Primary air average velocity tube flow meter
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This post was last edited by chensf123 on 2018-7-24 at 23:01. For the measurement of the air volume at the outlet of the boiler’s primary fan, the total length of the straight section in the outlet duct is less than 3 meters. The Delta Bar is installed in a section measuring 1200X1200 mm; the rest of the ducts consist of irregular bends or sections with changing diameters. However, the Delta Bar’s data sheet requires that the straight section ahead of it be 6.2D in length. Due to the unsuitable installation location, this flow rate has been fluctuating significantly. The designed pressure difference is 260 Pa, as the delta P is relatively low; it is unclear whether the use of pressure equalizing probes installed in a matrix configuration will improve the situation.
Using a differential pressure as low as 260 Pa for such a large diameter and flow rate is like trying to pull a heavy load with a weak horse; moreover, the installation of the pressure tapping tubes, three-valve assemblies, and differential pressure transmitters in bar-type flow meters is complicated, and the required straight pipe sections are not sufficient, so it’s no wonder there are significant fluctuations. Matrix-type pressure sampling essentially involves taking an average; it’s like using a weak horse to pull a heavy load. The matrix-style installation, which requires thorough disassembly, is even more complex, and problems are likely to persist. You can try installing several insert-type thermal mass flow meters in the same cross-section and taking an average value; the installation is simple, and the results will be much better.
The main issue here is an unreasonable design; the low pressure difference causes fluctuations in flow rate. It is recommended to increase the pressure difference as much as possible. Currently, domestic standards permit the use of three types of flow meters for this application: the differential pressure method, the thermal method, and the ultrasonic method; however, the differential pressure method is the one that is most widely used. As for the matrix method, it’s really unnecessary – it’s just a waste of money.
Stop with these nonsense concepts; they only disrupt the process. Just adjust the insertion depth as needed – according to the holographic method theory, there is always a value at the source point that can reflect the average flow rate across the cross-section of the main pipe.
If the pressure difference is small, should one buy an ultra-low pressure gauge? There are ones with a range of 25 Pa and ones with a range of 10 Pa – you have plenty of options to choose from, so there’s no need to worry. What are you afraid of?
The differential pressure generated by the matrix is not very large either; by using an insert-type multi-orifice flow meter, the fluid passes through multiple orifices, increasing the flow velocity and thus raising the generated differential pressure
Three years later, upon reading my own reply, I came up with new ideas. First, the flow meter used by the poster should already be of the line pressure-taking type; adding more of them in a matrix format will not solve the problem ； Second, the length of the straight pipe section is already like this; nothing can improve it. Third, the thermal method is complete nonsense; it’s not applicable at all. In summary, just endure it for now.