SME Annual Meeting

Feb. 21 - 24, 2016, Phoenix, AZ

1 

Preprint 16-010 

EXAMINATION OF A NEWLY DEVELOPED MOBILE DRY SCRUBBER (DS) FOR COAL MINE DUST CONTROL APPLICATIONS

J. A. Organiscak, NIOSH, Pittsburgh, PA

J. Noll, NIOSH, Pittsburgh, PA

D. Yantek, NIOSH, Pittsburgh, PA

B. Kendall, J. H. Fletcher & Co., Huntington, WV

ABSTRACT

A self-tramming remotely controlled mobile Dry Scrubber (DS) 

was developed by J. H. Fletcher and Co. under a contract with the 

National Institute for Occupational Safety and Health’s Office of Mine 

Safety and Health Research (NIOSH OMSHR) to reduce exposure of 

miners to airborne dust. NIOSH OMSHR laboratory testing showed 

that the DS averaged greater than 95% dust removal efficiency with 

the disposable filters and averaged 88% and 90%, respectively, for the 

optional washable filters in their pre-washed and post-washed test 

conditions. Although the washable filters could be reused, washing 

them generated personal and downstream respirable dust 

concentrations of 1.2 and 8.3 mg/m3

, respectively, for a 10-minute 

washing period. The DS’s velocity pressure regulated variable 

frequency drive (VFD) fan maintained a relatively consistent airflow 

near the targeted 1.43 and 4.25 m3

/s (3000 and 9000 ft3/min) airflow 

rates during most of the laboratory dust testing until reaching its 

maximum 60 Hz fan motor frequency or horsepower rating at 2610 Pa 

(10.5 in. w.g.) of filter differential pressure and 3.97 m3

/s (8420 ft3/min) 

of scrubber airflow quantity. Laboratory sound level measurements of 

the DS also showed that the outlet side of the scrubber was noisier 

and the loaded filters increased sound levels as compared to clean 

filters at the same airflow quantities. With loaded filters the scrubber 

reached a 90 dB(A) sound level at 2.83 m3

/s (6000 ft3/min) of scrubber 

airflow, indicating that miners should not be overexposed in relation to 

MSHA’s permissible exposure level (PEL) of 90 dB(A) at or below this 

airflow quantity (1). The DS’s washable filters were not used during 

field testing because of their lower respirable dust removal efficiency 

and the airborne dust generated from filter washing. Field testing the 

DS with the disposable filters at two underground coal mine sections 

showed that it can clean a portion of the section return air and provide 

about a 50% dust reduction at the face area downstream of the 

continuous miner (CM) operation.

INTRODUCTION

Coal mine worker overexposure to coal and quartz dust continues 

to be a problem at underground coal mining operations in the U.S. The 

U.S. Mine Safety and Health Administration (MSHA) recently changed 

the 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, effective as of August 1, 2016 (1). If more than 

0.1 mg/m3 quartz mass is found in the coal mine worker dust sample, 

the applicable respirable dust standard is further reduced to the 

quotient of 10 divided by the percentage of the quartz in the sample 

(1). Over 90% of the mechanized mining units operating in U.S. 

underground coal mines are continuous mining machines (2). MSHA 

inspector dust samples from 2009 to 2012 showed that 3.7% of 

continuous miner operators exceeded the 2.0 mg/m3 dust standard, 

while 8.8% of these dust samples exceeded the new 1.5 mg/m3 dust 

standard and 9.7% of these samples exceeded the reduced quartz 

standard (3). Additionally, roof bolter dust samples at these 

mechanized mining units during this same period have exceeded the 

2.0 mg/m3

, 1.5 mg/m3, and reduced quartz levels at 1.1%, 3.7%, and 

10.6%, respectively (3).

On-board flooded bed scrubbers on continuous mining machines 

are proven, efficient collectors of respirable dust (4-7). However, dust 

by-passing the scrubber may expose roof bolter operators who work 

downwind of the continuous miner to high levels of respirable dust (7). 

Respirable dust concentrations downwind of the continuous miner can 

greatly exceed regulatory standards and overexpose roof bolter 

operators, especially when the continuous miner does not employ a 

flooded bed scrubber (7). To combat the upstream continuous miner 

dust source, a portable stand-alone scrubber can be strategically 

placed to clean the dust-laden air before it enters the roof bolter 

workplace entry, providing the bolter operators with a cleaner air 

supply and thereby reducing their dust exposures.

J. H. Fletcher and Co. (Fletcher) of Huntington, WV has 

developed a self-tramming stand-alone Dry Scrubber (DS or simply 

referred to as “scrubber” throughout this report) under NIOSH Contract 

No. 200-2010-36164, “The Development of Dust Control Units for 

Underground Coal Mines” (8). This prototype scrubber shown in Figure 

1 was delivered to NIOSH OMSHR for operational and dust collection 

efficiency testing. The general specifications of this prototype DS are:

• Machine Dimensions : 1.22 m wide x 1.22 m high x 4.79 m

long (4 ft wide x 4 ft high x 15.7 ft long)

• Air Mover: 22.4 Kw (30 hp.) vane axial fan (480 V) with

variable frequency drive (VFD) speed controller

• Filtration: Dual 71-cm (28-in.) O.D. cylindrical air filters rated

at 99% efficiency for 2-μm particles

• Tram System: Crawler tram hydraulically controlled via

remote transceiver

• Hydraulic System: Remote transceiver controlled, 12 V pilot

solenoid function operation, variable flow axial piston pump

with designated 30 hp. electric motor (480 V)

Figure 1. Picture of the self-tramming Dry Scrubber (DS) prototype.

