Improved drill shroud capture of 

respirable dust utilizing air nozzles 

underneath the drill deck 

W.R. Reed and J.D. Potts 

Mining engineers at the Respiratory Hazards Control Branch, 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania
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Abstract 

This study tests the ability of a new air nozzle system, located under the drill deck shroud, to improve 

the dust capture of the drill deck shroud.The system consists of three air nozzles supplied with regulated 

pressurized air and located midway down the shroud in the off-inlet corners of the shroud. Laboratory 

testing produced operating curves for the system that showed an optimum operating point of 207 kPa 

(30 psi) and generated respirable dust reductions of 48% to 52% from the drill deck shroud. 

Introduction 

Surface mine blasthole drills have been shown to generate large 

amounts of respirable dust during dry drilling. A past NIOSH 

study documented that time-weighted average dust concentrations from area sampling in the vicinity of the drill shroud can 

potentially range from 8.68 to 95.15 mg/m3 (Organiscak and 

Page, 1995). Another study corroborated these results, showing 

area sampling respirable dust concentrations ranging from 1.04 

to 52.30 mg/m3 for dry drilling (Listak and Reed, 2007). These 

concentrations are highly variable and are dependent on the effectiveness of the dust controls used on the drill, with higher dust 

concentrations resulting from poor dust control methods. 

To put these dust concentrations into perspective, 2.0 mg/ 

m3 is the maximum allowable respirable dust concentration 

for exposure at coal mines. If the silica in the respirable dust 

sample is >5%, then the maximum allowable concentration 

is reduced to the quotient of 10 divided by the percent silica 

in the sample. Generally, emissions from drilling operations 

also contain silica, which continues to be an ongoing concern 

in the mining industry, as exposure to respirable crystalline 

silica dust can lead to silicosis. Silicosis is a respiratory disease 

that is often fatal and has no cure, except through its prevention (Porter and Kaplan, 2007). A review of the Mine Safety 

and Health Administration (MSHA) database of respirable 

dust samples containing silica from 2001 to 2004 shows that 

drilling operators and their helpers have some of the highest 

exposures to respirable silica dust, with overexposure rates of 

19% and 14%, respectively. Additionally, the blaster or shotfirer, who often works near operating surface mine blasthole 

drills, has an overexposure rate of 9% for respirable silica 

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dust (Joy, 2005). 

Wet drilling is an effective method to reduce dust concentrations,butwetdrillinghasdisadvantages,suchaswater-freezing 

issues in northern climates and rotary bit life reductions 

caused by the use of water during drilling. Therefore, many 

drilling operations use a dust collector system to control dust 

emissions. When operating properly, dust collector systems 

are very efficient. However, as operating parameters change 

or if the system is poorly maintained, there can be a significant reduction in system efficiency (Listak and Reed, 2007). 

Therefore, additional dust control methods for dry drilling are 

being investigated.

Dustfromdrillingoperationsgenerallyemanatesfromthree 

sources on the drill rig: the dust collector dump, the drill stem 

and the drill shroud (Maksimovic and Page, 1985). The focus 

of this work was directed toward eliminating dust emissions 

from the drill shroud. Prior research in this area focused on 

maximizing dust capture from the drill shroud through the 

evaluation of collector-to-bailing airflow ratios. This work also 

showed the importance of minimizing leakage from the drill 

shroud.Collector-to-bailingairflowratiosof4:1(i.e.,acollector 

airflow four times the amount of bailing airflow) were shown 

to efficiently reduce dust concentrations from the shroud area. 

However, most drills operate with collector-to-bailing airflow 

ratios of 2:1 (Page and Organiscak, 2004). Therefore, further 

investigations were conducted to find additional methods to 

reduce concentrations. Based on the results of this research, 

a new method of using air nozzles underneath the drill deck 

shows promising results for minimizing dust concentrations 

caused by leakage from the shroud without having to modify 
 

the collector airflow. 

Background of the problem

The drill deck or table, shown in Fig. 1, is located adjacent to 

the operator’s cab, which allows the operator access to perform 

maintenance on the drilling apparatus. 

Figure1 —Drilldeckwithshroudshowingleakage between 

the bottom of the shroud and ground surface. 

This platform is generally 0.91 m (3 ft) above the ground level. The gap between 

the bottom of the drill deck and the ground surface, which 

can range from 0.61 to 1.22 m (2 to 4 ft), is enclosed using 

conveyor belting or similar material. This enclosure is part of 

the drill dust collection system, which helps to contain the dust 

generated during drilling. The dust collector, connected to the 

drilldeckbylarge diameterwire-reinforcedhose,removesdust 

from under the shroud. The inlet to the dust collection system 

is generally located on the drill deck at the back corner of the 

side opposite the operator’s cab. Dust leakage from the drill 

deck enclosure can occur at three different sources. Two of 

the sources that were evaluated in this study are leakage from 

gaps between the bottom of the shroud and the ground and 

from gaps at the corners of the enclosure. The other source of 

dust is from leakage around the table bushing where the drill 

steel goes through the drill deck.

