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II

I n f l u e n c e

y o n T h o m a s

In dem vor]Je~

welche Art yon Fasern in der

flacher, werm das oberwasser

diese Beobachtun~en zuriick~

Fasern ungleichm~iBige und targ

Filterschicht am gr6l~t~n, Fein~

der Bereich der st~rksten Yerbei

k6nnten zur F6rderung yon obe

auszusch6pfen.

(6.5 sn., 9 figs., 2 tabs., 6 refs . )

Influence des cara~l~isllklm p a r ~iryanne Thomas Peters, ~

Cette fitude concerne t 'effet des c,

couche de fibres gre~si~es et d'u~

fibres mais la pe r te de charge est |

fractionnaire est ptus plate quand 1 i • z dentffies pour ex | iquer ces Obse~

P ~

la surface du, flltre, (3 )des Compo~,

filtre devient g6n~ra]emen~ pe~e~

et tangentielles ; eette dissipation,

aux fibres grossteres one I et~cac

nouveaux d e s i ~ s de fittre, banffs .s

l '~coulement de l 'air maxhnlsant ]

~ i !~e~ i ~i~ ~!!iiilZ~iiiill

This study invest igated the effect of upstream f low condit ions on the eff iciency of a reversible, two- layer f i l ter composed of one layer of coarse and one layer of f ine fibres. Efficiency was higher when the leading layer of the f i l ter contained f ine fibres, but the drop was the same regardless of which f ibres led. Efficiency was higher and the fract ional efficiency curve was

shal lower when the upstream velocity profi le was non-uni form and very turbulent . Three phenomena were ident i f ied that account for these observations: (1) non-uni formity of the

upstream velocity profi le, (2) mean velocities tangent ia l to the fi lter's surface and (3) f luctuat ing components of velocity, inherent to a turbulent a ir f low. Air enter ing a f i l ter becomes general ly perpendicular to the f i l ter face as f ibres dissipate non-uni form and

tangent ia l velocity components; this dissipation is greatest in the leading layer of the fi lter. Fine f ibres have a higher eff iciency that coarse fibres, so efficiency was higher when the region of greatest dissipation contained f ine fibres. Innovat ive housing designs, based on these observations, could be made to promote upstream a i r f low condit ions that maximize

f i l ter performance.

!

F m

CS o n

Thomas Peters, Maryanne Boundy & David Leith* University of North Carolina, Department of Environmental Sciences and Engineering,

Chapel Hill, NC 27599-7400, USA.

~Corresponding author - Tel: +1 919 966 3851; Fax: +1 919 966 7911; E-mail: david__leith@.unc.edu

single, two-layer filter can be thought of as two filters in series. Particle penetration through both

layers together should be the product of the penetration through each layer individually. Thus,

.... . . . . . . . in principle, a filter comprising one layer of fine fibres and one layer of coarse fibres should have the same

penetration regardless of which layer comes first. Contrary to these expectations, our work to evaluate industrial-scale mist collectors [1] led to some preliminary results that suggested filter orientation might affect penetration.

One explanation of this phenomenon might be that fine and

coarse fibres on the filter's leading face interact with entering air in different ways. To investigate this possibility, we constructed a two-layer filter and evaluated it in a large housing configured so that turbulence levels within the housing would be representative of those found in industry. Although considerable work has gone into describing the performance of

fibre filters, we have not identified work in the literature that investigates the effect of incoming air flow and turbulence levels on filter performance.

The objectives of this study were: (1) to measure velocity and turbulence profiles upstream and downstream for fine coarse and coarse-fine orientations of the same two-layer filter, (2) to identify the relationships among penetration, pressure drop, filter orientation, and upstream flow conditions, and (3) to investigate

whether filter theory could describe possible mechanisms for the results found.

IIETHODS

Test Filters The test filter was composed of a bed of metal mesh fibres typical

of coarse collection media used in the first-stage of industrial mist collectors. Two layers of fibres were arranged in series in a frame

with external measurements of 51.4 c m x 51.4 cm x 9.2 cm (20.25 inches x 20.25 inches x 3.6 inches). Table I lists characteristics of the metal mesh fibres used in each layer, including composition, density, width, and breadth. Furthermore, this table provides the total weight, solidity, and thickness of each layer. A coarse wire screen encased the fibre layers on the inlet and exit sides of the filter. To prevent changes in filter solidity

during testing, the filter was pre-compressed with a 3 kg weight and fixed in this position. This degree of compression corresponds to a higher pressure drop than occurred during filter testing.

