Impact of fan type for reducing respirable dust at an underground 

limestone crushing facility 

G.J. Chekan, J.F. Colinet & R.H. Grau III 

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

ABSTRACT: Researchers from the National Institute for Occupational Safety and Health have demonstrated 

that mine-wide ventilation in large-opening stone mines can be improved by using low-pressure, high-volume 

propeller fans and constructing stoppings in key locations to direct and control airflow. In light of these findings, a comparative study was conducted to determine if a portable diesel-powered propeller fan could perform more efficiently for dust dilution and transport than an axial vane fan for localized ventilation. The objective of this study is to evaluate both fan types for ventilation efficiency at an underground dump/crusher 

facility in a limestone mine. Results showed an improvement with the propeller fan to dilute both the respirable dust and the respirable silica dust around the dump/crusher facility. Overall, an average reduction of 20% 

in respirable dust and silica was observed at the dump/crusher location with the propeller fan. 

1 INTRODUCTION 

The National Institute for Occupational Safety and 

Health (NIOSH) at the Pittsburgh Research Laboratory is currently involved in various research projects related to worker health and safety in underground metal/nonmetal mines. A primary area of 

research involves health issues in underground stone 

mines, a growing segment of the aggregates industry. Methods for reducing worker exposure to 

noise, silica dust, and diesel particulate matter are 

being addressed through various research programs. 

One approach to reducing worker exposure to silica 

dust in large-opening mines is to adopt ventilation 

practices that will increase airflow and dilute and 

transport harmful particulate from major dust 

sources. 

Baseline sampling surveys have shown that underground dumps and crushers in limestone mines 

can be a significant source of dust generation 

(Chekan et al, 2002). Inhalation of excessive levels 

of respirable silica dust can lead to the development 

of silicosis in mine workers. Although the health 

hazards from silica dust have been well documented 

for many years, the problem of overexposure still 

persists for a number of job occupations in both underground stone and metal/nonmetal mining operations. Compliance sampling from the Mine Safety 

and Health Administration (MSHA) database, for a 

5-year period from 2000 through 2004 shows that, 

on average, 14% of all occupational samples exceed 

the permissible exposure limit. Occupations that exceed the average include mobile workers, crusher 

operators, and front end loader operators (MSHA, 

2000 – 2004). 

Typically, underground stone operations are drift 

mines developed after the economical quarry reserves have been exhausted. Room-and-pillar mining methods are utilized, with pillars of either square 

or rectangular dimensions ranging from 10.6 to18.3 

m (35 to 60 ft). The entries are considered large 

mine openings with entries widths ranging from 9.1 

to 18.3 m (30 to 60 ft) and entry heights on development ranging from 4.9 to 13.8 m (16 to 45 ft). 

After benching, entries can be over 18.3 m (60 ft) 

high. Due to these large openings, ventilation fan 

pressure is very low, even if significant quantities of 

air move through the mine. Ventilation pressures of 

less than 24.9 Pa (0.1 in w.g.) are common whether 

airflow is induced by fans, natural ventilation pressure, or a combination of both (Grau et al, 2002a). 

Depending on the extent of the workings, air velocities less than 0.13 m/s (25 fpm) are common, or in 

some idle areas of the mine, virtually nonexistent. 

In addition, airflow in the entries can be stratified, 

multi-directional, or readily affected by the movement of mine equipment. 

These large-opening mines require ventilation 

systems that are designed to effectively dilute airborne particulate by supplying sufficient ventilating 

air to primary locations, i.e., working faces, worker
locations, etc. To accomplish this, studies show 

(Grau et al, 2002a; Grau et al, 2002b; Head, 2001; 

