Paper manufacturers continually face increasing demands on paper quality due to changing customer expectations.
These demands are not just driven by the expectations of the final consumer regarding the print quality but are also influenced by increasing efficiencies, which require improved paper properties.
The capabilities of a remarkable number of paper machines, mainly older graphical paper machines, are increasingly
unable to satisfy these demands. They often struggle to keep their product in an ever more demanding market. As a result, even newer machines undergo modifications to allow for the production of different grades
than originally planned.
This paper presents an overview of rebuilds of former, press and dryer sections, as well as coating equipment and
the measures achieved.
The participants will see different original layouts of the various sections of production machines and the
rebuilds done along with numbers showing the actual potential of the machine, together with latest technology.
This will enable the participants to see parallels for their possible needs and options regarding an economical way
to get value through reasonable investment costs at low risk.
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This paper will give a brief overview of various rebuilds done on different production
machines in order to get increased value from existing equipment with a potential to produce enhanced product qualities.
Today, it will not discuss possible modifications on headboxes, calenders and reels, as the
potential of today's state-of-the-art equipment and/or controls is well known.
Far more demanding and a challenge to the teams of the supplier and the user are rebuilds
which affect the more complex equipment built into a paper machine.
Therefore, we will walk through a paper machine starting with rebuilds of former sections,
followed by press-, dryer sections and coating equipment.
Wire section rebuilds:
The concept of the blade-type hybrid former shown here is suitable for almost all paper
grades. This is especially suitable for rebuilds, as we will see on three practical and already implemented rebuild examples, along with achievements made.
Blade-type Hybrid Former [Figure 3]:
The initial drainage (fourdrinier part) consists of a forming board, several foil boxes and/or single foils and usually one or two wet suction boxes ahead of the top wire forming unit.
The drainage section in the twin-wire area consists of the top wire suction box, the forming
box in the bottom wire below the top wire suction box and the succeeding transfer box, at which the top and bottom wires separate.
The top suction box consists of three suction zones which are: the vacuum skimmer, the
first and second suction zone.
Due to the alternating pressure impulses in the area of the first suction zone, fibre flocs are
disrupted. This results in an excellent formation level which is achievable with this type of former which will be discussed later on based on actual examples.
Due to the single-sided dewatering process on a fourdrinier, there is a higher ash content on
the top side of the sheet. With a top former the dewatering process occurs towards the top and bottom side of the sheet and therefore the ash content is way more evenly distributed in z-direction of the sheet.
The first example is the rebuild of a forming section at a mill located in Germany producing
recycling copy paper using a furnish of a 100 % deinked pulp.
The 12-year-old roll-type hybrid former [Figure 4] was replaced by a blade-type hybrid
former [Figure 5].
The major target of the rebuild was to improve formation quality, which has been achieved
remarkably. The improvement in the Ambertec formation was approx. 30% compared to the formation before the rebuild.
In addition, MD/CD tensile ratio and CD basis weight profile were maintained or improved.
The twosidedness of the ash Z-distribution could be reduced to less than 1 % absolute and the MD/CD tensile ratio as well as the CD basis weight profile was maintained [Figure 6.].
This overhead again shows the reduction in the formation index from about 0.6 %(g/m²) Ambertec normalized to about 0.4 %(g/m²) [Figure 7].
A second noticeable effect of the rebuild was a significant increase in dewatering capacity
which allows for possible speed increase.
The second example will discuss the rebuild of a former of a paper machine in Sweden
producing printing and writing grades. Since the existing roll-type gap former [Figure 8] had very limited possibilities to adjust or improve formation, the former was replaced by a blade-type hybrid former [Figure 9] in order to allow formation adjustment and improvement.
Also in this case the achieved results were a big success for the customer. An improvement
of formation by 30 % [Figure 11], an ash distribution with a difference of less than 1 % absolute at no compromises in opacity were achieved [Figure 10].
The third example discusses a former rebuild of a machine in Germany producing envelope
paper from 100 % deinked pulp. A top former [Figure 13] was installed on the existing fourdrinier [Figure 12] in order to improve formation.
