â–¡ Chen Zhonghui Zhang Wenge Huang Xingliang Liu Wei Liu Junjie The PM2.5 measurement process is divided into the collection of particulate matter and the weighing of the membrane. The collection of particulate matter requires the use of aerodynamic principles to separate PM2.5 from larger particles.
The PM2.5 cutter can separate particles in the atmospheric particles with an aerodynamic diameter of less than or equal to 2.5 pm. For PM2.5 cutters, 2.5 pm is a statistical value, ie particles with an aerodynamic equivalent diameter of 2.5 pm have a 50% probability of passing through the cutter. The PM2.5 cutter is the core component of the PM2.5 measuring instrument and determines the size of the collected particles. The cutting characteristic detection of PM2.5 cutter is the core part of the accuracy study of PM2.5 monitoring results.
Generally, the cutters are classified according to the separation principle: an impact cutter, a cyclone cutter, and a virtual cutter. Among them, the most common PM2.5 cutters are impact cutters and cyclone cutters.
The working principle of the impact type PM2.5 cutter is that the gas containing particles is sprayed from the nozzle at a certain flow rate (constant flow rate) under the action of the air pump, and the particles obtain a certain kinetic energy, and the particles have a particle diameter of more than 2.5 pm. Because the inertia is large, it can be deposited on the trap plate through the airflow, and the particles with the particle size less than or equal to 2.5 pm will be collected on the filter membrane for particle analysis due to the small inertia. Thereby achieving the collection of particles.
The cyclone cutter is composed of a cylindrical hopper and a conical chamber. When a constant flow of air enters the cutter and spirals inside the body, particles larger than 2.5 pm are trapped under the action of centrifugal force. In the grit chamber, particles with a particle size of less than or equal to 2.5 pm can be collected on the filter for particle analysis as the gas stream passes smoothly.
Second, PM2.5 cutter cutting characteristics research method introduction Because PM2.5 measurement and research started late, for the cutter cutting characteristics detection method, there is no perfect technology and standards in China, and foreign countries have completed the cutting characteristics detection method Analysis and research. For example, the US Environmental Protection Agency (EPA) stipulates three methods for detecting PM2.5 cutting characteristics: elution method, static box method and split method.
The monodisperse particles were generated by the oscillating orifice aerosol generator, and the monodisperse particle mass Mfflter collected on the filter membrane, the monodisperse particle mass McS collected inside the cutter, and the monodisperse adsorbed on the inner wall of the cutter were respectively detected by a fluorophotometer. The particle mass MwaE ("less"), the capture efficiency of the cutter for different monodisperse particles is calculated. For example, the specific operation procedure of the static box method is to install the cutter to be tested into a static box and occur in a static box. Monodisperse aerosol particles, using aerosol detectors to detect changes in aerosol concentration in static tanks to ensure uniformity and stability of aerosols in the tank. The aerodynamic particle size spectrometer was then used to measure the concentration of monodisperse aerosol particles in the static tank and at the cutter outlet, respectively. For a given aerodynamic diameter, the formula for calculating the trapping efficiency of the cutter is as shown in equation (2).
The number concentration of aerosol particles at the cutter exit; the number concentration of aerosol particles in the nref static tank.
3. The specific operation procedure of the split flow splitting method is to form a monodisperse aerosol by atomizing a monodisperse PSL suspension, which is divided into upper and lower branches through a flow divider after gas drying and electrostatic neutralization. Where the tested cutter is installed in the downstream branch, the cutting efficiency corresponding to the aerosol of different particle sizes is calculated by testing the ratio of the aerosol concentration upstream of the cutter and the aerosol concentration passing through the cutter, and fitting The cutting characteristic curve of the aerodynamic diameter and the trapping efficiency is deviated from the geometric standard.
The definition formula of the capture rate of the split method is as shown in the formula 丨3丨. This method has a short cycle and a large amount of data, and it takes only two hours to obtain a complete cutting characteristic curve.
Among the three methods of PM2.5 cutting characteristics specified by the US EPA, there are technical difficulties in the design of the static box method. For example, the tank is difficult to maintain uniform and stable over time; the elution method cannot measure the particle size. Distribution, the sphericity of the particles can not be guaranteed, resulting in the value cannot be traced, and the measurement process is long and the operation is cumbersome; and the shunting device is simple to process, easy to operate, and the test time is short, which is faster, less time-consuming and labor-saving. Measuring method. Comprehensive analysis of the above three kinds of cutting characteristics detection methods, we have developed a set of PM2.5 cutter cutting characteristics detection device based on the principle of shunting.
The PM2.5 cutting characteristic detecting device we studied mainly consists of a gas source generating device, an atomizing generator, a dryer, an electrostatic neutralizer, a mixing tube, a shunt, and an aerosol particle size spectrometer. The device structure is as shown. .
The working principle of the PM2.5 cutting characteristic detecting device: the clean air passes through the atomizing generator to form a monodisperse polystyrene aerosol. After drying and static elimination, it is diluted and mixed in the mixing tube to achieve a uniform and stable state. After passing through the diverter, it is divided into upper and lower two channels. The upstream branch is directly connected to the particle size spectrometer, and the downstream is first connected to the particle size spectrometer through the PM2.5 cutter to be tested, and the electromagnetic valve is switched. Thereby, the particle size spectrometer is alternately measured to measure the concentration of aerosol particles in two ways, as shown.
According to the different aerodynamic diameters, the structure of the PM2.5 cutting characteristics of the capture efficiency is matched, and the characteristic curve of the PM2.5 cutter is obtained. By analyzing the D5. and geometric standard deviation of the cutting characteristic curve, it is judged whether the performance of the PM2.5 cutter meets the relevant standard requirements.