During the drilling operation, bailing air is sent through 

the center of the drill steel, exiting at openings on the drill 

bit. This bailing air is used to cool the drill bit and to flush 

the drill cuttings from the hole (Brantly, 1961). The air and 

the cuttings exit the drill hole at a high velocity. Once the air 

enters the drill deck enclosure, its velocity is reduced and the 

large drill cuttings drop out of the air stream. The respirable 

dust, because of its small size, continues with the airstream, 

as the reduction of velocity is generally not enough to allow 

these smallerparticlestofallout.The suspendedrespirabledust 

leaks from the shroud, resulting in respirable dust emissions. 

This study evaluated the use of air nozzles for directing dusty 

air to the scrubber inlet to improve dust capture and reduce the 

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amount of dust leaking from the shroud. Different air nozzle 

configurations underneath the shroud were tested on a drill 

shroud simulator that was constructed at NIOSH labs. 

Test facility 

NIOSH’s test facility used for the air nozzle testing has been 

thoroughly described in previous publications (Page and 

Organiscak, 2004; Organiscak and Page, 2005). This facility, 

which simulates the dust collection system of a blasthole drill 

rig, is used to evaluate dust control techniques for surface 

drills in a controlled laboratory environment. The laboratory 

environment includes a full-scale model of a drill deck and 

shroud as shown in Fig. 2.

Figure 2 — Drill deck testing facility at NIOSH. 

The simulator is contained in a large dust chamber, which 

serves as a containment and monitoring area for the dust that 

escapes the shroud. To prevent dust from leaking outside of 

the chamber, static pressure in the chamber is controlled and 

maintained by a series of louvered vents located on the top 

of the chamber. The simulated drill deck and shroud, whose 

dimensions are 1.52 m wide, 1.22 m deep and 1.22 m high 

(5 by 4 by 4 ft), are located in the center of the dust chamber. 

This shroud size is within the range found on medium-sized 

rock drills (rubber-tired or track-mounted) that drill holes 127 

to 203 mm (5 to 8 in.) in diameter, with about 178 to 222 kN 

(40,000 to 50,000 lbs) of drill pulldown pressure. The base 

of the shroud is fitted with hinged plywood slats that can be 

adjusted to simulate gaps between the ground and the shroud, 

from which dust escapes. The gaps can be set at a range of 51 

to 356 mm (2 to 14 in.).

Compressed air, using a Kaiser DSD125 air compressor 

capable of delivering 326 L/s at 758 kPa (690 cfm at 110 psi), 

is piped into the simulated drill steel to represent the bailing 

air of the drill rig during the drilling process. A 152-mm (6-

in.) steel pipe, which extends from 0.61 m (2 ft) above the 

simulated drill deck to 0.91 m (3 ft) beneath the floor of the 

test chamber, is used to simulate the drill steel. This 152-mm 

(6-in.) pipe is concentrically located in a 203-mm (8-in.) steel 

pipe. This 203-mm (8-in.) pipe simulates the drill hole with its 

opening at the chamber floor surface; it extends from the floor 

to 0.91 m (3 ft) beneath the floor of the chamber. The pressure 

and flow rate can be regulated to provide accurate bailing air 

velocities. To simulate the dust emissions from the borehole, a 

Vibra-Screw feeder delivers the amount of limestone rock dust 

(approximately25g/min)ataconstantrateintoacompressedair 
 

eductor, operating on a small separate split 

of air at 11.8 L/s (25 cfm). The limestone 

dust has a particle size distribution similar 

to the bulk dust at the dust collector dump 

point. Figure 3 shows the mass frequency 

of the particle size of the test material 

with the bulk dust collector material and 

illustrates the similarities in particle size 

distributions. 

Figure 3 — Mass frequency of particle diameters of limestone test material 

(dashed line) compared with bulk dust collector material (solid line). 

This dust is mixed with the 

remaining bailing airflow near the top of 

the 152-mm (6-in.) pipe and blown out the 

concentric opening between the 152-mm 

(6-in.) pipe and the 203-mm (8-in.) pipe. 

Therespirabledustconcentrationsescaping 

the shrouded area over this simulated drill 

hole is the response variable measured during these experiments, while maintaining 

a constant feed rate with the Vibra-Screw feeder. 

