mfety in the chemicd loboratory
edited by
MALCOLM M. RENFREW University of MOSCOW.
ldaho Idaho 83843
Updating Older Fume Hoods G. Thomas Saunders St. Charles Manufacturing Co., St. Charle The English call a fume haod a fume cuphoard. This was a most apt description for many, many years. After all, the hood was mostly a large hox-like chamber with a hole in the top and a door (or window) on the front. As time progressed, a baffle system was introduced in hack t o provide a more even ventilation pattern from top to bottom. Douhle-end walls were added t o hide mechanical piping and give more work space. In the late 1940's the Harvard Schoolof Public Health provided the biggest advancement to that date by adding the streamlined air-entrance shape and most particularly the hottom-front air foil. This simplistic piece of metal reduced fume-hood air turbulence significantly, and the old f u m ~ruphuard item sterred ro beeume a murewph~~licntcd and, at last, n much safer place in which to work. Concurrently, with the progress of this equipment, the hygienists were developing maximum permissible levels of the various airborne chemicals. Eventually came OSHA and EPA and regulations relating to safety for the worker. The amount of air utilized by fume hoods was douhled and then almost doubled again so as ta "insure worker safety." The "mare is better" fraternity came into existence and the face velocities recommended for fume hoods rose from 50 fpm to 150 fpm. Whereas, a t the onset of the onslaught of regulations energy costs were minimal, rising oil prices soon made the operation of hoods a major concern of costconscious laboratory managers. In 1978the great white charger came down the road in the form of ASHRAE Research Project 70 by Caplan and Knutson.' This
pnpcr challmg~dfnrc velocities as the doalinant meaiure of houd performance. It pnr. vided a comprehensive and quantitative procedure for measuring fume-hood performance. It also gave a method of testing hoods and evaluating the importance-or lack of same-f the various innovations that had shown up on laboratory hoods over the years. Reduced to a simple overview, ASHRAE RP70 (now ASHRAE Standard llOP) states:
Figure 2. Back baffle.
HOOD WURK SURFACE
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Journal of Chemical Education
Using the testing procedures of l l 0 P the importance of the relative opening sizes in the hack baffle can he readily demonstrated. Most hoods manufactured today have three slots in the back baffle (see Fig. 2).
Now, let us apply to older hoods in existing laboratories what has been learned and eventually was verified by l l 0 P . If the haod does not have a bottom-front air fail, add one. This is a fairly simple step and most likely will cost only $100 per hood, considering an average hood width of 6 ft. The air foil can be of any metal hut stainless steel does not add significant cost over painted steel and survives better-except in an HCI (or like family) "slop-chemistry"area The shape is shown in Figure 1 and can be fastened rigidly to the existing hoad work surface. Any good hood manufacturer should he able to provide a fairly standard unit to fit almost any existing hoad. Installation can be done by in-house maintenance personnel. (Increasing the air space to l5ls in. will not
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A178
Adjustment of t h e Back Baflle
1) Hood location in relation to room air patterns is accountble for a t least half of the ahilitv of the hood to oerform safely. 2) Face velocities should range from 60 t o lCil fpm. 3) Hood design is important to hood performance.
G. Thomas Saunders is director of research and develooment for St. Charles Manufactur ng Co St Cnarles, lhno s hns work mc udes des go ol many lypes of lao. Olalorl equ pment, m addltlon lo sldler an fume hood efficiency.Saunders is a graduate of St. Mary's College in California.
Caplan, K. J., and Knutson, G . W., "Development of Criteria for Design, Selection & In-Place Testing of Laboratory Fume Hoods &Laboratory Room Ventilation Air Supply," ASHRAE RP70, March, 1978.
hurt performance and will permit the insertion of older 3-prong plugs.)
