F1ELD OF THE INVENTION

The invention relates to a method of measuring particles and to an apparatus for measuring particles. BACKGROUND OF THE INVENTION

When measuring exhaust gases of a combustion engine, for example, the temperature of the gas flow is so high that it sets strict requirements for the structure of the measuring apparatus lt is challenging to provide the apparatus with adequate insulators and seals ln addition, the high temperature causes challenges regarding the measuring electronics.

One solution is to carry out the measurement in a lower temperature. The lower temperature, however, causes particle conversion through particle nucleation and condensation of volatile species in the exhaust. Particle conversion must be prevented using sample handling techniques. The sample handling techniques, however, are cumbersome and rather expensive solutions.

BR1EF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a new method and a new apparatus for measuring particles. The invention is characterized by what is stated in the independent claims. Embodiments of the invention are disclosed in the dependent claims.

ln the presented solution, particles are measured in a sample gas flow. Particles in the sample gas flow are charged and excess ions and/or a part of the charged particles are removed from the sample gas flow. The charging of the particles and the removing of the excess ions and/or the part of the charged particles are performed in a first temperature of the sample gas flow. The temperature of the sample gas flow is lowered to a second temperature and thereafter the charge carried by the particles is measured in the second temperature of the sample gas flow lt is rather simple and easy to provide the means for charging and the means for removing excess ions and/or a part of the charged particles that tolerate the rather high first temperature. When the charging and removing are performed in the rather high first temperature the need for using sample handling techniques for preventing particle conversion may be avoided. When the particles are charged and the excess ions and/or the part of the charged particles are removed the charge carried by the particles stays with the sample gas flow even though the temperature of the sample gas flow is lowered. Because the charge carried by the particles is measured in the second lower temperature there are no strict requirements for the measuring components, such as insulators, cabling, and electronics but the structure of the measuring means may be rather simple and thus economical.

According to an embodiment, the first temperature is higher than 250°C. ln such a high temperature it is possible to achieve very reliable measuring results.

According to an embodiment, the second temperature is lower than 100°C. ln such a low temperature the structure of the measuring means is easily implemented.

According to an embodiment, the lowering of the temperature of the sample gas flow is performed by guiding the sample gas flow through a channel whereby the wall of the channel has a temperature that is lower than the first temperature. Such a cooling structure is simple and reliable.

According to an embodiment, the lowering of the temperature is performed by using a cooling gas flow having a temperature that is lower than the first temperature.

According to an embodiment, the cooling gas is used as a shielding gas for insulators in the measurement apparatus. This embodiment decreases thermophoretic losses.

According to an embodiment, the cooling gas flow is mixed with the sample gas flow thereby diluting the sample gas flow. By mixing the cooling gas with the sample gas flow the size of the cooling means may be formed to be rather small.

BR1EF DESCRIPTION OF THE DRAW1NGS

ln the following the invention will be described in greater detail by means of embodiments with reference to the attached drawings, in which

Figure 1 is a schematic cross-sectional side view of an apparatus for measuring particles; and

Figure 2 is a schematic cross-sectional side view of another apparatus for measuring particles.

DETA1LED DESCRIPTION OF THE INVENTION

Figure 1 shows an apparatus 1 for measuring particles in a sample gas flow A. The sample gas flow A flows into the apparatus 1 through a sample gas flow inlet 2 at a first end of the apparatus 1. At the second end of the apparatus 1 there is a sample gas flow outlet 3 wherefrom the sample gas flow A flows out of the apparatus 1.

The apparatus 1 may be positioned at an end of an exhaust pipe of a combustion engine of a vehicle, for example ln such case, the sample gas flow inlet 2 may be connected to the end of the exhaust pipe. Thus, the whole or at least the majority of an exhaust gas flow in the exhaust pipe forms the sample gas flow A. Correspondingly, in such case the sample gas flow outlet 3 may lead the sample gas flow A to ambient air, for example.

The apparatus 1 may also be situated by a side of an exhaust pipe of a combustion engine of a vehicle, for example ln such case a part of the exhaust gas flow in the exhaust gas pipe is lead to the apparatus thereby forming the sample gas flow A. Thus, the sample gas inlet 2 may be connected to an opening in the side of the exhaust gas pipe, for example. Correspondingly, the sample gas flow outlet 3 may connected to another opening in the side of the exhaust pipe, for example ln such case, the sample gas flow A is thus returned back to the exhaust gas pipe.