As with a rotary drill rig, the simulator’s dust collector provides the negative pressure within the shroud to collect the dust 

as it is emitted from the hole at the base of the chamber. The dust 

collector was simulated using a baghouse connected through 

fiberglass tubing. The final connection to the drill deck was 

accomplishedusing203-mm-(8-in.-)diameterwire-reinforced 

tubing, which connected using a metal transition from 203-

mm- (8-in.-) diameter to a 457- by 190.5-mm (18- by 7.5-in.) 

rectangular opening in the drill deck. The collector is capable 

of providing variable flow rates up to a maximum flow of 944 

L/s (2,000 cfm). The collector’s inlet is located in the corner 

of the simulator, similar to those found on drills in the field. 

Preliminary testing

Preliminary experiments were conducted to determine the 

viability and the optimum placement of the air nozzles for 

maximized dust capture of the drill shroud and to narrow these 

variable ranges for a factorial design experiment. Variables 

investigated included spray location, orientation, quantity, 

type and pressure. The air nozzles evaluated were flat-fan and 

hollow-coneplastic airnozzles(SprayingSystemsCo.WindJet 

ModelsAA727-11andAA707-11,respectively).Thesenozzles 

were chosen because they do not require large quantities of 

air from the compressor and thus are able to generate airflow 

without affecting the bailing airflow quantity. 

Dust measurements were made with two RAM sampling 

devices located outside of the shroud on the front (on-inlet) 

and back (off-inlet) sides. The locations are shown in Fig. 4 

and are labeled as RAM-Front and RAM-Back. 

Figure 4 — Plan view of shroud inside the drill deck testing 

facility showing the locations of the gravimentric sampling 

locations(numberedlocations)andinstantaneoussampling 

locations (RAM-Front and RAM-Back). 

The locations 

of the gravimetric sampling, which was not conducted during 

the preliminary testing, are also shown and are numbered 

consecutively from 1 to 8. The RAM data were fed into a chart 

recorder and a data recorder to provide an instantaneous visual 

representationofdust concentrations andtoallowdownloading 

of data for post-test analysis. The preliminary tests followed 

an ABAB pattern, where A was the spray-off condition and 

B was the spray-on condition, with each segment lasting 10 

minutes. 

Initially,horizontalspraybarscontainingfourin-linenozzles 

were testedatvariouslocationsunderneaththe shroud, asshown 

in Fig. 5 (a). The variables considered in this evaluation were 

one and two spray bars (four or eight nozzles, respectively), 

hollow-cone and flat-fan nozzles and air pressures ranging 

from 34 to 276 kPa (5 to 40 psi). None of the locations tested 

produced consistent significant reductions of respirable dust 

outside of the shroud. An attempt was made to push all the 

airflow from the off-inlet side towards the collector by using 

20 air nozzles in a grid pattern on the off-inlet wall, which is 

opposite the collector inlet, shown in Fig. 5 (b). This evaluation utilized hollow-cone sprays with air pressures ranging 

from 138 to 552 kPa (20 to 80 psi). This configuration also 

resulted in unsatisfactory results, sometimes producing more 

respirable dust emissions, especially at higher pressures. Another configuration placed a set of four hollow-cone nozzles 

directed into the collector inlet, shown in Fig. 5 (c), at pressures ranging from 34 to 276 kPa (5 to 40 psi). These operating 

pressures produced dust reductions at the 69 and 138 kPa (10 

and 20 psi) levels. However, these reductions were inconsistent and varied from 3.0% to 26.0%, with the best reductions 

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occurring at 138 kPa (20 psi). Due to the inconsistency of the 

reductions, it was decided that these three test scenarios were 

not worth evaluating.

The nozzle orientation evaluated during preliminary testing 

that showed the most potential for respirable dust reductions 

was avortexconfigurationwiththenozzles 

placed midway down the shroud and oriented slightly off parallel (a slight angle 

of <10°) from the side of the shroud. The 

vortex configuration targeted airflow in a 

clockwisedirectionunderneaththe shroud, 

in a manner similar to the rotation of the 

drill steel. Initial testing involved as many 

as 20 sprays with pressures ranging from 

69 to 276 kPa (10 to 40 psi); however, 

dust reductions were not observed until 

the number of sprays was decreased to 

four.Throughoutpreliminary testing ofthe 

vortex orientation, three sprays produced 

as good or better results than four sprays. 

Two sprays demonstrated a marked drop 

in performance. Therefore, in the interest of minimizing air consumption but 

maintaining system integrity, the factorial 

experiment only examined a three-spray 

system. Figure 6 shows the placement of 

the three effective air nozzles. 

The impacts of spray pressure and 

nozzle type (flat-fan vs. hollow-cone) on 

dust levels were not as easily discernable 

duringpreliminarytesting.Thesevariables 

were included in the factorial experiment 

for further analysis. During the preliminary test, this three-spray configuration 

produced a stark contrast between the on 

and off conditions, reducing average dust 

levels at the front and back sides of the 

shroud by 67% and 54%, respectively. 

