Abstract View
Reevaluating the Filter Pressure Drop Model for Modern Respirators
PATRICK O'SHAUGHNESSY, Zoe Harris, Matthew Purdy, The University of Iowa
Abstract Number: 464
Working Group: Control and Mitigation Technology
Abstract
Among several mathematical models developed to estimate respirator pressure drop, a semi-empirical model by Davies developed in 1952 is still commonly used in the scientific literature. In addition to predicting pressure drop, that model is also used to determine an “effective” fiber diameter – a single fiber diameter that best represents the influence of all fiber diameters on pressure drop. Furthermore, the effective diameter is often applied to equations developed to estimate the particle collection efficiency of a filter because it influences the forces of diffusion, interception, impaction and static charge. Having an accurate pressure drop model is therefore important for predicting both pressure drop and effective diameter.
A study was developed to determine the physical characteristics of filter media associated with modern filtering facepiece respirators (FFRs) and to determine whether adjustments to the Davies model are needed to enhance its accuracy when used to predict pressure drop, and, conversely, the effective diameter of modern FFRs. This study focused on the primary filter layer of 10 FFR models. The pressure drop of 3 filter samples of each filter was measured at 4.7 cm/s and 10.5 cm/s; face velocities associated with low and high breathing rates. Filter thickness and solidity were measured for each FFR model. The median of fiber diameters were measured from 100 fibers counted using photos from a scanning electron microscope.
The measured results did not align with the expected curve from the Davies model. An adjustment was made to that model by minimizing the RMSE (root mean square error) between measured and modeled data. The adjusted equation provided a more accurate prediction of fiber diameter relative to measured diameter. Furthermore, the adjusted model reduces the estimate of filter pressure drop 16-fold, if median diameter is known, relative to the Davies model.