The objective of the process on the paper machine is to produce a continuous paper web of the required quality, uniform in machine (MD) and cross machine (CD) directions. The process equipment consists of the approach flow system, the headbox and the wire section, the press section, the dryer section, often a size press, sometimes a coating station, a calender and a pope reeler. After the web has been reeled up this full width mother reel is cut into smaller rolls with widths and diameters according to the customers’ requirements. The rolls are then wrapped and made ready for shipment.
The various paper and board grades require special furnish make-ups as well as paper machine layouts. For economic and quality reasons these machines are now tailor-made for graphic paper, packaging paper, board, specialty or tissue paper production. They differ for instance in wire, press and dryer section design, in machine width and in operating speed. Machine widths are dependent on the grade produced, since the width depends on the customer’s equipment such as printing or corrugating board machinery. For instance, for corrugating board ma¬chines widths of 2.5 m, or 3.3 m for the new generation, are standard. So a trimmed width of 10 m at the paper machine would fit both corrugating machine widths (4 V 2.5 m and 3 V 3.3 m).
The technical history of paper machines shows many major changes in their design and operation over the last three to five decades:
. • Use of recycled fibers putting higher demands for instance on fabric cleaning for high machine efficiency
. • Changing the pH value of the suspension in the approach flow system from acid to neutral, allowing the application of calcium carbonate fillers and ensuring more durable papers
. • Hydraulic headboxes enabling higher machine speeds, higher throughputs and easy application of twin wire gap formers
. • CD basis weight profiling in the headbox by dilution, resulting in improved paper uniformity and quality, thus enabling fiber savings by lower mean basis weights
. • Twin-wire formers providing more symmetrical z-structure of the paper and high machine speeds
. • Shoe presses with high dewatering capacity for high web dry content on enter¬ing the dryer section, thus saving drying energy and improving machine runn¬ability due to increased web strength
. • The one-nip shoe press reducing investment costs, space requirements and op¬erating costs to a minimum
. • No-draw press enabling higher machine speed and better machine efficiency
. • No-draw dryer section enabling higher machine speed and machine efficiency and changing the CD web shrinkage curve
. • Synthetic forming, press and dryer fabrics to best customize the fabrics to the requirements of application for uniform dewatering, smooth web surface struc¬ture and long lifetime
. • Process control with advanced measuring techniques and control strategies al¬lowing the operators to continuously monitor the operation, see trends in paper quality and machine condition and take appropriate action
. • Machine speed: five decades ago the typical paper machine ran at a speed of about 300–350 m min–1. Today maximum machine speeds are above 2100 m min–1 for tissue machines, more than 1900 m min–1 in newsprint production and above 1550 m min–1 for woodfree grades ( Fig. 6.1)
. • Machine width: The typical paper machine of five decades ago had a machine width of about 3–3.5 m. Today machines with 10 m trimmed width (packaging paper) or 10.35 m paper width (untrimmed) at the pope reeler (newsprint) are found as a maximum (Fig. 6.2)
. • Soft covers of calender rolls allowing integration of calenders into the paper machine at high machine speeds for high quality paper grades
. • Integration of coating (and calendering) into the paper machine even at high speeds of up to 1550 m min–1.
6.1 Overview of Paper and Board Machines
When coating and calendering are integrated into the paper machine the efficiency of the whole process decreases as there are two more sensitive process steps with their interfaces to the pre- and succeeding process steps. Here the weakest link in the chain limits the overall production efficiency. On the other hand in off-line coating and calendering the loss in high quality production due to the batch proc¬ess is high. Therefore there is clear trend towards on-line coating and calendering for most applications.
Today paper making is a continuous highly sophisticated process with high de¬mands on precision and reliability of all components. For instance the slice open¬ing of headboxes should have an accuracy of about 1/100 mm (or 10 mm) over a width of 10 m in order to obtain the required CD basis weight uniformity and uniform main fiber orientation in CD. Precision in calender roll grinding should be about 3 mm, which is equivalent to 5 % deviation when comparing it for in¬stance with a paper thickness of 60 mm.
Paper machines are in operation 24 hours per day throughout the year only interrupted by short planned or unplanned shutdowns. Both mean loss in produc¬tion which has to be minimized.
Predicted machine maintenance or wire and felt changes are examples of planned shutdowns and should be synchronized. Unplanned shutdowns are caused for instance by web breaks, or failure of machine components or energy supply.
The forces which the paper web undergoes during its run through the paper machine are as follows:
. • A force to release the web from a surface, which is dependent on the surface properties and on the moisture content and other properties of the web.
. • A vacuum force is built up in the vicinity of the detachment line which is de¬pendent on the detachment angle as well as on the machine speed.
. • A dynamic pressure force acting in a free draw results from any air flows and is dependent on their velocity and the size of the impinged unsupported area.
. • A force due to the dead weight of the web is dependent on the total web weight, the length of and the sag in the free draws.
. • The centrifugal force in the free draws is a function of the total web weight and the machine speed.
. • A dynamic pressure due to the entrained air builds up in the gap where the web is going to contact the next surface (the actual friction forces of the air may be neglected here) and is mainly a function of the machine speed.
. • The mechanical draw force which has to be applied must at least overcome the above acting forces to ensure safe web run.
The applied draw force must be well below the strength of the web to avoid web breaks. Force application results in web stretch, so the web should run faster. During drying the web shrinks, which means that the web runs more slowly if there is no force to counteract the shrinking forces. This force, again, has to be well below the web strength. At low sheet dryness the strength is lower whereas at higher dryness the shrinkage is greater. The stretch potential of the paper web at different dryness varies correspondingly and is lowest at high dryness. In these most sensitive areas of low strength or low stretch potential the felt speed differ¬ential between the individual press nips as well as the size and speed of dryer groups have to be adjusted accordingly. Furthermore free draws of the web have to be avoided at higher machine speeds. Local nonuniformities such as poor profiles of basis weight or moisture in CD and MD further raise the web break risk.
After a web break (or at the machine start up) the web has to be threaded through the machine as fast as possible to reduce production downtime. This is done by feeding either the web at full width or just a “tail”, a web strip of about 20 cm, at the machine tender side which is then widened to the full machine width after completion of tail threading. The equipment for web or tail feeding includes air blowing nozzles, suction rolls, wires and felts, rope guides or vacuum-sup-ported transfer belts.
The web width changes during the run through the paper machine by:
. • edge trim at the end of the forming section reducing the wet web width by about 150 to 200 mm,
. • running the web under tension (draw) and
. • shrinkage in the dryer section reducing the web width by about 2–8 %.
A paper machine comprises a framework at the tending side and one at the drive side of the paper machine supporting different kinds of rolls and stationary ele¬ments like dewatering elements or beams for sensors. Almost throughout the whole paper machine the paper web is in contact with forming wires, press felts
6.2 Rolls in Paper and Board Machines
and dryer fabrics. When worn fabrics are to be replaced, the front framework is opened (“cantilevered”) and an endless forming wire or press felt is installed. Al¬ternatively, in the press and dryer section a non-endless fabric is first thread throughout the machine part and then closed inside the machine with a seam.