Using results from the preliminary testing, 

the following factorial experiment design 

was formulated to further investigate the 

use of air as a means of improving dust 

capture under the shroud. 

Figure 5 — Spray locations that were ineffective in reducing respirable dust 

concentrations: (a) horizontal spray locations, (b) 20 pushing spray locations 

and (c) inlet spray locations. 

Test procedure

The drill deck simulator was used for 

testing the vortex air nozzle configuration. 

The shroud-to-ground gap was kept at a 

constant 51 mm (2 in.) throughout testing. 

Higher gap settings were not used because 

drill operators are generally conscientious 

about minimizing leakage between the 

ground and the bottom of the shroud. Bailing air was supplied to the facility in two

separate splits. The main split of air was 

regulated between 226 and 234 L/s (479 

and 495 cfm) averaging 230 L/s (488 cfm) 

after temperature and pressure correction 

for the deviations from the conditions at 

which the flow meter was calibrated. The 

main split temperature varied from 17° to 

26°C (62° to 78°F) and the pressure for all 

tests was 207 kPa (30 psi). The secondary 

split of air for the dust feed was regulated 

at 11.8 L/s (25 cfm) after temperature and 

pressure correction. The secondary split temperature varied 

from18° to28°C(65° to82°F) andthepressurewasmaintained 

at 345 kPa (50 psi). Therefore, the total bailing airflow rate 

varied between 237 and 246 L/s (503 and 521 cfm).

To provide the dust feed, the secondary air split was used 

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to drive a venturi eductor. The eductor and 

secondaryairsplitinjectedtestdustintothe 

main split of air at the head of the simulated 

drillpipelocation.Limestonesizedto100% 

<0.25mm(60mesh)was usedas a testdust 

and fed into the eductor by a Vibra-Screw 

feeder equipped with a 13-mm- (0.5-in.-) 

diameter auger feed screw. The feed rate 

ranged from 23.1 to 27.4 g/min, targeting 

a sustainable feed rate to be maintained 

within ±10% of the average of 25.1 g/min. 

This feed rate was selected over the lower 

feed rates previously used (Organiscak 

and Page, 2005) in an attempt to simulate 

the high dust concentrations encountered 

during previous area sampling of drilling 

operations (Organiscak and Page, 2005; 

Listak and Reed, 2007).

Adust-collector-to-bailing-airflowratio 

of 2:1 was targeted throughout the testing. Dust collector airflows ranged from 

447 to 480 L/s (947 to 1,018 cfm) with 

an average of 462 L/s (978 cfm). These 

airflows were calculated from velocity 

pressures measured before and after each 

test. Measurements were performed using a pitot tube inserted 

into the fiberglass tubing dust collector network. The resulting 

dust collector airflows produced collector-to-bailing airflow 

ratios ranging from 1.87 to 2.00, with an average ratio of 1.91. 

This dust-collector-to-bailing-air ratio is representative of the 

typical operation of a dust collector on a rotary blasthole drill 

(Page and Organiscak, 2004). 

Gravimetric samplers using 37-mm filter cassettes and 10-

mm Dorr-Oliver cyclones were used to measure respirable dust 

levels inside the chamber. Two samplers were used on each of 

the four sides of the shroud and located approximately 0.61 m 

(2 ft) above the floor level, yielding a total of eight gravimetric 

samples for each test. Figure 4 shows the gravimetric sampler 

locations, which are numbered consecutively from 1 to 8, 

and the instantaneous RAM sampler locations labeled RAMFront and RAM-Back. All gravimetric samplers maintained 

constant flow rates of 2.0 L/min using one vacuum pump. 

Consequently, all samplers were turned on/off simultaneously. 

All of the gravimetric sampler flow rates were calibrated prior 

to conducting the series of tests. Two RAM-1 instantaneous 

dust monitors were used at the on-inlet and off-inlet sides of 

the drill deck with the RAM sampler head located in the same 

proximityasthegravimetric samplers.TheRAMmonitorswere 

used to monitor the dust concentration prior to test initiation, 

during which the pre-test dust feed was adjusted to the proper 

levels and maintained throughout the test. The analog output 

of the RAM monitors were fed to a strip chart recorder for 

visual monitoring of the dust concentration stability and to 

a data-logging device to allow downloading of the data for 

later analysis. Dust sampling time was fixed at 30 min after 

verification that the chamber dust concentration had stabilized. 

The final chamber dust concentration was determined from the 

average of the eight gravimetric samples.

