I. Design and calculation steps of ventilation duct system
1. Draw the axle-side diagram of the ventilation system, number each section, and mark the length and air volume of each section. A duct with constant wind volume and speed is used as a section. Generally from the farthest section from the fan. Numbered in order of distance and proximity. The length of the pipe section is calculated by the length of the center line of the two fittings, without deducting the length of the fittings (such as elbows and tees).
2. Choose reasonable air velocity. The wind speed in the duct has a great influence on the economy of the system. High flow rate, small cross section of air duct, less material consumption and less construction cost; however, the pressure loss of the system increases, power consumption increases, and sometimes the wear of the pipeline may be accelerated. Low velocity, low pressure loss and low power consumption, but large cross-section of air duct, increased material and construction costs. For the dust removal system, how low the flow rate will cause dust deposition and plug the pipeline. Therefore, a comprehensive technical and economic comparison must be made to determine the appropriate economic flow rate. According to experience, for general ventilation systems, the wind speed can be determined according to Table 6-10. For dust removal systems, the minimum wind speed required to prevent dust deposition in the pipeline can be determined according to Table 6-11. For the duct after the dust collector, the wind speed can be reduced appropriately.
3. Determine the diameter (or cross-section size) of each section according to the air volume and selected flow velocity of each section, and calculate the friction and local pressure loss of each section.
When determining pipe diameter, the uniform specification of ventilation pipes listed in Table 6-2 and Table 6-3 should be adopted as far as possible to facilitate industrial processing and production.
The calculation of pressure loss should start with the most disadvantageous loop (i.e. the farthest exhaust point from the fan).
For the air duct after bag filter and ESP, the air leakage and back-blowing of the dust collector should be counted. Leakage rate of bag filter is generally about 5%.
4. Calculate the pressure balance of parallel pipeline. The general ventilation system requires that the pressure loss difference between the two pipes should not exceed 15%, and the dust removal system requires that the pressure loss difference between the two pipes should not exceed 10%, so as to ensure that the air volume of each branch pipe meets the design requirements.
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