Restricted by many factors such as paper machine conditions, paper varieties, and blanket performance, the applicability of the blanket to the paper machine is limited, but the requirements of the paper machine for the blanket are very strict. To obtain an ideal blanket, one must understand the characteristics of the paper-making blanket, and choose carefully and reasonably according to the structure of the paper machine, the production conditions, and the characteristics of the paper.
1. Selection and use of press felt
1. Select the content included in the press felt
1. Selection of variety, weight and size of press felt
The selection of varieties refers to the selection of blankets suitable for the machine and different parts among many different varieties of papermaking blankets.
Selection of gram weight After the selected felt type is determined, the square meter gram weight of the felt must be reasonably selected according to many factors of the paper machine.
Size selection The size selection includes the length and width of the papermaking felt.
Determining the length refers to the actual length of the papermaking felt that can meet the normal operation under tension on the papermaking machine, and the elongation change state of the selected felt should be fully considered.
The width is defined as the actual width of the papermaking felt under the tension of the paper machine during normal operation, and the change of the shrinkage rate of the selected felt should also be fully considered.
2. Selection of air permeability and density of press felt
Choose the air permeability. The accurate selection of the carpet air permeability suitable for this machine is the key to determine the normal and efficient operation of the paper machine and the life of the carpet. At the same time, the change curve of the air permeability of the selected felt during use should be fully considered.
The choice of density is an important indicator to measure the air permeability of the blanket on the side.
3. Special requirements when selecting the felt
Develop the special properties of papermaking blankets to meet the special requirements of special papermaking machines and special paper grades on blankets.
Second, the technical considerations for selecting the press felt
For the selection of felt in a certain position on a paper machine, the experience of the felt manufacturer and the papermaker should be based on the experience of the process test and production practice. For a new paper machine, the simulation test of the press felt is performed at the same position as the paper machine. After the press felt is in operation, it must be inspected and analyzed, and the original design modified if necessary. Papermakers are always interested in long press felt life and low cost per ton of paper production. Blanket manufacturers are willing to meet these requirements, although they do not necessarily emphasize the trend of reducing costs in many ways. More effective wet pressing can save steam, increase vehicle speed and reduce head breaks.
The choice of press felt depends on the variety, basis weight and surface modification requirements of the paper produced; the type of paper machine; the environmental conditions in which the press felt is used; and the existing blanket cleaning and purification equipment. When designing and selecting felts, we must pay special attention to the relevant production technical parameters of the paper machine. Only by properly adjusting the relevant process and technical parameters of the felt according to the actual use conditions of the paper machine can the paper felt be more effective in satisfying the paper machine's Actual demand. .
Now it is generally believed that paper blankets are customized. Each blanket is designed and manufactured according to the technical requirements of a certain paper machine and part, according to the speed, paper type, specifications, etc. Therefore, in the design and selection of press blankets At this time, the following factors must be paid attention to.
1. Selection according to the technical conditions of paper machine production
When choosing the press felt, we must pay special attention to the relevant technical parameters of the paper machine. Only by appropriately adjusting the related process and technical parameters of the felt according to the actual use conditions of the paper machine can the paper felt meet the actual needs of the paper machine.
1) The requirements of line pressure and mechanical load on the strength of felt
The linear pressure and mechanical load in the press area of the paper machine are directly proportional to the strength requirements of the felt. The greater the line pressure, the higher the pressure resistance of the selected papermaking felt in the life cycle. At the same time, the papermaking felt must have good elastic recovery performance, so that the felt can maintain good water filtration performance and water holding capacity for a long time. space. The greater the mechanical load, the greater the tensile (breaking) strength of the papermaking felt when transmitting power, and the smaller the elongation and shrinkage.
Figure 1 is a graph showing the linear pressure that paper felts with different bottom mesh layers can withstand.
2) Vacuum state
The degree of vacuum in the paper machine system and the setting state of the vacuum system directly determine the water content, cleanliness and dewatering performance of the papermaking felt, especially the bottom wire papermaking felt. If the vacuum is too small, it will cause poor dehydration of wet paper sheets, large moisture content of the blanket, the pressure of the nip cannot be increased, difficulty in leading the paper, and the cleaning of the blanket is not clean. Excessive vacuum will cause problems such as increased load on the paper machine, increased wear on the carpet, and wasted energy. Relevant knowledge about the setting of the vacuum system will be introduced in the following pages.
