Development of a lower-pressure 


water-powered spot scrubber for 


mining applications 


J.A. Organiscak and D.E. Pollock 

Mining engineer and mechanical engineer, respectively, NIOSH,Pittsburgh, ~enns~lvania 

Abstract 

Water sprays and water-powered scrubbers have both been utilized in the mining and nzilling industry to 

suppress airborne dust. Unconfined wnter sprays operated at lower water pressures of 5689 kPa (51 00 

psig) can be very eflective at wetting the mine product at the d1ist source and significantly reducing the 

amount of respirable dtist that becomes airborne. However, unconfined waters sprays can be somewhat 

ineffective in actually removing airborne dust from the air. On the other hand, wnter-powered scrubbers operating at higher water pressures of 12,724 kPa (250psig) in physical enclosures or ducts have 

previously been demonstrated to be very effective in removing airbonze dust from the air. These higher 

operating water pressures are uncommon in many mines and mills, so their use is limited. The National 

Institute for Occtrpational Safety and Health (NIOSI-I) recently investigated the performance of a lowerpressure, water-powered inline series spray scrubber for removing localized airborne dust emitted at the 

source. Resultsshowed noticeable improvements in airborne dust capture efficiency through he operation 

of multiple inline series hollow cone spray nozzles within a round duct or pipe at the same water pressure 

as a single spray. Operating hollow cone spray(s) at higher water pressures noticeably improved airflow 

through the scrubber and yielded some additional scrubber eflciency improvements. Th~is, in-line spray 

scrubber efficiency trade-offs were observed to be made by altering spray power components of water 

pressure andor quantity (number of sprays). Results show that, on average, up to 0.23 and 0.32 m3/s (484 

and 679 c~ift per miiz) of ai@ow at 0.81 and 0.69 d~istcapture eflciencies can be achieved with three 81" 

and 33" hollow cone inline series sprays, respectively, operating at 1,655 kPa (240psig). 

Introduction samples) for permissible exposure limits (PELs) or reduced 

Water spray systems that were previously developed for creat- dust standards due to quartz indicate that a notable portion of 

ing localized ventilation patterns (shearer clearer, underboom the airborne respirable dust samples collected in the mines 

tuned spray system, etc.) and for providing dust suppression continue to exceed the mandated quartz PELs. MSHA's coal 

(internal stageloader-crusher spray systems, drum sprays, mine dust PEL for quartz is a reduced mining research estabunderboom sprays, belt wetting sprays, etc.) have made lishment (MRE) equivalent respirable dust standard of (10 s 

significant contributions toward successfully reducing the %quartz) mg/m3 when there is more than 5% quartz present in 

average dust concentration for underground coal miners, the dust sample as determined by MSHA's P7 infrared method 

reducing these averages from more than 6 mg/m3 in 1969 to (Parobeck and Tomb, 2000; U.S. Code of Federal Regulabelow 2 mg/m3 today (NIOSH, 1995). However, the former tions, 2004). MSHA's metal/nonmetal mine PEL for quartz 

U.S. Bureau of Mines identified apoint of diminishing returns is a reduced dust standard of (1 0 + (%quartz + 2)) mg/m3 for 

for existing mine spray systems operating at higher supply respirable dust containing at least 1 % quartz as determined 

parameters (pressure and quantity) (Schroeder et al., 1986; by NIOSH's X-ray method (Parobeck and Tomb, 2000; U.S. 

Colinet et al., 1991) and indicated that these spray systems Code of Federal Regulations, 2004). The percentage of airborne 

are probably not adequately providing the silica dust control dust samples taken between 1990 and 1999 that exceed the 

needed for obtaining lower permissible exposure levels (Or- respirable quartz PEL for coal, metal and nonmetallic mining 

ganiscak et al., 1990). were 30.196, 12.4% and 7.0%, respectively (NIOSH, 2003). 

