Use of a Directional Spray System Design to Control

Respirable Dust and Face Gas Concentrations Around

a Continuous Mining Machine

Gerrit V.R. Goodman and Douglas E. Pollock

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health,

Pittsburgh, Pennsylvania

A laboratory study assessed the impacts of water spray

pressure, face ventilation quantity, and line brattice setback

distance on respirable dust and SF6 tracer gas concentrations around a continuous mining machine using a sprayfan

or directional spray system. Dust levels were measured at

locations representing the mining machine operator and the

standard and off-standard shuttle car operators, and in the

return airway. The results showed that changes in all three

independent variables significantly affected log-transformed

dust levels at the three operator sampling locations. Changes

in setback distance impacted return airway dust levels. Laboratory testing also identified numerous variable interactions

affecting dust levels. Tracer gas levels were measured on the

left and right sides of the cutting drum and in the return.

Untransformed gas levels around the cutting drum were significantly affected by changes in water pressure, face ventilation

quantity, and setback distance. Only a few interactions were

identified that significantly affected these concentrations. Gas

levels in the return airway were grouped by face ventilation

quantity. Return gas levels measured at the low curtain quantity

were generally unaffected by changes in water pressure or

curtain setback distance. At the high curtain quantity, return

airway gas levels were affected by curtain setback distance.

A field study was conducted to assess the impact of these

parameters in an actual mining operation. These data showed

that respirable dust levels may have been impacted by a change

in water pressure and, to a lesser extent, by an increase in

curtain setback distance. A series of tracer gas pulse tests

were also conducted during this study. The results showed that

effectiveness of the face ventilation was impacted by changes in

curtain flow quantity and setback distance. Laboratory testing

supported similar conclusions.

Keywords coal mine dust, engineering controls, exposure assessment, silica, underground mining

Address correspondence to: Gerrit V. R. Goodman, Pittsburgh

Research Laboratory, NIOSH/Building–03, 626 Cochrans Mill Road,

Pittsburgh, PA 15236; e-mail: gcg8@cdc.gov.

Federal regulations require that the concentration of

respirable dust in the mine atmosphere not exceed

2.0 mg/m3 for a working shift.(1) If the silica content

on the dust filters exceeds 5% by weight, the 2 mg/m3

dust standard is reduced according to the following expression:

10 divided by the percent silica. In either case, this maintains

silica dust levels at or below 100 μg/m3.

These regulations also require that methane gas levels in the

face area be maintained below 1%. Methane readings obtained

with a hand-held methane monitor are usually taken as close

to the face as practical, although they cannot be taken closer

than 0.3 m (12 inches) from the roof, face, ribs, and floor.

Whenever methane readings are 1% or higher, mining must

stop until concentrations drop below this level.

Many factors can affect respirable dust exposures and

methane gas concentrations during coal mining. The impacts

of face ventilation quantity are well documented in the literature and show that respirable dust levels around the mining

machine drop significantly as exhaust face ventilation airflow

increases.(2,3) A blowing face ventilation scheme generally is

used with a mining machine equipped with a flooded-bed dust

scrubber. Face ventilation in underground coal mining is typically provided by a curtain hung from the roof approximately

1 m from the left or right wall. The curtain mouth is the region

defined by the seam height and the distance from the curtain

to the wall.

Fresh air issuing from the curtain mouth and flowing toward

the mining machine constitutes blowing face ventilation. Contaminated air flowing from the machine into the curtain mouth

constitutes exhaust face ventilation. Studies have shown that

increased blowing face ventilation flow above the flow rating

of the scrubber unit sometimes led to excessive dust rollback.(4)

However, exhaust face ventilation designs are generally preferred for dust control because dust-laden air does not pass

over the machine operator. Higher airflow quantities further

dilute face gas and result in decreased face gas concentrations.
A blowing ventilation design typically provides a higher rate

of airflow to the mining face than a comparable exhausting

design.(5)

Water sprays become effective movers of air as water spray

pressure increases, and this airflow creates turbulence that can

improve dilution of face gas.(6) However, this turbulence also

can push dust toward the mining machine operator, and this

can lead to higher respirable dust exposures for this person.(7)

Ventilation parameters, such as face airflow quantity, water

spray pressure, and curtain setback distance (distance from

curtain mouth to cutting head), must emphasize control and

confinement and minimize dispersion of the dust cloud to

improve suppression and capture of these particles. To control

face gas levels, however, these parameters must be selected

to provide turbulence to improve dilution of the gas. This

suggests that ventilation airflows, spray pressures, and setback

distances needed to control respirable dust concentrations may

not effectively control methane gas at the face.

For this study, laboratory work assessed the effects of face

airflow quantity, water spray pressure, and curtain setback

distance on respirable dust exposures for the continuous miner

and shuttle car operators. Dust levels also were monitored in the

return airway. Corresponding tracer gas concentrations were

measured at conventional methanometer locations and in the

return airway. A similar study evaluated these factors in an

underground mine environment. Due to concerns about worker

health and safety, only changes in setback distance could be

assessed. A series of tracer gas tests examined movement of

the gas pulse into the return as curtain quantity and setback

distance changed.

