By Greg Oliver, Hartford Engineering LLC, Bellevue, Washington
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Separating dust and other fine particles from sand or crushed rock has long posed a challenge for aggregate producers. Higher quality concrete and non-frost-susceptible soils require a small amount of fines but methods used to separate them out have historically met with varying degrees of success. Feed material can be washed but this uses a lot of water; it can be heated (baked) and dried but this method uses a lot of energy. These methods also require large, fixed buildings at a facility – portable systems have generally not been available until recently.
Feed materials being separated into piles of varying particle sizes
The portable separator is capable of being delivered to any job site using standard over-the-road trucks and utilizes moving air to separate fines from the feed material. In the separator, rotating blades create circular airflow while feed material enters, gravity fed, from above. The material falls upon a rotating disk and is flung outward into the lifting airflow, where the finer particles are lifted by the air column and removed via a dedicated chute. Coarser particles fall and are removed via a separate chute.
Several factors affect the ability of the machine to separate fines, including the type of feed material (sand and gravel or crushed limestone-based aggregate, for example), the initial moisture content in the material, as well as ambient air temperature and humidity.
Recently, Hartford Engineering used FLUENT to optimize the existing air separator design and improve the internal air flow and particle-separation efficiency. The initial modeling work in GAMBIT was straightforward. A mesh of approximately 550,000 elements included multiple periodic zones/boundaries to accurately reflect the cylindrical separator while simplifying the model. The 3D, double-precision simulation made use of the RNG k-å model for turbulence and the multiple reference frames (MRF) model to account for the rotation (at 240 rpm) of the large blades inside the separator machine. Standard wall functions were used along with a wall roughness of 0.5.
Once satisfactory convergence was reached, it became apparent that the existing air separator system was afflicted with several areas of poor airflow, with numerous eddies & vortices. These flow anomalies led to material buildup internally, causing blockages and generally degraded separation performance.
Clogging in the separator prior to the air flow modifications
Certain key internal geometry elements were then modified in GAMBIT and new solutions were run to see the effect of each of these alterations on the air flow and fines-separation efficiencies. A number of suggestions for internal design and operational changes were proposed to the client. Once these changes were implemented, the unit achieved a markedly improved performance.
Velocity vector plot showing improved air flow
To date there has been a significant increase in the machine’s operating efficiency, which translates directly to less energy use, better quality of the final product, and lower costs overall. FLUENT has enabled Hartford Engineering to approach a complicated flow system and deliver tangible results to the client. In the future, the company will continue to use Fluent products to further refine both existing system designs and to provide additional guidance on next-generation machines for its clients.
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