d e t y in the chernikl icrborcrtory
edited by MALCOLM M. RENFREW University of Idaho Moscow, Idaho 83843
Synthesis-Laboratory Fumehoods Easy, Reliable Performance Evaluation and Importance of Sash Design John 6. Adams, Jr. Agricultural Products Department, Stine-Haskell Research Center. E. I. du Pont de Nemours &Company, Newark, DE 19714
Jokn B. Adam, Jr., is a Senlor Research Associate in the Discovery Section of Ihe Research and Development Division of the Agrlculhrral Products Depanment at the Du Pont Company's Stine-Haskell Research Center in Newark. Delaware. He hBs over 20 years of industrial synthesis experience.
We recently completed a period of intensive synthesis-hood experimentation involving changes that affected their performance and usefulness. These experiments included changes in baffling, face opening, face velocity, and sash type. I developed and used a hood-performance test that would quickly determine whether the hood, in its experimental configuration, would perform satisfactorily under real-use conditions. We used our experience to determine the type of hood sash that would enable the maximum productivity with the highest degree of safety in synthesis work. This information led us to the design and operation of our new synthesis hoods. Both good hood design and good hood practices1 are key elements for safe hood operation.
Performance Evaluatlon The hwd-performance test developed is the dry ice test. I t is fast, easy to conduct, cheap, reproducible, noncontaminating of the environment, and a reliable indicator of hood performance. Any laboratory worker can run the test with readily available materials. I t proved reliable for predicting whether a hood will satisfactorily contain chemical fumes under real-use conditions. In my experience, when a hood passed the dry ice test, it never failed to contain chemical fumes during actual synthesis operations. Compared to many other types of work in the hood, we consider synthesis operations to be more demanding on degree af needed hood performance. If the hood passes the test, it should contain chemical fumeswell; if not, it may still contain chemi-
cal fumes, hut not necessarily with the margin of safety desired. The vertically sashed hoods in our newly constructed synthesis facility were performance-tested only with the dry ice test; there have been no fumeescape prohlems since start-up in early 1988.
Sash Deslgn The superiority of vertically sashed aver horizontally sashed hoods far synthesis work became evident through our experiments in working with them hoth. For example, we observed that the horizontally sashed hoods: crowded the workers hy decreasing the work spareat the heneh (from 10 linear ft down to 5 h e a r i t for a 10-ft-long hood). made it difficult or impossible to move chemicals and apparatus from one end of the hood toward the other end without removal of the item from the hood; hand-to-hand passing of chemicals or apparatus behind asash was not a satisfactory solution. Wash-off of a drop of chemical from the gloves you were wearing hy their extension to a running-water tap in the hood, was often precluded hy a sash or two in the way. impeded access to chemical reactions requiring prompt attention, hecause sashes can often he moved in only one direction.. deoendine on how the" are stacked or on the location "fa ro-worker. Olrviuusly, acres$ should nor b c a t tempted if it cannot be done safely. gave full exposure (top-to-bottom) of the worker t o apparatuslchemicals within the hood when the sash was opened in front of the worker by any amount.
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T h e combination vertical/horizontal sash, which was not evaluated, would expectedly provide the same advantages as the vertical sash when used in that mode.
The Dry Ice Test for Evaluatlon of Hood Performance Volume 66
The hood must be tested with the sashes positioned a s they would be during actual use. For example the hood should not he tested only with the hood face half open, then actually used with it fully open. The term "smoke" as used below refers to the mixture of carbon dioxide vapor and water droplets that issues from the mixture of dry ice (solid carbon dioxide) and liquid water in the bowl. The smoke is released omnidirectiondy at low velocity, due to the Large opening of the howl. The smoke temperature is about 21 "C.
Summary of the Test Dry ice is dumped into a howl of water restine on the hood hench. If the resultine rmgnkc exit* the hood, the lest is rankc2 "fail"; if not, the tert id ranked "pass"; if it IS unclear whether smoke is exiting the hued, or wisps of smoke are seen occasionally exiting the hood, the test is ranked "passlfail", indicating the hood performance is a t the edge of failure for this test. A "pass" or "passlfail" indicates that chemical fume escape from the hood will be unlikely during actual synthesis work. Details of the Test; Standard Conditions Two hundred grams of dry ice pellets (about 250 mL, 5/~&. diameter, cylindershaped, contained in abeaker) is dumped all a t once into 1L of water (initial temperature = 43 "C), contained in a 3-qt stainless steel howl (with height of 4 in. and open diameter of in.), placed 6 in. into the hood (as measured from lip of howl to the nearest obtainable ooint of the hood sash). set Leftto-right on'the hood bench top &where you want tu test. If only larger pieces of dry ireare available, itsweight should headjurted through crperimentation t u pnwide a sublimation rare of nhour 1fiO glmin during the firsr minute,which nnrrespondst