Laboratory testing was initially conducted at NIOSH OMSHR to 

examine the DS’s respirable dust collection efficiency and its 

operational performance with disposable or washable filter cartridges. 

Functional and operational modifications were made during and after 

the laboratory testing to prepare it for further underground field testing. 
2 

Field testing was conducted at two sections in an underground mine to 

examine its effectiveness in reducing dust levels at the face area 

downstream of the continuous mining machine. This report describes 

the results from both the DS laboratory and field testing. 

LABORATORY TESTING 

The Fletcher DS was tested for operational performance and dust 

collection efficiency in the continuous miner gallery at NIOSH 

OMSHR’s laboratory in Pittsburgh, PA. DS testing was conducted in 

the intake entry of the continuous miner dust gallery as shown in 

Figure 2. Airflow measurements were initially made on the DS to 

validate its fan-controlled variable frequency drive response to its 

internal pitot tube velocity pressure measurement. Several 

modifications were initially made by Fletcher to the velocity pressure 

transducer and the VFD fan controller to improve agreement with the 

preset DS airflow quantity and the amount delivered as the filter loaded 

with dust. After the DS’s airflow controller response was improved, 

dust removal efficiency and filter loading tests were performed with the 

disposable filter cartridges and the alternative washable (re-useable) 

filter cartridges

Airflow Measurements

Eight air velocity sampling holes were drilled 7.6 centimeters (3 

inches) downstream of the fan discharge transition point along the top 

cross section of the 121.9-cm-wide by 30.5-cm-high (48-in.-wide by 12-

in.-high) discharge duct. This provided a stationary 32-point equal area 

air velocity sampling grid at 4 measurement heights and 8 horizontal 

locations across the duct for determining the average air velocity and 

air flow quantity of the DS. The DS’s fan had a VFD controller which 

was regulated by a pitot tube velocity pressure transducer 

measurement at the center of the discharge duct. A TSI Model 8346 

VelociCALC hot wire anemometer (TSI, Inc., Shoreview, MN) was 

used by NIOSH to measure the air velocities across the sampling grid, 

which were averaged and multiplied by the 0.372-m2 (4-ft2

) duct area to 

determine the air quantity of the DS at planned dust testing quantities 

of 1.42 and 4.25 m3

/s (3000 and 9000 ft3/min). A one-minute moving 

traverse was also made across the scrubber’s 0.372-m2 (4-ft2

) 

discharge area before and after the hot wire measurements with a 

high-speed vane anemometer (Davis Instrumentation Mfg. Co., 

Baltimore, MD) mounted on a 121.9-cm (4-ft) extension wand to 

measure its average air velocity and quantity at the preset airflows. 

Finally, a Series 166T telescoping stainless steel pitot tube with a 0 to 

1.0” water gage magnehelic differential pressure gage (Dwyer 

Instruments, Michigan City, IN) was also used to measure the velocity 

pressures and average air velocity across the sampling grid in 

conjunction with additional vane anemometer measurements at the 

scrubber discharge. These pitot tube measurements were also used to 

identify several stable velocity pressure monitoring grid locations for 

continuous monitoring of DS airflow during NIOSH’s dust removal 

efficiency and filter loading tests.

Dust Testing Procedures

Dust efficiency testing was conducted on the DS at a low and high 

airflow quantity of 1.42 and 4.25 m3

/s (3000 and 9000 ft3/min), 

respectively. The targeted airflow of 1.43 m3

/s (3000 ft3/min) is the 

minimum ventilation airflow quantity allowed to a working coal face 

where coal is being cut, mined, drilled for blasting, or loaded (1). The 

targeted airflow of 4.25 m3

/s (9000 ft3/min) is the minimum ventilation 

airflow quantity allowed at the last open crosscut of each set of entries 

or rooms (1), which is the planned operating location for the DS to 

clean a portion of the airflow for the roof bolting machine when 

operating downstream of the CM.

During laboratory dust testing, the velocity pressure inside the 

exhaust duct and the differential pressure across the filters were 

continuously monitored and recorded during the dust efficiency testing. 

The negative differential pressure across the filters was measured with 

either a 0- to 2490-Pa (0- to 10-in w.g.) or a 0- to 4980-Pa (0- to 20-in 

w.g.) magnehelic pressure instrument with a 4- to 20-mA output

(Dwyer Instruments, Inc., Michigan City, IN) connected with Tygon 

tubing to a copper tube inserted inside the filter cartridges between the 

gasket seals. The velocity pressure was measured with a 0- to 249-Pa 

(0- to 1-in w.g.) magnehelic pressure instrument with a 4- to 20-mA 

output (Dwyer Instruments, Inc., Michigan City, IN) connected to a 

Dwyer Series 166T telescoping stainless steel pitot tube inserted into 

the exhaust duct. The pitot tube was initially inserted at grid location 12 

where its velocity measurement was close to the grid average and later 

placed at a higher air velocity location of 14 during most of the dust 

testing to increase the velocity pressure resolution at the lower 

scrubber quantity of 1.42 m3

/s (3000 ft3/min). These instruments were 

electronically recorded by a Telog R-3307 seven-channel data 

acquisition system (Telog Instruments, Inc., Victor, NY).

