Issue link: https://www.e-digitaleditions.com/i/1344205
March 2021 / 19 Gas mover The gas mover is the source of the air or other gas flow into the con- veying line to entrain the solids and transport them to the destination point. There are three basic gas mover choices: a fan, a positive- displacement (PD) or centrifugal blower, and a compressor, as shown in Figure 3. Just as is the case with the line charger, the system spe- cifics will be the information that drives the decision-making process as to which gas mover type to use. Q: Our blower system is con- stantly popping the relief valve and we have to run the blower at very low rates to make the pop- ping stop. However, we need to get our production back up to where it used to be — what can you suggest? There are many line charger options to consider: pickup attach- ments or pans when using railcars, rotary feeders and airlocks, pres- sure tanks, double-gate feeders and lock hoppers, screw pumps, and eductors and venturis. Some of these are shown in Figure 2. Q: I am trying to understand why our pneumatic conveying sys- tem isn't working right — can you suggest a place to start? A: In my experience, having spent a long time troubleshooting prob- lems with pneumatic conveying, a good place to start is by checking the line charger. If the line charger is worn or not functioning correctly from a mechanical or electrical sequence point of view, the system won't work correctly. As you can imagine, each specific line charger type has its own areas to address. Q: Must a pneumatic convey- ing system run continuously or can the system be used to convey material in batches? A: If the material feed is a production line, which operates continuously and typically doesn't work on a batch or intermittent basis, the pneumatic conveying system will be continuous as well, which means the material coming off the production line always has a means of being taken away to the next step in the process. If the goal of the project is to make a specific material batch size, then the pneumatic conveying system may be a dedicated system for each material or a single system that handles multiple materials. For example, this system type would work well with a food product's recipe formulation that has four or five different ingredients that need to be transferred to a single point such as a mixer. Typically, the mate- rial destination will be a hopper over the mixer, which is on a weigh system that controls how much weight of each ingredient is loaded into the hopper until the total batch is weighed out. In this case, the pneumatic conveying system(s) are operating intermittently. Line charger The line charger is the equipment that gets the material to be con- veyed from the material feed into the conveying line, typically at a specific rate and against the pres- sure differential in the system. Q: Is a rotary feeder the best choice for a line charger? A: The line charger chosen depends on the specific application because the varying pneumatic conveying systems, modes of conveying, material characteris- tics, operating temperatures, and material flowrates all must be con- sidered when selecting the most cost-effective line charger. FIGURE 2 Line chargers feed the material from the material feed to the conveying line. Railcar attachment Rotary airlock Railcar pan Suction-dry solids inlet Discharge- air + solids Motive air Eductor Courtesy of Fox Valve Pressure tank Courtesy of Nol-Tec Double-gate feeder Courtesy of Plattco