Kissell and Volkwein., 2002; Timko, and Thimons, 

1987; Chekan et al, 2004a). that three key parameters should be included in the mine ventilation plan 

to improve airflow: (1) a main mine fan to establish 

air circuits on a mine-wide scale; (2) either permanent or brattice stoppings in key locations throughout the mine to more efficiently direct and control 

the airflow; and (3) the application of booster fans, 

to improve local ventilation in areas requiring a 

more direct and controlled volume of airflow to remove harmful particulate. These fans include axial 

vane fans, jet fans, and more recently low-pressure, 

high-volume propeller fans. Studies comparing fan 

performance have shown that propeller fans are 

more efficient at lower pressures and can produce 

larger air quantities at lower horsepower than axialvane fans (Grau et al, 2002a; Grau et al, 2002b; 

Krog and Grau, 2006). In addition, because of operational characteristics related to lower fan pressure 

and larger diameter, propeller fans provide better regional air coverage in large- opening mines than axial vane fans. In light of these findings, a study was 

conducted at an underground dump/crushing site to 

compare the effectiveness of an electric-powered axial vane fan versus a diesel-powered propeller fan 

for the dilution and transport of respirable dust from 

this facility.

2 FAN CHARACTERISTICS AND SAMPLING 

STRATEGY 

The two fans compared in the study are shown in 

figure 1. Air volume for each fan was measure using a moving traverse immediately downstream of 

the exhaust opening with a 10 cm (4 in) Davis vane 

anemometer. Both fans had approximately a 23.6 

m3

/s (50,000 cfm) output, keeping this operational 

parameter constant in the study. The axial vane fan 

is Joy Model AMF-100-50-26H. It is 460 volts, 

37.1 kw (50 hp) with a 0.91 m (3 ft) outlet opening 

and equipped with a flow straightener. The propeller fan is a Spendrup Model 152-764-HVLD. It has 

a 20.0 kw (27.2 hp) Duetz Model F2L1011F diesel 

engine. The fan is belt driven with a 1.52m (5 ft) diameter outlet opening. Figure 2 is a plan view of the 

study area showing information concerning the location of the fans, the dump and crusher, the operator 

control booth, ventilation stoppings, and dust sampling stations. 

The fans were positioned in the same location, on 

the left side of the haul road entry, approximately 

30.3 m (100 ft) from the dump/crusher. The fan disFigure 1. Axial vane fan and propeller fan used in study 

charge blew across the haul road entry and was oriented toward the center of the dump area and 

crusher. The width and height of the entry at the 

dump were 13.7 m and 18.3 m (45 ft and 60 ft), respectively. Generated dust was diluted and transported away from the crusher via the belt entry, into 

a return airway, and out of the mine. The belt entry 

is isolated from the main developments using both 

permanent and curtain stoppings in crosscuts along 

its entire length of approximately 152 m (500 ft) as 

shown in figure 2. The crusher is a 222.6 kw (300 

hp) jaw type rated at 907 metric tph (1000 short 

tph).

The dust survey consisted of two parts. The dust 

survey was first conducted with the axial vane fan, 

routinely used by the mine to ventilate the facility. 

After completion of the survey, the axial vane fan 

was replaced by the propeller fan which was positioned in the same location. The sampling survey 

was then repeated. As shown in figure 2, the sampling strategy was to collect dust samples at six key 

locations around the dump/crushing facility. This 

sampling array provided dust concentrations at the 

same locations to determine the effectiveness and 

performance of each fan as it relates to the dilution 
Figure 2. Plan view of the study area showing the location of 

the blowing fans (axial vane or propeller), the dump and 

crusher, the operator control booth, ventilation stoppings, and 

dust sampling stations. 

and transport of respirable dust and respirable silica 

dust. 

The instrument package at each sampling location 

varied depending on the information required at 

each location. Two types of instruments were used. 

The first instrument type that was used for measuring respirable dust was the gravimetric sampler operated at 1.7 liters/minute which complies with 

MSHA requirements for metal/nonmetal sampling. 

The respirable fraction of the airborne dust (<10 microns) was obtained with a 10 mm Dorr-Oliver cyclone and deposited onto a 37 mm PVC filter. The 

filters used for dust were weighed before and after 

sampling to calculate overall respirable dust concentrations (which includes all dust types and particulate) based on the sampling rate and time. The filters 

were then analyzed by an independent laboratory for 

silica using NMAM-Method 7500 to determine the 

silica weight. From this information, silica concentrations could be calculated. 

