Paper machine dryer section

Continuous paper machines have undergone extensive mechanical developments since the 1950s, although the principles employed have changed Utde. Cylinder machines stUl are operated and involve multiples of five to seven cylinders they are used to produce heavy multi-ply boards. Fourdriniers are common in the industry and can be used to produce virtually any grade of paper or paperboard. They vary from 1 to 10 meters in width and, including the press and dryer sections, may be more than 200 m long. 

As the machine speed has continuously has increased the dynamic forces to the web have increased and it has forced to support the web with felts to avoid web breaks. Before 30 s typically the web had open draw between press and wire section which was soon replaced with pick-up suction roll arrangement. In this arrangement web is picked up with a suction roll from wire and supported by press felt to the first nip. Most recent fast paper machines are using the same principle of suction roll and supporting fabric not only through press but also through dryer section. 

Kirk, L.A. 1979. Digital simulation of the ventilation of a paper machine dryer section, AIChemE Symp. Series, 184 86-93. 

The multicylinder dryer section of a paper machine consists of a number (up to 70) of large, hollow, cast iron or steel cylinders over which the web passes. These cylinders are nsed to alternately heat the two sides of the sheet. The major mode of heat transfer is the conduction through the steam-heated cylinder shells. 

The number of dryers and the auxiliary equipment used in a dryer section depend on the particular grade to be produced and the speed of the machine. Up to 70 cylinders may be used in a Kraft dryer section 50-55 would be adequate for a typical newsprint or fine paper dryer section. On the other hand, although the breaker stack and size press are necessary for Kraft bag and the hot press, size press, and intercalender dryers for linerboard, none of these is used for corrugating medium [4],... 

In a paper machine, the dryers are arranged in groups that have a common steam-supply header. Each group has to operate in such a condition that neither the condensate nor the noncondensables accumulate in the dryer. There are basically two approaches to design the dryer steam and the condensate system. One is the recirculation system in which most of the blow-through is recirculated and the other is the cascade system in which the blow-through steam from higher-pressure sections is used in lower-pressure sections. The recirculation system is more flexible whereas the cascade system is simpler and cheaper [4],... 

The mill had the choice either to shut the machine or upgrade to improve the stretch and TEA values and the profiles in the cross direction. The mill acted to address the CD profile problems by upgrading the dryer section of the paper machine to improve dryer pocket humidity profiles. At the same time, the mill also decided to upgrade the product quality from flat sack to semi-extensible sack papers. Semi-extensible sack paper is an ideal sustainable packaging solution where high pressure and high-speed filling are... 

L. Sivill, P. Ahtila, M. Taimisto, Thermodynamic simulation of dryer section heat recovery in paper machines, Appl. Therm. Eng. 25 (2005) 1273-1292. 

Drying of the paper web from about 35-50% dry content to about 90% dry content is performed in the dryer section of the paper machine where cylinders are heated with low pressure steam. For removal of moisture, supply air at 95°C is blown into the paper... 

Paper machine dryer section heat recovery system... 

In a paper machine heat recovery system (HRS) the exhaust air streams of the paper machine dryer section are utilised for heating different cold streams. Condensation of the air moisture causes strong non-linearities in the system, notably the heat flow rates per °C and the heat transfer coefficients in the exchangers. A mixed integer non-linear programming (MINLP) model has been developed that takes into account these non-linearities (Soderman et al., 1999). Additionally the heat transfer area prices can be given as concave price curves in the model and the climate of the mill location can be taken into account with a multiple period formulation. 

Different types of heat pumps can be applied to upgrade the secondary heat streams. The conventional compressor heat pump cycle (CHP) and mechanical vqiour lecompression (MVR) are widely utilised. With an absorption heat pump (AHP) or an absorption heat transformer (AHT) the compressor can be omitted, but the process becomes more complex. In fig. 2 an absorption heat transformer is applied for a paper machine dryer section. LiBr-H20 mixture can be used as working fluid. Around 50 % of the sum of the heat input in the evaporator and the desorber is obtained from the absorber at an elevated temperature. 

The paper machine dryer section is built with 30 % impingement drying and 70 % cylinder drying. It is assumed, that due to the impingement drying 6 kg/s of HR-steam becomes available for ORC. 

Process water heating in an ORC-plant condenser opens up new possibilities to utilise the heat from the paper machine dryer section. Heat can be used for example for reduced-pressure evaporation of wastewater to recover clean water. Water recovery evaporation plants are in operation in several mills in Finland and Sweden. 

The great advantage in favor of ASA is the high rate of cure in comparison with AKD size. The reaction with cellulose hydroxy groups takes place rapidly in the dryer section of the paper machine at less than 5 % web humidity. More than 90% of the attainable sizing potential is, in most cases, achieved before the size press unit. The reaction rate is influenced by the same parameters as the hydrolysis, that is primarily by pH, previous residence time, and stock temperature. 

End-of-pipe treatment for bled-out water is carried out in waste water treatment plants which are either owned by the mill or the pubhc (see Section 10.1). In a few cases mills have completely closed their water circuits, meaning there is no waste water produced at all and fresh water therefore is only fed at the same volume rate (approximately 1.5 1 (kg paper)" ) as it is removed by evaporation in the dryer section of the paper machine and with the rejects leaving the mill. 

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... 

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