Unloading of materials from dry bulk trailers is done through pneumatic Conveying lines on the trailer. Transport pipe at the unloading site either a truck-mounted or on-site blower provides conveying hair pressurize the trailer. When the discharge valves are open, the velocity of the Airstream is increased, and pneumatic conveying system begins. There are different phases of Conveying, and each material has unique characteristics that determine what air volume and line velocity are required.
Dense phase conveying is much like an extrusion process where low air velocity is used to push material through the uploading line. It is used over short distances for materials like cereal, where avoiding product breakup and degradation is required. As air velocity increases, there is a point where particles are picked up and carried into the Airstream, which is referred to as particle pick-up velocity. This velocity is unique to the material being conveyed must be exceeded to convey a product in the dilute phase. Dilute phase conveying allows powder like cement and flour to be conveyed over longer distances.
Saltation velocity, which is the Air velocity at which particles begin to fall from suspension, is also important where air velocity is reduced below the saltation velocity. Particles began to collect on the bottom of the unloading line. If the velocity is decreased further, the product will continue to collect and partially fill the pipe in heaps. Similar to dunes during intermediate or mixed, please conveying these heaps may move along the pipe because the open area of the pipe is reduced, the air velocity increases, causing the Air to pick up more materials. Conditions then become unstable, but the flow becomes self-regulating, and plugs are unlikely to occur.
Materials characteristics are not the only factors important to efficient Conveying. Other factors that affect unloading include tank pressure, which is limited by regulations and is normally 25 psi one bar in the United States. Line diameter allows for more area for material to be conveyed, and unloading line length, when doubled, can reduce the unloading rate by as much as 36% for a product like flour and cement.
On practical applications, the factors affecting unloading and Transport through a pipeline can be summarized by operating zones. The plugging zone is the line where the plugs line repeatedly. The slugging zone is where insufficient air volume causes the lines to stop and then start for a short while the surging zone where the flow slows then speeds up repeatedly, and the risk of plugging is low, which results from maximum unloading rate.