US3054664A

US3054664A - Apparatus for the catalytic conversion of vehicular exhaust gases

Info

Publication number: US3054664A
Application number: US854726A
Authority: US
Inventors: Frank V Purse
Original assignee: Universal Oil Products Co
Current assignee: Universal Oil Products Co
Priority date: 1959-11-23
Filing date: 1959-11-23
Publication date: 1962-09-18
Anticipated expiration: 1979-09-18
Also published as: DE1293792B; US3068079A; NL111996C; GB952757A

Description

Sept 18, 1962 F v PURSE 3,054,664

APPARATUS FOR 'THE OATALYTIC CONVERSION OF VEHICULAR EXHAUST GASES Filed Nov. 23, 1959 2 Sheets-Sheet 1 A TTORNEYS Sept. 18, 1962 F. v. PURSE 3,054,664

APPARATUS FOR THE CATALYTIC CONVERSION OF VEHICULAR EXHAUST GASES Filed Nov. 25, 1959 2 Sheets-Sheet 2 INVENTOR- Fran/r V. Purse A TTO/P/VEYS Figure United States Patent O 3,654,664 APPARATUS FR THE CATALYTHC CONVERSIN F VEEHCUIAR EXHAUST GASES Frank V. Purse, Skokie, lll., assigner to Universal Oil Products Company, Des Plaines, lll., a corporation of Delaware Filed Nov. 23, 1959, Ser. No. 854,726 13 Claims. (Cl. 23a- 288) This invention relates to catalytic apparatus and in particular to a combination catalytic converter and acoustic muflier useful in the conversion and/ or purification of the exhaust gases of internal combustion engines, especially those employed in automobiles, trucks, and other vehicles.

The desirability of removing or converting to harmless form the noxious components of vehicular exhaust gases has been well established. The principal offending constituents of such gases include the several oxides of nitrogen, carbon monoxide, and unburned or partially burned hydrocarbons; these materials, upon being expelled into the atmosphere, are known to combine under the action of sunlight to produce a wide variety of noxious, irritating compounds popularly and generically referred to as smog Of the many solutions proposed for solving the problem of atmospheric contamination, the use of a catalytic device inserted in the engine exhaust line is one of the most effective. In this method, the hot gases issuing from the motor exhaust manifold are mixed with a quantity of secondary or combustion air and the resulting mixture is passed through a fixed catalyst bed which effects more or less complete oxidation of the carbon monoxide and unburned hydrocarbons present in the exhaust gases, converting them to carbon dioxide and Water. Suitable catalytic apparatus therefor must -be constructed to meet a number of unique requirements. First, it is essential that the converter impose a low pressure drop on the exhaust system since engine efficiency falls off rapidly with increasing back pressure in the exhaust manifold. Since conventional non-catalytic mufllers suffer a pressure drop of the order of 3 to 7 p.s.i. at maximum motor speed, it is desirable that a catalytic converter have a pressure drop at least as low and preferably lower. Second, there is the problem of structural failure of the converter induced by large thermal gradients therein. High temperatures, substantially in excess of those encountered in conventional muiers, are produced as a result of exothermic oxidation reactions taking place within and around the catalyst bed. Depending upon the particular catalyst employed and the operation of the motor vehicle, i.e., whether the vehicle is being operated under conditions of idle, accelerate, cruise, or decelerate, converter temperatures may run as high as 1200 to 1800 F. Because of weight and size limitations necessarily imposed upon a vehicular muflier, the catalytic converter must be fabricated of relatively light gauge sheet metal, which, under these high temperature conditions, is subjected to severe thermal gradients. Muiers of conventional design, which have been merely modified to contain a bed of catalyst, and even when constructed of stainless steel, have been rapidly and virtually completely destroyed by deformation, split seams, etc. as a result of uneven thermal expansion. A practical catalytic converter should therefore be designed to provide for equalization of thermal stresses whereby differential stresses within its walls are kept well Within the elastic limit of the material of construction. Third, the converter internals should be arranged so that uniform distribution of exhaust gas flow through the catalyst bed is maintained in order to achieve maximum catalyst life and maximum conversion. Fourth, the converter should be capable of functioning not only as a catalyst holder but also as an 3,54,664 Patented Sept. 18, 1962 acoustic muffler whereby to obviate the need for an auxiliary muffler. Lastly, it is important that the physical size of the converter be minimized, yet provide a maximum catalyst volume, so that the apparatus may be installed in place of conventional vehicular mufflers without requiring modification of existing or projected designs of automobile frames or bodies.

