Testing local exhaust ventilation at controlled turbulence generation

by using tracer gas and a 3-D anemometer

Magnus Mattsson

Dec 2014

University of Gävle

2

Summary Local exhaust (LE) ventilation is a ventilation technique where contaminated air is locally extracted close to the contaminant source, usually with the purpose to reduce the exposure of a person doing work which involves the contaminant. There is a need for well-defined and appropriate methods to test the performance of LE constructions. The present study aims at contributing to the establishment of such tests. The study entails full scale experimental measurements that include 3-D air velocity measurements, tracer gas tests and controlled generation of air turbulence through physical movements of a vertical, human-sized plate. The tested exhaust hood (EH) was of circular, flat plate flanged type. One part of the study concerned the task of determining the 0.4 m/s distance, x0.4, at the EH; i.e. the distance from the EH opening to a point where the air velocity has declined to 0.4 m/s. This is a currently used measure of “safe zone” at an EH. It was found that practicable measurements of good accuracy seem to be attained by using the following fairly simple correction equation:

where Vm is a provisionally measured air velocity, preferably within the zone where Vm is within 0.35-0.45 m/s in front of the EH, and xm is the measured distance from the EH opening to the measuring point of Vm. The tracer gas tests implied injection of a neutrally buoyant tracer gas through a perforated sphere placed in front of the EH. The amount of tracer gas that escaped from the suction flow was measured in the room air, thus yielding a sensitive method for measuring the capture efficiency (CE) of the EH. The CE is the percentage of injected tracer gas that is directly captured by the EH. Measurements of CE was performed at several test cases, were exhaust flow rate, gas release distance, turbulence level and EH arrangement were varied. The recorded CE values varied between 75 to 100% and the response to the different test cases appeared trustworthy. The use of a 3-D sonic anemometer, that yielded both magnitude and direction of the air movement, proved very useful in analyzing the generated air turbulence. Its measurement data was also used to construct another measure of the local exhaust performance: Percentage Negative Velocities, PNV. This measure represents the percentage of the time when the air flow at the measuring point in front of the EH is directed away from the EH nozzle, i.e. when the velocity component in the direction towards the EH opening is negative. The recorded PNV values correlated well with the corresponding CE values, attained at the tracer gas tests. Thus, measuring PNV might be a convenient alternative or complement to tracer gas measurements.

x0.4= xm∙�Vm0.4

[ m ]

3

Nomenclature A Area of exhaust hood opening [ m2 ] CE Capture Efficiency – fraction of injected tracer gas directly captured by the local

exhaust [ % ] D Internal diameter of the circular exhaust opening [ m ] EH Exhaust Hood LE Local Exhaust MP Movable plate r Radial distance from the middle of the exhaust opening [ m ] PNV Percentage of Negative Vx air Velocity [ % ] Q Air flow rate in exhaust pipe [ m3/s ] QG Flow rate of injected gas mixture (here N2O + He) [ ml/min ] QTG Flow rate of injected tracer gas (N2O) [ ml/min ] QTGC Flow rate of injected tracer gas (N2O) that is directly captured by the EH

[ ml/min ] QTGE Flow rate injected tracer gas (N2O) that is escaping from the direct exhaust

capture air flow and spread to the room [ ml/min ] SA Sonic Anemometer TG Tracer Gas V Air velocity, absolute value [ m/s ] Vav Average air velocity in exhaust pipe (=Q/A) [ m/s ] Vx Air velocity in x-direction, horizontally aligned with the exhaust pipe, positive

towards its opening [ m/s ] Vy Air velocity in y-direction, in the horizontal plane, perpendicular to Vx, positive

in “right” direction when seen from above. [ m/s ] Vz Air velocity in z-direction, positive vertically upwards. [ m/s ] Vol Room volume [ m3 ] x Horizontal distance, direction aligned with the exhaust pipe, positive towards its

opening [ m ] y Distance in the horizontal plane, perpendicular to x , positive in “right” direction

when seen from above [ m ] z Distance in the vertical direction, positive upwards [ m ] α Horizontal angle for measurements at the exhaust hood (see Fig. 10) [ ° ] Δt Movement interval of the movable plate. τ Time of tracer gas injection [ min ]

4

Background Local exhaust (LE) ventilation is a means to locally extract contaminated air from a process, usually with the purpose to reduce the exposure of workers to dust, fumes or vapour, which could be hazardous to their health. The performance of an LE installation depends however on many influential factors, and there is not yet a standardized way (at least at the international level) to test LE constructions. Contributing reasons for this are difficulties to state and generate typical characteristics of important influences like physical movements and air turbulence in the nearby zone around the LE. In the present study, possible ways to test the performance of LE installations are evaluated through experimental measurements that include 3-D air velocity measurements and tracer gas tests.

Method

The test room The tests were performed in a test room pictured in Fig. 1. A drawing of the test room and its surroundings is shown in Fig. 2. The test room was one of three rooms in a test building, situated in a lab hall (Fig. 3) at the University of Gävle. The ceiling height in the test room was 3.00 m and its room volume Vol = 54.7 m3. The building envelope of the test building, including inner walls, consisted mainly of wooden boards with 5-10 cm mineral wool insulation in between. The temperature of the lab hall was maintained at about 20 ºC. Except ceiling lighting and measuring equipment there was no heating in the test building. The test building was ventilated only through the flow of the tested local exhaust. Three air inlet openings (550x100 mm, indicated as broad, dashed arrows in Fig. 2) were located at ceiling height between the test room and the control room. On the test room side, a 500 mm high plate, extending across the whole width of the room, deflected the incoming air downwards, as illustrated in Fig. 4 and Fig. 5. This arrangement intended to produce an inlet air flow that caused very little air turbulence in the test room, especially in the area around the local exhaust. The air to the control room was in turn sucked in from the lab hall through a 130 mm hole in the ceiling, as sketched in Fig. 2. Some air is likely to have entered the test room also through minor leaks e.g. at the doorways, none of which were close the local exhaust installation. All doors were closed during the tests. The exhaust air flow through the exhaust hood, EH, was regulated through the speed of a fan placed outside of the test room, eventually discharging the exhaust air to outdoors. The exhaust air flow rate, Q, was measured through the pressure drop over an orifice plate (Standard ISO 5167:2003) of diameter 68.5 mm in a straight duct of diameter 104 mm. The pressure drop was measured with a newly calibrated differential pressure gauge (SwemaMan 80, Swema AB, Farsta, Sweden), yielding a total uncertainty of the flow rate of about 2%.

