Analysis and Comparison of Several Technical Problems of Heat Meter Flow Sensors

Brief description: In many household heat meter products at home and abroad, due to many factors such as price and power consumption, the small-caliber mechanical hot water meter is generally used as the flow meter of the heat meter. The Ministry of Construction heat meter industry standard CJ128-2000 The requirement of the flow meter part also basically adopts the same requirements as the current hot water meter product performance.

The use and research practices have shown that there are a series of problems that need to be solved when directly using a small-caliber mechanical hot water meter as a flow meter for a heat meter. According to the research experience of the heat meter flow sensor, we found that the main problems of the small-caliber mechanical hot water meter as the heat meter flow sensor are: the range problem, the difference between the hot and cold water flow coefficient, reducing the initial flow and increasing the small flow In the case of accuracy problems, magnetic interference in magnetic transmission methods, high temperature out-of-step problems, and adaptability to the water quality of our heating systems. Based on the research and analysis results, we made a preliminary analysis of the above issues and proposed some solutions to discuss with the industry peers, in order to develop a flow meter with a suitable flow meter, and jointly improve the level of heat meter research in China.

1 Heat meter flow sensor range problems 1.1 Heat meter flow sensor measurement range Ministry of Construction heat meter industry standard CJ128-2000 Article 4.3.3 stipulates: "The heat meter's common flow should meet the GB/778.3 cold water meter requirements, commonly used The ratio of flow rate to minimum flow rate should be 10, 25, 50 or 100. For heat meters with a nominal diameter ≤ 40mm, the ratio of the common flow rate to the minimum flow rate must be 50 or 100."

The flow range of a caliber hot water meter of a certain factory (currently the heat meter manufacturer generally adopts the hot water meter of the factory) is shown in Table 1:

The indication error is 2% between the demarcation flow (inclusive) to the maximum flow, and 5% between the demarcation flow to the minimum flow. At the same time, it is stipulated that the error limit of flow sensors at all levels should not exceed 5%.

Taking the DN20 heat meter currently widely used as an example, the measurement error curve is shown in Figure 1.

1.2 Flow Design Range of Building Heating System According to relevant information, the flow design ranges of energy-saving and non-energy-saving building heating systems in northern cities in China are shown in Table 2 and Table 3.

According to the empirical data of the actual use situation, when the user implements the sub-room adjustment, the work flow will be reduced to 50% of the design flow.

1.3 Analysis conclusions and improvement measures Based on the above data, the small-caliber mechanical hot water meter is directly used as the flow meter of the heat meter. The following conclusions can be drawn:

a. The common flow rate of hot water meter is too large, more than 10 times of the design flow of the building heating system;

b. Most heat meters will operate below the demarcation flow, and heat meters with calibers above DN 20 will even operate near the minimum flow rate;

c. The flow meter's flow sensor will work in high error areas most of the time. If it is working below the minimum flow, the actual measurement error will exceed the 2% to 5% requirement of the CJ128-2000 standard;

d. The unmodified hot water meter is not suitable for direct use as a heat meter flow sensor;

e. The problems to be solved when using the hot water meter as the flow meter of the heat meter are: reduce the common flow rate to make it approach the design flow range of the building heating system; increase the sensitivity of the flow sensor, reduce the initial flow, and further reduce the minimum flow rate. It meets the requirement of 1/100 of the current commonly used traffic; effective measures are taken to increase the measurement accuracy of the following measurement ranges of the demarcation flow. In the process of developing the HM-1 heat meter, we have taken a series of technical measures to solve the above-mentioned problems. Including: take out the original hot water meter gear technology, reduce the impeller resistance (according to experimental data: remove the original hot water meter gear counter mechanism can reduce the resistance of about 30%), to increase sensitivity, reduce the initial flow, further reduce the minimum flow To achieve the current use of the flow of 1/100 of the purpose; to take one by one to the flow sensor calibration and dynamic measurement of corrective measures to achieve a table of coefficient, effectively increase the measurement of the following range of flow measurement accuracy. Design new flow sensors to reduce common flow rates and make them close to the design flow range of the building heating system. In order to reduce the pressure loss of the system, variable caliber measures can be taken. These should be the technical issues that continue to be focused on.

