Improving the performance of fan-powered dust collectors in 

stone cutting applications 

G.J. Chekan 

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

USA 

A.B. Cecala, D.E. Pollock, & J.A. Zimmer 

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

USA 

ABSTRACT: In the dimension stone industry, workers in processing shops may be exposed to harmful levels of 

respirable silica dust when conducting the different tasks in stone preparation, which include cutting, grinding, 

polishing, and thermaling. To protect workers from this health hazard, operators sometimes ventilate processing 

shops using fan-powered dust collectors to capture and filter dust. To investigate this issue, NIOSH, Pittsburgh 

Research Laboratory, in cooperation with a dimension stone operation, upgraded an off-the-shelf dust collector 

the operator was utilizing in a polishing/cutting shop. Initial dust surveys showed that the unit was under-designed 

for the amount of dust being generated in the shop. The objective of the research was to cost-effectively increase 

the air cleaning volume and improve dust capture efficiency of the unit with a larger motor and blower. The unit’s 

original stock 0.56 kw motor and blower was replaced with a 2.24 kw motor and matching blower to increase 

airflow. The unit was evaluated both in the laboratory and the shop before and after the retrofit to determine the 

improvement of the upgrade. The study showed that the 2.24 kw upgrade improved the performance over the 

original 0.56 kw motor resulting in more dust being cleaned from the ambient air in the shop. The 2.24 kw unit 

cleaned 19% more air and captured 32% more respirable dust than the 0.56 kw unit. 

1 Introduction 

Many types of respirable dust are potentially harmful to 

workers, but overexposure to respirable silica is extremely 

hazardous. Exposure to crystalline silica dust may lead to 

silicosis, which creates irreversible and progressive 

deterioration once the dust has been deposited within the 

lung tissue. This disease may be chronic, accelerated, or 

acute depending on the length and magnitude of exposure. 

Workers who develop silicosis have an increased incidence 

of lung cancer and pulmonary disorders (NIOSH, 2002). 

Recent data from the Mine Safety and Health 

Administration’s (MSHA) compliance sampling database 

indicate that several occupations in the metal/nonmetal 

industry are at high risk to respirable silica. Inspector 

samples collected from 2000 through 2004 reveal that 

occupations exhibiting frequent overexposures include 

mobile workers (such as cleanup man, laborer, utility man, 

and mechanics), which had an average of 15% of the 

samples over the permissible exposure limit (PEL). 

Crusher operators and bagging operators also have a high 

percentage of samples over the PEL, averaging 14% and
 

23%, respectively. In the dimension stone industry, 31% 

of the samples for the stone polishers/cutters exceeded the 

PEL (MSHA, 2000–2004). 

Reducing silica levels in large metal/nonmetal 

processing facilities has been a long-term objective in 

NIOSH research. Reduction in dust levels has been 

demonstrated at several mineral processing facilities by 

implementing total mill ventilation systems. These systems 

are designed to create a flow pattern to dilute, capture, and 

remove respirable dust from a structure (Cecala et al, 1993; 

Cecala et al, 2006). Another method is to clean the air with 

stand-alone fan-powered dust collectors. Some small 

operators in the dimension stone processing shops tend to 

rely on natural ventilation by opening doors and windows 

and using exhaust fans to remove dust, during the summer 

months. However, during the winter months, when the 

shop doors and windows are closed, dust levels tend to 

increase. To lower dust levels, some shops have adopted 

the use of fan-powered dust collectors as the primary dust 

control method. These units are ambient air cleaners that 

can be purchased off-shelf. Specifications vary depending 

on the manufacturer, so operators do not necessarily size 

units for a particular application based on the shop size and 

the quantity of dust being generated. To evaluate the issue 

outlined above, the objective in this study was to compare 

the performance and dust capture capabilities of an offshelf fan-powered dust collector to that of an upgraded unit 

in both the laboratory and the field. 

2 Background 

The unit evaluated in the study was an Aercology 

AmbientAer 2000 air cleaner.1

 This unit is a selfcontained, fan-powered dust collector, shown in Figure 1. 
 

