At a large metalworking company, around 300 machine tools are in operation. Each machine has an individual tank and filtration system for its metalworking coolant. The plant machines a wide range of materials, including stainless steel, steel and cast components.
Machines processing cast components were particularly affected. Graphite, corrosion-protection oil, slideway oil and hydraulic oil heavily contaminated the coolant. Because the oils are less dense than the coolant, they floated on the surface together with graphite and fine chips. This barrier reduced the coolant’s contact with oxygen. Within a relatively short operating period, bacterial counts rose sharply and the coolant had to be replaced.
Various systems had been tried to suppress anaerobic bacterial growth and extend coolant life. Because systems without tramp-oil contamination showed much lower bacterial loads, removing the surface oil layer offered a clear route to longer coolant service life.
Initial belt and round-cord skimmers did not deliver the expected result. Owing to their operating principle, they kept only a very small area—about 20 cm in diameter—free of oil. The surface layer was heavily loaded with graphite and fine chips, making it so viscous that it could no longer flow toward the skimmer without additional circulation.
A short-term reduction in bacterial load was observed at first. However, the severe contamination caused the systems to fail after relatively short use. Plastic parts wore quickly, while scrapers and drains repeatedly became blocked. Solids carried with the oil or emulsion accumulated as sticky sludge on the scraper and clogged the outlet.
To avoid further repairs and frequent coolant replacement, the plant tested a FRIESS 1U oil skimmer. Its operating principle differs fundamentally from conventional belt, disc and cord skimmers. An endless oil-collecting hose floats over most of the liquid surface and is driven by the skimmer. Floating oil adheres to the outside of the hose and is carried into the unit. A scraper removes the oil, while the cleaned hose returns to the surface to collect more. The separated oil falls vertically through a short chute into the collection container.
The oil-collecting hose travels across the entire tank surface, removing oil over a wide area instead of only locally. Its steady motion also creates gentle surface circulation, drawing even highly viscous oil and individual grease lumps into the collection zone. The coolant surface is left virtually free of oil and debris.
Because the separated oil drops vertically away from the scraper, fouling and blockage are reliably avoided and regular maintenance is no longer necessary. All parts in contact with the heavily loaded hose are made from highly wear-resistant ceramic or hardened stainless steel, so wear remains very low despite the coolant’s high solids content.
With the surface virtually free of oil, the coolant remains in continuous contact with oxygen and the bacterial load is substantially reduced. Coolant service life increased from only a few weeks to one year, while tramp-oil content fell below 0.5%. Labor-intensive tasks such as vacuuming the coolant surface or replacing the coolant were greatly reduced. The skimmer now requires cleaning and maintenance only once a year when the coolant is changed.
This gave the FRIESS 1U oil skimmer a payback period of less than six months.