Coal dust was introduced at the entrance of the intake entry of the 

continuous miner gallery as shown in Figure 2. The coal dust used was 

Pulverized Keystone Mineral Black 325BA (Keystone Filler & 

Manufacturing Co., Muncy, PA), which is -325 mesh (-44 μm) 

Pocahontas No. 3 coal dust, with 45% of this dust less than 10 μm in 

size. This pulverized coal dust was fed into the gallery through a screw 

feeder (Vibra Screw, Inc., Totowa, NJ) and two LH-1/2 brass eductors 

(Penberthy, Prophetstown, IL) operating between 4 and 6 psig of 

compressed air. The eductors aerosolized and discharged the dust 

through two hoses at mid-entry height located 1/3 of the entry width 

away from the left and right side of the entry. The dust cloud was 

drawn down the entry by the scrubber operating at air quantities of 

1.42 and 4.25 m3

/s (3000 and 9000 ft3/min). A sealed curtain wall was 

constructed around the scrubber (with a door) to separate the inlet and 

outlet ends of the scrubber, thereby isolating the upstream (dirty) and 

downstream (cleaner) airstreams inside the gallery (see Figure 2). An 

entrance door near the continuous miner gallery face area was left 

open during testing so that the pressure differential across the 

temporary wall around the scrubber would remain neutral when 

operating both the scrubber and gallery fan. The gallery’s ventilation 

airflow quantity was set at approximately 0.47 m3

/s (1000 ft3/min) 

higher than the targeted scrubber airflow quantity by adjusting the 

regulator doors in the shared return air course between NIOSH’s 

longwall and continuous miner galleries (see Figure 2). Airflow quantity 

of the gallery was determined and adjusted by measuring the average 

velocity across the 18.9 ft2 entrance area of the continuous miner 

gallery return with a vane anemometer (one minute moving traverse),

before the operation of the scrubber. After the gallery airflow quantity 

was preset for the test, the scrubber airflow was preset to the desired 

airflow quantity and measured with a vane anemometer traverse of the 

scrubber discharge before and after each test. All dust efficiency 

testing was conducted with the straight exhaust configuration. An 

optional 90-degree angled discharge duct is available to redirect the 

exhaust air to either side of the scrubber but was not used during the 

laboratory dust testing. 

Figure 1. Plan view of Dry Scrubber (DS) testing location in 

continuous miner gallery (not to scale).

Respirable concentrations were measured at two locations 

downstream and upstream of the DS as shown in Figure 2. Location 

numbers 1 and 2 were positioned 3.05 m (10 ft) downstream of the 

scrubber exhaust at mid-entry height and 1/3 of the entry width away 

from the left and right side walls of the entry. Location numbers 3 and 4 
3 

were positioned 3.05 m (10 ft) upstream of the scrubber inlet at midentry height and 1/3 of the entry width away from the left and right side 

walls of the entry. Gravimetric respirable dust concentrations were 

measured with coal mine dust personal sampling units (CMDPSU), 

comprised of an ESCORT-Elf constant flow air sampling pump pulling 

dust-laden air at 2.0 L/min through a 10-mm nylon cyclone (respirable 

dust classifier) and depositing the respirable fraction onto a preweighed 37-mm filter cassette (Zeffon International, Ocala, FL). A pair 

of dust samplers (CMDPSU) and one real-time Personal Data Ram 

(PDR model 1000) instrument were placed and operated at each of the 

4 sampling locations during each test. The average of the gravimetric 

dust concentrations measured downstream (from locations 1 and 2) 

and upstream (from locations 3 and 4) of the DS were used to 

determine the DS respirable dust removal efficiency for each test. 

Respirable gravimetric concentrations used in this analysis were not 

adjusted to MSHA’s MRE compliance sample equivalents (multiplied 

by 1.38).

DS dust testing was performed with a set of disposable and 

washable filter cartridges. The washable filters were also tested in their 

pre- and post-washed conditions. Each set of scrubber filters were 

successively tested during 4 one-hour test replicates at two scrubber 

preset airflows of 1.43 and 4.25 m3

/s (3000 and 9000 ft3/min). After 

completion of the one-hour tests, the filters were further evaluated 

during two-hour tests at the scrubber preset airflow of 4.25 m3

/s (9000 

ft3

/min) to observe for diminished scrubber performance from additional 

filter dust loading. The number of additional 2-hr tests performed on 

the disposable, pre-washed, and post-washed filters were 2, 3, and 1, 

respectively. The dust concentrations generated for these tests varied 

somewhat between the different airflow tests but averaged 17.8 + 3.3 

mg/m3, 17.4 + 3.7 mg/m3, and 19.1 + 3.9 mg/m3 (at the 95% 

confidence level), respectively, for the disposable, pre-washed, and 

post-washed filter tests. After the pre-washed filter tests were 

completed, the filters were removed from the scrubber and backflushed from the inside of the filter cartridge with a standard garden 

hose and twist nozzle, until the water running off the filters was 

relatively clear. A real-time Personal Data Ram (PDR model 1000) 

instrument was worn by the person washing the filter and another PDR 

was placed 10-ft downstream in the gallery from where the filters were 

washed over a 10-minute period. These PDR dust concentrations were 

adjusted or calibrated to the CMDPSU’s gravimetric concentrations 

measured during these laboratory tests. The post washed filters were 

set out and air dried for about 24 hours (filter media damp to the touch) 

and run for about another hour in the scrubber at 1.43 m3

/s (3000 

ft3

/min) to completely dry them out before post-washed filter dust 

testing.

