edited by MALCOLMM. RENFREW University of idaho Moscow. Idaho 83843
W. G. Mikell and L. R. Hobbs Experimental
Station, Central R e s e a r c h & D e v e l o p m e n t Department. E. I. du Pont de Nemours. Wilmington. Delaware 1 9 8 9 8
Introduction Laboratory fume hoods are an important safetv device. Thev . ~ r o v i d ethe chemist pnm~liun i r o m rxpmure l o cnemrcal fumes and particlrq whmh m.ght hr putmrially in. j u r ~ u u i11, h m l r h . The, a r t rompl,~rin their design and operation and have long been designed t o meet ''face velocity" standards. These standards have been arrived s t over the years b y largely empirical studies and, in general, lack a t r u l y scientific foundation based o n experimental data. T h i s does n o t i m p l y they have n o t been effective in protecting chemists, for the evidence indicates t h a t they have. However, in the current environment of occupational health concerns, it is time for a deeper assessment o f the prateetion they provide. T h i s study represents a beginning w i t h s t i l l much t o be done. Technology t o analyze and quantify key factors in hood performance has been reported in recent literature (1-3).Caplan and Knutson, Fuller and Etchells, Chamberlin and Leahy have dl contributed to techniques permitting a more quantitative measure of performance. These were applied in this work and further test develooment was n o t undertaken.
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Wllllam G. Mlkell is the Environmental Control Manager at the Du Pont Company Experimental Station in Wilmington. Delawars. This facility houses a large number of laboratories engaged in a wide variety of research and development activities for the Industrial Departments of Du Pont, as well as a Cenbai Corporate Research Facility. Dr. Mikeii was with the Textile Fibers Department prim to his present post. He was active With the National Research Council Committee on Hazardous Substances in the Laboratory engaged in developing their report on "Prudent Practices for Handling Hazardous Chemicals in Laboratories." Leonard R. Hobbe is a Supervisor in Projen Engineering at the Du Pant Company Experimental Station in Wilmington. Deiaware. The Project Engineering Group's responsibilities include design and construo tion of research facilities and technical a s slstance on engineering activiks. Mr. Hobbs was previously with the Textile Fibers Department. He is a registered professional engineer in the state of Delaware.
T h e objective of this study was t o assess and characterize anumber of the bench-type chemical laboratory hoods typical of those at t h e Du Pont Comoanv's , Exoerimental Statiw drrcrbninr the I c v ~ l ac a i pr..tcciiw p r w i d ~ dfur poczmnel w n r k ~ n g 11 t h r e h w d * . KP) vwibhles >uth a4 I R r~a (t q UII supply systems, face velocity, work practices, personnel movement, and the effect of equipment location i n the hood were explored.
I.,
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Laboratory Fume Hood Systems L i k e most large laboratory locations, there
are several kinds of hoods a t the Experimental Station. T h e two basic sizes of hoods (10-ft w i d t h and 5-6 ft width) were designed and b u i l t using engineering standards based o n smoke tests conducted in 1949. These tests fnrr v r i < r i t y of 61. r w t per h w e d that nninute ifpmr p n r w i 4 ndequatr. c.minlnmrnt uhrn used w t h 3 pn,pc-rl> uvbirned nlr distribution system. In a typical two-rnan laboratory, thereare two 10-ft hoods-one o n either side of the laboratory and located just inside the door.
Malcolm M. Rentrew draws on v m e d industrial and academrc experiences in his approach to good safety practices. After graduate study at the University of Minnesota, he was a superwsor of research and development with DuPont and General Mills; then an administrator and teacher at the University of idaho, his Alma Mater. He is active in the American Chemical Society. including service with the Committee on Safety and the new division of Chemical Health and Safety. He now is professor emeritus of chemistry and is patent director of his University's idaho Research Foundation. Inc.
LABORATORY AIR SUPPLY
'
PRIMARY
AIR
Fuller, F. H., and Etchells, A. W., "The Rating of Laboratory Hood Performance," ASHRAE Journal, October 1979. Caplan. K. J. and Knutson. G. W., "Development of Criteria for Design, Selection, end in-place Testing of Laboratory Fume Hoods and Laboratory Room Ventilation Air Supply: Final Report," ASHRAE RP-70, March 197R ~. .. Chamberlin, R. I. and Leahy, J. E., "Laboratory Fume Hood Standards," Recommended for the U. S. Environmental Protection Agency, Contract No. 68-01-4661. January
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1978.
