WO2013121096A1

WO2013121096A1 - Apparatus and method for particle measurement

Info

Publication number: WO2013121096A1
Application number: PCT/FI2013/050137
Authority: WO
Inventors: Kauko Janka; Juha Tikkanen
Original assignee: Pegasor Oy
Current assignee: Pegasor Oy
Priority date: 2012-02-19
Filing date: 2013-02-08
Publication date: 2013-08-22
Anticipated expiration: 2014-08-19
Also published as: FI20125189A7; FI20125189L; EP2815224A1

Description

Apparatus and method for particle measurement

Field of invention

The present invention relates to an apparatus for monitoring particles and especially to an apparatus as defined in the preamble of independent claim 1. The present invention also relates to a process for monitoring particles according to the preamble of independent claim 6.

Description of the state of the art

Fine particles having diameter between 1 nanometer and 10 micrometers are formed in many combustion processes. For various reasons these fine particles are measured. The fine particle measurements may be conducted because of their potential health effects and also for monitoring operation of combustion processes, such as operation of combustion engines, especially diesel engines. Due to the above reasons there is need for reliable fine particle measurement apparatus.

One prior art method and apparatus for measuring fine particles is described in document WO2009109688 Al. In this prior art method clean, essentially particle free, gas is supplied into the apparatus and directed as a main flow via an inlet chamber to an ejector provided inside the apparatus. The clean gas is further ionized before and during supplying it into the inlet chamber. The ionized clean gas may be preferably fed to the ejector at a sonic or close to sonic speed. The ionizing of the clean gas may be carried out for example using a corona charger. The inlet chamber is further provided with a sample inlet arranged in fluid communication with a channel or a space comprising aerosol having fine particles. The clean gas flow and the ejector together cause suction to the sample inlet such that a sample aerosol flow is formed from the duct or the space to the inlet chamber. The sample aerosol flow is thus provided as a side flow to the ejector. The ionized clean gas charges the particles. The charged particles may be further conducted back to the duct or space containing the aerosol. The fine particles of the aerosol sample are thus monitored by monitoring the electrical charge carried by the electrically charged particles. Free ions may further be removed by an ion trap.

In addition to the above mentioned fine particles, industrial processes and combustion processes form usually also particles having particle diameter greater than 1 μιη, or greater than 2 μιη, 3 μιη, 5 μιη or even greater. These coarse particles having particle diameter greater than 1 μιη may be formed in small amounts in normal operation conditions, but especially in special operation conditions such as during startups, shutdowns, malfunction conditions. The size distribution of the diesel engine exhaust particles generally shows three different modes: the nuclei mode consists of particles having a diameter of less than approximately 50 nm, the accumulation mode consists of particles having diameters between 50 nm and 1 μιη and in the coarse mode the particle diameter is greater than 1 μιη. A majority of the diesel engine exhaust particles is born after the exhaust gases escape from the exhaust pipe and these particles typically belong to the accumulation and nuclei mode.

One important demand for the fine particle monitoring apparatuses especially for onboard-diagnoses of diesel engines is small and compact construction. Furthermore, it is also preferable that these fine particle monitoring apparatuses may be operated long time periods without need for maintenance. In many applications, such as monitoring fine particles of combustion engines, it is further preferable that the monitoring apparatus may be operated continuously for conducting fine particle measurements in real-time.

In order to fulfill the long operation time requirement, it is essential that the fine particle monitoring apparatus is not blocked by particle, i.e. avoidance of apparatus soiling. One of the critical soiling areas is the inlet nozzle for sample flow inlet, leading to the actual measurement space.

US patent 4,307,061, Robert Bosch GmbH, 22.12.1981 describes a self-recovering soot detector, particularly to monitor carbon content in diesel engine exhaust gases. An insulating support body, for example an aluminum oxide ceramic, supports two electrodes spaced from each other by a small gap, for example 0.1 mm, which will have there between a high resistance. Upon collection of soot, the resistance between the electrodes across the gap will drop, which can be indicated by sensing current through the electrodes upon connection to a source of electrical energy. To remove soot upon termination of smoking, or soot contents in the gases, the electrodes are applied over, or embedded in a layer of essentially nonconducting catalyzing material which, in the presence of oxygen, catalyzes the oxidation of soot located in the gap between the electrodes to thereby remove the soot by oxidation and restore the resistance of the path between the electrodes and hence the restore also the sensitivity of the sensor for subsequent detection of accumulation of soot in the gap.