5

Figure 1. The test room with the local exhaust and movable plate arrangement. Set-up for test Case A.

Figure 2. Drawing of the test room (thick lines) situated in the test building. Supply air flows indicated by broad, dashed arrows. Measures in mm.

4200

4340

TEST ROOM

Control room

MP movement area

Local exhaust

6

Figure 3. The lab hall enclosing the test building. Figure 4. Side-view of the air inlet to the test room. Measures in mm. Slot width = 100 mm.

Figure. 5. Rear side of the test room, with supply air deflecting plate visible at the top, gas sampling point in the middle and mixing fan on the floor on the left.

Test room

500

7

Exhaust device The local exhaust (LE) device consisted of a 400 mm long straight aluminium cylinder with an inner diameter of 75 mm. The exhaust inlet opening was equipped with a 300 mm wide and 200 mm high flat plate, centred onto the inlet, thus forming a simple, exterior, flanged exhaust hood, EH. The EH was placed on a table (900 mm high, 1640 mm wide and 1220 mm deep). Two different vertical distances, h, between the EH centre and the table surface were tested: Case A: h=100 mm between the EH centre and the table (flange plate in contact with the table), Case B: h=500 mm between the EH centre and the table (Fig. 6), in principal constituting a free-standing EH. In case A and B the table was in contact with the wall. In a third set-up case, Case C, all equipment, incl. the table, EH and MP, was displaced 800 mm away from the wall, thus occupying a more central zone of the room; see Fig. 7. The height h of the EH was then the same as in Case A: 100 mm.

Figure 6. Case B: Exhaust hood and point of SA measurements and TG injection placed 500 mm above the table (free-standing exhaust hood).

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7. Case C: Equipment moved 800 mm away from the wall on the right. The exhaust air flow rates, Q, used in the tests and the corresponding average velocities in the exhaust pipe, Vav, are listed in Table 1. Vav = Q/A, where A = Area of exhaust hood opening. Exhaust air flow rate,

Q [ m3/h ] Exhaust air flow rate,

Q [ L/s ] Average exhaust velocity,

Vav [ m/s ]

50 14 3.1

100 28 6.3

150 42 9.4

190 53 11.9

250 69 15.7

Movable plate – turbulence generation In order to induce room air turbulence for challenging the contaminant capture efficiency, a vertical flat plate (Fig. 6-8) was set into motion in front of the EH table. The movable plate (MP) was 1900 mm high, 400 mm wide, 20 mm thick and placed upright on a carriage with its lower edge 200 mm above the floor level (same construction as specified for use in tests of fume cupboards, SS-EN 14175-3 2004). The plate thus had the approximate front-view dimensions of a grown-up human being. The carriage was connected to a timing belt, driven by a computer controlled stepping motor. When executing movements, the MP moved perpendicularly to the EH suction direction, and the speed of the plate as it passed in front of the EH was 1.0 m/s. This corresponds to a relaxed walking pace of a human being.

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Figure 8. Side-view of exhaust hood and movable plate arrangement. Place of air velocity measurements and tracer gas injection marked with an “x”. Measures in mm. The span of the MP movement was 2800 mm, centrally positioned between the two side walls. The acceleration and retardation of the MP was 2.0 m/s2, resulting in the sustained speed of 1.0 m/s prevailing over a 1800 mm distance, preceded by a 500 mm acceleration distance and ended by a 500 mm retardation distance. At each test with the MP in action, it was programmed to make repeated traversing moves like this at a steady movement interval, Δt. The intervals tested are listed in Table 2 below. A “movement” here means one passage across the room, being either to the right or to the left. Table 2. Movement intervals of the movable plate. Nominal movement interval, Δt [ s ]

Actual movement interval [ s ]

Movement frequency [ min-1 ]

4 4.4 14 (=Continuous movements) 10 10.4 6 30 30.6 2 60 60.8 1 120 121.2 0.5

The shortest movement interval tested, Δt=4 s, implied a nearly continuous movement, with the MP just resting a few 10th of a second at each endpoint of the path. Successively longer Δt were tested, in order to find an interval where the air turbulence had time to abate enough between the movements for them to be considered “single moves” in undisturbed room air. Initially it appeared that 60 s would enough for this, but later on it turned out that at least 120 s seems to be needed. Especially at the tracer gas measurements particular attention has been paid to the two most intense (Δt=4 and 10 s) and the two least intense (Δt=60 and 120 s) movement patterns.

1220 400

300 x

Movable plate

h

10

Air velocity measurements For measuring air velocities, a 3-D sonic anemometer (SA) of model TR92T/DA650 (Kaijo Sonic Inc.) was used; see Fig. 9. This anemometer sampled the air velocity in x-, y-, and z-direction at 20 Hz frequency. The distance between the sonic sensor-parts that constituted the measuring volume of the SA was about 30 mm, and in the direction of the expected main air flow the length of the measuring volume was 12 mm. The SA was always oriented such that the expected main air flow direction – towards the exhaust nozzle center – implied that no prongs were situated “upstream”, since they then partly would obstruct the measuring volume. Two different distances between EH and SA were tested: x=150 and x=225 mm, resp., corresponding to two and three diameters, respectively, of the exhaust nozzle (x=2D and x=3D). The SA was always placed at the same height above the table as the center of the exhaust nozzle.