Testing examined the effects of the following three variables: nozzle type (flat-fan or hollow-cone); pressure at 138, 

207 and 276 kPa (20, 30 and 40 psi); and vertical leakage, 

which represents possible gaps at the corners of the shroud (no 

leakage vs. vertical leakage). In other words, this procedure 

evaluated twelve configurations of nozzle type, pressure, and 

vertical leakage (2 x 3 x 2). Each configuration, including the 

baselinewherenonozzleswereused,was evaluatedthree times. 

This resulted in a total of 42 tests, which were conducted in 

random order. During testing, ambient air data, which included 

wet and dry bulb temperatures and barometric pressure, were 

measured outside the test chamber and recorded. 

The 138, 207 and 276 kPa (20, 30 and 40 psi) pressures 

were selected as showing potential for respirable dust reduction during preliminary testing. Lower pressures below 138 

kPa (20 psi) only produced slight dust reductions and therefore 

were not tested. Higher pressures tended to generate too much 

airflow under the shroud, which increased respirable dust 

emissions. Vertical leakage was examined to determine its effect on air nozzle operation, as many shrouds are constructed 

using one piece of conveyor belting per side, due to ease of 

installation and to allow for expansion as the drill cutting pile 

becomes larger. Where the pieces of conveyor belting meet 

at each corner, there is a split through which respirable dust 

emissions may escape. The vertical leakage was replicated by 

cutting two 25-by-800 mm (1-by-31.5 in.) slots in the off-inlet 

side of the drill deck simulator, each at a location nearest the 

corner. The slots were covered when vertical leakage was not 

desired during testing. 

Figure 6 — Air nozzle spray system using the vortex configuration that is effective at reducing respirable dust concentrations from the drill deck shroud. 

Results 

Table 1 shows the gravimetric and instantaneous respirable 

dust concentrations for each configuration, sorted by spray 

type, spray pressure and vertical leakage. The figures reported 

for the back and front gravimetric concentrations are averaged 

from the samples taken at locations 5 and 6 (back) and 1 and 

2 (front), respectively (see Fig. 4). The average gravimetric 

concentration is calculated from samples taken at all eight locations (individual sample results are not shown). In contrast, 

theback and front instantaneous concentrations are the average 

of all back (RAM-back) and front (RAM-front) instantaneous 

concentrations, respectively, that were recorded at 10-second 

intervals for the entire 30-minute testing period, with the average instantaneous concentration being the average of the back 

and front instantaneous concentrations. 

Figures 7 and 8 show the graphs of the gravimetric respirable dust concentrations versus air nozzle pressure for both 

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the flat-fan and hollow-cone spray nozzles, respectively. These 

graphs were created by averaging the time-weighted average 

gravimetric dust concentration results of the three tests of each 

case that was evaluated during this study and plotting these 

results with their corresponding pressure.

Table 1 — Respirable dust concentrations from air nozzle testing. 

