HORIZONTAL AIR BLOCKING SHELF REDUCES DUST LEAKAGE FROM SURFACE DRILL SHROUD

J. D. Potts, NIOSH, Pittsburgh, PA 

W. R. Reed, NIOSH, Pittsburgh, PA 

ABSTRACT

Installation of a horizontal air blocking shelf around the inside 

perimeter of the surface mining drill deck shroud is proposed to 

improve dust capture. Without the use of this shelf, a strong coanda 

effect airflow pattern results in dust leakage to the environment, 

especially at the shroud-to-ground interface. Bailing air exits the drill 

hole and travels along the drill steel to the underside of the table, 

where it fans out across the bottom of the drill table and continues 

down the sides of the shroud. This dust-laden bailing air then strikes 

the ground and fans out under gaps between the shroud and the 

ground. The proposed shelf prevents dust-laden air from striking the 

ground by redirecting it toward the center of the enclosure, where it 

can be captured. Laboratory tests demonstrated that the shelf reduced 

dust levels outside of the shroud by 81% under operating conditions 

consisting of a 1.9:1 collector to bailing airflow ratio, and a 5.1-cm (2-

in) gap between the shroud and ground. The shelf was 15.2-cm (6-in) 

wide and located at a height equal to 2/3 the distance measured from 

the bottom of the shroud to the bottom of the drill table. The air 

blocking shelf was also capable of overcoming the adverse impact of 

vertical leakage along the shroud seams as long as this leakage was 

confined to an area below the horizontal plane defined by the shelf 

location. 

INTRODUCTION

Overexposure to respirable crystalline silica dust can result in the 

development of silicosis, a serious and often fatal respiratory disease. 

The U.S. Mine Safety and Health Administration (MSHA) regulates the 

exposure of coal miners to hazardous substances and has set the 

permissible shift exposure limit for respirable dust at 2.0 mg/m3

 when 

the crystalline silica level does not exceed 5%. If silica in the sample 

exceeds 5%, the respirable dust standard is reduced to a level equal to 

the quotient of ten divided by the percentage of quartz in the sample, 

resulting in an effective silica exposure limit of 0.1 mg/m3

. Surface coal 

drill operators have had some of the highest frequency for noncompliance with the respirable dust standard, exceeding the limit 15-

19% of the time during the period from 2001 through 2006 (Joy, 2007). 

Compliance problems are exacerbated by both the level of respirable 

dust generated by the drilling machines and the fact that they are often 

being used to cut silica-bearing materials like sandstone and shale. 

One study found that time weighted average respirable dust levels in 

areas surrounding the drill shroud could exceed 95 mg/m3 (Organiscak 

and Page, 1995). Depending on wind direction and worker location, 

rotary drills can potentially expose all surface workers to elevated silica 

levels. A recent voluntary study of Pennsylvania surface miners 

showed that nearly 7% had radiological evidence of silicosis (Tyson et 

al., 1997). As demonstrated by the data above, additional technology 

is needed to reduce the exposure of surface miners to silica. A recent 

research project initiated by the National Institute for Occupational 

Safety and Health (NIOSH) identified improved drill shroud design as 

one of its main objectives for reducing the exposure of surface miners 

to respirable dust. Priority was assigned to designs that would 

minimize operational resources and maintenance. A method utilizing a 

horizontal shelf attached to the inside perimeter of the shroud to 

disrupt the coanda airflow pattern under the shroud shows promise for 

reducing dust leakage to the environment. 

BACKGROUND

Many surface mining operations use dry drilling techniques to 

eliminate problems associated with wet drilling, including maintaining a 

constant water supply, short bit lives, and freezing-related issues in 

colder climates. Dry drilling requires a dust collection circuit to remove 

dust from the bailing air. Components common to all dry dust 

collection systems are shown in Figure 1, including a shroud to 

enclose the area under the drill deck, an inlet and flexible duct to route 

the dusty air to a filtration device, and an exhaust fan to power the 

system. Dust emissions from this circuit can result from several 

sources including the collector exhaust, drill stem bushing leakage, 

shroud leakage and the dust collector dump discharge (Organiscak 

and Page, 2005). From a control standpoint, poor collector exhaust 

quality and drill stem leakage are largely a function of improper 

maintenance, which can be easily remedied. In addition, an effective 

dust control technique, which makes use of a simply constructed 

shroud, has already been identified for minimizing collector discharge 

dust generation (Reed et al., 2004). Therefore, NIOSH research 

efforts have focused on minimizing leakage from the shroud with field 

evaluations demonstrating that openings or gaps in the shroud can 

significantly diminish dust capture (Zimmer and Lueck, 1986; Zimmer 

et al., 1987). Oftentimes these gaps are unavoidable due to uneven 

and sloping ground surfaces and outward forces created by the 

accumulation of drill cuttings at the base of the shroud. 