The Federal Mine Safety and Health Administration's These data indicate that there continues to be respirable dust 

(MSHA's) dust compliance data (both inspector arid operator overexposures in the mining industry. 
Velocity Pressure Static Pressure 

Regulator Flow Meter 

Side View of Inline Spray Scrubber 

Figure 1-lnline spray scrubber test apparatus for determining airflow inducement. 

Directing spray nozzle flow through a small shrouded 

opening or duct induces airflow and significantly increases 

the capture efficiency of the spray droplets as compared to an 

unconfined area. This principle is the basis for the development 

of water-powered scrubbers by the Mining Research Development Establishment and U.S. Bureau of Mines (Jayaraman et al., 

1981;Grigal et al., 1982; Kelly and Muldoon, 1987; Jayaraman 

et al., 1989). Many of the water-powered scrubbers developed 

were small-diameter tubes with high-pressure water sprays, 

greater than 3,447 kPa (500 psig) to achieve higher airflows 

through the tubes at higher dust capture efficiencies. Although 

these scrubbers proved to be very effective for underground 

coal mine dust control on continuous miners (spot scrubbers) 

and shearing machines (ventilated shearer drum), mine application was limited because of their higher water-pressure 

requirements, between 3,447 and 10,342 kPa (500 and 1,500 

psig), for effective operation 

While high water pressure is advantageous for confined 

spray dust capture (in scrubbers), it is detrimental to dust 

capture with unconfined water spray systems commonly used 

on mining machinery. Laboratory and underground research 

have shown that as the number of spray nozzles and the 

water pressure are increased for unconfined spray systems, 

the dust capture effectiveness per gallon of water is reduced 

(Schroeder et al., 1986). The improved dust capture from 

smaller higher velocity droplets at higher spray pressures is 

offset by the additional dilution from spray-induced airflow 

in the unconfined space (reduced residence time or droplet 

dust interaction). Thus, more dust knockdown for unconfined 

sprays will be achieved with higher water volume rather than 

pressure. It was also found during this research that operating unconfined water sprays at high pressures can also cause 

undesirable localized air turbulence, pushing contaminated 

dusty air to worker locations (continuous miner rollback) 

(Jayararnan et al., 1984). 

To improve the airborne dust capture efficiency of water 

sprays in the mining industry, a water-powered scrubber with 

a lower operating pressure, i.e., 4,724 kPa (250 psig), was 

investigated and developed for more widespread user-friendly 

applications. Previous water-powered scrubbers were designed 

to achieve high airflow and dust-capture efficiencies by operating at water pressures greater than 1,724 kPa (250 psig) as noted 

above. To achieve higher scrubber airflow and dust-capture 

efficiencies at lower water pressures, i.e., <1,724 kPa (250 

psig), a multiple inline spray series water-powered scrubber 

was investigated. This report describes the design parameter 

investigation of a low-pressure inline series spray scrubber, 

consisting of a 152-mm- (6-in.-) diameter tube with wide and 

narrow angle hollow cone spray nozzle arrangements. 

Examination of scrubber operating parameters 

Some early research on water-powered scrubber development 

has shown that the scrubber airflow capacity can be increased 

at lower spray operating pressures by placing nozzles in series along the enclosed tube, or in a co-planer pattern over a 

cross-sectional area of a tube (Grigal et al., 1982). This work 

also indicated that the pressure and flow characteristic of the 

scrubber was cumulative for multiple sprays in a series or coplanar arrangement at the same water pressures. The co-planar 

scrubbers were selected for further development because their 

arrangement could cover larger cross-sectional areas or ducts 

and had higher air induction rates per unit of spray water 

used. No further development or dust capture measurements 

were made on the inline or series spray scrubbers at the time 

of the research. 

