TECHNICAL AREA

The present invention relates to a method for cleaning particle filters and in particular a method whereby it is assured that soot and other particles are removed in a safe way.

BACKGROUND OF INVENTION

There is an increased demand on the environmental aspects regarding pollutions in order to decrease the negative effects and thus the amount of pollutions. A source of pollutions in the environment is the exhausts from combustion engines such as cars, lorries busses and the like.

The exhausts have been less damaging during the last two decades due to more advance emission controls and catalysts. Also diesel engines have become cleaner due to catalysts. During recent years many diesel engines have been equipped with particle filters in order to further reduce the amount of pollutions in the exhausts.

The particle filters are very effective in collecting particles and many vehicles have a certain cleaning procedure at certain driving distance intervals, whereby the filter is heated in order to reduce the collected particles. However, even if some of the particles are burnt away during this procedure, it is not so effective such as to completely clean the filters and thus the filter has to be replaced after a certain driving distance. The replacement of the filter is rather costly for the owner of the vehicle and the filter should be able to be used further if it is cleaned because the material of the filter, stainless steel and ceramics, have not deteriorated during use, if the filter has not been exposed to physical damage and thus the filter could be reconditioned. This in turn could mean cheaper spare parts for the owners and reduced use of material resources.

The applicant for the present invention has developed a method for cleaning particle filters by burning off soot and other particles in the filter, which method is described in the document WO2008/091218. The disclosed method has shown to be very effective in cleaning many clogged particle filter. In some instances the described method does not provide a complete cleaning for different reasons, whereby it may be needed further measures in order to obtain a complete result. BRIEF DESCRIPTION OF INVENTION

The aim of the present invention is to provide a method which gives a very good cleaning result.

This aim is obtained with a method comprising the features of the independent claim 1. Preferable embodiments of the invention form the subject of the dependent claims.

According to a main aspect of the invention, it is characterised by a method of cleaning a particle filter, in particular for combustion engines, which particle filter comprises a casing and a particle filter core in the casing, comprising the steps of opening the casing in order to remove the particle filter core, to process the interior of the particle filter core during a certain time period in order to burn off the enclosed particles, and to reintroduce the particle filter core into the casing after which the casing is closed. According to one aspect of the invention, the processing step comprises adding heat to the interior of the particle core in order to burn off particles.

According to another aspect of the invention, the processing step comprises adding liquid material to the interior of the particle filter core during a certain time period in order to remove the burnt particles from the particle filter core.

According to a further aspect of the invention, the opening of the casing is performed by any of machining or cutting open.

According to another aspect of the invention, the closing of the casing is done by any of welding, soldering, metal strip joint, melting, gluing. According to another aspect of the invention, the liquid material comprises any or several of the types water under high pressure blown through the filter core for removing particles, steam under high pressure, water under vacuum, water with ultrasound, ice crystals with high velocity.

According to yet an aspect of the invention it further comprises the step of measuring the particle filter, whereby the measurements are compared with the values of an unused filter of the specific type. According to a further aspect of the invention, the measuring step is performed by measuring the pressure drop over the particle filter core.

Alternatively or in addition, the measuring step may be performed by filling the particle filter core with smoke and thereafter measure the opacity.

There are a number of advantages with the present invention. By removing the particle filter casing, a more controlled, improved access and thereby cleaning can be obtained. Preferably the casing is opened in a suitable manner that on the one hand allows withdrawal of the core but also a closing of the casing after cleaning. Thereby the casing can be divided by different mechanical machining methods or heat cutting methods, and reassembly of the casing parts can be done by for example welding. Preferably the processing is performed by heating and thereby burning off of the particles in the filter core. It is also feasible that liquid material is used for processing the core. An initial burning can also be complemented by the use of liquid material.

By different combinations of the different steps with burning, using of liquid material and preferably subsequent measurement for control of the degree of cleaning, a very good cleaning result is obtained, that at the same time is quality ensured with measurements and comparison with reference values. The liquid material is preferably water that may be in different phases such as steam, liquid or frozen and with different pressures and velocity depending on the desired application. It has been shown that adding of liquid material, in particular after burning of the filter, has a very good degree of cleaning.

The advantages with taking out the core from the casing comprises also that more cores can be placed in for example a heating oven or furnace, because they take up lesser space than with the casing still on. It may also be that the casing is damaged, for example by external impact, such that it cannot be re-used in a cost-effective way, while the core is un-damaged and very well can be treated for re-use. In this case only the relatively cheap casing is discarded. In other cases when the core is not taken out, the whole particle filter has to be discarded if the casing is damaged.

Further other components inside the particle filter may be replaced, for example fibre cloths or mats that may be placed between the casing and the core itself.

