CONTENTS INTRODUCTION...............................................1 CURRENT BASIS OF LEV DESIGN................................6 LIMITATIONS OF CAPTURE VELOCITY DESIGN....................15 THE CAPTURE EFFICIENCY CONCEPT............................18 CURRENT DESIGN PROCEDURE VERSUS BREATHING ZONE CONCENTRATION........................................20 VORTEX SHEDDING___. .___...___..........................25 IMPORTANCE OF REVERSE FLOW PHENOMENON IN WORKER EXPOSURE..29 A SIMPLE MODEL ADDRESSING REVERSE FLOW....................32 OBJECTIVE AND PURPOSE................___.................38 METHOD OF MODEL EVALUATION................................40 Wind Tunnel Description..............................40 Velocity Determination...............................41 Test Object Description..............................48 Sulfur Hexafluoride Generation.......................50 Determination of Zone Depth: Visualization of Test Smoke......................................51 Determination of Zone Depth: Concentration Versus Distance Curves.................................53 Determination of Zone Depth: Cbz = 0.5Co............74 Determination of Zone Depth: Calculation from Theory..........................................74 MANNEQUIN VERSUS MANNEQUIN 90 DEGREES.....................80 DISCUSSION OF MODEL EVALUATION............................87 Discussion of Test Smoke Observation.................87 Discussion of Actual and Theoretical Concentration Versus Distance Curves..........................88 Discussion of Mannequin Versus Mannequin 90 Degrees..91 Discussion of the Model's Ability to Predict De......92 11
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IMPORTANCE OF REVERSE FLOW PHENOMENON IN WORKER EXPOSURE..29
A SIMPLE MODEL ADDRESSING REVERSE FLOW....................32
OBJECTIVE AND PURPOSE................___.................38
METHOD OF MODEL EVALUATION................................40
Wind Tunnel Description..............................40Velocity Determination...............................41Test Object Description..............................48Sulfur Hexafluoride Generation.......................50Determination of Zone Depth: Visualization of
Test Smoke......................................51
Determination of Zone Depth: Concentration VersusDistance Curves.................................53
Determination of Zone Depth: Cbz = 0.5Co............74Determination of Zone Depth: Calculation from
MANNEQUIN VERSUS MANNEQUIN 90 DEGREES.....................80
DISCUSSION OF MODEL EVALUATION............................87Discussion of Test Smoke Observation.................87Discussion of Actual and Theoretical Concentration
Versus Distance Curves..........................88
Discussion of Mannequin Versus Mannequin 90 Degrees..91Discussion of the Model's Ability to Predict De......92
Discussion of the Model's Ability to PredictMixing Zone Concentrations......................93
CONCLUSIONS..............................................112Test Smoke Observation..............................112Theoretical Model...................................112Mannequin Versus Mannequin 90 Degrees...............118
RECOMMENDATIONS..........................................119Validation of Model.................................119Effects of Hands and Arms...........................119Turbulent Diffusion Effects.........................120Study of Concentration Decrease as a Function of
^'v-^x/ ^^1.SU BSTITUTION VVITH A TRAINING & EDUCATIONLESS HARMFUL MATERIAL (IMMEDIATE CLEANUP) (MOST IMPORTANT)(WATER IN PLACE OFORGANIC SOLVENT) 2. GENERAL EXHAUST 2 ROTATION OF WORKERS
VENTILATION (SPLIT UP DOSE)2. CHANGE OF PROCESS (ROOF FANS)(AIRLESS PAINT SPRAYING) 3 ENCLOSURE OF WORKER
3. DILUTION VENTILATION (AIR CONDITIONED3. ENCLOSURE OF PROCESS (SUPPLIED AIR) CRANE CABS)(GLOVEBOX)
4. INCREASE DISTANCE 4 PERSONAL MONITORING4. ISOLATION OF PROCESS BETWEEN SOURCE AND DEVICES (DOSIMETERS)
(SPACE OR TIME) RECEIVER (SEMI-AUTOMATICOR REMOTE CONTROL) 5 PERSONAL PROTECTIVE
5. WET METHODS DEVICES (RESPIRATORS)(HYDRO BLAST) , 5. CONTINUOUS AREA
MONITORING (PRESET 6 ADEQUATE MAINTENANCE6. LOCAL EXHAUST ALARMS) PROGRAMVENTILATION
(CAPTURE AT SOURCE) 6. ADEQUATE MAINTENANCEPROGRAM
The ductwork is the piping system through which the
contaminant-laden air flows. Its design and construction
are determined by many factors such as the type of materialconveyed, temperature, and plant layout, for example.
