Investigation of induced recirculation

during planned ventilation system maintenance

by C.J. Pritchard, D.F. Scott, J.D. Noll, B. Voss and D. Leonis

C.J. Pritchard, member SME, D.F. 

Scott and J.D. Noll are (Mining 

Engineer, Physical Scientist, 

Research Chemist) with the 

U.S. National Institute for 

Occupational Safety and Health. 

B. Voss, member SME, and D. 

Leonis are (Ventilation Engineer, 

Mining Engineer) with Newmont 

Mining Corp. Paper number TP14-014. Original manuscript 

submitted February 2014. 

Revised manuscript accepted 

for publication May 2014. 

Discussion of this peer-reviewed 

and approved paper is invited 

and must be submitted to SME 

Publications by Jan. 31, 2015.

Introduction

The Office of Mine Safety and Health Research (OMSHR) is examining 

methods to improve airflow delivery underground as mines face challenges 

due to production workings moving farther away from shafts, mining deeper 

ore reserves, increased production demands and finding additional reserves 

away from the original mining area. The use of booster fans and controlled 

recirculation are two potential options being explored.

Recirculation has been defined as “when any portion of the airflow passes 

by the same point more than once,” (Marks, 1989) and further as “the intended 

use of the recirculation principle within a ventilation system to improve conditions at the face.” Controlled partial recirculation was also defined by McPherson (2009) as “one in which a controlled fraction of the air returning from a 

work area is passed back into the intake, while, at the same time, the volume 

of air passing through the region is monitored to ensure that it remains greater 

than a predetermined minimum value.” This survey will analyze the effects of 

induced recirculation at a Nevada gold mine where shaft rehabilitation was 

in progress and one of the two main fans was shut down to allow reduced air 

velocity for safe shaft work.

Abstract n The Office of Mine Safety and Health Research (OMSHR) investigated ways to increase 

mine airflow to underground metal/nonmetal (M/NM) mine working areas to improve miners’ health 

and safety. One of those areas is controlled recirculation. Because the quantity of mine air often 

cannot be increased, reusing part of the ventilating air can be an effective alternative, if implemented 

properly, until the capacity of the present system is improved. The additional airflow can be used 

to provide effective dilution of contaminants and higher flow velocities in the underground mine 

environment. Most applications of controlled recirculation involve taking a portion of the return air 

and passing it back into the intake to increase the air volume delivered to the desired work areas. 

OMSHR investigated a Nevada gold mine where shaft rehabilitation was in progress and one of 

the two main fans was shut down to allow reduced air velocity for safe shaft work. Underground 

booster fan operating pressures were kept constant to maintain airflow to work areas, inducing 

controlled recirculation in one work zone. Investigation into system behavior and the effects of 

recirculation on the working area during times of reduced primary ventilation system airflow would 

provide additional information on implementation of controlled recirculation into the system and 

how these events affect M/NM ventilation systems. The National Institute for Occupational Safety 

and Health monitored the ventilation district when both main fans were operating and another 

scenario with one of the units turned off for maintenance. Airflow and contaminants were measured 

to determine the exposure effects of induced recirculation on miner health. Surveys showed that 

19% controlled recirculation created no change in the overall district airflow distribution and a small 

reduction in district fresh air intake. Total dust levels increased only modestly and respirable dust 

levels were also low. Diesel particulate matter (DPM) levels showed a high increase in district intake 

mass flow, but minor increases in exposure levels related to the recirculation percentage. Utilization 

of DPM mass flow rates allows input into ventilation modeling programs to better understand and 

plan for ventilation changes and district recirculation effects on miners’ health. 

Mining Engineering, 2014, Vol. 66, No. 10, pp. 43-48. 

Official publication of the Society for Mining, Metallurgy & Exploration Inc.

Technical Papers

www.miningengineeringmagazine.com Mınıng engıneerıng OCTOBER 2014 43
Background

The subject mine operates in the Carlin Trend west

of Elko, NV. Descriptions of the mine, mining methods,

ventilation system and future plans can be found in Araya

Figure 1

Study area underground district booster fan (photo by NIOSH).

