Visualization of aerosol deposition in HEPA filters via nuclear imaging. P.-C. Gervaisa,b, S. Poussierc,d, N. Bardin-Monniera,b, G. Karcherc,d and D. Thomasa,b a

Université de Lorraine, LRGP, UMR 7274, Nancy, 54001, France CNRS, LRGP, UMR 7274, Nancy, 54000, France c Université de Lorraine, NanCycloTEP, Vandœuvre-lès-Nancy, 54500, France d CHU Nancy, Service de Médecine Nucléaire, Vandœuvre-lès-Nancy, 54500, France (E-mail : [email protected]) b

Abstract The combination of Single-Photon Emission Computed Tomography (SPECT) and X-Ray Computed Tomography is used to visualize the behavior of a radioactively marked aerosol in pleated filters, under different operating conditions. We first validate this non-intrusive technique as a mean to comprehensively observe the filtration process in a HEPA filter at the macroscopic scale. We highlight the influence of the filtration velocity on the areas where the particles first tend to settle out in blank filter. We demonstrate that the pleating geometry and the local media permeability acts on the flow and accounts for the position of the deposit. Moreover, we show that the increase in the filtration rate leads to a more homogeneous distribution of the tracer on the entire height of the pleats and hence a more uniform arrangement of the flow.

Keywords. Filtration – Pleated filter – Aerosol deposition – Nuclear imaging. INTRODUCTION Pleated filters are widely used in areas related to air treatment such as air admission, nuclear containment, etc. In addition to the fact that they are easy to use and maintain, they provide excellent purification efficiency. Their lifetime is however conditioned by the increasing drop of pressure that hinders the filtration flow when the media is clogged by aerosol. Developing a thorough model requires full comprehension of all the physical processes involved: flow in porous media, particle deposition and particle/media interactions. Unfortunately, it remains very time-consuming to observe them experimentally due to the large number of parameters and the wide range of operating conditions to take into account. The implementation of an original technique to characterize aerosol deposition on a particular type of filter could be used to experimentally prove the efficiency of a numerical tool (Gervais, 2013).

MATERIALS AND METHODS Our experiments aim for the in situ visualization of the filtration process with no alteration of the 3-D structure 99m of the deposit. We based our approach on the determining of the position of a technetium-99m ( Tc, t1/2=6h) marked aerosol by combining SPECT with X-Ray CT. First, we wanted to highlight the influence of the filtration velocity on the preferential area of initial deposit in the blank filter. To achieve this, we have generated, filtrated and located a monodisperse radioactive aerosol, which traces the flow. In a second step, we focused on the dynamics of the deposition to determine the impact of the clogging level on the flow. Filters were preloaded and the radioactive aerosol was then generated, filtered and located.

Figure 1 : Schematic diagram of the experimental setup.

The experimental set-up (Fig. 1) is composed of a monodisperse aerosol generator (MAG 2010, Palas GmbH) able to produce DEHS based aerosol. In order to generate 99m radioactive condensation cores, the Tc solution is added to the sodium chloride solution in the MAG nebulizer. MAG is set in order to produce submicronic monodisperse aerosol, which traces the flow. The resulting radioactive aerosol is then characterized by an optical particle counter (Welas digital 2100HP, Palas GmbH). The particle size distribution ranges from 0.45 to 0.55 μm in volume.

For the second step of the study, a rotating brush generator (RBG1000, Palas GmbH) is used to produce solid aerosol of alumina. The mass median diameter of alumina particles is centred at 1.0 μm with a geometric standard deviation σg=3.2. The studied HEPA filters are H14 classified by the EN1822/2009. It 2 consists of a 15cm square stainless steel frame; the fibrous medium is made from fiberglass. To avoid the media/media contact and ensure uniform pleat spacing, 6 rigid separators are placed each 2.2 centimetres perpendicular to the pleats direction. The pleating characteristics are as follow: height of pleat h = 27.5 mm and distance between adjacent pleats p = 2.1 mm. Images acquisitions were performed using a conventional double-head γ-camera (Symbia T2 Siemens Medical Healthcare). The spatial dissociation of the filter in Volumes Of Interest (VOI) is performed in the CT images. It is then applied to SPECT images to measure the activity in the filter for post-processing. For each VOI, the mean voxel intensity (MVI) and the resulting standard deviation are calculated. The amount of 99m activity is related to the distribution of Tc and therefore to the deposition area of the traced particles.

RESULTS Figure 2 shows the MVI, relative to the total activity, depending on the position in the pleat depth for three filtration rates. Homogeneous concentrations of the aerosol as well as an incompressible flow are assumed. The initial deposit can be assimilated to the velocity profile. If the media permeability is homogeneous, the flow along the pleat could lead to a homogeneous deposit. However, we qualitatively observe a preferential deposit in the second third of the pleat. The shape of the distributions exhibits that the amount of aerosol is much lower as it is close to the folded areas of the media. The pleating process could induce a lower local permeability in the folded areas. The increase in the filtration rate leads to a more Figure 2 : Tracer distribution as a function of the homogeneous distribution of the tracer, and thus, of the flow. This could be explained by the appearance of position in pleat depth. Velocity influence. small sized eddies in the flow along the pleats. Filters face observations show that increasing the degree of clogging causes a deposit of particles onto the front surface of the filter. Figure 3 shows that, for a fluid velocity of 2.5 cm/s, the level of clogging should not impact the location of the radioactive aerosol. The shape of the three distributions is consistent with the experiments of initial deposit at the nominal flow rate. Note that the MVI is an averaged value on a filter slice, taking all the pleats into account. Each pleat may have a different local level of clogging, as we see on front face observations. This suggests that preferential channels for the radioactive aerosol appear due to the inhomogeneity in the growth of the solid particles cake.

CONCLUSION

Figure 3 : Tracer distribution as a function of the position in pleat depth. Level of clogging influence.

The results indicate that the flow depends on the local permeability of the medium. The larger the local airflow resistance, the smaller the tracer quantity. Moreover, the increase in the filtration rate subsequently leads to a more homogeneous distribution of the tracer on the entire height of the pleats and hence a more uniform arrangement of the flow. Surface observations of loaded filters show that an inhomogeneous growth of the cake induces the formation of preferential channels for the solid aerosol to flow in. Caution should then be taken when carrying out the tomographic analyses because of the competition between the local air resistance and the local efficiency that can prevent from determining the areas where the radioactive aerosol accumulates.

REFERENCES Pierre-Colin Gervais. Etude expérimentale et numérique du colmatage de filtre plissé. PhD thesis, Université de Lorraine, 2013.