Dryer fabrics have high air permeability, making them susceptible to flutter during the paper drying process due to air carried along by the fabric. This phenomenon is significantly impacted by the fabric’s placement. Currently, there are two types of fabric placement for the drying cylinders in the paper machine’s dryer section: 1. Single-wire (single-wire drying) and 2. Double-wire (double-wire drying). Double-wire drying generally presents no problems for slow-speed paper machines, but high-speed paper machines are prone to flutter, leading to sheet folding. This is caused by air carried along the dryer fabrics surface, particularly in the first and second sets of dryer cylinder in the paper machine’s dryer section. This can lead to unstable paper flow and become a key factor influencing paper machine productivity.
An analysis of the production performance of two high-speed newsprint machines at a client revealed different frequencies of web edge flutter and end breaks. This was due to differences in the length of open web in the dryer section of each machine, and the root cause was air flow. So, how can air flow be reduced? This requires an analysis of the dryer fabrics layout.
About Double Wire Drying for Dryer Fabrics
Double fabric dryer section: The dryer section use double row alternate arrangement of dryer cylinder, the upper dryer and bottom dryer are same. Every groups of dryer have top and bottom two group dryer fabric or dryer felt. The advantages of this type arrangement is compact structure, short paper machine length. But it is no attachment between the dryer cylinders, and the paper is prone to breakage, making it difficult to handle after breakage.
Single Wire Drying for Dryer Fabrics
Single fabric dryer section : It generally refers to the use of a single row arrangement of dryer cylinder, partially or entirely in the dryer section of high speed paper machine. The top is dryer cylinder , the bottom is vacuum cylinder, every group dryer only have one group dryer fabric. The advantage of single hanging is that within a group of dryer cylinders, the paper is constantly attached to the dry fabric during operation, making it less prone to paper breakage. After the paper breaks, it falls directly to the ground and will not damage the dry net between the dryer fabric and the dryer cylinder like in a double row cylinder.
The Single Wire Drying of the Dryer Fabrics is Not Easy to Shake or Break
Because the wet paper is held against the dryer fabric between the upper and lower dryer cylinders, single-wire drying virtually eliminates paper vibration, naturally reducing sheet breaks at the dryer cylinders. Furthermore, single-wire drying eliminates the need for the lower roller and wire guide required in double-wire drying, and eliminates the need for scrapers on the lower dryer cylinders. Drying with dryer fabrics can increase the wrap angle of the drying cylinder by about 37%, thus increasing the time the dryer fabrics presses the paper on the drying cylinder for drying and improving the heat conduction efficiency of the drying cylinder. It has obvious benefits in improving the uniformity of paper moisture, increasing paper roll width, increasing paper machine speed, ensuring the straight running of the dryer fabrics, reducing fabric consumption, and improving paper quality.
Selection of Dryer Fabrics
The key to single-wire drying lies in dryer fabrics selection. To achieve maximum performance, factors such as air permeability, thickness, surface properties, adhesion, and stretchability must be considered. Air permeability is particularly important for paper machine drying. It mainly depends on the papermaking conditions (machine speed, paper type, basis weight, beating degree, etc.). If the air permeability is too high or too low, the paper sheets will bulge at the inlet and outlet of the upper single-wire drying cylinder, and the higher the machine speed, the greater the impact. The thickness of the dryer fabrics should not be too thick in the first few groups of single-wire drying, because the position of the paper sheets in the upper and lower dryer cylinders is special. Thick dryer fabrics carrying paper through the upper dryer cylinder will affect the load on the upper and lower devices and the change in the paper web’s attraction. The dryer fabric selected for the single-wire drying should have appropriate adhesion on the side that contacts the paper sheet so that the wet paper will not peel off during operation.
The dryer fabrics for single-wire drying does not have great ductility, so the elongation should be controlled below 0.5%. The width ratio will shrink by about 1cm/m in both directions after being put on the machine, and the length ratio will increase by about 1cm/m compared with the general dryer fabrics to facilitate paper threading and keep the holes from deformation, so that there is reasonable room for adjustment after being put on the machine.
Like ordinary single-wire, single-wire drying can be used in all fourdrinier multi-cylinder paper machines. It is mostly installed at the wet end and the first group after sizing. The number of cylinders used is determined by the drying capacity and characteristics of the paper machine. Since single-wire drying will slightly reduce the drying efficiency, it is generally used on paper machines with relatively abundant drying capacity.
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