FEM + CFD: When there's a crash in the filter
Filter systems are boring? Not at Merkle CAE Solutions. And certainly not when they are mounted at a height of 14 meters under seismic loads – or when they have to shake off filter cakes using air jets. BOKELA GmbH has entrusted us with several projects that demonstrate how versatile and useful simulation can be – with both FEM and CFD.
Introduction
A disc filter is a machine that filters solids out of suspensions, such as wastewater or process fluid. It usually consists of several round discs mounted side by side on a shaft, like CDs on a skewer, with the lower half of the discs in a trough filled with suspension and the upper half exposed to the air.
How it works:
Filtering
The discs consist of individual segments, which in turn are covered with a filter cloth. The suspension in the trough is sucked through these filter cloths by means of a vacuum. The solid particles stick to the filter cloth, forming what is known as a filter cake.
Dehumidification & discharge
The shaft with the discs rotates continuously at up to six revolutions per minute. As soon as a segment emerges from the trough, only air is sucked in instead of the suspension – this dries the moist filter cake. Shortly before re-entering the trough, the filter cake is ejected from the segment by a short burst of compressed air, which slightly inflates the filter cloth, and falls through a chute onto a conveyor belt. At this point, the filter cake has less than 20% residual moisture, depending on the application.
Further filtering
The segment, now completely free of filter cake, is immersed in the trough again and a new cycle of suction – drying – discharge begins.
Why disc filters?
Because their compact design allows a large filter area to be achieved in a small space. This makes disc filters very efficient, compact, and suitable for continuous use—especially in the paper, mining, and chemical industries.
Part 1: When the earth shakes – FEM for emergencies
The first project dealt specifically with the question: Does the disc filter deliver what it promises—even in the event of an earthquake? The filter is not located on Lake Constance or in the Black Forest, but is to be installed in a 28-meter-high building in Greece—in a seismically active zone.
Our task: An FEM analysis for earthquake testing in accordance with Eurocode.
The procedure in brief:
- Model setup in SpaceClaim
- Networking and modeling in ANSYS
- Consideration of all contact conditions (linear and nonlinear contacts, screw connections, bearings)
- Loading with dead weight, pressure, seismic acceleration (according to EAK2000) and combinations
- Nonlinear evaluation including plastic deformation
Result?
Despite high stress due to horizontal acceleration of 2.84 m/s², all components remained below the permissible total elongation of 5%. The FEM simulation showed:
- Maximum local total elongation of 2.34%
- No impermissible exceeding of the limit values
- Safety even in the worst case (earthquake load with a load factor of 1.375)
The screws also remained tight: The M12 and M24 bolts were loaded to a maximum of 89% – also within the standard.
Part 2: When the cake flies – CFD for air distribution
While FEM focuses on deformation and strength, the second project deals with air—more specifically, the targeted “ejection” of filter cakes using bursts of compressed air.
The goal is to find out whether all cells of a rotating filter unit are supplied evenly with compressed air, even at increased speeds. The suspicion is that the last segment was no longer functioning reliably.
Our task:
CFD analysis of a rotating distribution system with multiple cells and porous media (filter cloth + perforated plate). Sounds technical? It is—but it's also exciting!
What did we do?
- CFD model with 17 million volume elements
- Rotation of the system through reference system change
- Simulation of pressure build-up and volume flow in the cells
- Analysis of pressure conditions, backflows, and flow topology
Conclusion?
The flow was stable, even, and reliable—even at higher speeds. The actual problem could not be attributed to aerodynamics.
A small bonus: even with changed filter resistance or a larger volume flow, the volume flow distribution remained almost identical. CFD says: everything is fine.
Part 3: When water remains inside – CFD under residual filling
And because we couldn't let go of the topic, a third project followed—this time focusing on a residual fill of approximately 1 liter of filtrate in the distributor.
The hypothesis here was: If water remains in the supply/drain line when the filter cake is dehumidified, it could be carried away by the compressed air blast and block the air path to the last segment.
Our task this time: multiphase CFD simulation with air and water.
Highlights:
- Dynamic observation of the filtrate volume
- Visualization of the flow velocity and water content
- Pressure differences at the outlets analyzed
- And what was the result?
Yes, there is slightly more air through outlet 2 – about 3% more. And yes, the water is gradually discharged. But the effect is moderate, and the pressure drop remains within limits (max. 0.31 bar at outlet 1).
Conclusion: Blockage by water is possible, but only relevant under specific conditions.
In summary:
FEM was used to test the filter for earthquake safety. CFD was then used to examine the air flow during filter cake discharge. Finally, the interaction of air and water during residual filling was analyzed.
Our conclusion: BOKELA has exploited the full range of numerical simulation with these projects. Whether safety, performance, or troubleshooting—simulation makes it tangible. And sometimes even visible.
Merkle CAE Solutions would like to express its gratitude for the trust placed in us—and we look forward to the next exciting challenges!
Yours Stefan Merkle
