edited by TIM CHAMPION
tips
Johnson C. Smith University Charlone,NC 28216
How Does Your Fume Hood Rate? New Assessment of Current Fume Hood Design and Operations Jerry Koenigsberg G.P.R. Planners, White Plains, NY 10603 On January 31,1990, without fanfare, OSHA, the Occupational Safety and Health Administration, under the Federal Department of Labor, handed down their long awaited landmark laboratory standard entitled, "29 CFR part 1910, Occupational Exposures to Hazardous Chemicals in Laboratories" (1).Considering that it took years to enact, it was surprising that OSHA allowed a mere three months for compliance. This standard covers all employees who work in laboratories where hazardous chemicals are handled routinely. The laboratories covered are those that are not part of manufacturing operations that were covered already by the myriad of standards dealing with specific chemical exposure limits. The uniqueness of this standard lies in the way OSHA chose to define the term "Hazardous Chemicals" as a means of broadening its authority. As defined under this standard, even the most wmmon laboratory reagents are potential candidates for this label, and institutions are obliged to establish procedures to minimize expos u r e to them once testing indicates t h e need. I t i s important to remember that, although students do not fit the definition of "employee", there is no reason to think that academic institutions are not covered by the OSHA standard. Coincident to this OSHAactivity, another federal agency, The Environmental Protection Agency (EPA),was support-
About the Editor Tim Champion is an Assistant Professor of Chemistry at Johnson C. Smith University in Charlotte. NC. He received his BS in chemistry in 1983 from the University of North Caroiinaat Chaoel Hill. He is currentlv cornoletino iis doctorate it the jnivers'lti ofhortnern~oio. raoo H s researcn nterests nc ~ d the e des gn and eva Latoon ot instructional materials. Champion lives in Charlotte with his wife Kathryn and dauuhter Julie. "safetyTps" isa column for allicles designed to help teachers improve safety conditions in academic laboratories at all levels. Of special interest are allicles describing previously unforeseen hazards in commonly performed experiments. Other appropriate submissions include ways to minimize alreadv recoonized safetv hazards and methods for lncorporatmg safety e"o~caton lnio laboratory managemenl systems that empnas ze safety lnreresleo persons sno- d s ~ b mat ancles to T m Cnampon a1 the aoove aadress ~
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ing legislation to address the concerns of many health and safety professionals for the quality of the air inside our offices and homes (Indoor Air Quality). In recent years, a number of studies have been published that point out that there are many facilities that exhibit internal pollution levels that exceed those found in the open air of some of our largest and most industrialized cities. The primary focus of these studies has involved three pollutants-formaldehyde, asbestos, and radon. Specific government regulation of exposure limits to these subst&ces has been sought and pniied. Efforts to date haw culminated in the Indoor Air CJuahtv Act of 1990. siencd into law on November 15.1990. which'tvould give the k ~ ~ t authority h e to establish a na: tional Droeram of research and education and to set ~erformance guidelines. Concurrently, a new professional subs~ecialtv-"IAQ has emereed in the eneineering and sEientific comm;nity in response to this concern. Of all the laboratories covered by the new OSHA standard, academic laboratories are probably most a t risk of failing to be in compliance with the new standard. As a rule, academic institutions are less likely than industrial or health-related organizations to be able to afford the significant wst involved in laboratory renovation and or new construction. Another contributing factor is that school labs generally are housed in buildings that were not designed to be lab buildings, which makes renovation, expansion, or relocation difficult or impossible. This inability to do much more than mere cosmetic or routine maintenance work on existing academic facilities would tend to explain why there are a significant number of "ancient" labs, many 50 years or older, still in use (2).In recent years, a significant number of blue ribbon panels have pleaded the cause for upgrading and expanding our laboratory facilities to improve the quality of our scientific education. To date there is little to show for these efforts since there has not been significant "grass-root"support to increase taxes for education. The new OSHAregulation, in concert with EPA's "IAQconcerns, may yet prove to be the catalyst for this much needed improvement. There is no ouestion that labs must be u ~ m a d e dif there is a ~otential lkalth risk to anyone who G h t be exposed; be they students, teachers, assistants or, housekeeping, maintenance, or security personnel, or anyone else working in or near the laboratory. &
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What You Can Do Now!
