Device for determining the particles in an aerosol

Abstract

To achieve improved analysis of particulate matter particles, the invention provides a method for analyzing aerosol particles, wherein: in a first measurement stage, the aerosol is supplied to an optical aerosol measuring device without the influence of a controllable centrifugal separator; in at least one further measurement stage, the aerosol is fed to the optical measuring device under the influence of the centrifugal separator rotating at least at a speed other than speed 0; and the properties of the aerosol particles are analyzed from the measurement signals received from the optical measuring device in the first measurement stage and in at least one further measurement stage;For this purpose, a device is formed to analyze aerosol particles flowing through a measuring volume by means of an optical particle detection sensor unit that forms the measuring volume, in which a separator for the separation of particles sensitive to size and / or mass is connected upstream of the sensor unit.

Description

[0001] The invention relates to a device for determining particles of an aerosol flowing through a measuring volume.

[0002] There are initially simple optical measuring devices for measuring fine dust, which capture the fine dust in the air in a general way, as it were, in a channel and display the measured value in the form of a voltage value on a display.

[0003] Furthermore, there are sophisticated aerosol spectrometers that count individual particles based on a signal generated by light scattering. The strength of this scattering signal is then measured in a multitude of individual channels, up to 256 channels. The strength of the scattered light signal serves as a measure of particle size, allowing particles to be determined and assessed based on their size.

[0004] However, with such an aerosol spectrometer, only the optical size or optical diameter of the aerosol particles can be determined based on the received light signal. This can be the same for particles with different refractive indices, shape factors (round / angular), and densities; in other words, particles of different sizes can produce the same scattered light signal, or, depending on the aforementioned influencing factors, particles of the same aerodynamic size (diameter) can produce different scattered light signals.

[0005] US 2014 / 0122021 A1 discloses a method and apparatus for determining the separation efficiency of a cyclone separator by collecting solid particles separated by the cyclone separator, analyzing the particle size of the collected particles, calculating particle characteristic parameters and determining the separation efficiency of the cyclone separator.

[0006] DE 10 2009 000 904 A1 relates to a method and systems for calculating the size distribution of small particles. It provides for the provision of a reference matrix of pre-calculated reference vectors, where each reference vector represents a discrete particle size or a particle size range of a particle size distribution contained in a dilute colloid.

[0007] WO 99 / 41588 A1 shows a device for particle measurement with an inlet tube to the sensor unit and a centrifugal separator placed in front of it.

[0008] Based on this, the invention aims to further develop a method and a device of the type mentioned above in such a way that they enable an improved determination of the fine dust fractions by optical measurement.

[0009] According to the invention, the aforementioned problem is solved by a device having the features of claim 1.

[0010] The device according to the invention enables the determination of particle separation efficiencies in an aerosol with the separation fan stationary and as a function of the rotational speed of the separation fan, optionally using a multi-channel spectrometer for the individual particle fractions according to particle size. Calibration can be performed beforehand using standard aerosols with predefined particle sizes.

[0011] The device according to the invention results in an improved determination of fine dust particles, in particular the fine dust fractions, i.e., the smaller particles in an aerosol along with larger particles, which are separated to a greater or lesser extent, sometimes completely, when the fan is switched on, depending on its delivery rate. This overcomes the previously mentioned problems. Thus, by determining the particle numbers of the individual fractions in a spectrometric measurement of the optical particle diameters at different separator forces, the aerodynamic diameters of the particles in the fractions can be deduced.

[0012] A separator for size- and mass-sensitive particle separation is connected upstream of the sensor unit in a secondary branch. Preferably, the separator is arranged in a secondary branch leading to a main inlet of the sensor unit.

[0013] In a preferred embodiment of the device according to the invention, it is provided that the separator acts as a centrifugal separator and separates light and heavy particles by centrifugal force.

[0014] According to the invention, the feed pipe is surrounded by an annular channel which has an inlet for a fan at the lower end of the separator and an inlet for the feed pipe to the sensor unit at the upper end. Alternatively or additionally, it can be provided that an annular channel has an inlet for a fan at its lower end and an inlet for a feed pipe to the sensor unit at its upper end, wherein, in particular, the annular channel has a guide plate resting on it, helically shaped for a maximum of one turn, above its inlet.

[0015] A method that is not part of the invention is characterized in that the particles are fed to an aerosol spectrometer, preferably with a channel number of up to 256.