Efficiency Measurements Filters were tested in a mist collector (Model OMC 010-D, S/N 95 263 1, Monroe Environmental Corp., Monroe, MI) sized to process 0.47 m3/s (1000 cfin) of air (Figure 1). These tests were conducted using a procedure described previously [2] with the

F i l t r a t i o n & S e p a r a t i o n D e c e m b e r 2 0 0 1 41

.... F ine Coarse

Composition Steel Stainless steel Density (g/cm 3) 7.76 7.76 Fibre width (~tm) 21 128 Fibre breadth (gm) 37.4 256 Weight (g) 155.8 243.5 Solidity 0.004 0.003 Filter thickness (cm) 2.0 2.5

p t f i l t e r _ Pthousing + filter

Pthousing

Penetrations were then converted to efficiency, where

E f f i c i e n c y = 1 - P e n e t r a t i o n

(l)

(2)

following modifications. A six-jet nebulizer (BGI Inc, Waltham, MA, USA) was used to introduce approximately 10 mg/m 3 of oil

mist alternately at points upstream (Position 2) and downstream (Position 1) of the collector. Droplets produced by the nebulizer were counted with an Aerosizer (TSI Particle Instruments Division, Amherst, MA, USA) ten times, five with the nebulizer in each position. To ensure that any count differences observed between the

positions were not caused by subtle shifts in the mist generation rate, the nebulizer was moved between the positions in a 2-1-1-2- 2 - 1 - 1 - 2 - 2 - 1 pattern, with the nebulizer in operation the entire time. This pattern resulted in five replicate measurements of pene- tration. The test protocol was then repeated, so that five additional

penetration measurements were made. From all ten tests, the average and standard deviation for penetration were determined for particles from about 0.5 mm to 5 mm, the region in which filter penetration decreased from nearly 100% to nearly zero.

Penetration as a function of droplet diameter was measured

for the empty filter housing and for the housing with the test

filter. Penetration for the test filter alone, Ptnlt~, was then determined by assuming penetration through the housing and the

filter together, Pthousmg. + fiher' was the product of the penetration through the housing and through the filter individually; that is,

Horizontal traverse positions

~ Air flow exit

Filter upper position

,Access door

II II Fl,terhousln0 266 cm (105") tall

Filter lower position

+ Drain

Air entrance in rear via 8" duct

Sloping floor for oil drainage

Tests were run with the filter installed at each of two locations: at the lower end of the collector housing near the air inlet, and also at the upper end of the housing near the air outlet.

When the filter was installed in the upper position, a perforated plate with 1.25 cm holes spaced 0.4 cm apart was installed in the lower position in an attempt to smooth out the velocity profile and to reduce turbulence at the filter's leading edge. For the tests with the filter in the upper position, the perforated plate was considered to be part of the empty housing when measuring filter

penetration.

Pressure Drop Measurements Pressure drop across the housing was measured using an inclined

manonaeter connected to the ductwork leading to and coming from the housing. Ten replicate measurements of pressure drop were made after adjusting flow to each of three filtration velocities and for each of the following conditions: across the empty housing, across the housing with the filter when fine fibres led, and across the housing with the filter when coarse fibres led.

The difference in readings with and without the filter was taken as the pressure drop due to the filter. From these measurements, the mean value and standard deviation of filter pressure drop in each orientation were determined.

Velocity Measurements Velocity was measured with a single sensor, straight probe, platinum hot-film anemometer (Model 1201-20, S/N 62-62, TSI Inc, St Paul, MN, USA) and a bridge/signal conditioner (IFA 100 Intelligent Flow Analyzer, TSI Inc, St Paul, MN, USA). The IFA 100 system consisted of a master cabinet (Model 158), a constant

temperature anemometer bridge (Model 150, S/N 950F), and a signal conditioner (Model 157, S/N 909). A portable oscilloscope (TDS, Tektronix, Winsonville, OR, USA) captured and digitized the voltage output by the signal conditioner. Using a custom Visual Basic (Microsoft Corporation, Redmond, WA, USA)

program, the captured voltages were then uploaded to a computer and converted to velocity using probe-specific calibration data. The anemometer system was calibrated in a 48.3 cm (19 inches) square, flanged duct for velocities ranging from 0.62 to 3.24 m/s and a 20.3 cm (8 inches) diameter, flanged duct for velocities ranging from 4.5 m/s to 30 m/s. An orifice meter was used to set the flow in the ductwork to achieve the desired

velocities during calibration. All velocity measurements were performed with a flow of