Figure 1. Dry Dust Collection Circuit. 

Drill shrouds commonly found in the field can be categorized into 

two main types: those constructed of a single piece of material 

wrapped around the drill table, and those constructed of sectional 

pieces. A typical sectional arrangement is constructed of four separate 

pieces of conveyor material hung from each of the four sides of the drill 

table. Another common arrangement is a single piece of material 

around the table with a hinged access door on the front side of the 

shroud. The access door allows the operator to move the drill to the 

next hole without forcing the cuttings into the hole. Shrouds 

constructed of a single piece of material are subject mainly to 

horizontal leakage under the shroud, whereas sectional configurations 

are additionally subject to vertical leakage along the seams. 

Airflow patterns within the shroud enclosure were investigated in 

a full-scale drill table simulator (described in the next section) to 

determine if dust capture could be improved through simple 
modifications. 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 simulator’s 

shroud was constructed of transparent acrylic plastic to allow 

observation of the area enclosed by the shroud. The tuft grid was 

created by attaching light-weight vinyl streamers to fencing material, 

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 Figure 2. 

Figure 2. Coanda Air Effect Under Drill Shroud. 

As a result of a strong coanda airflow effect, the bailing air exits 

the drill hole, travels up the middle of the shrouded area, maintaining 

its course along the drill steel to the underneath side of the drill table, 

where it fans out across the bottom of the drill table and continues 

down the sides of the shroud. A dead air zone was found between the 

drill steel and sides of the shroud. Dust emissions from horizontal 

leakage occurred when the air traveling downward along the sides of 

the shroud struck the ground and fanned out underneath the 

enclosure. This leakage was observed when smoke tubes were 

aspirated at the base of the shroud. 

A concept to reduce dust emissions caused by the above process 

was to disrupt this airflow and to redirect it toward the center of the 

enclosure to prevent air from striking the ground. This was 

accomplished, as shown in Figure 3, by installing a 15.2-cm (6-in) wide 

horizontal shelf along the inside perimeter of the shroud. This paper 

describes the laboratory development of the concept and provides 

important design criteria for its successful application. 

TESTING FACILITY

All preliminary and designed experimentation was conducted in 

NIOSH’s full-scale drill table simulator located at the Pittsburgh 

Research Laboratory. The simulator is shown in Figure 4 and 

described in detail in previous publications (Organiscak and Page, 

2005). Operating parameters similar to a medium-sized rock drill can 

be produced in the lab. The dimensions of the shroud, which is 

located in the center of the dust chamber, are 1.52 m wide by 1.22 m 

deep by 1.22 m high (5 ft x 4 ft x 4 ft). The dust chamber dimensions 

are 3.66 m wide by 3.05 m deep by 2.44 m high (12 ft x 10 ft x 8 ft). 

The base of the shroud is fitted with hinged sections of plywood, 

allowing simulation of horizontal leakage at the ground surface with 

shroud gap heights of 5.1, 20.3, and 35.6-cm (2, 8, and 14-in). In 

order to generate bailing airflow velocities typical of those found in the 

field, a compressor capable of generating 326 l/s (690 ft3

/min) at 758 

kPa (110 lb/in2

) is used to force air down the inside of a 15.2-cm (6-in) 

schedule 40 pipe, which simulates the drill steel. The 15.2-cm (6-in) 

pipe is concentrically located and suspended within a capped 20.3-cm 

(8-in) schedule 80 pipe, which simulates the drill hole. In this way, air 

is forced through the end of the 15.2-cm (6-in) pipe and then upward 

between the outside diameter of the 15.2-cm (6-in) pipe and inside 

diameter of the 20.3-cm (8-in) pipe at velocities exceeding 30.5 m/s 

(6,000 ft/min). In order to simulate dust generation, a Vibra-screw 

feeder is used to meter a constant flow of limestone dust (100% < 200 

mesh) into the bailing airflow using a compressed air eductor. The 

dust collector utilized in the simulator is capable of providing a 

maximum airflow of 944 l/s (2,000 ft3

/min). The collector inlet is 

rectangular, measuring 45.7-cm by 19.1-cm (18-in x 7.5-in), and is 

located in the corner of the drill table, which is typical of field 

conditions. 

