Headbox is known as the "heart" of paper machine. It is the key part of paper machine. It is the hub connecting the "flow" and "forming" parts. It is also one of the fastest developing parts of paper machine. In papermaking, the function of headbox is to send the required pulp to the forming net according to the requirements of the forming part of the paper machine, so as to provide the necessary premise for the good forming of the paper sheet.

　　Function and performance of headbox

　　The basic task of headbox is to provide a good prerequisite for paper sheet forming, that is, to evenly distribute the paper along the width of the paper machine, to ensure uniform pressure distribution, uniform speed distribution, uniform flow distribution, uniform concentration distribution and fiber orientation controllability and uniformity; to effectively disperse the pulp fiber and prevent fiber flocculation, according to the process Provide and maintain a stable sizing head and speed ratio.

　　In the long-term development process of pulp and paper technology, although there are many kinds of headboxes with different structures, their main functions are basically unchanged, namely pulp distribution, homogenization and spray.

　　The basic functions of headbox include five aspects

　　(1) To provide a uniform and stable jet flow to the whole width of the paper machine, avoid transverse flow, no directional branch or longitudinal flow;

　　(2) To provide a stable lip with the required geometric size, which is not affected by temperature, pressure and lip opening;

　　(3) A kind of fiber suspension with little flocculent and good dispersion was formed;

　　(4) A control system is provided to meet the technological requirements of the quantitative distribution of the banner, the dropping point, the spraying angle and the spraying speed;

　　(5) Provide convenient measures to keep headbox clean and easy to operate and maintain.

　　A good headbox must have the following properties:

　　(1) It can produce a stable and uniform web without longitudinal streaks;

　　(2) The lip area of headbox should not be affected by pressure, temperature and lip opening, so as to ensure uniform distribution of cross weight;

　　(3) The phenomenon of slurry hanging and adhering should be avoided in headbox;

　　(4) Easy to keep the box clean;

　　(5) The speed ratio of pulp to wire can be controlled.

　　Therefore, headbox must:

　　(1) The pulp can be evenly distributed in the direction of the paper machine;

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

　　Structure and function of headbox

　　The function of headbox, in a word, is to spray the web pulp with uniform pressure, speed, flow rate and as much as possible evenly dispersed fibers along the width direction of the paper machine at a certain jet angle, so as to ensure the required quality of paper .

　　In fact, the functions of any headbox are undertaken and coordinated by the distributor, rectifying unit and web device

　　Distributor:

　　According to the principle of hydrodynamics, the feed flow of the web is evenly distributed along the width direction of the paper machine with a certain pressure, speed and flow by using regular variable cross section.

　　Rectifier unit:

　　The rectification element and turbulence generator are used to generate turbulence of appropriate scale and intensity, which can effectively disperse the fibers, prevent fiber flocculation, and keep the non directional arrangement of fibers as far as possible, so as to make the fibers in the web material flow in a uniform dispersion state.

　　Paper feeding device:

　　In order to ensure the required quality of paper, it is necessary to spray the paper evenly, at a certain angle and at a certain speed to the predetermined landing point on the web, and to provide the quantitative adjustment of the width direction of the paper machine, the micro adjustment of the moisture content, the turbulence of the pulp flow and the control and adjustment of the flocculation scale.

　　Development and trend of headbox

　　The earliest headbox used was the open headbox with tipping gate, which was produced before fourdrinier brothers invented the fourdrinier paper machine in 1804. Its structure and form are relatively simple, the technical requirements are not high, it can only be applied to low speed paper machine, and the homogenization effect is poor.

　　With the increase of the speed of the paper mill, it is required that the static pressure head formed by the paper material in the weir must increase with the square of the speed of the mill. Therefore, when the speed of the paper mill increases, the required static pressure head must increase. At higher speed, if open headbox is adopted, it will become very large and complicated in volume, height, structure and weight. Moreover, it is impossible to efficiently distribute pulp, rectify and disperse fibers for such a large volume of paper in weir pond to provide high quality jet paper flow. In order to solve this problem, the closed headbox was developed in the late 1940s.

　　Since 1970s, the air cushion headbox developed has absorbed some advantages of the headbox design. Some new headbox technologies, such as the conical cloth main pipe, the hydraulic pulp rectifier element, the foam overflow and the upper lip plate structure, have been widely used in the medium speed long net paper machine and the folder paper machine.

　　Since the 1970s, in order to meet the needs of the development of new multi-layer forming technology, multi-layer hydraulic headbox has been developed. The multi-layer hydraulic headbox is a full flow hydraulic headbox with multiple channels. At present, the multi-layer hydraulic headbox can not only be used in the production of paperboard, but also can be used in the production of tissue paper and high-grade paper. It plays a good role in improving paper quality and rational use of fiber raw materials. Therefore, the multi-layer hydraulic headbox has a good development prospect. At present, multi-layer hydraulic headbox with various structures has been developed.

　　Since 1990s, the research and development of headbox dilution water concentration control and regulation technology is a significant achievement and progress in headbox common technology. The headbox adopts the dilution water concentration control and regulation technology, which causes the problems of bias flow and cross flow caused by weir plate regulation. At present, several systems have been developed, and this technology is mostly used in the newly designed headbox.

　　It can be seen from the development process of headbox that the main goal and driving force of headbox development is to make headbox adapt to the improvement of papermaking speed and produce higher quality products. Each headbox has the best speed range and the best product variety and quality range.

　　Throughout the modern new high-speed paper machine headbox, basically has the following main characteristics:

　　(1) High efficiency hydraulic slurry distribution rectifier is adopted. Through the design of different structure and size of hydraulic slurry distribution rectifier, different slurry flow conditions can be adjusted and controlled to meet the requirements of different process conditions.

　　(2) Attention should be paid to the role of stabilizing chamber. The slurry stabilizing chamber can eliminate and attenuate the energy fluctuation and stabilize the slurry flow. For the full flow air cushion combined headbox, the slurry stabilizing chamber is the place to produce air cushion and eliminate bubbles.

　　(3) The narrow channel is used to produce controllable micro turbulence. In the outlet section of headbox, the turbulent degree of the slurry flow at the outlet can be controlled by using a long and narrow channel to produce micro scale turbulent flow, which is more prominent in the flapping headbox.

　　(4) It is developing to the trend of primary slurry distribution and rectification. Most of the new headboxes are secondary distribution rectifiers, but there are also attempts to develop to primary distribution rectifiers. If the slurry flow can be well distributed, it may become a development trend in the future.

　　(5) Equipped with external pulse attenuation device. One weakness of full flow headbox is its sensitivity to pressure fluctuation. Therefore, in addition to the improvement of the headbox itself, an effective method is to add an automatic pulse attenuator to the headbox.

　　In the future, headbox design will continue to develop in the following trends:

　　(1) By adjusting the pulp concentration, the whole width transverse ration is controlled. The most remarkable new technology in the 1990s is the full width lateral quantitative controller. Compared with the traditional lip bending adjustment method of headbox, it can not only effectively adjust the uniformity of the whole width transverse quantitative, but also keep the uniformity of the whole width fiber orientation, which is a better quantitative control method.

　　(2) It is developing to adapt to all kinds of formers. In the transformation of the existing paper machine, it has become a trend to retain the original machine body and change the web part. Due to the different requirements of different shapers, the headbox must be designed to adapt to various shapers.

　　(3) To adjust more accurately, control more perfect direction.

　　(4) Further development of headbox with special performance, such as multi-layer headbox.