It is the principal object of this invention to provide a combination catalytic converter and acoustic muffler, hereinafter referred to as a catalytic mufiier which meets all of the aforesaid requirements.

One embodiment of this invention relates to a catalytic muffler comprising in combination an enclosed chamber defined by a longitudinal wall having an interior whose transverse cross section is a regular plane figure having more than four axes of symmetry co-planar therewith, a pair of perforated baffle members spaced along the longitudinal axis of said chamber, said baffle members being connected to the inner surface of said chamber and dividing said chamber into two manifold spaces separated by an intermediate space adapted to contain a mass of solid packing material, an inlet conduit and an outlet conduit entering said chamber and extending through at least a portion of said intermediate space and separately communicating with respective manifold spaces.

In a specific embodiment thereof, the present invention provides a catalytic muffler comprising in combination an enclosed chamber defined by a cylindrical diametrically elongated side Wall and end closure members therefor, a pair of axially spaced perforated discs connected to the interior peripheral surface of said side Wall and dividing said chamber into two manifold spaces separated by an intermediate space adapted to contain a mass of solid packing material, each of said discs having an enlarged opening therethrough the center of which is radially offset from the center of the disc and the centers of said enlarged openings being circumferentially spaced approximately apart with respect to the common central axis of said discs, an inlet conduit and an outlet conduit entering said chamber through said side wall, said conduits extending radially through a portion of said intermediate space and therein connecting with said enlarged openings whereby sai-d inlet and outlet conduits separately communicate with respective manifold spaces.

A basic feature which is common to all embodiments of the invention is the provision of an enclosed catalyst chamber whose transverse cross-section is substantially symmetrical with respect to its longitudinal axis and wherein the gas inlet and outlet points are substantially centrally disposed with respect both to the chamber itself and to the catalyst bed. 'I'he longitudinal axis of the chamber is defined as the central axis thereof which is parallel to the average direction of ow through the catalyst bed. In accordance with the requirements of symmetry, the transverse cross-section of the chamber may assume the form of a regular polygon of n sides, i.e., a pentagon, hexagon, Octagon, etc.; or, it may be circular which is preferred because of absolute symmetry and ease of fabrication. However, since a cross-section which is triangular or square deviates too far from a symmetrical configuration and does not provide sufficient equalization of thermal stress, as explained below, these shapes are excluded from consideration here. In general, then, the transverse cross-section of the chamber may be any regular plane figure having more than four axes of symmetry co-planar therewith; this generic description is definitive of, and is so intended to include, any regular polygon having five sides or more, as well as a circle. A second essential provision of this invention is the use of one end closure member or end wall as the inlet manifold and other end wall as the outlet manifold. As a consequence of the aforesaid provisions, an essentially 3 uniform temperature pattern, or temperature symmetry, obtains throughout the catalyst bed and, more importantly, throughout the end walls and side wall of the chamber during operation. In other words, an innite number of substantially circular concentric temperature isotherms, centered on the longitudinal axis of the chamber, exist in both of the end closure members, and an inlinite number of peripheral, longitudinally spaced isother-ms exist in the side wall. Thus the temperatures at all points equidistant from the longitudinal axis will, on the average, be equal. There will, of course, usually be a substantial temperature diierential between the centers of the end walls but the rate of change of temperature with path length in proceeding therebetween along the surface of the chamber is quite small, and the temperature proles of all minimum distance path lengths between the centers of the end walls will be substantially identical. Such temperature symmetry means that the thermal stress pattern within the walls and mufer internals will also be symmetrical so that excessive differential stresses within the structural members of the muffler are avoided; the structural members, particularly the side and end walls, may thereforebe safely fabricated of light gauge sheet metal without being subject to buckling or warping.

In a specific aspect, it is a particular feature of the catalytic muiiier to provide a transversely elongated catalyst chamber, the longitudinal wall-of the chamber being considerably shorter than the end closure walls, so that the thickness of the catalyst bed parallel to the direction of the gas flow therethrough is substantially less than the average width of thebed or diameter thereof in the case of a circular bed. This arrangement is desirable inthat it minimizes the differential pressure across the bed while providing a relatively large catalyst Volume, and further is conservative of the physical space required for the installation of the muifler, a consideration of no little importance in the case of automobiles having low clearance Ybetween oor board and road.

