Paper Machine Dewatering and Paper Making Machine handling Performance

Both wet-end dewatering and Paper Machine handling performance are very important to improve paper machine operating conditions. The strength of the paper web increases greatly with the dryness. In the wet end, the dryness is low and the strength is poor, which makes it difficult to run. Improving the wetness of the paper web fundamentally guarantees the running performance of the paper web.

Wet end dehydration mechanism

A constant function of the forming, pressing and drying sections of the paper machine is dehydration. In the wet section, the mechanical method is used for dehydration, and the drying section is dewatered by evaporation. The relative cost of taking out 1 ton of paper is: forming section: pressing section: drying section = 10: 12: 78; therefore, paper workers must strive to Improve the dehydration efficiency of the wet section, especially the efficiency of the press section to reduce the evaporation load of the drying section, and the lateral dehydration should be uniform.

The dehydration of the paper sheet is the process of curing. The network is formed in the wire section, and only the vacuum effect is used to make the dryness of the paper sheet of the roll roll reach 18-23%. In this moisture range, most of the free water has been removed. For further dehydration, the external dehydration should be completed by pressing to increase the internal water pressure of the compressed paper structure. During the pressing process, the void volume of the paper sheet is reduced after squeezing out the moisture in the paper web, and a certain degree of fiber compression occurs where the fibers are in contact with the fiber. The applied pressure helps to evenly distribute the moisture throughout the paper sheet.

Press dehydration is divided into four stages: the pressure of the press is the sum of the water pressure and the mechanical pressure, and the wet press nip is divided into four stages according to the interaction of the water pressure and the mechanical pressure.

In the first stage, the paper sheets and felt are discharged until the paper reaches saturated moisture and there is no residual air. At this stage, the water pressure in the paper sheet is not large, and there is no change in dryness.

In the second stage, the paper sheet is saturated with water, and the water pressure in the paper sheet increases, so that the water is transferred from the paper sheet to the felt. When the felt is also saturated, the water is discharged from the felt. In the second stage, the water is discharged from the middle of the nip. The pressure is highest.

In the third stage, the nip seam begins to expand and further dewatering until the water pressure in the paper sheet drops to zero, at which time the paper sheet is dry.

In the fourth stage, the paper sheet and felt begin to expand, and the paper sheet becomes unsaturated. At this time, due to the capillary action, the internal vacuum of the paper sheet, or the mechanism of void moisture regain, some water returns to the paper sheet. The exit sheet should be separated from the felt as soon as possible to reduce the effect of moisture regain.

Factors affecting press dehydration:

① The pressure exerted on the unit area;

② Duration of pressure application;

③ Number of pressing devices;

④Structure type of pressing roller;

⑤ The characteristics and condition of the press felt;

⑥Viscosity (or temperature) of water;

⑦Characteristics of the paper sheet, including fine fiber content, dosage, and air permeability of wet paper sheet.

The main factors that promote dewatering are the water pressure difference through the paper sheet, the residence time of the nip and the uniformity of the pressure. The product of the nip pressure and the residence time is called the press impulse; the main resistance is the flow resistance of the paper itself. When the water temperature is increased by 1 degree, the flow performance is increased by about 2.5%. During the pressing process, the temperature of the paper sheet is increased from 20 degrees to 30 degrees, and the dryness of the pressed paper sheet is increased by 2-3%.

Press dehydration is divided into pressure control type and flow control type. The pressure control type is mainly on low-quantity and high-freedom paper sheets. Because the paper is very thin, the structure of the paper sheet does not obviously restrict the flow of water. The possibility of rewetting from the blanket to the paper sheet after the midpoint is determined. The flow control type is mainly on high-quantity paper sheets, which are quite thick, and the paper sheet structure restricts the flow of water from the paper sheets. Paper density, temperature and blanket void ratio have a significant effect on water resistance, and the residence time in the nip becomes the limiting factor. Separate pressure-controlled dewatering and flow-controlled dewatering indicate the direction of efforts to improve the efficiency of pressing.

Crushing is the direct result of the lateral movement of the water flow in the sheet to form the water pressure. The slow pressure method can be used to avoid the occurrence, and the structure of the press roll and the selection of the felt can be improved to accelerate the dehydration speed, thereby avoiding the occurrence of crushing. Press dehydration also has a critical point problem. It is not economical to dehydrate by pressing beyond the critical point, or the paper sheet will be excessively compressed, and the water remaining in the paper sheet should be removed by evaporation. Depending on the type of slurry, pressing form and operating conditions, the critical point is 50-57%.

Paper machine performance

The papermaking performance of the paper machine refers to the convenient operation and stable performance of the paper machine, which is mainly reflected in the speed and stability of the transfer of the paper sheet, with the main purpose of reducing paper breaks. The transmission of the paper web requires the action of force, speed difference traction, vacuum suction and backing, etc.

The technologies used to improve the running performance are: vacuum transfer paper guide, paper transfer vacuum box, paper stabilization bellows, paper transfer bellows, single-hanging dry net, vacuum cylinder, bag area ventilation, automatic tension control, curved roller.

