Issue link: https://www.e-digitaleditions.com/i/1212417
March 2020 / 21 and often provides the opportunity for supporting the pipe. Often, the distance to the nearest wall is shorter than the required acceleration zone, and the convey- ing line turns vertically at the wall using a long-radius bend. In both dilute-phase and two-phase flow, this long-radius bend and the low material velocity cause the material to reflux in the bend. In many sys- tems, a long-radius bend can be the cause of line plugs, low conveying capacity, or high system pressure. If you think this configura- tion is causing a problem in your conveying system, try this simple test: Listen to the material flow- ing through the bend; if you hear a surging or slugging sound at the same time the conveying system's pressure surges, reflux is likely to be occurring. — of the conveying system's kinetic energy is dissipated as the mate- rial impacts bend elbows and must then be reaccelerated as it enters the downstream pipe. In addition, each bend becomes a logical location for wear when handling an abrasive material or for causing degradation when conveying a material that's friable. In the case of a cohesive material, something we might typ- ically see as a Type C material in the Geldart Classification 1 would be line scaling, creating buildup. So, respect your conveying line's con- figuration by making it as straight as possible. First bend location. Almost every pneumatic conveying system starts by running the conveying line toward the nearest wall or column. This is natural because running the line in this way doesn't obstruct walking or material-handling areas 2. The acceleration zone shouldn't contain any bends or inclined sec- tions because the settled fluidized layer in the conveying line's lower section should be given time to reach a steady-state condition before changing direction. Permeable dense-phase flow. In permeable dense-phase flow, shown in Figure 3, the material is acceler- ated to a lower velocity. However, the conveying gas drag doesn't accelerate the material; instead, the differential pressure exerted across the short slug, or piston, of material accelerates it. Design criteria for this system's acceleration zone include: 1. The conveying gas velocity, which is below the material's salta- tion velocity, isn't critical because the conveying gas drag doesn't accelerate the material. 2. The acceleration zone is very short and shouldn't contain any bends or inclined sections until the point where material slugs are formed (either in the first 2 feet of conveying line or in the pressure tank or line charger's discharge). Bends for all modes An ideal pneumatic conveying system has a minimum number of directional changes. Unfortunately, many systems are installed as a retrofit to an existing installation, compared to a greenfield site, and consideration must be given to rout- ing conveying line around every obstacle in the line's path. Each bend requires more energy for con- veying because much — if not all FIGURE 3 Permeable dense-phase flow