Application of thermal mass flowmeter in flow measurement of large diameter ventilation ducts

At present, the flow measurement of large-diameter ventilation ducts is mainly based on energy efficiency, instrument installation and maintenance, etc., to ensure that the measurement can meet the plant operation requirements. According to structure, the most widely used are plug-in flow meters and non-contact flow meters. This type of meters can minimize pipeline pressure loss, save resources, and facilitate later equipment maintenance. The author mainly discusses the application of thermal mass flow meters in flow measurement of large-diameter ventilation ducts. The content mainly covers instrument structure and principle analysis, instrument technology development and ventilation flow application analysis.

1. Analysis of the structure and principle of thermal mass flowmeter

Thermal Mass Flow (TMF) is an instrument that measures pipeline flow based on the principle of heat transfer. Its theoretical basis is derived from King's Law. The two most widely used ones at present are the constant power method and the constant temperature difference method. meter. The working principle of King's Law is to place two temperature sensors in the pipeline fluid. One sensor is used to detect the fluid temperature T1, and the other sensor is electrically heated with a certain power to make its temperature T2 higher than T1. As the pipeline fluid The flow continues to take away heat and the temperature T2 decreases. The flow rate of the pipe can be calculated based on the temperature difference.

2. Technology development and ventilation flow application analysis

At the beginning of the 20th century, Thomas, an American, designed an insertion flow measurement instrument for gas media, which was very suitable for the measurement of large flows. This flow meter directly contacts the heating coil and temperature measuring resistor with the gas being measured, so it is inevitable The belt brings a series of problems such as corrosion, wear and explosion protection. In the 1950s, a boundary layer flowmeter was developed that overcame the shortcomings of the Thomas flowmeter. However, the flow measurement results were easily affected by fluid physical parameters (such as thermal conductivity, etc.). With the advancement and development of science and technology, the demand for detection of large-diameter gas flow used in industrial production is becoming more and more urgent. In 1992, Kurtz improved and designed a new structure of thermal mass flowmeter. It developed rapidly once it came out and is now widely used in large-diameter gas pipeline flow measurement in various fields, especially in ventilation ducts. The field of application is more extensive.

Research on thermal mass flow meters in my country has mainly started after 2000. Research institutions and universities such as the Institute of Mining and Metallurgy, the Chinese Academy of Sciences, and Huazhong University of Science and Technology are all conducting research on thermal gas mass flow meters and have achieved certain results. After nearly 20 years of research and development, domestic technical research on thermal mass instruments has also developed rapidly. In the market application of domestic thermal mass flowmeters, foreign products still have a high share. Currently, only a few domestic manufacturers are producing them, and domestic products have a low market share.

The biggest features of thermal mass flowmeter applications are high precision, large range ratio, simple structure, small pressure loss, easy installation and maintenance, low failure rate, good accuracy and repeatability, and are not affected by temperature and pressure, but the instrument detection The response speed is relatively slow. When applied in large-diameter ventilation ducts, the measured results will be affected by the irregular flow velocity distribution of the flow field. It is necessary to adjust the instrument detection plan, such as using multi-point thermal instruments, increasing the number of detection instruments, etc., to improve the performance of such instruments. Detection accuracy for irregular flow field applications in large-diameter ventilation ducts. The current research directions in the field of thermal mass flow meters at home and abroad mainly focus on the following aspects:

a. Improve the sensor structure of the thermal mass flow meter to improve measurement accuracy and enhance environmental adaptability;

b. Research signal processing methods, such as adopting compensation algorithms or using different devices to process output signals;

c. Improve and develop multi-point thermal mass flowmeter to be more suitable for the measurement of gas flow in large-diameter pipelines.