2. The difference between the flow coefficient of hot and cold water is well-known. Rotary wing mechanical magnetic heat meter flow coefficient KV is related to the design, manufacturing precision and production commissioning. In the entire flow range of the hot water meter, the indication error is the flow rate (flow rate Change and change, see Figure 1. The research results show that the flow coefficient KV also changes with the change of water temperature, especially in the small flow area below the demarcation flow, the change is more significant. It is easy to understand that as the water temperature rises, the density of water decreases, its viscosity decreases, and the resistance of the impeller decreases; when the water temperature rises, both the housing and the impeller will swell, due to their different manufacturing materials, the expansion coefficient Differently, it will cause the gap between the inner cavity of the housing and the impeller to change. The calculation result shows that the influence of this change on the flow coefficient KV can not be ignored. In addition, as the water temperature increases, the resistance of the impeller and the bearing also changes. The combined effect of the above factors causes the flow coefficient KV to change with the change of the water temperature. For different hot water meters, the change rule will be different. Table 4 shows the variation of the flow coefficient KV at 85°C at room temperature and at room temperature.

Table 4-1 Flow rate of cold water Table 4 Variation of flow coefficient of hot and cold water In China, there is no hot water flow standard test device for hot water meter production plants. The factory inspection is performed on cold water devices, and almost no consideration is given to the flow coefficient of temperature. The impact, which is the main reason for the reduced accuracy of such water meters at high temperatures, has led to a general consensus in the industry; the direct use of hot water meters as heat meter flow sensors must be carefully monitored during prototype inspections. It is very unusual for selection to pass. We believe that the inspection of the product at the factory is performed on a chilled water device, and the designed product must be subjected to a cold and hot water comparison test to find the variation law between the cold and hot water flow coefficient KV of the product and the flow coefficient detected on the chilled water device. Make the necessary corrections to meet the requirements of the heat meter for hot water meters.

3. Magnetic interference in magnetic transmission methods, out-of-step problem Currently, the heat flow sensor developed in China generally adopts the magnetic transmission method to transmit the impeller rotation signal. Since the magnetic material is installed on the impeller, it is inevitably affected by the magnetic field. Article 6.11.3 of the CJ128-2000 standard stipulates that under normal working conditions of the heat meter, the flow sensor, the calculator housing, and the monitor should be placed in an environment with a magnetic field strength of 100 kA/m. Suddenly increase or decrease. Tests show that although domestic hot water meters have some anti-magnetic measures, some can meet the above requirements, but in the vicinity of stronger magnetic fields (such as neodymium-iron-boron strong magnets), impeller oscillation will generally occur, magnetic transmission failure. Phenomenon, unable to meet the requirements of the standard. When the hot water meter is improved as a heat meter flow sensor, this problem must be completely solved. In this regard, we have taken a dual measure of magnetic shielding and magnetic detection for the impeller box, which has solved the magnetic interference problem.

Research and practice have also shown that most magnetic heat transfer water meters generally suffer from high-temperature out-of-step phenomena, which means that under high temperature conditions (such as when the water temperature is above 85°C) there is a discontinuity phenomenon, and the measurement accuracy is significantly reduced and exceeded. When the temperature drops back to the original accuracy. Analyze the reason that this is caused by the decrease in the magnetic material installed on the impeller as the temperature increases. For this reason, we chose a magnetic material with better temperature characteristics and successfully solved this problem. China Heating Heating Heating Radiator Boiler Heating Furnace Water Heater Water Heater Tube Pump Valve Heat Exchanger Electric Heating Floor Heating Heat Meter Plumbing Equipment Best Promotion Platform

Of course, to completely solve the problem of magnetic interference in the magnetic transmission method, the most effective method is to cancel the magnetic transmission method and adopt a more advanced signal pick-up sensor. Due to technical reasons, there is a big gap between domestic and foreign countries in this regard. We are deepening. Research is expected to be resolved in the near future.

4. Adaptability to the water quality of our country's heating system The poor water quality of our country's heating system is a common problem. Whether the heat meter should adapt to poor water quality has been a matter of debate in the industry. We believe that: While calling for the relevant departments to improve water quality, the development of heat meters must take into account the adaptability to different water quality, and they should not refuse to install heat meters because of poor water quality. The actual application tests show that the impurities in the water for heating systems are mainly mud, debris, gravel, and red and black rust that remain in the pipeline. In response to this actual situation, we have taken measures to add special filters and magnetic filters to effectively filter out the above impurities and extend the use of heat meters. Of course, the filter and the magnetic filter must be easy to clean or replace. It is a problem that must be considered when designing. In the case that the water quality problem has not been completely solved, it is feasible to clean up every heating season once.