EXPERIMENTAL DESIGN

This laboratory work was conducted at a full-scale surface

test gallery simulating a cut 12.2 m (40 ft) deep, 5.5 m

(18 ft) wide, and roughly 2.0 m (6.4 ft) high. A full-scale

mockup of a continuous mining machine used for this testing

featured a 0.9 m (3 ft) diameter cutting drum rotating at 50 rpm.

The machine was positioned at the end of the sump or box cut

for testing. A coal slab measuring 2.4 m (8 ft) by 6.1 m (20 ft)

remained to the right of the machine (Figure 1).

The mining machine was equipped with a directional spray

system design (spray fan), which is a water-powered ventilation system originally designed for controlling methane gas

levels. This system contained several spray manifolds placed

on the continuous mining machine to direct fresh intake air to

the cutting face, sweep contaminated air across the face, and

direct this airflow into the return airway. The spray fan system

wasinstalled on the mockup of the continuous mining machine

using established guidelines.(6) Thirty-one BD3-3 hollow cone

nozzles (Spraying Systems, Wheaton, Ill.) were positioned

above, below, and along the sides of the cutting boom of the

mining machine (Figure 2). These sprays directed airflow to

the face to sweep dust and gas toward the return.(8) Water spray

pressures were 483 kPa (70 psi) or 1310 kPa (190 psi) measured

in the line feeding the spray manifold. Measured water flow

rates at low and high pressures were 78.1 L/min (20 gpm) and

124.7 L/min (33 gpm), respectively.

In practice, the spray fan system is used only with an exhaust face ventilation scheme. Exhaust curtain ventilation was

created in the laboratory by drawing air via a main gallery fan

through a curtain positioned on the left side of the test gallery.

Exhaust curtain ventilation flows were 2.8 m3/sec (6000 cfm)

or 5.7 m3/sec (12000 cfm), while curtain setback distances

were 9.2 m (30 ft) or 15.4 m (50 ft). Airflow was measured at the

mouth of the curtain using a conventional vane anemometer.

A mixture of silica dust in coal dust (average of 8.2% silica

by weight) was introduced into the gallery at the miner cutting

head via a compressed air/eductor system. Two LH-1

/2 brass

eductors (Penberthy, Prophetstown, Ill.) were connected to a

5 psi compressed air source. These eductors mixed the dust

mixture with the compressed air to deliver the dust to the cutting

head at a rate of 25 gpm. One eductor discharged along the

left front side of the cutting drum, while the other discharged

FIGURE 1. Full-scale test facility (plan view)
FIGURE 2. Spray locations on mining machine (plan view upper, side view lower)

along the right front of the drum. The rotating cutting head

drum insured adequate mixing of the dust with the ventilation

airflow.

For dust sampling, constant flow pumps pulled dust-laden

air through 10-mm nylon cyclone separators at a rate of 2 L/min

as specified by 30 CFR 70.205.(1) The respirable mass deposited onto preweighed 37-mm filters were subsequently postweighed. Dust levels were calculated from the net weight and

the total airflow. Selected filters were sent to an independent

and certified laboratory for silica analysis using XRD (X-ray

diffraction) infrared method number 7500.(9) A pair of gravimetric samplers were placed on the off-curtain (right) side of

the entry near the mouth of the exhaust curtain to represent the

location of the remote mining machine operator. This location

moved with curtain setback distance. Two pairs of samplers

placed 7.6 m (25 ft) outby the rear of the mining machine

on the left and right sides represented locations of the standard

and off-standard shuttle car operators, respectively. A standard

shuttle car operator sits on the left side of the car when traveling

toward the dump point. The operator of an off-standard car sits

on the right side when traveling toward the dump point. Finally,

gravimetric samplers placed downwind of the mining machine

monitored dust levels in that location (Figure 1).

A realtime aerosol monitor (RAM-1; Thermo Andersen

Inc., Smyrna, Ga.) dust monitor measured pretest (baseline)

conditions. Before each test, dust was introduced at the cutting

head for 10 min with the cutting head rotating and only the

exhaust curtain ventilation in operation. During this baseline

period before operating the water sprays, the dust feed rate was

adjusted to a level that produced uniform and visually stable

dust concentrations.

Testing also evaluated the impact of face airflow quantity,

water spray pressure, and setback distance on the removal

and dilution of hazardous gas concentrations. Due to safety

concerns, methane gas (CH4), which is typically found in all

underground coal mines, could not be used in this testing.

Instead, sulfur hexafluoride (SF6) was used as a surrogate or

tracer gas. This gas is tasteless, odorless, not found naturally

in the atmosphere, and is nontoxic in the amounts used in this

testing. It is also detectable in the ppm concentration level. SF6

was introduced at the cutting drum where the drum’s rotation

mixed the gas with the face airflow.