Sound Level Measurements

Sound level measurements were also taken on the DS operating 

inside the dust gallery with an unloaded disposable filter before dust 

testing and with a loaded washable filter at the completion of dust 

testing. A tripod-mounted Larson Davis LxT sound level meter was 

used to measure the equivalent continuous sound level 3.05 m (10 ft) 

from the inlet side of the scrubber and 3.05 (10 ft) from the outlet side 

of the scrubber at a height of 1.5 m (59 in.) with the microphone 

pointing at the scrubber. The measurement locations were 

approximately centered on the width of the scrubber. Initial sound level 

measurements were made at three scrubber airflow settings with the 

unloaded disposable filters (clean filters) before dust testing began. A 

second set of sound level measurements were made for eight scrubber 

airflow settings with the loaded washable filters (loaded filters) after 

completion of all dust testing. A vane anemometer was swept across 

the discharge of the scrubber to determine the airflow at each scrubber 

operating point. For each airflow setting, three 15-second-long sound 

level measurements were taken and the logarithmic average of the 

three sound level measurements was calculated by equation 1 shown 

below.

( ( )) 10/ 10/ 10/ 1, 2, 3, 103/1log10 10 10 LAeq LAeq LAeq LAeq = ++ (1)

LABORATORY TEST RESULTS

Table 1 shows the airflow quantities measured with the hot wire, 

pitot tube, and vane anemometer as compared to the preset DS 

airflows of 1.42 and 4.25 m3

/s (3000 and 9000 ft3/min). This table 

shows that there was initial disagreement between the lower DS 

airflow target and the measured airflow quantities. After Fletcher visited 

NIOSH OMSHR’s laboratory and made velocity pressure transducer 

and VFD fan controller modifications, better agreement was achieved 

between the preset DS airflow quantity and airflow measurements. 

Once the DS’s airflow controller response was improved, dust removal 

efficiency and filter loading tests were performed with the disposable 

filter cartridges and the washable filter cartridges.

Table 1. Comparison of hot wire and pitot tube measurements to vane 

anemometer discharge measurements.

Airflow Instrument,

Quantity Testing Period DS Target of 1.42 m3/s

DS Target of 4.25 

m3/s

Hot Wire (Vane Anemometer),

m3/s Initial DS 1.05 (1.11) 4.32 (4.33)

Pitot Tube (Vane Anemometer),

m3/s Initial DS 1.01 (1.16) 4.04 (4.38)

Hot Wire (Vane Anemometer),

m3/s Modified DS 1.42 (1.45) 4.34 (4.18)

Pitot Tube (Vane Anemometer),

m3/s Modified DS 1.34 (1.42) 4.03 (4.19)

Figures 3 and 4 show the DS’s dust collection efficiency results 

and average scrubber airflow measurements (vane anemometer) with 

the disposable filter cartridges and washable filters tested, 

respectively. Figure 5 shows the differential pressures of the filters 

tested with respect to the filter dust loading. Filter dust loading is the 

accumulated respirable dust mass that was put through the scrubber 

during the individual filter tests, amassed from the upstream respirable 

dust concentrations, average DS airflow quantities, and test times.

Figure 2. Disposable filter dust efficiency and airflow test results with 

respect to operating time.

Figure 3. Washable filter dust efficiency and airflow test results with 

respect to operating time.

The DS averaged greater than 95% dust removal efficiency with 

the disposable filters and had consistent dust removal efficiencies for 

the two airflows tested (see Figure 3). After 8 hours of operation, the 

scrubber airflow quantity with the disposable filters could not be 

maintained above 4.00 m3

/s (8500 ft3/min), indicating that the fan 
4

reached its maximum 60 Hz fan motor frequency or horsepower rating. 

After the 8th hour of dust testing, the maximum scrubber airflow 

quantity that could be achieved was 3.97 m3

/s (8420 ft3/min) at a filter 

differential pressure of 2610 Pa (10.5 in. w.g.). Figure 5 further 

illustrates decreasing DS airflow quantities with additional filter dust 

loading after 8 hours of testing. DS airflow quantities decreased to 3.80 

m3

/s (8056 ft3/min) for 1.98 kg of filter dust loading and 2690 Pa (10.8 

in. w.g.) of filter differential pressure, and decreased to 3.60 m3

/s (7630 

ft3

/min) for 2.27 kg of filter dust loading and 2750 Pa (11.0 in. w.g.) of 

filter differential pressure.

Figure 5. Differential pressure of scrubber filters with respect to filter 

dust loading (labeled points show the average scrubber airflow 

quantities measured for the last two filter dust tests). 

Operating the DS with the washable filters showed them to be 

less efficient and more inconsistent than the disposable filters (see 

Figure 4). The dust removal efficiency of these filters averaged 88% 

and 90%, respectively, for their pre-washed and post-washed test 

conditions. The efficiency of the pre-washed filters initially started at 

86%, peaked at 97% after 5 hours of testing, and dropped off to 66% 

after 14 hours of testing. Scrubber airflow remained fairly steady during 

all the prewashed filter dust testing. The efficiency of the post-washed 

filters initially started at 95% and inconsistently dropped to 86% at 

which time the airflow decreased noticeably to below 4.00 m3

/s (8500 

ft3

/min) for the final test. When washing these filters between the preand post-testing periods, the average personal and downstream PDR 

dust concentrations measured were 1.2 and 8.3 mg/m3

, respectively.