This chart shows the remarkable improvement in formation from about 0.7 %(g/m²) to 0.9 %(g/m²) Ambertec normalized to values of about 0.5 %(g/m²) depending on the basis weight [Figure 14].
The short initial drainage resulted in the very quick fixing of the fibers and therefore, the
desired increase of the MD/CD ratio of approx. 20 % was achieved [Figure 15].
Press Section Rebuilds:
Since 1984, closed shoe presses have been in operation on paper machines for different
paper grades and a variety of speeds. Meanwhile, these presses have certainly proved their influence on both production and quality.
With shoe presses the press pressure is created by a concave pressure shoe which presses
a polymer press sleeve against a press roll.
Compared to a conventional roll press, the longer press zone achieves a longer pressing time
, resulting in a higher press impulse.
For optimum product quality, the dewatering process during pressing should take place with
an optimum pressure development.
This chart [Figure 17] explains the difference between the press impulse of a conventional roll press and a shoe press. The far lower pressure gradient of a shoe press, along with a
higher dwell time resulting in a higher pressure impulse at lower maximum pressure, results in a higher dewatering capacity compared to a conventional roll press.
Advantages for the papermaker are:
- higher dryness after press
- higher bulk
- even density distribution in z-direction
- lower two-sidedness regarding roughness and oil absorption
- higher felt life
Combi Press with Shoe [Figure 18]:
Especially when facing space limitations as with a combi press with the pick-up suction roll
forming the first nip, rebuilds with shoe presses in the 2nd nip are the most suitable and often only solution and advantageous regarding capital investment because of the cost saving design:
- no pick-up roll and spare roll necessary
- less vacuum required
- reduced number of drives
- simple framing of felt run.
As a first example I would like to discuss a rebuild of a press section in a mill in Sweden
producing coating base paper on this machine.
The goal was higher drying potential for increased production whilst maintaining bulk.
The solution was to modify the existing 3 roll press with separate third press [Figure 19] into a shoe press in the second nip with new center roll [Figure 20]. The pick-up suction roll was reused and, according to the customer, the top roll of the third press had to be identical
with the top roll of the second press. A special challenge was to reuse major framing components.
Since the furnish also changed after the rebuild, it has been difficult to compare the
measures achieved with the values before the rebuild. However, the speed was increased from 830 m/min to 1,000 - 1,100 m/min. Depending on the grade, the dryness after the
press increased by approx. 3 %. The bulk was kept at acceptable levels [Figure 21].
At another mill in Finland the two existing free-standing roll presses [Figure 22] were replaced by a combi press with shoe in the second nip [Figure 23]. Due to less space
required by this solution, two cylinders could be added to the dryer section and the web run could be optimized. With this, the draws could be reduced from about 6 % to 3 – 4 %. The
dryness after the press increased by 4 %. The roughness on the sheet top side was decreased by 1.1 microns. The gradual increase in the oil absorption on the top side stayed
within the acceptable limits set by the customer [Figure 24].
The last rebuild I would like to discuss with you is a rebuild of a paper machine in Germany
producing SC grades. There, the existing 1-nip roll press with 2nd press [Figure 25] was replaced by a combi shoe press with press shoe in the 2nd nip [Figure 26]. The second press
was also rebuilt.
After the rebuild the production speed increased by 200 m/min to 1,000 m/min. Today, the
dryness after the press section is in the range of 52 % compared to approx. 44.5 % before the rebuild. The roughness level was also reduced, as well as the roughness twosidedness,
which is today about half of the value before the rebuild. There are also improvements in porosity, and the number of sheet breaks after the rebuild is close to zero [Figure 27].
Drying Section Rebuilds:
Not only on the wet end but also in the dryer section, productivity and quality can be
improved by rebuilds.
Sheet Stabilization [Figure 30]:
Due to limited efficiency of conventional and mostly older dryer sections regarding web run
and rope run, the production and quality problems faced are:
- increased number of sheet breaks
- loose edges
- unacceptable threading times
- risk of accidents
In order to tackle the above-mentioned points, systems are available based on experiences
gained with state-of-the-art dryer sections. Those systems can also be installed in modules on a step-by-step basis.
