Abstract:
A filtering apparatus having a plurality of passages made up of juxtaposed longitudinally extending channels. The passages are axially divided by a porous filtering media extending across the channels whereby the channels fluidly communicate through the porous filtering media. A pressure differential is created across the filtering media by restricting flow through an end of each channel, with each restriction being at an opposite end from that of the other channel. As flow restriction rather than outright blocking is used, some flow is possible along each passage directly along the channels even if the channels lose the ability to fluidly communicate through clogging of the filtering media.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to filtering apparatus and more specifically to apparatus for filtering particulate matter from a diesel engine exhaust gas stream.  
       BACKGROUND OF THE INVENTION  
       [0002]     Diesel engine exhaust gases contain “soot” (particulate matter including fine carbon particles). Under some operating conditions, a considerable amount of soot may be present. The soot, which can be seen as a blackish cloud emanating from an exhaust pipe, is objectionable. Accordingly if it can&#39;t be avoided in the combustion process, it needs to be removed.  
         [0003]     One manner of removing the soot is by passing the exhaust gases through a filter. A difficulty encountered with the use of a filter is that the filter may become blocked thus interfering with proper engine function by creating too much “back pressure” (i.e. flow restriction) in the exhaust thereby preventing proper outflow of exhaust gases.  
         [0004]     Accordingly it is an object of the present invention to provide a filtering apparatus for filtering soot from a diesel engine exhaust which allows at least some gas flow even if a filtering medium associated with the filtering apparatus should become completely blocked.  
       SUMMARY OF THE INVENTION  
       [0005]     In very general terms, the structure of the present invention comprises a plurality of passages made up of juxtaposed longitudinally extending channels. The passages are axially divided by a porous filtering media extending across the channels whereby the channels fluidly communicate through the porous filtering media. A pressure differential is created across the filtering media by restricting flow through an end of each channel, with each restriction being at an opposite end from that of the other channel. As flow restriction rather than outright blocking is used, some flow is possible along each passage directly along the channels even if the channels lose the ability to fluidly communicate through clogging of the filtering media.  
         [0006]     In its simplest embodiment, the channels are flow restricted by pinching one end. In more sophisticated embodiments, the channels are tapered lengthwise in opposite directions from the adjacent channels.  
         [0007]     More particularly, a filter apparatus is provided which has first and second longitudinally extending channels facing one another and separated by a sheet of porous filtering medium to define a filtering unit having first and second longitudinally extending passages fluidly communicating through the filtering medium. The first and second passages each have an inlet and an outlet at opposite ends thereof with the inlet and outlet of the first passage being respectively adjacent to the inlet and outlet of the second passage. The inlet of the first passage and the outlet of the second passage are each provided with a non-blocking flow restrictor to cause a pressure differential across the filtering medium in response to pressurized fluid being presented to the inlets of the first and second passages. This promotes fluid flow between the first and second passages through the filtering medium while also permitting some fluid flow directly along the first and second passages from their respective inlets through their respective outlets.  
         [0008]     The inlet and outlet flow restrictors may be selected to limit, to a predetermined amount, the maximum flow restriction caused by the filtering apparatus in the event that the filtering medium becomes blocked.  
         [0009]     A plurality of filtering units may be arranged in a side by side alternating arrangement to define a plurality of the first and second passages with the inlet of each of the first passages adjacent an inlet of one of the second passages on an opposite side of the filtering medium therefrom and the outlet of each of the first passages having an outlet of one of the second passages adjacent thereto on an opposite side of the filtering medium.  
         [0010]     The first and second channels may be formed in metal foil with the filter apparatus comprising alternating sheets of metal foil and the porous filtering medium being in one of stacked and wound configuration.  
         [0011]     The fluid may be diesel exhaust gas with the filtering medium selected to trap particulate matter, including soot, from the diesel exhaust gas.  
         [0012]     The flow restrictor may be a narrowing of the inlet of the first passages and the outlet of the second passages.  
         [0013]     The first and second channels may taper along their respective lengths.  
         [0014]     The first and second channels may have generally parallel sides with the narrowing being a result of the crimping of the inlet of each of the first passages and the outlet of each of the second passages.  
         [0015]     The channels may taper in a 10 to 1 convergence ratio. The inlets of the second passages and the outlets of the first passages may be about 10 mm (0.4 inches) wide, 2 mm (0.08 inches) high and 90 mm (3.5 inches) long.  
         [0016]     The metal foil may be an iron chromium aluminium alloy and the filtering medium may be an open cell foam or a sintered non-woven fibre fleece of iron chromium aluminium alloy.  
