CORRESPONDENCE
SIR: T h e article “Experimental Determination of Sizing Parameters and \Val1 Losses of Five Source-Test Cascade Impactors” [ES&T, 13, 726 (1979)]displayed calibration data f’cir the Xndersen Mark I11 and four other cascade impactors. I twlieve that certain clarifying comments need to he made so that the data in the article are not misinterpreted. (;r.c>nsr~ os. (;lass F i h r
T h e graphs of collection efficiency vs. \’$ for the various impactors are not directly comparable. Several of the impactors were calilirated using greased surfaces rather than glass f’iher.Although the authors note in the text that greased qurf’aces improve the collection efficiency of an impactor, it is difficult for the reader t o quantify the degree of improvement. Andersen has long recognized that greased substrates improve impactor performance, and we offer stainless steel foils ivith stencils t o facilitate greasing. Had the Andersen Mark I I I heen calihrated using these greased foils, the collection el’l’iciencieswould have been even better than those shown in Figure 4 ( i f the article. However, I helieve that the calibration of an in-stack cascade impactor with greased substrates is not appropriate for the vast majority of source sampling applications. In an extenhive test of greases and coatings used in source testing, all 01’ the greases were found to be unacceptable above a ternperature of 177 “ C (350 O F ) ( I ) . This severely restricts their usefulness in actual field sampling.
.Spticini: of S t a g e C u t p o i n t s As noted in the article, the collection efficiency curve of a given stage of an impactor reaches a maximum of less than 1 ()Or( and then begins to fall off to lower efficiencies for larger particles. In multi-stage impactors, this makes it important to know how various stage efficiency curves relate to one another. By graphically displaying only collection efficiency vs. \ I$. . the relationship between stage efficiencies is omitted. H y properly spacing the stage cutpoints, Andersen has reduced t he effects of large particle bounce-through. For example, stage 5 has it SOrn* cutpoint of approximately 2.5 p m with a 909 efficiency for 4.0-pm particles. T h e collection efficiency of‘ stage 5 for 7-pm particles falls to approximately 70c(.However. stage 4 is approximately 857~efficient on 7-pm particles and stage 3 is approximately 60% efficient on 7-pm particles. Therefore, the probability of a 7-pm particle going past stages 3. 4. and 5 in series is (1 - 0.7)(1- 0.85)(1- 0.6) X 100 = 2?> (see r e f 2 ) . Thank you for the opportunity to present the above clarif i ca t ions.
0013-936X/80/0914-101$01 .OO/O
@ 1980 American Chemical Society
Andersen Samplers, Inc 4215 Wendell Drive Atlanta. Ga. 30336
SIR: 1Ve have read with interest the letter from Mr. Smith and we feel that several comments are necessary. In regard to the use of greased collection plates as contributing to better impactor stage collection characteristics, we wholeheartedly agree and would have gladly calihrated an Andersen Mark I11 in this manner had the stainless steel foils existed a h i l e our calibration program was in progress. Unfortunately, these have been available from Andersen SamInc. for only a relatively short time. are surprised by the pessimism regarding the possibility of using greased collection plates. There is clear evidence that their use can result in more accurate particle size distributions, and our extensive experience with particle sizing a t a variety o f industrial sites would lead us to a conclusion that there is at least a 50%’or better chance that greased collection plates with cascade impactors could be used in a particular sampling program. In regard to the spacing of impactor stage cutpoints, we feel t h a t it must be emphasized that each source being sampled must be analyzed for potential problems from particle bounce and/or reentrainment. Even though only 2%’ of the 7-pm particles might pass t h e fifth stage, there is no certainty that this amount of material might not be equal to or greater than the particle mass passing stage five e c e n in a n ideal situation. l h e distribution of particles as a function of size for each source must therefore be studied. If particle bounce and/or reentrainment is suspected, cascade impactor tests at lower flow rates (more ideal collection characteristics) may reveal whether particle bounce has occurred. This would be evidenced by a skewing of the particle size distribution toward small particles for the higher flow-rate tests. Kenneth M. Cushing Southern Research Institute 2000 Ninth Avenue South Birmingham, Ala. 35205
Volume 14, Number 1, January 1980
101