Figure 9. 3-D Sonic Anemometer, SA, in front of the exhaust hood, EH. Since the SA produced a noise signal of random fluctuations within about ±0.02 m/s, which was considered influential on some of the data analysis, the raw air velocity data were after-filtered using the Matlab (MathWorks, Inc.) function “filter” set at low pass filter with a time constant of 0.2 s. The measuring uncertainty of the SA, after data filtration, was estimated at ±0.02 m/s or 5%, whichever is greater. The x, y and z directions of the air velocities measurements are defined in Fig. 10. The corresponding air velocities are denoted Vx, Vy and Vz, respectively.

Figure 10. Definition of directions and measuring angle relative to the exhaust hood. View from above. z-direction positive upwards.

x

y

Exhaust hood

α

11

Tracer gas measurements The contaminant capture efficiency of the exhaust hood was measured by means of tracer gas injection. Tracer gas (TG) was then injected through a sphere (Fig. 11) with a diameter of 40 mm (standard size table-tennis ball of celluloid, Stiga competition, Stiga sports AB, Eskilstuna, Sweden). The sphere was perforated with 14 evenly distributed circular holes of diameter 1.0 mm. The tracer gas used was N2O (laughing gas) mixed with Helium at a ratio He/N2O = 0.613 to achieve a gas mixture of neutral buoyancy. The gas injection sphere was always placed at the same height above the table as the center of the exhaust nozzle. The same two distances from the EH as used with the SA were used for the TG injection, i.e. x=150 and x=225 mm, resp., corresponding to two and three diameters, resp., of the exhaust nozzle (x=2D and x=3D). The total flow rate of the injected gas mixture was, with a few exceptions, 0.66 L/min. This flow rate was motivated by yielding capture efficiency measurements of good accuracy without consuming excessive amounts of gas, at the same time as it resulted in a nominal gas velocity (flow divided by cross-sectional area) of 1.0 m/s in the holes of the injection sphere. This gas velocity is the same as the speed of the movable plate, MP; thus, in origin, these velocities were of the same magnitude.

Figure 11. Tracer gas injection sphere in front of the exhaust hood (left) and in close-up view (right). The N2O and He gas flows were measured with rotameters (Rota, Wehr 2, L 2.5/100 and Fisher 2-A-150 3109. Fisher Controls Ltd. Crydon, England, respectively) which were newly calibrated against a bubble flow meter, yielding a flow uncertainty of about 3%. The rotameters and the gas monitor are shown in Fig. 12. The gas monitor (Innova 1412 Photoacoustic monitor, LumaSense Technologies A/S, Ballerup, Denmark) measured the N2O gas concentration in the middle of the test room through a 3 mm Teflon sampling tube, with a dust protection filter at the end (visible in Figures 5 & 7). Also the concentration in the control room was measured, for background-checking. The gas monitor was set up to measure also the water content and to cross-compensate the N2O readings for this. The sampling interval for each point thus became 70 s. The detection limit for N2O was 0.03 ppm, and the estimated uncertainty of the N2O measurements was about ±2%.

12

Figure 12. Gas monitor and rotameters. A powerful (140 W) double-jet mixing fan, visible in Fig. 5, was at times used for mixing the room air. Tracer gas measurements indicated full room air mixing within 1 min when running this fan.

Calculating the capture efficiency In deriving the calculation formula for the capture efficiency by means of the performed tracer gas measurements, we consider a certain time period, τ, when there is a steady exhaust air flow rate, Q, and a steady injection flow rate of N2O tracer gas, QTG, from the tracer gas injection sphere in front of the exhaust hood, EH. We further assume a certain level of air turbulence in the room, including the zone around the EH, that causes a fraction of the QTG flow rate, QTGE, to escape from the direct exhaust capture air flow and instead mix with the room air. The other fraction, QTGC is directly captured by the EH. Thus we have:

TGETGCTG QQQ += ( 1 ) The capture efficiency, CE, of the exhaust hood is defined by:

100Q

Q-Q100QQ CE

TG

TGETG

TG

TGC ⋅=⋅= [ % ] ( 2 )

If QTGE > 0 (i.e. CE

13

where Vol = room volume, t = time and Cs is the tracer gas concentration in the supply air to the room, if existing. Assuming Cs = 0 and integrating eqn. (3) yields for the concentration in the room after the time period τ:

( ) τττ ⋅⋅ ⋅+−= n-0n-TGEr eCe1QQC ( 4 )

where n is the air change rate in the room, n = Q/Vol, and C0 is the initial tracer gas concentration in the room, if existing. Eqn. (4) takes care of the fact that, during the time τ, some of the escaped tracer gas, QTGE, will be extracted through the EH, and that any initial concentration in the room, C0, will decay exponentially. Eqn. (4) yields that after a long time the concentration in the room will be Crτ = QTGE/Q, as would be expected. Rearrangement of Eqn. (4) gives:

( )τττ ⋅⋅ ⋅−⋅−=n-

0rn-TGE eCCe1QQ ( 5 )

Finally, with use of Eqn. (2), the capture efficiency can be attained from:

( ) ( ) 100eCCe1QQ1CE n-0rn-

TG

⋅

⋅−⋅

−⋅−= ⋅⋅

τττ [ % ] ( 6 )

In the performed tests on capture efficiency, room air turbulence was achieved through a steady movement pattern of the MP during a time period τ (between 5-20 min), after which the room air was mixed by the mixing fan and the tracer gas concentration in the room, Crτ, was measured. Then all terms in Eqn. (6) were known for calculating CE.

Results

Air velocity measurements

Ventilation induced background air turbulence level In order to check the background air turbulence level in the test room, caused by the supply air entering the room, the sonic anemometer, SA, was placed in the middle of the room, at the same height as the exhaust hood, EH, for test Case A & C, i.e. at 1.00 m above floor level. The EH was placed as in Case C, i.e. with the table and EH moved 800 mm towards the room centre. The exhaust flow rate, Q, (= ventilation rate of the room) was then gradually increased from 0 to 250 m3/h, and at the end the mixing fan was started, with its two jets passing at least 1 m away from either side of the SA. The result is shown in Fig. 13. It appears that there is a remaining noise in the velocity signals of around ±0.0015 m/s. Otherwise there seems to be virtually no effect of the ventilation on the air velocities; only at the highest flow rate, 250 m3/h, there is a tendency of a slight increase in Vy. The air velocities stay within about ±0.03 m/s, which is on the limit for measurable true air movements with the SA. It seems

14

unlikely that supply air movements should have caused higher air velocities in the vicinity of the EH zones, instead the table ought to have some damping effect. Thus the air supply installation appears to have worked well as regards low turbulence generation. The mixing fan, on the contrary, clearly induced substantial turbulence when switched on.