Spray Gravimetric concentrations Instantaneous concentrations

Spray pressure, Vertical Back, Front, Average, Back, Front, Average, 

Test type psi leakage mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3

32 FF 20.0 no 4.80 10.24 7.43 4.22 8.65 6.44 

35 FF 20.0 no 4.53 11.47 7.60 4.00 9.16 6.58 

20 FF 20.0 no 8.06 12.66 10.53 7.45 10.20 8.82 

41 EF 30.0 no 4.07 4.67 5.32 3.57 4.04 3.81 

15 FF 30.0 no 7.38 7.11 7.84 6.64 5.74 6.19 

3 FF 30.0 no 7.98 6.96 8.13 7.08 5.99 6.53 

24 FF 40.0 no 7.94 8.54 8.14 7.47 6.98 7.23 

34 FF 40.0 no 7.75 8.35 8.31 7.32 6.96 7.14 

31 FF 40.0 no 8.07 8.68 8.74 6.55 7.44 6.99 

23 FF 20.0 yes 12.50 9.23 11.35 10.69 7.49 9.09 

28 FF 20.0 yes 14.14 10.96 13.06 12.39 8.88 10.64 

4 FF 20.0 yes 14.01 10.70 13.20 12.24 8.76 10.50 

14 FF 30.0 yes 11.17 7.75 10.01 9.87 6.32 8.10 

17 FF 30.0 yes 10.86 7.99 10.23 9.91 6.25 8.08 

8 FF 30.0 yes 14.52 7.88 11.73 12.21 6.40 9.30 

1 FF 40.0 yes 11.27 9.20 11.92 11.65 7.56 9.60 

27 FF 40.0 yes 13.79 11.14 11.98 12.05 8.97 10.51 

33 FF 40.0 yes 18.33 14.73 15.65 15.88 11.90 13.89 

11 HC 20.0 no 7.19 5.50 7.49 6.96 4.49 5.73 

10 HC 20.0 no 7.70 6.43 8.15 6.53 5.35 5.94 

19 HC 20.0 no 6.13 12.12 8.72 5.49 9.55 7.52 

42 HC 30.0 no 4.12 4.60 5.44 3.44 4.38 3.91 

38 HC 30.0 no 5.16 8.44 6.95 4.54 6.83 5.68 

39 HC 30.0 no 5.38 8.04 7.34 4.35 7.13 5.74 

36 HC 40.0 no 6.29 6.45 6.78 5.32 5.07 5.20 

21 HC 40.0 no 6.85 6.63 7.48 5.86 5.66 5.76 

6 HC 40.0 no 8.32 7.53 8.07 6.97 6.09 6.53 

5 HC 20.0 yes 11.61 7.31 9.42 8.71 6.16 7.43 

12 HC 20.0 yes 11.23 7.36 9.57 10.57 5.86 8.22 

13 HC 20.0 yes 12.70 9.35 11.17 11.48 7.78 9.63 

13b HC 20.0 yes 14.13 12.27 13.14 12.77 9.82 11.29 

16 HC 30.0 yes 12.92 10.38 12.49 11.51 8.21 9.86 

29 HC 30.0 yes 15.97 9.84 13.24 14.27 8.41 11.34 

30 HC 30.0 yes 17.01 11.23 14.51 13.96 9.57 11.77 

37 HC 40.0 yes 12.47 9.77 10.48 11.65 8.02 9.84 

25 HC 40.0 yes 13.31 9.08 11.75 10.55 7.33 8.94 

9 HC 40.0 yes 14.08 8.63 12.32 11.89 7.13 9.51 

2 none - no 12.72 9.61 11.78 12.66 8.14 10.40 

22 none - no 10.86 12.50 12.70 9.97 10.42 10.19 

7 none - no 15.38 14.31 16.38 14.38 12.01 13.20 

40 none - yes 10.60 9.80 11.45 8.45 8.50 8.48 

18 none - yes 10.38 10.02 11.91 8.70 8.50 8.60 

26 none - yes 11.82 10.76 13.37 9.31 8.94 9.13 

Note: Average gravimetric concentration includes gravimetric concentrations from four drill deck 

samples, which are not shown. 

FF = Flat-Fan 

HC = Hollow-Cone 

The results in Fig. 7 show the operating performance curve 

that was fit to the gravimetric dust concentration data for flatfan nozzles. This curve uses a second-order polynomial to fit 

the data and had an R-squared value of 98.7%, demonstrating 

a good fit to the data. Use of the flat-fan air nozzles resulted in 

a dust concentration reduction outside the shroud of approximately 48% when the nozzles were operated at 207 kPa (30 

psi). Operating the nozzles at lower or higher operating pressures, i.e., 138 and 276 kPa (20 and 40 psi), resulted in less dust 

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reduction (37% and 38%, respectively). 

However, whenever vertical leakage was 

present in the shroud, the operation of the 

air nozzles did not appear to influence the 

dust concentrations. 

Figure 7 — Respirable dust reduction curves for flat-fan air nozzles, both with 

and without vertical leakage, using time-weighted average gravimetric data. 

Figure 8 shows the operating performance curve for the hollow-cone nozzles. 

Figure 8 — Respirable dust reduction curves for hollow-cone air nozzles, both 

with and without vertical leakage, using time-weighted average gravimetric 

data. 

This curve also used a second-order polynomial to fit the data and had an R-squared 

value of 99.3%, demonstrating a good fit 

to the data. Use of the air nozzles resulted 

in a dust concentration reduction outside 

the shroud of approximately 52% when the 

nozzles were operated at 207 kPa (30 psi). 

Operating the nozzles at lower or higher 

operating pressures, i.e., 138 and 276 kPa 

(20 and 40 psi), resulted in less dust reduction (40% and 45%, respectively). Again, 

however, vertical leakage appeared to 

negate the benefit of using air nozzles for 

reducing dust.

Figure9shows thegraphof the instantaneousrespirabledust concentrationsversus 

air nozzle pressure for both the flat-fan and 

hollow-cone spraynozzleswithoutvertical 

leakage. 

Figure 9 — Respirable dust reduction curves for flat-fan and hollow-cone 

air nozzles without vertical leakage using instantaneous dust concentration 

data. 

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This graph was created by averaging all the instantaneous respirable dust 

concentrations for the 30-minute testing 

period of the three tests for each case evaluated, then plotting these results with their 

corresponding pressure. Figure 9 supports 

the gravimetric results shown in Figs. 7 and 

8,producingdustconcentrationsoutsidethe 

shroud that are 51% lower when operating 

flat-fan nozzles at 207 kPa (30 psi) and 

55% lower when operating hollow-cone 

nozzles at 207 kPa (30 psi). Again, it is 

demonstrated that operating the nozzles 

at lower or higher pressures, i.e., 138 and 

276 kPa (20 and 40 psi), results in less dust 

reduction (35% and 37%, respectively, 

for flat-fan nozzles and 43% and 48%, 

respectively,forhollow-conenozzles)than 

operating at 207 kPa (30 psi). 

