Issue link: https://www.e-digitaleditions.com/i/1212417
20 / March 2020 powderbulk.com 3. The acceleration zone shouldn't contain any bends or inclined sections because the material isn't fully accelerated and can settle in the bends, especially going from horizontal to vertical or in the inclined sections. Such settling could cause refluxing, which occurs when material falls out of the gas stream, settles on the conveying line's lower wall, and slides down the incline. This material accumulates at the bot- tom of the bend or inclined sec- tion, is re-entrained in the con- veying gas, and must be reaccel- erated. Though refluxing doesn't occur in a horizontal-to-horizontal bend, the material does slow down and requires reaccel- eration after the bend. 4. It's also recommended to have a straight-line section with a minimum length of 10 pipe diame- ters before a feeding point. Indeed, a straight-line section prior to the feeding point helps in achieving a laminar flow of air. Turbulent air at the feedpoint can have a negative effect, reducing material accelera- tion and increasing the risk of wear in that turbulent zone. Two-phase flow. In two-phase flow (a combination of dilute-phase flow with a settled material layer), as shown in Figure 2, material doesn't have to accelerate above the saltation velocity. The conveying gas drag accelerates the material. Design criteria for this system's acceleration zone include: 1. The acceleration zone should be relatively short (typically in the range of 5 to 10 pipe diameters). FIGURE 2 Two-phase flow Go to www.powderbulk.com/video PBE articles tell you how equipment operates. Now see it in action. Videos on the PBE website show shredders, blenders, valves, packaging equipment, conveyors, and other bulk solids processing and handling equipment at work.