Figure 13. Air velocities in the middle of the test room at different ventilation rates.

Estimating the 0.4 m/s distance The zone within which the exhaust suction air velocity is 0.4 m/s or higher has by tradition been regarded a ”safe” zone, from which normally no contaminants escape. One method of characterising the performance of a local exhaust device is hence to identify and quantify the 0.4 m/s zone by air velocity measurements. This can however be quite time consuming to perform in an adequate way. The measurement of the air velocity in an apparent close-to 0.4 m/s point will – due to velocity fluctuations – need to be performed during a certain time period in order to establish a mean value of good enough accuracy. The resulting mean value is unlikely to be exactly 0.4 m/s, so repeated measurements may be needed until the 0.4 m/s position has been pin-pointed with acceptable accuracy. If instead a reliable correction algorithm can be applied to the first measurement, much time can be saved. This chapter presents measurements in this regard, and on the basis of the results such a correction algorithm is suggested. The air velocity around a hypothetical, imaginary point-sink in the free space, free of turbulence, is given by:

( 7 )

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0 2 4 6 8 10 12 14 16 18 20 22 24

Air v

eloc

ity [

m/s

]

Time [ min ]

Vx

Vy

Vz

Q=0 m3/h Q=50 m3/h Q=100 m3/h Q=150 m3/h Q=200 m3/h Q=250 m3/h Q=0 m3/h

Mixing fan on

V = Q

4∙π∙r2

15

where the denominator constitutes the area of a sphere of radius r around the point-sink. That is, the velocity decays with the distance as ~r-2. In the vicinity of a flat, flanged hood the air flows towards the exhaust nozzle more like within a half-sphere, where the air velocity can be expected to be twice as high as within a whole sphere, at a given flow rate Q and a given distance r. Further, in case the flanged hood rests on a table, the flow region gets closer to a quarter-sphere, within which another doubling of the air velocity can be expected. In the case of half- and quarter-spheres, the velocity will however still decay with the distance as ~r-2. A relationship of that kind could hence be expected for the tested EH. Figures 14 & 15 show examples of results of air velocity measurements at different distances from the EH and at different angles, α, in the horizontal plane, as specified in Fig. 10 (complementary diagrams can be found in the Appendix). No room air turbulence was induced at these measurements. Data for the 4-5 measuring points that were closest to the 0.4 m/s region were selected and for each measuring position was fitted a power-function curve, i.e. of the form:

( 8 ) The exponent p is of particular interest here. Table 3 includes all the attained values of p, where also values of test Case B (free-standing EH) are shown. The table shows that the value of p varies somewhat between the cases, but is of the order of -2, as could be expected according to the previous discussion. The variation in velocity profile between different measuring angles, α, also appears small. The mean value of p for test Case A is -1.90 and for test Case B -2.10, and these two yield a mean value of -2.00.

Figure 14. Air velocity, V, at different distances, r, and angles, α, at the EH. Case A, Q=100 m3/h.

y = 61.213x-2.171

y = 35.477x-1.928

y = 23.721x-1.808y = 19.797x-1.702y = 19.476x-1.678

y = 25.049x-1.757

y = 36.978x-1.88

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25

Air v

eloc

ity [

m/s

]

Distance [ cm ]

30

45

60

90

120

135

150

Angle [ degrees ]

V = k∙xp

16

Figure 15. Air velocity, V, at different distances, r, and angles at the EH. Case A, Q=190 m3/h. Table 3. Power-function exponent, p, for different test cases.

Case A Case A Case A Case B Case B Case B Case B α [ ° ] Q = 100 Q = 150 Q = 190 Q = 100 Q = 150 Q = 190 Q = 250

30 -2.17 -2.12 -2.15 45 -1.93 -2.00 -2.03 -2.14 60 -1.81 -1.79 -1.94 90 -1.70 -1.85 -1.91 -2.17 -2.22 -2.00 -2.11

120 -1.68 -1.94 -1.81 135 -1.76 -1.75 -1.89 -2.03 150 -1.88 -1.93 -1.86

Thus the measurements confirm that the velocity tends to decay with the distance as

( 9 ) at least in the 0.4 m/s zone (at distances close to the EH other functions apply). This can be utilized at estimations of the 0.4 m/s distance, x0.4. At exactly that distance we have:

( 10 )

y = 140.25x-2.154

y = 89.935x-2.025

y = 68.248x-1.935

y = 66.341x-1.912

y = 50.466x-1.81

y = 64.316x-1.892

y = 66.062x-1.861

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25

Air v

eloc

ity [

m/s

]

Distance [ cm ]

30456090120135150

Angle [ degrees ]

V = k∙x-2

0.4 = k∙x0.4-2

17

The constant k will differ between test cases and depend especially on the flow rate, Q, but at a certain test case it will be practically equal at two nearby measuring points. Suppose we have performed an air velocity measurement somewhere near the 0.4 m/s distance, yielding a measured air velocity Vm at a measured distance xm. For that measuring point then applies:

( 11 ) Dividing Eqn. (10) with Eqn. (11) and rearranging the terms gives the following relatively simple correction equation for estimating the 0.4 m/s distance x0.4 from nearby measurements of Vm and xm:

( 12 ) It is here assumed that the power function exponent is p= -2 while Table 3 shows p-values varying between -1.68 and -2.22. It seems however that in practice the uncertainty in x0.4 that this variation causes is marginal. Fig. 16 shows an example of what the estimated value of x0.4 would be for p= -2.0, but also for the “extreme” values -1.6 and -2.2, when using Eqn. (12) with insertion of the measurement values Vm = 0.50 m/s at xm = 13.4 m. Apparently the attained estimated value of x0.4 differs fairly little for the different p-values. This is analyzed in more detail in Fig. 17, which shows how the error in x0.4 – estimated with Eqn. (12) – depends on different measured air velocities, Vm, at different assumed “true” p-values for the air flow at the measurement point. The figure indicates that this error stays within about ±3% for measured air velocities within the zone 0.30-0.50 m/s, and within about ±2% for measured air velocities within the zone 0.35-0.45 m/s. In practice the uncertainty in most anemometers is greater than this uncertainty that is caused by an uncertain p-value, so the correction Eqn. (12) appears to be satisfactory.

