A Deep Dive into Open Scroll Decanter Centrifuge Design
Wastewater treatment facilities looking for savings frequently focus on creating a drier dewatered cake, reducing polymer consumption, and using less energy. A decanter centrifuge accomplishes all of these, primarily because it is more efficient than a settling pond, acting on solids with several times the force of gravity. The result is greater separation power and speed, more solids, and a drier cake.
If you have seen a centrifuge in action, you know there are many variables that affect output: RPM, bowl size, scroll speed, cone angle, pond depth, and slurry concentration to name a few. The design of the scroll itself can have a major impact on output too.
Increased Pond Depth
Pond depth, sometimes called the separation zone, is the level of the liquid inside the bowl. The more filling level in the bowl, the more hydraulic pressure on the separated solids. These results in more water being squeezed out of the slurry, and produces a drier cake. When the scroll is an open tube instead of a solid rod, there is additional space for a deeper pond, and separating power increases even more.
Gentle Acceleration, lower Shear Force
Sludge is introduced into the centrifuge through the feed pipe located at the center of the scroll. In a traditional centrifuge design with a solid scroll body, the sludge has to pass the body wall through openings, which exerts high shear forces to the flocculated feed. In consequence, this causes breaking up agglomerated particles into smaller ones, descending less easily.
But when there is no scroll body, the pond surface gets pass the open scroll up and very close to the feed pipe. The material feeds directly into the pond instead of through a feed chamber, which eliminates shear force to the flocculated particles. In the end this helps reduce the amount of polymer needed to hold solids together.
Efficient Acceleration and Rotation
The moment of inertia describes the amount of energy required to change an object’s rotation, which is affected by its mass and the distance of that object from the axis of rotation. The shorter the distance, the easier it is to get it rotating. In a shallow pond, material is farther from axis and requires more energy to accelerate. A deeper pond places the pond surface closer to the axis of rotation, which needs a smaller moment of inertia to overcome, in order to get it rotating and to maintain speed. This results in a more energy efficient operation.
In centrifugal separation, the details matter, and innovative engineering makes a difference. That is why at Flottweg, we are always looking for better methods for dewatering and separation – please contact us to see what our systems can do for you.