| The press section of paper machine is mainly composed of roller, press felt and wet sheet, whose main function is removing the moisture of wet sheet through the mechanical extrusion of the press roller with the help of a press felt, which takes the water away. The surface structure of press felt and the Micro System of Felt-Sheet(MSFS) in press section are complicated, which directly affect the efficiency of dewatering and sheet quality in paper machine. They have become one of the key considerations in modern paper machine system. But there are less researches on the structure and dewatering mechanism of MSFS at home. The domestic researches on simulation experiment of pressing have not been reported. The researches on the structure and dewatering mechanism of MSFS are helpful to optimize dewatering process, improve the sheet dryness of the press section and, thus, reduce energy consumption.Based on the brief discussions on the basic function, characteristic and the current production technology of press felt both at home and abroad, this paper mainly discussed research methods, means and achievements of researches on the structure character of the press felt and the characteristics of MSFS in the press section, which closely related to the dewatering efficiency and the sheet quality. Furthermore, it put forward the press felt technology research trends and application process. According to the above theories, then, in this paper, we designed a set of experimental scheme and three experimental devices to study the structure of MSFS, the stress distribution on the felt surface and the influence of felt structure to the dewatering process in press section. Based on all the results from the experiments concerned, this paper put forward a ‘interaction layer’ assumption and optimization suggestions about the press felt. The main results of this paper are as following:(1) Based on the process and dewatering mechanism of pressing, in this paper we designed three kinds of experimental apparatus, which were used to study the structure of Felt-Sheet system, the stress distribution on the felt surface and the influence of felt structure to the dewatering process in press section.(2) Through the experimental phenomena, it was found that the compaction of paper model is non-uniform in the Z-direction, which caused the stratification. The layer next to the permeable surface(Felt surface model) was the most compacted and some parts of the paper model were penetrating into the ‘pores’ of the felt surface model. The stratification had something to do with the compacting forces, which led to a reduction of the permeability of the whole paper model.During the compressing process, with the same compression velocity, the forces required to compress the paper model with different contact surfaces were different. The required compress force for the three felt surface model(the fine, medium and coarse felt surface model)was practically the same. When the paper model was compressed using the wire press surface(a ideally flat permeable press surface), a peak force about 5.8 times higher than that for different felt surface models was required.At all compression velocities, the required force increased with increasing compression. Generally speaking, different compress velocity resulted in considerable differences in required force. But the relationship between the pressure and velocity did not conform to the changes of Darcy’s law. The increasing trend was not absolute, it only right when the compress velocity reached to a point, which the stratification became important to required force. And then the required pressure increased with the increase of the speed.(3) Felt surface models required roughly one fourth of the force required by the wire press surface with intermediate layers. Compared to the case with no intermediate layers, the force decreased when the wire press surface was used but increased when the felt surface were used. These suggested that the property of paper model(the type of pulp) affected the required force.(4) The stratification was related to the press surface structure and the properties of wet sheet, such as the in-plane stiffness and the whole permeability. The factors that affected the model experiments on wet pressing and the required force included the compress velocity, the permeability of press felt and the whole permeability of wet sheet during the pressing process. The total pressure will vary locally at the interface between the felt and sheet, due to the non-uniformity of the pressure application of felt. The total pressure will also vary in the thickness direction. These resulted the existence of less compressed, high permeable areas in the compressed medium, which considerably facilitate dewatering when there are large amount of water needing to remove. But when mechanical force was removed, these uncompressed areas will cause the separation re-wetting.(5) According to the experiment 2 and 3, it can be found that every felt samples are corresponding to an average pressure and the average pressure has some relationships with the press felt structure and the dryness of sheet after pressing. So a bold proposal can be proposed, the average pressure can be used as a measure index of press felt comprehensive performance.(6) The existence of the press felt makes pressure distribution in press nip, which will weaken dewatering efficiency in press section. According to the experiment, it can be obtained that in the surface batt fiber diameter is the first impact element of design factors, the base weave design is the second element and the number of batt layers is the least important parameter. At the same time, the paper grammage and the pulp kinds also have huge influences on MSFS dewatering efficiency. For low grammage paper, a uniform, fine press felt with finer batt fiber, multilayer base weave and multilayer batt fiber is conducive to the dehydration process; For high grammage paper, a nonuniform, coarse press felt with coarser batt fiber, double or single base weave and less layers of batt fibers is advantageous to the dehydration process. It can be also found that the sensitivity to the press felt design for the chemical pulp and mechanical pulp is different. The mechanical pulp is more sensitive to the felt structure. For the mechanical pulp, below approximately 70g/m2 it was found that the felt with surface batt fiber diameter 11μm gave the highest dryness and that with 44μm gave the lowest dryness. At approximately 70g/m2, there was no difference in dewatering for the different felts. Thus the 70g/m2 can be defined as the ‘transition grammage’ for mechanical pulp. |
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