Figure 16. Example of estimated values of x0.4 from a measurement of 0.50 m/s. Q≈200 m3/h.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14 16 18

Air v

eloc

ity [

m/s

]

Distance, x [ cm ]

p= -1.6 p= -2.0 p= -2.3

Original measurement

Estimations of x0.4 fordifferent values of theexponent p:

Vm = k∙xm-2

x0.4= xm∙�Vm0.4

[ m ]

18

Figure 17. Error in estimated x0.4 at different measured air velocities and different hypothetically “true” exponent p-values. If a more certain estimation of x0.4 is deemed needed, a complementary measurement “on the other side” of the 0.4 m/s distance is recommended; i.e. if the first measurement gives a measurement value of Vm that is somewhat below 0.4 m/s, the second should be done where it is slightly above 0.4 m/s. Then an average of the two attained values of x0.4 will be an improved estimate of the true value.

Air turbulence generated by the moving plate, MP Fig. 18 shows a sequence of smoke-visualization photos of the air movements in front of the EH as the MP makes a single move across the room. The smoke was here produced by a smoke stick (Smoke pen, Björnax AB, Sweden), placed at 300 mm from the front table edge, with the EH at x=3D (225 mm) distance and the stick tip 50 mm above the table surface. The reason for placing it in a lower position than that of the SA measurements and the tracer gas injections (being 100 mm) was the buoyancy of the smoke, giving it an upward momentum. The first, top left picture visualizes the quite low air turbulence level when there were no movements in the room; the smoke streak indicates a more or less straight, laminar flow, directly towards the EH. The following sequence of pictures then shows how the MP, as it makes a move to the rear wall, generates an anti-clockwise, horizontal vortex around the smoke stick. The vortex is then dissolved and most of its smoke is captured by the EH within a few seconds, but a little smoke appears to escape from this direct extraction and diffuse to outside of the suction zone.

-10

-8

-6

-4

-2

0

2

4

6

8

10

-2.5-2.0-1.5-1.0-0.50.0

Erro

r in

estim

ated

x0.

4[ %

]

Value of "true" exponent p

0.30 m/s

0.35 m/ s

0.45 m/s

0.50 m/s

Measured air velocity

19

1.35 s

0.00 s 1.79 s

0.30 s 2.94 s

0.64 s 6.98 s Figure 18. Smoke visualization of air movements in front of the exhaust hood as the MP makes a move towards the rear wall. Case C, Q=100 m3/h, x=3D. The MP has reached its end position at about the time 0.40 s. When quantifying the effect of MP disturbances through SA and TG measurements, the MP was set into regular motion at the different time intervals, ∆t, previously listed in Table 2, with durations according to Table 4 below.

20

Table 4. Movement intervals and time of action for the movable plate, MP. Nominal movement interval, Δt [ s ]

Time of action [ min ]

4 5 10 5 30 5 (10 moves) 60 10 (10 moves) 120 20 (10 moves)

The position of the SA was then as shown in Fig. 8. In Fig. 19 is presented how the air velocity component Vx (directed towards the EH nozzle) varies in the case of the longest movement intervals, Δt=120 s, when there is no EH air flow: Q=0. The MP apparently causes both a positive and a negative velocity spike at each passage of the exhaust hood, followed by much smaller, but fairly long lasting fluctuations. Some variation in the signal from one move to the other can be seen, but on the whole the velocity pattern remains similar during the whole 20-min movement period. Fig. 20 includes also Vy and Vz in a close-up of the first 10 min. The figure shows that peaks even larger than those of Vx occur in Vy, i.e in the direction of the MP movements, with the peaks alternating in direction due to the MP switching between right and left movements. In fact, the Vy peaks occur in opposite direction to the MP movements, as also was indicated in the visualizations photos in Fig. 18. In Fig. 21 more details are shown of the velocity pattern occurring at a single move. Studying Vx, it seems that each passage of the MP first causes a “push” of air towards the EH, followed by a suction phase where the air moves away from the EH. The suction phase appeared clearly also in the pictures in Fig. 18, and that phenomena ought to be particularly influential on the capture efficiency of the EH, since it counteracts the intended extract air flow. It is also interesting to note that amplified turbulence episodes often seem to occur in between the passage spikes.

21

Figure 19. Air velocity Vx at MP movements at Δt=120 s, starting at t=2 min. Last movement at t=22 min. Case A, Q=0 m3/h, x = 2D.

Figure 20. Air velocities at MP movements at Δt=120 s starting at t=2 min. Case A, Q=0 m3/h, x = 2D.

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22

Figure 21. Air velocities at MP movements at Δt=120 s. Case A, Q=0 m3/h, x = 2D. Fig. 22 is similar to Fig. 19 but now with double as frequent moves, Δt=60 s, and with an exhaust flow rate of Q=100 m3/h. Similar peaks as in Fig. 19 can be seen, but the exhaust flow clearly “lifts” the curve and makes the air flow direction positive, towards the EH, almost all the time – a few negative Vx values indicate occasional reversed flow direction. The figure also indicates more turbulence remaining at the start of an MP move than at Δt=120 s. A close up of the signal, including also Vy and Vz is shown in Fig. 23, illustrating clearly the alternating sequence of the Vy peaks, while the Vx peak pattern is the same for both left and right MP moves: a “push” phase followed by a suction phase. “Bursts” of turbulence that tend to occur in between the MP passage peaks were noted at Δt=120 s and can be seen also here at Δt=60 s.