The second type of sampling instrument was the 

MIE personal DataRAM (pDR). The pDR measures 

and records the concentration of respirable airborne 

particulate from 0.1 to 10 microns (which includes 

all dust types, DPM, and water mist) using a light 

scattering technique. Light-scattering instruments 

offer a relative measure of concentrations, but provide a continuous record of particulate levels so that 

concentrations can be evaluated over any time interval during the sampling period. 

Table 1 identifies the types and numbers of dust 

samplers that were positioned at each location. 

Sampling packages on the ribs were positioned 

about 1.82 m (6 ft) above the floor. Figure 2 shows a 

plan view of the location of these sampling stations. 

Samples for both parts of the study were collected 

on 3 consecutive days for an average sampling time 

of about 5 to 6 hrs per shift. Other information related to dust production and migration was collected 

each day during the sampling period. During this 

time, the number of trucks that dumped and the tonnage processed through the crusher were recorded. 

Table 1. Dust samplers used in study. 

Sampling Instruments 

Site Location Gravimetric pDR 

1 Intake Entry 4 0 

2 Dump 2 1 

3 Crusher 2 1 

4 Belt 2 1 

5 Return Entry 4 0 

6 Adjacent Entry 2 0 

To monitor airflow from the crusher to the return 

airway, vane anemometer readings were taken at the 

end of the belt entry (site 4 in figure2) at a 0.91m by 

2.3m (3 ft by 7.5 ft) regulator in a permanent stopping leading to the return. Vane anemometer readings of the air velocity were also taken in front of 

the dump to compare the ventilation coverage provided by each fan. To determine the bulk percentage of silica in the rock, samples were collected 

from the belt during both parts of the study and sent 

to an independent laboratory analysis. Percentages 

of silica in the rock ranged from 37.8% to 44.4%. 

3 GRAVIMETRIC SAMPLING 

Figures 3 and 4 summarize the concentrations based 

on filter weights from gravimetric sampling at all six 

locations for the 3 sampling days for the respirable 

dust and silica dust, respectively. Both the respirable and silica dust concentrations were examined to 

determine if fan effectiveness varied depending on 

the type of dust. In figure 3, the respirable dust concentration is most likely composed of three main 
Figure 3. Average respirable dust concentrations from gravimetric sampling at all six locations for the 3 sampling days. 

Figure 4. Average respirable silica dust concentrations from 

gravimetric sampling at all six locations for the 3 sampling 

days. 

components: inert limestone dust or calcite, diesel 

particulate, and silica. Filters were sent to an independent laboratory for XRD silica analysis of three 

primary mineral components: quartz, cristobalite, 

and tridymite. The analysis only found quartz mineral on the filter. Figure 4 shows the concentrations 

of silica at the six locations around the crusher. 

For each sampling location, the mean and 95% 

confidence interval were calculated using a tdistribution. The graphs in figures 3 and 4 plot the 

mean concentration and the upper and lower confidence limit (UCL and LCL) for each sampling location. In examining all locations, the following is notable in comparing the axial vane to the propeller 

fan: 

Site 1- Intake: Gravimetric sampling showed a 

slight increase in both respirable dust and silica dust. 

This station was not influenced directly by the fans, 

but the low dust levels indicate that very little if any 

dust is migrating from the dump and crusher back 

into the main developments on the intake side. Observed concentration increases are most likely related to dust generated from vehicle traffic in the 

area. 

Site 2 - Dump: This station recorded a reduction 

in both respirable and silica dust of 23.1% and 

24.8% respectively, with the propeller fan. When 

compared to the crusher, the dump has low respirable and silica concentrations. This suggests that 

both fans are preventing dust rollback from the 

crusher as the trucks dump. 