One way to gauge your facility's ability to comply with the OSHA standard is to determine the number of fume hoods in use and evaluate how well they work. Remember the main thrust of this new standard is to minimize "Exoosure to Hazardous Chemicals" and the fume hood is widely recoenized as the most i m ~ o r t a nlaboratorv t safetv amlianc~specificallydedicated for that In &ct; the
new standard makes freauent references t o fume hoods i n terms of t h e number required per person, performance criteria, construction details, a n d t h e need t o instruct all personnel i n their proper use. Armed with this knowledge, you a r e now i n a position to begin t o evaluate your own situation. T h e following series of questions a n d answers, some specifically based on t h e new OSHA laboratory standard, were developed to assist you i n your effort.
Question 1:Are there enough fume hoods in each lab to allow each student access? Answer: OSHA recommends 2.5 linear feet of hwd space per person ifthey spend most of their time working with chemicals. Remember that a typical 6% fume hood has a 5-R. wide opening and is, therefore, appropriate to fulfill this requirement far two people. Question 2: Do you allow your hoods to be turned off! Answer: OSHA recommends that hoods should not be turned o f f ~ r t o x ~,hazardous, e substances are stored in it. The exhaust fan should run continuously Q u e s t i o n 3: Does your hood have a performance monitor? Answer: OSHArecommends that each hwd have a continuous monitorme device that Dermits mnfirmation of adeauate performance both pnur to and durmgoperatlon Smce the hood should never be turned off, rcgnrdlcss of whrrher nn expenment 1s bemg run in it or nor, the mumtor nlust be dcsngncd to
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Q u e s t i o n 4: A r e your hoods directly a d j a c e n t t o t h e student's work bench, or a r e they located elsewhere i n t h e room? Answer: The hood should he considered part of t h e student's work space to ensure safety and to maximize their use. Any significant separation will increase the potential for a n incident if chemicals and a ~ o a r a t u must s he moved around the rwm, and the fume hood will not he used fully. Q u e s t i o n 5:Are your hoods located i n a heavy traffic aisle or i n a quiet corner of t h e lab? Answer: Hwds should be in a location that permits escape from the room without having to pass in front of them in case of an emergency (4.5). Q u e s t i o n 6: Do you routinely store chemicals inside your hood? Answer: OSHA recommends that materials stored in the hood be kept to a minmum, and they should nor be allowed to block the arr flow entenng hame slots at the rear of the huud. Chemicals should be removed frum the hood and placed in properly designed and vented storage cabinets.
Question 7: Are t h e cabinets below t h e hood designed and used for chemical storage? Answer: Chemical storage cabinets fall into two categories: corrosion resistant and flammable (6).The fomer is lined with an acid-resistant liner material, and the latter is mnstructed in accordance with the standard "N.F.P.A. 30" (7). Both should be vented into the hood to avoid vapor build-up and exposure when the doors are opened (8). By storing the chemicals below the hood, we will minimize the movement of potentially hazardous chemicals inside the laboratory.
Answer: Transite is a light gray colored board that is smooth to the touch. Up to 1986, it was the most mmmon hood liner material used in this country. It has a tendency to exhibit crumbling a t its edges. If your hwd contains asbestos, it must be treated as any other ashestas problem. If it proves necessary to remove it, it should he done by a certified contractor and sent to an approved land fill.
Question 10: Is t h e hood physically sound a n d devoid of cracks in t h e liner a n d counter top t h a t could leak if chemicals were spilled ? Answer: All cracks should he fdled and all seams sealed to insure that the hood can he decontaminated effectivelv -~~~~~~ " in the event of spill. If the liner is badly stained or disfigured, it can be sealed with a n epoxy or urethane paint. AU seams can he sealed with dear silicone. All spills have the potential for increasing the possibility of chemical exposure. ~
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Question 11: Is t h e light fixture inside t h e hood operable, a n d is it vapor proof? Answer: Being able to view an experiment going on inside a hwd chamber and controlling its operation in a safe manner requires that the chamber be lighted adequately. To aceomplish this, the light fixture should he operable, and the lens cover should he cleaned to maximize the lighting levels. The light fixture should be sealed to prevent any flammable vapors that may be present in the hood chamber to enter it and be ignited by a spark (9). Q u e s t i o n 12: A r e a l l electrical devices, outlets, l i g h t switches, etc. mounted outside t h e hood chamber? Answer: As previously mentioned, it is imperative that there not be any source of ignition inside the hwd chamber especially if flammable vapors are present. The devices may be mounted on the hood's side post or on the base cabinets below (9).