Figure 3. Air Blocking Shelf Concept. 

During operation of the simulator, the dust concentration within

the test chamber reaches a state of equilibrium because make-up 

airflow equals collector airflow and dust feed is constant. Adjustable 

louvers on top of the chamber ensure that make-up air velocities are 

significantly high to prevent dust leakage from the chamber. This 

chamber configuration allows a direct comparison of the efficacies of 

various simulated drilling parameters and control technologies on 

shroud leakage. 

PRELIMINARY TESTING

Preliminary testing was conducted to determine if the horizontal 

shelf warranted additional experimentation. It was also used to 

determine whether basic design criteria, such as shelf width and 

location, as well as material loading on top of the shelf, played any role 

in the effectiveness of the shelf for reducing dust concentrations. 

Preliminary testing was conducted with two GCA real-time aerosol 

(RAM) instruments located outside the shroud at the front side (on 

inlet) and rear side (off inlet) of the drill table. Tests were conducted 

using an A-B format over a 30-minute sampling period, where the 

horizontal shelf was utilized during the first 15 minutes of the test and 

then removed for the last 15 minutes. All preliminary testing was 
conducted using a collector to bailing airflow ratio (airflow ratio) of 2:1, 

and shroud gap height at 5.1-cm (2-in). 

Figure 4. NIOSH’s Full-Scale Drill Table Simulator. 

The first series of tests were conducted to determine if blocking 

width affected dust capture. Horizontal shelves were constructed of 

plywood at widths of 15.2, 8.9, and 3.8-cm (6, 3.5, and 1.5-in). Each 

shelf was tested at a height of 81.3-cm (32-in) above the bottom of the 

shroud, which equated to 2/3 of the total shroud height. All of the 

boards significantly improved capture, decreasing average dust levels 

outside of the box by 92, 86, and 81%, respectively. Since decreasing 

the board width appeared to slightly diminish the effectiveness of the 

horizontal shelves, a 15.2-cm (6-in) board width was used for a 

subsequent two-level factorial designed experiment. 

A second series of tests were conducted to determine if board 

location would influence capture efficiency. For the first test, all of the 

8.9-cm (3.5-in) boards were placed at a lower location about 6.4-cm 

(2.5-in) above the ground. During this test, dust levels were reduced 

about 72%, indicating that this is a less desirable location because the 

8.9-cm (3.5-in) boards lowered dust 86% at the higher location. Also, 

the higher location is less likely to interfere with the cuttings pile 

generated during drilling; therefore, no further testing was conducted at 

the lower position. For the second test, the positions of the 15.2-cm 

(6-in) boards were offset to determine if gaps in placement would 

diminish performance. The front side board was lowered 10.2-cm (4-

in), while the rear side board was raised 10.2-cm (4-in). During this 

test, dust levels outside of the shroud were reduced by 86%, indicating 

that locating the boards on the same horizontal plane was not an 

important design criterion. This may be a beneficial attribute for 

installation because operators will not have to be concerned with 

placing all of the blocking shelves on the same horizontal plane as long 

as they cover the parametric cross-section of the shroud. However, for 

final testing, the 15.2-cm (6-in) boards were kept on the same 

horizontal plane. 

The last preliminary test was conducted to determine if the buildup of material on the shelf would negate the dust reduction effects of 

the horizontal shelf by altering the path of airflow within the shroud. 

For this test, sections of 20.8-cm (8.2-in) wide cardboard were secured 

between the sides of the shroud and the outer edges of the 15.2-cm 

(6-in) boards to simulate material collected on top of the shelf at a 43 

degree angle of repose. During this test, dust levels were reduced 

76%, indicating a somewhat diminished but still dramatic reduction in 

dust under the loaded condition. The unloaded condition was used for 

the designed experiment since any material buildup would fall off when 

the drill mast is lowered. 

TWO-LEVEL FACTORIAL EXPERIMENT 

As a comparison to the successful testing detailed above, a twolevel factorial experiment was designed to determine the dust 

reduction potential of the horizontal shelf under more severe operating 

conditions. Collector to bailing airflow ratios of 2:1 and 1.25:1 were 

tested, as well as shroud gap heights of 5.1 and 20.3-cm (2 and 8-in). 

Using these parameters and conducting experiments both with and 

without the use of the horizontal blocking shelf resulted in a matrix of 8 

(2 x 2 x 2) different tests. Each test was run three times for a total of 

24 tests. All 24 tests were conducted in random order (Table 1). 