Technical Papers

Abstracto n La	Oficina	de	Investigación	en	Seguridad	y	Salud	Ocupacional	en	Minería	de	los	Estados	

Unidos	(OMSHR	por	sus	siglas	en	inglés)	 realizó	investigaciones	para	aumentar	el	flujo	de	aire	hacia	

las	 áreas	 de	 trabajo	 en	minas	 subterráneas	metálicas/no	metálicas	(M/NM)	 de	modo	 que	 se	mejore	

la	 seguridad	 y	 la	 salud	de	 los	 trabajadores	mineros.	Una	de	 esas	 investigaciones	 es	 la	 recirculación	

controlada.	Debido	a	que	con	frecuencia	no	se	puede	incrementar	la	cantidad	de	aire	en	la	mina,	el	

reuso	de	una	parte	del	aire	de	ventilación	puede	ser	una	alternativa	eficaz	si	se	implementa	de	manera	

correcta,	hasta	que	la	capacidad	del	sistema	existente	sea	mejorada.	El	flujo	de	aire	adicional	puede	

ser	usado	para	proporcionar	una	eficaz	dilución	de	contaminantes	y	velocidades	de	flujo	mayores	en	el	

ambiente	subterráneo	de	la	mina.	La	mayoría	de	aplicaciones	de	la	recirculación	controlada	involucran	

tomar	una	porción	del	aire	de	retorno	y	pasarlo	nuevamente	por	la	boca	de	entrada	a	fin	de	incrementar	

el	volumen	de	aire	suministrado	a	las	áreas	de	trabajo	objetivo.	La	OMSHR	realizó	investigaciones	en	

una	 mina	 de	 oro	 en	 Nevada	 en	 donde	 se	 estaba	 rehabilitando	 el	 pique;	 para	 esto,	 uno	 de	 los	 dos	

ventiladores	principales	fue	apagado	a	fin	de	reducir	la	velocidad	del	aire	para	un	trabajo	seguro	en	el	

pique.	Las	presiones	de	operación	del	ventilador	secundario	se	mantuvieron	constantes	para	mantener	

el	flujo	de	aire	a	las	áreas	de	 trabajo,	induciendo	la	 recirculación	controlada	en	una	zona	de	 trabajo.	

Las	investigaciones	sobre	el	comportamiento	del	sistema	y	los	efectos	de	la	 recirculación	en	el	área	

de	trabajo	durante	los	momentos	de	la	reducción	del	flujo	de	aire	del	sistema	de	ventilación	principal	

proporcionarían	información	adicional	sobre	la	implementación	de	recirculación	controlada	al	sistema	y	

de	cómo	estos	eventos	afectan	los	sistemas	de	ventilación	en	minas	subterráneas	metálicas/no	metálicas	

(M/NM).	El	Instituto	Nacional	de	Seguridad	 y	Salud	Ocupacional	 realizó	 un	monitoreo	de	 la	zona	de	

ventilación	cuando	ambos	ventiladores	principales	estaban	operando	y	otro	escenario	con	una	de	las	

unidades	apagada	por	mantenimiento.	Se	midió	el	flujo	de	aire	y	los	contaminantes	para	determinar	los	

efectos	a	la	salud	del	trabajador	minero	debido	a	su	exposición	a	la	recirculación	inducida.	Los	estudios	

mostraron	que	19%	de	la	recirculación	controlada	no	creaba	cambios	en	la	distribución	del	flujo	de	aire	

general	en	la	zona	y	que	había	una	pequeña	reducción	en	la	zona	de	toma	de	aire	fresco.	El	nivel	total	

de	partículas	de	polvo	aumentó	ligeramente	y	los	niveles	de	polvo	respirable	fueron	también	bajos.	Los	

niveles	de	partículas	diésel	(DPM	por	sus	siglas	en	inglés)	tuvieron	un	considerable	aumento	en	el	flujo	

másico	de	la	 toma	zonal,	pero	pequeños	incrementos	en	los	niveles	de	exposición	 relacionados	con	

el	porcentaje	de	recirculación.	El	uso	de	índices	de	flujo	másico	de	DPM	permite	el	ingreso	de	datos	a	

programas	de	modelado	de	ventilación	para	un	mejor	entendimiento	y	planificación	de	los	cambios	de	

ventilación	y	efectos	de	recirculación	zonal	en	la	salud	de	los	trabajadores	mineros.

