Chemical press felt cleaning in the press section
The batt-fibers act as a type of filter and retain impurities from the production process. This is unavoidable, but such soiling can lead to a variety of problems. In most cases, these relate to a loss of paper quality. Sheet stealing is another negative side effect: Press dewatering can deteriorate, which leads to an upward trend in the number of sheet breaks. Increased vacuum in the suction rolls and boxes leads to higher energy consumption and felt wear. Should these problems become acute, the only solution is a felt change. The resultant additional shut downs naturally result in production losses. Today, chemical press felt cleaning is indispensable for paper makers.
What is the basis for the choice of chemicals?
There are a couple of general rules, e.g. calcium carbonate filler can often be easily removed with acid cleaning agents, while alkali agents are normally the best solution for organic contaminants. However, the most suitable cleaning agent in practice must be established in the course of laboratory testing.
It becomes apparent that optimum cleaning results are achieved through a combination of diverse cleaning phases. It is also interesting that the average consumption of cleaning chemicals varies considerably according to paper grade. In the case of tissue, specialty papers, board and packaging, it can be assumed that around 250g of chemicals per metric ton of paper will be needed for felt washing. Conversely, only 50 g/t are needed for graphic paper. This discrepancy mainly derives from the fact that there are many graphic production lines where no chemical washing is carried out, largely due to the clean raw materials employed. During the optimization of felt washing, should work closely with the chemicals supplier of the respective paper mill.
What methods are available for press felt cleaning?
Felts are always cleaned mechanically, i.e. with high and low pressure showers and suction boxes. Chemicals merely play a supportive role. In principle, one can differentiate between a continuous felt treatment with chemicals, which is relatively rare, and discontinuous treatment.
Occasionally, the felts are subjected to discontinuous treatment, which can be done either during production or shutdown. However, the most frequently applied method is cleaning during a shutdown. This offers the advantage that negative effects on production are avoided and the press felt can be treated with relatively high concentrations.
Recommendations for paper producers with felt contamination problems
A number of aspects require consideration during felt washing optimization, e.g. the position of the showers, the choice of chemicals and the coordination of subsequent cleaning steps. Clearly, the design of the press felts plays a major role how easily the felts get dirty and then can be cleaned.
In many cases, the best solution is when answers are sought during close cooperation between the paper mill and its suppliers of chemicals and clothing. This offers the best chances that paper quality and paper machine runnability can be noticeably improved.