An air-assisted gravity conveyor is a steel rectangular duct that is divided horizontally into two sections by a porous membrane. The area below the membrane is an air plenum and the area above the membrane is a material plenum. In practice the porous membrane allows low pressure air to flow from the air plenum, through the membrane and into the material to be conveyed. The air serves to essential fluidize the material being conveyed to thereby facilitate the material's movement. The conveyor is installed at a slight downward slope to allow gravity to do much of the work of conveying the material. An example of such a conveyor is FLSmidth's Airslide® gravity conveyor.
Air gravity conveyors are notable for achieving high transport rates combined with low energy consumption when conveying moderately fine particulate solids on a continuous downward slope. As such, it is known that air-gravity conveyors are utilized in a large ship loading procedure, which requires high material loading rates, typically 500 to 2,000 metric tons/hour to load dry, free flowing materials that fluidize well when air (or similar gas) is introduced to the material.
More specifically, materials that are advantageously loaded into a ship's hold utilizing an air gravity conveying means include products of small particle size and/or low density such as cement powder, barite, bentonite, diatomaceous earth, fly ash, talc, starch, flour, copper concentrate, and alumina, to name a few. The conveyors will extend from the dock to an area above a ship's hold, and will load such materials into a ship's hold from distances of 45 meters or more from the support tower. In such a process the air-gravity conveyor will have a closed top to reduce dust and to isolate the material being loaded from the elements, with the air-gravity conveyor having the shape of a rectangular box.
However, present ship loading systems that utilize an air gravity conveyor have disadvantages. At such high loading rates the air gravity conveyor, including its loading spout, is very heavy. Furthermore, an upset condition such as a plugged loading spout or conveyor section may cause the entire vertical spout conduit and horizontal conveyors to fill completely with the material being loaded. In order to support the air gravity conveyor as it extends into space from the support tower to above a ship's hold, it is necessary to employ bulky and costly truss work, designed with a large safety factor to accommodate a worst case load that may be present during an upset condition. Such truss work is expensive because it requires large amounts of steel in order to bear the heavy load of the conveyor while in operation. In addition, the truss work adds considerable height requirements to the support tower.
It is an object of this inventor, therefore, to develop a ship loading system that utilizes an air gravity conveyor that does not have the above specified disadvantages.