Offline measurement of honeycomb body filtration efficiency
Apparatus and methods are disclosed which are capable of being used to determine filtration efficiency of a filter body even in a clean state. Methods of determining a filtration efficiency of a filter including forcing an inlet flow comprised of a gas (such as air) flow into the inlet end of the filter at a set flow rate, introducing particles such as smoke particles into the inlet flow, and optically counting the number of particles entering and exiting the filter during a sampling event, such as with diffraction based optical particle counters positioned upstream and downstream of the filter. Preferably the gas flow is a soot-free flow stream which does not load the honeycomb filter body with contaminants that need to be removed or burned out. The filter body can thus remain in an essentially clean state even after testing its filtration efficiency.
1. A method of determining a filtration efficiency of a filter comprised of a plurality of intersecting porous walls, the filter having a longitudinal axis extending from an inlet end to an outlet end, and an outer peripheral portion extending parallel to the longitudinal axis and surrounding the plurality of intersecting porous walls, the method comprising:
sealing the outer peripheral portion to prevent gas flow across an outermost surface of the outer peripheral portion;
forcing an inlet flow comprised of a gas into the inlet end of the filter at a set flow rate;
introducing particles into the inlet flow; and
optically counting the number of particles entering and exiting the filter during a sampling event.
2. The method of claim 1 , wherein the particles introduced into the inlet flow have a particle size distribution in a range from 0.2 μm to 2.0 μm with a median particle size of 0.3 μm.
3. The method of claim 1 , wherein the particles introduced into the inlet flow have a particle size distribution in a range from 0.2 μm to 2.0 μm, with a peak particle size between 0.2 and 0.4 μm.
4. The method of claim 1 , wherein the inlet flow is maintained at a volumetric flow rate which does not vary by more than 10% during the sampling event.
5. The method of claim 4 , wherein the gas flow is maintained at greater than 35 Nm{circumflex over ( )}3/hr. and less than 60 Nm{circumflex over ( )}3/hr.
6. The method of claim 1 , wherein the gas flow is between 15 deg C and 50 deg C.
7. The method of claim 1 , wherein the particles are selected from the group consisting of cigarette smoke particles, atomized particles, liquid particles, nebulized particles, burnt organic powder, and combinations thereof.
8. The method of claim 1 , wherein the gas is filtered upstream of the introducing the particles into the inlet flow to remove contaminants from the gas.
9. The method of claim 8 , wherein the gas is filtered through a HEPA filter with a 99.97% filtration efficiency for a particle size greater than 0.3 μm.
10. The method of claim 1 , wherein the filtration efficiency of the filter is determined from the equation:
μ e =( n u −n d )/ n u ,
where μ e =filter efficiency, n u =particles upstream, and n d =particles downstream.
11. The method of claim 1 , wherein the plurality of intersecting porous walls comprise porous walls having a bulk porosity of 40 to 75% as measured by mercury porosimetry.
12. The method of claim 1 , wherein the plurality of intersecting porous walls comprise porous walls arranged in a honeycomb structure of 100 to 900 cells per square inch.
13. The method of claim 1 , wherein the filter has a soot loading of less than 0.1 grams/cubic inch of filter volume, both before, and immediately at the end of, the sampling event.
14. The method of claim 1 , wherein the filter is a ceramic filter comprising the plurality of intersecting porous walls, the plurality of intersecting porous walls forming a plurality of channels extending from the inlet end to the outlet end, the plurality of channels comprising a plurality of inlet channels and a plurality of outlet channels, at least some of the inlet channels plugged at or near the outlet end and at least some of the outlet channels plugged at or near the inlet end.
15. The method of claim 1 , wherein the sealing comprises inflating a bladder around the peripheral skin.
16. The method of claim 1 , wherein the sealing comprises placing the filter in a duct comprising a liner that seals the peripheral skin.
17. A method of determining a filtration efficiency of a ceramic filter, the method comprising:
positioning the ceramic filter in a duct, the ceramic filter comprising an inlet end and an outlet end, a longitudinal axis extending from the inlet end to the outlet end, a plurality of intersecting porous walls forming a plurality of channels extending from the inlet end to the outlet end, and a peripheral skin layer surrounding the plurality of intersecting porous walls;
sealing the peripheral skin layer to prevent gas flow across an outermost surface of the peripheral skin layer;
forcing an inlet flow comprising a gas through the duct and into the inlet end;
forming particles with a particle generator;
introducing the particles into the inlet flow, at least some of the introduced particles entering the ceramic filter and at least some of the introduced particles that enter the ceramic filter also exiting the ceramic filter; and
during a sampling event, optically counting the introduced particles entering the ceramic filter and the introduced particles exiting the ceramic filter.
18. The method of claim 17 , wherein the sealing comprises inflating a bladder around the peripheral skin layer.
19. The method of claim 17 , wherein the sealing comprises sealing the peripheral skin layer against a liner in the duct.
20. The method of claim 17 , further comprising filtering the gas prior to introducing the particles into the inlet flow.