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ity

[ m/s

]

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Vy

Vz

23

Figure 22. Air velocity Vx at MP movements at Δt=60 s starting at t=1 min. Last movement at t=10 min. Case A, Q=100 m3/h, x = 2D.

Figure 23. 2-min selection of Figure 22. Air velocities at MP movements at Δt=60 s, passing sensor slightly before whole minute. Case A, Q=100 m3/h, x = 2D.

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Air v

eloc

ities

[ m

/s ]

Time [ min ]

VxVyVz

24

Figures 24-26 show the air velocities at the most intense MP moves, Δt=4 s, essentially implying continuous back-and-forth movements. Apparently the air flow conditions are now more dramatic, but still the passages of the MP can be discerned both in the Vx and Vy signal. Occasionally rather augmented Vx peaks occur. This phenomena was observed also in other tests at Δt=4 s; sudden peaks of up to 1 m/s occurred, seemingly at random. Fig. 24 also shows several occurrences of negative Vx peaks.

Figure 24. Air velocity Vx at MP movements at Δt=4 s, starting at t=1 min. Last movement at t=6 min. Case A, Q=100 m3/h, x = 2D.

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ity ,

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25

Figure 25. Air velocities at MP movements at Δt=4 s. Case A, Q=100 m3/h, x = 2D.

Figure 26. Air velocities at MP movements at Δt=4 s. Case A, Q=100 m3/h, x = 2D.

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26

As shown in the pictures above, the movement of the MP may cause occasional negative Vx values, implying air movement in opposite direction to the intended suction flow. Since these occurrences appear to be particularly risky as regards failure of the EH to capture contaminants, the percentage of negative Vx values was calculated for the performed measurements and used as a potential measure of contamination risk. The measure is denoted PNV, Percentage of Negative Velocities, and represents the fraction of the time when MP is in action that Vx is negative. The time of MP action is then the whole time period specified in Table 4, thus including also the instances when the MP is at rest between the regular moves. Figure 27 summarizes the results of recorded PNV values for the different test cases, movement intervals and exhaust flow rates. Not all combinations of these were however tested.

Figure 27. Percentage of Negative Vx Velocities, PNV, at the different test cases, movement intervals, Δt, and exhaust flow rates, Q. (Note: Different scales on the PNV-axes for the x=2D cases (left diagrams) and the x=3D cases (right diagrams)). Three general trends appear clearly in Fig. 27: The PNV value gets higher by (1) a lower exhaust flow rate, Q, (2) a shorter movement interval, Δt, and (3) a longer the distance to the EH (2D vs. 3D). These relationships are thus as one might expect. It is also evident that PNV gets significantly higher when the EH is free-standing, away from the table: Case B results in

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[ %

]

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[ %

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Case C, x=3D

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27

considerably higher PNV than Case A. Moving the EH – with the table – away from the wall does however not seem to result in a clear change as regards PNV; Case C doesn’t differ significantly from Case A. PNV correlates well with the tracer gas capture efficiency, as will be presented in the later chapter on Tracer gas measurements.

Air turbulence generated by a walking person Also air turbulence generated by a real walking person was studied in order to see how it compares with that generated by the MP. The person was then walking back and forth at the same path as the MP for the Case A, Q=100 m3/h, x=2D. Both the cases with Δt=60 s and Δt=4 s were tested. By programming the MP carriage to run without the plate next to the walking person, he could imitate the movement pattern of the MP well. The person was 1.78 m high, weighted 72 kg, and had a maximum width of about 0.45 m at the shoulders and approximately 0.55 cm at the forearms as they were swinging slightly during the walks, which could be performed in a relaxed manner. Figure 28 shows how Vx varies as the person walks at Δt=60 s. Except for a strange initial peak the fluctuations are not very strong – considerably smaller than those caused by the MP, shown in Fig. 22. Especially the peaks when passing the SA are much smaller with the person. The 2-min selections in Figures 23 and Figure 29 show that also the fluctuations in Vy and Vz are substantially smaller with the person than with the MP.

Figure 28. Air velocity Vx at person walking at Δt=60 s, starting at t=1 min. Last movement at t=10 min. Case A, Q=100 m3/h, x = 2D.

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28

Figure 29. 2-min selection of Figure 28. Air velocities at person walking at Δt=60 s, passing SA sensor slightly after whole minutes. Case A, Q=100 m3/h, x = 2D. Figures 30 and 31 show air velocities occurring with a continuously moving person at Δt=4 s. Like the MP (Figures 24-26) the walking person now generates quite a turbulent air flow field. The magnitude of the velocity fluctuations is however again considerably smaller than at MP movements. The peaks when he person is passing the SA are blurred in Fig. 31 but can be identified, including the tendency of the Vy to change direction at every walk. Thus it is likely that also the human movements tend to create some kind of large scale, horizontal vortices, similar to those visualized in Fig. 18, although not that distinct.

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6.7 6.8 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7

Air v

eloc

ities

[ m/s

]

Time [ min ]

Vx

Vy

Vz

29

Figure 30. Air velocities at person walking at Δt=4 s, starting at t=1 min, stopping at t=6 min. Case A, Q=100 m3/h, x=2D.

Figure 31. ½-min selection of Figure 30. Air velocities at person walking at Δt=4 s. Case A, Q=100 m3/h, x = 2D.

