Central system grinding emulsion

Automatic backflush filter used for the first time as a primary filter in steel grinding – space-saving concept without filter consumables

At the chain production plant in Wil, it was time for Stihl to replace the central filtration system located in the basement beneath more than 60 tooth grinding machines after 20 years of operation. The main reason was not the increasing maintenance and repair costs, but rather the associated production downtime. Accordingly, the key requirement for the new system was clear: installation and commissioning had to be carried out while the old system remained in operation, in order to avoid production interruptions as much as possible.

The available space was not only significantly smaller than that of the previous system, but also required a corner installation, including the use of an adjacent room. Large-area vacuum filters, as previously used by STA, could no longer be accommodated. In addition, Stihl required improved filtration performance and an alternative to the maintenance-intensive briquetting press; grinding sludge briquettes no longer provided disposal advantages, as they floated in the melting furnace.

In the past, STA had consistently developed suitable filtration concepts for Stihl, both in Wil and at the second plant in Bronschhofen. Once again, a new filtration solution had to be found. STA therefore proposed a concept based on multiple automatic backflush filters combined with self-discharging centrifugal separators. The separators served both to dewater the backflushed solids and to provide proven fine filtration in bypass operation for particles < 5–10 µm, as previously achieved with manually emptied centrifuges.

Automatic backflush filters with eight filter elements, of which one is always in backflush mode, are common in emulsion central systems. However, they are typically not used as primary filters, but rather as bypass filters during main filter regeneration or as secondary “police filters” with similar or coarser filtration ratings.

When comparing the filter performance data with Stihl’s specifications, it became clear that a pre-filtration stage would be required to relieve the backflush filters. Due to the ferromagnetic grinding chips, magnetic drum filters with continuous sludge discharge proved to be an ideal solution.

A thorough validation process followed using test equipment. For simplicity, the sludge discharge from the magnetic drum filter was directed into the collection container for the backflush material. The A-25 centrifugal separator produced such a dry disposal sludge from both filter systems that it was decided to include this concept of a central sludge discharge point in the system design—despite the limited available space.

Unconventional filtration technology as a bridging solution

With the three selected separation technologies, a new challenge arose: none of the systems included integrated tanks. While this provided the advantage of space savings, a certain fluid volume was still required to buffer circulating emulsion volumes (so-called travel volume) and ensure sufficient residence time for air entrained during grinding to degas.

In the relatively highly concentrated emulsion, air separation occurred only slowly, leading to air inclusions and reduced performance of the supply pumps. Additionally, rising air bubbles caused the short grinding chips, which tended to interlock, to float.

STA successfully optimized the A-25 separators to minimize air entrainment and the resulting oil mist, as well as to ensure reliable discharge of the unexpectedly dry, lightweight, cotton-like grinding sludge. However, within the available timeframe, it was not possible to fully resolve the flotation of sludge in the tanks and the resulting clogging of the magnetic drum filters.

After approximately five years of operation, Stihl decided to replace the system again. As the space of the original filtration system was now available, a large-area vacuum belt filter could be installed and commissioned there—once again without production interruption, thanks to STA.