3) Washing conditions
The washing conditions include the configuration of the washing device, the requirements of the washing water quality, the setting of the washing water pressure and the degree of vacuum, etc. The washing conditions are directly proportional to the washing effect. When choosing a paper-making blanket, be sure to pay attention to the washing conditions of the paper machine. Under normal circumstances, we require that each paper machine should have good washing conditions to meet the requirements of nets and carpets, but the actual situation is that the conditions of each paper machine are different, and the conditions of each paper machine at different times are also Not the same. Therefore, when selecting papermaking blankets, it is entirely necessary to adjust the varieties, quantitative adjustments, structural proportions, production process adjustments and stereotypes adjustments of papermaking blankets within a certain range to meet the needs of papermaking machines. Generally speaking, low-pressure flushing water less than 0.3 MPa can only use low-quantity and thin blankets, and 0.3-0.5 MPa low-pressure flushing water can use ordinary blankets; those with both low-pressure flushing and high-pressure flushing (0.6-3.0 MPa) Can choose high-quantitative blanket and bottom net blanket. See the following chapters for the washing technology of paper felt.
4) Slurry situation
The quality of the pulp not only affects the quality of the paper to be copied, but also plays a vital role in the water drainage state and service life of the felt. The degree of pulp disintegration, the viscosity of the filler and the content of impurities are inversely proportional to the drainage performance of the blanket. In addition, the chemical fiber blanket is resistant to weak alkalis and acids. The level of the PH value of the pulp also has a certain effect on the blanket. When choosing the matching papermaking blanket, we must pay attention to the parameters of the slurry ratio, the degree of pulp dissolution and wet weight, the use of chemical products, the pH of the slurry and other parameters, which will directly affect the water filtration performance of the papermaking blanket And the service life also affects the surface viscous state of the paper felt.
5) Dehydration method
The press type of the paper machine determines the dewatering method of the wet paper sheet. This item has been detailed in the previous chapter. The different press types are suitable for different papermaking blankets. Paper felts with suitable performance must be selected according to parameters such as the linear pressure of the press, the degree of vacuum, the matching state of the felt, the setting state of the vacuum system, and the washing state. The above section on the performance of papermaking felts is also introduced. In general, the paper machine mainly based on horizontal dewatering should use a felt with greater compressibility and a smaller basis weight; the paper machine used for vertical dewatering with squeezing dewatering and vacuum suction is generally selected to have a lower compressibility, better elasticity and a fixed basis weight. Larger blankets. The selection of the bottom layer of the press felt is also determined according to the parameters of the press section of the above paper machine. Under better conditions, you can choose a blanket with more layers to extend the service life of the paper felt and increase the speed of the paper machine. And paper quality.
2. Choose according to the past use of papermaking blankets
For the normal operation of the paper machine, when re-selecting the paper-making blanket, it must be improved by referring to the previous situation when using the paper-making blanket, so that the special requirements of the blanket's variety, weight, air permeability, thickness and other aspects can be made correctly Selection to make the re-selected paper-making blanket more suitable for the needs of the paper machine, and to prevent the production abnormal factors caused by the inappropriate selection of the blanket.
3. Communicate closely with the technical staff of the paper blanket supplier
With the continuous improvement of papermaking technology and papermaking machinery and equipment, the performance requirements of the papermaking blankets matched with it are also getting higher and higher. Therefore, the performance of papermaking blankets is more refined and the varieties are more and more. At present, there are dozens of varieties of BOM paper-making blankets alone, and there are many factors that need to be adjusted in the process of producing paper-making blankets. These undoubtedly bring a lot to the engineering and technical personnel of the paper mill when choosing paper-making blankets trouble. Frequent technical exchanges with blanket supplier technicians are a good way to really make paper blankets designed by the paper machine.
Second, a few issues that should be paid attention to when choosing a paper-making blanket
According to the equipment of the paper machine, we should mainly consider the following aspects when selecting and designing the paper felt.