         [0017]     The invention further provides a filtering apparatus having first and second sheets formed into a plurality of longitudinally extending tapered channels having opposite inlet and outlet ends, with the channels alternating between narrowing and broadening along respective lengths thereof between the inlet and outlet ends. The first and second sheets may be stacked one above the other with the inlet and outlet ends corresponding and a sheet of porous filtering medium interspersed therebetween to define a stacked set. A longitudinally extending housing extends about the stacked set to define a fluid conduit extending between the inlet and outlet ends.  
         [0018]     The stacked set may be wound, along with a further sheet of the porous filtering medium to separate adjacent layers, in a spiral having an axis parallel to the channels with the housing at least substantially sealing about an outer perimeter thereof to avoid gas leakage therebetween.  
         [0019]     The filtering apparatus may comprise a plurality of the stacked sets arranged one above another in a block with a sheet of the porous filtering medium therebetween. The housing may have a parallelepiped cross-sectional shape closely conforming to an outer perimeter of the block to avoid gas leakage therebetween. The housing may be cylindrical or of rectangular cross-section. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]     Preferred embodiments of the invention are described below with reference to the accompanying drawings in which:  
         [0021]      FIG. 1  is a perspective view, partially cut away illustrating the basic structure and operating principles of a filtering apparatus according to the present invention;  
         [0022]      FIG. 2  is a top plan view of the structure of  FIG. 1  absent the cut away portion;  
         [0023]      FIG. 3  is a section on line  3 - 3  of  FIG. 2 ;  
         [0024]      FIG. 4  is a section on line  4 - 4  of  FIG. 2 ;  
         [0025]      FIG. 5  is an exploded view of the structure of  FIG. 1 ;  
         [0026]      FIG. 6  is an end view of an apparatus according to the present invention having a stacked configuration.  
         [0027]      FIG. 6A  is a partially cut away perspective view of a filtering apparatus similar to that of  FIG. 6  but having rounded inlet and outlet ends;  
         [0028]      FIG. 7  is an end view of an apparatus according to the present invention illustrating a wound configuration;  
         [0029]      FIG. 7A  is a perspective view of an apparatus according to the present invention in a wound configuration within a housing;  
         [0030]      FIG. 8  is a perspective view showing an end of two formed and two porous sheets alternatingly arranged as they might be prior to the stacking of  FIG. 6  or the winding of  FIG. 7 ; and  
         [0031]      FIG. 9  is an exploded view of an alternate embodiment of the present invention similar to  FIG. 5  but illustrating differently shaped formed channels.  
         [0032]      FIG. 10  is a graph of filter efficiency vs. residence time;  
         [0033]      FIG. 11  is a graph showing transit efficiency vs. velocity;  
         [0034]      FIG. 12  is a graph showing efficiency vs. inlet velocity;  
         [0035]      FIG. 13  is a graph showing soot concentration vs. time; and  
         [0036]      FIG. 14  is a graph showing back pressure vs. velocity. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]     A filter apparatus according to the present invention is generally indicated by reference  20  in the accompanying illustrations. The filter apparatus  20  has first and second adjacent and longitudinally extending passages  22  and  24  respectively, each defined by a first channel  26  and a second channel  28  which face each other (i.e. open toward each other). The first and second channels  26  and  28  respectively, are separated by a sheet of porous filtering medium  30  to define the first and second passages  22  and  24 .  
         [0038]     The first and second passages  22  and  24  fluidly communicate through the filtering medium  30 .  
         [0039]     Each first passage  22  has an inlet  32  and an outlet  42  at opposite ends thereof. Each second passage  24  has an inlet  34  and an outlet  44  at opposite ends thereof. The inlets  32  and  34  of the first passage  22  and second passage  24  respectively are adjacent one another. The outlets  42  and  44  of the same first passage  22  and second passage  34  respectively are also adjacent one another.  
         [0040]     Characterized another way, the first and second channels define a passage which is divided by the porous filtering medium  30  into first and second adjacent passages,  22  and  24  respectively which fluidly communicate through the porous filtering medium  30 .  
         [0041]     The filter apparatus  20  would in most applications typically have many first passages  22  and second passages  24 , in which case, as illustrated, a plurality of first channels  26  and second channels  28  would be formed in alternating sequence in a first sheet of formable material  50 . Similarly, a plurality of first channels  26  and second channels  28  would be formed in alternating sequence in a second sheet of formable material  60 . The filtering medium  30  would, as suggested above, comprise a sheet of porous material sandwiched between the first sheet  50  and the second sheet  60 .  