The PM2.5 cutter uses aerodynamics to separate PM2.5 from larger particles. The flow rate of the cutter directly affects the size of the cutting particle. The flow entering the cutter deviates from the specified flow value, causing deviations in the cutting point. In addition, it is important to select the appropriate standard particles in the PM2.5 cutter cutting characteristics study.
Therefore, in the design of the PM2.5 cutting characteristic detecting device, the stability of the flow into the PM2.5 cutter and the selection of standard particles are also considered.
In order to verify that the cutter intake air quantity meets the standard requirements, the PM2.5 cutter intake air flow test is performed.
Connect the calibrated flowmeter to the sampling port, start the particle size spectrometer and bypass the pump, set the bypass flow rate to 13.67L/min, and record the flowmeter's indication. The test results are shown in Table 1.
Table 1 PM2.5 cutter intake air test results The flow rate value is calculated, the average value of the intake air flow is 16.50L/min, the relative error is 1.01%. The US EPA and China's environmental protection standards require PM2.5 The volumetric flow rate of the cutter is 16.67 (15%) L/min, so it can be seen that the flow into the cutter meets the US EPA requirements for PM2.5 cutter flow.
The US EPA and China's environmental protection standards for the standard particles used in the test are selected from 8 standard particles (polystyrene latex balls) with aerodynamic equivalent diameters ranging from U.5 to 4.5) pm, as shown in Table 2. .
Table 2 Particle size requirements of test particles and selection requirements (!m) Select particle size (!m) specified requirements (!m) Select particle size (!m) clean dry gas through the atomizer to generate the above 8 standards A monodisperse solid aerosol of particles. After drying and removing static electricity, dilute and mix in the mixing tube to reach a stable state.
Then, the cutter to be calibrated is connected in the gas path, and the number of aerosol particles on the upstream and downstream of the cutter is alternately determined by a real-time aerosol particle size spectrometer. Remember and. Up to 8 kinds of PM2.5 atomized monodisperse solid aerosol particles were obtained and N. repeated operations were performed 3 times, and 8 sets of capture efficiencies were calculated according to formula (4).
Solid monodisperse particulate concentration upstream of the vessel; solid monodisperse particulate concentration downstream of the N-cutter; capture efficiency (=1, 2, 3) for a single measurement per particle size point.
The average value of the collection efficiency of the eight particle size points calculated by the equation (5) is shown in Table 3.
Table 3 Mean value and standard deviation of different particle size capture efficiency (!m) Average capture efficiency (.) Mean standard deviation (,) Calculate the capture efficiency of each aerodynamic particle size according to Bessel's formula The average standard deviation Ci, if C exceeds 10%, the capture efficiency test of the particle size point should be redone.
It can be seen from Table 3 that the calculated average standard deviation of each particle size capture efficiency is less than 10%, indicating that the test results meet the requirements, and the data can be applied to the result analysis.
The average of the obtained trapping efficiencies was fitted to the corresponding aerosol aerodynamic particle size (see Table 3).
From the cutting curve (see), the corresponding aerodynamic equivalent diameter Da16=2.886pm for the PM2.5 cyclone cutter with a capture efficiency of 16%, 50%, 84%, and the standard cutting of the D2.5 meter PM2.5 cutter Characteristic curve Table 4 Comparison of test fitting curve and standard cutting characteristic curve Test result Standard PM2.5 cutter result IV. Conclusion 1. According to the three cutting characteristics detection methods specified by the US EPA, comprehensive analysis of the above three cutting characteristics detection The method is based on the principle of shunting method, and a set of PM2.5 cutter cutting characteristic detecting device is developed.
The US EPA and China's environmental protection standards require that the volume flow rate into the PM2.5 cutter be 16.67. The average flow rate into the cutter is 16.50 L/min, and the relative error is 1.01%. Using the PM2.5 cutting characteristic detecting device, The cutting characteristics of a standard PM2.5 cutter were studied in combination with the selected 8 standard particles, and the test results were satisfied: = (2.50.2) m; geometric standard deviation: ct = 1.20.1. In addition, the experimentally fitted PM2.5 cutting characteristic curve is compared with the standard PM2.5 cutter standard characteristic curve, D5. The difference is 0.079 pm, The relative error is 3.21%. Geometric standard deviation A new type of membrane gas meter temperature adaptability device research program â–¡ Gao Chunyi, membrane gas meter different verification method comparison Membrane gas meter belongs to volumetric gas flow meter, mainly used for measurement The cumulative volume flow of gas is widely used in gas measurement of civil and industrial gas such as natural gas and artificial gas. The membrane type gas meter uses a flexible diaphragm metering chamber to measure the volumetric flow of the gas. Under the action of the pressure difference, the gas is alternately entered into the metering chamber through the distribution valve, and is filled to the outlet port after being filled, and simultaneously pushes the flexible diaphragm in the metering chamber to reciprocate. Movement, through the conversion mechanism, the circulation process of the inflation and exhaust is converted into a corresponding gas volume flow, and then transmitted to the counter through the transmission mechanism to complete the gas cumulative measurement function. The verification of the membrane gas meter is mainly based on the G577-2012 membrane gas meter verification procedure. At present, the general methods for verifying the membrane gas meter are roughly classified into the following three types, and the characteristics are compared as follows.
1. Using the bell cover method to verify the characteristics: the bell cover itself has a constant pressure, no need to additionally configure the voltage regulator source (when below 0 degrees, the bell jar needs to be raised)
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