 

 

 

 

Figure 1. Fan-powered air cleaner used in study. 

The outside dimensions of the unit are 1.3 m in length, 

0.66 m in depth, and 0.72 m in height. The intake side of 

the unit is 0.61 m by 0.61 m through which dust-laden air 

is drawn using a direct-drive centrifugal blower. The dust 

is then filtered in two stages. The first filter or prefilter is a 

0.61 m by 0.61 m by 0.10 m multi-vee high-capacity 

pleated filter. This filter media is composed of continuous, 

synthetic, and hydrophobic fibers and is 60% efficient for 

particle sizes > 1.0 microns. The second filter or primary 

filter is a 0.61 m by 0.61 m by 0.53 m vee-bag filter 

composed of ultra-fine glass fiber media. It is 

approximately 92% efficient at particle sizes > 1.0 

microns. The filtered air is then exhausted through the back 

of the unit via a 0.36 m by 0.19 m (0.068 m2

) louvered 

opening. The prefilter extends the life of the primary filter, 

but both filters require replacement, depending upon dust 

loading conditions. 

Figure 2 shows the floor plan for the processing shop 

and the location of six air cleaning units suspended 

approximately 1m from the shop ceiling. These air cleaners 

are equipped with a 0.56 kw, 120 Volt AC motor and 

direct drive blower. Fan curves show that the motor and 

blower should operate at an air volume of approximately 

1.70 m3

/s at zero Pa static pressure (no filters installed), to 

an air volume of 0.94 m3

/s at a static pressure of 622 Pa. 

However, measurements in the shop showed that the air 

volume, with no filters installed, was approximately 1.22 

m3

/s or 0.48 m3/s less than the air volume indicated by the 

fan curve. These initial dust surveys also found that air 

volume cleaned by the units decreased rapidly under dust 

concentrations between 0.75 mg/m3 and 1.3 mg/m3

 in the 

shop. Ventilation measurements at the exhaust louvers 

showed that the air volume was 0.94 m3

/s when new preand primary filters were installed and decreased to 0 24 

m3

/s after approximately six shifts of operation. 

NIOSH and plant personnel decided to retrofit one of 

the air cleaning units in the shop with an upgraded motor 

and blower combination that would provide additional air 

volume. This would improve performance by overcoming 

the pressure increase as both the pre- and primary filters 

were loaded with dust generated by the cutting and 

polishing of stone in the shop. 

To quantify this upgrade, both the off-shelf unit and 

upgraded unit were tested and evaluated at the NIOSH 

laboratory and then in the processing shop to assess their 

dust capture and air flow characteristics. First, tests were 

conducted in the laboratory on the off-shelf unit equipped 

with a 0.56 kw, 120 Volt AC motor and direct drive 

blower. The unit was then retrofitted with a 2.24 kw, 240 

Volt AC motor and direct drive blower also rated at 1.70 

m3

/s at zero Pa static pressure (no filters installed) and 1.42 

m3

/s at 622 Pa static pressure. The higher air volume at 

622 Pa static pressure would improve the air flow through 

the unit and consequently, the dust capture performance of 

the unit. 

3 Laboratory Comparison 

The air cleaner was tested by enclosing the intake and 

exhaust ends of the unit in ductwork as shown in Figure 3. 

This ensured that the dust introduced on the intake side 

would pass through the unit and be collected by the filters. 

The feed material was a standard limestone rock dust (40% 

of the feed > 250 microns), which was introduced into the 

intake side of the ductwork with a vibrator screw and 

compressed air feed to separate dust particles and minimize 

agglomeration. 