A model 1303 multipoint gas doser (California Analytical Instruments, Orange, Calif.) metered the SF6 into the

gallery at a rate of 6 mL/sec. Real-time measurement of the

SF6 gas levels was made with a model 1312 photoacoustic

gas monitor (California Analytical Instruments). Gas sampling

was conducted at locations representing typical methanometer

locations on a continuous mining machine: on the left and

right sides of the cutting boom approximately 0.6 m from

the cutting drum. Gas levels were also measured in the return

airway (Figure 1). For each test, gas was introduced into the

gallery in a series of four evenly spaced pulses, each pulse

lasting 12 min. These pulse durations were used to more accurately mimic a flow of methane gas into the underground mine

environment.

The water sprays controlling the respirable dust cloud would

not absorb sufficient quantities of either SF6 or CH4 to seriously

impact gas levels. SF6 and methane are soluble in water at a

rate of 5.4 cc and 30.1 cc per liter of water, respectively.(10) This

chemical property is typically measured in a closed system under equilibrium, whereas the full-scale test facility was neither

closed nor at equilibrium. For this reason, the solubilities of

these gases in water are less under test conditions. At the high

water flow of 124.7 L/min, the absorption rate of SF6 is less

than 10–4 mL/sec compared with the dose rate of 6 mL/sec, a
TABLE I. Untransformed Respirable Dust Levels Measured During Full-Scale Testing (n = 3)

Average Gravimetric Dust Concentration, mg/m3

(Standard Deviation)

Water Spray Exhaust Curtain Curtain Setback Continuous Standard Shuttle Off-Standard

Pressure (kPa) Flow (m3/s) Distance (m) Miner Operator Car Operator Shuttle Car Operator Return

483 2.8 9.2 0.73 (0.12) 0.56 (0.21) 1.27 (0.71) 5.53 (1.17)

1310 2.8 9.2 1.33 (0.33) 0.36 (0.07) 0.46 (0.09) 7.87 (6.95)

483 5.7 9.2 0.12 (0.03) 0.11 (0.02) 0.11 (0.02) 6.63 (1.23)

1310 5.7 9.2 0.17 (0.06) 0.14 (0.04) 1.08 (0.78) 7.54 (3.18)

483 2.8 15.4 0.19 (0.05) 0.50 (0.18) 4.52 (1.21) 5.26 (1.49)

1310 2.8 15.4 0.88 (0.17) 1.79 (0.25) 3.52 (0.59) 3.03 (0.35)

483 5.7 15.4 0.24 (0.07) 0.26 (0.05) 4.60 (0.89) 3.80 (0.38)

1310 5.7 15.4 0.42 (0.32) 0.45 (0.45) 3.75 (0.64) 3.10 (0.45)

loss of less than 1%. At the same water flow, CH4 is absorbed

by the water sprays at a rate of 0.29 mL/sec. For a typical CH4

dose rate of 1300 L/min,(11) less than 1% of the methane gas is

absorbed. Neither SF6 nor CH4 levels are sufficiently affected

through absorption by the water sprays.

Tracer gas release and sampling ran concurrently with dust

sampling. Disposable filters, placed at the ends of the gas sampling tubes, prevented dust from entering the gas monitoring

equipment.

Robust assessments of the effects of curtain quantity, water

spray pressure, and setback distance on respirable dust and

tracer gas concentrations were made by testing with the high

and low values of these variables. A randomized full factorial

design evaluated each combination of independent variables.

Two replicates of each test were made to obtain a measure of

the error associated with each combination.

The dust concentration at each sampling location was calculated using the average of the concentrations for each group of

samplers. The tracer gas concentration at each location was

the average of the concentrations recorded for each pulse.

Respirable dust and tracer gas concentrations for each series of

test conditions were calculated using the average concentration

for each test comprising that set of conditions.

RESULTS AND DISCUSSION

Effects on Respirable Dust Exposures

The dust concentrations at each sampling location displayed

a tendency toward a log-normal distribution. Following log10

transformations, Shapiro-Wilks statistics ranged from 0.86 to

0.96 (p-value from 0.004 to 0.48) for the laboratory data. This

test rejected the assumption of normality for data collected

at the off-standard shuttle car location. Levene’s statistics for

homogeneity of variances ranged from 1.40 to 4.55 (p-value

from 0.27 to 0.01). This test rejected the null hypothesis of

equality of variances across groups for data collected at the return sampling location. Although some sampling data showed

departures from normality, the F tests used in analyses of variance (ANOVA) are robust and little affected by such departures.

The balanced design used for these analyses also minimizes the

impact of unequal variances within each grouping of curtain

quantity, water pressure, and setback distance.(12) Analyses of

data with a nonnormal distribution or with unequal variances

may produce significance levels slightly higher than would be

otherwise found.

The untransformed data in Table I show means and standard

deviations for operator and return sampling locations. Analyses

of variance were used to define the effects of the independent

variables on log10 transformed respirable dust levels.