Preferably, the non-conductive catalyzing material is a mixture of platinum, or a platinum metal, or a platinum metal alloy and a metal oxide which is compatible with, or identical to the ceramic base, for example aluminum oxide. The essentially electrically non-conductive layer can be applied by thick-film technology, and the electrodes also by thick-film

technology there over, or the electrodes may be in the form of fine platinum wires extending through the catalyzing electrically non-conductive layer. The sensing element can be held in a housing or socket, similar to a spark plug socket. The described solution for soiling removal is, however, complex.

PCT application WO 2008/138849 Al, Robert Bosch GmbH, 20.11.2008, relates to a method for the detection of particles in a gas flow with a sensor element comprising at least two electrodes. A measurement voltage is applied to the electrodes of the sensor element during a measurement phase, wherein particle paths formed by the accumulation of particles short circuit the electrodes, and the resulting current flow, voltage reduction, and/or electrical resistance is measured and is given as a measurement of the concentration and/or mass flow rate. The invention is characterized in that during a regeneration phase following the measurement phase, the accumulated particles are partially or entirely removed by raising the measurement voltage applied to the electrodes to a regeneration voltage. This approach requires complex power supply design and construction.

There is a need for an improved particle measurement apparatus and process where soiling of the particle measurement apparatus, especially soiling of the inlet nozzle, can be avoided. There is also a need to use the particle measurement device at its optimal concentration measurement range, as in many cases the output signal of the particle measurement device gets saturated at high particle concentrations and respectively the output signal gets noisy at low particle concentrations.

Summary of the invention

The object of the present invention is to provide an apparatus so as to overcome the prior art disadvantages. The objects of the present invention are achieved with an apparatus according to the characterizing portion of claim 1. Another object of the present invention is to provide a process so as to overcome the prior art disadvantages, achieved with a process according to the characterizing portion of claim 6. The preferred embodiments of the invention are disclosed in the dependent claims. The invented process for fine particle measurement comprises pulling sample flow (F) into a measurement apparatus, feeding dilution flow (F_A) into the measurement apparatus, mixing the sample flow (F) with dilution flow (F_A), measuring particle concentration from the diluted sample flow (F+F_A) and adjusting dilution flow on the basis of the particle

concentration measurement result. The advantage of adjusting the dilution flow, which can be adjusted so that the F/F_A ratio can be varied in a large range, on the basis of the particle measurement result, is obvious for a person skilled in the art or fine particle measurements: it allows the use of the measurement apparatus at its optimal particle concentration

measurement range and greatly reduces or even completely hinders soiling of the inlet nozzle.

As the use of the measurement apparatus at its optimal particle concentration measurement range, and preventing apparatus soiling has greatest effect in long-term measurements, the invented process and apparatus are best suited for use with non-collecting particle measurement apparatuses, i.e. with measurement processes which do not deliberately collect particles.

Sample flow is beneficially pulled into measurement apparatus by using an ejector pump where essentially clean gas is used for the motive fluid flow and thus the measurement result is not erroneously affected by the particles in the motive fluid flow.

In some cases, i.e. in the case of abnormally high particle concentrations, the dilution flow would need to be set to a value which would lead to inaccurate concentration

measurement results, e.g. in the case where the dilution flow is much higher than the sample flow (F_A/F ratio larger than e.g. 100). In such cases the invented process comprises closing the inlet and/or outlet to and/or from the measurement apparatus and thus protecting the measurement apparatus from particle contamination in cases where measurement cannot be accurately made.

It is beneficial for the invented process that the response time for setting the dilution flow F_A is shorter than the response time of the measurement apparatus, or the measurement volume of the apparatus. Typically flow controllers with less than 100 ms response time are well suited for the purpose.

In one embodiment of the present invention the annular dilution flow F_A around the sample flow is laminar and thus the flows are mixed mainly due to diffusion only. With suitable flow velocities and nozzle geometries, especially with pretty thick laminar flow layer and short inlet nozzle length, the diffusion-based-mixing of particles is so slow that there is no considerable particle accumulation to the inner walls of the inlet nozzle. The fact that the diffusion rate of particles is much lower than the diffusion rate of gases, can be used to benefit aerosol sampling, e.g. the condensation of water vapour to the particle measurement apparatus can be significantly decreased by using dry dilution gas. Water vapour present in the sample gas is quickly diffused and mixed to the dilution gas while the particles remain mostly unmixed. Due to this slow diffusion of aerosol particles compared to gas components and heat transfer, the cooling (or heating) and dilution of gas component can take place in the flow, while particles does not mix into the outer core. This prevents particle deposition on the inlet tube surface.