The function of the air cleaning device is to removethe contaminant from the air stream before it is exhausted
to the outside environment. Many different types of
cleaners exist and proper selection depends on concentrationand particle size of contaminant, degree of collectionrequired, characteristics of the air or gas stream,
characteristics of the contaminant, energy requirements, andmethod of dust disposal [2].
The fan provides the means of inducing air flow by
creating a pressure differential between the atmosphere and
inside of the system. The magnitude of this pressure
difference determines the quantity of air entering thesystem. At the end of the LEV design procedure, a specificfan is selected that will move the required amount of airagainst the necessary pressure differential.
LEV has been utilized in industry since early in the
twentieth century. However, the basic parameters used todesign these systems have changed very little over the lastfifty years. Presently, new concepts are being exploredthat may significantly improve the current state of LEVdesign with the ultimate goal of providing the best possibleprotection for the employee at the lowest possible cost.
Active generation into zone ofrapid air motion Spray painting in shallow booths;
barrel filling; conveyor loading;crushers
200-500
Released at high initial velocityinto zone of very rapid air motion. Grinding; abrasive blasting, tumbling 500-2000
In each category above, a range of capture velocity is shown. The proper choice of values depends onseveral factors:
Lower End of Range Upper End of Range1. Room air currents minimal or favorable to capture. 1. Disturbing room air currents.2. Contaminants of low toxicity or of nuisance value 2. Contaminants of high toxicity,only.
3. Intermittent, low production. 3. High production, heavy use.4. Large hood—large air mass in motion. , 4. Small hood—local control only.
Additionally, to maintain the same velocity at a givendistance in front of the hood, less air is required when thehood rests on a plane than when it has no obstruction.
Within the last decade. Garrison [11] has evaluated thework of Dalla Valle and Silverman using much smaller inletsand higher velocities, i.e., high velocity/low volumesystems. Among other things, he concluded that theempirical equations published in the Ventilation Manual [2]from Dalla Valle's and Silverman's work were generallyappropriate. However, he disagreed with the flat 33 percent increase in centerline velocity velocity attributed toflanging the circular or rectangular inlets as isrecommended in the Manual. His data indicate that a more
realistic centerline velocity increase would be on the orderof 20 to 3 0 per cent. He also recommended that Silverman'sequations be restricted to centerline distance/hood diameter(or hood width) ratios of 0.4 or greater because, as wasmentioned previously, as x approaches 0, V becomesindeterminate.
In a later paper Garrison [12] provides the followingnon-dimensional equations that describe centerline velocitygradients in terms of distance, inlet end shape, andflanging:
(5) Y(near) = a(b) ^''^(6) Y(far) = a(Xdw)^
where Y = centerline velocity/hood face velocityXdw = centerline distance/hood diameter (or hood width
will not adequately describe the flow field over much of thecontaminant generation area.
Roach [18] states that "...it is inadvisable to designthe ventilation of an exhaust hood so as merely to produce astandard capture velocity or standard entrance velocity, as
the velocity chosen may be excessively high or, what wouldbe worse, not high enough."