As shown in Table II, curtain quantity and water spray pressure significantly affected (at a 0.05 confidence level) the log

transformed values of the respirable dust levels for the mining

machine operator. Not surprisingly, increasing curtain quantity

improved dilution of the dust cloud and led to the greatest

reductions at this position. Although reductions occurred for

all levels of water pressure, reductions were greatest when

testing with the highest water pressure. The reductions were

also high when increasing curtain airflow at the smaller curtain

setback distance.

Increasing water spray pressure increased dust levels for the

miner operator generally by increasing dust rollback levels.

Increasing water pressure had the most adverse effect on operator dust levels with low return curtain quantity. This supports

previous work regarding the importance of balancing airflow

and water pressure to control dust levels.(13)

Log transformed dust levels at the standard shuttle car operator were affected by changes in curtain quantity and setback

distance. Increasing the curtain quantity reduced dust levels by

improving dust dilution at the face. This effect was constant at

either setback distance. The 9.2 m setback distance produced

lower dust levels due to improved circulation of fresh air around

the mining machine.

Extending curtain setback distance produced statistically

significant changes in the transformed dust levels at the
TABLE II. Summary of ANOVA Statistics from LogTransformed Gravimetric Dust Data

Source of Variation

Sum of

Squares

Degrees of

Freedom F-Ratio

log10(machine operator)

A: water pressure 2.51 1 9.48A

B: curtain quantity 8.57 1 32.43A

C: curtain setback 0.08 1 0.29

Interactions

AB 1.00 1 3.80

AC 0.27 1 1.03

BC 3.58 1 13.53A

Error 4.49 17

Total 20.55 23

log10(standard shuttle car operator)

A: water pressure 0.62 1 1.83

B: curtain quantity 9.25 1 27.36A

C: curtain setback 3.49 1 10.33A

Interactions

AB 0.12 1 0.35

AC 0.87 1 2.58

BC 0.00 1 0.00

Error 5.74 17

Total 20.09 23

log10(off-standard shuttle car operator)

A: water pressure 0.11 1 0.28

B: curtain quantity 1.04 1 2.67

C: curtain setback 27.78 1 71.57A

Interactions

AB 3.09 1 7.97A

AC 0.75 1 1.92

BC 1.28 1 3.29

Error 6.60 17

Total 40.65 23

log10(return airway)

A: water pressure 0.10 1 0.81

B: curtain quantity 0.00 1 0.01

C: curtain setback 1.76 1 13.91A

Interactions

AB 0.03 1 0.24

AC 0.33 1 2.60

BC 0.14 1 1.08

Error 2.16 17

Total 4.52 23

Note: Critical F-value: F0.95,1,17 = 4.45. ASignificant at 0.05 level of confidence.

off-curtain side shuttle car operator by placing the curtain

mouth near this individual. Dust flowing near the curtain mouth

may have contaminated these samples. The interaction of water

pressure and curtain quantity produced significant changes in

dust levels at the off-curtain side operator. Increasing water

pressure at the higher curtain flow increased dust levels by

blowing dust back to this individual. The analytical results in

Table I show that dust levels at the off-standard shuttle car

location were generally much greater than levels measured at

the mining machine or standard shuttle car operator locations.

The inability to effectively protect the operator of the offstandard shuttle car limits its use with a sprayfan-equipped

mining machine.

Finally, changes in curtain setback distance resulted in statistically significant changes in transformed return airway dust

levels, a fact that contradicts the sampling results for the offstandard shuttle car operator. However, standard deviations

were sometimes very high when sampling in the return. This introduced considerable uncertainty into the analyses of variance

at this location making it difficult to identify significant

variations.

Silica dust levels at the mining machine and standard shuttle car operator sampling locations were generally below the

0.01 mg limit of detection or the 0.03 mg limit of quantification

for NIOSH Method 7500. Even at the off-standard shuttle car

location, silica dust levels were not detected for tests at the

smaller setback distance of 9.2 m. At the larger distance of

15.2 m, the respirable dust samples contained from 3.2 ±

1.5% to 4.3 ± 1.9% silica by weight. Silica dust contents

in the return varied from 2.4% to 5.0% by weight. However,

analyses of silica contents versus changes in water pressure,

curtain quantity, and setback distance showed no significant

differences at a 95% level of confidence.

Ventilation airflow quantity and setback distance are largely

ineffective for selective control of respirable silica dust. Previous work showed that high water pressure can provide added

control of smaller sized silica particles.(14) However, water

pressures recorded for that work were 17,250 kPa, much higher

than the pressures reported in the current work.

Effects on SF6 Levels

Although respirable dust concentrations were log transformed to improve normality and homogeneity of variances,

similar adjustments were not necessary for the tracer gas data.

Shapiro-Wilks statistics ranged from 0.86 to 0.94 (p-value

from 0.004 to 0.17) with nonnormality discovered at the return

sampling location. Homogeneity of variances was not a major

concern with Levene statistics ranging from 1.46 to 1.54 (pvalue from 0.25 to 0.22) for the gas concentrations.

