(239e) Influence of Leaks in Surface Filters on Particulate Emissions

Compliance with severe limit values of dust emissions is a main characteristic of surface filters. This characteristic is due to the high particle collection efficiency of surface filters. Especially the collection efficiency of particles in the lower micrometer range has to be mentioned. So it is possible to decrease dust concentrations from some 100 g/m3 to only few mg/m3 or less. Beside regular operation it is necessary to consider phenomena such as a ?pinhole? bypass through leaks in surface filters to ensure the above mentioned compliance with the limit values at all times. Pinholes are small holes which may be formed during cleaning or during filtration. These leaks can occur as a result of mechanical stress, abrasive particles or pyrophoric particles. They are of particular interest when the ?straight through? mechanism does not appear any more. The ?straight through? mechanism can be observed at the beginning of a filtration process, when the new, unconditioned filter is penetrated by air-borne particles. The effect of this mechanism is a clean gas concentration peak. While regenerating/removing the filter cake in order to reduce the resistance to flow resp. the pressure drop particles can also penetrate the filter media. But in this case the particles responsible for the ?seepage? mechanism were previously collected by the filter. Experimental research has been carried out to observe and understand the ?pinhole? bypass and the performance of pinholes over filtration time. The first systematic tests have been run with thermally bonded nonwovens (Polyester). Therefore one defined pinhole has been generated in each test filter. The apparatus used for tests corresponds in every way to the test device Type 1 described in the Guideline VDI 3926. To work out the influence of different filtration conditions the parameters pinhole diameter, filter face velocity and dust cake thickness were varied. The characterisation of the emissions has been done by analytical filters (integral dust concentration over the total test phase in mg/m3). Additionally, optical measurements with a particle counter and sizer have been made. Thus, time-dependent number concentrations and size distributions of the penetrating particles could be obtained. The results can be explained by formulas usually used to calculate volumetric flow rates of orifice gauges. By adapting these formulas to the filtration process and stating assumptions the collection efficiency can be calculated and shows good agreement with experimental data of integral clean gas side concentrations. The calculation leads to the conclusions that bigger pinholes decrease the collection efficiency and higher filter face velocities increase the collection efficiency of pinholed filters. In contrast, the data of similar experiments with sintered metal filters can't be explained by formulas because of the transient pinhole performance. Due to three metallic wire-cloths with quadratic apertures (aperture width = 1 cm) for stability reasons at the back of the filter layer, the pinholes tend to close. The pinhole-passing particles are separated by the wires. The pinholes clog with filtration time by the effect of inertial impaction at the wires. A distinct closing was observed at lower filter face velocities. This observation of potential clogging is in accordance with the filter theory of intact filter media: Higher filter face velocities decrease the collection efficiency.