Question 13: Is t h e hood's front door (sash) glazing intact a n d transparent? Answer: It is important to understand that it is the sash that is the primary barrier preventing vapors from escaping and not the air flow capacity. While it is true that the inflow of air into the hood's chamber daes contain vapors, it cannot cope with a major gas generation, flash, flame, or heat. Therefore all sashes must be closed fully when there are no hands-on operations taking place, and they must be clear and clean enough, to allow visual inspection of the experiment without having to open it. Q u e s t i o n 14: Does the sash overate with relative ease? Answer: If you cannor effonlessly close it, you will not try and, therefore, not get the full benefit of the hood's m n t l n ment potential. Replace worn components such as rollers and weight cahles that may have become frayed. Consider retrofittingvertical sashes with a horizontal type (10). Q u e s t i o n 15: Is t h e sash fully closed except when one m u s t reach inside? Answer: You wouldn't operate your rchgerator or oven wlth the dour upen, why would you runsrder uperatmg your hood that s,ay? Hemernbrr, at i s thc barraerrrratcd by theglnsa panel in the sash that offers the maximum user protection, so take advantage of it (10).
Answer: OSHA specificallystates that hwds should not he used as a means of disposing volatile chemicals. They should he collected and placed in an approved safety container while awaiting removal fmm site. Consider that, under ideal conditions, small quantities of common organic reagents (e.g., acetone) can pmduce enough energy when ignited, to destroy any fume hood (9).
Question 16: Does your hood's exhaust system evacuate vapors efficiently? Answer: OSHA discusses hood performance in terms of containment, not specific face velocity. Containment is determined by measuring the amount of tracer gas that escapes the hood's chamber relative to the amount generated within during a controlled test procedure (11). Most professionals believe that a face velocity of 100 FPM (feet per minute) is reasonable and adequate under most conditions. Keep in mind that by closing the hwd sash you would he minimizing the potential far exposure regardless of face velocity.
Question 9: Are your hoods at least 6 years old? Do you know if a r e they lined with transite (a fire-resistant material containing asbestos)? How can you tell?
Q u e s t i o n 17: What is t h e possibility t h a t t h e hood's contaminated exhaust will t o be re-circulated back into t h e building?
Qnestion 8: Do you allow chemicals to be disposed by allowing t h e m to evaporate in t h e hood?
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Answer: It is imoerative to avoid havine the fume hood's exhaust m-enter the building through the fresh ar mtake of the building's air eonditiunmg system. I t is equally important to avoid having the hood's exhaust re-mtcr the building through open windows or other openings, especially if the building is not air conditioned (12).
Question 18: Do you constantly check the fume hood's performance and integrity? Answer: OSHA suggests that fume hoods be evaluated every three months or whenever the system is modified. Although not explicit, it implies that the face velocity should be checked hecause it is the only variable that can be measured directly %and when,the exhaust system were to degrade (13). Also. it is recommended that all hmds he tested with the ASI I I ,American ~ ~ Society of Heating, Refrigerntmn, Aircondit~nningEngineers, approach to determine its level of conrainment (11,. Once this value is drtrrmined, follow-up face velocity testing is appropriate Current Trends in Fume Hood Design In the last 20 years, fueled by the enormous increases in the cost of energy, the major advances in fume hood technolow have em~hasizedthe need for makine hoods more energy efficient; not necessarily safer. This need has produced both the auxiliary air and the V.A.V. (variable air volume) fume hood (14). The auxiliary air type attempts to save energy by pumping un-air conditioned$treet ai; into the room at the face of the hood to minimize the amount of expensive air conditioned air that the hood will "waste". The V.A.V. hood incor~oratesa verv exoensive (53000 oer hood) electronic or pneumatic control system that attempts to vary the amount of air the hood will exhaust in ~ r o ~ o r t i oton the oDen area ex~osedbv the sash. As the operator lowers the sash, the control system reduces the exhaust volume in order to maintain 100 F.P.M. through whatever part of the sash remains open. This means that if the hood were closed fully, the hood conceivably could operate with as little as 100 CFM (cubic feet of air). This is obviously a fraction of the amount of air normally required to operate a hood when the sash is fully opened. While both hood designs have demonstrated a potential for saving energy, serious questions have been raised a s to whether they enhance fume hood performance. ARer all, the hood's primary function is to contain and puree all vapors geneiated $thin and to be a n inteqai of the rwm's ventilation svstem: the mission of which is to maintain "IAQ.While there is no question that the cost of energy is important, i t must be skbordinated to the overriding need to protect laboratom personnel. Therefore. other a l t e m a t i ~ e ~ m ube s t evaluated that combine energy efficiency and enhanced hood containment. In 1978, the first generation of a novel new h w d design, the "HOPEC" (Hand Operated Positive Energy Control) fume hood, was introduced in direct response to the need for energy conservation (2, 3, 14, 15).The uniqueness of this design lies in the fact that for the first time the hood's sash was made an active part of the hood's operation and not merely a door. The third, and current, generation of this hood design incorporates a combination horizontallvertical sash that permits the operator to use the sliding panels as a full body shield. Recently published papers have indicated that, not only was this hood design energy efficient, by consuming half the amount of energy that a traditional hood requires, but also, exhibited performance hieher (16). levels (containment) that were sienificantlv . This hood design is currently available, a t no premium, from a number of manufacturers in this countw. The following is a description of the basic concept of this hood design.
