Take apart the heart of the paper machine - headbox

The headbox is known as the "heart" of the paper machine. It is the key component of the paper machine. The function of the headbox in papermaking is to send the pulp that meets the requirements to the forming wire according to the requirements of the forming part of the paper machine, so as to provide the necessary prerequisite for the good forming of the paper web.

The function and performance of headbox

As the combination of "flow" and "forming" of the paper machine, the basic task of the headbox is to provide good preconditions for sheet forming, that is, to evenly distribute the paper stock along the width direction of the paper machine, to ensure uniform pressure distribution, Uniform speed, uniform flow, uniform concentration, and fiber orientation controllability and uniformity; effectively disperse pulp fibers, prevent fiber flocculation, and provide and maintain stable sizing head and pulp web speed ratio according to process requirements.

In the long-term development of pulp and paper technology, although there have been various structural forms of headbox, its main functions are basically unchanged, namely pulp distribution, homogenization and shotcrete.

Specifically speaking, the basic functions of the headbox are shown in five aspects:

(1) Provide a uniform and stable ejected pulp flow to the entire width of the paper machine, avoiding transverse pulp flow, no directional tributary flow or longitudinal stripe flow;

(2) Provide a stable lip with the required geometric dimensions, which is not affected by temperature, pressure and opening of the lip;

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

(4) Provide a control of quantitative profile distribution, drop point, shot angle and shot speed that can meet process requirements;

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

A good headbox must have the following properties:

(1) It can produce a paper web with a uniform and stable quantification, and there is no longitudinal stripe on the paper web;

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

(3) The phenomenon of hanging and sticking pulp should be avoided in the headbox;

(4) It is easy to keep the inside of the box clean;

(5) The ratio of pulp to wire speed is controllable.

Therefore, the headbox must do:

(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.

The structure and function of headbox

The function of the headbox, in a nutshell, is to spray a certain uniform pressure, speed, flow rate and internal fibers as much as possible to the wire part along the width direction of the paper machine at a certain injection angle. It can provide a uniformly dispersed upper mesh flow, and at the same time provide quantitative and fine adjustment of moisture in the width direction of the paper machine, so as to ensure the required quality of paper sheets.

In fact, the functions of any form of headbox are specifically undertaken and coordinated with each other by the three major parts of the pulper, the rectification part, and the paper material online device. The functions of these three major parts are:

Spreader:

According to the principle of fluid dynamics, using the regular variable cross-section, the paper stream on the Internet is evenly distributed along the width direction of the paper machine with a certain pressure, speed and flow.

Rectifier:

The rectification element and turbulence generator are used to generate turbulence of appropriate scale and intensity, which can effectively disperse fibers, prevent fiber flocculation, and maintain the non-directional arrangement of fibers as much as possible, so that the fibers in the web stream are uniformly dispersed.

Paper material online device:

The paper material is uniformly sprayed to the predetermined screen point on the wire at a certain angle and shot speed, and provides the quantitative control of the width direction of the paper machine, the fine adjustment of moisture, the turbulence of the pulp flow, and the control and adjustment of the flocculation scale to Guaranteed to obtain the required quality of paper.

The development situation and development trend of headbox

The earliest used headbox was the open headbox with the flap ram, which was produced before the 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 machines, and the homogenization effect is poor.

With the increase of the speed of the paper machine, it is required that the static pressure head formed by the paper material in the weir must increase with the square relationship of the speed of the vehicle. Therefore, when the speed of the paper machine increases, the required static pressure head must increase. At higher vehicle speeds, if an open headbox is used, it becomes large and complex in terms of volume, height, structure, and weight. Moreover, it is impossible to efficiently disperse, straighten and disperse fibers in such a large volume of paper stock in the weir pond to provide a high-quality web-jet paper stream. To solve this problem, closed (air cushion) headboxes were researched and developed in the late 1940s.

After the 1970s, the developed air-cushion headbox absorbed the advantages of some headbox designs, and adopted some new headbox technologies such as conical distribution headers, hydraulic distribution rectifiers, elimination of foam overflow, optimization of upper and lower headboxes. The lip structure, etc., has been greatly improved in performance, so it is also widely used in medium-speed Fourdrinier wire paper machines and sandwich wire paper machines.

Since the 1970s, in order to meet the needs of the development of new multi-layer forming technology, a multi-layer hydraulic headbox has been researched and developed. Multi-layer hydraulic headbox is a multi-channel full flow hydraulic headbox. At present, the multi-layer hydraulic headbox can be used not only for the production of cardboard, but also for the production of tissue paper and high-grade paper. It plays a good role in improving the quality of paper and rationally using fiber raw materials. Headbox has a very good development prospect. At present, multi-layer hydraulic headboxes with various structures have been researched and developed.

Since the 1990s, the research and development of headbox dilution water concentration control and regulation technology has been a major achievement and progress in terms of headbox common technology. The headbox adopts the dilution water concentration control and adjustment technology to adjust the bias flow and cross flow caused by the weir plate. Several systems have been developed, and most new headbox designs use this technology.

As can be seen from the development history of the above headbox, the main goal and driving force of the development of the headbox is to make the headbox adapt to the increase in the speed of the paper machine and produce better quality products. Each headbox has the best range of operating speeds and the best range of product varieties and qualities to produce.

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

(1) Adopt high-efficiency hydraulic rectifying element. Through the design of different structures and sizes of the hydraulic distribution and rectification elements, different pulp flow conditions can be adjusted and controlled to meet the requirements of different process conditions.

(2) Pay attention to the role of pulp stabilization chamber. The pulp stabilization chamber can eliminate and attenuate energy fluctuations and stabilize the pulp flow. For the full-flow air-cushion combined headbox, the pulp stabilization chamber is also a place to generate air cushions and eliminate air bubbles.

(3) A narrow and long flow channel is used to generate controllable fine turbulent flow. The use of narrow and long flow channels at the outlet of the headbox can control the degree of turbulence of the outlet slurry to generate micro-scale turbulence, which is more prominent in the floating blade headbox.

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

(5) Configure the external pulse attenuation device. A weakness of the full flow headbox is that it is very sensitive to pressure pulsations. Therefore, in addition to improving the headbox itself, the effective method is to add an automatic pulse attenuator outside the machine.

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

(1) By adjusting the pulp concentration to control the full width transverse quantitative. The most striking new technology of the 1990s was the full-width lateral dosing controller. Compared with the traditional headbox lip bending adjustment method, it can not only effectively adjust the uniformity of the full width transverse quantitative, but also maintain the uniformity of the full width fiber orientation, which is a better quantitative control method.

(2) Towards the direction of adapting to various formers. In the renovation of existing paper machines, it has become a trend to retain the original machine body and modify the wire section. Since different formers have different requirements for the headbox, the headbox must be designed to accommodate various formers.

(3) Towards the direction of more precise adjustment and more perfect control.

(4) Further develop headboxes with special properties, such as multi-layer headboxes.