Optimisation of a sampling cyclone to the new international sampling convention for respirable dust

J. Aerosol Sci., Vo|. 26. Suppl I, pp. $795-S796, 1995

Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00

Pergamon

OPTIMISATION OF A SAMPLING CYCLONE TO THE NEW INTERNATIONAL SAMPLING CONVENTION FOR RESPIRABLE DUST G Lid6nl, A Gudmundsson2 1National Institute of Occupational Health, Solna, Sweden 2 Lund Institute of Technology, Division of Working Environment, Lund, Sweden

Keywords: Cyclone, Optimisation, Sampling convention, Standardisation INTRODUCTION Recently a new sampling convention for respirable dust has been internationally adopted (1,2). The adoption implies that instruments currently in use have to be modified if they deviate considerably from the new sampling convention. We have optimised the Higgins & Dewell cyclone by varying flowrate (Q) and vortex outlet pipe dimensions (pipe lengt, L, inner and outer diameter of pipe, ID & OD). METHODS A preliminary experiment was done to determine the experimental domain. It was found that the optimum would reside within the ranges Q=1.7-2.51/min, L=3-18 mm, ID=2.0-2.7 mm, OD=3.4-5.0 mm. The experiment was designed for a determination of the re sponse surface of the cyclone penetration's dependence of flowrate and pipe dimensions, i.e. a full factorial design with four factors, including all linear, interaction, and quadratic effects. Vortex outlet pipes with these dimensions were machined. A neutralized bimodal polydisperse polystyrene aerosol with a larger-mode CMAD = 4 - 5 I.tm was used in the experiment. The cyclone penetrations were determined with a TSI Aerodynamic Particle Sizer. For each combination of parameters were five consequetive samples taken, respectively with and without the cyclone attached to the APS inlet. For each combination of vortex pipe and flowrate the penetration values were fitted with the regression model In ((1-P)IP) = W o + W l

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where P is the measured penetration value, d~ the corresponding aerodynamic diameter, and Wo, W1 & W2 are the regression coefficients. For all combinations were good fits obtained. In half of the runs the regression passed the strict test of Lack of Fit at the 5% level. With decreasing inner diameter of the vortex outlet pipe, the curvature of the penetration (W2) became more pronounced. For each parameter of the penetration model (Wo, W1 & W2) a response surface, showing the influence of the four experimental factors (Q, L, ID, & OD), was determined with regression analysis. Non-significant effects were discarded from the subsequent use of the model. The flowrate and the vortex inner diameter were the factors most influencing the cyclone's penetration curve. The dependence ofWo, Wl & W2 on the four experimental factors (Q, L, ID, & OD) was used to model the cyclone's penetration curve over the experimental domain. Each modelled penetration was compared with the sampling convention at 24 aerodynamic diameters corresponding to 24 evenly spaced sampling convention values in the penetration range [0.04-0.96]. The optimal modelled penetration was $795

$796

G. LIDI~N and A. GUDMUNDSSON

determined as that which, over the 24 diameters, gave the lowest root-mean-square (RMS) of the diffe-rence between the modelled penetration curve and the sampling convention. The 25 best modelled penetrations were vitually identical, and all had RMS-values = 0.02. RESULTS AND DISCUSSION Figure 1 shows the modelled optimised penetration curve together with the sampling convention for respirable dust it emulates. The fit is very good, except at the upper and lower ends of the penetration curve. Recently Maynard optimised the Higgins & Dewell cyclone, based on his own results in combination with the results of Lid6n & Kenny, and Bartley et al. (3, 4,5). This optimisation was achieved by varying only the flowrate, which effectively optimises only the ds0 of the penetration curve. Maynard's optimum penetration curve is also plotted in Figure 1. A comparison between the two optimisations shows that by also optimising the vortex outlet pipe, both the slope and the dso of the penetration curve, will agree much better with the sampling convention. In the next step of the optimisation, a vortex pipe with the optimal dimensions will be machined, and the optimum verified. 1,00

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Dae [I.tm] Fig 1. International sampling convention for respirable dust, and two different optimisations of the Higgins & Dewell cYclone. REFERENCES 1. American Conference of Governmental Industrial Hygienists (ACGIH), Threshold Limit Values and Biological Exposure Indices, Cincinnati, USA 1993 2. Comit6 Europ6en de Normalization (CEN), EN481 Workplace Atmospheres - Size Fraction Definitions for Measurement of Airborne Particles, Brussels, Belgium, 1993 3. AD Maynard, J. Aerosol Sci. 24:$457-$458, 1993 4. G Lid6n; LC Kenny, Ann. occup Hyg. 35(5):485-504, 1991 5. DL Bartley; CC Chen; R Song; TJ Fishbach; Am. Ind. Hyg. Assoc. J. 55(11):10361046, 1995