Views: 2 Author: Site Editor Publish Time: 2021-12-09 Origin: Site
The forming process of the paper web of the paper machine is the process in which the fiber suspension is dehydrated and deposited on the forming wire surface. It is mainly a hydrodynamic process; chemical forces and colloidal chemical forces also have a certain impact, especially in the paper material. The influence of fine substances is more significant. The hydrodynamic process mainly includes three processes: water discharge, directional shear and turbulence.
Water discharge process of paper machine:
Discharge is the flow of water through the forming wire. The direction of the water flow is basically but not completely perpendicular to the plane of the wire, and its characteristic is that the flow rate changes with time. The main function of the drainage is to dehydrate the fiber suspension so that the fibers in the pulp are deposited on the forming wire to become a layer.
Water discharge is carried out by two mechanisms: filtration and concentration. Filtration occurs when the fibers in the suspension are mobile or can move freely without interference with each other. When the water is discharged under the filtering mechanism, there is a clear boundary between the deposited fiber layer and the thin suspension close to the layer. The concentration of the suspended solid on the layer is basically a constant value.
1. The drainage of the filtration mechanism has two effects on paper forming: uniform distribution and layer-by-layer deposition.
Because the drain flow always has the largest flow in the path with the least resistance, more mobile fibers are brought to this place with the least resistance, that is, where the fiber layer is thinner, which makes the layer more uniform. The trend of growth, this is the effect of uniformity.
Since the drainage of the filtration mechanism deposits the fibers in a monomer state layer by layer on the forming mesh, the interweaving and interweaving of fibers in the layered structure is relatively small, and the layering is more obvious. This is the layer-by-layer deposition.
2. When the fibers in the suspension become an intertwined network state and the fibers are not easy to move, concentration occurs. At this time, there is no clear boundary between the layer and the suspension, and the concentration of the suspension increases closer to the layer. The fiber network in the suspension is the same as the laminate, which is gradually compressed as the concentration process. The main mechanism of suspended solids is filtration. However, the two drainage mechanisms of filtration and concentration actually exist at the same time, and the drainage is only local and temporary according to the concentration mechanism.
In the drain water, the fibers are mechanically blocked by the forming mesh and accumulate on the mesh, which creates a layer, and the layer grows and is compressed as the drain progresses. The friction between the fibers in the laminate and between the fibers and the forming wire gradually increases during the draining process, so that the fiber structure of the laminate is stabilized and can resist the flow in the suspension above the laminate. Disturbance and recoil from the water below the forming wire. The growth and compaction of the build-up increases the accumulation of fibers and fine substances.
Directional shearing process of paper machine:
Directional shear is the shear flow in the fiber suspension that has not yet drained. It is characterized by a clearly visible gradient of flow direction and average flow velocity. Directional shear has a dispersing effect, a directional effect and a concentration effect.
1. The dispersing effect of directional shear is that due to the existence of the flow velocity gradient, the fiber network in the suspension is deformed under the action of shearing force, so that it is finally dispersed.
2. The directional effect of directional shear is that the directional shear exhibits obvious flow directionality, so that most of the fibers deposited in the layer are arranged along the advantageous shear direction.
3. When the non-uniform strong shear flow pattern is included in the discharge range, the fiber shows a concentration trend corresponding to the shear field mode, which is the concentration effect of directional shear. This effect is undesirable in forming, so high-intensity small-scale turbulence should be used to suppress it.
Turbulent process of paper machine:
In forming, turbulence is theoretically the non-directional fluctuation of the flow velocity in the free suspension without water discharge. But in fact, the turbulent flow pattern in the free suspension is not truly non-directional, but it is not enough to produce significant directional shear. Its effect on the structure of the paper is very similar to the effect of the real non-directional fluctuation of the flow velocity. approximate. This is a pseudo-turbulent flow pattern. It is also a natural result of flow relaxation with high flow directionality.
The main function of turbulence in the forming process is to disperse the fiber network, to a limited extent, to make the fibers move easily in the suspension to reduce the degree of flocculation, and as a means to attenuate the directional shear.
Both directional shearing and turbulence are shearing processes, and the difference lies in the difference in the degree of directionality within a given scale (a scale that is large enough compared to the length of the monomer fiber). In any shaper, the above three hydrodynamic processes all occur and exist simultaneously, and they are not evenly distributed in time and space, nor are they completely independent of each other.
The above is the mechanism of paper web forming on a paper machine.