Fletcher and Johnson [19] demonstrated that the removal
of gases and submicron particles released at low velocitieson the centerline of LEV hoods can be predicted by thetraditional concept of capture velocity. However, when thedirection of contaminant release is away from the hood at
velocities greater that about 0.21 m/sec the capture
velocity concept is not valid in that a higher velocity isneeded at the source to entrain the contaminant than that
published in the Ventialtion Manual [2].
Heinson and Choi [20] list the following flaws in thecurrent design methods:
(1) Contaminant concentration in the vicinity of thesource cannot be predicted.
(2) The effect of changes in design (such as systemdimensions or volumetric flow rate) on the performance of asystem cannot be estimated.
(3) Even though the performance of a particular system
is known, the effect of geometrically scaling it up or downin unpredictable.
Consider a worker in a spray paint booth. In a well
designed booth, virtually all of the contaminant is
eventually captured, giving a capture efficiency of 100 percent. However, most spray painters are required to wearrespiratory protection because a significant quantity ofcontaminant passes through their breathing zone before beingremoved.
Clearly, a method of LEV design that somehow relatesdesign parameters to breathing zone concentration would bemost useful in protecting employees. However, before such amodel can be developed, a fundamental interaction must beinvestigated; that of the worker with the flow field.
Previous analytical models describing flow fields intohoods [16,17,20] have used potential theory as thetheoretical basis. In potential flow, the assumptions aremade that the fluid is both incompressible (the volumeexpansion is negligible) and irrotational (negligible localangular velocity) [21]. These assumptions are valid in thefree field where no object is present to obstruct the flow.While these models have certain applications, instances
arise when the worker becomes a significant obstacle in thepath of air flowing into the hood.
An object (such as a person) in the flow field
questions the validity of the potential theory approach intwo ways. First, by its very presence the object acts as anobstacle, a physical obstruction to the flow of air into the
boundary- The authors also showed that the ratio of
longitudinal spacing between vortices to the diameter of thecylinder is about 4.27.
Bloor [26] demonstrated a stable range of vortexformation at Re below 200. That is, in this range, the flowis laminar everywhere. In the Re range of 200 - 400, thewake begins to disintegrate to turbulence. The onset ofwake turbulence moves closer towards the cylinder as Reincreases. At Re greater than 400 the separated boundarylayer becomes turbulent even before it rolls up into avortex. Thus, the vortices are turbulent upon fomnation.However, at Re between 400 and 1300 the point of transitionof turbulence remains constant relative to the cylinder.Finally, at Re of about 1300, the length of the laminar flowregion begins to decrease again until, at Re of about50,000, it is almost to the shoulder of the cylinder. Thepoint at which turbulent motion reaches the separation pointof the boundary occurs at Re of about 300,000. This pointis called the critical Reynolds number. However, a definiteshedding frequency is still observed, even at the criticalReynolds number.
Bearman [27] demonstrated regular vortex shedding at Reup to 550,000. However, at a Re of 300,000, the sheddingfrequency as described by the Strouhal number showed a sharpincrease from its relatively constant value of 0.21. TheStrouhal number leveled off to about 0.46 for Re greaterthat 400,000. The author points out that any small change
in the surface smoothness of the cylinder can significantlydisrupt the separation causing fluctuations in the sheddingfrequency.
Achenbach and Heinke [28] also noted the sharp increasein shedding frequency at Re of 300,000. This increasebecomes less prominent as cylinder surface roughnessincreases.
arising from movements of the body. He concludes thateither of these two wake structures can completely destroythe intended beneficial effect of a LEV and that no
consideration appears to be given to this problem instandard ventilation design.
In studying push-pull ventilation systems, Hampl and
Hughes [30] also demonstrated the effect of a person in theflow field of a ventilation system. They observed thecollection of smoke by a standard LEV hood with variousorientations of air jets used as "pushing" air streams. Foreach orientation where a test mannequin obstructed thepushing jets, smoke was observed in the area in front of themannequin. However, when the jet was placed between thesmoke and the mannequin, no smoke was observed in thebreathing zone and all smoke was captured by the hood. Theyconcluded that the "push jet should be located so that theair impinging on the worker or other obstruction should beminimized".