[0016] Aerodynamic diameters of the particles of the aerosol can be determined in particular as a function of the rotational speed of the centrifugal separator and thus of the electrical voltage applied to it d=f(do , D) or d=f(do , U), where d is the aerodynamic particle diameter, do is a measured optical particle diameter, D is the rotational speed of the centrifugal separator and U is the electrical voltage applied to it.

[0017] A method that is not part of the invention provides that particles of the aerosol are selected according to size by centrifugal force of the fan and / or that particles of the aerosol are selected according to size by centrifugal force of the fan in an ascending ring flow of the aerosol.

[0018] Furthermore, a further development of a method that is not part of the invention provides that the fractional separation efficiency FAG i = cn (dp ),Li / cn (dp ),L 0 of individual size fractions of the particles of the aerosol is determined, wherein cn (dp ),L i is the concentration of the fraction n=1...N, N=1...256 at measurement step i with a predetermined rotational speed switched on and cn (dp ),L 0 is the concentration of the fraction n at the first step 0 with the centrifugal separator switched off and thus at rest.

[0019] Further advantages and features of the invention will become apparent from the claims and from the following description, in which exemplary embodiments of the invention are explained in detail with reference to the drawing. This shows: Fig. 1 A schematic sectional view of a first embodiment of the device according to the invention; Fig. 2 A detailed view of the measuring unit of the Fig. 1 Fig. 3 shows a further embodiment of the device according to the invention; Fig. 4 shows the size distribution of the particles measured in the particle counter of the device according to the invention as a function of their diameter when the separator is stationary; Fig. 5 shows the fractional separation efficiency of a device according to the invention in Ab dependence on rotational speed; and Fig. 6 a representation of the signal profile of the particle counter for small particles or large particles over time with switching on and off of the separator.

[0020] A device 1 according to the invention of the Fig. 1 It has a sensor unit 2 with particle measuring device and a separator 3 positioned upstream of it.

[0021] The separator 3 has an inlet in the form of an inlet head 3.1, which, under a closed hood 3.11, has an annular double wall 3.1.2 through which unknown aerosols from the environment U can flow into the device, which surrounds an inlet 3.2.1 of a feed tube leading to the flow tube 2.1. The inner and outer rings of the double wall 3.1.2 each have circumferentially offset openings 3.1.3, which homogenize the incoming particle flow.

[0022] Below the inlet head 3.1, an outer tube 3.1.4 is arranged, in which a feed tube 3.2 to the flow tube 2.1 of the sensor unit 2 is arranged. An annular channel 3.4 is formed between tube 3.1.4 and flow tube 2.1.

[0023] A feed chamber 3.3, also with an inlet from the inlet head 3.1, is arranged to the side of the pipe 3.1.4. A fan 3.3.1, with a horizontal axis of rotation, is located in the lower part of this chamber. The annular channel 3.4 forms a return and separation channel from the fan 3.3.1 to the inlet of the feed pipe 3.2. Immediately below the inlet of the annular channel 3.4 from the fan 3.3.1, an annular channel and a helically supported guide plate are arranged, from which the feed pipe 3.2 extends by at most one turn.

[0024] The sensor unit 2 can be designed in the usual way, for example according to DE 3641716 A1 ( Fig. 3 ) or also the EP 2 717 035 B1.

[0025] The sensor unit 2 essentially comprises a flow tube 2.1 with a particle measuring device 2.2, to which a gas conveyor 2.3 is attached for drawing a particle-containing aerosol through the flow tube 2.1 and the particle measuring device 2.2. A gas conveyor is a conveying device for transporting a medium whose carrier fluid is gas. Aerosols also fall under this category of medium. Specifically, the gas conveyor can be designed as a fan or a pump.

[0026] The particle measuring device 2.2 - Fig. 2 The particle measuring device 2.2 has a known illumination unit 2.2.1, with which the particles flowing through the flow tube 2.1 are illuminated in a virtual measuring cell 2.2.2 formed by the light beam of the illumination unit in the flow tube 2.1. Furthermore, the particle measuring device 2.2 has a sensor or detector 2.2.3, by means of which scattered light from the aerosol particles flowing through the flow tube 2.1 is detected and, particularly when the measuring device is configured as a particle counter or spectrometer, light signals of the detected particles are counted or spectroscopically detected in individual channels corresponding to the particle sizes. A converging lens can be arranged downstream of the light source to generate a parallel beam. A converging lens is arranged upstream of the deflector 2.2.3 to focus the scattered light onto the deflector 2.2.3. A counting or evaluation unit 2.2 is arranged upstream of the detector.4 assigned, by means of which the particles detected by the detector are counted size-selectively in, for example, up to 256 channels according to their (optical) size given by the intensity of the light scattered by them, and thus a size distribution of the particles can be measured and output (. Fig. 3 ).

