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.