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LETTERS
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on best the
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1006 Environmental Science & Technology
Dlelectrophoretlc filter fortunately, they did not publish as far Dear Sir: The filter described and il- as we can tell, and the HarvardIAEC report (NYO-1592) mentions none of lustrated, “NRL Dielectrophoretic Filter” (€S&T, June 1975, p 505),is a duplicate them. In two of our papers we have in every respect of the “Electro-Polar made specific reference to the ElectroFilter” developed and built under my di- Polar unit as described in the Harvard/ rection in 1953 by Western Precipitation AEC report. With the advantages of 20 years’ Corp. (now the Western Precipitation hindsight, we have often tried to reconDiv. of Joy Mfg. Co.) in Los Angeles. It was tested by Charles Billings, cile the success of the NRL sutdy with Richard Dennis, and Leslie Silverman at the rejection of the Electro-Polar dethe Air Cleaning Laboratory, School of vice. Our reasoning may be instructive: Public Health, Harvard University, in (1) The full-sized industrial unit (3500 Boston, at the request of the New York cfm) was too large to allow easy and Operations Office of the US. Atomic frequent adjustment of experimental Energy Commission, under Contract No. variables: (2) the electric field (6 kV/cm) AT(30-1)841, the Harvard report being was less than the NRL figure of 11 kV/ issued July 15, 1954. Some 175 or 200 cm; (3) the use of 3 or 4 different aerocopies of the report were sent to vari- sols, of two aerosol concentration inous organizations, including the Naval dexes, and individual tests extending as long as 5’/2 days, almost precluded-in Research Laboratory. The following excerpts are quoted our view-precise and reproducible asfrom the report: “The presence of di- sessment of the interactions of all operelectric fibers within an electric field ating variables: (4) at some points the produces divergencies in field intensity data are difficult to evaluate because such that the regions of highest field the performances of the Electro-Polar strength are concentrated about the fi- and a prefilter were measured as a sinbers. Dust particles, which become po- gle parameter; and (5) some spark dislarized by passing through the electrical charge through the filter was anticifield, migrate toward the regions of pated. We attribute much of our success highest field intensity . . . Higher dust concentrations in the immediate vicinity with the NRL dielectrophoretic filter to of the fibers cause increased agglomer- real-time near-instantaneous measureation and enhance the probability of ment of aerosol concentrations by means of the NRL light-scattering capture by inertial mechanisms.” “Tests with copper sulfate micro- meter. This instrument shows perforspheres (MMD = 1.8 micron) and PF mance augmentation of HP-100 glass 316 and PF 105 Fiberglas mats show fiber filter medium, used at the manuincreases in collection efficiency of 91 facturer’s rated air flow of 20 cm/sec to 97 and 97 to 99 % , respectively, with or 0.4” wg pressure drop, of about 20application of screen voltage. These fold for 0.3 micron DOP aerosol and data correspond to a three-fold reduc- about 30-fold for 1 micron DOP aerosol. tion in penetration or effluent concen- Future development should render the sparking tendency negligible. tration.” Wayne T. Sproull, Consultant 3lendale. Calif. 91208
Filter, response Dear Sir: The NRL team is happy to ?espondto Dr. Sproull’s letter. We beieve that he is right on all counts, although the Electro-Polar filter actually nay have performed somewhat better :han he states. We should add that we Nere not aware of the Electro-Polar reiort until our study was well underway. 3. Sproull and his Western Precipita:ion Corporation colleagues-among :he most distinguished figures in the his:ory of air filtration by electrical meansjeserve more credit for this noteworthy nnovation than they have received. Un-
J. K. Thompson, H. F. Bogardur, R. C. Clark, G. H. Fielding Naval Research Laboratory Washington, D.C. 20375
Book correction Dear Sir: “Ecology in the Bible” ( E S T , June 1975, p 597) was quoted at $3 per copy for this booklet. Neot Kedumim U.S.A. decided that this booklet will not be available for general public sale and it is now given as a gift to each individual who is joining a group of Neot Kedumim Friends during 1975. Yaakov Reshef, director general Neot Kedumim Ltd. New York, N.Y. 10017