[0027] During measurement, the light intensity of the scattered light, and thus also the resulting electrical signal strength, is a measure of the particle size of the aerosol particles, to which a particle diameter is accordingly assigned. The measured particle size distribution is a function of the particle diameter.

[0028] When the fan 3.3.1 is switched off and therefore at rest, the entire aerosol, with all its particle size fractions, is drawn exclusively by the gas conveyor 2.3, which acts as a suction pump for the sensor unit 2. This aerosol is drawn through the inlet 3.2.1, the feed pipe 3.2, the flow pipe 2.1, the measuring unit 2.2, and the gas conveyor 2.3 to an outlet 2.6 of the sensor unit 2, and thus through the (virtual) measuring cell in the measuring unit 2.2. In this way, all scattering particle fractions of the aerosol flow through the measuring cell and are detected by the evaluation unit.

[0029] When the separator 3 is switched on by activating the fan 3.3.1, aerosol is drawn in by the latter through the feed chamber 3.3 and forced through the return channel. The particles are initially subjected by the fan 3.3.1 to centrifugal or radial forces and thus radial accelerations, which are greater for larger particles than for smaller particles. This causes larger particles to be separated, while only smaller particles enter the lower part of the annular channel 3.4 and are drawn by the gas conveyor 2.3 through the upper inlet of the feed pipe 3.2, the flow pipe 2.1, and thus the measuring unit 2.2, thereby contributing to signal generation in the measuring cell.

[0030] The aerosol stream is guided tangentially by the fan 3.3.1, via a guide plate 3.3.3, into the annular gap 3.4 forming a separation channel between the feed pipe 3.2 and the surrounding wall, thus generating a swirling flow. At the upper end, the aerosol is drawn off to the particle counter / aerosol spectrometer. The aerosol fed from the annular channel 3.2 to the fan 3.3.1 and the (residual) aerosol drawn in by the gas conveyor 2.3 are thus set into a helical upward flow, which is supported by the aforementioned guide plate. This allows further coarse particle fractions to settle, while only the smallest particles rise, enter the feed pipe 3.2 through its upper inlet, and are finally guided through the sensor unit 2. Only these small particles are detected and analyzed by the evaluation unit.

[0031] With fan 3.3.1 switched on, aerosol entering the feed tube from above is drawn through a guide plate 3.3.3 in the lower section of the feed tube 3.2 by the fan 3.3.1 via the annular channel 3.4 back to the inlet of the feed chamber 3.3. This ensures that, when the fan is switched on, virtually all of the aerosol passes through it and is subject to its separation action, allowing large and coarse particles to settle and smaller particle fractions to continue flowing in the aerosol. Separation can thus occur both directly by the fan and in the annular channel. The respective proportions depend on the specific aerosol and the fan speed. However, the main separation takes place directly at the fan. The separation channel forms an additional element that further improves separation efficiency. The annular channel therefore acts as a gravity separator.

[0032] The aerosol stream exiting separator 3 via flow tube 2.1 is drawn by pump 2.3 through measuring cell 2.2.2 of measuring unit 2.2 and illuminated there by lighting unit 2.2.1. Light scattered by individual particles of the aerosol strikes the sensor or detector 2.2.3 via the converging lens. The corresponding opti-electronically converted detected signal is then evaluated in the evaluation unit 2.2.4, in particular spectrometrically analyzed by a spectrometer with regard to particle size, for which, for example, 256 (size) channels can be available.

[0033] The Fig. 3 shows another embodiment of the device according to the invention, in which the structure of the sensor unit 2 is basically the same as in the embodiment of the Fig. 1 .

[0034] During the design of the Fig. 1 The fan is aligned with a vertical axis, as fan 3.3.1 exhibits in its design the Fig. 3 and 5 a horizontal axis.

[0035] The sensor unit 2 can be used in the design of the Fig. 3 be the same as with the Fig. 1 and 2 and described these figures.