Several types of instruments were used to monitor dust 

concentration, pressure, and velocity during the tests. Dust 

concentrations were monitored on the intake side and 

exhaust sides of the air cleaner using a real-time aerosol 

monitor, called a Personal DataRam (pDR), manufactured 

by Thermo Anderson Inc.1

 The pDR measures and records 

the concentration of respirable airborne dust using a lightscattering technique. Light-scattering instruments offer 

only a relative measure of concentrations; however they 

also provide a continuous record of dust levels so that 

concentrations can be evaluated over any time interval 

during the sampling period. A useful feature of this 

instrument is that dust concentrations can be viewed in 

real-time via laptop computer. Two pDRs were used in the 

tests, one was located at the intake side of the air cleaner 

and the other at the return side by the exhaust louver. A 

recording micromanometer was used to monitor pressure 

drop across the primary filter as it loaded with dust. Air 

velocity at the exhaust louver was measured using a 

recording vane anemometer so that the volume of air 

cleaned could be calculated. All instruments were set to 

display values each second and record the average at oneminute intervals. 

To limit the time of the tests in the laboratory, dust was 

introduced into the air cleaner at a much higher level than 

actual field conditions so that both filters would load in a 

reasonable timeframe. Dust was introduced at a rate to 

cause similar air volume loses observed in the processing 

shop. Shakedown tests with limestone dust showed that 

respirable dust concentrations ranging from 40 to 50 

mg/m3

, measured at the intake side of the air 
 

Figure 2. Shop floor plan with sampling locations. 

Figure 3. Intake and exhaust sides of air cleaning unit 

enclosed in ductwork for laboratory tests. 

cleaning unit by the pDR, would yield similar air volume 

losses as observed in the processing shop in about 150 

minutes of testing. Based on this, the criteria selected for 

testing was an initial air volume of approximately 0.90 

m3

/s and a final air volume of between 0.10 and 0.35 

m3

/s. The dust concentrations, as observed on the pDRs 

located on the intake and exhaust sides, were used to 

determine the length of each test. When the pre- and 

primary filters were loaded to capacity, pDR 

concentrations on both the intake side and exhaust side 

began to increase rapidly, indicating that the filters were 

no longer cleaning the intake air. The dust and air volume 

ranges were used for all the tests, however the average 

concentrations as logged by the pDRs did vary slightly, 

most likely due to variations in feed rate and air 

turbulence. 

Five tests were first conducted with the 0.56kw motor 

and blower. The unit was then retrofitted with the 2.24 kw 

motor and blower and five tests conducted with the 

upgrade. Both the pleated filter (prefilter) and vee-bag 

(primary filter) were changed after each test. 

Table 1 compares the performance of the 0.56 kw and 

2.24 kw motor as volume of air cleaned per test. The 

“Initial Air Volume” and “Final Air Volume” are 

calculated by taking the initial and final velocities from 

the recording vane anemometer and multiplying these 

values by the area of the louvered opening which is 0.068 

m2

. The average of the values is then multiplied by the 

test time, in seconds, to calculate the volume of air 

cleaned. 

The graph in Figure 4 compares the vee-bag pressure 

and the air velocity losses for an average test for each 

motor type. A comparison of results from table 1 and 

Figure 4 reveals the following: 

1)	 From Figure 4, the final pressure drop across the 

v-bag for the 0.56 kw motor was approximately 

570 Pa versus 820 Pa for the 2.24 kw motor. 

2)	 From Figure 4, the final air velocity measured at 

the 0.068 m2

 louvered opening for the 0.56 kw 

motor was 2.1 m/s versus 4.2 m/s for the 2.24 kw 

motor. 

3)	 From table 1, the average filter loading time was 

132 minutes for the 0.56 kw motor versus 283 

minutes for the 2 24 kw motor. 

The upgraded air cleaner was installed at the 

processing shop and sampling surveys were 

conducted during the winter months when the shop 
 

doors were closed. Sampling surveys were conducted 

for eight shifts for 

Table 1. Comparison of motors in laboratory tests. 

Figure 4. Pressure and air velocity characteristics for an 

average test for each motor. 

approximately seven hours per shift. Figure 2 shows the 

locations of the two units compared in the study as well as 

the location of the four other 0.56 kw motor air cleaners 

in the shop. The location of the air cleaners in the shop 

were chosen because of their proximity to highest dustgenerating source, the computerized cutting saw. Two 

types of instruments were used to compare the 

performance of the units: 1) gravimetric samplers, placed 

at four locations as shown in Figure 2 to measure 

respirable dust concentrations, and 2) recording vane 

anemometers, mounted at the discharge of each collector, 

to calculate the volume of air cleaned by each unit. Figure 

5 shows the positioning of these instruments on the two 

air cleaning units. Each air cleaner had the same 

instrument setup. 

