I. Commonly used measurement principles in compressed air flow measurement
Measurement of airflow in compressed air, that is, the flow measurement of compressed air, in industrial measurement, especially in energy measurement is of great importance. For compressed air flow measurement device, can be roughly divided into several categories: First, the throttle device class, such as nozzle flowmeter, orifice plate flowmeter, rotor flowmeter, etc.; Second, through some principles to measure the gas flow rate for the purpose of measuring the gas flow rate, indirectly calculating the gas flow rate of the measurement method, such as the turbine flowmeter, vortex flowmeter, electromagnetic flowmeter, etc.; Third, through some principles, the direct measurement of the gas quality of the flow rate, and later Third, through some principles, directly measuring the mass flow of gas, and then converted to volumetric flow measurement methods, such as Coriolis flowmeter, thermal mass flowmeter and so on.
With the development of science and technology, electronic information industry leap, some mechanical instruments have been slowly eliminated in the development of the times, leaving some of the decades of application in the practice of verification and selection, the survival of the fittest, and gradually formed the current mainstream in the industrial application of several measurement principles of flowmeter.
1. A more widely used is based on the vortex principle of flowmeter. Vortex measurement principle, also called vortex measurement principle, that is, the use of Carmen vortex body, when the air flow through the Carmen vortex generator, it will be formed in the downstream of the rule of two asymmetric vortex columns, and the vortex frequency and the air flow through the vortex generator when the flow rate has a definite relationship, so when the shape and size of the vortex generator is determined, you can measure the flow rate of the vortex generating frequency to measure the flow rate, and then with the pipe diameter size, temperature, temperature, and other factors, the flow rate is measured. Therefore, when the shape and size of the vortex generator are determined, the velocity of the airflow can be measured by measuring the frequency of the vortex generation, and then combined with the pipe size, temperature, pressure and other parameters, the flow rate can be calculated. The principle of vortex measurement is shown in the figure:
According to the vortex principle of flowmeter design has some universal advantages, such as: simple structure, stable measurement, no moving parts, long service life, in a certain range of flow rate measurement is more accurate. At the same time, the vortex measurement principle for the measurement of the medium is compressed air quality requirements are not high, in a certain measurement position even if there is water or some impurities does not affect the stability of the measurement. The vortex is also one of the earliest principles used in air measurement, measurement technology is mature, many manufacturers, manufacturing costs are relatively inexpensive, these characteristics make the vortex flowmeter in compressed air flow measurement in a wide range of applications. However, with the diversity of measurement needs and the improvement of measurement accuracy requirements, in the compressed air measurement needs have gradually revealed some of the disadvantages of the vortex flowmeter: pipeline installation is relatively more complex to use, the relative limitations of the measurement range and vibration for the measurement of the interference is greater, and so on.
2. A thermal mass flow meter based on the principle of thermal balance. When the thermal sensor around the flow of the medium will have the transfer of heat, in the steady state thermal sensor dissipated into the surrounding gas heat and gas mass flow rate has a definite relationship. By measuring the temperature change of the surrounding medium, the mass flow rate or volume flow rate of the gas can be obtained. A sketch of the thermal mass measurement principle is shown below:
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Thermal flow meters are further divided into types according to classification such as heat loss type i.e. constant power type, thermal temperature difference type i.e. constant temperature difference type and thermal pulse type. With the development and practical test of thermal mass flow meter, immersion type or called probe type thermal mass flow meter occupies the dominant thermal flow meter. Especially in compressed air flow measurement, the probe type thermal mass flow meter is used in the majority of industrial enterprises in the market for compressed air flow measurement.
Thermal mass flow meter because of its heat transfer principle, with some other principles of flow meters do not have and irreplaceable advantages. First, the measurement range is wide, because of the sensitivity of heat transfer, thermal mass flow meter usually range ratio in 100:1 or more, the excellent thermal flow meter can reach 300:1, so that the thermal mass flow meter for the measurement of small flows, especially small flows have a natural advantage; Second, the measurement of the thermal flow meter is directly measured by the mass flow rate of the gas, can be converted to get the volume flow rate of the gas. The conversion process is only based on the density of the gas in the standard condition of the calculation, do not need to measure the temperature and pressure of the gas and other operating conditions, reducing the measurement error, thereby directly improving the accuracy of the measurement; Third, the typical probe type thermal mass flowmeter is generally designed for insertion in the form of the installation and use in the installation and use of the user provides a convenient and fast installation form, especially suitable for some of the existing pipe Improvement of the measurement needs. But at the same time, the thermal mass flow meter also has some obvious shortcomings, for example, for the quality of the medium and humidity requirements are higher, for some gases containing impurities or water content, the measurement will be affected by the more obvious. Therefore, in the compressed air flow measurement, some special position measurement, such as air compressor out of the position of the flow measurement, and is not suitable for the use of thermal mass flow meter.