A better understanding of theseV and specific aspects of the catalytic muler may be gained with reference to the accompanying drawing which isV presented as illustrative of the invention but is not intended to be restrictive of its generally broad'scope. FIGURE 1 of the drawing is an elevational section of a preferred embodiment of the catalytic muiiler and FIGURE 2 is a sectional plan view of the apparatus shown in FIGURE l. FIGURES 3 and 4 are plan views of alternative embodiments of the muffler. 1^

With reference rst to FIGURES 1 and 2, the muffler comprises a circular, diametrically elongated longitudinal wall 1 to which are connected upper and lower end closure members or

heads

2 and 2 respectively, thereby forming an enclosed chamber of circular cross section having a central longitudinal axis a-a. It is understood that the terms upper and lower, etc. are used here purely for descriptive convenience since the muier may be mounted in any position, although for its primary Vpurpose of being installed in a vehicle exhaust line, it is preferably oriented as illustrated. Upper and lower horizontalbaiile plates 3 and 3' containing perforations 4 and 4' respectively are connected to side wall 1 and divide the chamber into an upper inlet manifold space 10, an intermediate space containing a bed of catalyst 11, and a lower outlet manifold space i'. The perforated baille members 3 and 3' may instead be woven wire screens or screens reinforced by perforated baille plates or discs. An inlet conduit 5 enters circular wall 1 and extends radially through catalyst bed 11 a distance of slightly less than one-half the diameter of the bed, then terminates in an upwardly directed 90 bend connecting with an enlarged opening 7 in plate 3. In a similar manner, an outlet conduit 6 enters circular Wall 1 at a point therein substantially diametrcally opposite the entrance point of conduit 5 and extends radially through catalyst bed 11, then terminates in a downwardly directed 90 bend connecting with an enlarged opening 7 in plate 3'. Conduits 5 and 6 are reinforced, as well as retained in the desired position of alignment, by means of supporting clips 17 and 17' which are connected to their respective conduits and to end heads 2 and 2' respectively. Conduits 5 and 6 are in abutting tangential contact at point 16 and are mechanically joined thereat, as by welding, so that the two conduits form a comparatively rigid longitudinal supporting member for the whole muler which substantially relieves the side wall, end heads, and perforated discs from having to withstand flexing or twisting stresses or to absorb vibrations or road shocks incident to the operation of a motor vehicle; this arrangement also provides a central tension member connecting

end heads

2 and 2 which additionally reinforces these elements against internal pressure.

The immediate area of discs 3 and 3 surrounding openings 7 and 7 is made imperforate or is sealed olf by suitable sealing means such as imperforate annular baille plates 8 and 8'; their function is to prevent the existence of a preferential ow path between openings 7 and 7 through bed 11, thereby promoting better distribution of the exhaust gases owing into and out of the catalyst bed.

With the muffler installed as illustrated, disc 3 supports the weight of the catalyst bed; when desired, additional support for disc 3 may be furnished by means of supporting members such as a plurality of radially extending, circumferentially spaced braces 9 which are disposed between disc 3 and lower head 2' and are suitably connected thereto. Further reinforcement for both discs 3 and 3' may be provided by a number of circumferentially spaced longitudinal strut members or tie-rods 12 which are bolted or welded to each disc and extend therebetween; struts 12 lend rigidity to the assembly of perforated discs and act to prevent longitudinal deiiection thereof.

The exterior of the catalyst chamber may be covered,

i in whole or in part, with a layer of insulating material 15, which insulation may comprise a mineral wool. A number of spacer members 14 are connected to the exterior surface of the chamber and project through insulating layer 15, serving as a supporting and spacing means for outer skin 13, the latter being fabricated of sheet metal and having the same general shape as the catalyst chamber. Skin 13 serves to retain the insulation and to protect it from road dirt, water, stones, etc. The purpose of the insulation is (l) to conserve heat and permit rapid activation of the catalyst following engine start-up, and (2) to protect the floor board, wiring, brake lines and other automotive equipment adjacent the mufller from excessively high temperatures. In warm climates it may i only be necessary to insulate the top half of the muffler.