Table 2 shows the average respirable dust 

concentrations measured at the four sampling locations 

for each sampling day. The concentrations show that dust 

levels at the shop floor and upper bay area were fairly 

consistent for each day of sampling. This would suggest 

that the arrangement of the units has resulted in a uniform 

mixing of the air within the shop. The shop floor had the 

lowest average concentration at 0.382 mg/m3

, while the 

2.24 kw unit had the highest average concentration at 

0.444 mg/m3

. Although this is only a 16% increase in 

concentration, dust levels would be somewhat higher in 

the upper bay than near the shop floor since the units are 

pulling airflow toward them. Also, natural airflow 

currents of heated air would move dust upward in the 

shop. 

Figure 5. Location of gravimetric samplers at the intake 

side and the recording vane anemometer at the louver 

exhaust side. 

Table 2. Dust concentrations measured at sampling 

locations for each shift. 
 

 

 

 

 

Figure 6. Comparison of 0.5 6 kw and 2.24 kw motors for 8 sampling shifts in processing shop. 

The graph in Figure 6 compares the airflow 

performance measured in m/s3

 for the 0.56 kw and 2.24 

kw motors. The air volume data show each unit’s 

performance as a timeline based on total minutes of 

sampling over eight sampling shifts. A description of the 

sampling history is as follows: 

1) A new vee-bag and prefilter were installed in 

both units, followed by NIOSH sampling for 

shifts 1 and 2. 

2) The units operated for seven shifts with no 

NIOSH sampling. Prefilters were not changed 

during this time. 

3) When sampling resumed at the start of shift 3, a 

large decrease in air volume for both units was 

measured resulting from the prefilters becoming 

loaded with dust. Also, airflow data are missing 

from the 1300 to the 1450 minute mark due to 

low batteries in both recording units, but this did 

not affect the results of the tests. 

4) Prefilters were changed in the middle of shift 4 

as air volume on both units began to decrease. 

As a result, new gravimetric samplers were used 

for the second half of the shift on each unit. This 

scenario is represented in table 3 with shift 4 

being split into two sampling times. Figure 6 and 

table 3 show that changing the prefilter resulted 

in a substantial increase in air flow for both 

units. 

5) Sampling continued for the remainder of shifts 4 

and 5 with no significant changes in air volume. 

6)	 The units were operated for 12 shifts with no 

sampling. 

7)	 NIOSH sampling resumed for shift 6, during 

which time it was learned that the prefilter had 

been changed on the 0.56 kw unit by shop 

personnel the previous day. In order to ensure 

that both units were operating under equivalent 

conditions, NIOSH also changed the prefilter on 

the 2.24 kw unit. 

8)	 Sampling continued for shifts 6, 7, and 8 without 

any additional changes to the units. 

The graphs in Figure 6 show that the 2.24 kw motor 

outperformed the 0.056 kw motor by cleaning more air 

during the sampling time. Table 3 compares the 

performance of the two motors as volume of air cleaned 

per sampling day. Using the same method as in the 

laboratory tests, the volume of air cleaned was calculated 

by multiplying the shift sampling time, in seconds, by the 

average air volume calculated for that shift. 

Using the information from table 3, the percent increase 

in air volume cleaned and amount of respirable dust 

captured, can be estimated using the average gravimetric 

concentration. For the 0.56 kw motor, the total volume of 

air cleaned and the average shop respirable dust 

concentration was 217817 m3

 and 0.401 mg/m3, 

respectively. For the 2.24 kw motor, this is 259570 m3 

and 0.444 mg/m3

, respectively. Based on this information, 

the 2.24 kw motor cleaned 19 percent more air and 

captured 32 percent more respirable dust than the 0.56 kw 

motor during the 3915 minutes of dust sampling in the 

shop. 
 