Table 1. Two-Level Factorial Experiment Matrix and Average 

Gravimetric Results. 

Airflow ratios of 2:1 and 1.25:1 were attempted by targeting

collector volumes of 472 and 295 l/s (1,000 and 625 ft3

/min), while 

keeping the bailing airflow volumes constant at 236 l/s (500 ft3

/min). 

After adjusting for temperature and pressure changes, actual bailing 

airflow volumes averaged 242 l/s (513 ft3

/min), ranging from 236 to 245 

l/s (500 to 519 ft3

/min). For tests targeting a collector volume of 472 l/s 

(1,000 ft3

/min), the actual levels varied between 437 and 464 l/s (925 

and 984 ft3

/min) and averaged 450 l/s (953 ft3/min), resulting in an 

average airflow ratio of 1.9:1. Tests targeting a collector volume of 295 

l/s (625 ft3

/min) varied between 278 and 300 l/s (589 and 636 ft3/min) 

and averaged 288 l/s (610 ft3

/min), resulting in an average airflow ratio 

of 1.2:1. Experimental airflow ratios were slightly lower than targeted 

ratios due to a slight drop in collector airflow observed during the 

course of each test, which occurred due to material loading on the 

baghouse filters. 

Respirable dust levels outside of the shroud were measured using 

samplers consisting of 37-mm filter cassettes and 10-mm Dorr-Oliver 

cyclones operated at a flow rate of 2 l/min. Two samplers were placed 

on each side of the box, approximately 40.6-cm (16-in) from the box 

and 61.0-cm (24-in) above the ground. The RAM samplers described 

in the preliminary testing section were also used during testing to 

determine when dust levels in the chamber had stabilized. At this 

point, the gravimetric samplers were energized for a period of 30 

minutes for each test. The limestone dust feed rate averaged 23.8 

g/min during the factorial experiments, ranging from 21.4 to 25.0 g/min. 

This level allowed sufficient gravimetric filter loading during low dust 

concentration tests. The final chamber dust concentration was 

determined from the average of the eight gravimetric samples. 

ANALYSIS OF TWO-LEVEL FACTORIAL EXPERIMENT

Use of a two-level factorial design experiment facilitated an 

analysis of variance (ANOVA) to determine the relative importance of 

the independent variables, including airflow ratio, shroud gap height, 

and the use of the blocking shelf on the dependent variable of dust 

concentration outside the shroud. Table 1 shows the experiment 

matrix and average gravimetric results. Table 2 shows the ANOVA for 

the experiment. Each independent variable was found to be highly 

significant (95% confidence interval did not include zero) and had a 
similar impact, in terms of magnitude, on dust levels outside of the 

shroud. Results from the experiment suggest that the use of a 

horizontal blocking shelf can overcome the detrimental effects on dust 

concentrations resulting from increasing the horizontal gap between 

the bottom of the shroud and the ground from 5.1 to 20.3-cm (2 to 8-

in), or from decreasing the airflow ratio from 1.9:1 to 1.2:1. 

Table 2. ANOVA and Regression Statistics for Two-Level Factorial 

Experiment. 

Figure 5 shows dust levels under various operating conditions 

both with and without the use of the air blocking shelf. Figure 6 shows 

relative percentage dust reductions measured when using the shelf. At 

an airflow ratio of 1.9:1 and a 5.1-cm (2-in) shroud gap height, the use 

of the shelf decreased dust levels outside of the shroud by 81%. The 

reduction achieved with the shelf was 70% at a ratio of 1.9:1 and a 

20.3-cm (8-in) gap, and 77% at a ratio of 1.2:1 and a 5.1-cm (2-in) gap. 

A reduction of 38% was achieved through the use of the shelf under 

poor operating conditions consisting of an airflow ratio of 1.2:1 and a 

20.3-cm (8-in) shroud gap height. The independent variables 

examined for this experiment accounted for approximately 90% 

(adjusted R2

) of the difference in measured dust levels outside of the 

shroud. 

Figure 5. Dust Concentrations Outside the Shroud With and Without 

the Air Blocking Shelf, Standard Deviation Bars Shown. 

ANALYSIS OF THE BLOCKING SHELF WHEN VERTICAL 

LEAKAGE OCCURS 

Two separate experiments were conducted to test the 

effectiveness of the air blocking shelf for reducing dust when vertical 

leakage was present in the shroud design. As discussed earlier, 

vertical leakage scenarios can arise when sectional conveyor belt 

pieces are incorporated into the shroud design. The first experiment 

examined full-seam vertical leakage, where the vertical leakage 

extended from the base of the shroud to the bottom of the drill table. 

