Issue link: https://www.e-digitaleditions.com/i/1212417
22 / March 2020 powderbulk.com In general, a good rule is to avoid an upwardly inclined conveying line. But why avoid such a line when an incline is perfectly satisfactory in dilute-phase conveying in which conveying is taking place above saltation? Saltation velocity in an inclined line is higher than in a hori- zontal line. Thus, if conveying is just above saltation velocity in a hori- zontal line, entering an inclined line, where the saltation velocity is higher, could cause the material to fall out of the dilute-phase flow and reflux. The incline angle is important because the refluxing depends on the material not only settling in the line but also sliding down the line and being entrained in the gas stream. For example, a conveying line inclined upward at 10 degrees from horizontal wouldn't affect a two-phase flow system and proba- bly wouldn't affect a dilute-phase system. But if the conveying line is inclined at a steep angle, such as 45 to 75 degrees from horizontal, the material's sliding will acceler- ate against the gas flow and cause severe refluxing. What about a conveying line inclined downward? Apparently, a downward incline isn't detrimental continues to slow down with each successive bend. Overcoming this effect requires less material loading or a higher gas velocity. Be aware that using flexible hose in your conveying system can create the same multiple-bend problem, but it isn't as obvious. For exam- ple, conveying system connections between railcars and pneumatic unloading systems or hose switch stations often have long flexible hose runs. The hose typically lays on the ground with many random bends. At one site, tests revealed that such a flexible hose configura- tion reduced the pneumatic unload- ing system's conveying capacity by 28 percent. Thus, never have more than two bends in a series without adequate straight conveying line for reacceleration between the bends. Typically, we would recommend a range of 5 to 10 pipe diameters as that straight section. Inclined conveying lines and angles. When designing a pneu- matic conveying system, we usually calculate pressure drop for a hori- zontal or vertical system, but what should we do for a conveying line at some angle to horizontal? You can solve the problem in three ways: 1. Reduce the conveying line diameter in the acceleration zone and in the first bend. The reduced diameter will increase the material velocity. 2. Use a short-radius bend, which has a very short sec- tion of inclined line. The short incline can't accumulate material and so will reduce refluxing. 3. In a dilute-phase system, increase the distance from the feedpoint to the first horizontal-to-vertical bend, which will allow the material to accelerate to dilute phase. Multiple bends in series. Any directional change in the three- dimensional world can be made with a maximum of two 90-degree bends. It seems that conveyed material knows this, because when- ever the material encounters three bends, the conveying line plugs, the pressure drop increases, or the conveying capacity drops. Because material decelerates in each bend and doesn't have enough straight conveying line to reaccelerate between the bends, the material