The flow box is known as the "heart" of the paper machine, a key component that connects the "flow delivery" and "forming" parts, and is one of the fastest developing components on the paper machine. In papermaking, the function of the flow box is to deliver the qualified pulp to the forming wire according to the requirements of the forming part of the paper machine, providing the necessary prerequisites for the good formation of the paper sheet.

　　The role and performance of the flow box

　　As the junction of the "flow delivery" and "forming" of the paper machine, the basic task of the flow box is to provide good prerequisites for the formation of the paper sheet, that is, to evenly distribute the paper material along the width of the paper machine, ensuring uniform pressure, speed, flow rate, concentration, and controllability and uniformity of fiber orientation; effectively dispersing the pulp fibers, preventing fiber agglomeration, and providing and maintaining a stable headbox pressure and wire speed ratio according to process requirements.

　　In the long-term development of pulping and papermaking technology, although various structural forms of flow boxes have emerged, their main functions have remained basically unchanged, namely pulp distribution, homogenization, and spraying.

　　The basic functions of the flow box can be specifically described in five aspects:

　　(1) Provide a uniform and stable jet of pulp across the entire width of the paper machine, avoiding lateral flow, non-directional side streams, or longitudinal streaks;

　　(2) Provide a stable lip plate with geometric dimensions that meet requirements, unaffected by temperature, pressure, and lip opening degree;

　　(3) Form a fiber suspension with minimal flocculation and good dispersion;

　　(4) Provide control over the cross-sectional quantitative distribution, drop point, spray angle, and spray speed that meet process requirements;

　　(5) Provide convenient measures to keep the flow box clean and easy to operate and maintain.

　　A good flow box must have the following performance:

　　(1) It can produce a quantitatively uniform and stable paper sheet without longitudinal streaks on the sheet;

　　(2) The lip area of the flow box should not be affected by pressure, temperature, and lip opening degree to ensure uniform cross-sectional quantitative distribution;

　　(3) The flow box should avoid the occurrence of pulp hanging and attachment;

　　(4) It should be easy to keep the interior of the box clean;

　　(5) The wire speed ratio should be controllable.

　　Therefore, the flow box must achieve:

　　(1) It can evenly distribute the pulp towards the paper machine direction;

　　(2) The ejected pulp should have a uniform velocity profile, concentration profile, and turbulence profile.

　　Structure and function of the flow box

　　The function of the flow box, in summary, is to spray a uniform and consistent pulp flow required by the paper machine's papermaking process at a certain angle along the width of the paper machine, with a certain pressure, speed, and flow rate, while ensuring that the fibers inside are as evenly dispersed as possible, and providing fine adjustments of quantity and moisture in the width direction of the paper machine to ensure the desired quality of the paper sheet.

　　In fact, the function of any form of flow box is specifically undertaken and coordinated by three main parts: the pulp distributor, the rectifying section, and the paper material webbing device. The functions of these three main parts are as follows:

　　Pulp distributor:

　　According to the principles of fluid dynamics, it uses a regular variable cross-section to evenly distribute the incoming paper material flow along the width of the paper machine at a certain pressure, speed, and flow rate.

　　Rectifying section:

　　It uses rectifying elements and turbulence generators to produce turbulence of appropriate scale and intensity, effectively dispersing fibers, preventing fiber agglomeration, and maintaining the non-directional arrangement of fibers as much as possible, so that the fibers in the incoming paper material flow are in a uniformly dispersed state.

　　Paper material webbing device:

　　It sprays the paper material evenly at a certain angle and spray speed to the predetermined webbing points on the wire, and provides fine adjustments of quantity and moisture in the width direction of the paper machine, as well as control and adjustment of the turbulence and agglomeration scale of the pulp flow, to ensure the desired quality of the paper sheet.

　　Development status and trends of the flow box

　　The earliest flow box used was the open flow box with a flip gate, which emerged before the Fourdrinier brothers invented the long wire paper machine in 1804. Its structure and form are relatively simple, with low technical requirements, suitable only for low-speed paper machines, and it has poor homogenization effects.