With the exception of skin 13, which may -be iron, steel, aluminum, etc., the muiller internals including the side wall, end heads, baifles, conduits and brace work are preferably constructed of a heat-resistant alloy such as one of the various stainless steels, Inconel, etc. Prior toV assembly, the muffler is lled with the desired oxidation catalyst. By way of illustration, suitable oxidation catalysts include the metals and their oxides of groups I, V, VI and VIII of the periodic table, particularly copper, silver, vanadium, chromium, iron, cobalt, nickel and platinum. These components may be used singly, in combination with two or more, or may be composited with an inorganic refractory oxide such as alumina, silica-alumina, silica-boria, silica-alumina-zirconia, and the like. The physical shape of the catalyst may be in the form of spheres, cylinders, cubes, pellets, etc., typically having a major dimension of s to M: inch. Such catalysts have an activation temperature in the rangeof 30G-900 F., and normally operate in the range of 900-1400 F., although during deceleration or under unusual circumstances, excessive concentration of combustibles may periodically run the temperature as high as 1800 F.

The catalytic mufer may be installed in the vehicle exhaust line in place of a conventional muffler or may be positioned more closely adjacent the engine exhaust manifold. The portion of the exhaust line upstream of the muier may be insulated to conserve heat.

In operation, exhaust gases from the engine exhaust manifold are mixed with secondary air from a suitable injector or compressor; the resulting mixture, which is combustible when in contact with the catalyst, enters conduit 5 and, in being conducted through a portion of the bed 11, pre-heats the bed by indirect heat exchange whereby to approach the reaction temperature and thus effects a more rapid activation of the catalyst after start-up from cold conditions. The inowing gases pass through the pipe elbow and are directed upwardly into inlet manifold 10, the major portion of the gases impinging on the inner surface of top head 2. Within manifold the gases undergo a 90 change of direction and are uniformly distributed outwardly across the upper surface of baflie 3. The gases then undergo another 90 change in direction, flow downwardly through the catalyst mass, therein being oxidized, collect in outlet manifold 10' and discharge through conduit 6 to the tailpipe. The net 180 reversal of gas iiow which occurs both at the inlet and the outlet of the bed advantageously promotes uniform distribution of ow therethrough. Although the function of conduits 5 and 6 could be interchanged, it is preferred to effect a downow, rather than upow, of gases through bed 11 since iiuidization of the bed with resultant channeling of gases is thereby avoided. Downilow operation is also advantageous in that upper head 2, forming the inlet manifold, is normally several hundred degrees cooler than lower head 2'; since upper head 2 is most closely adjacent the oorboard of the automobile, the latter will not be subjected to unusually high temperatures in the event that insulation is not employed.

An important feature of the invention is the inward concavity of head 2 (also of head 2'); the inner surface, at least, of heads 2 and 2' may be circular as shown in FIGURE 1, conical, elliptical or in fact any surface of revolution which is concave toward the catalyst bed. This design provides a manifold zone of decreasing thickness at increasing radial distance from central axis a-a. As the gases flow first radially outwardly and then downwardly through perforations 4 into bed 11, lesser quantities of outflowing gases remain in manifold 10 at increasing radii; the decrease in cross-section presented to the gas ow in manifold 10 tends to compensate for the reduced gas iiow which will minimize the transition of velocity head to pressure head, thereby encouraging uniform -iiow through the catalyst bed. The same considerations apply in reverse to outlet manifold 10'. Here the gases flow radially inwardly after passing through perforations 4 in baffle 3', and the cross-section of manifold space 10' presented to their flow increases with decreasing radius from axis a-a; the increased cross-section tends to compensate for the increased gas How which will minimize the transition of pressure head to velocity head.

It has been previously pointed out that the present invention operates not only `as a catalytic converter but also as an acoustic muffler. Such muier .action is achieved by the particular `disposition of conduits 5 and 6 as shown in FIGURES 1 and 2. Since the inlet and outlet conduits are oppositely directed, sound pulses cannot travel through the rnuflier undeflected as would be true in the case of axially aligned conduits, but the pulses are instead broken up. Equally important is the fact that openings 7 and 7' are also not in axial alignment, but the centers thereof are radially offset a short distance from axis a-a and are spaced 4approximately 180 apart; this arrangement makes the enclosed chamber, as a whole, acoustically asymmetric and prevents the generation of standing Waves therein.

FIGURES 3 and 4 of the drawing are plan views of other possible modifications of the catalytic muffler wherein the transverse cross-section of the catalyst chamber is polygonal, rather than circular. In FIGURE 3, longitudinal w-all 20 forms a pentagon, land in FIGURE 4, wall 21 forms an ootagon. Obviously, the higher .fthe order of the polygon, the more perfect the symmetry of gas flow and temperature distribution. For this reason, as well as ease of manufacture and lower first cost, a circular cross-section is preferred. However, in accordance with the teaching herein, other regular shapes may be employed when desired.

