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6.6 Dryer Section
6.6.1

Overview
The purpose of the dryer section is to increase the dry content of the paper web, usually to 90 to 98 %, by evaporation. During drying the fibers develop hydrogen bonds which provide the natural strength of the paper. Drying is a coupled heat

6.6 Dryer Section
and mass transfer process so heat has to be transferred from a heat source to the paper and the evaporated water has to be carried off. During drying, the paper web which has been picked up from the press section has to be guided safely through¬out the dryer section to the reel where it is wound up. Some dryer sections include a size press for paper strength improvement and/or a breaker stack for pre-cal-endering. Depending on the type of paper machine different heat transfer and drying principles as well as their combinations are applied.

Drying Principles

6.6.2.1 Contact Drying by Steam Heated Cylinders
With this most common principle in paper drying steam condenses at the inner surface of the cylinder wall, the heat is transferred through the wall to the paper web, the web is heated and water is evaporated. Air flow takes up the evaporated water. The heat transfer rate from the steam to the cylinder shell depends on the flow pattern of the condensate motion. This flow pattern is mainly dependent on the machine speed, and to a lower degree on the amount of condensate volume in the cylinder and on the cylinder diameter. At low speeds of up to about 300 to 500 m min–1 a pond of condensate is found in the cylinder. At higher speeds – above the “rimming speed” – the condensate builds up a ring. Acceleration during “ascending” of the condensate ring is against the rotating direction, and in the rotation direction when descending. This results in a swinging condensate motion relative to the cylinder with the effect that the condensate ring velocity is lowest and thus its thickness is highest at the culmination point and not in the bottom position. For high machine speeds the condensate motion and thus heat transfer decrease. For heat transfer enhancement and uniform drying spoiler bars induc¬ing turbulence to the condensate layer are installed (Fig. 6.52). The condensate is removed from the inner surface of the cylinder to its axis by syphons (Fig. 6.53) in the form of a two-phase flow of steam and condensate. To overcome the high centrifugal forces and to generate the two-phase flow a pressure difference of about 0.3 to 0.5 bar is necessary. The syphons either rotate with the cylinder (for higher machine speeds) or are stationary. Heat transfer through the wall depends on the thickness and conductivity of the cylinder material which is mainly cast iron (in some cases steel). Higher steam pressure increases the temperature difference and thus the drying rates. Accumulation of air in the cylinder has to be avoided as it would reduce the condensing temperature according to the partial pressure. Good heat transfer from the cylinder to the paper web is obtained by pressing the web tightly to the cylinder e. g. by means of dryer fabrics.
 
