Seals used in mixers and kneaders must often withstand extreme temperatures and aggressive chemicals. To extend service life, reduce maintenance costs and improve productivity, Hermann Linden selected Kalrez Spectrum perfluoroelastomer O-rings for its double-arm (Z) series kneaders – and gained a six-fold+ increase in maintenance intervals compared with the flat seals originally used.
For over 60 years, Hermann Linden Maschinenfabrik GmbH & Co. KG, of Marienheide, Germany, has supplied double-arm (Z) kneaders to industrial customers for high-viscosity product mixing. These kneaders typically process aggressive chemicals at temperatures from 100 to 300 °C in the chemical, pharmaceutical, paints, plastics and other industries.
Originally, both sides of the mixing trough were bolted on and sealed with asbestos-free sheet gaskets of aramid fibres, fillers and high-temperature-resistant asbestos substitutes. These seals were prone to occasional leaks, however, that in some cases could only be temporarily repaired. Even special flat seals with metal inserts failed to achieve more than a slight increase in service life.
The challenge for Linden was to find a long-term method of sealing the trough and side walls. Although one side could be welded directly to the trough, eliminating the seal, the second side still had to be removable – demanding a special solution to the sealing problem.
Linden decided to work with customers to seek alternative solutions that would extend service life, reduce maintenance costs and improve productivity.
One suggestion was to caulk the groove with soft metal. However, this was rejected as being costly to replace and not very maintenance-friendly. Instead, an O-ring seal was proposed, since it would be easy to fit and replace and required far lower contact pressure to seal reliably, compared with the flat seal.
Good heat and chemical resistance essential
„We were looking for a sealing material that could withstand kneader operating conditions over several years,“ explained Alexander Schneider, general manager of Linden. „In these applications, the seal must resist temperature cycles from 100 to 300 °C and exposure to a variety of chemicals.“ Linden asked customers to test a silicone seal, a graphitised aramid-fibre cord and a perfluoroelastomer seal under typical operating conditions. Although the silicone seal stood up to high temperatures, it was severely attacked by the process chemicals. The graphitised aramid fibres resisted the chemicals but charred at high temperatures. Only the perfluoroelastomer seal withstood both aggressive chemicals and high temperatures.
„The mixing trough in one application had a working capacity of 2750 l, requiring an O-ring with a diameter of more than 1.5 m,“ explained Johannes Löcher, former general manager of Linden and now a consultant to the company. „We searched for a manufacturer that could produce perfluoroelastomer O-rings in this size without any problem. We finally chose O-rings made of DuPont Dow Kalrez Spectrum 7075 perfluoroelastomer because of its availability in the required dimensions at short notice.“
Kalrez Spectrum 7075
In view of the challenging operating conditions, DuPont Dow recommended O-rings made of its Kalrez Spectrum 7075 perfluoroelastomer. The result is an Kalrez O-ring with a cord thickness of 9.86 mm and a diameter of 1705 mm optimised for the specified groove dimensions.
„We are enthusiastic about the performance of the Kalrez perfluoroelastomer O-rings and calculate at least a six-fold increase in maintenance intervals compared with the flat seals currently used,“ says Schneider. „We are planning to use Kalrez for our other kneaders and mixers operating in similarly challenging conditions.“
About Kalrez Spectrum perfluoroelastomer parts
Kalrez Spectrum perfluoroelastomer parts are tailored to the requirements of the chemical and petrochemical industries. They combine very good elastic properties with exceptionally broad chemical resis-tance, and are ideally suited for sealing applications requiring excellent resistance to chemicals and high temperatures.
Kalrez Spectrum 7075 sets new standards for seals used in challenging chemical processes. The DuPont Dow perfluoroelastomer offers even lower compression set (15 % after 70 h at 204 °C), higher thermal stability (continuous service at up to 326 °C) and better chemical resistance than Kalrez 4079, the previously accepted industry standard. In addition, it provides an exceptionally smooth surface. For applications requiring even broader chemical resistance, DuPont Dow recommends Kalrez Spectrum 6375 for continuous service temperatures up to 275 °C.
About Hermann Linden
Hermann Linden Maschinenfabrik GmbH & Co. KG was founded in 1940 as a family business pro- ducing mixers and kneaders for laboratory use. Today, with some 42 employees, Linden manufactures mixers, kneaders and agitators in laboratory, pilot-scale and production scale sizes with capacities from 1 to 6000 l, for worldwide use.
Typical industry applications include chemicals, pharmaceuticals, plastics, cosmetics, paints and coatings, confectionery, cements and sealants. All machines are individually produced to exact customer requirements, and are delivered fully assembled on special transporters.