[0036] Otherwise, the design of device 1 is according to the Fig. 2 basically the same as with the Fig. 1 Therefore, identical parts are marked with the same reference symbols, and existing identical features are also referenced in the representation of the Fig. 1 is referred.

[0037] A nozzle tube 3.1.4 is interposed between the inlet head 3.1 and the feed pipe 3.2 to ensure the aerosol flow is as laminar as possible. The feed chamber 3.3 with the fan 3.3.1 is arranged parallel to the feed pipe, which contains a guide plate 3.3.3 that becomes effective when the fan 3.3.1 is operating.

[0038] The operating principle is essentially the same: When the fan 3.3.1 is switched off and at rest, the aerosol is drawn solely through the gas conveyor 2.3 via the inlet head 3.1 into and through the feed tube 3.2, through the flow tube 2.1 and the measuring unit 2.2, and discharged via the gas conveyor 2.3. The entire aerosol, with all the particles it contains, especially particle fractions, passes through the measuring cell, so that all particles of the aerosol in the environment and entering the flow tube 2.1 scatter the light source and can therefore be detected.

[0039] If, on the other hand, the fan is switched on and rotating at high speed, the flow occurs via the secondary branch. The aerosol is swirled by the fan 3.3.1; larger particles acquire a higher radial velocity and are separated, according to the design of the Fig. 2 in the middle below the fan, while only smaller particles are sucked into the inlet head 3.1 by the gas conveyor 2.3 and pass through the measuring cell.

[0040] The foregoing is from the Fig. 3 bis 5 evident: The Fig. 3 shows the device 1 according to the Fig. 1 , or 2 measured particle distribution cn , i.e. the particle concentration as a function of the particle size d, specifically with the upper curve with the separating fan switched off or stationary. This shows that a significant signal still occurs with larger particles.

[0041] In contrast, when the fan is switched on, a greater decrease in the particle size distribution with larger particle diameters occurs, as shown by the middle curve at a fan speed of 1000 rpm, indicating a significant reduction in the concentration of larger particles. Increasing the fan speed to 4000 rpm results in a further reduction in the concentration of larger particles, as seen in the lower curve.

[0042] In the Fig. 5 The separation efficiency or fractional separation efficiency FAG i = cn (dp ),L i , is given as a function of the upper particle size.

[0043] The Fig. 6A sensor light signal, again showing scattered light from particles, is displayed as a function of time, with curve A for small particles and curve B for large particles. The fan is switched off until time T1, switched on at T1, and switched off again at T2.

[0044] It appears that when the fan is switched on, the measurement signal B is significantly reduced in the case of coarse particles.

[0045] The measurement results with the fan switched on and off, as well as with the fan stationary (speed 0) and rotating at various speeds, provide additional information compared to a purely optical measurement by a particle counter without any influence on the aerosol or the separation behavior for particles of different sizes, due to the aerodynamic separation behavior of the aerosol particles, particularly their size, mass, and / or shape factor. This is in contrast to previous purely optical measurements. While the above descriptions the relationships qualitatively, the behavior of the particles as a function of fan speed is empirically determined using calibration fine dusts with predefined sizes and other properties. This calibration of the influence of fan speed thus allows for an improved calculation of the fine dust fractions of an aerosol.

Claims

1. A device for determining particles of an aerosol flowing through a measuring volume, which device has an optical sensor unit (2) forming the measuring volume for detecting particles, wherein a separator (3) for size-and / or mass-sensitive separation of particles is arranged in a secondary branch upstream of the sensor unit (2), having an inlet (3.1) to the surroundings and wherein the feed tube (3.2) is surrounded by an annular channel (3.4) which has an inlet of a fan (3.3.1) on the lower end of the separator (3) and an inlet to the feed tube (3.2) to the sensor unit (2) on the higher end.

2. The device according to claim 1, characterized in that the separator (3) acts as a centrifugal separator and separates light and heavy particles by means of centrifugal force.

3. The device according to any of claims 1 or 2, characterized in that the annular channel (3.4) has a helical overlying deflector plate over its inlet, which deflector plate is helical over a maximum of one turn.

4. The device according to any of claims 1 to 3, characterized in that the sensor unit (2) has a light source (2.2.1) irradiating a measuring cell (2.2.2) of the sensor unit (2), and has a scattered light detector (2.2.3) and an electronic evaluation unit (2.2.4).

5. The device according to claim 4, characterized in that the evaluation unit (2.2.4) has a spectrometer, in particular for particle size determination.

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