Van Wagenen [31] studied the effects of positiveairflow (blowing rather than exhausting air) onconcentrations of various contaminants in a welder's
breathing zone. He demonstrated that when directional airflow comes from directly behind the welder, concentration offume in the breathing zone was equal to or higher than thebreathing zone concentration with no directional air flow atall. He attributed this to the eddy and convective currentsaround the welders body. He also noted that positive
The flow rate out of the zone is controlled by theshedding of vortices such that
(13) Qv = fV
Where V is the mixing zone volume and f is the frequencywith which this volume is removed by vortex shedding. Ifone assumes that the vortices are approximately circularcylinders of height H then the volume can be given by
(14) Vv = (pi)(De^H/4where De is the diameter of an average vortex. However, onemust account for the fact that the zone is composed of twovortices which are alterntely formed on each side and sheddownstream in accordance with the Strouhal number. Thus,
when a vortex is shed, it takes with it one half of the
volume of the zone. Therefore, the actual volume out of thezone is
(14a) V = (pi) (De^ (H)/8and the flow rate out of the zone is given by
(15) Qv = [(0.2)(U)/(D)][(pi)(De)(H)/8]
Substituting into equation (12) gives the followingrelationship for concentration within the zone
(16) Co = [3.57/De]*2[(Qs)(D)/(U)(H)]
Solving for the theoretical diameter of a vortex gives(17) De = 3.57 sq.rt.[(Qs)(D)/Co(U)(H)]
The following assumptions are made; (a) the diameter ofa vortex is essentially the same as the diameter of the zoneof reverse flow (that is, De = s) and beyond this point nocontaminant is drawn back towards the cylinder, (b) the zone
the tunnel cross section. This increases the velocity inproportion to the amount of tunnel area blocked by theobject. Therefore, a "blockage ratio" must be calculated.A blockage ratio is basically a factor by which theunblocked velocity must be multiplied to get the true tunnelvelocity. To determine the blockage ratio, the amount oftunnel cross section blocked by the object must beestimated. The following formula can then be applied:
(19)Corrected = Measured X BlockageVelocity Velocity Ratio
Notice from table (3) that by appling the blockageratio the corrected velocities are 265 fpm, 152 fpm, and 49fpm for the mannequin and 292, 167, and 54 fpm for thecircular cylinder.
The Re for air flow around the objects at thesevelocities is also given in table (3). These velocitieswere selected because the Re are in the same range as thosefor air flowing around an industrial worker in a uniformflow such as a spray paint booth.TEST OBJECT DESCRIPTION
The circular cylinder used in this project wasconstructed of sheet metal and was 48 inches tall and 12inches in diameter. Two holes were drilled in the cylinder,one in the front about 15 inches from the top and another inthe back about 6 inches from the bottom. One end of a one
*The mannequin diameter is not actually a diameter becausethe mannequin cross section is more elliptical than circularin shape. The value reported here as the diameter is thebreadth of the mannequin chest as measured just under thearmpits and at the same distance from the floor (27") as thesource of SF6.
evaluated. Each of the above listed values was known for
the cylinder and the mannequin. Once again, the same twovalues for Co mentioned above were used in this calculation.Table (8) gives the calculated values of De from thepredictive model.SUMMARY
Table (9) summarizes the results of the modelevaluation experiment by comparing the depth of the zone asdetermined by;
1) the observation of test smoke,
2) the concentration versus distance curves, and3) the theoretical equation.
Another way of examining the effect of boundary layerseparation on breathing zone concentration is to monitorconcentration in the breathing zone under conditions inwhich the boundary layer will interact with the contaminantsource and compare the result to a situation in which itdoes not. As has been mentioned, in the typical orientationof a worker with respect to LEV, the separated boundarylayer can possibly extend downstream far enough to cause thecontaminant to be drawn back into the breathing zone.However, if the worker stands at right angles to the flow(figure (32)), whatever reverse flow zone is formed will beless likely to reach out and interact with the contaminantsource but will rather extend downstream.