0.47 m3/s through the collector. Horizontal traverses were

conducted at three vertical locations: 2.5 cm upstream and 6.0 cm downstream of the filter in the lower position, and 2.5 cm upstream of the filter in the upper position (Figure 1). Traverse locations, shown in Figure 2, were located at the center of each of 16 equal areas, whose total area equaled the area of the filter. At

42 D e c e m b e r 2 0 0 1 F i l t r a t i o n + S e p a r a t i o n

each traverse location, 1500 data points were collected at a sampling frequency of 1 kHz. From the acquired velocity trace,

the average velocity (Vg) , root mean square velocity (Vrm~), and turbulence intensity (TI) were determined. Turbulence intensity

was calculated as

(3)

where u(t) is the measured velocity at time t and the overline bars represent averages over the measurement period [3].

Flow visualization was conducted to supplement hot-wire anemometer velocity measurements with directional information.

A tuft wand consisting of a 0.2 cm diameter, 1 m long rod with a 3 cm piece of string attached to one end was inserted into the housing below the filter. Directional information was discerned by viewing the string through an eight-inch port covered with a

transparent plate.

Efficiency Model l ing A standard model for filter efficiency was modified to incorporate the anemometer velocity measurements as input. Calculations

were carried out using an Excel (Microsoft Corporation, Redmond, WA, USA) spreadsheet. Efficiency, 1], for a given

droplet diameter was calculated as:

7

D

Air flow entrance

61.0 cm (24")

Side B ~ ,

Filter opening 45.7 cm x 45.7 cm 08"x 18")

/ Side A

l ° Traverse location I

(-4o~Ezt) q =1 - exp - -

( x(1--~)df (4)

where, 0~, is filter solidity, E z is single-fibre efficiency, t is filter thickness, and dris fibre diameter [4]. Single fibre efficiency was

calculated as:

E~ = I - ( 1 - E R ) ~ - E , X 1 -E v )~ -E DeX 1-E c ) (5)

from the approach of Davies [5] where E represents single-fibre efficiency and the subscripts represent 52 for sum, R for

interception, I for impaction, D for diffusion, DR for diffusion

interception, and G for gravity. ER, El) , EDR , and E G were calculated using equations 9.21, 9.24, 9.27 and 9.30 from Hinds

[4]. E l was calculated using equation 10-11 from Lee and Ramamurthi [6]. A theoretical efficiency curve was obtained by

calculating overall efficiency for droplets ranging in size from

0.5 /Jmto 10 pm. To accommodate filter behavior as a function of depth, the

filter was treated as three distinct layers: (1 a) the leading edge of the fibre layer oriented toward the airflow, or upstream fibre layer, ( lb) the remaining portion of this upstream layer, and (2)

the second, or downstream fibre layer. Each of the three modelled layers allowed independent input for solidity, fibre diameter, thickness, and air velocity. The model was further expanded to treat the filter as a collection of 16 parallel, multi- layer filters to account for non-uniformities in the horizontal velocity profile corresponding to the 16 equal areas for which velocity measurements were made. In this manner, velocity measurements were input for each of the three layers for the 16 individual velocity cells that comprised the model filter.

7 =;

1.01

3.8

0.6

0.4

0.2t

/ 7 _~,e ~ i b . , 7 / I

f

/

1 10 Droplet diameter (pm)

1ESULT!

