and 9,000 ft3

/min. Respirable dust concentrations 

were measured downwind and upwind of the DS with 

coal mine dust personal sampling units (CMDPSU). 

These were made up of an air sampling pump pulling 

dust-laden air through a cyclone and depositing the 

respirable fraction onto a pre-weighed filter. Each 

sampling location contained two CMDPSUs and one 

real-time Personal Data Ram (PDR) instrument. The 

dust concentrations measured downwind and upwind 

of the DS were used to determine its respirable dust 

removal efficiency.

The disposable scrubber filter cartridges installed in 

the DS were successively tested during four one-hour 

tests at scrubber airflows of 3,000 and 9,000 ft3

/

min. The filters were further evaluated twice during 

subsequent two-hour tests at the scrubber airflow 

of 9,000 ft3

/min to observe scrubber performance 

during additional filter dust loading. A sound pressure 

level meter was used to measure the sound pressure 

level of the DS while operating at multiple scrubber 

airflow settings with unloaded and loaded filters.

Underground field testing was performed at two 

active mechanized mining units (MMUs) that used 

blowing face ventilation with flooded-bed scrubbers 

on the CM. The DS was placed in the last open

crosscut of the section return to examine its

effectiveness at lowering respirable dust

concentrations downwind of mining activities

Background 

Coal mine worker overexposure to coal and quartz 

dust continues to be a problem at underground coal 

mines. Effective in 2016, the U.S. Mine Safety and 

Health Administration (MSHA) changed the

regulatory standard for coal mine worker

respirable dust exposure from 2.0 mg/m3

 average 

during an eight-hour shift to 1.5 mg/m3

 average 

over the actual working shift. MSHA inspector 

dust samples from 2009 to 2012 showed that 

3.7% of roof bolter operators exceeded the new 

1.5 mg/m3

 dust standard, and 10.6% of roof 

bolter operators exceeded the quartz standard.

On-board flooded bed scrubbers on CMs are proven, 

efficient collectors of respirable dust. However, dust 

bypassing the scrubber, as well as exhaust air from 

CMs not equipped with scrubbers, may expose roof 

bolter operators who work downwind of the CM to 

high levels of respirable dust. These dust

concentrations can greatly exceed regulatory

standards and overexpose roof bolter operators. To

combat this, the National Institute for Occupational 

Safety and Health (NIOSH) contracted with

Fletcher to develop a self-tramming, stand-alone 

dry scrubber (see Figure 1), which can reduce roof 

bolter operator dust exposure by cleaning dust-laden 

air before it enters the bolter workplace entry.

Laboratory and Field Testing

Laboratory testing was conducted at NIOSH to

examine the DS’s operational performance. This

included airflow measurement, dust removal

efficiency, filter loading, and sound pressure level 

testing. All laboratory testing was conducted with a 

straight exhaust duct. A 90-degree angled discharge 

duct is also available to redirect the exhaust air to 

either side of the scrubber. Field testing was conducted 

in an underground mine to examine its effectiveness in

reducing dust levels at the face area downwind of

the CM.

To determine the average air velocity and airflow 

quantity of the DS, a 32-point equal area air velocity 

sampling grid across the discharge duct was used. A hot 

wire anemometer and pitot tube were used to measure 

the air velocities across the sampling grid to determine 

the air quantity of the DS at planned dust testing 

quantities of 3,000 and 9,000 ft3

/min.

Dust removal efficiency testing was conducted on the 

DS in NIOSH’s CM gallery. Coal dust was introduced 

at the entrance of the gallery intake and was drawn 

down the entry by the scrubber at airflows of 3,000 

Summary

A self-tramming, remotely controlled 

mobile dry scrubber (DS) was developed 

by J.H. Fletcher and Co. (Fletcher) under a 

NIOSH contract to reduce exposure of

miners to airborne dust. Roof bolter 

operator overexposure to respirable dust 

can occur when bolting downwind of the 

continuous miner (CM) during its

operation. Laboratory and underground 

testing shows that the DS can provide 

cleaner air at these downwind roof bolter 

locations.

By Scott Klima

No. 559 September 2017

Milestones in Mining Safety and Health Technology

NIOSH MINING PROGRAM

Technology News

Figure 1. Photo 

of the dry scrubber, 

commercially available 

from Fletcher.

