent dustbox chambers passes through to the 

filter. Typically, filters are cleaned by shaking or tapping against 

the rubber tire of the bolter or a hard surface. This method of 

cleaning often creates a respirable dust cloud that contaminates 

the breathing area of the operators. The operator must take care 

not to damage the filter, filter seal, or dustbox door seal during 

the cleaning cycle, as dust not captured by the filter or that 

bypasses the filter seal is exhausted into the mine air. In 

addition, care must be taken to stay upwind of the dustbox 

during cleaning. 

Approach 

A laboratory study was conducted to evaluate the use of 

vacuum bags in the dust collector for improving dust capture 

efficiency. A Fletcher® dust collector box, typical of many 

found on underground bolters, and a model AP–228 dustbag 

were configured in the lab to simulate roof bolter drilling dust 

collection. The bags are MSHA-accepted for retrofit in 

Fletcher® dustboxes. Testing was performed with and without 

the bag installed. Airflow through the box was provided by a 

vacuum pump rated at 60 cfm at 20 inches Hg. Dust was fed into 

the box using an AccuRate bulk dust feeder with a 21⁄4-in screw. 

Fifty pounds of ground limestone, similar in size distribution 

to dust collected from bolter dust collectors in use, was used as 

feed material for each test. The bags and canister filter were 

weighed after each test to determine the dust capture inside the 

box. Dust concentrations in the exhaust were recorded continuously in a 6-in-diam pipe using a RAM–1 instantaneous dust 

monitor. Aerodynamic particle sizes were measured in the 

exhaust using a TSI Aerodynamic Particle Sizer (APS). A laptop 

computer was interfaced with the APS and data acquired using 

the available TSI software. Aerosol was drawn isokinetically 

into the diluter at 5 L/min through a 4-ft length of 0.31-in ID 

conductive tubing. Particle sizes were measured in the 6-in-diam 

pipe, roughly 8 ft away from the entry of the 2-in hose into the 

PVC pipe. Vacuum pressures within the box and across the filter 

were also recorded continuously during each test. A total of 60 

tests were conducted (30 with the bag installed and 30 without). 

No. 523 February 2007 
Figure 1.—Condition of main chamber before test (top), after test 

without bag (center), and after test with bag (bottom – note retrofit 

kit installed). 

Results 

 The laboratory dust collector tests showed that 99.6% of the 

test dust fed into the collector was captured by the dustbags. 

Figure 1 shows the condition of the main chamber before and 

after testing. Total weight gain on the canister filter was over 

Figure 2.—Comparison of total weight gain on canister filter after 

30 tests for each condition. 

five times greater without use of the bag (Figure 2). The RAM–1 

unit showed respirable dust levels in the collector exhaust to be 

over two times higher when tests were conducted without the 

bags in place. The APS showed that the number of total dust 

particles emitted from the exhaust was two times greater when 

the tests were conducted without the bags. Since nearly all the 

dust is contained in the bag, operator exposure is improved when 

emptying the collector box’s main chamber. Filter loading is 

greatly decreased when using the bags, enabling longer periods 

of drilling without filter removal/cleaning. Pressure drop across 

the filter for all tests ranged from 3.0 to 3.3 in w.g. when the bag 

was used and from 4.0 to 8.4 in w.g. without the bag. Filtered air 

emitted from the collector has less respirable dust and fewer 

total dust particles when the bags are used. These results show 

that benefits from use of the bags are realized in all areas of 

operator exposure. In order to use these bags in underground 

coal mines, the dustbags must be accepted by MSHA as an 

optional item for the specific dust collection system and machine 

model (30 CFR 33). In addition, the collector must be equipped 

with a predump option and must have a retrofit kit installed 

inside the collector to connect to the bag. 

For More Information 

 For more information on roof bolter dust collector bags, 

contact Jeffrey M. Listak (412–386–5082, JListak@cdc.gov) or 

Timothy W. Beck (412–386–4776, TBeck@cdc.gov), NIOSH 

Pittsburgh Research Laboratory, P.O. Box 18070, Pittsburgh, PA 

15236–0070. 

To receive other information about occupational safety and 

health topics, call 1–800–35–NIOSH (1–800–356–4674), or 

visit the NIOSH Web site at www.cdc.gov/niosh. 

Mention of any company name or product does not constitute 

endorsement by the National Institute for Occupational Safety 

and Health.

DHHS (NIOSH) Publication No. 2007–119

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