         [0042]     The respective materials of the first sheet  50 , second sheet  60  and filtering medium would be selected based on the service requirements. For filtering apparatus  20  intended for use in diesel engine exhaust filtering applications, the first sheet  50  and the second sheet  60  may be an iron chromium aluminium alloy (FeCrAl). Such material is commonly used for catalyst support substrates and the aluminium serves both as a corrosion inhibitor and a convenient surface on which to anchor a catalyst coating. Both properties are desirable in the present application. Initially experimental parts used nickel foam as the filtering medium but it was found that pure nickel oxidizes quickly towards the upper end of a diesel exhaust temperature range. Better results have been obtained using Inconel or FeCrAl as the material. Acceptable results have been obtained using either an open cell foam or a sintered non-woven fibre fleece. Either material is available in both forms. Specifically, materials have been tested with approximately 90% porosity and a mean pore or void size of about 300 μm. Thickness has varied between 0.2 and 4 mm (0.008 and 0.16 inches).  
         [0043]     In order to cause a pressure differential across the filtering medium  30 , the inlet  32  of the first passage  22  and the outlet  44  of the associated second passage  24  present different flow restriction than the respective inlet  34  of the associated second passage  24  and outlet  42  of the first passage  22 . More particularly, the inlet  32  of the first passage  22  would impose greater flow restriction than the inlet  34  of the associated second passage  24 . The outlet  42  of the first passage  22  would impose less flow restriction than the outlet  44  of the associated second passage  24 .  
         [0044]     The disparity in flow restriction may be achieved by making the inlet  32  of the first passage  22  of smaller cross-sectional area than the inlet  34  of the second passage  24  and the outlet  42  of the first passage  22  of greater cross-sectional area than the outlet  44  of the second passage  24 . This will have the general effect of causing, for a given gas pressure at the respective inlets  32  and  34 , greater pressure in the first passage  22  than in the second passage  24 . Accordingly this will promote gas flow from the first passage through the filtering medium  30  into the second passage  24 . Such is illustrated by arrows  70 .  
         [0045]     Although it is expected that there will be general fluid (gas) flow from the second passage  24  into the first passage  22 , this may not be the case along the entire length of the first passage  22  and second passage  24 . Computer modelling suggests that there may be regional anomalies wherein flow actually takes place in the reverse direction, i.e. from the first passage  22  to the second passage  24 . Such may be a function of operating conditions, channel configuration and the degree of blocking of the filtering medium  30 . Accordingly, the arrows  70  should be considered as illustrative of a general flow direction rather than a specific actual flow direction.  
         [0046]     As a practical matter, filtering occurs upon passing exhaust gases (or other fluid) through the filtering media  30 . Accordingly, regional anomalies may not affect the overall performance of the filter apparatus  20 .  
         [0047]     The first channels  26  and second channels  28  may be tapered as illustrated in  FIGS. 1 through 8 . Alternatively, they may be parallel sided as illustrated in  FIG. 9 . In the former arrangement, flow restriction is inherent in the tapering of the first and second passages  22  and  24  respectively. In the latter arrangement, flow restriction may be achieved by pinching the inlets  32  of the first passages  24  and the outlets  44  of the second passages  22 .  
         [0048]     A benefit to the tapered arrangement illustrated in  FIGS. 1 through 8  is that it provides more “robust” performance characteristics. In other words, the tapered arrangement performs well over a large range of velocities as it provides a more uniform velocity profile through the filtering medium  30  than the “pinched end” version of  FIG. 9 . While it is believed that the pinched design works principally via the restriction of exhaust gas flow at the pinched end, other factors may be relevant to flow in the tapered design. The tapered design causes gas flow to accelerate along the length of the filtering media  30 . The very low angle of incidence also allows for long residence times within the filter media.  
         [0049]     As the arrangement of the present invention doesn&#39;t entirely close either the inlet  32  of the first passages  22  or the outlet  44  of the second passages  24 , it is a non-blocking arrangement. In other words, should the filtering medium  30  become blocked, flow will still occur along the first passages  22  and the second passages  24  directly from their respective inlets  32  and  34  through their respective outlets  42  and  44 . Preferably any flow restriction will be selected to provide a predetermined maximum back pressure. Accordingly, any blockage will still enable operation of an engine associated with the filter apparatus  20  of the present invention.  
         [0050]     In order to cause flow through the first and second passages  22  and  24  respectively, the arrangement of the first passages  22  and second passages  24  are mounted within a housing  80  which fits closely about the passages to minimize gas flow between the housing  80  and the passages.  
         [0051]     The first sheet  50  and the second sheet  60  may be placed on either side of the filtering medium  30  with a further sheet  30  of filtering medium therebelow and then wound about a mandrel into a spinal such as illustrated in  FIGS. 8 and 9 . In such an arrangement the housing  80  may be of cylindrical cross-section as illustrated. Alternatively the sheets  30 ,  50  and  60  may be wound into an elliptical configuration (not illustrated) in which case an elliptical housing  80  would be required. It is also possible to form the structure by simply placing a first sheet  50  over the filtering medium  30  and winding the two sheets together. In effect, the first sheet  50  of one layer acts as the second sheet  60  relative to an overlying layer.  
