mfety in the chemicd lcrborcltory
edited by
MALCOLM M. RENFREW University of idaho Moscow, idaho 83843
Upgrading Laboratory Fume Hoods G. Thomas Saunders Geneva Research, 126 Montrose Drive, Durham, NC 27707 Laboratory fume h~xsdsare purely and simply safety devices. Their rule in laboratory safety is to protect the worker, the work proper, thelaboratory environment, and the building occupants. They are expensive to purchase, more expensive to operate, and, in spite of this, are not well understood by users. hv olanners. hv eneineers. and last (but ., not least) hy purchasing agencic*. To these four dewred antups uf people. I dedicate thisahort and simple overview. Wc will treat hoods as part of an overall system and inspect their operation in six topic areas:
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1. adequate room conditions, 2. adequate hood design, 3. adequate equipment distribution (in the hood). ~-~~ ~-~ 4. adequate face velocity, 5. adequate maintenance, and 6. adequate operator education.
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Adequate Room Conditions In this regard, you have three major considerations: 1. make-up air, 2. room air-supply patterns, and 3. hood location in the room.
Make-up Alr A fume hood cannot exhaust air if the laboratory proper does not have sufficient make-up or supply air. This is pretty basic hut often it is a major hood problem. How do you know? Do this: Turn on all of the fume hoods in the laboratory. Then go to the door. If it swings inward, see how hard it is to close. If it swings outward, is it hard to
.. n e w Re~earch.was ongtorm airemor of researcn and awelopmen1 far St. Charles Manufactur ng Co.. St Char es, IL, wnere he was responsible for the design of many
types of laboratory equipment including fume hoods. He has panlclpated in many studies of hood efflclency and laboratory ventilation. He Is a graduate of St. Mary's College in California.
sponsorea by tne ACS Dlvlslon of Chernlcal Health an0 Safety and m e Cammms on Cnemlcal Safety, 1 April 1987. Denver. CO.
Journal of Chemical Education
open? Either way you have some degree of trouble. To quantify the problem, open the door and with the aid of a velometer take a velocity profile of the door opening. By the way, learn how to use a velometer properly. Be certain youare as far out of the arealair stream area as possible; otherwise, the area is reduced by your body size, and your recordings will be faulty. If the velocity of the air through the door opening is significant (100 fpm or more), then you must address this matter of supply air as a real problem, one that must be solved in conjuuctiou with the facilities engineering and maintenance people. If you cannot solve it, you should consider eliminating some hoods or instituting designs that reduce exhaust volumes (i.e., install horizontal sliding sashes or safety shields or sash stops).
Rwrn Air-Supply Panerns Let us assume that you have sufficient air in the lahoratory proper. Haw does it enter the room? Is it a hurricane aimed at the hood face acting like an aspirator for the hood, or is it calming Hawaiian breeze that soothes your hood into being a purring pussv cat? The purring pussy cat hood is more the exception than the rule. There are many mare roaring lions. To determine this very quickly, use a 30-second smoke homb; place it in the hood, and see what happens. If all the smoke stays in the hood chamber, congratulations. If not, look around and see where the supply air is coming into the room. Are the air-supply diffusers located so that they are aimed a t the hood face? Are there wall grille located at right angles to the hood face? Whatever you have, evaluate, and change if you can. The air supply must not create air currents a t the hood face. Get a new-style diffuser, hang some perforated metal pieces (even decorative metal panels from the local hardware) from chains or wires to break up the air-current pattern. You must solve the problem. Period. If you have to relocate the hood (or the diffusers or grill), so he it.