44 OCTOBER 2014 Mınıng engıneerıng www.miningengineeringmagazine.com

(2012a; 2012b; 2012c). The active mining district begins at 

the Carlin East Pit, trending north along multiple mining zones that were accessed as discovered, presenting 

a ventilation challenge commonly seen during the life 

of an underground mine. These additional ore 

zones were discovered between the pit and the 

main deposit, and further ore zones have been 

located along the trend further north. The study 

area mining district is part of the ore reserves 

between the main deposit and these north area 

reserves. A northern shaft is being sunk into 

these reserves and will be intercepted in 2015. 

Until then, the ventilation system is challenged 

to provide adequate airflow, and maintenance 

of the present air shafts is needed to ensure 

ventilation capacity until the future shaft is 

available.

The two main fans operate in parallel and 

are located on the mine level at the bottom of 

the exhaust shaft. Because of the high velocity 

associated with full airflow, shaft maintenance 

is difficult. Shutting down one fan lowers velocities to safely accomplish this task. During these 

shutdown periods, mining district booster fans 

(Fig.1) operate at normal pressures, inducing 

recirculation from the district return into the 

intake. After preliminary discussions, OMSHR 

and company employees proposed studying this 
ventilation district during both

recirculation and normal operation to further understand

system behavior and its effect

on miner health and safety.

Survey plan

The mine was sampled during two days with recirculation

and one day where both main

fans were operational illustrating the “normal” condition. The 

first day revealed unplanned

changes in the ventilation system, so the study protocol was

adjusted for the next two days.

The areas monitored were 

the district intake entrance, raise intake to the working

level, district exhaust return from the working level, and

recirculation ramp between the district intake and return

(Fig. 2). Respirable dust area samples were measured using conventional pumps, cyclones and filters at 1.7 L/m,

and total dust was measured using passive personal data

rams (PDRs) (Volkwein et al., 2004). Sampling times were

approximately five hours. Due to the utilization of diesel

mining and haulage equipment, contaminants measured

were carbon monoxide (CO), nitrogen dioxide (NO2) and

oxygen using a recording Industrial Scientific multi-gas

meter (Industrial Scientific, 2013); average diesel particulate matter (DPM) area sample concentrations by NIOSH

method 5040 using SKC DPM impactors (Noll et al., 2012;

NIOSH, 2006) and real-time DPM concentrations by a

FLIR/Airtec (FLIR, 2013; Noll et al., 2013). Airflows

were determined using standard vane anemometer traverse methods.

Preliminary ventilation network modeling showed

that the tracer gas (SF6) released in the district intake

airway during recirculation would return in a very short

time due to high velocities and close proximity to the

three local production areas and associated exhaust raises. A full lecture bottle of tracer gas was released just

inby the district intake sample point and then sampled

in the district exhaust prior to the recirculation crosscut

after passing through the work areas via booster fans

Samples were taken in 10-ml evacuated bottles every

two minutes in the district intake and return. Gas analysis showed varied residence times due to multiple nearby

production areas and extended development headings

mining towards the future shaft bottom. SF6 data confirmed pre-test modeling, distinctly showing recirculation spikes approximately every six minutes from these

nearby mining areas. There was negligible effect after

one hour, showing quick movement through the system.

.

Airflow analysis

Figure 2 and Table 1 show results of normal and recirculation airflow measurements in the ventilation district.

The district recirculation airflow results are:

Figure 2

Survey schematic — Airflow m3/s: Normal (recirculation). Intake (blue), return (red).