The current research study focused on several design variables of an inline series water spray scrubber. The scrubber 

design variables examined included spray discharge angle, 

number of inline sprays, inline spacing distance between sprays, 

water spray operating pressure, scrubber air pressure-airflow 

characteristics and dust capture eff ciency. The preliminary 

scrubber design research classified the air pressure-quantity 

relationships generated by in-series sprays arranged inside a 

3.05-m- (10-ft-) long, 152-mm- (6-in.-) diameter round pipe 

enclosure. Figure 1 shows the laboratory test setup (bench 

testing) to examine scrubber airflow inducement characteristics of various spray designs and inline spacing arrangements. 

This research focused on hollow cone spray nozzle designs 

because they produce the best water droplet atomization as 

compared to other single-fluid full-pattern spray types (flat 
fan, full cone, etc.). They also provide 

the best dust capture per gallon of water 

(U.S. Bureau of Mines, 1982; McCoy et 

al., 1985). 

Two hollow cone nozzle designs were 

tested in the scrubber. One was a narrow discharge angle nozzle, Spraying 

Systems1 UniJet No. % TTD4-46 with a 

33"-spray angle and an orifice diameter 

of 1.6 mm (0.063 in.). Its manufacturer's 

flow specifications are 3.0 L/milz (0.78 

gpm) of water flow at 551 kPa (80 psig) 

gauge pressure with a calculated discharge 

coefficient of 0.74 (actual flow divided 

by theoretical orifice diameter flow). The 

sdcond nozzle tested was a wider discharge 0.00 

angle nozzle, Spraying Systems UnitJet No. 

% TTD6-45 with an 8l0-spray angle and 

an orifice diameter of 2.4 mm (0.094 in.). Figure -Spray Its manufacturer's flow specifications are 

3.1 Llmin (0.83 gpm) at 551 kPa (80 psig) 

gauge pressure with a calculated discharge 

coefficient of 0.35. 

Up to three spray nozzles were spaced at either 152-mm 

(6-in.) or 305-mm (12-in.) distances apart, starting 610 mm (2 

ft) away froni the discharge end of the pipe. At the discharge 

end of the pipe, a perpendicular end plate was used to stop and 

remove the water droplets. The end plate was moved through 

various open positions, i.e., 12.7, 25.4, 50.8, 102 and 203 

mm (0.5, 1, 2,4 and 8 in.), to measure the air pressure-flow 

characteristic generated by the spray arrangement at various 

scrubber airflow resistances. Initially, each spray arrangement 

was measured at two water pressures of 55 1 and 1,103 kPa (80 

and 160 psig). The operating spray arrangements tested were 

a single spray closest and furthest away from the scrubber 

discharge, the two sequential sprays closest and furthest away 

from the scrubber discharge and all three sprays. 

Using a pitot tube, scrubber airflow or velocity measurements were made eight diameters or 1.22 m (4 ft) from the 

scrubber entrance. Static pressure differential pressure measurements created by the sprays nozzle arrangements were 

made between the midpoint of the pipe, i.e., 1.52 m f5 ft) 

from the entrance or discharge, and 305 mm (1 ft) from the 

discharge end of the pipe. Water flow and pressure were also 

monitored downstream of a pressure regulator in the supply 

line to the spray nozzles. These scrubber parameters were 

continuously recorded on a four-channel data logger for five 

minutes and averaged during each test condition. These test 

data were conlpiled into 40 air pressure-flow characteristic 

curves. The 305-mm (l-ft) distance between sprays was 

found to yield higher air pressure-flow characteristic curves 

for the 33" hollow cone sprays than the 152-mm (6-in.) spacing, whereas there was no noticeable spacing effect for the 

81" hollow cone sprays. Thus, all the inline spray testing for 

dust capture was limited to the 305-mm (l-ft) spacing for 

simplification. Several of the spray configurations, i.e., one 

spray and three sprays at 305 mm (12 in.) apart, were also 

tested at 1,654 kPa (240 psig) to boost scrubber airflow even 

more. Scrubber airflow quantities greater than 0.24 m3/s (500 

cu ft per min) were achieved with three spray nozzles of either 

type operating at 1,654 kPa (240 psig). 