Increases in curtain flow quantity and curtain setback distance significantly impacted SF6 tracer gas concentrations on

the off-curtain or right side of the cutting drum (Tables III

and IV). The data generally showed lower gas levels with

increasing curtain flow quantity suggesting improved dilution

at this higher flow. Increasing curtain setback distance had

an adverse effect on gas levels by reducing dilution airflow

quantity to the face. The interaction of curtain flow quantity

and water pressure significantly impacted off-curtain side gas

levels. Increasing curtain flow produced the greatest drop in

off-curtain side gas levels when coupled with high water pressure. No other interactions were statistically significant.
TABLE III. Untransformed SF6 Tracer Gas Concentrations Measured During Full-Scale Testing (n = 3)

Average SF6 Tracer Gas Concentration, ppm

(Standard Deviation)

Water Spray Exhaust Curtain Curtain Setback Cutting Drum, Cutting Drum, OffPressure (kPa) Flow (m3/sec) Distance (m) Curtain (left) Side Curtain (right) Side Return

483 2.8 9.2 2.80 (0.24) 0.45 (0.15) 2.22 (0.08)

1310 2.8 9.2 2.07 (0.15) 0.60 (0.19) 2.37 (0.23)

483 5.7 9.2 2.51 (0.12) 0.20 (0.09) 1.10 (0.08)

1310 5.7 9.2 1.46 (0.02) 0.09 (0.04) 1.15 (0.05)

483 2.8 15.4 2.88 (0.09) 0.73 (0.11) 2.17 (0.17)

1310 2.8 15.4 2.38 (0.16) 1.03 (0.06) 2.29 (0.12)

483 5.7 15.4 2.79 (0.19) 0.44 (0.08) 1.03 (0.30)

1310 5.7 15.4 1.73 (0.07) 0.27 (0.03) 0.73 (0.18)

Water pressure, curtain quantity, and curtain setback distance all significantly impacted tracer gas concentrations on

the curtain (left) side of the cutting drum. Tracer gas concentrations decreased as water spray pressure and exhaust curtain

quantity increased, likely a result of improved dilution. This

follows from previous work showing that higher water spray

pressures create additional airflow and that this airflow can

be used to dilute and remove harmful quantities of gas.(6,15)

Gas levels were slightly higher at the larger curtain setback

distance suggesting that dilution airflow quantities decreased.

The interaction of water pressure and curtain airflow quantity

significantly impacted gas levels on the left side of the cutting

drum. Increases in curtain quantity had a slightly greater impact

to reduce gas levels when water pressure was highest. Remaining interactions were not significant. This testing also showed

that gas concentrations were highest on the curtain side of the

cutting head, indicating that the sprayfan effectively “scouredout” the right or off-curtain side. Similar results were reported

by other researchers.(8)

Although return airway gas levels exhibited homogeneity of

variance, the distribution of gas concentrations was nonnormal.

The data in Table III show a distinct grouping of return gas concentrations at flow rates of 2.8 and 5.7 m3/sec. Examining these

two groupings of return gas levels individually improved the

normality of the data. Shapiro-Wilks statistics improved to 0.93

(p-value = 0.38) for data at curtain flows of 2.8 and 5.7 m3/sec.

Levene statistics were only slightly affected (statistic = 2.20,

p-value 0.16).

At a curtain flow of 2.8 m3/sec, analyses of variance showed

no statistically significant impacts of water pressure or curtain

setback distance on return gas levels, suggesting that return gas

levels were largely a function of this curtain quantity alone.

At the larger curtain flow of 5.7 m3/sec, changes in curtain

setback distance produced statistically significant reductions

in return gas levels, although this did not necessarily indicate

improved ventilation at this larger setback distance. Rather, this

showed that poor ventilation at the face likely caused a plug of

gas to form between the cutting head and return curtain. This

phenomenon also explains the increased gas concentrations at

the cutting head sampling locations. The lower gas levels in the

return were due to the ventilation airflow diluting the periphery

of the gas plug.

Field Evaluation of Dust and Gas Concentrations

The effects of varying ventilation parameters on respirable

dust and face gas levels also were assessed at an underground

coal mining operation. Mining activity occurred in five entries

that were an average of 2.7 m high and 5.5 m wide. Face

ventilation was provided by an exhaust line curtain hung along

the left rib. Curtain flow varied with entry location but averaged 3.3 m3/sec (7200 cfm). The mining machine used 26

water sprays arranged in a spray fan pattern with each spray

delivering 2.4 L/min at 689.5 kPa (0.63 gpm at 100 psi). Spray

pressure varied from 621 kPa to 793 kPa (90 to 115 psi) as

measured on the cutting drum of the mining machine.