 

Table 3. Performance of each unit as volume of air cleaned per sampling day. 

4 Summary and Conclusion 

Stand-alone fan-powered dust collectors can be a feasible 

method to lower dust levels in small dimension stone 

processing shops. However, this requires that air cleaning 

units be properly sized to clean the anticipated dust loads 

and keep overall shop levels at acceptable levels during 

the winter months. Both laboratory and field testing 

demonstrated that a 2.24 kw motor improved the 

performance of a cleaning unit over the original 0.56 kw 

motor when considering the volume of air cleaned by 

each of these units. The study also showed that costeffective retrofit options are available if the dust collector 

is under- performing. The cost of the upgraded motor and 

blower was $1,100 and took 8 man-hours to install. 

Laboratory tests showed that, on average, the 2.24 kw 

motor cleaned over twice the amount of air as the 0.56 kw 

motor for the five tests conducted--4278 m3

/s versus 9993 

m3

/s. This improvement is because the upgraded air 

cleaner was able to clean air for a longer time before the 

final air volume criteria (0.15 and 0.3 m3

/s) was reached. 

From table 1, the average time for a test was 132 minutes 

for the 0.56 kw motor and blower versus 283 minutes for 

the 2.24 kw motor and blower. As a result, this additional 

performance would extend the life of both filters, 

requiring them to be changed less often under normal 

operating conditions. 

Results from the processing shop tests, conducted 

under normal dust loads, showed a similar trend as in the 

laboratory tests. This resulted in more dust being cleaned 

from the ambient air in the shop. The 2 24 kw motor 

cleaned 19 percent more air and captured 32 percent more 

respirable dust than the 0.56 kw motor during the eight 

shifts of sampling. 

The survey data also show that the vee-bag filter can 

be used much longer than prefilters. As shown in Figure 

6, when the prefilters were changed in the middle of shift 

4, the airflow increased to levels near those observed at 

the start of shift 1. This demonstrates that the airflow 

reductions observed within the units resulted from dust 

loading on the prefilter. Therefore, to effectively achieve 

the air moving capacities of these units, it is critical to 

replace the prefilters on a regular basis. 

Based on these study results and observations, an 

optional design change to consider would be collector 

position and/or location. With all the dust collectors 

mounted at the roof, it would be expected that higher dust 

levels would be measured near the roof, with lower dust 

levels at the floor around the shop workers. The relatively 

uniform dust levels measured at the different sampling 

locations in this study would suggest that the units are 

mixing air within the shop, but not pulling dust-laden air 

from the breathing zone of the workers. If feasible, a more 

effective use of the collectors may be achieved by 

changing spacing and orientation of the units mounted at 
 

 

the shop ceiling, extending intake ducting toward the 

floor to improve dust pickup, or moving the collectors 

closer to the dust-producing sources. 

References 

Cecala AB, Klinowski GW, and Thimons ED, 1993. 

Reducing respirable Dust Concentrations at Mineral 

Processing Facilities Using Total Mill Ventilation 

Systems. US Bureau of Mines RI 9469. 11 pp. 

Cecala, A.B., Zimmer J.A., and Timko, R.J., 2006. 

Lowering Respirable Dust at an Iron Ore Plant 

Through Improved Ventilation Practices. 

Proceedings of the 11th US/North American Mine 

Ventilation Symposium, The Pennsylvania State 

University, University Park, Pennsylvania, June 5-7, 

2006, pp. 189-196. 

Mine Safety and Health Administration (MSHA), 2000-

2004, Metal/Nonmetal Mine Inspection Data 

(MNMID), available from the MSHA Metal and 

Nonmetal Safety and Health Office in Arlington, Va. 

NIOSH 2002. NIOSH Hazard Review – Health Effects 

of Occupational Exposure to RespirableCrystalline 

Silica. US Dept of Health and Human Services, 

Centers for Disease Control and Prevention. DHHS 

(NIOSH) Publication No. 2002-129. 127 pp. 

Mention of any company name or product does not 

constitute endorsement by NIOSH.