Application of hot gas mass flowmeter in large diameter field

At present, the flow measurement of large diameter ventilation pipe is mainly taken into full consideration from the aspects of energy efficiency, instrument installation and maintenance, so as to ensure that the measurement can meet the operation requirements of the workshop. According to the structure, the most widely used are the plug-in flow meter and non-contact flow meter, which can greatly reduce the pipeline pressure loss, save resources, and facilitate the later maintenance of equipment. The author mainly discusses the application of thermal mass flowmeter in the flow measurement of large diameter ventilation pipe. The content is mainly in the structure and principle analysis of instrument, the development of instrument technology and the application analysis of ventilation flow.

1. Structure and principle analysis of thermal mass flowmeter

ThermalMassFlow (TMF) is a kind of pipe flow measurement instrument based on the principle of heat transfer. Its theoretical basis is dry King's law. At present, constant power method and constant temperature difference method are the most widely used two kinds of instruments. 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 heated by a certain power, so that its temperature T2 is higher than T1. As the pipeline fluid flow continues to take away heat, the temperature T2 decreases, and the flow rate of the pipeline can be calculated according to the temperature difference.

2. Technical development and ventilation flow application analysis

In the early 20th century, Thomas, an American, designed a kind of plug-in flow measuring instrument for gas medium, which is very suitable for large flow measurement. This flowmeter directly contacts the heating coil and temperature measuring resistance with the measured gas, which inevitably brings a series of problems such as corrosion, wear and explosion-proof. In the 1950s, boundary layer flowmeter was developed to overcome the defects of Thomas flowmeter, but the flow measurement results were easily affected by fluid physical parameters (such as thermal conductivity, etc.). With the progress and development of science and technology, it is more and more urgent to detect large diameter gas flow used in industrial production. In 1992, Kurtz improved the design of a new structure of the thermal mass flowmeter, once came out has been rapid development, now has been widely used in various fields of large diameter gas pipeline flow measurement, especially in the field of ventilation pipe application more widely.

China's research on the thermal mass flowmeter mainly since 2000, research institutions and universities such as Research Institute of Mining and Metallurgy, Chinese Academy of Sciences and Huazhong University of Science and Technology have carried out research on the thermal gas mass flowmeter, and made certain achievements, after nearly 20 years of research and development, domestic thermal mass instrument technology research has also had a rapid development. In the domestic market application of thermal mass flowmeter, the share of foreign products is still high. At present, there are only a few manufacturers in the domestic production, and the market share of domestic products is low.

The main characteristics of the application of thermal mass flowmeter are high precision, large range ratio, simple structure, small pressure loss, easy installation and maintenance, low failure rate, good accuracy and repeatability, not affected by temperature and pressure, but the response speed of instrument detection is relatively slow. When applied in large diameter ventilation pipes, the measured results will be affected by the irregular flow distribution of the flow field, so it is necessary to adjust the instrument detection scheme, such as adopting multi-point hot instrument and increasing the number of detection instruments, so as to improve the detection accuracy of such instruments in the application of irregular flow field in large diameter ventilation pipes. At present, the research directions in the field of thermal mass flowmeter at home and abroad mainly focus on the following aspects:

a. Improve the sensor structure of the thermal mass flowmeter to improve the measurement accuracy and enhance the environmental fitness;

b. Study signal processing methods, such as adopting compensation algorithm 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.