Percent recirculation 

(25.3/(31.1+128.8-25.3) 8.8% 

Recirculation district airflow change 

@ district exhaust/intake -3.7/+9 m3

/s

Percent total airflow change to district 

— intake/exhaust -2.3/+6%

Decrease in district intake air 

(121.4+29.5)-(128.8+31.1) -9 m3/s

Percent increase in district intake air 

(128.8+31.1)/(121.4+29.5) 6%

Decrease in fresh air 

(121.4+29.5)-(128.8+31.1-25.3) -16.3 m3/s

Percent decrease in fresh air 

(16.3/(121.4+29.5)) -10.8% 

Total intake and exhaust airflow in the district was

measured inby the recirculation ramp and showed essentially the same total airflow, with the intake showing

slightly more and exhaust slightly less during recirculation. This was well within the accuracy of anemometer

and area measurements. District fresh air composed of

the intake raise and outby district intake fell from 150.9

m3

/s to 134.6 m3/s or 11% during recirculation.

Baseline/recirculation contaminants

Mine gases CO and NO2 showed no significant changes or measured levels during either scenario and were

eliminated from further analysis.

Total dust levels, shown in Fig. 3, exhibit increased concentrations comparable to the recirculation percentage

(19%), with much higher levels seen in the recirculation

ramp. Airflow here reversed and increased in magnitude

with the source changing from intake to return. High levels of roadway dust were noted in the district exhaust airway, which would be more susceptible to total dust from

local haulage activity and much lower airflow quantities

than the cooler, well-maintained, watered main drifts of

the district intake. The raise intake had no mining activity the day that no recirculation was used. District intake

roadways had higher traffic but were better maintained

www.miningengineeringmagazine.com Mınıng engıneerıng OCTOBER 2014 45
Figure 3

Total dust levels during normal and recirculation ventilation.

46 OCTOBER 2014 Mınıng engıneerıng www.miningengineeringmagazine.com

than district exhaust, as shown in total dust measurements.

Respirable dust levels, on the other hand, were either

constant or decreased, except in the raise intake, which

showed no production activity on the normal ventilation

day (dust or DPM), as evidenced by measured dust and

DPM levels (Fig. 4 and Table 1). District intake levels

were constant and low, and district exhaust levels were

constant and higher than most others measured as would

be expected in the main return close to mining activity.

The recirculation drift concentration can be related to the

change in relative airflow, essentially doubling the airflow

during recirculation, dropping dust levels in half. Intake

Table 1

Normal and recirculation airflows with measured and mass flow rates of DPM.

DPM was corrected from 5040 method elemental carbon (EC) to total carbon (TC)

by multiplying by 1.3 (Noll, 2013).

				Note:	+	Airflow	is	towards	intake	

Normal airflow Airflow m3/s DPM - 5040 EC* 

1.3 μg/m3

Total DPM Mass 

flow- μg/s

District	intake 121.4 41 4979

Raise	intake 29.5 0 0

District	exhaust 159.2 87.2 13879

Recirculation	ramp -11.4 76.9 875

District	fresh	air 150.9

Recirculation

District	intake 128.8 

(103.5	Fresh)

51.9 6685 (3320 

Fresh)

Raise	intake 31.1 32.5 1012

District	exhaust 155.5 114 17722

Recirculation	ramp +25.3 132.9 3365

Total	district	fresh	air 134.6

dust levels show good attention

to roadway maintenance and

dust control.

Results

DPM level results are 

shown summarized in Tables 

1and 2, and can be visualized 

in previous Fig. 2 airflow and 

Fig. 5 DPM mass flow (ug/s), 

computed by multiplying average DPM concentration (ug/

m3) (Table 1) by airflow (m3/s). 

NIOSH 5040 analysis elemental carbon (EC) values were 

multiplied by 1.3 (Noll, 2013) 

to correlate to total carbon 

(TC) values, from which the 

U.S. Mine Safety and Health 

Administration (MSHA) enforces the 160 ug/m3

 DPM exposure limit (Code of Federal Regulations, 2013). Equating DPM concentration to mass flow, the contribution 

of mining work area emissions may be determined by 

difference from total intake and exhaust district airflow 

concentrations and flows. 

