Johnson Screen in Pulp Screening And Fractionation
We are a professional Johnson v wire screen filter manufacturer in China. Our company can provide customized and design services for pulp mills all over the world.
Pulp screening is an essential process in pulping. Johnson screen is one of the important components to realize pulp screening.
Johnson type pulp screen structure: Vee wire + Support rod. V-shaped wire and support rod are welded together by ring welding process.
The type of Johnson pulp screen determines the screening effect, screening capacity and the operation of the screening system.
Johnson pulp screen VS Perforated pulp screen
There is another type of screen used in pulp screening – Perforated screen.
Perforated screen hole size: 1 – 2mm
Johnson screen slot size: 0.4 – 1mm
Therefore, the screening efficiency of the slotted screen(wedge wire screen) is higher, and the quality of the screened pulp is better.
The sieve plate is also divided into smooth sieve plate and wave surface sieve plate. The screening ability of smooth sieve plate is affected by the area of opening holes. Increasing the screening ability can be achieved by increasing the aperture or the number of holes.
However, the larger pore size affects the clarity of the good pulp and reduces the screening efficiency. At the same time, the increase of the number of holes and the density of holes are too high, which will weaken the rigidity of the sieve plate.
Special v-wire structure, Johnson screen panel has continuous slotted structure, it can improve the screening ability under the same screening efficiency.
Although the aperture rate of slotted sieve screen is small, the sieve plate adopts the wavy surface structure. In addition to the fluid tensile stress and pulsating local stress, the pulp is also affected by the same direction eddy shear stress at the seam, which makes the fiber network disperse, closer to the fluid characteristics or fluidization, and the fluidized pulp is easier to be screened, so that the flow rate of the pulp through the seam is increased.
Although the micro turbulence on the sieve screen seems to increase the width of the sieve plate gap and decrease the screening efficiency for the fiber particles, the decrease of the screening efficiency can be compensated by reducing the width of the sieve plate gap.
For the same screening efficiency, the wave Johnson screen has higher production capacity than the smooth one.
For the operation of screening, because the opening rate of slotted screen is smaller than that of perforated screen, the pressure difference increases correspondingly, and the pressure pulse produced by the same rotor is greater. In addition, the eddy current effect at the slit is favorable for the passage of long fiber and the elimination of slurry blocking at the slit.
Under the same conditions, two kinds of corrugated sieve plates with a slot size of 0.254m m and a diameter of 1.143mm were studied, it was found that no matter what kind of pulp was used, the formation of fiber network could not be observed by Johnson screen, while the fiber flocs could be observed hanging on the edge of the hole by the perforated screen, and it would take several pulses to make them free into the main flow area. Therefore, the use of Johnson screen is conducive to the stability of operation and prevent pressure screen from blocking.
At the same time, in the screening process of ordinary rotors, there is a thickening phenomenon from the feed end to the discharge end, that is, the thickening gradually increases with the feed side of the sieve plate, resulting in a gradual decrease in the concentration of good pulp, which makes the screening process in different areas. When the pulp concentration is higher than 1.5%, the energy consumption of fluidization increases with the increase of pulp concentration. When the pulp concentration is higher than 3%, high energy input is needed.
In fact, only 1 / 3 or 1 / 2 of the screen plate surface can achieve effective screening effect because of the thickening phenomenon on the screen plate surface. In the lower part of the screen plate surface, the consumption of electric energy is only used for stirring slurry.
By using the rotor of the widened action surface, the thickening phenomenon in the screening process is eliminated, the slurry is fully fluidized in the screening area, the suction / pulse effect is enhanced, and the uniform screening conditions along the whole screen plate surface are ensured. The concentration of the good slurry is almost the same as that of the feed slurry and the slurry.
The uniform screening conditions make it possible to increase the screening concentration. With the increase of screening concentration, the screening quality and screening ability will be improved accordingly. At the same time, the high concentration screening washing equipment is improved.
We can also customize the wedge wire rotary drum screen(Johnson trommel screen) used in the dehydration equipment for the paper mill.