Although there are numerous analytical and empirical equations available in literature to predict the pressure drop of filters with unimodal fiber diameter distribution,, there are practically no simple relations for filters composed of fibers with a bimodal fiber diameter distribution. This work builds up on the pioneering work of Brown and Thorpe on finding an equivalent unimodal filter geometry that can be used to calculate the pressure drop of bimodal filters. In agreement with Brown & Thorpe we concluded that unimodal filters obtained by area-weighted averaging of the fiber diameters, d_(eq)~((2)), can be used to estimate the pressure drop of bimodal filters. However, the accuracy of this estimation rapidly declines as ratio of fiber diameters R_(cf) increases or number fractions of coarse fibers F_c decreases. To this end, we proposed a modification to the above equivalent diameter in terms of d_(eq)~((2)) ,R_(cf), and F_c. Our modified unimodal equivalent fiber diameter, d_(eq), is shown to be accurate for a wide range of R_(cf) and F_c when used for predicting the pressure drop of bimodal filters.
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