Hood Location in the R w m Far convenience to the huilder a Lot of hoods are located adjacent to the main laboratory doorway. Not only do you have disruptive traffic patterns, hut also if the lab is starved for make-up air, then the door isleft open and you have a hood that does not operate to its design capacity. If the hood
does not work under these conditions, do not blame the manufacturer: out it (the blame) ~. sauarelv on the arehitkt and t h e technical laboratory planners; they should haw known better. That is not solace now but a good lesson for the future. If this is the problem, minimize it by getting sufficient make-up air into the lab area, and then keep the door closed. If you are fortunate to have two doorways and one is and one is not neat to the hood, close the offender and use it only as an emergency exit. ~~~
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Adequate Hood Design There are many things you can cover in hood design. If you were just starting to buy new hoods, it would be well to pay a lot of attention to the sales presentations of various manufacturers. We are not really placing ourselves in this position, and I suggest that we review what I consider the three major area. that ran ire upgraded easily, inexpensively, and produce marked performance improvements: 1. baek-baffle adjustments, 2. hattcm.fnmt eirfuil, and A. inward leakage of air.
Back-baffle Adjustments This area in fume hood construction is maligned and then ignored. You can read all kinds of sales halonev about "heavier than a d ' n n d "lighter than air" adlustmenr, single pumr ndpstment, remute (eaterlor) ad justing levers, and on and on. The baek-haffle is of extreme importance to proper hood performance. The top adjustment should be set from % in. to % in. open. The bottom adjustable baffle piece should he removed and thrown away, its only function is as a troublemaker. If you look at the size and location of the hackbaffle slots, you first recognize that the top baffle slot is located closest to the source of maximum suction and the bottom slot at the point of (potentially) least suction. Under normal usaae you would like to have about the same voiume out the top slot and out the bottom slot. (If vou have three baffle slots. then you dewably should have equal \olumc from eachune I Ifthey are not set to the proper openings, you will never get an acceptable air distribution.
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There are few, if any, lighter-than-air gases used in fume hoods. You conceivably
could have a heat problem that makes the exhaust blower exhaust less dense air; under the continued use of a hood with a very high heat load, you may want to open the top baffle to 1in. or 1'I4in., but this will happen rarely. The smaller opening far the top adjustment should be the norm. I highly recommend setting the baffles as we have discussed and then using screws, bolts, or something to keep them in place. If you leave them so that you can turn a knob and change the setting, you can bet that they will be changed by people who have absolutely no idea of what they are doing.
BonomFront Alrfols The hottom-front airfoils added to older hoods, or present on newer designs, are vital to securing acceptable hood performanee. They eliminate front-edge (entrance) turbulence and allow the hood to address other problems without compounding the trouhles. If you do not really believe this, go to a hood without afoil, get a cake or pie pan and fill it halfway with warm water and then put some dry ice in the water. You will see the COI and water cloud crawl to the hood front; it may even spill out of the hood and head for the floor. Now take a 4- or 5-in.-wide piece of cardboard the same length as the hood opening, and place it so that it resembles a bottom airfoil; remember to leave a gap between the cardboard and the work surface. And guess what, the COz and water vapor move hack into the hood just like magic. Inward Leakage of Air On hwds with verticd-rising sashes (and this is probably 90% of installed hoods) the manufacturers left a %- to %in. gap in the area behind the sash and in front of the hood chamber. This is so that the sash can go up and down and not bump or bang into the hood proper. It just so happens that this present. an area for air to leak into the hood without going through the hood face. Under this condition the exhaust blower is doing its job, but the face velocity is at least 10% Low. There is a simple solution: You can fill this gap with a Teflon strip, 3- to 5-mil thickness. cut to size and held in dace with two metal strips. This is easy and inexpensive and dms help inrrea.ie face velocities. Or in rewrse this will rave energy dollars by exhausting less room air.' Adequate Equipment Dlstrlbutlon (In Hood) Too many fume hoods resemble a countyfair flea market. What does not fit someplace else winds up in the hood where it will be easier to see in case you may need it someday. The fume hood is probably the most expensive piece of laboratory equipment you have, barring instrumentation and some