Site 3 - Crusher: This location had the highest 

concentrations of both respirable and silica dust. Of 

interest is the fact that respirable concentrations increase threefold from the dump to the crusher location, a distance of roughly 18.2 m (60 ft). This station recorded a reduction in both respirable and 

silica dust of 17.5% and 17.9% respectively, with 

the propeller fan. Observation from inside the operator’s booth showed that a large plume of dust was 

created as trucks dumped regardless of the fan in 

use. The propeller fan appeared to be more effective 

in diluting the dust than the axial vane fan. Stratification or layering of the air may be causing this effect as both fans are lifting and entraining the dust, 

but the propeller fan is more effective in dust transport. 

Site 4 - Belt: Both the respirable and silica dust 

concentrations at the belt location are less than half 

of the levels at the crusher, at a distance of approximately 152 m (500 ft). Dust reductions were 3.7% 

and 12.8% for the respirable and silica dust, respectively. However since the concentrations mean values show little change, this confirms that the dust is 

well diluted and uniform when it reaches the end of 

the belt before it passes through the regulator in the 

stopping. This indicates that both fans are slowly 

moving the air down the entry with about the same 

efficiency. 

Site 5 - Return: This location behaved much the 

same as the intake location with low respirable and 

silica concentrations showing that very little dust is 

migrating from the dump/crusher back into the main 

developments. This site is the only station to record 

conflicting concentration levels in that the respirable 

dust decreased with the propeller fan but the silica 

dust increased. Similar to site1 intake, this station 

was not directly influenced by the fans and observed 

concentration increases are most likely related to 

dust generated from the tramming of haulage trucks 

past the sampling station. 

Site 6 – Adjacent Entry: Since dust concentrations were similar to the dump and the belt locations, 

this indicates that considerable air leakage is occurring through the line curtains along the belt entry. 

As a result, dust is being transported through the 

stopping line and has the potential to be carried toward the working faces. Reductions in both respirable and silica dust were recorded when the propeller fan was in use but dust concentrations in this 

adjacent entry can easily be reduced by tightening 

the curtain line to prevent the escape of dust. 
Figures 3 and 4 show that the propeller fan outperformed the axial vane fan in the dilution of respirable dusts as evidence for the lower mean values 

at sampling sites most influenced by the fans. This 

includes the dump and crusher, belt, and the adjacent 

entry. However, since at all sampling sites the error 

bars (upper and lower confidence intervals) overlap, 

the dust reductions are not statistically significant 

for the amount of data collected. For larger dust 

particles or nuisance dust (>10 microns), visible dust 

from the crusher still appeared to linger in the air 

with relatively slow removal from this area regardless of fan type. Although total dust measurements 

were not collected, the primary reason for this observation is that the area of the entry at the dump increases dramatically at the crusher, which results in 

a significant drop in air velocity. 

4 PERSONNEL DATA RAMS 

The pDR graphs in figures 5 and 6 illustrate the difference in dust patterns between dump, crusher and 

belt for a typical day of sampling. These graphs 

Figure 5. Dust patterns for axial vane fan at the dump, crusher, 

and belt for a typical day of sampling. 

Figure 6. Dust patterns for propeller fan at the dump, crusher, 

and belt for a typical day of sampling. 

compare pDR data from the third day of sampling 

with the axial vane fan, and the first day of sampling 

with the propeller fan. These days were select because tonnage processed through the crusher those 

days were similar at 3890 metric tons (4290 short 

tons) and 3820 metric tons (4210 short tons), respectively. The pDR concentrations in the figures 

have been corrected to the average gravimetric concentrations at those locations. This is simple correction by calculating a ratio of average gravimetric 

concentration divided by the average pDR concentration for the entire sampling period. This ratio is 

then used to adjust each individual concentration in 

the pDR log. The pDR was configured to log a concentration every 10 seconds over a 285-minute sampling period. This equates to 1710 sampling points. 