DPM mass flow was used to better determine the area 

contributions to DPM generation. Average concentration 

is good for the determination of exposure level, but mass 

flow assists in identifying where the particulate generating sources are and in helping investigators to better 

understand the effect when airflow rates change, as in the 

district recirculation environment. 

Mining activity also changes shift to 

shift and day to day, and the effect 

can be better isolated by using mass 

flow data. DPM mass flow can then 

be entered as a contaminant source 

into a ventilation network program 

to better determine exposures for

miner health and safety management.

Recirculated district intake 

location DPM concentration increased comparably (+27%) to the 

recirculation percentage of 19% 

(Table 1), but was still well below 

the MSHA 160 ug/m3

 DPM exposure limit. During recirculation 

testing, and in comparison to the 

normal ventilation condition, additional DPM over and above that 

from recirculated air was contributed from production sources outby 

the raise intake and from an increase in mining work area activity. 

Recirculated DPM mass flow (3365 

μg/s) added a considerable amount 

to intake fresh air (4333 μg/s) — a 

78% increase in DPM mass flow 

above district intake air — but was 
a relatively low concentration level (51.9 vs 41

μg/m3) increase over

normal intake airflow

when compared to an

MSHA-allowable exposure of 160 μg/m3

.

District exhaust levels

were also below allowable exposure limits in both scenarios.

Mass flow information

added considerably to

understanding of DPM

sources over that from

concentrations alone.

Conclusions

Often, changes in the mine ventilation system cause

upsets that may induce recirculation. At one Nevada gold

mine where shaft rehabilitation was in progress and one o

the two main fans was shut down to allow reduced air velocity for safe shaft work, shutting down one main fan and

leaving underground boosters

at normal operating pressures

did result in recirculation. Consequences of that action on the

mining district ventilation were

investigated by measurement

of SF6 tracer gas recirculation

times, airflow, mine gasses, dust

and DPM.

f

SF6 concentrations showed

a very quick turnaround time

in recirculated air due to the

close proximity of three mining sublevels and high airflow

velocities, confirming network

modeling results, which gives

confidence to using modeling

for effective network predictions. This finding also demonstrates the need for carefully monitoring fire contaminants

and control of recirculation fans, as they will quickly enter

the recirculation zone, affecting miner health and safety.

Airflow surveys showed that 19% recirculated air generated no significant change in district airflow distribution. 

However, the induced recirculation from shutting down 

one of the two main fans created a 23% reduction in district intake fresh air. Analysis of mine gasses CO, NO2 and 

O2 showed no concerns. Total dust levels increased modestly in conjunction with the recirculated air percentage 

except in the reduced airflow recirculation drift, where 

increased levels were controlled by airflow quantity. Respirable dust levels were also low. Contaminant increases 

noted were from the raise intake due to mining activity. 

DPM levels showed increased district intake mass flow 

but only minor increases in exposure levels related to the 

recirculation percentage and from work area activity. 

Previous research has shown that success of a conFigure 4

Respirable dust concentrations during normal and recirculation conditions.

trolled recirculation is predicated on an adequate supply 

of fresh air. These researchers also recommend a reliable 

monitoring system, toward which this mine has made considerable progress (Araya, 2012c) having installed airflow, 

gas and temperature monitors. In this study, intake DPM 

concentrations increased (+27%) similarly to the perTable 2

Mass flow of DPM (μg/s)—Airflow (m3/s) times average NIOSH 5040 

concentration (μg/m3).

Intake 

(Fresh)

Exhaust Recirculated Work Area

Ventilation DPM μg/s DPM μg/s DPM μg/s DPM μg/s

Normal 4979 13879 N/A 8900

Recirculation 4333 17722 3365 10025

Recirculation ramp -11.4 76.9 875

District fresh air 150.9

www.miningengineeringmagazine.com Mınıng engıneerıng OCTOBER 2014 47

centage of recirculated air (19%), although intake fresh

airflow dropped by a slightly lower percentage (11%). If

intake fresh airflow and production activity had remained

constant, DPM levels may have been close to the same,

confirming the above.

