High aspect ratio solid particulate filtering apparatus and method of filtering

A solid particulate filtering apparatus employs a plurality of unidirectional flow type honeycomb filter elements operating in parallel to provide improved particulate loading and useful operating time than was provided by the single, long, narrow type of unidirectional flow filter previously employed. The effective collective diameter of the plurality of elements is greater than their effective length and, preferably, their individual effective diameters approximate or exceed their individual effective lengths to maximize the improved particulate loading capacity and operating time characteristics of the apparatus. The plurality of smaller elements are also subjected to lower thermally induced stresses than would be a single larger filter of the same effective aspect ratio and the described filter housing fosters flow about the outside of the elements further reducing the occurrence of stresses induced by thermal gradients within the filters. The apparatus is particularly useful for diesel engine exhaust and other solid particulate filtering applications involving fluids at elevated temperatures. A method of filtering solid particulates from fluids by use of the apparatus comprises dividing a contaminated-fluid-carrying conduit into an upstream portion of a downstream portion with a partition, then positioning the plurality of filter elements through the partition, and thereafter passing the fluid through the conduit and filters.

BACKGROUND OF THE INVENTION 
It is widely known that honeycomb structures formed from ceramic or other 
porous materials can be used to filter solid particulates and larger 
particles from fluids passed therethrough including carbonaceous 
particulates from the exhaust gases of diesel engines. There are two basic 
types of honeycomb solid particulate honeycomb filters: unidirectional 
flow and cross-flow. This invention relates to the former. 
A unidirectional flow type honeycomb filter is formed by a matrix of thin 
interconnected porous walls defining an inlet and an outlet end face on 
opposing outer surfaces of the filter and a multiplicity of hollow cells 
extending through the filter between the two end faces. An inlet group of 
cells is formed by closing the open ends of some of the cells near the 
outlet end face. An outlet group is similarly formed by closing the open 
ends of other cells near the inlet end face. A porous outer wall is 
typically provided around the matrix and between the end faces. Fluid 
enters the filter primarily at the inlet end face through the inlet cells 
but may also enter through the outer wall, if porous. The thin walls of 
the matrix are provided with internal interconnected open porosity of a 
volume and size sufficient to enable the fluid to flow at least across 
their narrow dimensions and, if desired, through their longer dimensions 
between adjoining and/or neighboring cells while preventing at least a 
significant portion of the particulates and larger particles from flowing 
in any direction across and through the thin walls. Trapped particulates 
are deposited on and within the thin wall surfaces forming the inlet 
cells. Diesel exhaust particulate filters (hereinafter referred to as 
"DPF's"), as well as molten metal and heat recovery wheel filters of the 
unidirectional flow type are described in a pending application Ser. No. 
165,646, filed July 3, 1980 and assigned to the assignee hereof, which is 
incorporated by reference herein. 
Unidirectional flow type honeycomb filters are preferred for diesel 
particulate filter application because they are relatively straightforward 
to manufacture and can be mounted in a housing and inserted into an 
exhaust system like a muffler or catalytic converter. Prior practice has 
been to maximize the cross-sectional diameter of a DPF transverse to its 
cells to the extent allowed by vertical and lateral vehicular clearances 
and then to extend the length of the filter to provide the volume required 
to accomplish the desired filtration of the exhaust gases. This has 
typically resulted in DPF's having lengths greater, often many times 
greater, than their diameters. 
It is desirable to maximize the useful operating time and/or particulate 
loading capacity of DPF's to minimize the cost and inconvenience 
associated with their replacement and/or regeneration. Both 
characteristics are effectively limited, among other factors, by the back 
pressure generated or flow rate allowed by the filter. A filter has an 
initial pressure drop (i.e., the difference in pressure between the 
contaminated fluid upstream and filter fluid downstream caused by the 
presence of the filter therebetween) which increases during use with the 
entrapment of particles and particulates in and on the thin walls forming 
the filter's inlet cells. Similarly the filter also has an initial flow 
rate which decreases with particulate build-up. Depending upon the 
application, either may control when the filter must be replaced or 
regenerated. The useful operating life of a DPF is generally controlled by 
the maximum back pressure which can be sustained by the diesel engine with 
which it is used. 