This solution shows the modification of single-tier and possible double-tier sections into a
single-tier concept. The lower cylinders will be drilled and web stabilizers operating at a low vacuum will be installed on the top of the drilled cylinders. This ensures a secure run of the
web with the dryer fabric also at remarkably higher speeds than were possible before a rebuild. This solution is possible on any type or design of the lower cylinders [Figure 31].
This solution is specified by the replacement of the dryer fabric roll in order to allow the
installation of web stabilizers. Even at high speeds, the web is supported far better, resulting in fewer sheet breaks [Figure 30].
Module: ropeless threading
This solution allows the ropeless threading throughout the entire dryer section whether it is
a single or a double-tier dryer section or a combination of both.
Case studies show that the pay-off of the above-mentioned solutions is possible within 8-12
months, depending on the total rebuild scope.
Sheet stabilisation benefits are [Figure 32]:
- stable web run at high speeds
- low production loss due to fewer sheet breaks
- less shrinkage
- no ropes (maintenance and cost)
- minimized risk of accidents
- short rebuild times
At a mill in Finland producing coated papers, along with a rebuild of the press section the
dryer section was also modified. The initial installation was 2 single-tier drying sections and 3 conventional sections. For a stable web run, the lower cylinders of the single-tier groups
were grooved and drilled and web stabilizers were installed in those groups. The threading was done ropeless in the first two groups and with rope in the conventional groups [Figure 33 top].
During a shutdown, the first conventional group was rebuilt into a single-tier group along
with drilling of the lower cylinders and the installation of web stabilizers. In the fourth group, in order to create a optimum web run, the draws were shortened by an asymmetric
installation of the dryer fabric rolls and new web stabilizers. The threading was converted into ropeless threading for the entire dryer section [Figure 33 bottom].
With the installation of a shoe press and the described modifications to the dryer section the
operating speed was increased from 1,272 m/min to 1,400 m/min. The maximum speed before rebuild was 1,300 m/min. Just three months after rebuild a speed of 1,475 m/min was
achieved and today the maximum speed is 1,525 m/min.
The number of sheet breaks in the single-tier groups was reduced from 16 to 2 per month.
The average threading time fell from about 13 minutes down to 3 minutes.
Calculations show that the savings for maintenance of the ropes resulted in about 80
minutes more production time per month. The entire rebuild, including rebuild of the press, took 15 days [Figure 34].
Stabilizers with a "release" zone:
A further optimisation would be the installation of web stabilizers with a web "release" zone
for a vacuum-assisted web run with the following advantages:
- web stabilizing right after release from the cylinder
- short release distance
- reduced web tear
- reduced draw
[Figure 35, 37]
The high vacuum in the release zone gives reduction in draw and improved quality and runability. Benefits include speed increase, fewer breaks, less sensitivity to process
variations, quality improvements and an opportunity for higher filler contents [Figure 36].
This chart [Figure 38] shows the results at a mill in Germany where stabilizers of this type
have been installed. Due to the release of the web by higher vacuum it requires less draw between the press and the dryer section, leading to significant speed increases.
Coater Section Rebuilds:
To close, I would like to present a modularly structured coater, which I will describe below.
This is a kind of building block principle with different coating units, and several drying and tension control modules.
This concept [Figure 40] is particularly suitable also for rebuilds due to its very compact
- Jet Coater
- Film Coater
- Curtain Coater
- Modular Dryer incl. Gas or Electrical IR, Air dryer or HPC dryer
Draw Control Modules:
- cylinder dryers
- s-wrap groups
The comparison of two off-line coating machines – at the top of the conventional type and
at the bottom based on the Modular Coater concept [Figure 41] – clearly shows the differences between the two systems and the advantages [Figure 42]:
- integrated IR, heated Air Turn, Flotation Dryers and Web Cooling System
- approx. 65 % fewer paper rolls
- reduced machine and building dimensions
- reduced energy consumption
- improved machine availability
The following example is of the rebuild of an off-line coater in Finland [Figure 43, 44] with the following results:
- successful integration of precoating in existing OMC within tightest space (12 m)
- all production and quality targets reached
- today's max. operating speed is above 1,700 m/min
This paper was designed to give an overview of rebuild possibilities for increasing quality and
production. The objective was to give one option or another that can possibly be adopted into your own system.
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