Although the fresh air quantity was reduced during

ventilation system maintenance and air was recirculated

back into district intake air, only modest increases in contaminants were measured. The critical area found during

the in-mine testing was the isolated recirculation drift

where airflow reversed. Should the system balance be

close to neutral, this area or others could encounter minimal airflow, creating an area of low airflow and potentially

high concentrations. If this reversal is critical to the system, air doors or a regulator could be installed to prohibit

or control recirculation.

When these ventilation events are envisioned, suitable ventilation surveys and modeling exercises should
be undertaken in advance of events to ensure a safe and

healthy environment is maintained. Critical areas in the

ventilation system are not usually in main airways, but

sampling information from face workers in conjunction

with knowledge of total fresh air intake dilution capacity

should drive decision making regarding production levels

affected by system ventilation upsets. Should face mining

area exposures be near the allowable limit, activity should

be reduced. Utilization of DPM mass flow allows input of

contaminant flow into a ventilation network model, giving a better idea of potential contaminant levels during

ventilation changes and district recirculation. Should a

permanent system of controlled recirculation be used, the

design should monitor for and minimize the recirculation

of mine fire contaminants to assure the safety of miners. ■

Acknowledgments

Dr. Gerrit Goodman, Dr. Charles Kocsis and John

Marks were instrumental in reviewing the manuscript.

Thanks to Ken Strunk for graphics support. Many thanks

to mine company officials and employees for field support.

Disclosure

The findings and conclusions in this report are those of

the authors and do not necessarily represent the views of

the National Institute for Occupational Safety and Health.

Mention of any company or product does not imply endorsement by NIOSH.

References

Araya, S., 2012a, “Updating Leeville Mine ventilation system to support future growth,” Proceedings of the 14th United States/North

American Mine Ventilation Symposium, pp. 183-188.

Araya, S., Terrillion, T., 2012b, “Diesel particulate matter (DPM) control

strategies at Leeville,” Proceedings of the 14th United States/North

American Mine Ventilation Symposium, pp. 197-204.

Araya, S., Danninger, D., Chik, S., Moorhead, M., Loup, M., Smith, M.,

2012c, “Developing ventilation management system at Leeville

Mine,” Proceedings of the 14th United States/North American

Mine Ventilation Symposium, pp. 517-523.

Code of Federal Regulations, 30 CFR, 2013, http://www.msha.

gov/30cfr/57.5060.htm

FLIR, 2013, http://www.flir.com/thermography/americas/us/

view/?id=57067

Industrial Scientific, 2013, http://www.indsci.com/products/multi-gasdetectors/ventis/

Marks, J.R., 1989, “Rationale and methodology in designing controlled

recirculation ventilation systems,” presented at the American Mining Congress Mining Convention ‘89’, September, 10 pp.

McPherson, M.J. 2009, Subsurface Ventilation Engineering, published by

Mine Ventilation Services Inc., Fresno, CA. Chapter 4, pp. 102-110.

NIOSH, 2006, http://www.cdc.gov/niosh/docs/2003-154/pdfs/5040.pdf.

Noll, J.D., 2013, personal communication.

Noll, J.D., Cecala, A.B., Organiscak, J.A., 2012, “The effectiveness of

several enclosed cab filters and systems for reducing diesel particulate matter,” Transactions 2011, Society for Mining, Metallurgy and

Exploration Inc.

Noll, J.D., Janisko, S., Mischler, S., 2013 “Real-time diesel particulate

monitor for underground mines,” Anal. Methods, Vol. 5 (12), pp.

2954-2963.

Volkwein, J.C., Vinson, R.P., McWilliams, L.J., Tuchman, D.P., Mischler, 

S.E., 2004, “Performance of a new personal respirable dust monitor 

for mine use,” Report of Investigations 9663, Pittsburgh, PA, U.S. 

Department of Health and Human Services, Centers for Disease 

Control, National Institute for Occupational Safety and Health, 

Pittsburgh Research Laboratory, June.

48 OCTOBER 2014 Mınıng engıneerıng www.miningengineeringmagazine.com

Technical Papers