Efficiency Fractional efficiency as a function of droplet diameter is shown in Figures 3 and 4 for the filter installed in the lower and upper

positions. Each figure presents data for superficial velocities through the filter of 2.3, 1.15 and 0.57 m/s and for two filter orientations: fine fibres followed by coarse (F-C) and coarse fibres followed by fine (C-F). In all cases, filter efficiency increased with

F i l t r a t i o n 8, S e p a r a t i o n D e c e m b e r 2001 43

1.0

0.8

o~ -~ 0.6

~ 0.4 m

0.2

G

gement , 1 ~ / ~ T i i - - 4 ~

C-Fo enT%1, ' i l I C-F °pen symb°Is b / / ( - / + 1.1,~ m/s 17

io.s:7m,s, i { I f t / : ,1

............ //I-77 7, -

% ± i

40 Droplet diameter (l~m)

12o • " m

80

60 - -

g

~. 40

°

6.0 cm downstream of a lower position

2.5 cm upstream of a upper position

C / F --- 6:64 + 2.53 2.00 + 0.55 32.6 + 9.1

F / C --- 2.71 + 1.06 0.33 + 0.2 12.0 + 4.22 C / F --- 3.41 + 1.52 0.50 + 0.34 13.5 + 5.71

P --- 3.30 + 1.22 1.34 + 0.46 41.2 + 6.81

P F /C 2.19 + 0.32 0.59 + 0.12 27.2 + 5.68 P C/F 2.23 + 0.39 0.60 + 0.10 27.4 + 4.32

a = Flow rate through the housing nominally 0.47 m3/s (1000 cfm) in all cases, corresponding to a superficial velocity of 2.3 m / s through the filter. b = _+ values indicate one sample standard deviation. c = P indicates perforated plate.

increasing superficial velocity, consistent with theory for particles

in the impaction regime. Efficiency was close to zero for particles

less than 0.5 pm and rose steadily with increasing diameter to

nearly 100% over the range of particle sizes investigated.

Figures 3 and 4 both show that efficiencies in the middle parts

of all curves were substantially and significantly higher when fine

fibres rather than coarse fibres led. Comparison of Figures 3 and

4 shows that the increase in efficiency when fine fibres led was

less when the filter was in the upper position. Also, efficiencies in

the upper position were somewhat lower and the efficiency

curves had a somewhat steeper slope.

Pressure Drop Figure 5 shows pressure drop for the filter in the lower position,

at each velocity tested and for both filter orientations. Pressure

drops were not significantly different if the coarse or fine

fibres led at the lowest and highest velocities, 0.57 m/see

and 2.30 m/sec, respectively. At the middle velocity

(1.15 m/sec) , pressure drop was slightly higher when the coarse

fibres led.

Velocity Table 2 presents average velocity, rms velocity, and turbulence

intensity from the horizontal velocity traverses; average velocity

profiles and turbulence data are plotted in Figures 6 and 7,

respectively. Because velocity measurements were nearly identical

for both filter orientations, Figure 6 shows data only for the

orientation with fine fibres leading. Figure 7 presents a

representative velocity time trace showing the magnitude of velocity

fluctuations for each of the three profiles presented in Figure 6.

4 4 December 2001 Fi l t rat ion+Separat ion

Figure 6A shows that the horizontal average velocity profile upstream of the filter in the lower position was highly non- uniform with average velocity ranging from 3 to 12 m/s . Furthermore, velocity fluctuated rapidly and substantially as a function of time (Figure 7A) with average velocity, rms velocity, and turbulence intensity measured as 8.21 m/s , 1.75 m/s , and 21.4%. The horizontal velocity profile downstream of the filter, Figure 6B, was much flatter. The velocity time trace (Figure 7B) indicates a much calmer airflow alter the filter than upstream, with average velocity, rms velocity, and turbulence intensity measured as 2.01 m/s , 0.13 m/s , and 6.7%.

Figure 6C shows the horizontal average velocity profile after the perforated plate, upstream of the filter when in the upper position. This profile was very flat with an average velocity, rms velocity, and turbulence intensity of 2.19 m/s , 0.59 m/s , and 27.2%. Although the velocity time trace (Figure 7C) was horizontally uniform, the airflow had a relatively high fluctuating component that led to the large turbulence intensity.