 6.6.2.2 Air Impingement Drying
This drying principle is mainly used in tissue production or in coating machines but also for enhancing the drying capacity of drying cylinders in multi-cylinder dryer sections. Hot air is blown through a nozzle plate at high velocity onto the paper. The impinging air transfers heat to the web and takes up the evaporated water. The air is then sucked back into the hood. Heat transfer in impingement drying increases with increased air temperature and velocity and by reducing the spacing between the nozzle plate and the paper surface.
6.6.2.3 Through Air Drying
This method is used in the drying of tissue and nonwovens. Hot air is sucked or blown through the air permeable paper web supported by a heat resistant wire. Heat is transferred directly into the fiber network and the evaporated water is carried off. Through air drying results in the highest drying rates.
6.6.2.4 Infrared Drying
This heat transfer method is mainly used to enhance the drying capacity in coaters when the web is wet. Infrared heaters are usually gas fired. The gas heats a mesh to a temperature of about 900 to 1100 °C. The low thermal inertia of the mesh allows fast control of the mesh temperature and the heating rate as well as preventing fires in the case of sheet breaks. In some cases electrical heaters are in use with temperatures up to about 700 °C, exhibiting a fast cool down of the emitter plates. Infrared drying units need sufficient air flow in order to carry off the evaporated water and to prevent coat quality problems.
6.6.2.5 Press Drying
This method is a combination of pressing and drying. First the web is dewatered mechanically in a press nip and brought into tight contact with the hot surface on one side. At the opposite side the web is covered by a permeable belt such as a felt or a wire which continues to press the web to the hot surface over a longer dis¬tance. The vapour escapes through the permeable cover or is stored therein. There are only a few installations of this dryer type worldwide.
6.6.2.6 Impulse Drying
This method is also a combination of pressing and drying. The process takes place in a press nip (for instance with a shoe press) where one surface, which is in direct contact with the web, is heated. The other web side is in contact with a felt. The wet web is compressed and thus mechanically dewatered. The vapor generated at the hot surface pushes the water through the compressed capillaries towards the felt and finally the generated vapor can flow freely through these channels. This kind of process is still in development.
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6.6.3
Dryer Sections
6.6.3.1 Multi-cylinder Dryer Section
6.6.3.1.1 Types of Multi-cylinder Dryer Section
Multi-cylinder dryer sections consist of double-tier or single-tier groups or a com¬bination of both. In double-tier dryer sections (Fig. 6.54) the paper web runs around a large number of cylinders (up to about 60 in graphic paper machines and up to 90 for board and packaging paper machines) arranged in an upper and a lower row. The paper is in contact with each cylinder at an angle of about 220 to 240°. Dryer fabrics, one for each top group and one for each bottom group of cylinders, press the paper against the cylinder surface for improved heat transfer. Even together with stabilizers the dryer fabrics can only partly support the paper web during its transfer to the next cylinder. Tail threading in conventional dryer sections is usually done by means of ropes.
The state-of-the-art dryer section for a high-speed paper machine has a single-tier configuration with steam heated cylinders in the top row and suction rolls in the bottom row. The paper web is always supported by the dryer fabric and runs jointly around the drying cylinders and suction rolls (Fig. 6.55). In the top row the paper web is pressed against the drying cylinders by the dryer fabric. In the bottom row it is held on the fabric by reduced pressure in the suction rolls. Stabilizing elements ensure a safe web run along its path between cylinder and suction roll where the web is not in contact with the drying cylinder (Fig. 6.56). For high speed paper machines tail threading with only doctors and air jets is standard (Fig. 6.57). Many paper machines have some single-tier groups for runnability reasons fol¬lowed by double-tier dryer groups. Cylinder diameters today are usually 1.8 m, in older machines they are also 1.5 or 2.2 m.
The dryer section is split into several drive groups to control web tension and to account for web stretch and shrinkage. Each group has its own dryer fabric and separate drive. The steam and condensate system is also split into groups to con¬trol the heating curve. Temperature and steam pressure in the different heating groups have to follow an ascending curve so, usually, a steam cascade system is applied, supplying the blow through steam of the last drying group to the last but one etc. The remaining steam from the first group is condensed and any air ex¬tracted from the steam-condensate system.
In the field of dryer fabrics excessive air entrainment at high machine speeds had to be reduced without losing drying capacity by decreased air permeability of the fabrics. Modern dryer fabrics provide high contact area, low caliper, high stabil¬ity and abrasion resistance. Good cleaning of the dryer fabrics ensures uniform evaporation, less sheet picking and improved effectiveness of the web run stabi¬lizers. Shutdowns for cleaning can be avoided or cleaning intervals increased by appropriate cleaning devices as mentioned in Section 6.4.
 