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30

Tracer gas measurements Tracer gas (TG) measurements were performed in order to quantify the capture efficiency (CE) of the exhaust hood (EH), as regards capturing a contaminant of neutral buoyancy emitted in front of the EH, such that the contaminant doesn’t escape to other parts of the room. Besides tests with no intentional turbulence generation in the room, tests were performed with the movable plate (MP) in action in a similar manner as at the tests with the sonic anemometer (SA). The MP was then set into regular motion at the different time intervals, ∆t, previously listed in Table 4 (except the case Δt=10 s). When testing without any MP movements or other intentional turbulence generation in the room, no escape of tracer gas could be detected, i.e. CE = 100%. Because of this, TG injection through the injection sphere could be started a bit before the start of the MP movements. TG was then injected throughout the time of MP action. In the two cases of long movement intervals, Δt = 60 and 120 s resp., the TG injection also continued after the last MP move another half time interval, i.e. Δt/2 s longer. This was done in order to reduce the risk of the very last movement being too decisive for the total amount of escaped TG. Hence the total time of TG injection, τ, which was inserted into Eqn. (6) for calculating the CE, was as listed in Table 5. For the cases Δt = 60 and 120 s, the time of MP action counts from the start of the first move until the end of the last move, thus being e.g. 9 min for 10 moves at Δt=60 s. Table 5. Time of tracer gas injection at different movement intervals of the MP. Nominal movement interval, Δt [ s ]

Time of MP action [ min ]

Time of TG injection, τ [ min ]

4 5 5

10 5 5

60 9 (10 moves) 9.5

120 18 (10 moves) 19

The stopping of TG injection was done by dragging the injection sphere into the EH nozzle by use of strings (visible in Fig. 11.) that could be pulled in the control room. Since there continuously was some air suction in the EH, no TG could escape to the room when the injection sphere was inside the nozzle. (The alternative to just cut the TG flow might result in some TG remaining in the sphere continuing to diffuse out of it for a while.) Directly after having stopped the TG injection the room air was mixed by switching on the mixing fan, and at the same time the exhaust flow Q was reduced to about 40 m3/h. Thus a homogeneous concentration of any existing TG in the room was attained, and this was used as Crτ in Eqn. (6). Fig. 32 shows an example of a tracer gas test, where the four different movement intervals (Δt) of the MP were tested in a sequence. It appears that during the first test, with Δt= 120 s, there is a slight – but rather clear – TG concentration increase of about 0.3 ppm. Calculations according Eqn. (6) yields CE = 99.7%. This indicates that the test method is quite sensitive in detecting that CE is below 100%. Then, clearly, during the two last, intense movement periods, considerably more TG is escaping into the room, resulting in CE=89.1% and 86.7%, respectively. As mentioned, Eqn. (6) corrects for the existence of any initial TG concentration in the room at the start of a test. Since there was always some exhaust air flow, the TG

31

concentration decreased gradually during the time of room air mixing and the few minutes preceding the next movement test, as is can be seen in Fig. 32.

Figure 32. Example of tracer gas test, where the four different movement intervals (Δt) of the MP are tested in a sequence. The results of the tracer gas tests yielding CE values are summarized in Fig. 33 a-d. Included in the diagrams are also the air velocities, V, previously measured at the TG injection point with the SA, when no turbulence (MP movements) was induced. The figures show that in general only little TG escapes into the room (high CE) at the “single move” conditions, Δt = 60 and 120 s, whereas considerable TG escape (low CE) frequently occur at intense movements, Δt= 4 and 10 s. With the TG injection point farther from the EH, x=3D instead of 2D, the CE is reduced dramatically. Also having the TG injection point and the EH away from the table (Case C, h=500 mm) results in substantially reduced CE. However, Figures 33 c & d indicates that moving the table 800 mm away from the wall, to a more central position in the room, doesn’t result in an evident change in CE; some CE values are lower in Fig. 33 c and some are lower in Fig. 33 d, with no clear tendency of a difference trend.

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er g

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[ pp

m ]

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32

Figure 33 (a). Capture efficiency, CE, attained at tracer gas test for Case B, x=2D. The velocity, V, is the air velocity at the TG injection point when no turbulence (MP) was induced.

Figure 33 (b). Capture efficiency, CE, attained at tracer gas test for Case B, x=3D.

99.4 99.7

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33

Figure 33 (c). Capture efficiency, CE, attained at tracer gas test for Case A, x=3D.

Figure 33 (d). Capture efficiency, CE, attained at tracer gas test for Case C, x=3D.

In the tests above, the total gas injection flow rate, QG , was 0.66 L/min. This injection flow rate has been argued for above. However, also half this flow rate, 0.33 L/min was tested, as well as 1.00 L/min. The results, which are shown in Fig. 34, indicate a tendency for the CE to be lower the higher the gas injection flow rate. The tendency is however quite vague, suggesting that the tracer gas injection rate is not very crucial in CE tests, at least with the present set-up.

98.5 98.5

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34

Figure 34. Repeated tests with total gas injection flow rate 0.66 L/min (filled bars), one test with 0.33 L/min (striped bar), and three repeated tests with 1.00 L/min (checked bars). The general indications above of local exhaust capability at different test cases have been rather similar for both of the performance measures CE (Capture Efficiency) and PNV (Percentage Negative Velocities). It is hence reasonable to see how these measures compare to each other. Fig. 35 shows the combined data of PNV and CE for the cases where both have been measured. With the exception of one outlier (Case B, x=3D, 50 m3/h) the figure shows a remarkably strong correlation between PNV and CE; the relationship appears more or less linear, with CE roughly decreasing 5%-units for every 10%-units increase in PNV.

Figure 35. Relationship between PNV (Percentage Negative Velocities) and CE (Capture Efficiency) at different test cases, x-distances and exhaust flow rates, Q (m3/h).

