Understanding Uhle Boxes and Covers with Chad Thomack

Uhle boxes, sometimes known as felt cleaning assemblies, dewatering elements, or Uhle pipes, and Uhle box covers are an integral part of every paper machine’s press section. I spoke with Chad Thomack, director of sales for Kadant Solutions, with 16 years of pulp and paper experience, to understand Uhle boxes and covers, how they work, and what the future holds for this important component used in paper making.

V:    What is a Uhle box?

C:    A Uhle box is a stainless-steel structure located in the press section of a paper machine. The structure is equipped with a cover that contacts the machine clothing. The Uhle box, connected to a vacuum source, maximizes water removal from the felt. This helps prevent felt filling by extracting water, fibers, and contaminants, ensuring cleanliness, maintaining permeability, and optimizing press felt performance. Some Uhle boxes rotate and typically feature a lubricating shower to help seal and reduce the coefficient of friction between the Uhle box cover and machine clothing.

V:    What is a Uhle box cover?

C:    A Uhle box cover is a low friction capital or consumable wear surface which interfaces with machine clothing. Uhle box covers can be manufactured from a wide variety of materials but are commonly constructed of polyethylene or ceramic. Uhle box covers feature a specific open area to meet the dwell time and dewatering requirements of a given application. They are typically offered in engineered patterns like drilled covers or herringbone covers or can be full-width strips.

Kadant Solutions has three different polyethylene materials that can be made into Uhle box covers or strips: the Durwear™ HTb, Durwear A+, and Durwear XW. We also offer ceramic cover/wear strip options.

V:    You mentioned earlier that the Uhle box is located in the press section of a paper machine. Why is the Uhle box used in the press section?

C:    The press section felt has a unique construction compared to other fabrics on the paper machine. While high-energy water jets effectively clean other machine clothing, the same approach cannot be applied to press felts because it would damage the batt structure.

Uhle boxes play an important role in making sure the machine clothing is conditioned. Needle jet showers operate at lower energy levels, gently loosening contaminants from the press felt. This process allows the vacuum from the Uhle box to effectively extract water and trapped process particles from the felt structure, ensuring the felt returns cleaner and drier to the press nip.

V:    What is the most common problem you see with Uhle box covers?

C:    Lack of optimization is the most common challenge mills face with Uhle box covers. It’s common to see Uhle box covers with patterns that aren’t engineered for the correct coverage or do not meet dwell time requirements. As a result, these applications are not maximizing water removal and imparting a negative moisture profile to the sheet. We also see set up issues where the Uhle box cover orientation and lube shower coverage is incorrect as depicted in the graphic below.

V:    How often should Uhle box covers be replaced?

C:    There are a couple indicators: Need to be sure to change the Uhle box cover or wear surface prior to them being structurally compromised, especially with ceramics as machine clothing can be damaged or fail. Caliper measurements and visual inspections for sharp edges and failing segments help capture these conditions. Another indicator is when moisture profiles deteriorate. This condition can occur with herringbone covers when the slot is excessively worn, and water is injected back into the felt. Though you never want to be in this condition, elevated press felt batt loss can indicate a cover or wear surface issues as well.

Low grade poly Uhle box covers can wear out in as little as one outage cycle, especially if the system is not installed correctly. Kadant’s higher-end poly offerings will typically double or triple the life of low-grade poly. Some applications will last a full year. Ceramic wear surfaces will last several years but can be more expensive and require extra care to avoid damage.

V:    Can you discuss a specific challenge that an optimized Uhle box cover design can help paper makers overcome?

C:     Many paper and tissue machines are operating at production rates beyond original design. Making sure each application is optimized for the current machine goals is key. There are proprietary patterns that have been proven to reduce felt deflection and wear which can extend machine clothing life.

V:    What makes Kadant Solutions Uhle boxes and covers stand out from others?

Kadant Solutions has over 70 years of experience manufacturing Uhle systems. We are one of the few suppliers that can provide an entire Uhle system that includes the stainless-steel structure, base/case, rotation mechanism, and lubricating shower along with the poly or ceramic cover. We have one of the largest installation bases in the U.S. This background and knowledge sets us apart in our ability to help optimize an entire Uhle system.

V:    What does the future hold for Uhle boxes and covers?

C:    Most recently, engineering and R&D teams at Kadant Solutions developed new proprietary drilled cover patterns. Our goal was to design Uhle box covers that help extend machine clothing life and improve moisture profiles. These new Uhle box covers offer some of the lowest variations in open area and felt deflection we’ve ever developed. Initial results have shown a 10-20% improvement in dewatering.