Mobile Dry Scrubber Provides Cleaner Air for Downwind Roof Bolter

Photo by J.H. Fletcher & Co.
For More Information

operating upwind of the CM as compared to 1.80 and 1.60 mg/m3, respectively, 

when operating downwind of the CM and upwind of the DS. Overall, the DS was 

able to clean a portion of the section return air, reducing the dust concentrations at 

the face areas downwind of the CM for all cuts by 49.1% at MMU 1 and 50.5%

at MMU 2.

In order to ascertain the DS’s respirable dust removal efficiency underground, the 

face area CMDPSU and PDR instruments were placed directly into the DS exhaust 

discharge air for 2 to 5 minutes at the end of several shifts. The measured DS 

exhaust concentrations at MMU 1 and 2 averaged 0.13 and 0.01 mg/m3

,

respectively, while their corresponding upwind dust concentrations averaged 1.87 

and 1.08 mg/m3

, thereby yielding DS dust collection efficiencies of 93.2% and 

99.2%, respectively. This shows that the clean DS exhaust airflow was mixed with 

some of the dirty return airflow being delivered to the face. This airflow mixing 

was further exhibited by the DS’s 1.4 to 1.7 face airflow quantity improvement over 

the initial airflow quantities without the DS operating. 

Based on the above results, the DS appears to be a new viable dust control method 

to combat roof bolter operator dust exposures when operating downwind of the 

continuous miner.

Mobile Dry Scrubber Availability

The DS is engineered to operate between the mine rib and ventilation curtain, 

downwind of the CM and upwind of the roof bolter. It is commercially available 

from Fletcher and is MSHA permissible. A tubing discharge configuration is also 

available for the dry scrubber. 

(Figure 2). The roof bolter was monitored using area sampling to examine respirable dust concentrations when working upwind and downwind of the CM during

production, and other dust sampling locations were used for evaluating the 

respirable dust reductions when operating the DS. PDR dust concentrations were 

gravimetrically calibrated to determine dust concentrations during the particular 

mining activities. Time studies and section locations of the CM and roof bolter 

machines were used to coordinate and determine the gravimetrically adjusted 

PDR dust concentrations during scrubber operation and roof bolter activities with 

respect to the CM operation.

Test Results 

NIOSH laboratory testing of the DS showed that it averaged greater than 95% dust 

removal efficiency with disposable filters. The scrubber fan maintained a relatively 

consistent airflow near the targeted 3,000 and 9,000 ft3

/min airflow rates until 

reaching 10.5 in. water gauge of filter differential pressure at 8,420 ft3

/min of 

scrubber airflow quantity. At this point the scrubber airflow decreased with

additional dust loading because the fan had reached its maximum horsepower 

rating. Laboratory sound pressure level measurements for the scrubber showed 

that loaded filters increased sound pressure levels as compared to clean filters. 

With loaded filters at 6,000 ft3

/min of scrubber airflow, the DS reached a 90 dB(A) 

sound pressure level measured 10 ft from the scrubber outlet.

Field testing of the DS with disposable filters was completed at two underground 

MMUs. Time-weighted average respirable dust concentrations were measured 

at each MMU with respect to CM activities. The average roof bolter machine dust 

concentrations at MMU 1 and 2 were 0.59 and 0.17 mg/m3

, respectively, when 

For more information on the dry scrubber, contact Scott Klima (412-386-6468, sklima@cdc.gov), John Organiscak (412-386-6675, 

jorganiscak@cdc.gov), or the NIOSH Mining Program (mining@cdc.gov).

To receive NIOSH documents or for more information about occupational safety and health topics, contact:

1-800-CDC-INFO (1-800-232-4636), 1-888-232-6348 (TTY), or request information at the CDC website at 

www.cdc.gov/info, or visit the NIOSH website at www.cdc.gov/niosh.

DHHS (NIOSH) Publication No. 2017–208

Disclaimer: Mention of any company or product does not constitute endorsement by NIOSH.

Figure 2. Underground mine application of the DS cleaning air downwind of the continuous miner and upwind of the roof bolter.

Accessible Version: https://www.cdc.gov/niosh/mining/content/dryscrubber.html