88.7 89.490.0 89.2 90.0

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35

Discussion The present study has shown that both PNV and CE, attained through the tested procedures, seem to be relevant performance measures of an exhaust hood. The fact that the depicted PNV- and CE-curves show monotonous trends in the expected directions regarding the influential factors strengthens the credibility in the attained measurement data. The observed good correlation between PNV and CE suggests that measuring PNV might be a convenient alternative to tracer gas measurements for indicating CE. A direction sensitive anemometer is however needed to measure PNV, and complementary tests at other set-ups also seem advisable to ascertain the correlation between PNV and CE before considering PNV a reliable substitute. In the present study, care was taken to minimize the background level of air turbulence in the test room, such that only the disturbance by the controlled MP movements caused turbulence of importance for the exhaust hood performance. With only the very low background turbulence level present, the capture efficiency of the hood was recorded to be 100% at al test cases. This seems a reasonable result, since in principal there are then no room air currents bringing the injected tracer gas in any other direction but towards the EH. At real installations, however, the background turbulence level is usually higher, e.g. due to air jets from ventilation supply air diffusers. A possible extension of the present study could hence be to have a higher level of background turbulence, onto which additional disturbances like MP movements would be superimposed. But the level and characteristics of that background turbulence will then be very crucial for the results, and must hence be possible to be generated in a controlled and repeatable manner, and also possible to be described in a well-defined way. Such controlled turbulence can be a challenging task to achieve, and a suitable target level seems nontrivial to establish. Background turbulence is discussed more in the next paragraph. A possible alternative to the repeated MP movements could be to induce just one single, short disturbance, like one move with the MP, then switch of both tracer gas injection and exhaust flow rate, mix the room air and measure the concentration in the room. A crucial question then is however when to consider the disturbance accomplished. As has been shown in this study, small scale turbulence is lingering on quite a while after the first large vortices that are produced by an MP move. This turbulence constitutes part of the disturbance and will delay the extraction of some of the tracer gas. So a definite cut-off time for the disturbance isn’t trivial to settle. Again the background turbulence level seems important. With no background turbulence but only air movements caused by a single MP move, these air movements may be confined to a limited zone around the EH, not spreading escaping tracer gas much to other parts of the room. With some background turbulence, escaping tracer gas will instead be spread more homogeneously, which would simplify the tracer gas measurements for quantifying CE and make them more adequate. In the present study, especially the frequent MP moves (Δt=4 & 10 s) seems to have caused turbulence enough to yield good air mixing in the whole test room (curves of TG concentration increase like in Fig. 32 has verified this assumption). That kind of disturbance thus also generates “background” turbulence, which appears desirable for CE measurement of the performed kind. As mentioned previously, achieving well-defined background turbulence in other ways seems a difficult task. In conclusion: In CE tests, a disturbance like the tested frequent MP movements appear to be a suitable kind of disturbance in that it both generates strong, challenging turbulence at the local exhaust, at the same time as it tends to mix the room air into a more or less homogeneous concentration of any escaping tracer gas.

36

In comparison with real human walking, the turbulence pattern generated by the MP proved however substantially different at the performed tests. A bit surprisingly, considerably less turbulence was produced by the walking person, although being a bit wider than the MP and (gently) swinging the arms, which ought to enhance turbulence. The human body shape, being a bit more “streamlined”, might be a reason for the lower turbulence generation. Hence, if it considered essential in the present kind of tests to imitate real human movements, another profile of the moving object seems to be needed. Further, especially during the most intense MP movements, occasional quite high air velocity peaks occurred, typically at random, long intervals. A more consistent turbulence pattern would be desirable, which might be achieved with another shape and/or movement pattern of the moving object. Extended tests in this regard might thus be worthwhile. A correction formula was developed for use at velocity measurements intending to establish the 0.4 m/s zone in front of the EH. The measurement data in Figures 33 (a-d) suggests that 0.4 m/s indeed would yield 100% capture efficiency also at the most vigorous MP movements; the 0.4 m/s-threshold even appears to be too much on the safe side. However, in practise a human being, working by the table in front of the EH, will obstruct the flow of room air towards the EH, possibly causing a wake were contaminants may reach the breathing zone. Then 0.4 m/s might be needed at the contaminant position. The situation with a human-like obstacle appears worth considering in extended studies on EH performance measurements, e.g. including air quality measurements in the breathing zone of a thermal manikin. Such measurements could also be used to validate the appropriateness of the test methods tried in the present study. Other factors that seem relevant to include in extended studies are buoyant contaminant release, as well as other MP movement sets-up, especially movements perpendicular to those performed here, i.e. along the sides of the table, parallel to the EH suction direction. It was shown in connection with Fig. 32 that several tracer gas tests could be performed in a sequence, without evacuation of all the tracer gas in between. Highest accuracy is however achieved when starting from zero concentration in the test room, since the baseline then is more certain. Further, in the present study N2O was used as tracer gas. A common alternative tracer gas in ventilation studies is SF6, which however is a very heavy gas that requires careful mixing to attain a neutrally buoyant gas mixture to inject.

Acknowledgements The moving plate and its control system were constructed by Rickard Larsson and Hans Lundström, resp., at the University of Gävle. Much of the presented air velocity data were collected by the University students Ahmed Abdillahi and Chris Clement Igiraneza.

37

Appendix

Air velocities at different distances Figures complementing Figures 14 and 15.

Air velocity, V, at different distances, r, and angles at the EH. Case A, Q=150 m3/h. (Outlier at α=30º, however also giving a p-value close to -2.)

Air velocity, V, at different distances, r, and angles at the EH. Case B at four different flow rates: Q=100, 150, 190 and 250 m3/h.

y = 106.82x-2.122

y = 64.737x-2.002

y = 37.895x-1.79

y = 43.662x-1.853

y = 59.134x-1.94

y = 35.582x-1.753y = 53.56x-1.926

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y = 74.995x-2.004

y = 90.203x-2.201

y = 79.132x-2.052

y = 62.287x-1.991

y = 58.064x-2.153

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Q=150, 90 degrees

Q=150, 135 degrees

Q=100, 90 degrees

Flow rate & Angle

SummaryNomenclatureBackgroundMethodThe test roomExhaust deviceMovable plate – turbulence generationAir velocity measurementsTracer gas measurementsCalculating the capture efficiency

ResultsAir velocity measurementsVentilation induced background air turbulence levelEstimating the 0.4 m/s distanceAir turbulence generated by the moving plate, MPAir turbulence generated by a walking person

Tracer gas measurements

DiscussionAcknowledgementsAppendixAir velocities at different distances