I Figure 1. Fume hood showing air foil
A is normally adjustable from 2 in. open t o fully closed. B'is a fixed size and should he approximately 1%in. C may or may not be adjustable; if adjustable it may be in the range of 2-3 in. open t o fully closed. To obtain maximum performance of the hood adjust A to '/z in. to 3/4 in., and then using screws or bolts fasten it into this fixed position. If the top adjustable piece is missing-r
the hood did not have one in the first place-make one. This single action has the ~otentialof improving performance by a factor of 1000. The one and only condition that would warrant an opening more then the 'J2 to Y4 in. is a uer, high heat k d ; this means a 5 to 7 kW Load andlor temveratures of the hwd air volume in the rangeof from 120' to
With the sash raised, air from the room ircumvents going into the hood via the face ,pening, gws through the gap, and, therefore, ioes not add to h o d performance, only #asks expensive room air that has been treated to room quality (7Z°F or so). This gap is easily closed (see Fig. 4).
ifinow -"" -.
If the h o d does not have s B slot, do not worry. This is not a necessary part of the baffle system. Contrary to many advertised claims, testing by the llOP procedure shows minimum effect of this slat If your hood d m not have one, forget it. Slot C really should not he adjustable. If your hood has an adjustment, remove the adjustable piece. The gap should he approximately 2 to 3 in., if your h o d has a smaller opening, make it larger. The goal is toachieve approximately equal slot velccities for all (two or three) slots in the hack baffle. The Industrial Ventilation Manual, published hy the ACGIH2,has recommended roughly 2000 fpm (in the fumeh o d world it will be closer to 800 fpm.)
Hood Alr Leakage Older hoods-and many new ones, depending on the manufacturer-have air leakage that uses at least 10%more air than is needed. Here is why, and it is easily corrected. Where the vertical rising sash goes past the h o d chamber proper, there is a gap to allow this up-and-down motion. I t is commonly from 314 to 2 in. depending on the manufacturer (see Fig. 3).
Figure 3. Gap around vertical sash.
2 "Industrial Ventilation," 17th ed., March, 1982, American Conference of Governmental Industrial Hygienists, P.O. Box 16153, Lansing, MI 48901.
Figure 4. Closure of gap m u n d sash
Light level The use of a cement-board liner in a nonpainted condition gives a dull gray chamber, not overlv illuminated, and results in an unattractive, dismal cave. To improve the illumination, get some Windex or ammonia or a chisel and clean the glass panel that covers the light. Get on top of the hood, raise the light, and do the same. Most hoods, new or old, accumulate a lot of process dirt andgrime or junk left over from initial installation that blocks available light; proper maintenance on a continuing basis is necessary. Next, get an air-dry, white, chemical-resistant paint; Sherwin-Williams and other paint manufacturers have commercial products that are adequate. The addition of this coating can he done by brush, roller, or spray and makes a world of difference. It is not expensiv-xcept the hood is out of service for a day or two. Do not paint a hood liner of a perchloric acid hood-all paints are organics and this plus perehloric acid spells deep, deep trouble. If you have a stainless steel-lined h o d , clean it with a cleanserBan Ami or the like. If it is a perchloric acid hood, be sure it is well washed down prior to cleaning, and even when cleaning use lots of water constantly. Perchlorates in solution offer minimal problems; when dry they present a real hazard.
(Continued on page A1801
Volume 62
Number 6
June 1985
A179
Lastly-Face
Velocities
Caplan and Knutson in ASHRAE llOP point to face velocities in the range of 6&100 fpm. This was seconded in 1982 by the ACGIH in the 17th edition ofthe Industrial Ventilation Manual. If you, or your hygienist, or your safety pruple have an etched-in-stone policy of a f a ~ evelcrcrty rhar exceeds dU-100 rpm, reconrtdrr. Thls "more is better" uh~loauvhv cannot he substantiated-or &en condoned. Recent test data are more in the 80 fpm area-all things considered-hut rarely, very rarely, over 1W fpm. If you (or your hygienist or safety person) have an inclination to think, "Boy, we've got nasties and must have 150 fnm" (H la OSHA).. ston . and read ouhlished dora. Fume-hod pert'ormance is poorer nt 150 fpm than it is nt 100 fpm Hy thus reduring the exhaust rate you can improve performance and save money, much money. Do not settle for more; settle for less. You may even become a hero in the process.