3. Another commonly mentioned flow meter is based on the ultrasonic principle. Ultrasonic principle is through the detection of ultrasonic pulse propagation and counter-current velocity difference to detect the size of the wind speed, and then combined with the temperature, pressure and other parameters of the working conditions to calculate the gas flow. According to the different calculation formulas can be divided into time difference method, frequency difference method and phase difference method. Ultrasonic principle of flowmeter has some unique advantages, because the measurement process can be outside the pipeline through the sensor affixed to the pipeline, so the measurement of the pipeline is not broken, measuring no pressure loss, no pollution of the medium, while the range is wide, can be measured in both directions, and other characteristics are also very advantageous.
Ultrasonic principle flowmeter with the development of microprocessor technology and signal processing over the years, has been widely used in water, oil and other media measurement, but the measurement of gas is still in the development stage, there is no large-scale application. The main reason is still limited to ultrasonic measurement principle of measurement is limited by the site of changing conditions (such as flow, pressure and temperature) and a variety of different types of “noise” interference.
In addition to the above, the nozzle flowmeter, electromagnetic flowmeter, Coriolis thermal flowmeter, orifice plate flowmeter and other principles of flowmeter in the field of gas measurement have certain applications, but in the compressed air flow measurement, the application is relatively limited, this paper will not be discussed one by one. And for the Pitot tube principle flowmeter in compressed gas applications, the fourth part of this paper will focus on research.
Second, compressed air flow measurement needs
Compressed air flow measurement compared to simple air velocity measurement, with complexity and specificity. Comparison of the standard industrial compressed air system mainly contains the following parts: compressed air production such as air pressure station, compressed air transportation such as transportation pipe network and compressed air use of compressed air production and use of the workshop. For the different components of the compressed air flow measurement will have different needs, need to be differentiated in the application and according to the different needs of the choice of the appropriate measuring principle of the flowmeter, in order to achieve the purpose of accurate measurement.
1. Compressed air production side of the flow measurement needs. For the production side of the air pressure station that the flow measurement needs are more complex, usually need to be broken down gradually from simple to complex. First of all, for the compressed air production side of the total production capacity needs to have a very clear grasp, so for the air pressure station room total outlet pipeline must be flow measurement, which is also the “compressed air station design specification” GB 50029-2014 in the clear requirements. Measurement in the total gas pipeline is generally thicker, the flow rate fluctuations, and because the gas is generally through the air pressure station room some drying filtering equipment, gas is cleaner, so here shall be used in a wider range, not subject to the lower limit of flow rate limit flowmeter, thermal mass flowmeter is the preferred choice. Secondly, if the operating efficiency of the air compressor and exhaust capacity needs to be analyzed and monitored, the air compressor exhaust position should also be configured flow meter for flow measurement, which is also clear in the design specifications. For the air compressor exhaust location of the flow measurement has always been a difficult point, because the compressed air here, with high temperature, high humidity, high flow rate, lack of straight pipe section and other restrictions.
Industrial enterprises also try to use a variety of principles of flowmeter to measure, but have their limitations, thermal mass flowmeter because of the requirements of the measurement medium is not suitable for flow measurement here, and vortex street, orifice plate and other flowmeter due to installation, vibration and other factors, the measurement is not good, Pitot tube flowmeter is more comprehensively suited to the application here. Again, for some energy-saving needs of users, in some special equipment in a specific location also need to carry out some measurements. For example, in the “air compressor unit and air supply system energy saving monitoring” GB/T16665-2017 in the air pressure station room energy saving monitoring requirements, to the compressed air dryer gas consumption rate should be monitored. This requires users to measure the flow rate before and after the compressed air dryer to calculate the air consumption rate of the dryer. For the measurement of the dryer before and after the two ends, the needs are different, because the gas medium at the front end of the dryer is usually high in water content, while the back end is dry air, so it is not possible to use the thermal mass flow meter for measurement. Overall, the compressed air production end of the flow measurement has a certain degree of diversity, should be appropriate to consider the different needs of the application of different principles of flow meter measurement.
2. compressed air transportation side of the flow measurement needs. Compressed air transportation side mainly includes the transportation network, valves and some regulators. For this end of the flow measurement, the general purpose is to grasp the station out of gas transported to various departments of compressed air after the respective amount of compressed air, so as to carry out compressed air cost analysis and management. In these measurement positions, the general measurement requirements are thicker pipes, flow with the fluctuation of production dosage and fluctuation, most industrial enterprises use compressed air are dried and filtered. And in some of the more relaxed management of the industry, there are still some use without any treatment of compressed air, such as iron and steel, petrochemical, electric power and other industries, so here you need to choose according to the individual situation of the business user in accordance with the actual situation. For example, after drying and filtering air, you can consider the thermal mass and vortex flowmeter; without drying and filtering air, you can consider the vortex flowmeter and Pitot tube flowmeter; for the pipeline is thicker, the pipeline installation is not convenient, you can choose from the thermal mass and Pitot tube and other flowmeter can be realized by the insertion of the installation.