Figure 5 further illustrates that the pre-wash filters had lower 

differential filter pressures as compared to the disposable filters and 

experienced no significant scrubber airflow reductions from 2.6 kg of 

filter loading with 2010 Pa (8.1 in. of w.g.) of filter differential pressure. 

The post-washed filters exhibited notably higher filter differential 

pressures during testing, and scrubber airflow notably decreased to 

3.69 m3

/s (7820 ft3/min) for 2.01 kg of filter dust loading and 2690 Pa 

(10.8 in. w.g.) of filter differential pressure for the final test.

The results of the sound level measurements are shown in Figure 

6. This graph clearly shows that the scrubber operating with loaded

filters had higher sound levels than with clean filters for similar 

scrubber airflow quantities. Sound levels were also higher at the 

scrubber outlet location as compared to the scrubber inlet location. 

Because 90 dB(A) is MSHA’s permissible exposure level, the sound 

levels (labeled in the graph) at 2.83 m3

/s (6000 cfm) of scrubber airflow 

indicate that miners should not have noise exposures that exceed the 

PEL for airflow rates at or below at 2.83 m3

/s (6000 cfm) (1). If the 

machine is used at airflows near the upper end of its operating range, 

miners working nearby could be overexposed to noise. However, 

miners’ exposure would vary depending on the distance from and the 

time spent near the scrubber.

UNDERGROUND FIELD TESTING

After laboratory testing the scrubber was returned to Fletcher’s 

research facility for inspection and preparation for underground testing. 

Several modifications were made to the DS prototype to facilitate 

completion of the underground coal mine testing portion of the NIOSH 

contract. The primary modification needed was substituting and 

reprograming an existing MSHA-approved remote control unit for the 

desired remote control unit tested in the laboratory, which was 

currently pending an MSHA approval for underground coal mine use. It 

was also decided to use only the disposable filters during underground 

testing, since they provided the highest respirable dust removal 

efficiency and they eliminated the option of generating respirable dust 

from being cleaned underground. NIOSH visited Fletcher’s research 

facility to examine these modifications and to re-measure the DS 

airflow quantity output as compared to the machine-selected airflow 

quantity with the new remote control unit. These pitot tube 

measurements were used to correlate a reliable sample location for 

measuring the scrubber airflow quantity underground. Scrubber field 

testing was conducted at two continuous miner sections in an 

underground coal mine.

Figure 6. Sound level measurement results for the Dry Scrubber with 

clean and loaded filters.

Airflow Measurements

NIOSH airflow measurements made at Fletcher’s research facility 

were again made along the 32 equal area sampling grid inside the 

121.9-cm-wide by 30.5-cm-high (48-in.-wide by 12-in.-high) discharge 

duct using a Series 166T telescoping stainless steel pitot tube and a 0 

to 1.0” water gage magnehelic differential pressure gage (Dwyer 

Instruments, Michigan City, IN). The DS’s VFD controller was operated 

at preset airflow quantities of 1.42, 2.83, and 4.25 m3

/s (3000, 6000, 

and 9000 ft3

/min). Pitot tube measurements at the three DS preset air 

quantities were conducted with the straight exhaust duct and with the 

optional 90° angled discharge duct. Average duct velocity to grid point 

velocity ratios were examined for the most consistent and reliable 

sample locations for underground scrubber airflow measurements. A 

pitot tube measurement at a reliable sampling location was used to 

determine the scrubber airflow velocity during the underground field 

studies.

Airflow quantities to the face area cleaned by the scrubber were 

also measured during the study to indicate the scrubber’s operational 

impact on face airflow quantity. A one-minute moving traverse was 

made over the cross-sectional area of the face curtain opening with a 

vane anemometer (Davis Instrumentation Mfg. Co., Baltimore, MD) 

without and with the scrubber turned on.

Underground Dust Sampling

Underground field testing investigated the dust reductions 

realized from operating the DS downstream of the continuous miner at 

two producing mining machine units (MMUs). One MMU was on the 

right side of a 9-entry super section and the other MMU was on the 

right side of another 12-entry super section. Fish tail ventilation was 

used at both super sections to supply intake air to the MMUs studied. 

The MMUs used blowing face ventilation with flooded-bed scrubbers 

on the continuous miners. In order to increase the DS operating time 

and data collection underground, the DS was placed in the last open 

crosscut of the section return to examine its effectiveness on lowering 

respirable dust concentrations at the face areas of entries 9 and 12, 

downstream of all mining activities. This test strategy minimized 

moving the DS around in the section while providing the least 
5

interference with production. Figure 7 illustrates the DS location in the 

last open crosscut of entry 9 at the first super section studied and the 

areas that were dust sampled during the field studies. The optional 90° 

angled discharge duct was used during the underground studies to 

direct the DS’s exhaust air along the blowing face ventilation curtain. 

Dust sampling locations sampled during the study included (as shown 

in Figure 7): A) the intake to the bolting machine; B) the bolting 

machine near the left-side operators’ location; C) the last open 

crosscut just upstream of the DS; D) the downstream face area 

cleaned by the DS (entry 9 or 12); and E) the section return entry. The 

bolting machine was monitored to examine the respirable dust 

concentrations at this location when working upstream and 

downstream of the continuous miner during the production shift. The 

other dust sampling locations were used for evaluating the respirable 

dust reductions realized from operating the scrubber. The sampling 

instrumentation used at each sampling location during this field study 

was identical to those used in the laboratory (two CMDPSUs and one 

PDR) as described above. Respirable gravimetric concentrations used 

in this analysis were not adjusted to MSHA’s MRE compliance sample 

equivalents (multiplied by 1.38).