To evaluate the effect of a 90 degree orientation withrespect to ventilation on breathing zone concentration, amethod similar to that previously described was used. Themannequin was again placed in the wind tunnel. However,this time it was oriented at 90 degrees from the flow. Thatis, the wind was flowing from its side rather than aroundits back. A set of SF6 concentration versus distance curves
was obtained in exactly the same manner as before for 2 65fpm, 152 fpm, and 49 fpm.
The curves obtained with the mannequin at 90 degreesare graphically compared to the curves obtained in theprevious experiment in figures (33), (34), and (35). A
*Note that these velocities are not entirely accurate sincethey were based on the blockage ratio at the maximummannequin cross section. However, for comparitive purposesthey are quite close.
experimental curves (figures (16) - (31)) does not support
the idea of complete, uniform mixing throughout the zone.For the cylinder at the highest velocity (292 fpm - figure(18)), very little mixing seems to occur. The concentrationappears to fall off exponentially with increasing distancethroughout the zone, dropping to essentially zero by thetime the edge of the zone (as determined from test smoke) isreached. However, at 167 fpm (figure (17)), experiment 1
data shows that a little more mixing is occurring, givingconcentrations that are relatively uniform up to 4 to 6
inches before falling off. The best mixing within the zoneseems to occur at 54 fpm (figures (16) and (24)). Here, asomewhat uniform concentration can be observed out to
approximately 8 to 12 inches before a sharp decrease occurs.
The mannequin data indicate some mixing in the zone out toabout 6 to 8 inches for all velocities (figures (20), (21),
(22), (28), (29), (30)). Note that the mannequin provides amore realistic view of the effect of the turbulent zone
phenomenon on human exposure because of the hands and armswhich extend into the zone. The hands extend a distance of
approximately three inches from the body of the mannequin.This may account for the better mixing observed for themannequin at lower flows. Perhaps some smaller scaleturbulent zone is formed downstream of the arms themselves.
The motion of air around arms and hands is an important
determinant in breathing zone concentration for workers atlaboratory hoods [34].
In addition to the assumption of a well mixed zone, the
model under study also postulates that the edge of the
mixing zone is considered to be the point at which the
initial concentration (Co) falls to one half of its originalvalue (see figure (8)).
In table (9), the zone depth as visualized by the testsmoke is compared with the distance at which the
concentration actually fell to one half of its initial value(as determined from the log-log plots of concentration
versus distance). Recall that Co was defined in two ways;
first as the breathing zone concentration at a source
distance of 0.5 inches and second as the maximum breathing
zone concentration value on the curve. Note from table (9)
that in all but one case the zone depth as determined from
Cbz = 0.5Co is considerably less than that visualized by thetest smoke. The difference ranges from a factor of about
two to a factor of more than ten. The one exception is in
the second mannequin experiment at a velocity of 49 fpm whenCo is defined as the breathing zone concentration at a
source distance of 0.5 inches. This zone depth determinedfrom this point (10.1") is almost exactly equal to that
visualized by smoke (10"). However, note from table (9)that this value is considerably out of line with pointsobtained from the other log-log plots. Observation of
figure (28) indicates a very low initial concentration for
this particular curve when compared to the curves at thisvelocity for the first mannequin experiment and bothcylinder experiments. Therefore, it is concluded that theclose agreement at this single point is not significant.