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The HOPEC Fume Hood
Simply stated, the modem laboratory fume hood is not just that "box" that sits on the countertop that mav or mav not have an operable dwr. It, in fact, consists of fi;e major components: base cabinets, countertop, hood superstructure, sash, and the room air supply/&haust system. All must be properly installed, maintained, and utilized to assure o p t i u b containment of potentially hazardous chemicals, which is the current OSHAcriteria for evaluating the margin personnel safety. Base cabinets should be designed and constructed to store chemicals. It is recommended that if the hood were wide enough to require two cabinets, one should be dedicated to corrosive chemicals and the other for flammable materials. The corrosive cabinet must be lined with an inert material. such a s non-asbestos transite or oolvoropylene, that &ll not be attacked by acids, common to most labs. The flammable cahinet must be constructed in aeeordance with the N.F.P.4. 30 regulation that will protect its contents from a potential fire in the laboratom. Each cabinet should be vented into the fume hood us*g a 1 112-in. diameter duct that pierces its back wall and penetrates the countertop behind the hafile. The duct should extend a t least 12 in. above the counter to assure that the vapor concentration within the cahinet would be minimal and not present a potential health problem every time one opens the cahinet door (8).Placing chemicals beneath the hood in properly designed and efficiently vented cabinets has the added advantaee of minimizine the movement of chemicals within the-laboratory since most, if not all, work involving such materials should be performed in the hood. The countertop should be fabricated of a material that is non-reactive and physically capable of supporting the weight of the hood and the apparatus anticipated. In most cases epoxy resin is the material of choice. The use of stainless steel is generally limited to those hoods that are designed to handle perchloric acid or radioactive chemicals. Regardless of the material selected, the surface of the countertop should be depressed to assure that any spills will be contained within the hood. The fume hood suoerstructure should be built laree enough to house anticipated experiments while still allowl ine the sash to be closed fullv. In addition. it should incorporate the following features: a liner that is inert and sealed to permit cleaning. o~erablebaffle slots that allow changes in hood exhaust&~racteristics,light fixtures (fluorescent type and vapor sealed), electrical switches and outlets mounted outside the hood, and all plumbing services having remote control. front-loadine tvoe ?. fixtures that are aee&sible outside the hood for maintenance. The front-to-back clear dimension inside the hood's suDerstructure should be at least 30 in. deep to allow ope>ators to move their work back at least 6 in. inside the hwd, (a universally r e c o ~ i z e dwork practlcc that enhances hood containment levels,. All openings in the hood's superstructure, with the exceotion of the sash. should he sealed (no hvpass) to avoid'the potential of leakage and make the hood more energy efficient (14). As previously stated, the sash (the hood's door), has finally received its well-deserved recognition as the major contributor to enhancing the hood's ability to contain vapors, which is the current criterion for evaluating hood efficiency Although the horizontal sash alone would create the desired effect, by making the sash a combination type (both horizontal and vertical), the operator is allowed to oDen the hwd fullv when necessarv for set uo and vet eain the advantage of gorizontal operagon. ~houlb.one fbol&ly attemot to use the hood in the full oDen ~osition.the correct hood monitor would sense this condigon and $et off an