Location Now. let us consider the other half of rumc-huud perfcmnanre: the location o f t h r hood in a nmm relative to traffic and tu nrom air.
Journal of Chemical Education
It is tough to reroute established traffic past hoods, particularly if the hood is adjacent to the access door; so forget it. However, make-up air is a correctable commodity. First you must have enough air going into the room to take it out by way of the hood or hoods. If the hood only has an "adequate" faee velocity when the lab door is open andlower with thelah door closed, get a good HVAC engineer and see what can be done. Open doors with hoods adjacent to the d w r normally spell a real problem. You could he aspirating out of the hood and have a daneerous situation. A smoke homh (30-s variety available imm the Vernon Hill 60.in Snn Vrat~ciscwcan tell you this ina hurry. I t the smoke pourv into rhe room uirh the d w r open, you need an HVAC engineer worse than you think or maybe a new insurance carrier. If your lab has sufficient make-up air, see where it enters the room. A fairly standard variety of air diffuser injects air into the lab a t a fairly high velocity. These pieces of vent-system equipment l w k similar to that shown in Figure 5 and can be round, square, or rectangular.
-~
~~~
Figure 5. Air dlffuser
The problem is "velocity"; it should be from % to 314 of the faee velocity of the hood. The chances are that it is in the range of 2 to 5 times the face velocity. The location of these diffusers often is at the convenience of light locations, beam or piping locations, or simply the duct system proper. If you can increase the sue andlor location of the diffuser, should it be a problem, do so. If you cannot, and this is most likely the case, take some perforated metal and slow down the air that is aimed at the hood face. This may take a little ingenuity, hut, even if you hang some perforated material on small chains or wire, you can do some wondrous things. Each situation is different, but the approach to solving the problem is essentially the same. I t is not expensive, except in time, and it may save injury from air-borne toxic substances.
Establishing and Maintaining Hood Performance Maintenance of the mechanical system is a must. Blowers must have periodic checks-motors, belts, impellers, ductingthe whole thing. The maintenance people should set up a program of inspection; whether once or twice a year is up to each laboratory, but it should be done. This is not what you would call a standard procedure consideringall laboratories, hig and small, yet it is important for all laboratories. Each hood should also have a periodic inspection to see that whatever faee velocity determined as adequate actually is being maintained. There are many instruments on the market to measure face velocity; Alnor is nrobablv the mast nrominent manufacturer ~~~~, and has a vast array uf devires. There in. spections a h d d be documented ]US1 in case an a c r i d ~ n tmight occur and cause* utre being sought. Last, but not least, the laboratory personnel using the hood should have some quick reference system to determine that the hood is actually operating. Attaching a manometer of some sort to the exhaust duct can confirm that the blower is sucking out air; Dwyer Manufacturing Co. makes a wide range of these items, and they are not necessarily expensive. The main thing is they should he mounted an the hood for ready reference by the operator. This means eyk high on the front of the hood, not on the wall or some area not easily seen. For more critical areas, alarm systems do monitor exhaust suction and are available from all fume-hood manufacturers. Fisher Scientific is now investigating a simple, visible device to indicate the face velocity. Whatever you use, even a piece of Kleenex taped to the bottom of the sash, use something. A parting challenge: fume hoods were not designed to be storage areas. Pound for pound the hood is the most expensive piece of furniture in the laboratory. Crammed hoods do not perform as do those with a planned apparatus arrangement. Keep the hood as a safetv device not a trash-storaze area. Take goodcare of your hood and it will take care of you-that is the bottom line.
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