Although the drawings show conduits 5 and 6 as extending through side wall 1 at substantially diameterically opposite points therein, or spaced about 180 apart, such provision is not vessential to the invention and various other configurations will be apparent to the designer. For example, in order to conform the position of the inlet and outlet conduits to certain existing automobile exhaust and tailpipe arrangements, it may be desirable to position said conduits `from to 160 apart, depending upon the diameter of the mufer and the particular application thereof. In other instances, it may be feasible lto space the conduits 90 or less apart. A particullarly desirable embodiment for use with trucks involves extending the conduits through end heads Zand 2' instead of through .side wall 1; for example, conduit 5 may enter lower head 2', and extend longitudinally upwardly through bed 11 to communicate with opening 7 in upper plate 3, while conduit 6 is extended through upper head 2 and longitudinally downwardly through bed 11 to communicate with opening 7 in lower plate 3'. The conduits may be joined together `at their point of contact, as described above. The preferred downilow operation, the reversal of flow at inlet and outlet, and the acoustic asymmetry are all features which are retained by this alternative design. All of these variations will achieve the desired temperature symmetry, provided, of course, Ithat openings 7 :and 7' are substantially centrally disposed.

It is contemplated that contact materials other than catalysts may be employed in the :muffler at the option of the designer, for example, the muiller may contain, in whole lor in part, activated carbon, silica gel or other adsorbent. I-t is not necessary that the contact material be granular or particle-form, but it may comprise other well-known types of ceramic, refractory, or metal packing material such as Raschig rings, partition rings, spiral partition rings, Berl saddles, grid packing, honeycomb -ilter blocks, and the like, upon which may be deposited a suitable catalytically active composite or adsorbent. Where the 1apparatus is to be used strictly as a muflier land not as an lexhaust gas purifier, the catalyst may be omitted in which event the solid packing material may be catalytically inert and will function primarily as a sound-absorbing medium. To achieve such function, the packing material may be in the form of a plurality of closely spaced perforated partitioning members to provide a .tortuous dow path therethrough, or simply a mass of mineral wool, such as glass or asbestos fibers. It is, of course, obvious that the catalytic muer may 'be used with diesel and natural gas engines as well as with gasoline-powered engines yand may be utilized in connection with the larger stationery prime rnovers commonly found in refineries, chemical plants, and pipe line compressor stations.

I claim as my invention:

l. A muier comprising a longitudinal wall and spaced end closure members forming a closed chamber whose transverse cross-section i-s substantially symmetrical with respect to its longitudinal axis, a pair of parallel perforated bafes connected to the inner surface of said 'longitudinal wall and spaced from `each other and from said end closure members to form within said chamber a first manifold compartment between one of said closure members and one of said baies, a second manifold compartment between the other closure member and the '7 other bathe and an intermedia-te compartment adapted to contain a mass of solid material between said first and second compartments, an imperforate inlet conduit ex# tending through said longitudinal Wall into said intermediate compartment an-d terminating in a 90 bend communicating with said first manifold compartment near the longitudinal axis of said chamber, and imperforate outlet conduit extending through said longitudinal wall into Said intermediate compartment in spaced relation to the inlet conduit and terminating in an oppositely directed 90 bend communicating with said second manifold compartment near the longitudinal axis of said chamber.

2. A mufer comprising a cylindrical Wall and spaced end closure members forming a closed chamber of circular cross-section, a pair of axially spaced parallel perforated bales connected to the inner surface of said cylindrical wall and spaced from said end closure members to form within said chamber a first manifold compartment -between one of said closure members and one of said baies, a second manifold compartment between the other closure member and the other bathe and an intermediate compartment adapted to contain a mass of solid material between said rst and second compartments, an imperforate inlet conduit extending through said cylindrical wall into said intermediate compartment and terminating in a 90 bend communicating with said rst manifold compartment near the axis of said chamber, and an imperforate outlet conduit extending through said cylindrical wall into said intermediate compartment in spaced relation to the inlet conduit and terminating in an oppositely directed 90 bend communicating with said second manifold compartment near the axis of said chamber.

3. The muler of claim 2 further characterized in that said inlet and outlet conduits extend through said cylindrical wall at approximately diametrically opposite points of the wall.

4. The mulier of claim 2 further characterized in that the diameter of said chamber is greater than its thickness along the centr-al axis thereof.

5. The muffler of claim 1 further characterized in that the transverse cross-section of said chamber is a regular polygon having at least five sides.