In another embodiment of the present invention, the dilution flow F_A is blown essentially radial towards the sample flow F. "Essentially radial" means in this case that the dilution flow F_A has a flow velocity v_r towards the sample flow which is at least 10% from the dilution flow's (F_A' S) velocity parallel to the sample flow v_p.

It is obvious for a person skilled in the art that different embodiments can be combined and a combined embodiment of the present invention may e.g. comprise a first part of the inlet nozzle with a first dilution flow F_AI blown essentially radially towards the sample flow F and a second part of the inlet nozzle with a laminar annular second dilution flow F_A2 around the sample flow and the first dilution flow F+F_Ai. Such an arrangement ensures minimum particle accumulation to the entrance of the inlet nozzle and is especially beneficial when the sample flow is taken from a hot environment where thermophoretic particle accumulation is a problem well known to a person skilled in the art.

In the case of a hot sample cooled and diluted by cool and clean gas, the

thermophoretic force to the particle directed from the hot core to the cooler outer region yields particle-deposition tendency to the flow-channel surface. The mentioned radial component of clean gas flow toward this thermophoretic force (and motion) can prevent the corresponding thermoforetic deposition.

In the preferred embodiment of the present invention, the suction which drives the sample flow through the inlet nozzle is generated by an ejector which is placed inside the particle monitoring apparatus. Furthermore, the preferred embodiment the motive fluid of the ejector is ionized and the motive fluid flow is - in addition to suction generation - used to charge the particles entering the particle monitoring apparatus. This ensures that the

measurement method described in WO2009109688 Al can be used for particle measurement.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the 5 appended schematic drawings, where

Fig. 1 shows a schematic drawing of a particle measuring apparatus with means for dynamic dilution;

Fig. 2 shows an embodiment with a laminar annular flow F_A escaping through a slot;

.0 Fig. 3 shows an embodiment with a laminar annular flow F_A escaping through holes;

Fig 4 shows an embodiment with annular flow F_A directed at least partly towards the sample flow;

Fig 5 shows an embodiment with annular flow F_A directed by Coanda effect;

.5 and

Fig 6 shows an embodiment where the control means are situated outside the particle sensor.

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the

[0 invention and which are obvious for a person skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention.

Detailed description of preferred embodiments

Figure 1 shows a schematic drawing of a particle measuring apparatus 1 with an dilution flow F_A. The particle measuring apparatus 1 comprises means 7,8 for ionizing

[5 essentially clean gas flow F_c which is fed into apparatus 1 from gas conduit 4 and through channel 5 and is ionized by corona charger 7 which is electrically insulated by insulator 8 so that a high voltage can be applied to the corona charger 7. "Essentially clean" means in this case that the particle number concentration, expressed in 1/cm , of the essentially clean gas is much lower than the particle number concentration in the sample flow F. Preferably the particle number concentration in the essentially clean gas, N_cg is 10^"1 ... 10^~6 times the particle number concentration in the sample flow, N_s and more preferably N_cg = 10^"3...10^"6 x N_s. The clean gas flow enters into an ejector which consists of ejector inlet 9a, ejector throat 9b and ejector outlet 9c. The clean gas flow forms the motive fluid flow of the ejector and generates a negative pressure (as compared to the space from which the sample flow F is taken) to the inlet nozzle 14, which negative pressure ensures sample flow F to the particle measuring apparatus 1. Ionized clean gas charges particles flowing into the apparatus 1 with the sample flow F in the mixing chamber formed by the ejector inlet 9a and ejector throat 9b and charged particles escape the apparatus 1 through outlet channel 3.

Apparatus 1 comprises a sensor 101 whose output signal corresponds to the concentration of particles flowing through apparatus 1, i.e. the concentration of particles in the sum of the sample, dilution and clean gas flow (F + F_A + F_c). Knowing the value of the individual flows F, F_A and Fc allows determining particle concentration in the sample flow F only and thus provides means for particle concentration measurement. Sensor 101 is connected to a functional unit 102 which provides information on the particle concentration. The term "functional unit" means that the unit 102 is functionally an essential part of the present invention but can physically be constructed in various ways, i.e. by analogue or digital means and it can situate either inside or outside the actual measuring apparatus 1. The functional unit 102 is connected into another functional unit 103, which controls the dilution flow through flow control unit 6*. Functional unit 103 can physically be constructed in various ways, i.e. by analogue or digital means and it can situate either inside or outside the actual measuring apparatus 1 or it can be integrated into flow control unit 6*.

