February 2008
By Martin Koepenick
Every mill is looking for techniques to reduce sheet breaks and improve the sheet quality of the paper it produces. An increasing number of mills have looked to mist elimination as a means to minimize wet end breaks, boost efficiency and customer satisfaction. Mist is also problematic, if not hazardous for operators, who may slip on wet end steps, handrails and other surfaces.
The primary mission of any wet end hygiene program is to effectively clean both the forming fabric surface and the interstices of the forming fabric. A side effect of high-pressure showering, especially on high speed machines and machines that have top-wire formers, is the generation of a tremendous amount of mist that has paper fines, fiber and fillers in it. Various forms and designs for mist elimination have been around for years, however, with the increased importance of fabric cleaning and machine efficiency a new era of mist elimination systems has emerged.
According to the vice president, sales and marketing with Kadant AES, Tom Vaughn, "Mist eliminators improve forming section hygiene that helps a machine run cleaner, which improves overall efficiency and productivity.
That's always a plus. While investing mist elimination, systems can increase productivity and profitability. In addition, mist minimization is a major safety boost, because of cleanliness of catwalk rails and stairs. When safety improves, maintenance takes a leap forward, especially around the paper machine."
All Types of Configurations
Mist elimination systems can be configured to fit on all types of top wire formers, twin wire gap formers and tissue machines. Depending on the machine configuration and customer goals, single-sided and double-sided mist eliminators are custom designed for each application. These systems consist of a combination of high-pressure oscillating showers, one or two vacuum boxes that can be cantilevered to assist in fabric changes, air/water separators and a low vacuum exhauster.
One recent mist eliminator installation was part of a major rebuild that included a new headbox and top wire former. This 320-in. lightweight coated (LWC) machine runs faster than 3,000 ft/min. The results of the installation are significant. Wet end wash ups have been reduced from one or more per week to three weeks or more. Fiber drop offs have been virtually eliminated which has greatly reduced holes and wet end breaks. Machine uptime has been improved dramatically. Finally, a significant increase in forming fabric cleanliness is obvious and this affects fabric life, wet end drainage, and uptime. This mill now has mist elimination systems on all three of its paper machines.
Another 207-in. fine paper machine faced top-wire fabric cleaning issues due to the limitation of showers running at only 150 psi due to mist. After the switch to the new mist eliminator, cleaning was possible at 350 psi. It has also seen an increase in fabric life with the improved cleaning efficiency. The mill management now believes they will be able to increase machine speed from the current 3600 ft/min to more than 4,000 ft/min.
Adds Vaughn, "Anyone who has spent time at the wet end of a paper machine understands that the mix of water vapor and fiber in the air ends up coating everything. Effective mist elimination literally clears the air to make the workplace better and safer. Every time the mill environment improves, mills see gains in productivity. Mill management confirms that operators and technicians appreciate the additional time to focus on other machine issues and not worry about the next wet end break."
Installing high performance mist elimination systems is clearly a trend for the future for all grades of paper. Tissue machines are particularly vulnerable to extraordinary mist problems including the corrosion of machine frame and overhead building supports. Increasing profitability by reducing wet end breaks and reducing product loss due to poor formation and holes, are obvious reasons to move in this direction. Fewer wash ups of the paper machine, and the potential for fewer safety incidents are equally valuable to running clean and incident free.
Martin Koepenick is a freelance writer based in Atlanta, GA
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