Discussion 

At the start of the design process, it was 

thought to be important to orient the 

nozzles to target the airflow in a clockwise 

direction, similar to the drill steel rotation. The reasoning was that the nozzles 

produced a vortex underneath the shroud 

that would help move the dust from the 

off-inlet sides of the drill deck shroud towardsthe collectorinlet. However,further 

investigation of the airflow underneath 

the drill deck revealed that this was not 

the case. A qualitative airflow model was 

developed using smoke tubes and a tuft 

grid technique (SAE, 1986), which uses 

a mesh and streamers to visualize the flow 

directions. One side of the shroud was 

constructed of transparent acrylic plastic 

to allow observation of the area enclosed 

by the shroud. The tuft grid was created by 
attaching lightweight vinyl streamers to a wire mesh, which 

was cut to the same dimensions as the horizontal parametric 

cross-section of the shroud enclosure. Airflow patterns were 

observed at various heights under the shroud during operation 

of the drill simulator to develop the qualitative model shown 

in Fig. 10. The arrows in Fig. 10 represent the bailing airflow, 

with the large downward arrow at the top center of the drill 

shroud representing the bailing airflow through the drill steel 

and other arrows depicting the airflow after the bailing air 

leaves the drill hole. 

Figure 10 — Airflow patterns underneath the drill shroud 

(arrows represent airflow). 

The airflow underneath the drill shroud, from the drill hole, 

was discoveredtofollowalongthedrillsteeluptothe bottom of 

the drill table. Once the air hits the underside of the drill table, 

the airflow radiates 360° from the drill steel and follows the 

bottom side of the drill table. Once the airflow hits the shroud 

at the edge of the drill table, it flows down the shroud on all 

sides, hitting the ground surface. Smoke tubes demonstrated 

that a portion of the air hitting the ground dispersed outside 

of the shroud, resulting in dust leakage into the environment. 

The space underneath the shroud between the air flowing up 

along the drill steel and the air flowing down the side of the 

shroud is predominantly dead air space, with airflow drifting 

toward the collector inlet. In evaluating the placement of the 

nozzles and the airflow underneath the drill deck, it is more 

likely that the air stream from the nozzle disrupts the airflow 

coming down the shroud and redirects it into the dead air space, 

therebycausingthe reductionofdust concentrations emanating 

from the bottom of the shroud. 

The operating curves for the air nozzle system shown in 

Figs. 7 and 8 use a second-order polynomial to fit the data. 

This provides the best fit as shown by the R squared values 

(98.7% flat-fan, 99.3% hollow-cone). A linear curve would 

not provide a good fit to the data as seen by the rise in dust 

concentrations at the higher operating pressure of 276 kPa 

(40 psi). Examining Figs. 7, 8 and 9 shows that the optimum 

operatingpointfordustreductionis at207kPa (30psi).Beyond 

the 207 kPa (30 psi) operating point, the dust concentrations 

begin to rise. The increasing pressure causes an increase in 

airflow from the nozzles underneath the shroud. This extra 

airflow into the shroud area can result in a lowering of the 

collector-to-bailing airflow ratio (i.e., < 2:1 from the lab experiments), which can lead to a reduction of the efficiency of 

the dust collection system (Page and Organiscak, 2004). The 

additional airflow also reduces the amount of negative pressure in the system, creating a condition of positive pressure, 

which results in higher levels of leakage and dust. Testing was 

not performed to determine whether this optimal operating 

point was universal for other configurations and dimensions 

of the drill deck shroud component of the dust collection system. Therefore, these performance curves for the air nozzle 

system may only be applicable to drill deck shrouds that are 

similar in configuration and dimensions of the shroud in this 

test facility. 

Figure 9 depicts both operating curves for the flat-fan and 

the hollow-cone nozzles using the instantaneous data recorded 

during testing. These curves corroborate 207 kPa (30 psi) as 

an optimal operating pressure for the air nozzle system. However, they additionally show that the type of spray is not very 

important in the operation of the system. The curves from the 

gravimetric dust concentration data also support this view. 

Hollow-cone nozzles are shown to be slightly more efficient 

at dust reduction than the flat-fan nozzles, but this advantage 

is not significant when looking at the dust reductions of 48% 

and 52% for flat-fans and hollow-cones, respectively. It would 

be preferable to use hollow-cones, but the placement of the 

nozzles in the corners and maintaining their parallelorientation 

with the side of the drill shroud is more important. 