Mention of any company name or product does not constitute 

endorsement b, ~ Institute ~ for ~~ i ~safety ~ and Health. 

0.05 0.10 0.15 0.20 0.25 0.30 0.35 

ScrubberAir Quantity, m3/s 

configuration static pressure and air quantity curves. 

Figure 2 shows the air pressure-flow characteristic curves 

for the scrubber configurations to be tested in the dust chamber. 

These were selected for the dust chamber testing because they 

represent the scrubber configuration design range for examining 

dust capture efficiency effects. These curves show that scrubber 

airflow output is increased by the number of sprays nozzles 

and their operating pressures. They also show that the nanower 

angle hollow cone sprays consistently provide a higher static 

air pressure-quantity curve than the wider-angle hollow cone 

sprays tested at similar scrubber configurations. 

Spray nozzle design effects on water droplet size distributions and droplet velocities can be observed from separate 

Phase Doppler Particle Analyzer (PDPA) measurements made 

on each of these two hollow cone nozzles operating at 55 1 and 

1,103 kPa (80 and 160 psig). Although these were individual 

unconfined spray nozzle droplet measurements, their confined 

scrubber tube droplet characteristic changes with respect to 

operating pressure are expected to be relatively similar. Water droplet size characteristics are expected to be relatively 

larger for both nozzles inside the scrubber tube due to droplet 

coalescence in a confined air stream and multiple inline spray 

interactions. The PDPA measurement method description can 

be found in Gemci et al., 2003. 

Figures 3 and 4 show the Sauter mean diameter (SMD or 

D32) and mean droplet velocity measurements for the 33" hollow cone spray and for the 8 1" hollow cone spray, respectively, 

0.3 m (1 ft) away and on a radial pattern from the centerline of 

the nozzle orifice. The actual measurements were made at the 

points connected by the solid lines, which were asymmetrically 

projected to the other side of the spray centerline. The darker laes 

are the SMD curves and the lighter lines are the mean droplet 

velocity curves. The thinner lines are the droplet characteristics 

for the 552 kPa (80 psig) water pressures and the thicker lines 

are the droplet characteristics for the 1,103 kPa (160 psig). 

Arrows indicate which y-axis the curve belongs to. 

The Sautermean diameter (SMD 0rD3~) is the summation of 

the total volume of sampled spherical droplet diameters divided 

by the summation of their total surface area. This represents 

the mean diameter ratio of droplet volume to droplet surface 

area generated by the nozzle and is the most commonly used 

~

mean droplet diameter representation in mass transfer reaction ~ ~ ~ l ~ ~ ~ i 

applications. 
Spray Fe~iterline 

Spray Pattern Location, Inches 

Figure 3 --Spray droplet characteristics for the 33"hollow-cone spr -ay nozzle. 

Spray Pattern Location, Inches 

Figure4 -Spray droplet characteristics for the 81" hollow-cone spray nozzle. 

by the removal rate of a known amount of 

dust in a known volume of the chamber 

over the application time period. The dust 

removal rate or chamber dust concentration 

decay rate was determined by (Ruggieri et 

a1.,1983; McCoy et al., 1985) 

When solved this reduces to 

where 

C is the final dust concentration 

(mg/m3>, 

C, is the initial dust concentration 

(mg/m3), 

Q is the air quantity scrubbed (m3/s), 

Vis the fixed volume of closed system 

(In3), 

t is the time (seconds), 

qQ is the dust removal or cleaned 

airflow rate (m3/s) and 

q is the dust capture efficiency (%). 