Assessment of Respirable Dust Concentrations

A combination of gravimetric and continuous-recording

instruments measured dust levels on and around the mining

machine. While the gravimetric samplers were identical to

those used in the laboratory phase of this study, the continuousrecording samplers were personal DataRam (pDR) dust

monitors (Thermo MIE, Inc., Smyrna, Ga.). A permissible

air sampling pump pulled dust-laden air through a 10-mm

nylon cyclone at a rate of 2 L/min. This aerosol then passed

through a light-scattering chamber where the amount of scatter

represented a respirable dust concentration. This pDR device

contained an internal data logger that recorded dust concentrations every 10 sec.

Two shifts of exposure data were collected during the underground phase of this work. For the two shifts, coal productions

were 696 and 960 tons while actual operating times of the

mining machine were 305 and 218 min, respectively. A third

shift was invalidated due to excessive downtime spent repairing

a bad trailing cable.
TABLE IV. Summary of ANOVA Statistics from SF6

Tracer Gas Data

Source of Variation

Sum of

Squares

Degrees of

Freedom F-Ratio

Off-curtain side location

A: water pressure 0.01 1 0.99

B: curtain quantity 1.24 1 107.90A

C: curtain setback 0.47 1 41.2A

Interactions

AB 0.21 1 18.22A

AC 0.003 1 0.27

BC 0.03 1 2.99

Error 0.20 17

Total 2.16 23

Curtain side location

A: water pressure 4.20 1 203.47A

B: curtain quantity 1.02 1 49.39A

C: curtain setback 0.34 1 16.32A

Interactions

AB 0.29 1 14.09A

AC 0.02 1 0.93

BC 0.01 1 0.49

Error 0.35 17

Total 6.23 23

Return airway—2.8 m3/sec

A: water pressure 0.05 1 2.05

C: curtain setback 0.01 1 0.51

Interactions

AC 0.00 1 0.02

Error 0.50 8

Total 10.35 11

Return airway—5.7 m3/sec

A: water pressure 0.04 1 1.40

C: curtain setback 0.18 1 5.54A

Interactions

AC 0.05 1 2.93

Error 0.50 8

Total 10.35 11

Notes: Citical F-value for off-curtain side and curtain side readings:

F0.95,1,17 = 4.45. Critical F-value for return readings: F0.95,1,8 = 5.32. ASignificant at 0.05 level of confidence.

Dust sampling packages consisting of a pDR dust monitor

and a gravimetric sampler were positioned together in the return airway and on a researcher shadowing the mining machine

operator. This operator typically stood away from the machine

on the right side of the entry. These sampling packages also

were placed on the left and right rear corners of the mining

machine. Two standard shuttle cars (no off-standard cars) were

used to haul coal at this operation. However, dust levels were

not measured on this equipment. Left rear and right rear corner

sampling locations on the mining machine were used in lieu

of shuttle car sampling locations.

The results given in Table V show that machine operator

dust levels were fairly low. Dust levels at the left and right

rear corners of the mining machine are potential indicators

of dust rollback from the cutting drum. These concentrations

were highest on the left side, not unexpected with the exhaust

curtain hung on the left side of the entry.

Right rear corner dust levels can serve as an indicator of

potential exposure for the machine operator; as this individual

stands nearest to this corner. First and second shift data showed

similarities between dust concentrations at the right rear corner

and those measured around the operator. Intake dust levels for

the first and second shifts were 0.26 mg/m3 and 0.14 mg/m3,

respectively.

The percentage of silica dust on the samples increased from

the machine operator sampling location to the return airway

sampling location. Silica contents at each location were similar

for the two sampling days.

A booster pump failed to operate the first shift, resulting

in a water pressure of 621 kPa measured with a dial gauge on

the cutting head. This pump operated during the second shift

and water pressure increased to 793 kPa. Dust levels decreased

from the first shift to the second shift, although the reductions at

the machine operator and right rear corner sampling locations

approximated the reduction in intake dust level (Table V).

However, the reductions seen on the downwind side of the

mining machine (left rear corner and return) far exceeded the

reduction in intake dust. This suggests that perhaps even this

small increase in water pressure on the second shift had a positive impact on downwind dust concentrations. Previous work

showed that small increases in water pressure improved dust

removal capabilities.(16) Although slightly smaller increases

were noted at this operation, the prior work suggests that even

this may be beneficial from a dust control standpoint.

However, it is understood that many factors can affect dust

concentrations around a continuous mining machine. These include, but are not limited to, production rate, cutting technique,

variations in seam geology, and so on. The aforementioned

tonnages and operating times yield similar production rates

for both shifts. Also, a majority of the development work for

these two shifts occurred with the mining machine cutting

straight-ahead, as opposed to this machine turning in the entry

to create an air connection to the adjacent entry. This suggests

that similar levels of dust generation may have existed during

each shift.