These and other aspects of, and advantages with, the present invention will become apparent from the following detailed description of the invention and from the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

In the following detailed description of the invention, reference will be made to the accompanying drawings, of which

Fig. 1 shows a flow chart of the method according to the invention,

Fig. 2 shows schematically an example of a burning process comprised in the invention,

Fig. 3 shows schematically another example of a burning process comprised in the invention, and

Fig. 4 shows schematically a test set-up for particle filters cores. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a schematic flow chart of the method according to the present invention. The particle filters that are to be cleaned are first inspected for visual defects such as cracks, damaged attachment brackets and the like. Then the filters are cleaned externally. This may for example be done by washing the outer surface with hot water after all holes have been plugged. Possible corrosion or other foreign matter that have not been removed by the water is removed by blasting. After this pipes and possible contacts and damaged bolts are removed. A main step according to the invention is to open the casing in order to take out and clean the core of the particle filter. The opening of the casing may be done in a number of different ways such as cutting with a cutting disc, to cut it open with a gas torch or to cut it open with a pair of scissors for sheet metal, just to mention a few. When the casing later has been opened the filter core can be taken out for cleaning.

The next step is then to clean the inner of the particle filter core. According to one variant, this is performed by burning away the soot and carbon compounds that have been deposited on the filter surfaces. Figure 2 shows an example of a burning step. The particle filter core 10 is first preferably arranged in a space 12 of insulating heat- resistant material and is then connected to a gas source 4. A control unit 16 for mass flow is arranged in the conduit for the gas inlet in order to control the volume flow and the mixing. Then the gas is preferably pre-heated before it is fed into the space and into the core. The core is heated to temperatures above the exothermic reaction of the soot when it is burnt away, but not too high, which thereby avoids the damage of the ceramic filter surfaces. Temperature sensors (not shown) may be arranged in order to monitor the above mentioned temperatures.

During this step the carbon compounds are burnt away from the filter surfaces while the volume flow and O2-concentration is controlled depending on exhaust and inlet temperatures. The oxidation process can be measured and monitored by an O2- sensor and temperature control at the front part of the filter and in the flowing gas stream. Figure 3 shows a second example of burning step. Here several particle filter cores that are to be cleaned are placed in spaces 18, which spaces are arranged in a furnace 20. Temperature sensors 28 may be placed in connection to the respective core. After this the oven is heated with a heating unit up to suitable temperature, that may be in the area 450 - 600 °C depending of the filter type. Then control valves 24 for air are opened so that it flows through a loop of piping 26. The air in the loop will be heated to the same temperature as the oven when it then is brought against and into each core. The air provides a burning whereby particles in the filter are burnt away. Preferably the air supply in the loop to the cores is low so that the burning is not too fierce so that the temperature is raised too much in the cores, which

otherwise could damage the cores. The flow is preferably measured by a flow meter 24 and controlled by the control valve 22. Further the temperature sensor 28 may measure the temperature continuously so that it does not become too high. If the temperature however is in the progress of reaching an upper acceptable limit despite the air flow being throttled completely, carbon dioxide may be fed from a pressurized container 30 with carbon dioxide into the filter in order to reduce the burning. Control valves 32 to the respective filter via branches 34 in a circuit 36 with carbon dioxide is opened whereby carbon dioxide is fed into the filter in order to reduce the burning and thereby the temperature. It is to be understood that other gasses can be used that are capable of reducing the burning in the filters. It is thus possible to control the burning process by controlling the supply of air, carbon dioxide, oxygen and other gasses as well as controlling the flow of gas in order to obtain an optimal cleaning process.

Because the burning processes in the filters are exothermic, i.e. heat is produced, not so much power input is required in order to keep the oven at work temperature. The oven is only switched on full power when it is to be heated. When the process then is running, the oven is only to be kept at constant temperature, whereby only about 25% of available power is utilized.

The end of the oxidation process may be performed in a few different ways. The most simple is to terminate the heating after a certain time period, which time period is based on empiric studies of certain types of particle filters. Another way is to terminate the heating when the 02-concentration at the outlet is equal to adjusted inlet concentration. When the filter has been treated during a certain time period the particles have been burnt and are to be removed. This can either be done by blowing air into the filter whereby the burnt away particles are removed. It is however to be understood that vacuum can be used instead, whereby the burnt away particles are sucked away by suitable suction means. According to the above the flow of gas through the filters may be controlled in many ways in order to obtain an optimal result. Thus it is conceivable to increase the flow during the burning process during maintaining of a proper burning temperature in order to remove burnt particles such that a preceding blow or suction process becomes unnecessary or at least is reduced. When the filter core has been cleaned and controlled it may according to alternative I in Fig. 1 , be re-mounted in the casing, which preferably has been cleaned on the inner surfaces before re-mounting. The casing parts are then joined together in a suitable fashion by welding, soldering, metal strip joint, gluing, melting, just to mention a few ways.