The dust removal rate or dust capture 

efficiency measured during the experiment is the cumulative dust capture of 

all the mechanisms taking place in the 

dust chamber. This includes the scrubber as well as other background removal 

mechanisms of the chamber (settling, 

impact-adhesion, etc.). Generally, all 

these background removal mechanisms 

become a notably smaller portion of the 

dust capture measurement when water 

spray or scrubber testing is conducted at 

higher dust in theenclosed 

dust chamber (Ruggieri et a1.,1983; McCoy et al., 1985). Thus, most of the dust 

removal rate or dust capture efficiency 

Both of these droplet size and velocity figures notably 

show that the narrower hollow cone spray angle nozzle had 

larger droplet SMDs and higher droplet velocities throughout 

the spray pattern as compared to the wider-angle hollow cone 

nozzle. Also, these spray droplet size and velocity figures show 

that by increasing nozzle operating pressures, SMD decreased 

and average droplet velocity increased throughout the spray 

pattern range. Thus, larger water droplet SMDs and higher 

droplet velocities appear to be the driving energy source for 

higher water-powered scrubber airflow inducement for the 33" 

hollow cone nozzle as compared to the 81" nozzle. 

Laboratory dust capture experiments 

Dust chamber experiments were conducted on various scrubber configurations to examine the key factors in in-line series 

spray dust capture. The inline series dust collection efficiency 

testing was conducted in a 2.44-m- (8-ft-) high, 2.44-m- (8-ft-) 

wide, 2.44-m- (8-ft-) deep closed system dust chamber. An 

air inductor into the closed system dust chamber injected dust 

until it reached a desired dust concentration ceiling. A 1.1 -m3/s 

(2,300-cu ft per min) mixing fan in the chamber was used to 

disperse theinjecteddust throughout the chamberbefore the test. 

Airborne dust capture efficiency of the scrubber was determined 

measurement can be attributed to the scrubber. 

This closed-system dust-chamber testing procedure was used 

successfully in the past to conduct comparative dust capture 

rates of various types of unconfined sprays (air-atomizer, hollow cone, full cone, flat fan) and a water-powered scrubber 

operating in the center of the chamber (Ruggieri et a1.,1983; 

McCoy et al., 1985). It was also used for comparative dust 

capture testing of a hollow cone spray on various bituminous 

coal types (Organiscak and Leon, 1994). An advantage of a 

closed dust chamber system is that a comparable dust removal 

or cleaned airflow rate (qQ) can be measured for open sprays, 

enclosed scrubber sprays and partially enclosed sprays. Another 

advantage is that the same aerosol sampling equipment utilized 

in field sampling call be located at a position away form the 

spray pattern or highest area of airflow turbulence, whereas 

testing in an open wind tunnel system requires changing the 

dust sampling equipment to meet the velocity conditions in 

the wind tunnel (iso-kenetic sampling). One disadvantage or 

drawback of the closed system approach is that some level 

of dust gradients exists in the chamber as compared to the 

mathematical model assumption of uniform dust concentrations throughout the chamber (Organiscak and Leon, 1994). 

However, past experience has shown that this drawback can 
Figure 5 -Scrubber dust capture efficiencies. 

be reduced by sampling in a more stable area (usually in the 

center) of the dust chamber. 

Dust-capture performance measured during this scrubber 

testing was conducted by personal gravimetric dust samplers 

for a given time period before and after operating the waterpowered scrubber, with instantaneous real-time dust sampling 

conducted during the complete test. Particular to this experiment, Keystone Mineral Black 325 BA of particle size less 

than 44 pm (Keystone Filler and Manufacturing Co., Muncy, 

Pennsylvania) was injected into the dust chamber to achieve 

a little more than 100 mg/m3 of respirable dust concentration, 

as measured with an instantaneous real-time dust monitor 

(RANI- 1, MIE, Inc., Bedford, Massachusetts). The RAM-1 

was operated at 2.0 Lllnin with a Dorr-Oliver 10-mm nylon 

cyclone to measure the respirable size fraction of dust. When the 

instantaneous respirable dust concentration naturally decayed 

to 100 mg/m3, two personal MSAcoal mine dust samplers were 

run for a three-minute interval to determine the initial average 

respirable gravimetric dust concentration for calibrating the 

RAM-1 dust concentration at the beginning of the scrubber 

decay (C,). The MSA coal mine dust samplers were made 

up of an Elf personal sampling pump drawing 2.0 L/min of 

air through a DOIT-Oliver 10-rnm nylon cyclone, which collected the respirable fraction of airborne dust on an MSA coal 