6.6.3.1.2 Web Handling
In order to reduce the forces acting on the web at high machine speeds a single-tier dryer section is applied where the paper web is continuously supported by a fabric. In the critical areas where the web has to be released from the drying cylinder surface stabilizers support the safe web run. Stabilizers are also used in double-tier dryer sections. In the most sensitive areas of low strength or low stretch potential the size and speed of the dryer groups have to be adjusted according to the strength, stretch potential and web shrinkage. This enables fine-tuning of sheet quality and improves machine runnability.
6.6.3.1.3 Dryer Hood
The whole dryer section is enclosed in a drying hood with doors which can be opened e. g. for inspection. It allows controlled flow of the hot and dry make-up air as well as of the vapor laden exhaust air. The pressure inside the hood should be balanced in such a way that a minimum of air is blown from the hood into the machine hall or sucked from the machine hall into the hood. For effective pocket ventilation the hot air enters via blow boxes or blow rolls and flows to both sides of the machine where it is sucked off. To prevent condensation the hood walls are insulated and make up air is supplied along both hood sides from underneath. These measures allow a low amount of make-up air, a high air dew point of the exhaust air and effective heat recovery.
6.6.3.1.4 Paper during Drying
The web strength increases with increasing dryness due to build up of hydrogen bonds between the fibers. The increase in strength from about 50 % to 95 % dry¬ness is about a factor of 10. Stretch before rupture decreases with drying and also depends on the web structure and how far the web was allowed to shrink.
The paper web shrinks during drying. The extent of shrinkage depends on the type of stock, degree of beating, the fiber orientation, and on forces that restrain the shrinkage. The shrinkage in the machine direction can be controlled by stretching the web. CD shrinkage is nonuniform and is higher at the sides than towards the center of the web. The single-tier dryer configuration changes the CD shrinkage profile compared with a conventional double-tier one (Fig. 6.58). The profile is now flatter over a large part of the sheet between the drive and tender side with low shrinkage, accompanied by less stretch potential and higher dimensional stability in this area. On the other hand there are distinctly steeper slopes in the shrinkage curve with high shrinkage numbers at the edges.
6.6 Dryer Section
 Fig. 6.58 CD shrinkage profile for single-tier and double-tier dryer sections.
Paper curl is an undesirable effect when paper undergoes heating or moisten¬ing, e. g. in copy machines or in printing. Curl is due to nonsymmetrical residual stresses in the z-direction of the web which date back to nonsymmetrical drying of its top and bottom sides. The amount of curl is influenced by the degree of non-symmetrical drying. The impact increases towards the end of the drying process. The direction of curl (MD, CD or diagonal curl may occur) is defined by the paper and fiber structure. Curl can be overcome by varying the operation of the top and bottom drying cylinders if there is a conventional double-tier after-dryer section. With a pure single-tier dryer section curl can be controlled by additional tools such as by moistening (water, steam) or by additional drying (air impingement dry¬ing).
A uniform CD moisture profile of the web at the end of the dryer section is an important quality requirement so the heat transfer of the cylinders to the web as well as the vapor exhaust conditions close to the web have to be uniform. Addi¬tional CD moisture control tools such as sectioned cylinders, blow boxes or mois¬turizers are also used.
6.6.3.2 Tissue Dryer Section
Conventional tissue drying is a combination of contact and impingement drying. Today a combination with through drying is often used. So a tissue drying system consists either of a tissue cylinder wrapped by a hood or of a system with an additional through air dryer section ahead of the tissue cylinder. In some cases only through air dryers are used.
6.6.3.2.1 Tissue Cylinder
Tissue cylinders (Fig. 6.59), also called Yankee dryers, have a width of up to 8.2 m and a diameter of 3.6 to 5.5 m, and in special cases even up to 6.3 m. The inside of a tissue cylinder shell is usually ribbed for maximum drying rates. The drying rate is further enhanced by turbulence generators. Condensate removal is performed by two-phase flow through a sodastraw syphon system. The cylinder shell, made of special cast iron, is the most sensitive part as regards safety against catastrophic failure. It undergoes a mainly two-dimensional combined static and dynamic
 Fig. 6.59 View inside a ribbed tissue cylinder with soda straw siphons for condensate removal (source: Andritz).
stressing. This is due to inside steam pressure, high temperature gradient across the shell thickness, high centrifugal forces and dynamic stress due to press roll loading so the admissible steam pressure has to be limited. For good creping effect a coating consisting of hemicellulose and/or synthetic agents has to be established on the cylinder shell surface. This also reduces wear; prolonged cylinder lifetime can also be achieved by covering the surface with a metal spray coating. One (or two) press roll(s) dewater the web and bring it into good contact with the cylinder for intense heat transfer. The shell shape in CD depends on the operating condi¬tions. These must be defined in advance in order to grind the adequate crowning on both cylinder and pressure roll(s) for uniform web pressing and dewatering in CD. As operating conditions may vary in a certain range, the cylinder shape and in CD the moisture profile may also show some deviations. To end up with a uniform moisture profile and creping quality modern tissue drying includes a press roll which can better adjust to cylinder shape deviations as well as a dryer hood with a CD moisture profiling system.
6.6.3.2.2 Tissue Dryer Hood
The dryer hood spans the tissue cylinder by about 220 to 260° and consists of two halves both of which are retractable. The nozzle plate is concentric to the cylinder and includes exhaust openings. The diameter of the nozzles is about 5 to 7 mm depending on the spacing between the nozzle plate and the cylinder. The ratio of nozzle diameter to spacing has to be optimized with regard to minimum fan energy consumption and maximum drying rate. The minimum spacing is limited for runnability reasons to about 20 mm. Air temperatures of up to 700 °C and air blow velocities of up to 160 m s–1 are used. For CD moisture profile control the hood is divided in several sections across its width. The hood is insulated. Its design has to account for the large temperature differences when heated up. The air system with burners, fans and heat exchangers for heat recovery is located in a separate place outside the hood. A tissue dryer hood and its air system is shown in Fig. 6.60.

6.6.3.2.3 Through Air Dryer
This type of cylinder can be built with diameters up to 5.5 m, or in special cases up to 6.7 m, and widths of up to 9 m. It has a free outside surface area of up to 96 % in order to have a uniform air flow through the paper web. During its contact with the cylinder the paper web is supported by a revolving wire. The cylinder can run at speeds of up to 3000 m min–1. Air is usually sucked into the cylinder by reduced pressure and is supplied by a hood wrapping the cylinder by about 250°. The temperature can be up to 300 °C. The design has to withstand the heat up and cool down cycles during its operational life and always has to ensure a uniform geome¬try and drying conditions. Figure 6.61 gives an overall view of a through air drying cylinder and a closer look at the construction.