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alarm (3).Remember containment, not face velocity is the Examine the hood's structure and repair any defects. ultimate criterion for hood efficiency. Seal all joints and create a spill barrier a t the front of the countertop if a dished surface does not exist. Replace Knowledgeable engineers should establish the air condilights, clean lenses and seal light fixtures to create a tioning load for a laboratory by determining which of the vaporproof barrier. Adjust andlor repair sash mechanisms following variables has the greatest impact on their deto permit effortless operation. sign: the-number of air changes per hour ;equired to maintain a healthy environment (IAQ), the anticipated heat Wash down he hood's chamber. If the liner is transitp and load generated by laboratory equipment, and the amount removal is not possible, seal the chamber walls with a light of air required by all exhaust systems consisting of hoods, colored chemically resistant epoxy or urethane paint. canooies. snorkels. and other local exhaust a ~ ~ a r a t(12). us Remove all chemicals not specificallyinvolved in the experiIn essence, all the' air introduced to a laboraiory mu& be ment at hand 'om the hood chamber. exhausted whether through a hood or the building's exStridlv enforce the directive that all sashes must be fullv closed"whcnthe hood 1s unatrcndcd. haust system. If not enough air is available in the room, Hemovr all sourccs ofclectr~callyinduc~digmtmn (spark, by the exhaust system will attempt to draw the required removmg all electrical outlets from wtfh the hood's chamber amount from adjacent space though doorways and win~onside;cuttingdown on the volume of chemicals used for a dows (17). eiven exneriment bv incamoratine micm scale teehniaues The building exhaust system, which includes the fume wherever possible. l'hc cost of ncu apparatus will hr maw hood, must be designed, first and foremost, to maintain the t h a n omset by rrducing not only the cost of reagents; hut, "IAQ of the laboratory Assuming that, it is important to more s~~mificantly, the cost of disposal. point out that the hood and its exhaust capacity is specifically responsible for containing chemical vapors within the Moderate Cost Recommendations hood and effectively enhancing the quality of the room's Purchase and install fume hood performance monitors. environment. I t is sueeested that the electronic "throueh the wall" ambiI n the past, fume hood manufacturers have offered their ent mass flow sensor be selected along w%h a monitor that hoods with integral exhaust fans mounted in the superhas three independent settings: operating. caution, and structure. In addition, many institutions have installed both a visual &d audible low cow harm. ($700 per hood). their hood's exhaust fan inside the laboratow creatine Modifv the hood's base cabinets bv lining them with aoboth a noisy and potentially dangerous situation. ~ h g propriaie materials for the storage of sp&ific chemicais ~racticeshould be sus~endedbecause it ~ u t the s duct svsand connect them to the hood's chamber with a -pipe to cretem under positive pressure that could Force vapors back ate a vented condition ($100 per hood). into the room. Today, i t is generally recommended that, Re~lacehood base cabinets with an N.F.P.A. (National with few exceptions, hoods should be ganged together to Fire Prevention Association) Code #30 approved flammacommon exhaust svstems with the exhaust fan mounted ble storage unit and vent them. ($800 per cabinet). on the roof of the bklding. This design offers a number of simificant benefits: it minimizes maintenance by reducing Modify the hood's sash to make them horizontal type (10). n i th;? number of fans and roof penetrations, minimizes : Replacement materials are locally available from glass supply houses. This will not only make your hood more efficient, tial cost by reducing the amount of duct work required, albut also it will cut total demand of the hood by 50%. This lows for the introduction of heat recovery capability, and means that you auld potentially double the number of hoods enhances the ability of the designer to isolate the exhaust on an existing system without the need to increase exhaust air from the fresh air intake (12, 18). capacity nor require additional air conditioning capacity be In recent years, a new category of fume hood, the "Ductadded ($300 per hood). less" type, has been marketed that purports to offer the r as the traditional hood. same level of o ~ e r a t o orotection Significant Cost Recommendations Its most enticing feature is that it relies on charcoal fdters Contract with a certified contractor for the removal of all and molecular sieves to eliminate the need for ductwork. asbestos contaminated hoods and make sure that the units Once filtered the air is recirculated back into the room. are deposited at an approved land fill. This type of unit is not recommended as a universal