On the basis of these experiments, it appears that thefundamental assumption of a well mixed turbulent zone maynot be accurate. The data seems to indicate that evenwithin the zone a definite variation in concentration withdistance is observed. Perhaps the periodic shedding ofvortices from this zone is not the only contaminant removingprocess involved. Other phenomenon, such as turbulentdiffusion, may also affect concentration, giving rise togradients within the zone.DISCUSSION OF MANNEQUIN VERSUS MANNEQUIN AT 90 DEGREES
Perhaps the most dramatic indication of thesignificance of the reverse flow phenomenon on workerexposure can be seen in the comparison of breathing zoneconcentration between the mannequin at typical orientationwith respect to LEV and the mannequin oriented at rightangles to the flow. From figures (33) through (35) it isobvious that standing 90 degrees to the direction of airflow significantly reduces the concentration of contaminantin the breathing zone. Concentrations measured with airflowcoming from behind the mannequin ranged from 6 to 43 timeshigher than those measured at corresponding distances withthe mannequin at 90 degrees to the air flow.
R e g r e s s i o n 0 i.i t p:) i..( t. nConstant 1„029587Std Err of Y Est 0„144758R Squared 0«7S7622M o, o f 0 b s e r V a t :i. g p. s 6Degrees of Freedom 4
X Coe-f -f i c i en t < s) 0, 000025S t. d E r r o t C o e f . O.. O O O 0 0 6
play a significant role in the flow rate of contaminant outof the mixing zone. Therefore, at these Re, the K factor,which in effect is accounting for turbulent diffusion, doesnot work as well at predicting the concentration (figures(44) and (47)). On the other hand, at Re greater than10,000, turbulent diffusion may predominate since thevortices are becomming more and more turbulent and thus themodel (including the K factor) predicts concentration withinthe zone very nicely (figures (42), (43), (45), and (46)).
There are also indications in the literature that the
distance over which the turbulent zone entrains contaminant
may increase with increasing Re. Gerrard [40] suggests thatthe entrainment flow of the turbulent wake is governedmainly by the length of the turbulent shear layer as itcrosses the axis of the wake and that this flow increases
with increasing Re. Gerrard terms the thickness of thisshear layer the "diffusion length". This could explain theincreasing zone depth with increasing Re observed with thetest smoke.
To summarize, it appears that the two crucialassumptions involved in the development of the model are notcompletely correct. The distance downstream to which theturbulent mixing zone extends seems to be greater than thediameter of a vortex. The turbulent wake appears to effectcontaminant concentrations in the zone at distances beyondthe region of vortex formation. Consequently, the alternateshedding of vortices do not appear to be the sole mechanism
effects into the model is essential if this work is to
applied to actual situation in the workplace.TURBULENT DIFFUSION EFFECTS
Since turbulent diffusion is suspected to be asignificant mechanism involved in the transport ofcontaminants in turbulent wakes, it is desirable to
incorporate this process into the model in a meaningful,theoretical way rather than to simply fit it to the datawith a mixing factor. To do this it will be essential tostudy the current literature on this subject to gain a morethorough understanding of this phenomenon. To proceed withthis research without attempting to account for turbulencewould seem to be unreasonable.
STUDY OF CONCENTRATION DECREASE AS A FUNCTION OF DISTANCE
Another entirely different approach to predictingbreathing zone concentration from LEV parameters issuggested by noting the decrease in concentration as thesource moves farther and farther away from the samplingprobe. Figures (48) through (59) illustrate log-log plotsof concentration versus absolute distance for the data
collected in this experiment. [Absolute distance is definedas the distance from the tip of the sampling probe to thepoint source of contaminant.] Note from these graphs thatthere appears to be a reasonably strong inverse relationshipbetween log concentration and log distance. Regressionlines give values for R squared ranging from 0.84 to 0.99.
The 95% confidence interval for the slopes of theseregression lines (table (12)) indicate that there is nostatistically significant difference between the slopes ofthe lines at 54 fpm, 167 fpm, and 292 fpm for the cylinderdata. The average value of the slope being approximately -1.6. Likewise for the mannequin data, slopes at all threevelocities are statistically the same with the exception ofthe 49 fpm line from experiment two data. For the mannequinthe average slope is about -2.4. Thus, for a given objectgeometry, the slope appears to be independent of velocity.