1. Selection of blanket varieties
As the functions of papermaking blankets are refined, their varieties will gradually become more diverse. There are about dozens of varieties of existing BOM paper felts and other ordinary press felts. Therefore, when we choose the felt, we should choose different press types, different parts, different paper types, and different washing conditions according to the paper machine. And other factors, to choose different varieties of blankets. It is best to choose the blanket based on the recommendation of the blanket manufacturer's engineer.
2. The choice of blanket weight
The weight of the felt should also be selected according to factors such as the press type of the paper machine, the position of the felt, the paper grade produced, the washing conditions, and the degree of vacuum. If the weight is too large, it may cause problems such as large moisture content of the blanket, the speed of the paper machine is not up, and the blanket is easy to be dirty; if the weight is too small, it may cause the blanket to have too much air permeability, not wear-resistant, rough surface, bottom net Traces and other shortcomings. Paper machines with better pulp conditions, pressing and washing conditions can choose paper blankets with a slightly larger square gram weight to improve the durability of the paper blankets.
3. Choice of blanket size
Papermaking blanket is a kind of fabric, the size of which is made, the size installed on the paper machine and the size after use are slightly different, so it should be decent in determining the order size. The determined order size should be the size under tension.
(1) Determination of length
The length of the felt used is proportional to the strength requirement of the felt. The longer the felt, the greater the strength. When choosing the length of papermaking blankets, when using BOB papermaking blankets, it must be considered that the elongation is relatively large, generally in the range of 1 to 1.5%; when choosing BOM papermaking blankets, especially when choosing more than double BOM , There is no need to worry too much about its elongation and shrinkage. The elongation of BOM paper felt is generally not more than 1%, and the width change will not exceed ± 20mm under normal circumstances. Generally, when measuring the length of the papermaking felt on the papermaking machine, the tensioner is placed at 2/5 of its tensioning range, and the actual size of the loop running along the felt using the non-stretchable wire is taken. The order length of the blanket is required.
(2) Determination of width
Subject to the increase of -5 ~ + 5 cm in width and width of the press roll. If the papermaking blanket you order is too wide, the edges of the blanket are likely to be worn away. After the edges of the blanket are worn away, the exposed bottom net (base fabric) thread is easily thrown into the wet paper sheet, causing the paper sheet Broken heads are frequent; in addition, it is also easy to tear the blanket. If the papermaking blanket is too narrow, it will cause the blanket to be unstable during operation, and it is easy to run out of air. After the standard line is slightly skewed, the blanket is easily narrowed, so that it cannot carry the entire paper web. Generally speaking, the order width of BOB papermaking felt is slightly wider, basically the width of the press roll surface,
Table 1 Requirements for strength and dimensional change of different papermaking felts
Or add 3 ~ 5cm width on the basis of the width of the press roll, so that when the felt is tensioned on the machine, its width is basically the same as the width of the press roll. For BOM paper felts, the order width is generally the same as the width of the press roll surface, or slightly narrower than the width of the press roll surface of 3 to 5 cm, because the lateral shrinkage of the BOM blanket is very small.
Table 1 lists the different varieties of papermaking blankets, their dimensional change requirements and breaking strength requirements.
4. The choice of air permeability
The choice of air permeability should be determined according to the comprehensive technical parameters of the paper machine, which is a relatively empirical value. In actual production, the air permeability of the new blanket, the air permeability when the blanket is in the best condition, and the air permeability when the blanket fails when off the machine are different, and the air permeability will become worse and worse. When we choose the air permeability of the paper-making blanket when ordering, it should be slightly larger than the air permeability of the blanket in the best condition. If the air permeability is too large, the vacuum of the vacuum system will not go up, the moisture content of the blanket is too large, and the moisture of the paper sheet Problems such as not being able to take it out, blanket penetration, etc .; if the selected paper-making blanket has too little air permeability, it will cause the blanket to jump at the vacuum suction box, the vacuum system is overloaded, the blanket wear is increased, and the paper sheet is difficult to dehydrate.
Figure 2 shows the change of air permeability when 1200g? M-2 double-layer press felt is used on a paper machine with a speed of 400m? Min-1, for reference.
5. The choice of density
The density of the blanket is usually measured by the thickness when it is selected, which is more intuitive. The size of density is inversely proportional to air permeability, and its selection principle is similar to air permeability.
Figure 3 lists the thickness change of 1200 g? M-2 double-layer press felt during use on a 400 m? Min-1 paper machine.