Leading at the roll: The variables of the paper machine that affect the peeling of the wet paper sheet from the roll are the peeling angle, the speed of the paper machine, the relative adhesion of the formed sheet and the forming wire. The minimum tension peeling angle is usually between 60 and 120 degrees;

The adhesion to the forming wire depends on the initial forming conditions, such as on-line concentration, the degree of pressure forming, and the cleanliness of the wire. There are two types of open paper and closed paper at the roll roll. Closed paper is divided into three types: ordinary roll paper, vacuum roll paper and vacuum paper press. The limitation of the strength of the wet paper web makes the open-type drawer the operating speed lower. When the speed exceeds 7.6m / s, the tension of the speed on the sheet quickly becomes the dominant factor. When the speed reaches 10.2m / s, the open-type drawer 60% of the wet paper web tension is caused by the speed of the car.

In the ordinary roller closed paper guide, the paper sheets and felt in the paper guide part must be quite wet to be passed through, which is easy to cause embossing, so it is only suitable for low-weight paper grades that are slightly pressed. Vacuum threading uses a vacuum roller on the inside of the threading felt to apply the suction force required to remove the wet paper sheet from the web, while further dewatering the paper by the pressure applied by the threading roller. Because the speed of the net is the same as the speed of the felt, the paper is transferred without changing the traction force.

Machining performance of the dryer section: The instability of the paper sheets in the dryer section is due to the flow of air. In addition to the ventilation, the flow of the air is also related to the moving surfaces such as the drying net, drying cylinder and guide rollers. These surfaces carry air Disturb the paper sheet, and the problem is further exacerbated as the speed of the vehicle is increased. In order to stabilize the paper sheet, a larger speed difference is used to tighten the paper sheet, which increases the frequency of paper breaks and also loses a lot of paper sheet performance. For this reason, non-traditional arrangements are used in high-speed paper machines.

From the press section to the dryer cylinder, the paper sheet transfer blower box: the first group of turning rolls of the dry net is as close as possible to the last press roll. The paper sheet reaches the bottom of the dry net only after a short distance of no support section, and the special blower The box is arranged on the bottom surface of the dry net, and the nozzle emits a pressure jet to control the air in the interface layer carried by the dry net to form a negative pressure suction force, which adsorbs the paper web against the dry net and enters the drying section.

Drying section stable paper blower box: the paper coming from the upper cylinder tends to follow the dryer, causing air pockets to form in the paper sheet and causing wrinkles; the dry web carries a large amount of air in the interface to form a positive pressure in the entrance area of the lower dryer, and The quilt was forced into the collection area of the mesh cylinder, a lot of air was pressed into the dry mesh, and was trapped between the dry mesh and the paper sheet along the lower dryer cylinder, so that the paper sheet floated away from the dry mesh and lost stability; when the paper sheet entered the upper cylinder When the trapped air is forced out, it may also cause wrinkles. Use a blower box to blow against the running direction of the drying net to remove the interface gas. The negative pressure generated by the blowing blows the paper tightly on the drying net and no longer traps air. There are two types of blow boxes, one is a unilateral blow box and the other is a bilateral blow box. The unilateral blow box blows out of the upper cylinder banner, the lower dryer cylinder banner and the sides of the mesh, forming a unilateral vacuum zone on the upper and lower dryer cylinders. The double blow box does not blow on the bake.

specific measure

Specific measures to ensure the operating performance of the paper machine during production include:

Vacuum transfer paper, adjust the width of the suction area that is suitable for the width of the comrades of the paper, to avoid banding and dropping;

The paper transfer vacuum box should make the whole width evenly remove the moisture from the press felt, and attract the paper web to the down felt, to achieve stable separation and then lead to the next process, pay attention to the necessary vacuum degree and can not be partially blocked during use;

To stabilize the paper vacuum box, it is necessary to adjust the air volume to ensure that a suitable negative pressure is generated to stabilize the paper web and avoid the paper web from being folded and drifting;

The paper transfer bellows should adjust the air volume to ensure that a suitable negative pressure is generated to stably transfer the paper web;

For the single-hanging dry net, it is necessary to scrape off the glue to maintain proper air permeability so that it can be combined with the vacuum cylinder and the paper stabilization bellows to realize the drying of the wet paper web of the first entry and no open operation support;

Vacuum cylinder: pay attention to the dredging of the holes and the adjustment of the vacuum exhaust valve to allow the paper web to run stably against the dry network;

For ventilation in the bag area, the ventilation and air supply volume should be adjusted to balance the air pressure in the bag area of the dryer cylinder to avoid excessive jitter and paper breakage caused by the unbalanced and unstable air pressure on both sides of the paper web;

For automatic tension control, it is necessary to set appropriate setting values according to the quantitative and position, and automatically adjust according to the setting values to ensure the smooth running of the paper web;

For curved rollers, the angle of the curved rollers should be adjusted according to the transverse tension of the paper web to stretch the paper web and maintain uniform tension across the web to avoid creases.