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MALCOLMM. RENFREW University of Idaho Idaho 83843
MOSCOW,
Laboratory HVAC Systems-A Troika Earl L. Walls Earl Walls Associates, Laboratory Programming and Design Consultants, 5348 Carroll Canyon Road, San Dlego, CA 92121 her eo-workers, and the neighbors are safe. Chances are excellent that, if you ask labRight? Of course not. The user is now free to oratory workers to describe the HVAC sysapply a little common sense and perhaps a tem for their laboratories. thev will never few calculations to what the" want to do. He include themselves as a n&t o i t h e svstem. ~,~~~~~~ or shr e now frre to apply pdgment. Yet, they are as much a pnrt of the succexsful opcrarionofthesy+temnsthe fume hood The m o s commmdy accepted te%t for fume hood performanre is the ASHKAE 110 itself. Other papers discuss fnme-hood face test that utilizes a spectrometer to detervelocities, exposure rates in the breathing mine the parts per million of Freon a t the zone, impingement effects of supply air and user's breathing zone at s specified release other cross drafts, and the effect ofappararate in the fume hood at a point 6 in. hack of tus in the hoods. Thev direetlv address two the hood sash. Two important p o i n t e a of the major campon&ts of the laboratory soeeified release rate and 6in. into the h w d . ventilation system-the supply air, includTo increase the release rate or 1,) reduce the ing perhaps auxiliary air supply to the fume hood, or other exhaust devices, and the exdepth from the sa5h wlll result in different, and higher, exposure rates. A hood that haust system. But, they only look briefly at passes the ASHRAE test with 80-fpm face this third component-the user. All the engineering in the world is not velocities would probably back up a user going to do any good if the systems designed who might release a standard smoke bomb and installed are not properly used. And in the hood. The device can be overpowered. If procedures are going to release copious how manv lab workers have ever been instructed ;n the proper w e of a fume hood quantities of anything to the atmosphere and its assoriaced systems? How many pro(with some containment, it is hoped), check fe~stlrshave W C r inrtructed a rcudent or e the protection devices out carefully. From within the lab, s snorkel system utilizing graduate student in the use of these syshigher capture velocities may be a much tems? Would they know what to teach? For more effective device than the fume hood in instance, would they advise them to use a method of protection other than the fume protecting the nser. But do not forget that hood? What can he expected in the way of most, if not all, general fume-hood exhaust user protection and protection of the envisystems discharge their content to the atmoronment from the conventional chemical sphere without treatment other than dilufume hood and its extraction system? Such tion. With regard to dilution, our most reliedquestions can be thrown out ad infiniturn, upon "protector" of the environment, we hut the point is that laboratory workers are find that a laboratory that, far whatever reaalmost totally ignorant of what their HVAC son, is on 100% fresh air supply, has much systems are about and what is more, what higher dilution factors if a combined exthey can expect of them. haust system is used. By this, we mean that For the purpose at hand, assume that the all the air being discharged from the lahoraarchitects, engineers, and facilities people tory is through the fume-hood exhaust syshave done their homework and provided a tems, not through separate systems. For the first-rate fume hood and extraction system, sake of some statistics in this paper, eonsidproperly balanced with a supply air system er that for a 5-ft-horizontal-sash fume hood that is free of cross-draft influences. In othwith a face velocity of 80 fpm, 500 cfm is er words, two of the components are right. required. Ethyl ether released in such a Now the nser is free to do anything he or she hood would have to he vaporized a t the rate wants to in that hood, and the user, his or of 6% odmin to reach the lower threshold limit (explosive) for this hood alone. If all the room air, plus all the other hoods and all Earl Walls, who currently is chairman of theother raomair,is beingdischarged in the the board for lhe company he founded. same system, then you can imagine the ultireceived a BS in civil engineering at the mate dilution at the discharge point. But University of West Virginia and is a regiswhat of the discharge point? In the precedtered professional engineer in a halfdozen ing, ethyl ether was used as an illustration. states. He was employed as an architect But what about more toxic or noxiousmateand enaineer bv Monsanto and now is a rials, such as hydrogen sulfide or gallium board memoer of revera technolog cal arsenide? Here the hood user must. accent. companler on La Jo a. Calltarnla He the reiponsihility ot all-around prorrc. Serves 85 a cons~ltsnlor many mdustrlal tion- himself, his co-workrrs, and theenviand academic establishments. ronmrnt. if rrlcaie rates arc too hlgh for the ~~
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A314
Journal of Chemical Education
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potential dilution factors, then alternative devices and systems are in order. This responsibility must he borne hy the user-the experimenter. If release rates are in excess of what is considered safe far the material being used with the dilution potential, then one of three options is open. One, do not do the experiment. Two,change the quantities. Three, use methods other than dilution, such aa filtration or scrubbing. The next responsibility of the experimenter: can the material be filtered or scrubbed on a feasihle basis? I am here equating feasible to cost in terms of first and operational costs, as well as that of the chosen removal media. There is little argument that any exhaust system having the need to filter or to scrub the effluent should he as small as possible (in terms of cfm). But there are ways of using both filters and scrubbers on local systems that utilize booater fans to connect to coml~inedexhaust systems. Hy way ofexplanation: Air handling syawms are like hydraulicsvstemvinthat h ~ h d e awithfluids. l involving resistance or pressure drops, in the design of the conveying and driving components of the systems. The common combined laboratory exhaust system is a parallel system, meaning that the pressure drop, such as through fume hoods, is a fairly constant value. The addition of either filters or scrubbers adds pressure drops that, in reality, if not provided with some booster technique, will not work if they are simply added to the general system. These can he provided on a local basis for combined exhaust systems. There are instances, for example, radioisotope usage, where filtration is an effective protection system for the environment though they may not be a t all effective for the user. For the user, in this instance, local shielding and face velocity compose the prime protection. Scrubbing, the use of a liquid (though not necessarily) to capture the air-borne contaminant, is effective for s ~ e c i f i annlications. c I t does reauire stud" tbdewrmiA the hert 8crubhlng media, surh sswatcr,causric,nr aridirrdutionr. Fur any of these, surh treatment s h d d be as e.osr to the source of contamination as possible. Put them a t the hoads-not at a remote loeation that can result in contaminated duet work between the source and the removal medium. For reasons also relating to the pressure drop of duct work and fan systems, it is frequently found advantageous to use what is referred to as a thimble connection to pick (Continued on page A316)
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