Environ. Scl. Technoi. 1983, 17, 418-424
Design and Performance of Miniature Cyclones for Respirable Aerosol Sampling Bernard E. Saltsman* and John M. Hochstrasser Department of Envlronrnental Health, University of Clncinnati, Cincinnati, Ohio 45267
Fifteen miniature cyclone presamplers were studied, each with a different combination of one of three cone lengths, three outlet tube inside diameters, and three outlet tube outside diameters. A single simple generalized equation with one constant for each cyclone accurately represented the effects of flow rate upon cut size for all 15 and also for 3 dual inlet stainless steel cyclones. Another single simple generalized equation with one constant for each cyclone accurately represented pressure drops for all. Effects of the cyclone dimensions on the constants were small. Normalized particle sizes for all 15 plotted as a single straight line from 5% to 80% efficiency when plotted on log probability paper. These equations should be useful for accurate design and prediction of performance of miniature cyclones.
Introduction The health hazards of many dusta strongly depend upon their mass distributions of sizes. Particles larger than 10 pm are deposited mostly in the nose and 5 pm mostly in the bronchii, and only those of 2 pm and smaller can penetrate efficiently into the lower lung and alveoli. When the nonciliated portions of the lung are the critical organ for toxic action, the “respirable” portion of the dust serves as the basis of evaluation of ita health hazard. Standards for allowable “respirable dust” are based upon a standard curve defined by the American Conference of Industrial Hygienists relating particle size and percent removed in a presampler (I). Miniature cyclone presamplers are frequently used to remove and discard the larger particles, because they can effectively collect substantial amounts of dust. Since the performance curve depends upon flow rate and the cyclone design and dimensions, it is important to determine the effects of these variables in order to best approximate the standard curve. In this study the characteristics of 15 cyclones were determined for different flow rates. Three values of cone length, three values of outlet tube inside diameters, and three values of outlet tube wall thicknesses were used in various combinations, and the remaining dimensions were kept constant. Some relevant theoretical and empirical formulas (2-6) were examined to see if they fit the data obtained. The complete details of this and of the experimental work have been reported elsewhere (7). Generalized performance curves were empirically fitted to these data and found also to fit published data (8)for other types of miniature cyclones. These curves clarify and help to predict the effects of the variables upon cyclone performance. Experimental Apparatus Each test cyclone assembly consisted of three basic sections, illustrated in Figure 1. The main body, 3, contained the air inlet and had a diameter of 1.905 cm. Three different cone length bodies, 4, three different cap sections, 2, with different sized outlet tubes and two bushings for the insides of the outlet tubes were constructed. Dimensions are given in Table I. These parts were machined from yellow brass to very close tolerance (e.g., 0.002-in. concentricity), and inside surfaces were polished. The 418
Environ. Scl. Technol., Vol. 17, No. 7, 1983
Table I. Dimensions of Cyclone Assemblies
item
fixed sym- dimns, boP cm
body diam inlet ht inlet width exit tube inside length main body length dust outlet diam cone length exit tube inside diam exit tube outside diam
optional dimns, cmb a
b
C
4.762 0.952
2.381 0.794
1.429 0.635
1.111 0.952
0.794
1.905 0.952 0.381 0.952 2.858 0.714
See Figure 1. Cyclone construction permitted assemblies with three different cone lengths, three different exit tube outside diameters, and three different exit tube inside diameters.
assemblies were mated with O-rings and gaskets in 15 various combinations. The listing of data in Table I1 indicates, after each cyclone number code, letters representing the cone length selection, the outlet tube inside diameter selection, and the outlet tube outside diameter selection. The outlet tube of the cyclone was connected to a 47-mm in-line filter holder (RAC 2353-2-1A) containing a type AA membrane filter (Millipore AAWPO 4700). The bottom dust outlet was connected to glass culture tubes, 25 mm 0.d. X 150 mm long (Kimax 45066). A hole was drilled into a tube cap to allow it to be cemented over the dust outlet boss. The tubes could then be attached and removed with ease. Test aerosol was supplied by a spinning-disk aerosol generator (ERC Model 8330), containing a 5 mCi 85Kr radiation source to neutralize any electrical charges on the particles. It was supplied by a variable-speed infusion pump (Harvard Apparatus Co. 975) with a ferric oxide solution (defined in U.S.Patent 3 480 555). This solution has been shown (9-11) to produce spherical particles with a density of 2.56 g/cm3 and was kindly provided by research scientists at New York University (12). An aerosol chamber was fabricated from a 55-gdon steel drum coated inside with epoxy paint. Aerosol entered through a 5-in. side port near the top and exhausted through a 4-in. side port near the bottom. A 7-in. hole cut in the top was connected with a flexible rubber duct to a Plexiglass leveling plate containing fittings from which the test cyclone was suspended. Pressure drops were measured between taps on the cyclone inlet and outlet by means of Magnehelic gages (Model 2001 for