　　With the increase in the speed of paper machines, the required static pressure head for the formation of paper material in the weir pool must increase in relation to the square of the speed. Therefore, when the speed of the paper machine increases, the required static pressure head must also increase. At higher speeds, if an open flow box is used, it becomes large and relatively complex in terms of volume, height, structure, and weight. Moreover, it is impossible to efficiently distribute, rectify, and disperse fibers in such a large volume of paper material in the weir pool to provide a high-quality incoming spray of paper material. To solve this problem, closed (air cushion) flow boxes were developed in the late 1940s.

　　After the 1970s, the air cushion flow boxes developed absorbed some advantages of flow box design and adopted new technologies such as conical pulp distribution pipes, hydraulic pulp rectifying elements, eliminating foam overflow, and optimizing the structure of the upper and lower lip plates, resulting in significant performance improvements. Therefore, they are still widely used in medium-speed long wire paper machines and sandwich paper machines.

　　Since the 1970s, in order to meet the needs of the development of new multi-layer forming technology, multi-layer hydraulic flow boxes have been developed. Multi-layer hydraulic flow boxes are full-flow hydraulic flow boxes with multiple channels. Currently, multi-layer hydraulic flow boxes can not only be used for the production of cardboard but also for the production of thin paper and high-grade paper, playing a good role in improving paper quality and rationally using fiber raw materials. Therefore, multi-layer hydraulic flow boxes have a good development prospect. Various structures of multi-layer hydraulic flow boxes have been researched and developed.

　　Since the 1990s, the research and development of dilution water concentration control technology for the flow box has been a significant achievement and progress in the common technology of flow boxes. The flow box uses dilution water concentration control technology to address issues such as bypass and cross flow caused by weir plate adjustments. Several systems have been developed, and many newly designed flow boxes adopt this technology.

　　From the development history of the flow box mentioned above, it can be seen that the main goal and driving force of flow box development is to enable the flow box to adapt to the increasing speed of paper machines and produce higher quality products. Each type of flow box has an optimal operating speed range and produces the best variety and quality of products.

　　Looking at modern new high-speed paper machine flow boxes, they generally have the following main characteristics:

　　(1) Use of efficient hydraulic pulp distribution rectifying elements. By designing different structures and sizes of hydraulic pulp distribution rectifying elements, the flow conditions can be adjusted and controlled to meet different process requirements.

　　(2) Emphasis on the role of the stabilizing chamber. The stabilizing chamber can eliminate and attenuate energy fluctuations, stabilizing the pulp flow. For full-flow air cushion combined flow boxes, the stabilizing chamber is also the place where the air cushion is generated and bubbles are eliminated.

　　(3) Use of narrow flow channels to generate controllable fine turbulence. In the outlet section of the flow box, narrow flow channels can control the turbulence level of the outlet pulp flow to produce fine-scale turbulence, which is particularly evident in the floating sheet flow box.

　　(4) Development towards primary pulp distribution rectification. Most new flow boxes are secondary pulp distribution rectification, but there are also attempts to develop primary pulp distribution rectification. If the pulp flow can be well distributed, this may become a future development trend.

　　(5) Configuration of external pulse attenuation devices. One weakness of full-flow flow boxes is their sensitivity to pressure fluctuations. Therefore, in addition to improving the flow box itself, an effective method is to add an automatic pulse attenuator externally.

　　In the future, the design of flow boxes will continue to develop in the following trends:

　　(1) Controlling full-width transverse quantification by adjusting pulp concentration. The most notable new technology in the 1990s was the full-width transverse quantification controller. Compared to the traditional lip plate bending adjustment method of flow boxes, it can effectively adjust the uniformity of full-width transverse quantification while maintaining uniform fiber orientation across the full width, making it a better quantification control method.

　　(2) Developing towards adaptability to various formers. In the current paper machine renovations, retaining the original body while modifying the wire section has become a trend. Since different formers have different requirements for flow boxes, the flow box must be designed to adapt to various formers.

　　(3) Developing towards more precise adjustments and more complete controls.

　　(4) Further development of flow boxes with special properties, such as multi-layer flow boxes.

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