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Fibrous Filters for Air Sterilization. Small Seale Equipment.. Role of Turbine Impellers.. Testing of Filters,.
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Control of Oxygen Uptake.. Aeration Effectiveness.. Microbiologieal Transformalion OF Sferoids
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ELMER L. GADEN, Jr., and ARTHUR E. HUMPHREY’ Department of Chemical Engineering, Columbia University, New York 27, N. Y.
Fibrous Filters for Air Sterilization Design Procedure Proper design of sterilizing filters makes possible considerable reduction in size, accompanied by greater rdiability ,To design a sterilizing filter, information i s needed, from either observation or specification, on concentration of organisms in the inlet air (maximum expected value, if estimated), operating period between filter sterilizations, and allowable penetration probability (the acceptable chance that a single entering organism will penetrate during filter life). With these points established, fibrous media may be tested in the laboratory to establish the relationship among penetration (efficiency), pressure drop, and air velocity. A number of possible filter designs may be proposed and the most economical selected. Filter design must avoid channeling (by providing maximum bed uniformity and effective seal between bed and wall) and bed movement (by constructing the bed to prevent shifting and settling and to minimize fragmentation).
CERTAIN
chemical process applications require substantial amounts of air which is sterile, or nearly SO. T h e aeration of fermentation broths is the 1 Present address, School of Chemical Engineering, University of Pennsylvania, Philadelphia, Pa.
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most important now, but others are under development which may also need large volumes of pure and sterile air. M a n y schemes for sterilizing air have been proposed and some have been employed commercially with success (7, 74). At present, however, the most common method is filtration through beds of fibers (cotton, glass, and steel wool), granular solids (carbons), or specially prepared filter media. Despite their widespread use, the design of fibrous filters is still largely a matter of judicious guesswork tempered with experience. About the only generalization that can be offered with respect to current sterilizing filters is that they are usually greatly “overdesigned” from the standpoint of size, yet are unreliable in performance. A major difficulty has been the great concern over bed depth as the focal point of filter design. Little or no attention has been given to the manner in which the filter bed is made u p and maintained during operation. Yet experience indicates that these factors are a t least as important as the physical dimensions of the filter in determining its performance. In recent years a great deal has been learned about the mechanics of filter action and the roles of design and operat-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing variables, particularly with respect to aerosols (3, 76). A number of studies (4, 8, 70, 75) with bacterial aerosols have given information applicable to design of sterilizing filters. In one of these (8) the effect of air stream velocity on the efficiency of specially compounded glass-fiber beds in removing bacterial spores (B.subtilis) from air was examined. From the data obtained i n these experiments and the generalizations already available in the literature, a general procedure for the design of fibrous, airsterilizing filters has been established.
Principles of Air Filter Design Any filter design represents a compromise between two somewhat incompatible criteria : high efficiency in removing organisms and low pressure drop ( 8 ) . Practical considerations add to these a third: reasonable service life a t the specified efficiency. I t is, of course, impossible to guarantee complete removal of organisms from a n air stream with any filter bed whose interstices are larger than the organisms to be removed. Only a statistical prediction of penetration can be made and no design can be better than the “calculated risk” involved in arbitrary