(Continued on page A166)
Figure 1. Laboratory air supply. Volume 5 8
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hoods were conducted using a modified ASHRAE method and test geometry essentially as reported in the October 1979 ASHRAE Journal by Fuller (2).A mannequin was used to simulate an actual person a t the hood
TEST GEOMETRY sash varieties exist, recent practice has been t o install the horizontal sliding sash variety whose effective face opening is limited to one-half the total face area. Lip ducts or airfoils are used to control air flow across the bench top. The 5-6 ft hood is installed, generally, one to a lab and also has a minimum average face velocity of 60 fpm. For the two-man labs, fully conditioned air, called primary air, is supplied t o the room from a location remote from the hood and is shown in Figure 1. This source heats and cools the lab and provides 30% of the air exhausted by the hoods when operating. Secondary air, or make-up air, is supplied from a ~ l e n u mwith a perforated metal plate in the ceiling over and in front of each h w d (Fig. 1). This air is not humidity controlled hut is conditioned t o room temperature and provides 70% (2500 efm for the [2] 10-ftvertical sash h?ods) of the total volume exhausted by the hoods. With the 5-6-ft hood design, allof the air utilized by the hood is fully conditioned and supplied from a location remote from the hood and is the same air used t o supply the laboratory room.
Performance Tests Characterizations of bench-type chemical
--
Figure 2. Test geometry.
as shown in Figure 2.Freonm 12 was released (rate 1llmin) in test hoods by means of a gas diffuser a t a 12,000 ppm concentration. Emission calibration was performed by taking samples with syringes and using the Miran" 1A infrared unit for analysis. Outside the hood, concentrations were sampled by a gas detector probe and results were recorded utilizing a Miranm 1A infrared analyzer and recorder. The instrument was calibrated a t freouent intervals for the concentration ranee h i l y d ~ w r t e dThe . entire nssemhly was a;,ached to a mounting srnnd to mamain a twtd rclationrhip fm all hmde tested. Rep. lication tests were run to check test procedures and equipment. A hood index as defined by Fuller (2) was used ta compare performance. I t is defined as the negative loglo of the average concentration (ppm) outside the hood divided by the average concentration (ppm) inside. Thus, an index difference of 1in comparing two lshoratory hoods means there is a 10-fold difference in the relative coneentration detected outside the hood with a constant emission inside. Finally, face velocity was determined by measurement with an Alno? thermal anenometer and averaged over 10 points for vertical sash hoods. The anenometer was maintained within instrument limits by frequent calihration. The test program was conducted using the procedure described above. First, the performance indices were determined for a number of hoods. This was followed by investigation of hood variables such as air supply systems and location of equipment. Finally, the effect of work practices while using a hood were explored.
Hood Characterization Twenty-eight hoods in several different buildings were selected randomly for inves~ tigation. The results of the investigations, which consisted of a series of three different tests for each hood, are shown in Figure 3. For HOOD CHALLENGES
Figure 3. Results of hood challenges,
the original test, which included the mannequin and involved po disruptive influences to the test hoods, 82% of the hoods checked had a index of greater than 6.1. The indices were greater than 6.1 because no Freon" was detected a t the limit of instrument sensitivity of 0.01 ppm. The remaining 18%of the hoods ranged from an index of 5.8 down to 4.1. The poorest performing hood had an outside concentration of 0.9 ppm. For the range of face velocities tested (48-127 fpm), no correlation with performance index was observed under the test conditions used. The next two tests involved challenges or disruptive influences to the hood. In the first challenge, the test setup was the same as for the original test except that a person walked past the hood at one-minute intervals during the test. In the second challenge, the mannequin was removed and a person again walked past the unattended haod a t oneminute intervals. The results in Figure 3 show that pedestrian traffic was confirmed as a detrimental influence to hoad performance as indicated by the number of hoods with indices lower than 6.1 (39%) as well as the increase in maximum outside concentration (3.6 ppm) detected for a single hood. Removing the mannequin eliminated eddy currents caused by the mannequin in the vicinity of the detector probe and returned the performance to a level similar to that of the original test. In this case, 82% of the hoods had indices greater than 6.1 and the maximum outside concentration noted was 0.2 ppm. For all of the hoods in this test, performance was judged to be acceptable for the standard conditions imposed by the test procedure. Direct correlation with hood operating conditions and configurations to actual performance could not be made. There isnoquestion, aslater data will show, that an important contributor was the location of the diffuser 6 in. in from the plane of the sash.