mine filter cassette. Following the completion of the initial 

three-minute sampling period, the scrubber was operated for 

various time periods to reduce RANI-1 dust concentrations to 

around 30 mg/m3. After the scrubber operation was stopped, 

another two personal MSA coal mine dust samplers were run 

for a 10-min period to determine the final average respirable 

gravimetric dust concentration for calibrating the RAM- 1 dust 

concentration (C) at the end of the scrubber decay. 

The scrubber dust capture efficiency performance configurations tested in the chamber were conducted in the operating 

ranges of the scrubber air pressure and quantity characteristic 

~urves~developedpreviously and shown in Fig. 2. The experimental factors included hollow cone discharge angle (33" vs. 

81°), spray number (one vs. three), spray operating pressure 

(552 vs. 1,655 kPa or 80 vs. 240 psig), and scrubber dernister 

resistance (50.8 vs. 203 mm, or two vs. eight in exit opening). 

A midpoint scrubber configuration tested within these ranges 

was one 33" and 8 1" (spaced 610 mm or 2 ft apart), operating 

at 1,103 kPa (160 psig) and a 127 mm (5 in.) demister exit 

opening. Seventeen scrubber configuration experiments were 

randomly conducted for at least three repetitions for each 

configuration. 

The initial scrubber was modified by cutting off the first 

1.22m (4 ft) of scrubber inlet used in developing the air pressure and quantity curves (see Fig. 1) to shorten it to 1.83 m (6 

ft) in length so it would horizontally fit into the dust chamber. 

During these experiments the water flow, water pressure and 

scrubber static pressure were continuously recorded. The 

scrubber airflow (Q) was indirectly determined by using the 

average scrubber static pressure upstream and downstream 

of the spray region to determine the scrubber quantity from 

the previously developed performance curves in Fig. 2. The 

static differential pressure for three inline series 33" HC sprays 

operating at 1,655 kPa (240 psig) appeared to be a little lower 

for the shorter scrubber (1.83 m or 6 ft) than the previously 

measured curve for the longer scrubber (3.05 m or 10 ft), so 

the air quantity was determined by extrapolating the existing 

curve to the lower pressures. Using the scrubber air quantity 

(Q), chamber volume (V), scrubbing time (t),initial dust concentration (C,) and final dust concentration (C), the scrubber 

dust capture efficiency (TI)could be determined from the above 

decay model. 

A previous scrubber efficiency study determined that the 

enclosed chamber test method resulted in a slight overprediction 

of scrubber efficiency (3% higher) as compared to the intake 

and exhaust gravimetric testing of the scrubber (McCoy et al., 

1985).The test chamber usedin the present experiments showed 

about a 1 %background decay efficiency after the chamber was 

wetted with a water spray, dust injected, and mixed with the 

1.1-m3/s (2,300-cu ft per min) mixing fan (no spray operating) 

over the same time periods that the scrubber was tested. Thus, 

background dust chamber decay efficiencies were considered 

negligible in the scrubber efficiency determinations. 

Dust capture results 

The inline series water-powered scrubber test results are 

shown in Fig. 5 and indicate that both water pressure and the 

number of inline sprays noticeably affected the water-powered 

scrubber performance. Figure 5 shows the scrubber efficiency 

averages and standard deviations for the configurations tested. 

A single 33" HC spray showed somewhat lower average dust 

capture efficiencies (39% to 42%)when operating at 552 kPa 
Figure 6 -Scrubber dust removal or cleaned airflow rates. 