Impact of Curtain Setback Change

Another facet of this study assessed the impacts of setback

distance changes on respirable dust levels measured on and

around the mining machine. This required the measurement of

dust levels for discrete time periods corresponding to specific

curtain setback distances. The pDR devices provided such a

time-based output of dust levels at the left and right rear corners

of the mining machine, in the return airway, and near the mining

machine operator. The ratio of gravimetric concentration to

average pDR dust level at each location provided a means to

calculate equivalent gravimetric dust concentrations for these
TABLE V. Gravimetric Analysis of Field Data

Sampling Respirable Dust Silica Dust

Sampling Location Time (min) Concentration (mg/m3) Content (%)

First shift

Continuous miner operator (shadow) 484 0.39 4.0

Mining machine—right rear corner 481 0.56 4.0

Mining machine—left rear corner 481 1.26 6.3

Return airway 480 2.72 7.5

Second shift

Continuous miner operator (shadow) 426 0.22 3.1

Mining machine—right rear corner 427 0.40 5.6

Mining machine—left rear corner 427 0.60 5.6

Return airway 380 1.75 7.2

time periods. Multiplying this ratio by the average pDR reading

for the time period in question gave a weighted gravimetric dust

level for that period.

Time study data compiled during this underground study

showed that the average curtain setback distance was either

2.4 m (8 ft) or 4.9 m (16 ft), depending on the position of the

machine within the cut. This analysis, therefore, examined the

impact of these two curtain setback distances on pDR-adjusted

respirable dust levels at the previously mentioned sampling

locations.

Table VI shows that only slight increases in pDR-adjusted

dust levels were seen with curtain setback distance. The largest

increases in dust levels were found at the left and right rear

corners, suggesting that the increase in curtain setback distance

affected the amount of dust rollback from the cutting head.

Increases in dust levels were similar at the machine operator

sampling location suggesting that movements of this individual

during mining were consistent for the two shifts.

Assessment of Face Gas Levels

This study also assessed the effects of varying curtain setback distance and return curtain quantity on face gas concentrations. A more precise measurement of these impacts

was possible using sulfur hexafluoride tracer gas. Ten cubic

centimeters of the gas were released on the right side of the

TABLE VI. Summary of pDR-Adjusted Dust Levels vs. Setback Distance

2.4 m Curtain Setback Distance 4.9 m Curtain Setback Distance

Average pDR-Adjusted Average pDR-Adjusted

Number Dust Concentration, mg/m3 Number Dust Concentration, mg/m3

Sampling Location of Samples (Standard Deviation) of Samples (Standard Deviation)

Mining machine operator 2 0.26 (0.04) 2 0.31 (0.04)

Machine right rear corner 2 0.48 (0.08) 2 0.62 (0.01)

Machine left rear corner 2 1.08 (0.64) 2 1.20 (0.78)

Return airway 2 3.67 (0.14) 2 3.71 (0.19)

mining machine about 1.5 m away from the cutting drum.

Gas samples then were collected in small evacuated bottles

at a location 4.5 m outby the mouth of the return curtain

(Figure 3).

Gas samples were taken in the following manner: 6 samples

at 5-sec intervals, 3 samples at 10-sec intervals, 4 samples at

15-sec intervals, and 2 samples at 3-sec intervals, resulting in

15 samples in 180 sec. This scheme was similar to that used

in previous testing.(17) Frequent sampling at the beginning of

the interval caught the rise and decay of the gas pulse as it

traversed the face and entered the return airway. Less frequent

sampling at the end caught the tail of the pulse.

The mining company stated that the most appropriate machine location for the tracer gas testing was at the end of a

3.0-m cut on the right side of the face (Figure 3). During these

tests, the cutting drum of the machine rotated and all water

sprays operated. Four different face ventilation configurations

were then assessed with the machine at this location.

1. Baseline conditions with the curtain mouth 0.6 m from

the face and 4.2 m3/sec measured at the curtain mouth.

2. Curtain mouth 3.6 m from the face and 4.2 m3/sec measured at the curtain mouth.

3. Curtain mouth 3.6 m from the face and 0.7 m3/sec measured at the curtain mouth. This low curtain flow condition occurs when the roof bolters raised the ventilation
FIGURE 3. Gas release and sampling locations (plan view)

curtain to move their machine around the mining machine.

4. Curtain mouth 0.6 m from the face and 0.7 m3/sec measured at the curtain mouth.

This release strategy differed from the laboratory evaluations, in that shorter term pulses were used. Given the highly

dynamic nature of the underground mine environment, releasing longer pulses of tracer gas may not have provided

additional insight. Given the shorter term pulses used in this

underground testing, the ventilation could not be assessed in

a conventional manner that is, by calculating and comparing

average gas concentrations. Instead, other parameters were

used.

Previous analyses of tracer gas data considered the time

required for measurable gas levels to reach the return curtain

sampling location. This time reflected the effectiveness of the

face ventilation for moving gas from the cutting face to the

return curtain.(16) Other measures include the duration or residence time of the gas pulse at the face as a gauge of face

ventilation efficiency.