SF5 CONCENTRATION VS. SOURCE DISTANCECYLINDER IN TUNNEL 1 67 FPM
2.8
2.6
2.4
2.2Ea.
•5 2
5 1.8H
i^ 1.62Ulo 1.4z
8 1.2U)h.at 1
z_i
0.8
0.6
0.4
0.2
a DATA POINTSLN ABSOLUTE DISTANCE (IN.)
+ REGRESSION POINTS
R e g r e s s i D n 0 n t p u t. sConstant _, 5,. 650873Std Err o-f Y Est 0.084051R Squared 0.. 989556!n|o. of 0bser^ Vat i ons 13Deqrees of Freedom 11
enter the area of increasing pressure on the downstream side
of the object and thus it separates. However, in front of a
hood it would be expected that the additional accelerationgiven to the air as it moves around the object would alterthe characteristics of the turbulent zone. It is possible
that this additional acceleration given to the decelerating
boundary layer may act to decrease the size and influence ofthis downstream vortical mixing zone [22].
A preliminary experiment was designed by Flynn [22] as
a first step in understanding the effect of this reverse
flow area on breathing zone concentrations of workers in
front of local exhaust hoods (accelerating flows). The
basic set-up of the experiment is illustrated in figure(60).
Basically, this experiment was designed to test the
effect of object size relative to hood size on the formation
of the reverse flow zone. A smooth cylinder of diameter Do
was placed in front of a local exhaust hood of diameter Dhon the hood centerline a distance z from the hood face. The
hood was operated at flow rate Q. A source of test smoke
(titanium tetrachloride as described previously) was also
placed on the centerline of the hood between the hood face
and the object. As with uniform flow, when the smoke source
was close to the hood face, all the smoke was drawn directly
into the hood. However, when the source was close to the
cylinder, the smoke was pulled back towards the cylinder bythe reverse flow. By moving the source slowly back andforth,the point at which the smoke first began to be drawnback toward the cylinder could be estimated. This distancefrom the source to the cylinder at which backflow starts tooccur is called s. This was considered to be the depth ofthe reverse flow zone.
Appendix V tabulates values of s for variouscombinations of one hood diameter (6"), three cylinderdiameters (1.5", 6.0", and 12.0"), and five flow rates (1075cfm, 995 cfm, 910 cfm, 810 cfm, and 700 cfm). At higherflow rates, values for s were measured at z distances(centerline distance from cylinder to hood face) rangingfrom 2" to 18". However at lower flow rates, the z distanceranged from 2" to only 10" because the influence of the hoodflow field does not extend as far away from the hood facefor the lower flow rates.
A complete analysis of the data contained in Appendix Vhas not yet been conducted. However, preliminary analysissuggests that the size of the turbulent mixing zone may varyas the size of the object relative to the size of the hoodvaries. Figures (61) and (62) illustrate s/Do versus z forflow rates of 1075 cfm and 995 cfm respectively. Thisindicates that, relatively speaking, the size of the zone ismuch larger when the object is small relative to the hooddiameter and becomes smaller as the object gets largerrelative to the hood. This relationship holds true for the
other flow rates as well. In an industrial situation, theeffect of the reverse flow phenomenon on breathingzoneconcentrations may be more significant for employeesworking in front of a large hood as opposed to a smallerhood.
Perhaps as the hood gets large relative to the object,the zone formation approaches the uniform flow situation asdiscribed previously whereas when the object is much largerthan the hood, most of the flow comes from the side ratherthan around the object. However, this explanetion isspeculative. Much empirical and theoretical work is yet tobe done to adequately explain this phenomenon.
1. National Safety Council, Fundamentals of IndustrialHygiene. 2nd Ed., New York (1979).
2. American Conference of Governmental IndustrialHygienists, Industrial Ventilation - A Manual ofJ^ecommended Practice. 19th Ed., ACGIH, Cincinnati, OH(1986).