The apparatus 1 may be applied also to other embodiments. Examples of such embodiments are static power plant emission measurements, marine power plant emission measurements, furnace measurements and high temperature gas phase chemical reactor monitoring.

The apparatus 1 comprises a frame 4 through which a sample gas channel 8 is provided. The sample gas flow A flows through the frame 4 along the sample gas channel 8.

A clean gas flow B is provided to flow along a clean gas channel 5 in the frame 4. The clean gas forming the clean gas flow B may be clean air, for example.

A charger 7 is provided in the clean gas channel 5. The charger 7 may be a corona discharge needle, for example. The charger 7 ionizes the clean gas flowing in the clean gas channel 5 thereby forming an ionized clean gas flow C.

The ionized clean gas flow C may form a main flow of an ejector thereby forming an ejector flow. The ejector flow may generate an under pressure for driving the sample gas flow A into the apparatus 1 via the sample gas flow inlet 2. Another solution, however, is to provide the system with one inlet flow and one outlet flow and form the flow with a pump ln such case, the operation of the ejector would be redundant and would not affect the flow. The ionized clean gas flow C and the sample gas flow A are mixed in the sample gas channel 8. Thereby the ions in the ionized clean gas flow charge particles in the sample gas flow A.

lnstead of mixing an ionized gas flow with the sample gas flow particles in the sample gas flow may be charged by other means, also. Examples of these other means are photoionization, radioactive ionization and x-ray ionization.

The apparatus further comprises means for removing excess ions and/or a part of the charged particles from the sample gas flow A. ln the embodiment disclosed in Figure 1 the means is a voltage trap 9. The voltage trap 9 removes small charged particles due to their high electrical mobility in an electrical field. Thus, the voltage trap 9 may remove free ions from the sample gas flow A. The voltage trap 9 may also remove charged particles having a particle diameter smaller than a trap cut-off diameter. The size of the trap cut-off diameter may be controlled by adjusting the voltage of the voltage trap 9, for example lt is also possible to control the size of the trap cut-off diameter by adjusting the distance between trapping electrodes in the voltage trap 9, for example.

The apparatus 1 also comprises means for cooling the sample gas flow A. The means is positioned after an insulator 12 in the direction of the sample gas flow A. The voltage trap 9 may be annular. The insulator 12 insulates the voltage trap 9 from the frame 4. Thus, the insulator 12 may be a structural component and an electric insulator.

ln the embodiment disclosed in Figure 1 the means for cooling comprises a cooling sleeve 10. A cooling gas flow D is arranged to flow along a cooling gas channel 6 in to the cooling sleeve 10. The cooling sleeve 10 is positioned around the sample gas channel 8.

The cooling gas flow D may consist of a clean gas such as clean air. The wall of the sample gas channel 8 may comprise apertures 11. ln such case the cooling gas flow D is mixed with the sample gas flow A thereby diluting and cooling the sample gas flow A. The cooling gas flow D may also be guided to a shielding gas for shielding the insulator 12.

The cooling gas flow D also cools the wall of the sample gas channel 8. lf there are no apertures 11 in the wall of the sample gas channel 8 the cooling gas may be another cooling gas than clean gas. Thus, in such case the cooling gas may be ambient air, for example ln such case also, the apparatus 1 should be provided with a channel leading out of the cooling sleeve 10 to ambient atmosphere, for example.

lnstead of a cooling gas, another cooling fluid may also be used. Thus, the cooling fluid may also be cooling liquid such as water or oil, for example.

After the temperature of the sample gas flow A is lowered the charge carried by the particles is measured by a measuring device 13. The measuring device 13 may comprise a particle filter collecting at least a fraction of the charged particles, for example. Another embodiment is to use inductive measurement principle whereby by modulating the ionizing element or the trapping element variation of charged particle concentration is achieved and an inductive detection element detects the change in charge between the electrodes as current.

For ensuring a sufficient flow through the apparatus 1 a pump 14 may be provided in the apparatus 1. However, a pump is not necessary but a sufficient flow may be arranged using an ejector pump, for example ln such case the clean air to the charger should be pumped by using an external pump, for example.

