Calculation of compressed air pipe diameter?

\"No one\"? ? ? ? ? ? ? ? ? ?

Based on a value of 10–20 m/s, HG/T20570 adopts a relatively more conservative approach; for compressed air used in economic purposes, a velocity of 15 m/s is taken as the standard

This post was last edited by zpg on 2011-5-9 at 15:45. Reply to 1# zmg813: There are many posts in this forum asking about the appropriate flow rate to use. The choice regarding such issues depends mainly on two factors: safety and cost. Safety refers to the presence of toxic, flammable, or explosive substances, such as acetylene or pure oxygen in pipelines; if the flow rate is too high, it can pose safety risks such as explosions. Cost, on the other hand, relates to economic considerations – for example, compressed air is generated by compressors, which consume electricity or steam, and the cost of electricity must be taken into account. The optimal flow rate is one for which the pressure drop resulting from that flow rate does not exceed acceptable levels, thus staying within economic limits. What is economics? To illustrate this with a simple example using the data from your post: the compressed air has a pressure of P=0.8 MPaG and a temperature of T=30℃ (this is roughly the temperature after the air compressor is cooled); its density is ρ=10.4 kg/m3. The flow rate under standard conditions is V0=1000 Nm3/h, while the flow rate under actual operating conditions is V=125 m3/h. The mass flow rate is W=1292 kg/h. The pipe dimensions are 57X3.5 mm, with an inner diameter of di=50 mm; the pipe length is L=100 meters (including the equivalent length of fittings). The absolute roughness of the pipe is 0.2 mm, and the friction coefficient λ is taken as 0.03. The power of the air compressor is 110 kW. The data shown above can be obtained easily at the construction site by the person in charge. With a simple calculation using these values, it is possible to determine what constitutes an \"economical flow velocity\": the pipe flow velocity is u=17.6 m/s. So, is this flow velocity considered economical or not? It depends on the proportion of pressure loss in the compressor’s power. The pressure loss ΔP = ρ·λ·(L/d)·(u²/2) = 96788 Pa = 0.097 MPa; in other words, after passing through a pipe section 100 meters long, the pressure drops from 0.8 MPaG to around 0.7 MPaG. The pressure loss can be converted into an energy loss ΔW = G·λ·(L/d)·(u²/2) = (1292/3600) × (9346/1000) = 3.35 kW. This represents 3.35/110 = 3.05% of the compressor’s total energy consumption. See? After traveling 100 meters, 3.35 kW of power is lost; due to this section of pipeline, 3.35 kWh of electricity is wasted each hour. Over the course of a year, with 8000 hours of operation, that amounts to 26,800 kWh. At a cost of 0.5 yuan per kWh, this alone results in a loss of 13,400 yuan per year – a loss that occurs silently. If you replace this pipe with a DN38 pipe of 45x3.5 dimensions, the pressure after 100 meters of pipe will be only 0.2 MPaG; the pressure cannot be maintained, and the resulting power loss is even greater, reaching 20 kW. This results in a loss of 83,000 yuan per year, which is an unacceptable and intolerable loss. Naturally, thickening the pipes reduces resistance losses, but it increases the cost of the steel pipes. Steel pipes have a limited lifespan; if the savings in electricity costs over that period are not enough to offset the additional cost of using thicker pipes, then smaller pipes should be used, otherwise larger pipes are necessary. The principle is the same whether building a highway or a dirt road: highways allow for higher speeds and thus save on fuel costs, that is, operating expenses. However, the initial investment for highways is high; in other words, it’s a matter of the ratio between operating expenses and investment costs. Building a dirt road requires much less money, but the speed is lower and fuel consumption is higher. More than 2,000 years ago, during the Qin Dynasty, the military highway that ran from Xianyang to Jiuyuan – the Qin Straight Road – was 700 kilometers long. It was designed to be as wide as possible, with the shortest distance between points, and without any unnecessary turns; the goal was simply one word: “speed.” Cavalry could travel from Xianyang to the Mongolian steppes, located over 700 kilometers away, in just 3 days, while supplies could reach there in 7 days. This is essentially the same principle as that behind the diameter of pipelines. Only after the Qin Straight Road was completed did Meng Tian’s army manage to completely defeat the fierce and formidable Huns’ cavalry. . The key is to be careful: for a pipe with a length of only 100 meters, DN50 is an appropriate size, but what about if the pipe length is 1 kilometer? 2 kilometers? Even having to cross mountains and deserts, what about the Gobi grasslands? For example, in long-distance pipelines such as those for transporting gas from the west to the east, a flow rate of 17 m/s is clearly unacceptable. In fact, the flow rate in long-distance high-pressure gas pipelines is only 3 m/s; due to the pressure losses, air compressors cannot reach such distances, and all 110 kW of energy consumption is wasted. But if you assume a flow rate of 3 m/s for pipes that are 10 meters long inside the factory building, the engineering audit department will definitely reprimand you: \"Kid, making the pipes that thick will result in a penalty for you!\" ! ”In such situations, it’s fine to use 30 m/s – the distance is short, so the resulting losses are acceptable. There isn’t a fixed value for the flow rate; what’s most appropriate is to spend as little money as possible. When working on technical issues, one shouldn’t focus excessively on the technical aspects alone; an understanding of engineering economics is also necessary. Otherwise, such an engineer cannot be considered qualified. . Achieving the goals that technology can accomplish with the least amount of money is the right approach, and it’s also the best way to do it. Don’t you see that the Americans spent trillions to kill one person – is that worth it? Haha. .

This post was last edited by ZHOUDAOZD on 2011-9-25 at 10:00. I’m very grateful for zpg’s explanation; I just started working in this department, and it involves the installation of pipelines for underground air compressors, which has helped me understand many things right away. Our design department followed the table for standard flow rates and selected a DN159 pipeline for the air compressor with a capacity of 64.3 m3/min; I requested that a DN325 pipeline be used, and I am currently in discussions with them regarding this. I also have a question for the seniors: where can I find the exact formulas for calculating power loss and resistance loss? I still don’t know

This expert is really great; they’ve said so much

Follow what foreigners do; many things in China are quite conservative, and many of them are copies of foreign ideas.

Reply 4# zpg: There are so many talented people here

Thanks zpg, I’ve learned a lot

What an expert; the explanation is simple yet thorough – truly impressive!