In the preferred embodiment of the present invention particle concentration measurement is based on measuring the current escaping with the charged particles as explained in prior art publication WO2009109688 Al.

In one embodiment of the present invention, apparatus 1 for fine particle

measurement comprises the inlet nozzle 14 for sample flow F inlet. The nozzle 14 comprises the gas conduit 4 for supplying dilution gas flow F_A as well as clean gas flow F_c into apparatus 1, the mixing chamber formed by the ejector inlet 9a and ejector throat 9b for mixing the sample flow with dilution flow (and clean gas flow Fc) and means 12, 13 for arranging essentially particle free annular flow F_A essentially around the sample flow F. The phrase "essentially around" means that the annular flow F_A surrounds the sample flow F in such a way that no considerable particle accumulation takes place from the sample flow F on the in inner wall of the inlet nozzle 14. This prevents clogging of the inlet nozzlel4.The annular gas chamber 12 has an average radial surface area A_r and the nozzle 13 providing essentially particle free annular flow F_A essentially around the sample flow F has a total surface area A_FA, wherein the average radial surface area A_r preferably is more than five times and more preferably more than ten times the total surface area A_FA. This ensures that the flow F_A is distributed uniformly from the nozzle 13.

In another embodiment of the present invention, apparatus 1 comprises at least one annular slot 13 in the means 12, 13 for arranging essentially particle free annular flow F_A essentially around the sample flow F, as shown in Figure 2. The annular slot 13 ensures, with a suitable slot width and dilution gas composition and flow F_A, controlled by a flow controller 6, that the annular flow F_A is laminar, i.e. its Reynolds number, a dimensionless number well known to a person skilled in the art, is preferably less than 3000, more preferably less than 1000 and most preferably less than 250.

In yet another embodiment of the present invention shown in Figure 3, essentially circular holes 13, in the means 12,13 for arranging essentially particle free annular flow F_A essentially around the sample flow F, are used to guide the essentially particle free annular flow F_A essentially around the sample flow F. The advantage of using such circular holes is that manufacturing such holes with tight tolerances is easier than setting an annular slot.

In yet another embodiment of the present invention, shown in Figure 4, at least a fraction of the annular dilution flow F_A is directed towards the sample flow F. "Towards" means in this case that the angle between the sample flow F and the fraction of the annular flow F_A directed towards the sample flow F is higher than 0°, preferably higher than 30° and most preferably higher than 60°. Such directed flow effectively drives the particles away from the surface of the inlet nozzle 14. In one embodiment of the present invention, the directed flow is arranged by a porous conduit 13, which can be manufactured e.g. from sintered metal powder. In another embodiment of the present invention, the directed flow is arranged by a perforated conduit, which can be manufactured by various techniques obvious for a person skilled in the art of machining. In yet another embodiment of the present invention, shown in Figure 5, the annular flow F_A passes into the inlet nozzle 14 through a slot 13, which is arranged to the vicinity of a curved surface (convex contour) 15. The curved surface 15 works as a Coanda surface, hugging the gas stream F_A to the convex contour 15 when the flow F_A is directed at a tangent to that surface and thus preventing particle accumulation on the inner walls of the inlet nozzle 14. Such an embodiment has the further advantage that the Coanda surface works also as an ejector pulling the sample flow F into the particle measurement apparatus 1.

