The design of a vacuum system for a felt conditioning system starts on the paper machine at the uhle box. To properly size a uhle box the maximum speed that the paper machine will run must be known first. TAPPI recommends that a minimum of 2 milliseconds up to 4 milliseconds is sufficient to dry a fabric. Any less than 2 milliseconds will not allow enough time for proper water removal and over 4 milliseconds has been found to hold no significant increase in water removal, and can in fact, add a drag load on the paper machine.

Once the slot width is established for the correct dwell time, the open area of the slot is calculated and is multiplied by the TAPPI recommended vacuum factor, 15-20 ACFM/square inch. This establishes the total vacuum airflow required for the uhle box to operate correctly. The body of the uhle box is designed so that the velocity is limited to 3,500-4,000 ft/min within the uhle box. This ensures that there will not be a vacuum drop from one end of the uhle box slot to the other for an even vacuum profile across the paper machine felt. The vacuum piping from the uhle box should match the size of the uhle box or be larger to prevent vacuum losses at the vacuum piping runs.

Often a "formed" uhle box is needed to both meet the air velocity requirements and the physical envelope that exists on the paper machine. By using a square or rectangular design which will provide an equivalent cross sectional area to a round design, most applications on the paper machine can be fitted properly. Due to the expense of a new paper machine, it will be upgraded for decades increasing the speed and requiring much more vacuum than originally designed for.

All uhle box applications should have an inlet separator installed before the vacuum pump to prevent surging and buildup in the vacuum pump. When the air flow is processed by the inlet separator, the water and contaminants are removed from the vacuum system, this is now considered "dry" air. After the separator, the velocities can increase to 5,500-6,000 ft/min as per TAPPI recommendations for dry air applications. Not following these rules for velocities can be an expensive mistake, if the application on the machine needs 18"Hg but the vacuum pump has produce 20"Hg due to friction losses. This increased horsepower to make up for vacuum losses can be significant to a mills annual utility costs.

Sizing the correct vacuum pump for a uhle box application consists of finding the air capacity needed by the uhle box, (vacuum factor X slot area), and to make sure the correct vacuum pump cone port for uhle boxes is used. Uhle boxes run at lower vacuum levels when the felt is new and more open, anywhere from 9-12"Hg. As the felt fills with fiber the vacuum level increases during the felt's life cycle. The felt is commonly changed when the uhle box vacuum level begins to read 18"Hg, this shows that the felt is so full of fiber it will not drain properly. A Vooner "M" cone or a Nash #2 cone vacuum pump is for medium vacuum and has a very flat power curve. A Vooner "L" cone or a Nash #1 cone has a power curve that matches the vacuum level. This means that at lower vacuum levels the brake horsepower will also be lower and the pump will be running at the maximum horsepower for only a short time right before the felt is changed.

A lubrication shower for a uhle box is more than just for lubricating the wear surface. A lubrication shower is used to guarantee a proper vacuum seal for optimum cleaning and drying. If the vacuum pump is sized properly any water that is added to the process will be removed by the uhle box. Every uhle box should have a "WET SEAL" shower.

The location of the uhle box to the high pressure shower should be clear without a roll between the two. The high pressure shower is designed to loosen the felt particles which will allow the uhle box to easily remove them. If a roll is between the shower and uhle box the particles will be compressed back into the felt and hinder cleaning.

CVN Vooner, www.cvnvooner.com

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