Figure 7. Plan view of DS operation downstream of the continuous 

mining machine on the 9-entry super-section studied using blowing 

face ventilation (right-side MMU, capital letters indicate dust sampling 

locations).

The PDRs’ instantaneous dust concentration data was timerecorded every 10 seconds in its internal memory and was 

downloaded to a computer after the sampling shift. The PDR real-time 

dust concentrations were gravimetrically calibrated by multiplying each 

10-second reading by a gravimetric to PDR dust concentration ratio, 

determined by dividing their average concentrations measured over 

identical sampling periods. These adjusted PDR dust concentrations 

were be used to determine dust concentrations during the particular 

mining activities. Time study and section location of the continuous 

miner and roof bolter machines were recorded during each sampling 

shift. This information was used to coordinate and determine the 

gravimetrically adjusted PDR dust concentrations during time 

segments of the scrubber operation and roof bolter activities with 

respect to the continuous miner operation. The DS was moved and 

parked outby in the section return entry when the continuous miner 

was cutting in return entries 9 and 12 so as not to inhibit shuttle car 

haulage. The dust samplers in the last open crosscut, the face area, 

and the section return were also relocated while mining in return 

entries 9 and 12.

Field Test Results

The pitot tube measurements made inside the exhaust duct of the 

scrubber at Fletcher’s research facility showed the average duct 

velocity to grid velocity ratios were the most consistent near the center 

of the duct for the average airflow quantities of 1.42, 2.73, and 3.90 

m3

/s (3000, 5790, and 8260 ft3/min) measured with both exhaust 

configurations (straight and 90 degree). Figure 8 shows the average, 

minimum, and maximum velocity ratios for each of the stationary 32-

grid sampling locations (4 measurement heights at 8 horizontal 

locations across the exhaust duct). As can be seen in this figure, some 

of the smallest velocity ratio variations were at grid locations 15, 16, 

and 19, having duct velocity/grid velocity ratios of 0.76, 0.85, and 0.71, 

respectively, with the lowest measured velocity ratio ranges of 0.04. 

Since the airflow velocity ratio at location 16 was nearest the grid 

average, it was selected as the pitot tube location for measuring the 

DS airflow quantity underground. The DS airflow quantity was 

determined from multiplying the airflow velocity measurement by the 

0.85 ratio and 4-ft2 duct area.

Figure 8. Average duct velocity to grid velocity ratios measured along 

the DS’s exhaust duct sampling grid.

Table 2 shows the time-weighted average respirable dust 

concentrations measured with respect to the continuous mining 

activities at both of the sections studied. These concentrations 

represent gravimetrically calibrated PDR dust levels averaged for the 

time periods when roof bolting machine was operating upstream and 

downstream of the continuous mining machine. Average dust 

concentrations at the roof bolting machine at sections 1 and 2 were 

0.59 and 0.17 mg/m3

, respectively, when operating upstream of the 

CM as compared to 1.80 and 1.60 mg/m3

, respectively, when it was 

operating downstream of the CM and upstream of the DS. The dust 

concentrations at the last open crosscut of the scrubber location at 

sections 1 and 2 were 2.77 and 2.43 mg/m3

, respectively, when the 

roof bolting machine was upstream of the CM as compared to 1.85 and 

1.35 mg/m3

, respectively, when the bolting machine was downstream 

of the CM. The higher crosscut dust concentrations measured when 

the roof bolter was upstream of the continuous miner was likely due to 

the CM operating physically closer to the last open crosscut sampling 

location, thereby reducing the time for dust dilution and dispersion of 

the CM return concentrations by the section ventilation.

Table 2. Time-weighted averages of roof bolter and DS dust 

concentrations with respect to the CM location.

Bolter 

Location

Mine Test 

Section

Intake Dust 

Conc. to Roof 

Bolter

A

Roof Bolter 

Dust Conc.

B

Crosscut Dust 

Conc.

Up-stream DS

C

Face Dust Conc.

Down-stream DS

D

Section Return 

Dust Conc.

E

‡

DS Face Dust 

Reduction 

Efficiency

MMU mg/m3 (cuts) mg/m3 (cuts) mg/m3 (cuts) mg/m3 (cuts) mg/m3 (cuts) %

Upstream CM

†

1 0.54 (6) 0.59 (6) 2.77 (6) 1.32 (6) 1.99 (6) 52.9

2 0.22 (9) 0.17 (9) 2.43 (5) 0.91 (5) 0.96 (5) 59.9

Downstream CM

& Upstream DS

†

1 1.96 (6) 1.80 (6) 1.85 (7) 0.99 (7) 1.51 (7) 46.3

2 1.58 (4) 1.60 (4) 1.35 (3) 0.66 (3) 1.20 (3) 50.7

Downstream CM

& Downstream DS

†

1 NA (0) NA (0) NA (0) NA (0) NA (0) NA (0)

2 1.00 (4) 1.10 (4) 1.69 (4) 1.06 (4) 1.25 (4) 36.8

Average of

All Cuts

†

1 1.32 (12) 1.26 (12) 2.24 (13) 1.13 (13) 1.72 (13) 49.1

2 0.71 (17) 0.71 (17) 1.90 (12) 0.88 (12) 1.11 (12) 50.5

†

The DS scrubber intermittently kicked off and was restarted during nearly half of the cuts 

studied.

NA – Not Available ‡

Determined as the time-weighted average of the DS face dust reduction efficiencies for the 

individual cuts.

