Measurements of single fibre efficiencies at critical values of the Stokesnumber

The single fibre collection efficiency is a key parameter to describe the f iltration behaviour of unloaded fibrous filters, but it is and has always b een difficult to achieve data about it experimentally. In a previous invest igation [1] in which single collection efficiencies for a wide range of bot h biological and non-biological particle materials were measured under cert ain operational conditions, values much lower than those expected from mode l calculations were found. With special regard to this effect, in the prese nt study, again under well defined conditions with even more effort and sys tematically excluding the remaining sources of possible errors, single fibr e collection efficiencies in the range of critical stokes number St where b ouncing effects occur were determined. A single fibre fixed in a special handling device was exposed to a well def ined gas-particle stream. With an upstream optical particle counter; the ve locity and the number of particles approaching the fibre was measured in si tu. Together with the particle number collected on the fibre determined via CLSM micrographs, the single fibre collection efficiency was obtained in a very direct way. The results showed that beginning at critical St numbers (St > 0.8), single fibre collection efficiency, after having reached its ma ximum value of 22%, decreases significantly. Comparisons with published the oretical considerations gave remarkable disagreement. Common equations over estimate especially the adhesion efficiency by a factor of up to 6. This le ads to much higher values for the single fibre collection efficiency at St > 1 than found here experimentally.