I ISCUSSIO I I

V e l o c i t y P r o f i l e s

Continuity dictates that the average upward velocity through the filter must be 2.3 m/s for a flow of 0.47 m3/s through the collector. An average velocity tbr a given traverse plane (Table 2) substantially above this velocity indicates that the airflow must have substantial velocity components tangential to the surface of the filter. The average velocity for the upstream traverse with the filter in the lower position (Figure 6A) was 7.04 re~s, indicating a substantial tangential component of velocity. Flow visualization confirmed that the elbow located just prior to the housing entrance (Figure 2) created an elongated, clockwise swirling pattern to the incoming air. This swirling pattern explains the high velocities observed at the right and the left sides upstream of the filter as well as the low velocities in the central region. The sloping floor of the collector also undoubtedly contributed to velocity non-uniformities.

The filter itself substantially flattened the velocity profile and reduced the magnitude of the observed velocities as shown in Figure 6b; however, because the average velocity downstream of the filter ranged from 2.71 m/s to 3.41 m/s , still greater than 2.3 m/s , a small component of tangential floss, remained. The lowest velocities downstream corresponded in location to the lowest velocities upstream, suggesting that filter pressure drop was insufficient to flatten the velocity profile entirely.

A very different velocity profile existed upstream of the filter installed in the upper position, Figure 6C. The pressure drop imparted by the perforated plate caused the flow to distribute evenly across the housing, resulting in a very flat velocity profile. Flow obstructions directly upstream of the traverse plane did not disrupt the average velocity profile but may have contributed to the substantial fluctuating velocity component shown in Figure 7C.

~0

Ef fec t o f V e l o c i t y P r o f i l e s on E f f ic iency Filter efficiency was strongly affected by differences in the upstream flow conditions. For a given flow rate and given leading fibre, the efficiency curves became shallower where the velocity profile was less uniform (lower filter position - Figure 3)

Upper Pes: F~; Lower Pos: per~ated plate

c. u ~ upper Posi~on '~

'if I c.o0,°0,,-,m ,:F.cupper ,,t,onl Lower pos: perforated plate

0 i I 1

12 I lB. d owos,

.~ | Upper pos: empty I Lower pos: F-C

, .upper pos: ~mp~y I A. upstream lower position Lo,~,erpos:F-Cf I ' ~ ~ = i

o o,s 1.o l.s 2.0 2.s l~me (sec)

F i l t r a t i o n & S e p a r a t i o n D e c e m b e r 2 0 0 1 4 5

1.0

0.8

o c- Q)

70 0.6

c - O

0.4 U_

0.2

I 10

Droplet diameter (gm)

Figure 8: Model results of efficiency vs. droplet diameter for a two-layer f i l ter wi th a uniform upstream velocity profile at each of the three f i l trat ion velocities.

o c-

c- O

c~

U-

1.0

0.8

0.6

0.4

0.2

1 10 Droplet diameter (gm)

Figure 9: Model results of efficiency vs. droplet diameter for a two-layer f i l ter at a nominal face

velocity of 2.3 m/s and each orientation of the test f i l ter (results are provided for uniform and non-uniform

upstream profiles).

compared to those where the velocity profile was more uniform (upper filter position - Figure 4). Non-uniformity of the velocity profile caused areas of decreased or increased efficiency where the velocity was correspondingly lower or higher. The efficiency of the whole filter, an integration of these varying efficiencies over the lace of the filter, resulted in a relatively shallow curve.

Filter orientation caused differences in the measured efficiency curves but did not affect the pressure drop through the filter or the upstream velocity profile. Thus, efficiency differences with filter orientation did not arise due to alterations of the upstream airflow profile. With both filter

orientations, the leading edge of the filter experienced relatively high tangential mean velocities that were redirected as the air passed vertically through the filter. Exposure to these higher tangential velocities caused enhanced collection due to impaction at the leading edge of the filter. Because impaction increases as fibre size decreases, this effect was more pronounced when fine fibres faced upstream.

The effect of filter orientation on efficiency was reduced, but still present, when the filter was placed in the upper position (Figure 4). Although the upstream velocity profile was uniform in this position (Figure 6C), it still contained velocity vectors tangential to the surface of the filter in the form of a relatively large turbulent rms velocity (Figure 7C) and a turbulence intensity of 27%. Once again, the leading filter layer was the region of greatest rms velocity dissipation, and once again efficiency was higher when the leading layer was comprised of smaller fibres.

