in the Chemical laboratory Edited by NORMAN V. STEERE, School of Public Health, University of Minnesota, Minneapolis, Minn. 55455
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doors, windows, pedestrian traffic, and air supply grills. Design and specification of new hoads should be based on critical analysis of present ventilation needs and an imaginaamwe rowtaut exhaust,. 111 addition to tive rtndysis of future needs, withoot rethe expense of sitah long operation, storage stricting design requirements to past of chenlirnl eont,ainers in hoods will reEdibr's Note: oustam. Some of the questions that oat, strict performance by reducing work space be asked before specifying new hoods This is the second i~,sl.allmc~it of the and by obstructing air flow so that velociinclude the following: article which began on page A000 of t.he ties may be too low to retain toxic maShould hoods incorporate aut,omatic fire August, 1965, issue. The Campm terials within - design rubinets which will require less air flow equipment and to be more convenient and musiderat,ions on which reaction and and which can be exhaosted by a separate safe far short people? comment are sought. p m d h ~ gfurther system 24 hr every day. Are there enough hoods to meet present action. T h r design roosiderations Laboratory hoods are not generally and anticipated needs? were developed by the Laboratory de~iguedlo contain explosions of a high Are hoods specified when other encloSafety Committee of the .4ssoriation order, even thongh laboratory personnel sures would do a bebter job a t less cost? with the i~ttentionof providing a tiniqoe may thinkso. For general use, lnborstor,v Each laboratory hood should generally distillatiou of experience and ideas hoods should be constructed of materials have its own exhaust fan, switch, and pilot which co~lldserve as useful guides for to withstand any fire which may occur light; multiple hoods in a laboratory designing better and safer lahoratories. and should be able to maintain struct,ural should be interconnerted in some manner Commeuts may be d i r e c t 4 (preferintegrity so as to confine a fire iu~t,ili t so that one may not pull air down through ably in triplicate) to the Chairman of another not operating. can he exlingtlished. t,he Labomtory Safet,y Committee, The amount of exhaust sir needed for a Successful performance of a lnhoratory G. W. Mxmiuehky, Division of Safety, hood depends primarily an the velocity of laboratory operation is reduced if the Indiana University, 801 Pi. Jordiln air moving through. This is affected by operation can be moved from a hood to a. Ave., Blomningt,on. Indiana. partial or complete enclosure such ss a. crosscurrent,s, entrance shapes, thermal loading, mechanical action of part,ieles, glove box requiring lit,t,le ventilntion, or exhanst slot design, and obstructions in to a, vacnum- or inert gm box, which rethe hood. Successful performance also quires almost none. depends on corrosion resistance, cleanA glove box can he safely ventilated ability if contaminated, and in some cases wit,h a n air flow of only $0 en ft per min I n order t,o meet. the needs of both safety the collection of eont,aminants, such as for each square foot of open door ares. and economy, laboratory ventilation sysImproved safety, great flexibility, and radioisotopes and pathogens. terns must effectively remove airborne economy are advantages oCfered by glove Ilesign criteria have been established toxic and Rammsble materials aud a t the boxes and other special enclasiwes. New for hoods in radioactive service, including same time exhaust a minimum valrime of laboratories shauld be designed t o provide sarnbben and filters wish gauges to show air. Makeup air should he supplied t,o ventilation connections for mch enclosures. pressure drop across the filter as i t loads laboratories to replace the air removed by Spot ventilation-exhausting contsmiup. exhaust systems so t,hat such systems work nants near their point of origin-can Hoods in which hot concentrated perproperly, and air exhausted from lsboraprevent inhalation hazards from labora n , ~ d e d ards is one of the most important functions of a laboratory building and design of the 112 edc fur Ilghly loiw ~znrrrinlsma). rethrough the freah air supply system. buildine should allow streamlined duetquire f.we \.(.Io,.IIBsranyinp, f n m 123 lo St,oragtge of chemicals and gas cylinders work and shaftways large enough far 200 ft per min. in laboratory hoods is a commoli practice future additions of ductwork. Since crosscurrents outside a hood can which may be both wasteful and danMaterial for hood exhaust ducts should iuterfere with the operation by countereemus. If ohemicals and cvlindem are resist corrosion by chemicals and moisture acting the c ~ p t u r i n gvelocity, i t is imstored in hoods t o gwrdag&t leaks, then portant to minimize air currents from (Continued on page A666) the hoads should operate 24 hr per day tu
XIX. Laboratory Design Considerations-Part II
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Volume 42, Number 9, September 1965
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to which the duets will he exposed and fire shotdd not he able to spread from one duet to another. A minimum duct velucily of 21100 i t per min is recommended for vapors and gases; a velucit,y of 2,50(E10l10 f i per min is suggested t o scavenge rwrdensed moisture. Transport velocilies for pnrtirrtlate ms, terial will range from 3500-4600 i t per min. Good practice wndd he t o determine t,he appropria1.e velovit,y from the Indust,rinl Ventilation i\Iatnval. The hest locntiuu for hood exhaust fans is on the n n f of ihe huildil~gSIB that the
duct system will he w d e r $negative pressure. This practice makes certain that any leaks which may develop will not allow contmninants to escape into the huilding. Consideration should he given to c m t,amination or air pollution problems which could result from dirert. discharge of hood exhaust to the atmosphere. There is an increasing need to provide filters, collectors, condensers, senlhhers, or other aircleaning equipment. A lahoratmy ventilatim system with the best hoods and best transport system is s failure if the exhaust returns t o t,he lahorntory through windows or the fresh air system. Discharge ontlets and discharge velocities should he designed so
I l m exhaust is effectively dispersed nnd nnisancr prevented. If conditioned sir from offices, classrooms, s t ~ dcorridors of laboratory huildings, and other air generally uneontaminat,ed by lahoratory operations is rerirmlated, two s p e d precautions should he observed to stop recirculation in case of emergency. The first consists of aut,omatir equipment, fi~sihlelinkfire dampers r m exhaust louvers, and smoke detection equipment which can shut down the entire rerircnlalion system to prevent spread of smoke and fire. The second preratltion reromrnended ronsiits of a readily aecessihle rontntl for laboratory fresh air supply or re&s~lation systems so t,hnt lahnrat,ory pe~.sorn~el can immediately st,op the s y s ten, in rase of an emergency snrh as a spill or release of toxic, radiaaet,ive, oi. flammable materials. The emergency control iov llre ventilntion system should he similar to n fire alarm pull station, properly Inheled, and connected to the evat,~ation slmm s?.stem. \Iunituring the operat.ional performance of laboratory hoods is desirable. The filat thing to know is whether the hood is operating. 4 pilot light. earl he provided to show that the exhaust fan motor has heen turned on, or a device can be h d t in to a s s u e thal the motor is turning, or s gauge ran he instnlled t,o indicate that the inn l ~ s vanes s uud is drawing air. If filters are part of the exhaust system ur fan vanes are liable lo corrode seriously, s manometer which shows the pressure drop across the filters or in the exhaust duct a m he wed to judge the need for replacmmnt of the filters or the fan. The erlrnusl from lahoratory hoods, enclusures, and spot ventilators should )rot berecirculabrd. Theeontinuallyaecellerating pace of research and development aetivitiossnd technulogical progress make i t irnpossihlc lo predict what chemicals and rcnvtions will he used in lnhrmtories in