         [0052]     From a manufacturing standpoint, it is simpler to form the structure using a single corrugated sheet such as the sheet  50  and a single layer of filtering medium  30 . Preferably the filtering medium  30  and the corrugated sheet ( 50  or  60 ) are brazed together using an amorphous nickel brazing foil. As a practical matter, the process of winding will not yield appropriate alignment of all of the first channels  22  and second channels  24  and accordingly all of the flow won&#39;t be exactly as illustrated. In the case of tapered channels there will be some overlap which may result in flow from one channel into two adjacent channels or vice versa, depending on the velocity profile in the affected region. The net result will however provide effective filtering. Should it be desirable to provide good alignment, then a stacked rather than a wound configuration would be preferred.  
         [0053]     As an alternative to winding, the basic structure comprising the first sheet  60  and the second sheet  70  may be replicated in a stack  72  such as illustrated in  FIG. 6 . Such will require a housing having at least a rectangular cross-section. Other stacking arrangements may be feasible such as diamond, hexagonal or octagonal or other cross-sectional shapes. The corresponding housing will typically be of parallelepiped shape, as illustrated in  FIG. 6A , and preferably fitting tightly about the stack to avoid leakage therebetween. Alternatively, a sealing medium may be provided between the housing  80  and the stack  72 . The sealing medium may for example be an inwardly extending flange or a resilient, heat resistive fibrous material (not illustrated). As with the wound arrangement, the configuration may also be achieved by layers comprising a first sheet  50  (or a second sheet  60 ) stacked one above the other with each successive layer being offset so that a narrow opening is above (or below) a wide one.  
         [0054]     As suggested above, better results have been obtained at least in initial testing with tapered rather than parallel sided first and second channels,  22  and  24  respectively. Although the specific dimensions selected will depend on the particular application by way of example early testing has shown favourable results with a convergence ratio of about 10:1 with a height of the first channel  26  and the second channel  28  of about 2 mm (about 0.08 inches), a length of about 90 mm (about 3.5 inches) and a breadth of about 10 mm (about 0.4 inches) for the inlets  34  of the second passages and outlets  42  of the first passages. It is expected that a lower limit for the convergence ratio would be about 4:1 defined by minimal effectiveness. It is expected that the upper limit for the convergence ratio would be about 20:1 in consideration of tooling demands.  
         [0055]     The invention is further illustrated by way of the examples set out below.  
         [0056]     Bench testing and computer modelling were carried out utilizing a wound filtering apparatus having tapered channels with a convergence ratio of 10:1 with the inlets being approximately 10 mm (0.4 inches) by 2 mm (0.08 inches) high and the channels being about 90 mm (3.5 inches) long. An Inconel® foam media was utilized having a pore size of approximately 540 microns with approximately 300 cells per square inch.  
       EXAMPLE 1  
       [0057]      FIG. 10  illustrates filtration efficiency vs. residence time. It can be seen that residence time is a very important factor in efficiency. For example a residence time of 0.001 seconds has an efficiency below 20% whereas increasing the residence time to 0.005 seconds yields an efficiency of greater than 50%.  
       EXAMPLE 2  
       [0058]      FIG. 11  illustrates a relationship between transit efficiency and velocity over an operating range of from 2 to 20 m/s (6.6 to 66 ft/s). The transit efficiency is approximately 0.8 at 10 metres per second (about 33 ft/s) and tapers off at around 13 metres per second (about 43 ft/s) to just under 0.9.  
       EXAMPLE 3  
       [0059]      FIG. 12  is a graph illustrating the filtering efficiency vs. velocity. Unlike transit efficiency, the filtering efficiency diminishes as the velocity increases. No doubt this is a function of the reduced residency time at higher velocity. There may however be other flow factors involved.  
       EXAMPLE 4  
       [0060]      FIG. 13  illustrates measured particulate emissions, baseline and with a device, with measured reduction being about 30%.  
         [0061]      FIG. 14  illustrates the relationship between back pressure and velocity. As one would expect, the back pressure increases significantly with velocity. It is interesting to note that while there is a rather sharp increase from between about 5 metres per second (about 16.4 ft/s) and 7 metres per second (about 23 ft/s), the increase is relatively linear beyond that point up to the limit of the design operating range.  
         [0062]     The above description is intended in an illustrative rather than a restrictive sense. Variations may be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined by the claims set out below.