Figure 6 shows an embodiment of the present invention which clarifies the meaning of functional components 101, 102 and 103. Apparatus 1 consists of various components and most of them are not placed inside sensor 1* which comprises the actual particle measurement sensor 101, but also comprises heater He for heating sensor 1*, thermocouple Tc for measuring temperature of sensor 1* and thermal switch TSw for preventing overheating of sensor 1*. Clean gas flow, in this case clean pressurized air flow, is fed into sensor 1* from conduit 4 through oil separator OS, water separator WS, filter F, active carbon filter ACF and membrane dryer, which guarantee that the clean gas is particle and moisture free. The clean air pressure is adjusted by pressure regulator PR, and the pressures upstream and downstream the regulator are measured by pressure meters PM. The moisture level is controlled by a dew point meter DPM. The clean gas flow Fc into sensor 1* can be switched ON and OFF by a solenoid valve SV. The clean gas flow Fc generates suction to the sensor 1* inlet and thus sample flow F is drew into sensor 1*. The volumetric value of sample flow F depends in normal operation only on the pressure set by the pressure regulator PR. Sample flow F is combined with dilution gas flow F_A which is fed from the clean gas channel via solenoid valve SC, flow controller 6* and check valve CV. The diluted sample flow, which is the sum of the sample flow and the dilution flow, F+F_A, is further directed to sensor l*via heater He and ball valve BV. Temperature of (essentially) heater He is measured by thermocouple TC and heater He is protected by thermal switch TSw, which sets heating power OFF in case of overheating. Flow outlet from sensor 1* passes through ball valve BV before it is exhausted. The temperature of the outlet tubing can be monitored by a thermocouple TC.

The ball valves are operated by pneumatic actuators PA, which are set to different positions by pressurized gas entering actuators PA through solenoid valves SV. When the solenoid valves are set OFF, pneumatic actuators return to OFF position due to the spring force and pressurizing gas exits from the actuator via silencer S.

The output signal of sensor 6*, corresponding to particle concentration in the flow passing through sensor 1*, i.e. combined flow F+F_A+F_C, is fed from sensor 1* into a programmable logic which contains the functional units 102, 103. Based on the concentration result, functional units 102, 103 set the dilution flow F_A to a value which keeps sensor 1* working in the optimal measurement area. The actual figures depend very much on the particle measurement apparatus 1 construction, but e.g. for a Pegasor^® PPS, Pegasor Oy, Finland particle sensor, the clean gas pressure PR is set to 1... 2 bar (as compared to ambient air pressure), sample flow F is 3-10 1/min, clean gas flow Fc is 2-8 1/min and dilution flow F_A is from zero to F. Dilution flow F_A is typically, but not necessarily, set by a mass flow controller based either on thermal or Coriolis measurement. The selection of the flow controller 6* depends mainly on the dynamic requirements set to the controller. Generally it is beneficial for efficient dilution flow control that the response time of the flow controller 6* is less than the response time of sensor 1*. Typically the response time of the flow controller 6* has to be less than 100 ms.

If the sensor detects very high or otherwise abnormal concentration levels, it is possible to close at least the inlet ball valve and thus prevent exposing sensor 1* to these abnormal concentrations.

It is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims herein below.

Claims

1. Apparatus (1) for particle measurement, comprising:

a. means (9a-c) for pulling sample flow (F) into apparatus (1);

b. means (6*)for feeding dilution flow (F_A) into the measurement apparatus; c. means for mixing sample flow (F) with dilution flow (F_A);

d. means (1*) for measuring particle concentration; and

e. means (102,103) for adjusting dilution flow on the basis of the particle

concentration measurement result;

characterized in that the means (102, 103) for adjusting dilution flow has a faster response time than the response time of the means (1*) for measuring particle concentration.

2. Apparatus ( 1 ) of claim 1, characterized in that the response time of the means (102,103) for adjusting dilution flow is less than 100 ms.

3. Apparatus ( 1 ) of claim lor2,characterized in comprising non-collecting means (101) for measuring particle concentration.

4. Apparatus (1) as in any of the previous claims, characterized in comprising: a. means for feeding essentially clean air into apparatus (1); and

b. ejector (9a-c) where the essentially clean air flow forms the motive fluid flow.

5. Apparatus (1) as in any of the previous claims, characterized in comprising: means (BV) for closing the inlet and/or outlet flow to/from apparatus (1).

6. Process for particle measurement, characterized in comprising:

a. mixing sample flow (F) with dilution flow (F_A);

b. measuring particle concentration; and

c. adjusting dilution flow on the basis of the particle concentration measurement result with the dilution flow adjustment response time being faster than the response time of the particle concentration measurement.

7. Process of claim 6, characterized i n comprising measuring the particle

concentration without deliberately collecting particles.

8. Process of claim 6 or 7, characterized in comprising:

a. providing essentially clean air for the motive fluid flow of an ejector; and b. using the ejector to suck sample flow for particle concentration measurement.

9. Process as in any of the claims 6 to 8, characterized in comprising closing the inlet and/or outlet to and/or from the measurement apparatus in case of abnormal particle concentrations.