1. Field of the Invention
The present invention relates to a plate-like filter element, a filter unit formed by stacking up a plurality of the filter elements or a filter device installed therein, the filter unit being used to mainly trap or remove particulates contained in an exhaust gas discharged from a diesel engine which is mainly used for various vehicles, vessels or industrial appliances.
An exhaust gas discharged from a diesel engine contains particulates including carbon particles as a main component at a fairly high content, which causes air pollution.
2. Description of the Related Art
Various devices have been proposed to trap or remove the particulates contained in the exhaust gas by using a filter means.
For instance, Japanese Unexamined Patent Publication No. 124417/1981 discloses a filter unit 20 made of ceramics as shown in FIG. 9. The filter unit 20 shows an outer configuration of a rectangular prism as a whole and comprises a plurality of (eight in FIG. 9) rectangular plate-like bodies 21, 22 placed in parallel to each other, vertical end ribs 23, 25 and a number of intermediate ribs 24, 26.
All the plate-like bodies 21, 22, ribs 23, 25 and intermediate ribs 24, 26 or at least the plate-like bodies 22 are made of gas-permeable porous thin ceramic wall having a filtering function. The plate-like bodies 21 constitute the upper and lower walls of the filter unit 20, and the plate-like bodies 22 constitute the intermediate walls. Between adjacent plate-like bodies 21, 22 and between adjacent plate-like bodies 22, 22, the end ribs 23 and the intermediate ribs 24 are respectively interposed so as to be in parallel to one side of the plate-like body 21. The upper edges of the ribs 23, 24 are jointed to the upper plate like body 21 or 22, and the lower edges of the ribs 23, 24 are jointed to the lower plate-like body 22 or 21, whereby a plurality of particulate-containing gas passages 27 with both ends opened are formed. In one side of the plate-like body 22, the ribs 23, 24 are formed, and in the other side of the same plate-like body 22, the ribs 25, 26 are formed extending in the direction perpendicular to the ribs 23 24. The ribs 25, 26 are formed substantially in the same manner as the ribs 23, 24 and differ in that they are formed in the direction perpendicular to that of the ribs 23, 24. Thus, a plurality of particulate-containing gas passages 27 and clean gas passages 28 with both ends opened are alternately formed, where the direction of passages of the former is perpendicular to the passages of the later.
In the filter unit, both ends of the particulate-containing gas passages 27 are opened, but the openings at their one side of the passages 27 are directly or indirectly closed, and an exhaust gas from a diesel engine is introduced from the openings of the other side of the filter unit 20. Or, alternatively, the exhaust gas from the diesel engine can be introduced from the openings opened at the both ends simultaneously. The plate-like bodies 22 function as a filter wall, and particulates are trapped on the surface of the plate-like bodies 22 at the side of the particulate-containing gas passages 27, whereby a clean gas without particulates passes through the plate like bodies 22 and then to the outside of the filter unit via the clean gas passages 28. The trapped carbonaceous particulates in the exhaust gas are removed by burning when the plate-like bodies 22 are heated at appropriate time intervals. Thus, the filter unit 20 is refreshed.
In the prior art technique as described above, the carbon particulates collected are burnt off by heating the plate-like bodies 22 having a filtering function to the ignition temperature of the particulates to thereby remove them. However, the plate-like bodies 22 are exposed repeatedly to a high temperature when the particles are periodically burnt off. Accordingly, sintering of the plate-like bodies is further accelerated, whereby a distribution of pore size originally formed changes to thereby increase a pressure loss during the particulates trapping operation. This causes difficulty in maintaining a stable performance of the filter unit. Further, a damage by fusion of the plate-like bodies 22 may take place when they are subjected to a high temperature at the time of burning off the particulates. When such damage occurs, it is no more possible to trap the particulates.
The particulates of exhaust gas from the diesel engine contains not only carbon but also an unnegligible amount of non-combustible components (for instance, 1%-5% by weight to the total amount of the particulates), and these non-combustible components are also trapped by the filter unit. Furthermore, the exhaust gas contains components such as SOx and NOx, which corrode materials constituting conduit elements to produce non-combustible solid components which deposit on the filter unit. These non-combustible solid components can not be removed by burning and accumulate in the filter unit to cause clogging, whereby properties of the filter unit are deteriorated.
Japanese Unexamined Patent Publication No. 225721/1984 discloses a dust removing apparatus for a hot dust-containing gas. The dust removing apparatus comprises a plurality of vertical filter tubes having both ends opened. A dust-containing gas is introduced from the upper portion of the filter tubes. Dusts hindered by the filter tube walls from passing therethrough are moved downwardly in the filter tubes and are collected in a dust hopper provided below the filter tubes, while a clean gas obtained by passing through the filter tube walls is discharged from the side walls of the filter unit. It is understood that the dust removing apparatus disclosed in this document is suitable for treating a large flow rate of gas containing noncombustible combustible dusts discharged from an apparatus such as a converter in an iron plant. However, the document does not suggest application of the apparatus to the treatment of an exhaust gas from a diesel engine.
Supposing this kind of system for a compact filter unit, it is easy to manufacture the simple shaped filter tubes, but there is such a problem that an assembling process requiring much labor and time is needed in order to fix a number of thin filter tubes to support plates with spaces among the tubes and it is not easy to manufacture a filter unit having a large capacity.