Table 2 also shows that the scrubber reduced the dust 

concentrations at the face areas downstream of the CM for all cuts by 
6 

49.1 and 50.5% at sections 1 and 2, respectively. At section 2, when 

the roof bolter was in entry 12 downstream of the CM and DS, the dust 

reduction at the face was somewhat lower at 36.8%. The average roof 

bolter machine dust concentration (1.10 mg/m3

) was nearly identical to 

the average face dust concentration (1.06 mg/m3

) when downstream of 

the CM and DS. The lower percent dust reduction realized at the face 

when the bolting machine was present may have been due to some 

additional dust generated by the bolting machine in this face area.

In order to ascertain the DS’s respirable dust collection efficiency 

underground, the face area sampling package was placed directly into 

the DS exhaust discharge air for short period of time (2 to 5 minutes) at 

the end of several shifts to determine its respirable dust collection 

efficiency (concentrations not shown in Table 2). The measured DS 

exhaust concentrations at sections 1 and 2 averaged 0.13 and 0.01 

mg/m3

, respectively, while their corresponding upstream dust 

concentration averaged 1.87 and 1.08 mg/m3

, thereby yielding DS dust 

collection efficiencies of 93.2 and 99.2%, respectively. These dust 

collection efficiencies were found to be very similar to the scrubber’s 

laboratory test results with similar disposable filters. Consequently, the 

scrubber’s reduced dust reduction effectiveness at the face sampling 

locations demonstrate that its clean exhaust discharge air was being 

mixed with some of the dustier section return air to ventilate the face 

area.

Airflow quantity measurements at the last open crosscut and face 

areas likewise showed that the scrubber cleaned a small amount of the 

section return air and mixed this cleaned air with some of the return air 

for ventilating the face. Airflow quantities in the last open crosscut 

during these studies ranged from 5.32 to 13.1 m3

/s (11270 to 27720 

ft3

/min), while the scrubber was operated at airflow quantities of 1.28 to 

2.31 m3

/s (2720 to 4900 ft3/min). Initial ventilation airflow quantities 

delivered to the face areas ranged from 0.784 to 2.31 m3

/s (1660 to 

4890 ft3

/min) without the scrubber operating and were enhanced by the 

scrubber to an operating range of 1.09 to 3.88 m3

/s (2300 to 8220 

ft3

/min), thereby providing a 1.4 to 1.7 increase over the initial face 

ventilation airflow quantities without the scrubber. These face airflow 

changes from operating the scrubber also indicate that its exhaust 

airflow was being mixed with some of the dusty return airflow.

Several DS operation issues were observed during the study at 

section 1. The scrubber unexpectedly shut down and was restarted 

during nearly half of the cut periods at section 1, and sometimes its 

measured airflow quantity deviated up to nearly 2760 cfm from its 

preprogramed targeted amount. These operational malfunctions were 

attributed to communication issues encountered from the substitution 

of an approved MSHA underground coal mine remote control unit with 

the original unapproved scrubber remote control unit, which had been 

thoroughly and successfully tested in the lab. The original scrubber 

remote control unit was pending an MSHA approval and could not be 

used during the underground coal mine field testing. During the second 

field study at section 2, the DS was operated more continuously by 

using the fan motor’s frequency control mode, thereby eliminating the 

intermittent DS shutdowns experienced during the first field study. The 

DS’s fan motor was operated at 25 to 35 Hz to deliver scrubber 

airflows of 1.28 to 2.13 m3

/s (2720 and 4510 ft3/min) at differential filter 

pressures of 473 to 971 Pa (1.9 and 3.9 in. w.g.).

The DS operating parameters measured at Section 2 indicate that 

the disposable filter cartridges did not reach their fully loaded dust 

capacity during underground testing. At section 1 the DS was operated 

for 461 minutes with an average dust concentration of 2.24 mg/m3

, and 

at section 2 the DS was operated for an additional 509 minutes with an 

average dust concentration of 1.90 mg/m3

. Previous laboratory testing 

indicated that the disposable filters approach their useful life when they 

were loaded with an average dust concentration of 17.8 mg/m3 for 729 

minutes. An accumulated filter dust mass loading of 2.27 kg was 

determined from this laboratory testing to reach the maximum 

differential filter pressure of 2750 Pa (11.0 in w.g.) at 3.60 m3

/s (7630 

ft3

/min) of airflow quantity. Estimation of filter dust loading times or 

useful filter life at various scrubber airflows and dust concentrations 

less than or equal to these maximum laboratory DS operating 

parameters can be calculated by using equation 2 shown above. Thus, 

operating the scrubber at 4000 ft3

/min in dust concentrations of 2 

mg/m3 (comparable to the underground field study conditions) 

indicates a longer filter dust loading time of 167 hours as compared to 

the 16.2 hours of actual underground field testing time. Given that the 

DS on the last shift of testing exhibited a maximum filter differential 

pressure of 971 Pa (3.9 in. w.g.) at 2.13 m3

/s (4510 ft3/min) while 

operating at 35 Hz, these filters did not appear to be at their maximum 

filter life as compared to laboratory test results.

( ) ( )

( ) ( ) hr

s s Airflow Quantity m m

mg Dust Concentration

kg mg kg

Filter Dust Loading Time hr

3600

000,000,127.2

3

3  × × 

 









 



 ×

=

(2)

CONCLUSIONS

NIOSH laboratory testing of the Fletcher Dry Scrubber showed 

that it averaged greater than 95% dust removal efficiency with the 

disposable filters and averaged 88% and 90%, respectively, for the 

washable filters in their pre-washed and post-washed test conditions. 