3. McDermott, H.J., Handbook of Ventilation forContaminant Control. Ann Arbor Science, Ann Arbor, MI(1976).
4. National Institute for Occupational Safety and Health,The Industrial Environment - Its Evaluation andControl, U.S. Government Printing Office, Washington,D.C. (1973).
5. Dalla Valle, J.M., and Hatch, T., Studies in the Designof Local Exhaust Hoods, presented at the 6th AnnualWood-Industries Meeting, Winston-Salem, NC, Oct. 15-16, 1931 of the American Society of MechanicalEngineers.
6. Silverman, L., "Centerline Velocity Characteristics ofRound Openings Under Suction", Journal of IndustrialHygiene and Toxicology. Vol. 24, No. 9, pp. 257-266.(Nov. 1942)
7. Silverman, L., "Centerline Velocity Characteristics ofNarrow Exhaust Slots", Journal of IndustrialHygiene and Toxicology. Vol. 24, No. 9, pp. 267-276.(Nov. 1942)
8. Fletcher, B., "Centerline Velocity Characteristics ofRectangular Unflanged Hoods and Slots Under Suction",Annals of Occupational Hygiene. Vol. 20, pp. 141-146(1977).
9. Fletcher, B., "Effects of Flanges on the Velocity inFront of Exhaust Ventilation Hoods", Annals ofOccupational Hygiene. Vol. 21, pp. 265-269 (1978).
10. Fletcher, B., and Johnson, A.E., "Velocity ProfilesAround Hoods and Slots and the Effects of an AdjacentPlane", Annals of Occupational Hygiene,. Vol. 25, pp.365-372 (1982).
11. Garrison, R., "Centerline Velocity Gradients for Plainand Flanged Local Exhaust Inlets", American IndustrialHygiene Association Journal^ Vol. 42, No. 10, pp. 739-746 (1981).
12. Garrison, R., "Velocity Calculation for Local ExhaustInlets - Empirical Design Equations", AmericanIndustrial Hygiene Association Journal. Vol. 44, No.12, pp. 937-940 (1983).
13. Garrison, R., "Velocity Calculation for Local ExhaustInlets - Graphical Design Concepts", AmericanIndustrial Hygiene Association Journal. Vol. 44, No.12, pp. 941-947 (1983).
14. Ellenbecker, M.J., Gempel, R.F., and Burgess, W.A.,"Capture Efficiency of Local Exhaust VentilationSystems", American Industrial Hygiene AssociationJournal. Vol. 44, No. 10, pp. 752-755 (1983).
15. Esmen, N.A., Weyel, D.A., McGuigan, F.P., "AerodynamicProperties of Exhaust Hoods", American IndustrialHycfiene Association Journal. Vol. 47, No. 8, pp. 448-453 (1986).
16. Flynn, M.R. and Ellenbecker, M.J., "Capture Efficiencyof Flanged Circular Local Exhaust Hoods", Annals ofOccupational Hygiene. Vol. 30, NO. 4, pp. 497-513(1986).
17. Flynn, M.R. and Ellenbecker, M.J., "The Potential FlowSolution for Air Flow into a Flanged Circular Hood",American Industrial Hygiene Association Journal. Vol.46, No. 6, pp. 318-322 (1985).
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When the appropriate software is used, the Dataloggercan "dump" the recorded data into a personal computer for asubsequent hard copy report. The following report types areavailable; overall statistics, time history, amplitudedistribution, raw data, and multiple channel.
For this project, the time history report was the mostuseful. This report gives the minimum, maximum, and averagevalues for each averaging period of the logged data. Agraphical representation is also provided by the computerwhich graphs a minus (-) sign for the period's minimumvalue, a plus (+) sign for the period's maximiim period, andan asterisk (*) for the period's average value. The graph'saccuracy and resolution depend on the variation (spread) ofthe test data. Please note attachment (1) for an example ofa time history report.