E f f i c i e n c y M o d e l l i n g Theoretical efficiency predictions for a two-layer filter with uniform velocity profile at three filtration face velocities (Figure 8) can be compared to the experimental measurements presented with the filter installed in the upper position (Figure 4). The predictions in Figure 8 were made with the 16 velocity inputs in layers la, lb, and 2 set to the superficial face velocity. The trend that efficiency increases as face velocity increases is reflected in the calculations; however, the magnitude of the increase observed experimentally was substantially greater than that predicted by the theory. Furthermore, the experimental efficiency curves are substantially shallower than their theoretical counterparts.

Figure 9 show's the efficiency predictions from theory for a two-layer filter for the highest face velocity (2.3 m/s) and for each filter orientation. For the curves identified 'non-uniform' in the legend, the velocities in layer l a were the upstream velocity measurements from Figure 6a that varied substantially with position. Velocities in layer lb were the average between the upstream and downstream velocity measurements from Figure 6B where the downstream velocities were much more uniform. Velocities in layer 2 were taken as the downstream values. For the curves identified as 'uniform', the velocities in layer la, lb, and 2 were taken from Figure 6C where the velocities were very uniform.

In the uniform case presented in Figure 9, all 16 filter compartments behaved identically, thereby resulting in a sharp efficiency curve. For the non-uniform case with coarse fibres leading, the theoretical efficiency increased only marginally in contrast to the same measurement determined experimentally (compare efficiency for the C-F filter orientation at a face velocity of 2.3 m/ s in Figure 4 to that in Figure 3). When fine fibres led, the theoretical efficiency was higher; the magnitude and direction of this efficiency shift with filter orientation agrees with the experimental results presented in Figure 3. Additionally, non-uniformity in the velocity profile caused a unique efficiency in each of the 16 compartments and caused a shallower curve when integrated to obtain overall efficiency. These calculations support the idea that the leading layer of a filter dissipates tangential velocities. Furthermore, a fine layer was more effective at translating this velocity dissipation into gains in collection efficiency.

4 6 D e c e m b e r 2 0 0 1 F i l t r a t i o n + S e p a r a t i o n

:ONCLUSION! IEFERENCE~

Upstream airflow conditions influenced filter efficiency. Collector geometry caused a non-uniform velocity profile, substantially elevated mean velocities tangential to the filter surface, and increased levels of turbulent velocity fluctuations. Non-uniformities in the velocity profile upstream of the filter promoted areas of decreased or increased efficiency where the velocity was lower or

higher, resulting in a relatively shallow overall efficiency curve. Tangential mean velocity components and turbulent velocity fluctuations were dissipated with the movement of air from the front of the filter to the rear. This dissipation occurs throughout, but is most substantial at the leading edge of the filter. Thus, collection efficiency is highest when the region of greatest velocity dissipation coincides with a filter layer composed of small diameter fibres. Compared to a uniform velocity profile, non-uniform conditions caused the fractional efficiency curve to shift toward smaller diameters and to become shallower. This effect was shown in experiments and was mirrored qualitatively using theory based on measured velocity values.

Although upstream airflow conditions affected filter efficiency, they did not substantially affect pressure drop. As a result, alteration of upstream airflow conditions through innovative modifications to housing geometry may provide a way to enhance filter performance.

1. Boundy M, D Leith, D Hands, M Gressel & G Burroughs. 2000. Performance of Industrial Mist Collectors OverTime, Appl Occup Environ Hyg, 15(12): p.928-935.

2. Leith D, P C Raynor M G Boundy & S J Cooper. 1996. Performance of industrial equipment to collect coolant mist. AIHAJournal, 57, p. 1142-1148.

3. Schlichting H & K Gersten. 2000. Boundary layer theory, 8th edition, Springer: NewYork, USA.

4. Hinds W C. 1999. Aerosol technology: properties, behavior, and measurement of aribome particles, John Wiley & Sons Inc, New York, USA, p.182-205.

5. Davies C N. 1973. Airfihration, Academic Press: London, UK. 6. Lee KW & M Ramamurthi. 1993. Filter collection, in Aerosol

measurement, K Willeke & P Baron, Eds. , John Wiley & Sons: New York, LISA.

kCKNOWLEDGMEN1

We are grateful to Ford Motor Co and the United Auto Workers, who provided a gift to the University of North Carolina that made this work possible.

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