DEFINITIONS 
As DPF's and other such unidirectional flow type honeycomb filters may be 
provided in various configurations reference will be made hereinafter to 
their "effective diameter" and "effective length". The former is the 
diameter of a circular area equal to the cross-sectional area of a filter 
transverse to the central longitudinal axes of its cells. The latter is 
that length which, when multiplied by the transverse cross-sectional area, 
equals the volume of the filter. The "effective diameter" of a plurality 
of filters (i.e. plurality of individual filter elements) acting in 
parallel is the diameter of a circular area equal to the sum of the 
transverse cross-sectional areas of the plurality of filters. The 
"effective length" is again that length which, when multiplied by the 
collective transverse cross-sectional area of the plurality of filter, 
equals the collective volume of the filters. By comparison, a plurality of 
filters operating in series have an effective diameter which provides a 
circular area equal to the average transverse cross-sectional area of the 
filters in series and an effective length which, when multiplied by the 
average transverse cross-sectional area, equals the collective volume of 
the filters. The aspect ratio of the filter(s) is the ratio of their 
effective diameter to their effective length. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide a novel solid particulate 
filter apparatus having improved particulate capacity. 
It is yet another object of the invention to provide a novel solid 
particulate filter apparatus having increased operating time. 
It is yet another object of the invention to provide a novel solid 
filtering apparatus having reduced rate of pressure drop buildup and/or 
flow rate reduction during use. 
It is yet another object of the invention to provide a novel diesel 
particulate filtering apparatus. 
It is yet another object of the invention to minimize the magnitude of 
radial thermal gradients in diesel particulate filtering apparatus 
employing increased aspect ratio filtration. 
SUMMARY OF THE INVENTION 
I have discovered that pressure drop buildup and/or flow rate reduction 
during the operation of a unidirectional flow type honeycomb filter is 
directly related, inter alia, to the reciprocal of the aspect ratio (i.e. 
to the ratio of effective length to effective diameter) of the filter and 
that by increasing the aspect ratio (i.e. effective diameter to effective 
length ratio) for any filter of a given composition, volume and thin wall 
surface area, the rate of pressure drop buildup or flow rate reduction 
during use is correspondingly reduced and, more importantly, the 
particulate loading capacity and useful operating time of the filter to 
reach a given maximum pressure drop or minimum flow rate is increased. 
Accordingly, my invention is a solid particulate filtration apparatus 
employing a plurality of unidirectional flow type filter elements in 
parallel so as to achieve an increased effective diameter to effective 
length ratio. The invention is described in terms of a diesel engine 
exhaust filtering apparatus which provides a reduced buildup of pressure 
and improved particulate capacity and useful operating time performance. 
Because thermal stresses are directly related to thermal gradients and the 
likelihood of radial thermal gradients increases with increased filter 
diameter, the use of a plurality of elements in the filtration of diesel 
exhaust and other fluids at elevated temperatures instead of a single 
filter of equivalent aspect ratio also reduces the thermal stresses to 
which each element is exposed. Embodiments are described each having a 
plurality of filter elements mounted in a housing divided by a partition 
into an inlet and outlet chamber. Each filter element is positioned across 
the partition with its inlet end face exposed to the contaminated fluid 
entering the inlet chamber and its outlet end face in communication with 
the outlet chamber so that the plurality of elements act in parallel to 
filter the fluid. 
According to yet another important feature of the invention, the 
particulate capacity and useful operating life of the apparatus are 
further improved by providing a plurality of filter elements each having 
an effective diameter greater than its effective length.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1a, 2 and 3 depict a preferred embodiment of the invention, a diesel 
engine exhaust gas solid particulate filtering apparatus, comprising a 
housing 10 formed from a pair of shells 12 and 14 sealably mating with one 
another at flanges 16. The depicted housing 10 is mounted beneath the 
chassis 2 of a vehicle 1 and across an exhaust pipe 5 or conduit leading 
from the manifold 4 of a diesel engine 3 in the vehicle 1 like a muffler 
or catalytic converter. The shells 12 and 14 form an inlet 18 and outlet 
24 to the housing. The inlet 18 is connected by suitable means such as 
welding or clamps to the end of the pipe 5 opposite the manifold 4 which 
carries exhaust gases, indicated generally by the arrow 22, generated by 
the diesel engine 3 to the housing 10. The outlet 24 also mates by similar 
suitable means to a downstream portion of the exhaust system which carries 
the filtered exhaust gas, represented by an arrow 28, from the housing 10 
to disposal into the atmosphere and comprising a connecting pipe 6, a tail 
pipe 8 open to the atmosphere and a muffler 7 connected therebetween. 