Air Supply Investigated next were the effects of different methods of supplying air to the hood. The first test involved a comparison of hood performance with air supplied both from the secondary supply in the ceiling over the hood as well as from the primary supply a t the rear of the room versus that from the primary (remote) supply alone (Fig. 1). The total volume of air supplied in both cases was the same. In order to compare performance levels, the distance from the detector prohe to
the source of emissions was reduced to ensure that some concentration was picked up for all tests. The average index (5 measurements) of hoods having the secondary supply over the hood was 4.5 versus 3.1 for those hoods with only aremote air supply. This 1.4 difference in indices represents a 2JX difference in performance. These results illustrate that the method of supplying air to the hoodcan be an important factor in achieving better hood performance. On the other hand, this does not imply that secondary air supply is required to ohtain satisfactory hood performance. Earlier data showed hoods employing only a remote air supply had satisfactory performance indices.
Effects of Emission Location The following series of tests represent an attempt to define the effectiveness of the hood with the emitting source in different positions in the hood. These testsexplore the relative effects of placing the source and probe at different locations. In the first test (Fig. 4), the diffuser was fixed a t a point 6 in. inside the haod and the distance of the detector from the diffuser was varied from 1in. (5 in. inside hood) to 13 in.
HOOD INDEX rt LOCATION OF SOURCE 60
4
I" I
0
2 4 6 8 1 D I 2 DISTANCE - DIFFUSER TO PROBE l inthtrl
Figme 4. Hwd index versus location of source when probe was moved and diffuser was fixed at 6 in. Outside of plane of sash.
rhe h ~ d , .l'lw n m m q u i n . ~ n dn in.? wlm 11, t i 10 fpnl w,%
t F 1". m t . . ~ I t -
nut w.rd
selected. Note that a t 9 in. from the diffuser no Freon* was detected. At the plane of the sash, the concentration of Freone was approximately 1 millionth of the concentration inside the hood. Thus, the user receives better protection if he remains outside the hood and places a source further into the hood. I n t h e second test (Fig. 5 ) , the probe was fixed a t the plane of the sash and the diffuser source was moved into the hood. This test was done without the mannequin but with a face velocity of 61 fpm. Again, the results are similar to those in Figure 4, although a erester rate of ehanee of hoad index is ohwwd.'rhvc~.m .uegrct that with hiahcr taw vrlw~llta,r l l.,rsvr mlnlmum d~.tan,c trum (Continued on page Al68) Circle No. 6 on Readers' inquiry Card
Volume 58
Number 5
May 1981
A167
HOOD INDEX n LOCATION OF SOURCE
the sash to the source could be tolerated and
7
is not necessarily true
HOOD INDEX va LOCATION OF SOURCE
ous two in showing the effect of emission source placement within the hood. I t is apparent from the above tests that the distance of the source of emissions inside the hood has a pronounced effect on haod performance and therefore on personnel exposure. With the hood design studied, a distance of 6 in. measured from the plane of the sash appears to reduce the potential for contaminant escaping from the hood while maintaining adequate working space. I t has been recommended that laboratories place a line 6 in. inside the haod as a guide for hood UGers.
5.0
SENSITIVITY
Work Practices
QL-LA~ - -
DIFFUSER
- - - - - - -. . -. -
I 00
2
4
6
8
1
0
1
2
OISTPNCL-DIFFUSER TO PROBf Unrhall
PLANE OF SASH ,DIFFUSER .--.-----..----
0
2 4 6 8 1 0 DISTANCE -DIFFUSER TO PROBE ItnChCSl
Figure 5. H w d index versus location of source when diffuser was moved within the hood and the probe was fixed at plane of sash.
Figure 6.H o W index versus location of source when diffuser is moved within the hood and probe was fixed 4.4 in. outside of the plane of sash.
hood. T h e diffuser was moved into the hood a n d the face velocity was again 61 fpm. T h e rate of change of hood index with distance shows that the mannequin in front of the hood essentially negates the effect of higher face velocities. This test confirms the previ-
In the next test, the effect on work praetices employed in working a t a hood were inve~tigated.A typical Y-min procedure involving pouring, transfer, and filtration of acetone was selected. An attempt was made to "standardize" the procedure a t Y-min duration by repetition and timing. In this work, the Miranmdetector robe was cliooed . . to the ~ l l l l d r ~ ~ f t h c i ~ p c r" .( ~ ; , t~~d~' $r .v ,A ~ T R I . I I C C ~ dvimtd J< ~ I I I I W ; , l t l l l w o r ~ e awe s kept dl I h a l r in.in:& tI~,.pli~!,r i r l lhc arb.
work practices consisted of ( 1 ) locating the acetone source onlv 2-3 in. inside the haod. (2) havine all the lashes m e n . and (31 the
face velocity as well as the use of secondary air over the hood as opposed to the use of remote air only to supply the hood. T h e results showed that all of the tests performed using goad work practices resulted in no detectable concentrations of acetone at the mannequin. Neither changing face velocity aver the range of 60,100,120 fpm or the method of supplying air to the hood resulted in a detectable concentration. Thus, all hoods had a performance index of greater than 6.1 with good work procedures. T h e tests using mod work oractices are somewhat ideal in ;:c.ncrpt 6,r'thcrc arr m f t man). h s d u r w uho u v k 1hl3way a11 < i t h p t m r 'l'her~tore. 1he e f f w t s ur ,,OM N, rk p r u t i w < \ \ < re ol..~ examined.