A274
Journal of Chemical Education
constant-temperature apparatus. Do not use hoods for storaee and exnectthem to act as fume hoods. spread the k i o u a pieces of equipment and apparatus aroundso they do nut form impregnable walls. Then do a little quick step that makes your lion more like a pussy cat. Make small platforms out of plasto tic, plywood, or metal panels, and put 1'/~?.-in. legs on them. Use corks, rubber stopners. wooden blacks. or dowels with d u e to ?s&n them 'toeether. ~"~~ If vou , have a oyeee of equipment with a nice tlat bottom, glue the legs, stoppers, or whatever to the equipment proper. This will permit the laminar flow of air along the bottom of the hood. At a major research center I once saw a whole series of ring stands with stone bases going from one side of a hood ta the other. The hood just did not work well: The ringstand bases totally blocked the bottom hack-baffle slot; also the bottom slot had been adjusted to partial closure, and this added to the problem. ~
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Adequate Face Velocity The range of face velocity from 60 to 100 fpm provides adequate containment 99% of the time. Possibly 125 fpm is needed when high heat Loads decrease the volume of roolr air exhausted. Good work oractices are essential at all face velocities especially at low velocities. Here is the recommendation that I make to my consulting clienls. l'se a program of limits: lower limit 60 fpm upper limit 100 fpm average velocity 80 fpm with a maximum variation of 10 fpm across face
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This says that you have selected 80 10 fpm as your norm, that 60 is the single lowest acceptable reading, and 100 is one you need not exceed. Some hood users and facilities designers disagree, but I offer this as aviable program. Ifthe numbers do not fit with your program, make some adjustments. But please do not make a statement like a hygienist made a t s meeting I was attending a t a large research lab-"We've got lots of 'nasties' so the design face velocity on all our hoods is 150 fpm." That is just being both dumb and stupid. Containment tends to he poorer a t 150 fpm than a t lower exhaust rates. Now I have one last request to make regarding face velocity: after it has been measured with some degree of accuracy, do somethine so that the hood user can visuallv recognrze that the hood is working at or near the predetermined face velocity. An indicating manometer tapped into the exhaust ducting is the easiest and least expensive. You can buy the manometers or make them using glass tubing and a light oil. Or use table-tennis balls on a string or paper tissues taped to the bottom of the sash, anything so that the hood user can see that this is an operating hood with a face velocity close to specification. A hood user who relies on "air noise" ta tell whether the hood is functioning is going to be injured some day when the noise persists but the air flow has been cut off. I will not go into all the ways this can happen, but it can. Believe me. For testing quantitatively the containment hy installed hoods under service conditions, I reeommend the widely accepted
procedure ASHRAEIANSI 110-1985.This may he a costly test, hut it could be essential for a good future when laboratories are working witb highly toxic substances.
Adequate Maintenance I t is appalling how poorly exhaust systems are maintained in many laboratory buildings. I was in a major research facility recently and better than 50% of the fume hoods had "Do Not Use" signs pasted on closed sashes. The maintenanee supervisor said he just did not have the manpower to monitor the fans and their controls. As a result they had downtime, sometimes a long downtime, for many systems except for office air conditioning and heating systems in season. I t is not right to injure workers in order to keep the office areas comfortable. Demand a comprehensive maintenance program. If you already have one, review it witb the proper people to know two things:
1. Is it being scheduled? 2. Do you have competent maintenance people?
Adequate Operator Education During your scholastic years you are exposed to a variety of techniques, apparatus, etc., and as a result you become a trained scientific investigator. Were you instructed in the proper use of fume hoods?? Did anyone ever show you how to use a fume hood m a t efficiently? Were you ever instructed in good-hetter-best hood design? Chances are your answers are mostly, if not totally, "No." So I challenge you: if you do nothing else, atart the ball rolling toward a real honest-to-heaven safety program regarding fume h w d use and testing. Do this and many people will gain. Sit on your hands (and your knowledge) and you might well hear some personal responsibility if a fellow worker or student is injured due to something you could have prevented by insisting on a good hood safety program.
Mikell. W. G.; Drlnkard. W. C. J. Chem. Educ. 1984, 61, A13.
Volume 64
Number 11
November 1987
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