Two primary observations can be made from 

these graphs. First, unlike the concentrations from 

the gravimetric samplers (figure 3), the average concentrations at each of these three locations were 

similar, regardless of fan type. These results are not 

uncommon, since the pDR a light scattering instrument which yields a relative concentration value depending on the aerosol being sampled. Differences 

in the content of the aerosol, (in this case limestone 

dust, silica dust, and diesel particulate matter) can 

vary during the sampling period and may account 

for the similarity in corrected concentrations. 

Second, although these light-scattering instruments offer only a relative measure of concentrations, they provide a continuous record of dust levels 

so that concentrations can be evaluated over any 

time interval during the sampling period. Examining concentration trends over this time period is the 

most useful way to interpret the data. In comparing 

the two graphs, concentration trends both the dump 

and belt sampling stations tend to have a similar pattern during the dump and crushing cycle. What is 

most notable in the figures is the differing trend at 

the crusher. The peak and valley concentrations are 

consistently higher and lower with the propeller fan. 

The dump and crushing cycle was a continuous 

process with only about a 30-second delay between 

trucks arriving to dump. The crusher was usually 

behind the dump cycle and trucks had to wait for the 

crusher to process most of the rock before dumping 

into the hopper. The trucks would idle at the dump 

before being signaled to dump. Figure 7 inset picture shows a truck waiting at the dump. 

Figure 7 compares the three-shift average of 

anemometer readings taken directly in front of the 

dump. The three fixed point measurements were 

spaced approximately 4.6m (15 ft) apart and 1.5 m 

(5 ft) above the floor. These reading were taken before the shift or when no trucks were at the dump. 

Due to the operational characteristics of each fan, 

the axial vane fan did not provide air coverage at the 

center and right side of the dump as did the propeller 
fan. As a result, with a truck waiting at the dump, it 

is likely that the airflow became more turbulent with 

the propeller fan. As airflow moved around the 

truck, increase turbulence may have entrained more 

Figure 7. 1) Three shift average of anemometer readings taken across the dump; 2) three shift average air velocity at the regulator 

at the end of the belt; 3) inset picture showing a truck waiting at the dump. 

Table 2. Daily production for each sampling shift. 

Axial Vane Fan Propeller Fan 

Shift number 1 2 3 1 2 3 

Gravimetric Sampling time, min 216 322 298 325 312 297 

Number of trucks dumped 55 110 110 108 107 108 

Measured tonnage, metric tons (short tons) 1945 

(2145) 

3890 

(4290) 

3890 

(4290) 

3820 

(4210) 

3785 

(4170) 

3820 

(4210) 

dust, leading to higher concentration values logged 

by the pDR (Chekan et al, 2004b; Xu and Bhaskar, 

1995). When the truck left the dump, the airflow 

provided by the propeller fan diluted the dust produced by the crusher more effectively, as evident by 

the immediate reduction in dust levels. 

Figure 7 also shows the three-shift average air velocity at the regulator at the end of the belt. The air 

flows are comparable, indicating that air losses 

through the brattice stoppings isolating the belt are 

similar for each fan. Figures 5 and 6 show that dust 

levels at site 4 (belt) are comparable in trend and 

concentration. This indicates that both fans are 

equally effective in transporting the dust once it has 

been diluted at the dump/crusher area. 

5 RESPIRABLE DUST GENERATION AND 

PRODUCTION 

Another method to evaluate the data, other than concentration, is to calculate dust generated based on 

the tonnage produced during the sampling period. 

This will normalize dust generation to production, 

and offer an alternative approach to compare the effectiveness of each fan. This value is calculated by 

taking the average milligram weight gain collected 
on the filter for a shift, and dividing it by the metric 

tons processed through the crusher for the duration 

of the sampling period. It is expressed as milligrams 

of respirable dust per metric ton of rock crushed per 

minute. Table 2 provides specific details about sampling time and tonnage processed by the crusher for 

each sampling shift. For this analysis, the following 

four sampling sites most influenced by fan airflow 

were examined: the dump, the crusher, the belt, and 

the adjacent entry. 