Although the washable filters could be reused, washing them 

generated notable amounts of dust. The personal and downstream 

dust concentrations measured averaged 1.2 and 8.3 mg/m3

, 

respectively, over the 10-minute washing period. Laboratory dust 

testing showed that the scrubber fan also maintained a relatively 

consistent airflow near the targeted 1.43 and 4.25 m3

/s (3000 and 9000 

ft3

/min) airflow rates until reaching 2610 Pa (10.5 in. w.g.) of filter 

differential pressure at 3.97 m3

/s (8420 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 with the VFD operating the fan at 60 Hz motor frequency. Sound 

level measurements made on the scrubber in the lab showed that the 

outlet side of the scrubber was noisier and the loaded filters increased 

sound levels as compared to clean filters at the same airflow 

quantities. With loaded filters the scrubber reached a 90 dB(A) sound 

level at 2.83 m3

/s (6000 ft3/min) of scrubber airflow, indicating that 

miners should not be overexposed in relation to MSHA’s permissible 

exposure level (PEL) of 90 dB(A) at or below this airflow quantity (1).

Field testing the DS with the disposable filters at two underground 

coal mine sections showed that it can clean a portion of the section 

return air and provide about a 50% dust reduction at the face area 

downstream of the continuous miner operation. The average roof 

bolting machine dust concentrations at sections A and B were 0.59 and 

0.17 mg/m3

, respectively, when operating upstream of the CM as 

compared to 1.80 and 1.60 mg/m3

, respectively, when it was operating 

downstream of the CM and upstream of the DS. A 36.8% dust 

reduction was measured at section B when the roof bolter was 

operating downstream of the CM in the face area being cleaned by the 

DS. The DS’s dust collection efficiencies at sections A and B were 

93.2% and 99.2%, respectively, showing that the exhaust airflow was 

mixed with some of the dusty 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 scrubber operating. Therefore, the DS appears to be a new viable 

dust control method to combat roof bolter dust exposures when 

operating downstream of the continuous miner.

REFERENCES

1. CFR (2015), “Code of Federal Regulations,” Washington, DC: US

Government Printing Office, Office of the Federal Register.

2. MSHA (2015), Information provided by Philip Lindahl, Mine Safety

& Health Specialist, Coal Division of Health, Mine Safety & Health

Administration, Headquarters, Arlington, Virginia.

3. U.S. Department of Labor (2013), Mine Safety and Health

Administration, Program Evaluation and Information Resources,

MSHA Standardized Information Systems, Arlington Virginia.

4. Fields, K.G., Atchison, D.J., and Haney, R.A. (1990), “Evaluation

of Combined Face Ventilation System Used with a Remotely

Operated Mining Machine,” Proceedings of the 3rd Symposium on

Respirable Dust in the Mineral Industries, Oct. 17-19,1990,

Pittsburgh, PA, Published by the Society for Mining, Metallurgy,

and Exploration, Inc., Littleton, CO, Chapter 43, pp. 349-353.
7

5. Colinet, J.F. and Jankowski, R.A. (1996), “Dust Control

Considerations for Deep-Cut Faces when Using Exhaust

Ventilation and a Flooded-Bed Scrubber,” SME Transactions, Vol.

302, Littleton, CO: Society for Mining, Metallurgy, and Exploration,

Inc., pp. 104-111.

6. NIOSH (2011). Evaluation of Face Dust Concentrations at Mines

Using Deep-Cutting Practices. By: Potts, J.D., Reed, W.R.,

Colinet, J.F. Pittsburgh, PA: U.S. Department of Health and

Human Services, Centers for Disease Control and Prevention,

National Institute for Occupational Safety and Health, DHHS

(NIOSH) Publication No. 2011-131, Report of Investigations 9680,

2011 Jan, pp. 1-94.

7. NIOSH (2013). Impact on Respirable Dust Levels When

Operating a Flooded-bed Scrubber in 20-foot Cuts. By: Colinet

J.F., Reed W.R., and Potts J.D. Pittsburgh, PA: U.S. Department

of Health and Human Services, National Institute for Occupational

Safety and Health, DHHS (NIOSH) Publication No. 2014-105, RI

9693, 59 pp.

8. Kendall, B. (2015). Final Report for Fletcher Mobile Dry Scrubber

(DS). NIOSH Contract No. 200-2010-36164, “The Development of

Dust Control Units for Underground Coal Mines,” available from

NIOSH, Office of Mine Safety and Health Research, 22 pp.

ACKNOWLEDGEMENTS

The authors express their appreciation to Jason Driscoll (Physical 

Science Technician), Scott Klima (Mining Engineer), Milan Yekich 

(Engineering Technician), and Jeanne Zimmer (Physical Science 

Technician) of NIOSH OMSHR for their assistance with equipment

preparation and data collection during this research. The authors also 

express their appreciation to the Fletcher design team that worked on 

the Dry Scrubber throughout the contract, including Gene Wilson, 

Dave Hickman, John Coleman, Joe McQuerrey, Carl Sanns, Bob 

Anderson, Bill Schwab.

DISCLAIMER

The findings and conclusions in this paper are those of the 

authors and do not necessarily represent the views of the National 

Institute for Occupational Safety and Health. Mention of any company 

name, product, or software does not constitute endorsement by 

NIOSH.