Conclusion 

A new air nozzle system was devised to help improve dust 

capture from underneath the drill deck shroud. This system 

wouldbe installedunderneaththedrilldeckshroudandconsists 

of three nozzles located midway down the shroud with each 

nozzle located in the off-inlet corners of the deck as shown 

in Fig. 6. They would be oriented to provide airflow slightly 

off-parallel to the side of the shroud (a slight angle <10° away 

from the shroud). Each nozzle is supplied with regulated pressurized air from the drill’s air compressor. 

The advantage to this system is that it has been shown to 

greatly reduce respirable dust concentrations from the drill 

deck shroud. For the drill deck shroud configuration tested, 

i.e., 1.52 m wide, 1.22 m deep and 1.22 m high (5 x 4 x 4 ft), 

operating the air nozzles at 207 kPa (30 psi) reduced dust 

concentrations by 48% and 52% for flat-fan and hollow-cone 

nozzles, respectively. It was important during the operation 

of this system to minimize any vertical leakage, as the presence of vertical leakage negated the benefits of this air nozzle 

system. Operating the nozzles at lower or higher pressures did 

not generate dust reductions as great as the optimum operating 

point of 207 kPa (30 psi). Further testing would be required 

to determine the optimum operating point of the system if the 

drill deck shroud configuration were to be changed.

Disadvantagesfortheuseofthissystemarethatitrequiresthe 

nozzles to be plumbed into the existing compressed air system. 

This requires additional air lines and regulators to operate and 

control the system. However, the airflow requirements of the 

nozzles should not encumber the compressor system, as the 

three air nozzles use less than 14.2 L/s (30 cfm) of air, com8
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bined, at the 207 kPa (30 psi) operating point. Additionally, the 

nozzles are located underneath the drill shroud. Therefore, they 

are susceptible to damage when the drill operates in locations 

where the ground surface undulates greatly. Also, this location 

underneath the drill shroud makes it difficult for the operator 

to know when damage has occurred, which could render the 

systemineffective.Becauseofthesedisadvantages,thissystem 

would require additional attention in the form of inspections 

and maintenance to ensure proper operating condition.

Laboratory testing suggests that use of this air nozzle spray 

system would significantly reduce mine dust exposures from 

surface mine blasthole drilling operations. This research has 

investigated the airflow conditions underneath the drill deck 

shroud when drilling occurs. From these investigations, an air 

nozzle system has been proposed which can reduce respirable 

dust concentrations by 48% to 52%. The next step should be 

to conduct additional field studies on surface mine blasthole 

drills to validate these reductions as seen in the laboratory. 

In an effort to reach a goal that mine dust exposures can be 

fully eliminated from drilling operations, research continues 

to explore additional innovative methods to eliminate dust 

exposure from the drill deck shroud, including field trials on 

surface mine blasthole drills. 

References 

Brantly, J.E., 1961, Rotary Drilling Handbook, 6th Edition, Palmer Publications, 

Los Angeles, California. 

Joy, G.J., 2005, Personal contact about surveillance analysis of MSHA’s dust 

sampling database housed at the US Department of Labor, Mine Safety and 

and Health Administration, Program Evaluation and Information Resources, 

MSHA Standardized Information System, Arlington, VA. 

Listak, J.M., and Reed, W.R., 2007, “Water separator shows potential for reducing respirable dust generated on small-diameter rotary blasthole drills,” 

International Journal of Mining, Reclamation, and Environment, Vol. 21, No. 3. 

Maksimovic, S.D., and Page, S.J., 1985, Quartz Dust Sources During Overburden 

Drilling at Surface Coal Mines, United States Bureau of Mines Information 

Circular 9056. 

Organiscak, J.A., and Page, S.J., 1995, Assessment of Airborne Dust Generated from Small Truck-Mounted Rock Drills, United States Bureau of Mines 

Report of Investigations 9616. 

Organiscak, J.A., andPage,S.J., 2005,“Development of adust collector inlethood 

for enhanced surfacemine drill dust capture,” International Journal of Surface 

Mining, Reclamation, and Environment, Vol. 19, No. 1, March, pp. 12-28. 

Page, S.J., and Organiscak, J.A., 2004, “Semi-empirical model for predicting 

surface coal mine drill respirable dust emissions,” International Journal of 

Surface Mining, Reclamation, and Environment, Vol. 18, No. 1, pp. 42-59. 

Porter, R.S., and Kaplan, J.L., eds., 2007, The Merck Manual of Diagnosis and 

Therapy, Online Edition, accessed at www.merck.com/mmpe/index.html, 

Merck Research Laboratories. 

Society of Automotive Engineers (SAE), 1986, “Aerodynamic Flow Visualization 

Techniques and Procedures,” SAE Information Report HS J1566 JAN86, 

Society of Automotive Engineers, Inc., Warrendale, Pennsylvania. 

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