Catalytic conversion may be provided by filters 30 within the housing 10 
or by means of a separate converter element installed across the exhaust 
gas stream. The shells 12 and 14 may be fixedly joined (e.g. welded, 
brazed or riveted) at the flanges 16 for a disposable apparatus 10 or 
joined in a disassemblable manner with nuts and bolts or other removable 
fasteners (neither depicted) for disassembly of the housing 10 and 
replacement or regeneration of its contained filter(s). 
FIGS. 2 and 3 are diagrammatic overhead and laterally sectioned views, 
respectively, of the housing 10 revealing a plurality of unidirectional 
flow type honeycomb filter elements 30 mounted within. Each filter element 
30 is formed by a matrix of inter-connected thin walls 32 defining an 
inlet end face 34 and outlet end face 36 at opposite ends of the element 
30 and a multiplicity of hollow passages or cells 38 extending in a 
substantially mutually parallel fashion longitudinally through each 
element 30 between its end faces 34 and 36. One or both ends of each of 
the cells 38 are closed near the inlet or outlet end face 34 or 36, as is 
indicated by shading in all three figures. A group of the cells 38 in each 
element 30 are inlet cells 38a which are open at the inlet end face 34 and 
closed near the outlet end face 36. Another group of the cells 38 of each 
element 30 are outlet cells 38b which are closed near the inlet end face 
34 and opened at the outlet end face 36 of the element 30. A smooth outer 
wall 39 surrounds the thin walls 32 and extends between the end faces 34 
and 36. The elements 30 are of conventional construction and may have any 
cellular geometries, cellular arrangements, and numbers and/or 
arrangements of inlet and outlet cells as desired. Pending patent 
applications Ser. No. 165,646, filed July 3, 1980 and Ser. Nos. 350,998, 
350,995, 350,994, 351,126, 350,993, 350,997 and 350,996 filed Feb. 22, 
1982, assigned to the assignee hereof and incorporated by reference 
herein, each describe and claim various unidirectional flow type honeycomb 
filters which may be used in practicing the subject invention. The filter 
elements 30 are fabricated in a conventional fashion from porous materials 
and preferably monolithically with a porous outer wall interconnected with 
the thin walls from extruded, sintered cordierite ceramic materials having 
cordierite cement plugs to close the cell ends, as is described in the 
aforesaid application Ser. No. 165,646. Such filters are chemically and 
mechanically stable with combustion exhausts and most other fluids to 
temperatures approaching 1200.degree. C. and thus may be used to filter 
fluids at elevated temperatures. Still other combinations of cordierite 
materials which can be used to fabricate filters stable to temperatures 
above 1300.degree. C. are described in yet another pending application 
Ser. No. 295,612, filed Aug. 24, 1981, assigned to the assignee of this 
application and incorporated by reference. 
For diesel particulate filtration, the thin walls 32 of the filter are 
desirably less than about 0.06 in. (about 1.5 mm.) to minimize filter 
volume and thermal shock problems. The cordierite thin walls can be formed 
in thicknesses as narrow as about 0.002 in. (about 0.05 mm.) and are 
preferably formed in thicknesses of between about 0.010 and 0.030 in. 
(0.25 and 0.76 mm.). The volumetric open porosity is at least 25% of the 
bulk volume of the thin walls 32 and preferably between about 40 and 70% 
to minimize hydraulic resistance to the fluid flow. The open porosity may 
be formed by pores having mean diameters of between about 1 and 60 
micrometers (i.e. microns) although a range of 10 to 50 microns is 
preferred, depending upon the application. Useful transverse 
cross-sectional cellular densities for diesel particulate filters may 
range from between about 10 and 300 cells/in..sup.2 (about 1. and 46 
cells/cm..sup.2) with densities of between about 100 and 200 
cells/in..sup.2 (about 15.5 and 31 cells/cm..sup.2) preferred with the 
indicated preferred open porosity and wall thicknesses. 
The housing 10 is further provided with a partition 40 through which the 
elements 30 are mounted. The partition 40 separates the interior of the 
housing 10 into an inlet chamber portion 42 and an outlet chamber portion 
44 and prevents the contaminated fluid 22 entering the inlet chamber 
portion 42 from bypassing the filter elements 30. The inlet chamber 42 
also serves as a means for directing the horizontal flow of the incoming 
contaminated fluid 22 (horizontal in FIGS. 1 and 3) downward into the 
elements 30 as indicated by arrows 22a. The outlet chamber 44 serves a 
similar purpose to divert the flow of the filter fluid exiting the outlet 
end face 36 of the elements 30 vertically (this flow being indicated by 
arrows 28) to the horizontal direction and through the outlet 24 of the 
housing 10. The elements 30 are mounted through the partition 40 with the 
inlet end face 34 of each exposed in the inlet chamber 42 in order that 
the elements 30 operate in parallel to filter the particulate contaminated 
gas 22. The outlet end faces 36 are each similarly exposed in the outlet 
chamber 44 so that fluid is not repeatedly filtered within the housing 10. 