According to preference, also a further/an alternative cleaning step can be used, see Fig. 1 , or instead of burning off according to above, if this is desired. The

next/alternative cleaning step is done by adding liquid material into the inner of the filter core. It may, as non-limiting examples, be one or several of following materials:

- water under high pressure is blown through the filter in order to remove particles,

- steam under high pressure,

- water under vacuum,

- water with ultra sound,

- ice crystals with high velocity.

These measures remove the soot and particles in the core. If a previous burning step has been done, remaining particles that have not been removed during the previous burning step are removed. The filter core is thereby attached I a suitable space arranged with inlet and outlet for the media that is to clean the core. The inlet is further connected to a source of the media and a pressure increasing or reducing device depending on the type of cleaning method according to above. After this the material is fed into the filter core whereby residual particles are removed by the flow of the material. The material may be brought in from one side or from both sides. It is also conceivable to pulse the material when it is brought into the particle filter core. Further, if water is introduced, also pressurised air may be added in order to increase turbulence in the inner of the filter. If water is used then it may preferably be demineralised in order to avoid limestone deposits and the like. After treatment with liquid material, the filter cores may be placed in a suitable device where a centrifugal step is performed. This centrifugation removes a lot of the liquid from the inner of the particle filter core. In certain cases, air may be used for removing particles from the core instead of or in addition to liquid. This air may be pressurized or even pre-heated for quicker drying of the filter cores. According to alternative II in Fig. 1 , the core may be re-mounted in the casing according to above. When the cleaning step (-s) has been terminated the cleaning result may be measured and evaluated for quality control. This may be done according to the invention in a number of ways.

Figure 4 shows a schematic setup.

One way is measuring the differential pressure over the particle filter where high values of he difference pressure is an indication of a dirty filter because the filter is clogged with soot and other carbon compounds. The measured differential pressure is compared with the differential pressure of a clean unused filter of the specific type. Pressure sensors 36, 38 are then placed at the inlet and the outlet of the filter and a pressure source (not shown) for applying a pressure to the filter. The pressure sensors are connected to suitable mans for handling the signals from the sensors and for comparing them with predetermined pressure values. A test unit is shown in Fig. 4, which comprises two inlet pipes 40, 42 connected to a three way valve 44, where one of the inlet pipes 40 are connected to an air source and a fan (not shown) and the other is connected to a smoke generator (not shown). A further pipe 46 is connected to the three way valve. To this pipe the inlet of a particle filter 48 may be disconnectively attached.

The outlet of the filter is connected to a straight pipe 50. The pipe 50 is in one end arranged with a light transmitter 52 for example a strong LED and in the other end arranged with an optical receiver 54. The light transmitter is connected to suitable drive means for driving it and the optical receiver is connected to signal handling means. An outlet pipe 56 is connected to one end of the straight pipe and arranged with a valve 58. A gas meter 60 is further arranged to the particle filter. The system is intended to function as follows. The three way valve 44 is positioned such that the smoke inlet 40 is connected to the further pipe 46. Smoke that is generated by the smoke generator is fed through the inlet, fills the particle filter core and the straight pipe 50. The outlet valve 58 is closed. The gas meter 60 is activated and measures the gas content such has CO, CO2, HC and O2. The light source 52 is activated and when a stable concentration of gas is obtained the strength of the transmitted light through the straight pipe is measured by the receiver 54 as a measure of the opacity. The measured opacity is then compared to the value of a clean unused filter. The opacity measure gives an indication that the filter is cracked, which gives high values.

When the measurements have been performed the three way valve 44 is positioned such that the air intake 42 is connected to the further pipe and the outlet valve 58 is opened. The system is now cleaned from smoke and is ready for testing of a subsequent filter.

As mentioned above the measured values are compared with earlier measured values and earlier set values of a new filter. If the measured values deviate from the earlier set values with a certain value, this indicates that the cleaning has not been completely successful. In this case the filter is exposed to a second cleaning operation and is measured. This may be repeated until the measured values are within acceptable areas in relation to the earlier set values. It is however to be understood that if a filter is cracked, no further cleaning processes will be made. In this case the filter is either scrapped of the position of the cracks are detected and the cracks are repaired.

When this is done it is transferred to a finishing step where the filter is exposed to surface treatment and is given a unique test number for later traceability of the specific filter.

Even if some examples of cleaning and test equipment have been mentioned above it is to be understood that other types of equipment, system and principles can be used for performing the method according to the invention. The embodiments described above and shown in the drawings are thus only to be regarded as non- limiting examples of the present invention and that it may be modified within the scope of the patent claims.