6. The muifler of claim 1 further characterized in that a layer of heat insulating material covers at least a portion of the exterior of said chamber.

7. The mufller of claim 2 further characterized in that the 90 bends of said inlet and outlet conduits are in abutting tangential contact and are mechanically interconnected at the point of contact whereby said conduits form a comparatively rigid longitudinm supporting member for said chamber.

8. A muffler comprising a cylindrical wall and spaced end closure members for-ming a closed chamber of `cir-` cular cross-section, a pair of axially spaced parallel perforated discs connected to Vthe inner surface of said Wall and spaced from said end closure members to form within said chamber a rst manifold compartment between one of said closure members and one of said discs, a second manifold compartment between the other closure member and the other disc and an intermediate compartment adapted to contain a mass of solid material between said first and second compartments, each of said discs having an enlarged opening therethrough near the axis of said chamber, said openings being out of axial alignment but overlapping along the axis of the chamber, an imperforate inlet conduit extending through said wall radially into said intermediate compartment and terminating in a bend communicating with said iirst manifold compartment through the opening of one of said discs, and an imperiorate outlet conduit extending through said Wall radially into said intermediate compartment in spaced relation to the inlet conduit and terminating i-n an oppositely directed 90 bend communicating with said second manifold compartment through the opening of the other of said dises.

9. The muier of claim 8 further characterized in that said inlet and outlet conduits extend through said cylindrical wall at approximately diametrically opposite points of the wall.

10. The muffler of claim 8 further characterized in that the interior surfaces of said end closure members are surfaces of revolution which are concave toward said intermediate compartment.

11. The muler of claim 8 further characterized in that said discs are reinforced by a plurality of circumferentially spaced longitudinal strut members connected to each and extending therebetween.

12. The muiiler of claim 8 further characterized in that the portion of each of said discs immediately surrounding said enlarged opening is imperforate.

13. The muler of claim `8 further characterized in that said 90 bends are in abutting tangential contact and mechanically joined at their point of contact and each of said bends is mechanically connected to one of said end closure members to form a comparatively rigid longitudinal supporting means for said chamber.

References Cited in the le of this patent UNITED STATES PATENTS 1,465,904 Herdle Aug. 21, 1923 1,761,471 Forstel .Tune 3, 1930 2,071,119 Harger Feb. 16, 1937 2,260,578 Murray Oct. 28, 1941 2,409,825 Baringholtz Oct. 22, 1946 2,833,615 Kollgaard May 6, 1958

Claims

1. A MUFFLER COMPRISING A LONGITUDINAL WALL AND SPACED END CLOSURE MEMBERS FORMING A CLOSED CHAMBER WHOSE TRANSVERSE CROSS-SECTION IN SUBSTANTIALLY SYMMETRICAL WITH RESPECT TO ITS LONGITUDINAL AXIS, A PAIR OF PARALLEL PERFORATED BAFFLES CONNECTED TO THE INNER SURFACE OF SAID LONGITUDINAL WALL AND SPACEED FROM EACH OTHER AND FROM SAID END CLOSURE MEMBERS TO FROM WITHIN SAID CHAMBER A FIRST MANIFOLD COMPARTMENT BETWEEN ONE OF SAID CLOSURE MEMBERS AND OF SAID BAFFLES, A SECOND MANIFOLD COMPARTMENT BETWEEN THE OTHER CLOSURE MAMBER AND THE OTHER BAFFLE AND AN INTERMEDIATE COMPARTMENT ADAPTED TO CONTAIN A MESS OF SOLID MATERIAL BETWEEN SAID FIRST AND SECOND COMPARTMENT, AN IMPERFORATE INLET CONDUIT EXTENDING THROUGH SAID LONGITUDINAL WALL INTO SAID INTERMEDIATE COMPARTMENT TERMINATING IN A 90* BEND COMMUNICATING WITH SAID FIRST MANIFOLD COMPARTMENT NEAR THE LONGITUDINAL AXIS OF SAID CHAMBER, AND IMPERFORATE OUTLET EXTENDING THROUGH SAID LONGITUDINAL WALL INTO SAID INTERMEDIATE COMPARTMENT IN SPACED RELATION TO THE INLET CONDUIT AND TERMINATING IN AN OPPOSITELY DIRECTED 90* BEND COMMUNICATING WITH SAID SECOND MANIFOLDER COMPARTMENT NEAR THE LONGITUDINAL AXIS AND SAID CHAMBER.