Conventional methods and materials used in the construction of mufflers, 
catalytic converters, other diesel particulate filters and exhaust systems 
generally are used in fabricating and joining the upper and lower shells 
12 and 14 and partition 40 and in fixing the elements 30 in the partition 
40 and closing or otherwise sealing any space between the elements 30 and 
partition 40 to prevent blowby of the particulate contaminated gas 22. It 
is envisioned that the shells 12 and 14 and partition 40 will be formed of 
metal and the partition 40 affixed to the lower shell 12 by welds 42 or 
brazing or joiners (not depicted) and form a compression seal against the 
upper shell 14 to complete the division of the housing into the two 
chambers 42 and 44. 
The effective cross-sectional area of the plurality of elements 30 is 
represented in FIG. 2 by the circle 46, in phantom, having an effective 
diameter D. The circle 46 is equal in area to the sum of the 
cross-sectional areas of the plurality of elements 30 transverse to the 
central longitudinal axes 48 of their cells (see FIG. 3) as the elements 
30 act in parallel to filter the fluid 22. The effective length L of the 
plurality of elements 30 is indicated in phantom in FIG. 3 and is equal to 
the length 1 of the individual elements 30, which are equal in this 
embodiment but need not be so. The effective length L of the plurality of 
elements 30 is less than their effective diameter D. The effective length 
1 of each element 30 may also be approximately equal to (or less than) its 
effective diameter d to maximize the improved particulate capacity and 
operating time characteristics provided by this this apparatus, although a 
range of 0.4.ltoreq.1/d.ltoreq.2.0 is acceptable and 
0.7.ltoreq.1/d.ltoreq.1.4 is preferred. A significant benefit of the 
invention is that the chambers 42 and 44 allow the hot exhaust gases to 
circulate around the outer wall 39 of each element 30 thus minimizing the 
occurrence of either thermal gradients in each element. This factor, 
combined with the use of several small filter elements 30 in parallel, 
greatly reduce the likelihood of filter damage due to thermal shock and 
extruded high temperature operation while providing a high aspect ratio 
filtration system. 
The apparatus of FIGS. 1a, 2 and 3 is envisioned to be mounted as is 
depicted in FIG. 1 on the underside of the vehicle 1 in which a diesel 
engine 3 is installed with the filters 30 arrayed in a horizontal plane 
with their greater dimension d parallel to the plane to minimize the 
height of the housing 10 beneath the vehicle. The elements 30 may be 
varied in number, size, and shape and the arrangement of the elements 30 
may be varied from that depicted so as to conform to the housing 10 to any 
spatial limitations which may be imposed. Moreover, the inlet 18 and 
outlet 24 need not be essentially parallel to one another and 
perpendicular to the central longitudinal axes of the cells 38 of the 
elements 30, as depicted in FIGS. 1a, 2 and 3, if other arrangements are 
more useful in installing the apparatus. 
FIG. 4 depicts diagrammatically a second exemplary apparatus 50 for 
mounting directly beneath a diesel engine. Exhaust gas represented by 
arrow 52 flows downward from an upstream pipe 5', in phantom, leading from 
the engine through an inlet 54 and passes through a plurality of 
unidirectional flow type filter elements 56, indicated in phantom, 
positioned through a partition 57 (also depicted in phantom) dividing the 
housing 50 into an inlet chamber 58 and outlet chamber 60. The filtered 
exhaust gas, represented by the arrow 62 passes from the housing 50 
through an outlet 64, into a downstream pipe 6', in phantom, and through 
the remainder of the exhaust system. The outlet chamber 60 acts to convert 
the momentum of the exhaust gas from the vertical to horizontal 
directions. 
While several embodiments of the invention have been described and other 
embodiments and modifications thereto have been suggested, it should be 
understood that other modifications could be made in the structure, 
composition and/or arrangements of the described elements without 
departing from the scope of the invention which is more fully defined in 
the following claims.