3. Compressed air end of the flow measurement needs. In industrial enterprises, compressed air end of the flow measurement is often overlooked, but here the amount of gas used is related to the important aspects of corporate productivity. Compressed air is an expensive secondary energy source, which is not as obvious as water or electricity, but more expensive and easily wasted. Therefore, the cost of compressed air in industrial enterprises is often very high, in the compressed air end of the flow measurement can enable business users to effectively monitor the enterprise compressed air usage, so as to accurately analyze the cost of compressed air use, for the enterprise’s costing and energy saving to provide the basic data. Flow measurement at the use side of compressed air is similar to the measurement at the transport side, but with a few additional branches and more refinement. A special application is the measurement of leaks in compressed air systems. Compressed air leaks are costly for companies, not only in terms of wasted compressed air, but also in terms of pressure drop and degradation of compressed air quality. Enterprises can consider the use of some can measure the flow of small flow meters for compressed air leakage measurement.
Third, the basic principle of flow measurement and development of Pitot tube
Pitot tube measurement is based on classical fluid dynamics a measurement of fluid point velocity measurement principle. The principle is based on the Pitot tube to measure the difference between the full pressure and static pressure of the measured medium, according to Bernoulli’s equation:
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Where P is the static pressure, P0 is the full pressure, ρ is the density of the medium and V is the flow rate of the medium. For air measurement, according to the above equation, plus the need to consider the error between the value measured by the Pitot tube and the true value due to the limited accuracy of the measuring element, a calibration coefficient α needs to be introduced, and finally the flow rate of the air is obtained along with the full and static pressure equations obtained from the measurement:
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Where ΔP is the micro-pressure difference measured by the micro-pressure gauge, i.e. the difference between full pressure and static pressure.
Pitot tube measurement principle structure is simple, the measurement of pressure measuring element is inexpensive, manufacturing and use is also more convenient, for the smoother air flow measurement accuracy is higher. Because of these characteristics, the Pitot tube measurement principle is widely used in aerodynamic and wind power related fields of gas flow rate measurement, according to this principle of gas flow meter in the market is also more common. Especially the S-type Pitot tube, with dustproof and anti-clogging, temperature and pressure resistance, etc., widely used in refining, coal power and other high-emission, environmentally complex large flue gas measurement in the flue gas.
However, there are not many applications of Pitot tube flow meters in compressed air measurement applications. Until 2016, SUTO, a manufacturer of compressed gas measurement equipment, first released a Pitot tube flow meter based on the Pitot tube measurement principle and applied it to compressed gas flow measurement. After the principle of the product is gradually recognized by the market and successive manufacturers to follow suit, and now the Pitot tube measurement principle of the flowmeter has been more commonly recognized and applied by industrial enterprises, especially compressor manufacturers. For compressed air flow measurement, it is the innovation of Pitot tube flow meter in industrial application. Compared with the traditional also is based on the principle of differential pressure rotor flowmeter, orifice plate flowmeter, etc., has some irreplaceable advantages, such as easy to install, high measurement accuracy, can also have temperature and pressure compensation and other characteristics.
Fourth, Pitot tube flow measurement in compressed air measurement applications
Pitot tube flowmeter used in compressed air flow measurement, should focus on the consideration of several characteristics of the Pitot tube flowmeter: high temperature and high humidity gas measurement, temperature and pressure measurement can be integrated at the same time, the Pitot tube insertion design. As mentioned earlier, a particularly typical measurement application in compressed air flow measurement is the flow measurement at the exhaust position of an air compressor. In addition to considering the characteristics of the measurement principle, the realities of construction and installation should also be taken into account. For example, in the energy-saving renovation project of air compressor system, in order not to affect the normal operation of the existing system, the insertion type flowmeter can be installed and dismantled under pressure is very important. And compared to the structure of the more massive pipeline flowmeter, compact and simple design, more sophisticated structure of the insertion of the Pitot tube flowmeter can be more recognized by the user.
Pitot tube flowmeter can be simultaneously integrated pressure, temperature characteristics can also greatly simplify the flow measurement process. And through some working condition parameter setting and compensation, can also realize some more professional gas flow measurement. For example, for the air compressor unit volume flow measurement, that is, in addition to the requirements of the air compressor exhaust position of the flow measurement, but also need the air compressor suction position of the temperature, humidity and atmospheric pressure, and these parameters can be through the Pitot tube flowmeter parameter setting and flow measurement, after the calculation of the target measurement value.
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For general compressed air flow measurement, it is necessary to pay attention to some characteristics of the Pitot tube flow meter has limitations. For example, the measurement range of Pitot tube flowmeter is relatively not wide, especially large pipes, low flow rate measurement needs, for such a measurement position, pay attention to the measurement range of the Pitot tube flowmeter to cover the required measurement of the flow rate, otherwise there may be a flow rate is too low and the Pitot tube flowmeter can not measure the data of the situation.
Overall, Pitot tube flowmeter, as a more comprehensive performance of a flowmeter, in compressed air flow measurement in the universality of good, is a comprehensive performance is good choice of flowmeter.