Previous work with residence time distributions is limited

to analyses of chemical reactors where a pulse of nonreactive tracer material is injected into the reactor input stream

and sampled in the effluent stream.(18) Residence time analyses calculated the lengths of time required to remove various

elements of the SF6 pulse from the face area. The distribution of these times is called the exit age distribution, E(t), or

the residence time distribution of the tracer gas. By definition,

the area under the E(t) curve is unity. The mean residence time

(MRT) is defined as the first moment of the residence time

distribution.(19) A well-ventilated face will not only show a

FIGURE 4. Residence time distributions and mean residence

times (MRT) for the four test conditions

quick arrival of measurable gas levels at the return sampling

location but also will show a short MRT indicating quick

removal of the gas from the face area.

Residence time distributions for the four different ventilation conditions show that arrivals of measurable gas concentrations were similar for the four different ventilation conditions (Figure 4). This figure also shows that the shortest mean

residence time of 23 sec occurred with a curtain setback of

0.6 m and a curtain flow of 4.2 m3/sec. Increasing curtain

setback distance to 3.6 m lengthened the MRT to 35 seconds, suggesting that the gas pulse resided longer at the face.

Decreasing exhaust curtain flow to 0.7 m3/sec dramatically

increased the capture time to 76 seconds, showing that the

gas pulse was being slowly removed from the face area. Reextending curtain setback distance to 0.6 m reduced the MRT to

72 sec.

The shapes of the E(t) curves reflect the efficiency of the

ventilation flow for removing tracer gas from the face area. The

curve obtained with a setback distance of 0.6 m and a curtain

flow of 4.2 m3/sec shows a pronounced spike suggesting that

the tracer was quickly removed. This shape is indicative of

flow exhibiting little mixing where each portion of the tracer

gas moves uniformly through the face area.(18) The two E(t)

curves with curtain flows of 0.7 m3/sec never reach zero on

the vertical axis. This suggests that measurable quantities of

tracer gas were still found behind the exhaust curtain at t =

180 sec. Because the tracer gas concentration will eventually

reach zero at some time beyond 180 sec, the MRT analysis

used earlier is a conservative estimate for these curves. The

gas concentrations measured beyond t = 180 sec would extend the mean residence time beyond those values shown in

Figure 4.

The data in Figure 4 show that curtain quantity had the most

dramatic effect on the behavior of the tracer gas pulse, a result

confirmed by the full-scale testing presented earlier. Curtain

setback distance had a much less pronounced impact on the

pulse curve. This also was evident from the full-scale laboratory results. In the laboratory testing, only a minor interaction

was observed between curtain quantity and curtain setback

distance at a curtain flow of 5.7 m3/sec. The underground tests
showed that varying curtain setback distance had a greater

impact at the higher curtain flow rate of 4.2 m3/sec than at a

rate of 0.7 m3/sec. At this flow rate, the effectiveness of the

face ventilation to remove the tracer gas was marginal and

changing curtain setback distance from 0.6 to 3.6 m produced

little change in mean residence time.

CONCLUSIONS

The laboratory study showed that increases in water spray

pressure led to increased levels of dust rollback, particularly at the lower return curtain quantity. This resulted in

higher exposures at the mining machine operator and standard

shuttle car operator. Increases in curtain quantity generally

led to reductions in dust levels, although some increases were

noted at the off-standard shuttle car operator. Increases in

curtain setback distance resulted in significant increases in dust

exposures at all sampling locations. Exposures for the mining

machine operator were also impacted by the interactions of

water pressure and curtain quantity and curtain setback distance. The standard shuttle car operator was impacted by the

interaction of curtain quantity and curtain setback distance. No

interactions were found for the off-standard shuttle car operator

or return airway.

Tracer gas testing showed that increased curtain flow and

reductions in curtain setback distance provided additional dilution to reduce SF6 gas concentrations at the two cutting head

sampling locations. The benefits of higher spray pressure were

evident only at the curtain side sampling location. Interactions

between spray pressure and curtain flow quantity led to statistically significant effects at the curtain and off-curtain locations

only.

Return gas levels were grouped according to face ventilation

flow. Those gas levels measured at the low ventilation were not

affected by any changes in water pressure or setback distance.

Curtain setback distance affected gas levels measured at the

higher airflow.

Underground testing showed that a small increase in water

pressure may have reduced dust levels on the downwind side

of the mining machine and in the return airway. The study

showed that increases in setback distance also led to slight

increases in gravimetric dust levels for the miner operator.

Dust levels measured on the left and right rear corners of the

machine also increased. The laboratory data showed similar

results.

Tracer gas tests showed that increases in setback distance

and decreases in curtain quantity both negatively impacted the

effectiveness of the face ventilation to remove the gas pulse.

However, reductions in curtain quantity impacted the ventilation much more seriously than increases in curtain setback

distance.

ACKNOWLEDGMENTS

The authors would like to thank Emery Chilton for his

assistance in quantifying the solubilities of the SF6 and

CH4, and the coal company and its mine workers for assisting

with the underground portion of the study.

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