soluReplace contaminated hoods with 6-ftwide Hopec 111 tion for adding additional hood capacity because of serious style hood systems as described above ($6000 per hood). limitations of the filtration media and the inability to anRemember, Hopec is not a brand name, rather a hood type, ticipate its effect on the "IAQ (19). and, therefore, available from most hood manufacturers upon request. Recommendations Add enough hoods to satisfy the OSHAsuggestion of 2.5 I t is too bad that we cannot merely sit back, wave a R of hood per person ($3000 per hood as sum in^ - 2 students maeic wand and wish awav all the sins of the ~ a screated t per 6-Rhood)' by Escal restraint and theshort sightedness or ineptitude Examine the roof-mounted hood exhaust system and of those ~rofessionalsinvolved in lab desien and construcmake sure it does not discharge near the fresh air intake tion over the last 50 years. While complete rejuvenation of the building air conditioning system. If it does, re-direct mav take vears. there are thines one can do immediatelv the d i ~ c h a r e ~ u ~ w and a r d &ease the velocitv to throw that will improve your current circumstances witholt the contam&;d air off the roof. breaking the bank. We, therefore, offer the following pack-. Modifv the WAC (Heatine Ventilatine Air-conditionine) age of recommendations that are broken down into three system to supply enough air ta support the hood population levels of potential cost: minimal. moderate. and signifiand elminatp systems that re-cycle lab aw hack rntn theeencant. tral system as is commonly done s,ith office or classroom oriented systems. Minimal Cost Recommendations Completely renovate the laboratory in response to the safety issues raised as well as making it more efficient in terms of Establish a oroeram to educate all students in the current technolow. Laboratorv HVAC svstems should be deproper use of the f-e hood and how it relates to the issue signed and mnst&ted to use100%fresh or outside air and of containment-the abilitv to minimize exDosure to wtenuse the hoods and other exhaust apparatus to purge the room air. tially hazardous chemicais.
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May 1992
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Conclusions In the next decade, many institutions will be required to upgrade their facilities to comply with increasingly restrictive government regulations and to avoid potential liability. This process is even more painful because funding may not be readily available. Today, many of our most renowned institutionsare being forced to make hard decisions regarding where to apply their ever shrinking resources. It is hoped that this report has offered some insight into a number of relevant issues and that some of the suggestions made can be used to alleviate or remove potential problems. Remember, the one constant that we all can count on is that things will continue to change. If you are currently considering significant modifications to your laboratory facilities, regardless of your motive, consider making your project truly "adaptable"; that is, be able to modify it in the future; quickly and a t minimal cost i n dollars and down time. For now, thanks to our governmentk recent action, we have been given our march-
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ing orders to move in a direction from which there is no turning back. Literature Cited 1. F&rolR&L 11990,(Dec 31,1990).33003335. 2. Koenigsberg,J.Phnm. Eng. 1991,(101,15-20. 3. Koenigsberg,J.HeafhgiPlpingiAirCondifioning,1991,(918142. 4. N.F.P.A. National k%reRoteetionkraodationNo. 45,1986. 5. Koenigsberg.J.Am.Lob 1987,(5),66106. 6. Koenigsberg.J.Am.Lob. 1988,(71 42-46. 7. N.FP.A. National ArePmtedion AsswiationNo. SO,1985. 8. Koenigsberg,J.,unpublished results. 9. Koenigsberg,J.Am Lab. 1988,(2)89-96. 10. Koenigsberg.J.H~ofinglPlpinglAir Condftioning, 1987,(10).77-92. 11. ASHRAE, Methodoftesting performance of Labarstay fume h o d s . ANSWASHRAE Standard 110-1985. 12. A S H W , HYACApplir.tions1991,14,1-17. 13. Koenigrberg, J.ChosNoBs 1991.9, 1. 14. Koenigaberg. J.Am.Lob.1984,(10),59-65. 15. Koenigsberg,J.Am Lab. 1982,(8)42-51. 16. Koenigsberg.J.Euduoting the Performanee ofH~ppeFumH d Lk&gn; Am. Bio. Safety Assoc., Od. 1988. 17. Koenigsberg,J.Res Deu. 1985,(2).194-195. 18. Koenigsberg,J.;Seipp, E. Ashme J. 1988,(21,434. 19. Koenigsberg,J.H~ofingiPlpingiAirConditioning, 1980,(21.63-75.