FACE YILOCITY, lpm
REMOTE ACETONE HOOD C O W pPn INDEX
SECONDARY LCETOYE HMYl C O K . ppn INDEX
Figure 7. Results for "poor" work practices studies.
Over-all, the results of the poor work practice test in Figure 7 show a dramatic difference when compared with the results of the good work practice tests. The difference
A168
Journal of Chemical Education
in average indices is greater than 2, indicating a difference of mure than twoorders of magnitude or a t least 100X. Thus, the protection provided a hood user by a good fume hood can very significantly depending upon his or her work procedures. Conversely, the effectiveness of a good haod can be substantially reduced by poor work practices. Within the work practice tests, the effects of face velocity and air supply were explored. Results comparing a remote air supplied hood versus one with a secondary air supply when used with poor practices are shown in Figure 7. Only when the hoods are operated below 60 fpm face velocity are differences due to air supply significant, i.e, at 40 fpm, the hood with secondary air has a higher index and lower average concentration detected. Examining next the effect of face velocity of a hood with a remote air supply, there is an improvement in going from 40 fpm to 60 fpm but the differencein m i n e to 100 f ~ ismnot significant. The differ&esamong &60, and 100 fpm face velocities on a hood with secondary air are also not significant. These data show an adequate level of hood performance a t a face velocity as low as 40 fprn when equipped with a secondary air supply and that face velocity by itself does nut adequately describe hood performance. Consideration must be given to other factors such as the air supply system. Another interesting way of examining these data is to look at the peak (transient) concentrations (Fig. 8) encountered during "POOR* WORK PRKTICES-PEAK CONCENTRATIONS
Figure 8. Peak concentrations during "poor" work practices studies.
the 9-min period of these tests instead of the average. First, the peak levels are much bigger for eachfaceveloeity for a hood using the remote air distribution as compared with a hood having a secondary air distribution. This shows the increased effectiveness of a secondary air distribution. Secondly, with a remote air system, there is adecreasein peak concentration with an increase in face velocity. With t h e secondary air system, however, the effect of face velocity is not significant in the region of face velocities tested. In summary, the results show: (1) the importance of good work practices, (2) the performance superiority of a hood incorporating a secondary air supply, (3) the insensitivity of both avcrage and peak external concentrations to face velocity for hoods having secondarv air suoolv. .. . and (4) the sensitivity of peak concentrations to face velocity for hoods employing only a remote air supply.
Chemical Spill
in. in from the sash and 300ml of acetone was poured into the tray followed by a clean~up with paper towels which were then placed in a iar. This was develooed into a 3-min DrO-
I
the air supply to the hood were compared. Results are shown in Figure 9. All hoods CHEMICAL SPILL and CLEANUP 22 TRAY 2'FROM SASH PROBE -0PERATER'S COLLAR REMOTE AIR SUPPLY TIME- 3 HiNUTES
40
60 80 100 FACE VELOCITY, fpm
120
Figure 9. Exposure to acetone versus hood face velocity tor a bmin spill and cleanup period.
did an effective job of handling the fumes from the spill with a low level of exposure to the operator, The average exposure for the 3-min period was 17 ppm, compared to a TLV (1979) for acetone of 1000 ppm. For a haod with remote air supply, increasing the face velocity from 40 to 60 fprn produced a reduction from 17 ppm to 2 ppm in average level detected. Further increases in face velocitv resulted in relstivelv small
with a lower level of average concentration. T h e average exposure for a 40 fpm face velocity was 7 ppm versus 17 ppm for the hood with remote air suoolv. These soill studies represent a begin;& and shoild be expanded to include other types of operating errors or lab accidents.
Summary Although these studies are of a scouting nature, and need to be done in greater depth, they nevertheless permit significant eonclusions to be d r a m :
.
Overall, it was concluded from the hood performance tests that, when good work practices are employed, the hoods protect the users from toxic fumes. T h e method by which air is supplied can significantly affect hood performance. Good work practices are very important in reducing exposure potential for hood users.
T h e ability of a hood to handle a chemical spill was also exam~ned.A tray was placed 2 Volume 58
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