A recirculation flow E may be formed to flow along a recirculation channel 15. Providing the system with the pump 14 and the recirculation channel 15 provides for the feature that the system keeps the hot inlet flow equal to the cool outlet flow. This is beneficial because there is no need to control the inner flows accurately and the sample flow easily remains constant.

The apparatus may be provided with a counter flow heat exchanger 21. The heat exchanger 21 may adjust the temperature of the recirculation flow E. The heat exchanger may be used for removing moisture or water from the sample gas flow A. A water separator may also be used for removing moisture or water. For controlling the moisture, it is also possible to insulate the system such that the temperature of the sample gas flow A remains on a level high enough. The temperature of the sample gas flow A may be arranged to remain above 60 °C, for example.

The apparatus may also be provided with a passive water absorber.

The passive water absorber may comprise water absorbing or water adsorbing material provided in a capsule structure. One example of such material is silica gel.

Preferably the passive water absorber is positioned to such part of the apparatus where the temperature of the gas flow is rather low. Thus, according to an embodiment the passive water absorber is positioned after the measuring device 13. ln such case the passive water absorber is closer to the gas flow outlet 3 than the gas flow inlet 2. According to an embodiment the passive water absorber is positioned in the sample gas channel 8 after the measuring device 13. According to an embodiment the passive water absorber is positioned in the recirculation channel 15 whereby it does not have substantial harmful effects on the gas flow in the sample gas channel 8. According to an embodiment the passive water absorber is positioned at the front part of the recirculation channel 15 near the gas flow outlet 3.

During cold start cycle, for example, the temperature of the gas exhaust pipe may be rather low. This could cause condensation and build-up of water to the exhaust system. When the exhaust system temperature rises, the excess water is evaporated into the exhaust, causing higher dewpoint than expected by the combustion process alone. The passive water absorber absorbs or adsorbs the excess water or moisture and thereby decreases or removes the need to raise the temperature of the gas flow in the device. The capacity of the passive water absorber is high enough to remove the excess water or moisture formed during the cold start cycle. After the cold start cycle the dew point of the gas flow is reduced whereby the material of the passive water absorber releases the water from it to the gas flow. Thereby the passive water absorber recovers and is ready to absorb water or moisture during the next cold start cycle, for example.

A filter 16 is used for cleaning the recirculation flow E. Thereby a clean gas flow B is formed.

The temperature of the sample gas flow A is first adjusted and balanced or stabilized to a rather high first temperature ln this connection, the first temperature means that the temperature is higher than a certain temperature. According to an embodiment, the first temperature is higher than 200 °C. According to another embodiment, the first temperature is higher than 250 °C.

The means for stabilizing the temperature of the sample gas flow A to the first temperature may comprise sample gas flow heater 17 and a clean gas flow heater 18, for example. The heaters 17 and 18 may be electric heaters comprising heating resistors, for example. The sample gas flow heater 17 may be positioned around the pipe that lead the sample gas flow A from the exhaust pipe to the apparatus, for example. The clean gas flow heater 18 may be positioned inside the frame 4 of the apparatus, for example. The means for cooling the sample gas flow A lower the temperature of the sample gas flow A to a second temperature ln this connection, the second temperature means that the temperature is lower than the first temperature. According to an embodiment, the second temperature is at least 50 °C lower than the first temperature. According to another embodiment, the second temperature is lower than 100 °C. lf the temperature of the sample gas flow A always remains above the condensation point of the sample gas there is no need for separate means for removing moisture or water ln such case the structure of the system is simple. Typically, the condensation point of an exhaust gas in an engine using liquid fuel is lower than 60 °C. Thus, according to an embodiment at least the second temperature is more than 60 °C and preferably the temperature of the sample gas flow throughout the apparatus is maintained above 60 °C.

ln the embodiment disclosed in Figure 2 the sample gas flow heater 17 is positioned inside the frame 4 of the apparatus 1. Furthermore, the sample gas flow inlet 2 and/or the pipe leading the sample gas flow A to the apparatus is provided with a thermal insulator 19.

ln addition, the means for removing excess ions and/or a part of the charged particles from the sample gas flow A is a diffusion trap or a diffusion collector 20. Other examples of the means for removing excess ions and/or a part of the charged particles from the sample gas flow are a cyclone and an impactor.

lt will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.