(80 psig) than the 8 1" HC spray (47% to 52%), but its average 

efficiencies inore notably increased and were slightly higher 

(65% to 73%) than the 81" HC spray (60% to 61 %) when 

operated at 1,655 kPa (240 psig). To gain some insight into 

this spray nozzle performance difference, previously measured 

water droplet formation characteristics for each nozzle can 

be examined (see Figs. 3 and 4). These figures show that the 

33" HC spray had a notably larger increase in average droplet 

velocity, and a notably larger decrease in SMD, throughout the 

spray pattern in comparison to the 81" HC spray at increased 

operating water pressures from 552 to 1,103 kPa (80 to 160 

psig). This indicates that both higher droplet velocities and 

smaller water droplet sizes are components for increased dust 

capture (Calvert, 1977) and airflow through the scrubber at 

higher water spray pressures (see Fig. 2). Water quantity is also 

another dust capture efficiency component related to increasing operating pressure, since the quantity passing through a 

nozzle or orifice is directly proportional to the square root of 

the higher operating pressure divided by the square root of the 

lower operating pressure. 

The independent scrubber water quantity effect of adding 

additional spray nozzles, irrespective of the operating pressures, 

can also be seen from Fig. 5. Increasing the number of the same 

sprays in the scrubber at the same spray operating pressures also 

showed an increase in the dust capture efficiency. The average 

dust capture efficiency for the 33" HC spray operating at 552 

kPa (80 psig) rose from about 40% to more than 60% when 

two more sprays were added at that same pressure. The average 

dust capture efficiency for the 81" HC spray operating at 552 

kPa (80 psig) rose from about 50% to more than 60% when 

two more sprays were added at that same pressure. Scrubber 

dust capture efficiencies increased to between 68% and 86% 

when using three of either type of water spray nozzle operated 

at 1,655 kPa (240 psig). The two-spray 33" HC and 81" HC 

combination inline scrubber operating at 1,103 kPa (1 60 psig) 

water pressure also showed an average dust capture efficiency 

of 63%, which is a good dust capture efficiency compromise 

between the single- and three-spray configurations operating 

at 1,655 kPa (240 psig). 

Another way of examining the inline spray scrubber 

performance is to examine dust removal or air cleaning rate 

qQ with respect to scrubber configurations (McCoy et al., 

1985). This performance measure is determined irrespective 

of scrubber airflow quantity by using above dust chamber 

decay equation. Figure 6 shows the averages and standard 

deviations of this scrubber performance measure (qQ) for the 

different spray configurations tested. This figure shows that 

both water pressure and the number of water sprays (water 

quantity irrespective of pressure) clearly increases qQ. The 

33" HC spray also shows a noticeable increase in qQ over the 

8 lo HC spray at the higher water spray pressures (1,655 kPa 

or 240 psig). This is due to the higher scrubber air quantity 

throughput of the 33" HC spray over the 8 1"HC spray at higher 

water spray pressures and elevated dust capture efficiencies 

(see Figs. 2 and 5). 

The water quantity impact of the various scrubber spray 

configurations on qQ can be more clearly seen in Fig. 7.This 

graph shows the scrubber water quantities measured during 

dust capture testing, with each cluster of points representing 

distinct spray configurations at their multiple demister end 

plate settings. As can be seen in this figure, water quantity 

increases with both pressure and the number of sprays added. 

The biggest increases to qQ with respect to water quantity 

were from changes in pressure. Using three sprays at lower 

water pressures and higher water quantities as compared to a 

single spray at higher water pressures and lower water quantities yielded about the same qQ. Thus, similar qQs can be 

achieved by adding more sprays at lower pressures or by using 

one spray at higher pressures. 