Figures 8 and 9 compare the 3 shift average of 

both the respirable dust and the respirable silica dust 

for each fan. The decrease in respirable dust based 

on tonnage is apparent when the propeller fan was 

used. These results are similar to the percent decease in dust concentrations at the dump, crusher, 

belt, and the adjacent entry shown in figures 3 and 4 

and offer further evidence that the propeller fan outperformed the axial vane fan for dust dilution. 

Figure 8. Three-shift average comparing respirable dust generated based on the tonnage produced during the sampling period. 

Figure 9. Three-shift average comparing respirable silica dust 

generated based on the tonnage produced during the sampling 

period. 

6 CONCLUSIONS 

Based on gravimetric sampling, the study showed an 

improvement with the propeller fan to dilute both 

the respirable dust and the respirable silica dust 

around the dump/crusher facility. Although the dust 

reductions are not statistically significant at a 95% 

confidence, concentration values show that the propeller fan outperformed the axial vane fan in the dilution of respirable dusts as evident by the percent 

reduction in mean concentration values at sampling 

sites most influenced by the fans (i.e., the dump, the 

crusher, the belt, and the adjacent entry). In addition, when examining dust generation as it related to 

production the percent reductions offer further support that the propeller fan outperformed the axial 

vane fan. Since reductions for both the respirable 

dust and the respirable silica were approximately the 

same whether comparing concentrations (figures 3 

and 4) or production (figures 8 and 9), this shows 

that the propeller fan’s effectiveness did not vary 

depending on dust type. 

However, one observation needs to be addressed. 

For larger dust particles or nuisance dust (> 10 microns), visible dust from the crusher still appeared to 

linger in the air with relatively slow removal from 

this area regardless of fan type. The reason for this 

observation is that the area of the entry at the dump 

increases dramatically at the crusher, which results 

in a significant loss of air velocity. 

The pDR concentration showed that concentration trends at the dump and belt sampling stations 

tended to have a similar pattern during the dump and 

crushing cycle. However, when comparing the concentration trends at the crusher, the peak and valley 

values are consistently higher and lower with the 

propeller fan. Due to the operational characteristics 

of each fan, the axial vane fan did not provide air 

coverage at the center and right side of the dump as 

did the propeller fan. As a result, with a truck idling 

at the dump, it is likely that the airflow became more 

turbulent with the propeller fan. When the truck left 

the dump, the airflow provided by the propeller fan 

diluted the dust produce by the crusher more effectively, as evident by the immediate reduction in dust 

levels. Data indicates that both fans are equally effective in transporting the dust once it has been diluted at the dump/crusher area. This is based on 

similar air velocities measured at the regulator and 

comparable concentration values and trends from the 

gravimetric and pDR samplers. 

This survey was conducted to evaluate two different types of fans for dust dilution and transport 

from an underground crushing facility during normal 

production activities. Dust concentrations around 

the crusher and down the belt entry were higher than 

desired for either fan and could be reduced with improved dust capture. The current fan location, approximately 30.3 m (100 ft) from the dump/crusher, 
is performing a function by diluting the dust at the 

dump and keeping it from recirculating back to the 

main developments. The use of brattice around the 

crusher to better direct and increase airflow would 

provide an inexpensive method to possibly reduce 

dust levels. Another alternative to consider would 

be to increase fan capacity using either a push-pull 

system with two auxiliary fans or a fan-powered 

dust collector. The push-pull system would require 

an axial vane fan to be placed outby the crusher in 

the belt entry with exhaust tubing positioned as close 

to the crusher as possible to maximize dust capture. 

Tubing would then be attached to the exhaust side of 

the fan to transport captured dust directly to the return airway. The second alternative would involve 

the installation of a fan-powered dust collector with 

filtration system to remove airborne dust and discharge clean air. With the additional fan capacity, 

either system should increase dust capture at the 

crusher, thus lowering dust levels at the crusher and 

in the belt entry. 

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