This condition was simulated by attaching hinges to the topside of the 

plywood on the off-inlet side of the shroud and propping the shroud 

open a distance of 15.2-cm (6-in) at the base. The second experiment 

examined partial vertical seam leakage, where the vertical leakage 

extended from the base of the shroud to a point just below where the 

blocking shelves were installed. Figures 7 and 8 show the shroud 

configurations for both the partial and full vertical leakage scenarios, 

respectively. During these experiments, both the airflow ratio and 

shroud gap height were kept constant at 2:1 and 5.1-cm (2-in), 

respectively. A total of six tests were conducted for each experiment, 

alternating between using and not using the air blocking shelf. 

Figure 6. Measured Dust Reductions Using the Air Blocking Shelf. 

Figure 7. Partial Vertical Leakage Scenario. 

For the full vertical leakage experiment, dust concentrations 

outside of the shroud averaged 10.69 mg/m3

 (0.50 s.d.) when using the 

shelf, and 10.61 mg/m3

 (0.44 s.d.) when not using the shelf. 

Calculation of the 95% confidence interval (0.08 + or – 1.04) for 

comparing the difference between two means with equal variance 

could not exclude zero, indicating that no true difference between the 
means existed. Therefore; the air blocking shelf was not effective for 

controlling dust under the condition of full vertical leakage. 

Researchers hypothesized that air traveling down the sides of the 

shroud struck the shelf and was forced out through the gap created to 

simulate vertical leakage during the experiments. 

Figure 8. Full Vertical Leakage Scenario. 

For the partial vertical leakage experiment, dust concentrations 

outside of the shroud averaged 2.06 mg/m3

 (0.26 s.d.) when using the 

shelf, and 9.60 mg/m3

 (0.65 s.d.) when not using the shelf. Calculation 

of the 95% confidence interval (-7.54 + or – 1.11) for the difference 

between these two means excluded zero, indicating that a true 

difference did exist. The air blocking shelf reduced dust levels outside 

the shroud by 79% when the vertical leakage was contained below the 

shelf plane. 

The results from the vertical leakage experiments underscore the 

importance of maintaining tight shroud seam seals at least to the 

horizontal plane created by the lowest air blocking shelf. Angle iron 

installed at the seam corners may be suitable for creating these seals. 

CONCLUSIONS

One objective of a recently initiated NIOSH project was to develop 

an improved shroud design for lowering dust liberation from surface

mine drilling machines. Emphasis was placed on a design that would 

minimize resources and maintenance. During the course of the 

project, researchers discovered a coanda effect airflow pattern under 

the shroud enclosure created by the bailing air, which traveled upward 

along the drill steel, fanning out across the bottom of the drill table, and 

flowing down the sides of the shroud. This dust-laden bailing air struck 

the ground and fanned out under gaps between the shroud and the 

ground, resulting in respirable dust leakage to the environment. 

Laboratory tests showed that installation of a horizontal air blocking 

shelf around the inside perimeter of the shroud disrupted this coanda 

pattern and improved dust capture under the shroud. The blocking 

shelf was 15.2-cm (6-in) wide and located at a height equal to 2/3 the 

distance measured from the bottom of the shroud to the bottom of the 

drill table. Under operating conditions consisting of a 1.9:1 collector to 

bailing airflow ratio, and a 5.1-cm (2-in) shroud gap height, the air 

blocking shelf reduced dust levels outside the shroud by approximately 

81%. The air blocking shelf was also shown to be capable of 

overcoming the adverse impact of vertical leakage along the shroud 

seams, as long as this leakage was confined to an area below the 

horizontal plane defined by the shelf location. Performance of the shelf 

was diminished to a 38% reduction in dust levels when the operating 

conditions were poor, including a 1.2:1 airflow ratio and a 20.3-cm (8-

in) shroud gap height. 

Laboratory testing also identified several important design criteria 

for a horizontal air blocking shelf. First, a 10.2 to 15.2-cm (4 to 6-in) 

shelf width appeared to be sufficient. Second, lowering the shelf did 

not improve performance. Third, the shelves, which were located on 

each side of the shroud, did not have to be placed on the same 

horizontal plane in order to be effective. Rather, coverage of the 

parametric cross-section of the shroud seemed to be the important 

design criteria. Finally, testing also demonstrated that material loading 

on top of the shelf only slightly diminished performance. Future 

research efforts will focus on evaluating the air blocking concept for 

dust control on actual surface drilling machines. 

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