The applicant of this application proposed (in Japanese Patent Application No. 232360/1987) a filter device for an exhaust gas from the diesel engine as shown in FIG. 10 in order to solve the above-mentioned problem.
A typical example of such device will be described.
A filter unit 33 having a rectangular solid form which is substantially the same as the filter unit shown in FIG. 9 is used.
The filter unit is secured in a casing 31 having openings at upper and lower parts and a side port by interposing sealing members 32 as shown in FIG. 10. In passages and respectively have one end closed and the 34 which vertically pass through the filter unit 33 (as shwon by solid line arrow marks in FIG. 10) and clean gas passages 35 (as shown by broken line arrow mark in FIG. 10) which cross the particulate-containing gas passages and respectively have one end closed and the other end opened, and each passages are defined by plate-like bodies 22 made of a gas-permeable porous material.
An inlet duct 37 is formed at the upper part of the casing 31 to introduce an exhaust gas from the diesel engine. An outlet conduit 38 is connected to the casing 31 at a side where the clean gas passages 35 are open. The outlet conduit 38 is provided with a throat portion 39 having a reduced diameter part, and the portions contiguous to the upstream and the downstream of the throat portion are gently expanded. A nozzle 40 for ejecting a pressurized gas, which is used for back washing, is provided near the downstream side of the throat portion 39 so as to open toward the upstream side.
A particulate receiving section 41 is provided at the lower part of the casing 31. The particulate receiving section 41 is provided with a tray 42, a filter plate 43 with an electric resistance type heater 46, an ash component removing port 44 with a removable cover plate 47 (it is usually closed), and a gas duct 45.
A part of the bottom of the tray 42 is perforated and the filter plate 43 is fitted to that part so that the tray 42 and the filter plate 43 surround as a whole all the lower open ends of the particulate-containing gas passages 34 of the filter unit 33. The ash component removing port 44 is open at the bottom of the tray 42, and the gas duct 45 is placed below the filter plate 43. The filter plate 43 is made of a gas-permeable porous material. The ratio of a filtering area of the filter plate 43 to that of the filter unit 33 is such that about 20% or lower portion, especially about 0.5%-5% portion, therefore, small portion of the total exhaust gas introduced from the inlet duct 37 passes through the filter plate 43, and the remaining portion of the exhaust gas passes through the plate-like bodies 22 of the filter unit 33 to be flown to the outlet conduit 38.
The exhaust gas from the diesel engine is introduced from the inlet duct 37 via the upper open ends into the particulate-containing gas passages 34 of the filter unit 33. The most part of the exhaust gas passes through the plate-like bodies 22, the clean gas passage 35 and is flown to the outlet conduit 38. However, particulates in the exhaust gas are trapped by the plate-like bodies 22 and deposit on the inner surfaces of the particulate-containing gas passages 34. A part of the particulates flow to the particulate receiving section 41 together with a part of the exhaust gas through the lower open end of the particulate-containing gas passages 34. The part of the exhaust gas passes through the filter plate 43 to the gas duct 45. However, the particulates in the part of the exhaust gas can not pass through the filter plate 43 and deposit on the inner surface of the filter plate 43.
After a continuation of the particulate trapping operation for an appropriate period, a short time back washing operation is carried out. In the back washing operation, a pressurized gas, especially pressurized air is ejected from the nozzle 40 for a short time such as about 0.1 sec-1 sec. The ejected gas induces the gas around the nozzle 40, and the gas of several times as much as the original ejected gas is flown in a pulse form to the clean gas passages 35. The pulse gas flow is flown to the particulate-containing gas passages 34 through the plate-like bodies 22. Then, the particulates accumulated on the inner surface of the particulate-containing gas passages 34 are peeled off. A part of the particulates drifts in the particulate-containing gas passages 34, however, the most part drops into the particulate receiving section 41. In the particulate receiving section 41, a stream of gas is produced to pass through the filter plate 43, and being carried with the gas flow, the most part of particulates deposit and accumulate on the inner surface of the filter plate 43.
Thus, the particulates trapped on the inner surfaces of the particulate-containing gas passages 34 during the particulate trapping operation are moved onto the inner surface of the filter plate 43 by the back washing operation to thereby refresh the filtering function of the filter unit 33. The particulates on the filter plate 43 are burnt and removed by heating the electric resistance type heater 46.
After a relatively long term use of the apparatus, there occurs accumulation of non-combustible residue or ash. In this case, the cover plate 47 is opened to remove the residue or ash. Alternatively, they may be forcibly removed by a suitable scraping means.
However, the above-mentioned filter device had a disadvantage as follows. When the back washing gas was introduced in a pulse form into the clean gas passages 35 repeatedly, a high gas pressure head such as about 0.3-0.4 kg/cm.sup.2 is instantaneously applied to the inside of the clean gas passages 35. Each application of the pressure head induces a stress to the plate-like bodies 22 which separate the clean gas passages 35 from the particulate-containing gas passages 34 in the filter unit 33 and finally damages the plate-like bodies 22.
Further, the conventional filter unit as shown in FIG. 9 had a complicated structure which requires much labor and long manufacturing time and the yield rate is low.