Large paper machine Composition and principle of operation(2)

Update:27 Jul 2019
Summary:

The application principles of the thick and white water […]

The application principles of the thick and white water systems are described below.
Thick slurry system
Mixing and agitating single or multi-component slurries, including related wet-end additives, is often not very effective for large systems, uneven agitation, poor flow paths, and the introduction of air reduces the performance of the wet end.
Compact slurry mixer
The compact wet end system solves the problem of uneven mixing of the slurry. The compact slurry mixer can replace the mixing and slurrying tanks to improve the stability of the thick slurry system. In this device, a variety of slurry components and wet-end additives can be effectively mixed, and the residence time of 1~3min can meet the requirements of related chemicals and other processes.
The compact slurry mixer is close to the headbox on the operating surface, reducing the size of the system. The compact slurry mixer can operate the plug flow for frequent and efficient changes when frequent paper type changes occur.
Absolute dry pulp flow control
In order to ensure the stability of the slurry flow of the compact slurry mixer, it is necessary to control the flow rate of the dry slurry. Here, the measured concentration and flow rate are used to control the speed of the slurry pump and accurately measure the amount of dry fiber entering the paper machine.
In this way, the fibers can be transferred directly to the mixing pump or the slag pump, eliminating the conditioning chamber while the process remains stable. The compact slurry mixer operates at a concentration of 4% to 5%, which eliminates a concentration control point on the machine. As the concentration increases, excess water from the paper machine decreases.
White water and slurry dilution system
The short circuit of the wet end of the machine must effectively remove the entrained air. When designing a larger volume of under-the-white puddles and storage bins, a lower flow rate must be used to provide the residence time required to eliminate air bubbles. Typically, these devices are inefficient at reducing the entrained air content at acceptable air levels in white water.
For some high quality fine paper and newsprint, in order to eliminate air, a mechanical system needs to be installed to achieve the required degassing requirements.
In addition, for inefficient degassers, under-white puddles and silos, due to the lower flow rate, it will encourage the growth of bacteria. In order to control the growth of bacteria, expensive fungicides, bactericides and defoamers are used in conventional systems.
Large systems vary in frequency and it is difficult to manage them effectively. These changes can cause large fluctuations in the longitudinal quantification of the paper machine. The compact wet end system solves these problems by using a centrifugal degasser, a pressurized white water distribution system, and white water reuse to effectively reduce the size of the system.
Centrifugal degasser
The degassing of white water in a compact wet end system is accomplished by a centrifugal degasser. The white water in the forming section can immediately remove the air by this means, so that the under-white puddle of the forming section can be eliminated.
Because the degassing quality of the centrifugal degasser is better, replacing it with a mechanical degasser can further reduce the scale of the system and save a lot of energy. For paper machines that do not have air-removing white water in their current operation, a compact wet end system with a centrifugal degasser can improve the formation and quality of the paper.
Pressurized white water distribution system
The white water from the centrifugal degasser is water that is truly free of entrained air and enters the pressurized white water distribution system under pressure.
This pressurized header supplies degassed white water to the slurry pump and the cleaner pump and other systems that require white water, respectively.
There is an overflow high level tank at the top of the distribution header to keep the pressure of the entire system stable.
White water separation system
The concentrated white water and the diluted white water are respectively processed by the deaerator and transferred to the distributed header under a certain pressure, and the concentrated white water is consumed first after entering the header, and is consumed after the diluted white water. The white water header is a closed, airless, hydraulic system with a higher water flow rate than the conventional white water system, thus minimizing bacterial growth and scale formation.
Flexible cleaner arrangement
Another opportunity to reduce the size of the wet end is the arrangement of the slag remover. The arrangement of the slag remover in the compact wet end system can reduce the size and energy consumption of the equipment and further reduce the volume of the system.
The flexible first-stage slag cleaner is suitable for minimal headbox overflow, which improves the purification efficiency, and the secondary good pulverized again in the entire series arrangement.
As the flow rate of the headbox slurry increases, the secondary good slurry can be opened in series operation and automatically feed forward to meet the higher slurry flow demand. When the slurry flow exceeds the flow rate of the primary and secondary good slurry, white water dilution can be added from the distribution header.
This slag cleaner arrangement optimizes slurry flow and slag removal efficiency, reduces equipment, reduces power requirements, and streamlines control and piping.