To examine the dust capture efficiency of all these spray 

configurations on more comprehensive terms, the power input 

of the spray(s) was normalized by the scrubber airflow for 

each individual test and is shown in Fig. 8. Because water 

spray power is a product of nozzle operating pressure and 

water quantity, they are jointly factored into spray power. This 

figure indicates that a general direct logarithmic relationship 

exists between scrubber spray power input per airflow induced 

and the capture efficiency during these experiments. It also 

shows that spray power per unit of airflow can be achieved by 

increasing water pressure and/or quantity. The spreads in the 

individual points are likely due to the various water droplet 

characteristics present during the different spray configurations 
0 33HC Spxay(s) 0 33&8 1HC Sprays A 8 1HC Spray(s) 

0.00 

0 5 10 15 20 

Water Flow, LJmin 

Figure 7 -Water flow rate relationship to scrubber cleaned airflow rate. 

, 33HC Spray(s) CI33&81HC Sprays A 81HC Spray(s) 

100% 

Spray Power per Airflow, ~lm'ls 

Figure 8 -Spray power and efficiency relationship. 

tested and experimental error of the individual tests. Again, 

Figs. 3 and 4 illustrate some of the noticeable differences in 

spray droplet characteristics for the 33" HC and 8 1" HC nozzle 

designs, with pressure increases from 552 to 1,655 kPa (80 to 

240 psig). However, quantifiable spray droplet effects on dust 

capture efficiency cannot be easily assessed from the limitations of these experiments. 

Conclusions 

Inline series water-powered scrubber laboratory tests show 

that spray operating pressure and number of inline sprays 

noticeably affected the water-powered scrubber airflow and 

dust-capture efficiency. The narrower 33" HC spray nozzle 

angle consistentljr developed higher static differential pressures and scrubber air quantities than the 8 1" HC spray nozzle 

for all the operating pressures and number of sprays used. 

Independent phase doppler measurements of spray droplets 

generated from unenclosed hollow cone nozzles showed that 

the narrower 33" angle hollow cone spray pattern had a larger 

droplet size distribution with faster velocity droplets than the 

wider 8 lo angle hollow cone nozzle, which likely explains its 

better scrubber airflow inducement. Dust capture efficiencies 

for either spray operating at 552 kPa (80 psig) were increased 

on average from 39% to 52% to between 62% and 86% by 

increasing water pressure to 1,655 kPa (240 psig) andlor adding two more sprays in series operating at 552 to 1,655 kPa 

(80 to 240 psig) water pressures. 

Increasing the spray pressure and/or the number of sprays 

also increased the air-cleaning rate of the scrubber (qQ). 

Similar air cleaning rates (qQ) were observed for one spray 

operating at 1,655 kPa (240 psig) as compared to three sprays 

operating at 552 kPa (80 psig). Increasing the water pressure 

appeared to make greater increases in the air cleaning rates 

(qQ) with respect to water flow rate rather than adding sprays 

at the same water pressure. This seemed to be a result of higher 

scrubber airflow inducement at higher water pressures and 

scrubber efficiencies. 

Examining scrubber performance on more comprehensive 

terms -of spray power input per unit of airflow induced -illustrated a direct logarithmic relationship with scrubber dust 

capture efficiency (q). Higher scrubber dust capture efficiencies 

were achieved by increasing spray power input through either 

increased water pressure and/or number of sprays. Thus, inline 

spray scrubber efficiency trade-offs can be made by altering 

spray power components of water pressure and/or quantity 

(number of sprays). Results show that on average up to 0.23 

and 0.32 rn3/s (484 and 679 cu ft per min) of airflow at 0.81 

and 0.69 dust capture efficiencies can be achieved with three 
81" and 33" hollow cone inline series sprays, respectively, 

operating at 1,655 kPa (240 psig). 

The in-line series spray scrubber operating at lower water 

pressures, i.e., ~1,724 kPa (5250 psig), than previously developed water powered scrubbers should be more user-friendly for 

more widespread mining application. They are portable scrubbers that can be located in high dust concentration areas near 

generation sources while capable of being operating by many 

existing water supply systems. Future mining applications that 

should be targeted with these scrubbers are continuous mining 

machines, shearers, crushers, and stone cutting machines. The 

scrubber(s) can be integrated into the existing spray systems 

to improve its overall dust capture effectiveness. 

References 

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