Screening Apparatus

The present invention relates to a screening apparatus which is intended primarily for screening and purification of pulp and more specifically paper pulp. This screening apparatus comprises a vessel, a cylindrical screen in the interior of the vessel, a rotor which moves in the vicinity of the screen surface, an inlet for the pulp to be screened, an outlet for the reject and another outlet for the screened pulp, which is called the accept.

In U.S. Ser. No. 472,742, a cylindrical screen plate is described which has an inlet on one side for introducing the unscreened pulp, and an outlet in the opposite side for removing the reject portion. Means are provided for moving the unscreened pulp along one first direction of flow. The screen plate has grooves in the side of the inlet recessed in the screen surface, the first direction of flow being essentially transverse to the grooves. The grooves are formed of an upstream side plane, a downstream side plane and a bottom plane. The bottom plane is essentially parallel to the envelope surface of the screen plate. The grooves have perforations in the bottom plane. The upstream side plane of the grooves, as seen standing from the bottom of the grooves, is substantially perpendicular to the envelope surface and the downstream side plane of the grooves forms an angle of 60.degree.-5.degree. against the envelope surface. According to a preferred embodiment, the angle between the downstream side plane of the grooves and the envelope surface of the screen plate is about 30.degree.. This configuration is called a "contour profile" .

In the paper making process, pulp is produced by cooking wood which separates the wood into fibers. Due to the different properties of the wood even from the same tree, some of the fibers do not separate and are dispersed as fiber bundles usually called debris, shives or slivers which form the reject. There are also other impurities, such as bark, which must be removed. The screen must separate the undesirable impurities and debris called the rejects from the accept portion. In order to avoid substantial losses of fibers which could be carried over together with the debris in the reject portion, it is necessary to remove the impurities efficiently and selectively.

It should be stressed that different applications have different requirements. In some applications, it is necessary to achieve a high content of long fibers, especially secondary fibers, in the accepts because the long fibers give strength to the final product, for instance paper. In other applications, on the other hand, the contrary is true. For instance, in virgin or pulp mill fibers, it is desirable to concentrate the long fibers in the reject for reject refining.

A great deal of work has been carried out in connection with the screen plates and the rotors and it has been recognized that means to create pulsations on the rotor will increase the efficiency of the apparatus. U.S. Pat. No. 3,363,759 and U.S. Pat. No. 4,318,805 describe drum rotors with a bumped surface which provides pulsations. In U.S. Pat. No. 4,318,805, the bumps take the form of pins projecting from the rotor with enlarged heads, the heads providing the pulses while the pins offer little resistance to flow.

U.S. Pat. No. 4,447,320 and U.S. Pat. No. 4,200,537 describe rotors which carry blades or vanes moving in the vicinity of the screen which produce a large positive pulse. Other patents describe other types of rotors, for instance U.S. Pat. No. 3,726,401 uses a rotor with bumps or protruberances which produce about equal positive and negative pulses. According to this patent, any form of bumps may be used provided it produces the desired pulses, the bumps and the depressions between them creating positive screening and negative screen cleaning pressure pulses.

U.S. Pat. 3,400,820 describes a rotary member made up of a plurality of separate segments joined together and forming a selected undulating pattern which produces about equal positive and negative pulses.

One object of the present invention is to provide a rotating element which increases the intensity of the pulses generated near the openings, either orifices or slots within the screen plate, for the purpose of creating the negative pulses which are necessary to backwash the screen, and to prevent plugging, thus increasing the flow of the fiber suspension through the openings of the screen.

Another object is to provide a rotating element with very high frequency pulses in addition to sufficient amplitude.

Another object is to provide a rotating element which produces sharp and steep negative pulses, thus resulting in high intensity.

Still another object is to reduce the power requirements.

Another object is to provide a rotor which permits to operate with smaller orifices in the screen thus improving the screening efficiency.

Still another object is to obtain an accept portion of consistency as close as possible to the consistency of the pulp introduced at the inlet.

Another object is to provide blade type segments to be used instead of a rotor in certain applications in which the continuous surface of a rotor is not desirable, for instance when fractionation of the long fibers should be avoided and a larger inlet screening zone is preferable.

Still another object is to provide a rotor and blade type segments which may be advantageously used in conjunction with the screen plate described and claimed in U.S. Ser. No. 472,742.

It has now been found that a specific shape of the rotor surface and the blade type segments as described hereinbelow, is particularly advantageous in producing the higher intensity pulses and in creating sufficient negative pulses so that plugging of the screen is minimized.

The present invention will be illustrated in more detail by reference to the accompanying drawings of which:

FIG. 1 illustrates a conventional pressure screen using rotating hydrofoils for producing pulses;

FIG. 2 illustrates the pulses which are produced by the rotating hydrofoil screens of FIG. 1;

FIGS. 3 and 3a illustrate the contour surface of the rotor in accordance with the present invention;

FIGS. 4 and 4a illustrate the nature of pulses which are produced from a rotor or from the blade type segments with a contour surface according to the present invention, as shown in FIGS. 3 and 5;

FIG. 5 illustrates the embodiment of the blade type segments instead of a rotor, with a contour surface according to the present invention;

FIG. 6 shows the contour profile of the screen plate according to U.S. Ser. No. 472,742.

FIG. 7 illustrates the intensity factor which is the ratio of the change of amplitude over a unit of time with a conventional rotating element.

FIG. 7a illustrates the intensity factor for a rotating element according to the present invention.

FIG. 8 is a comparison of the pulse profile of conventional rotor and the rotor according to the present invention.

As shown in FIG. 2, the rotating hydrofoils produce both negative and positive pulses The symbol O pressure designates the value of the pressure near the screen surface or the screening zone between the rotating foil and the screen cylinder. When the pulp pressure is greater than the reference O point, the pressure has a positive value, which is at maximum at the point A and when it is less than the reference point, the pressure has a negative value, the latter being at a maximum at the point B.

The rotating hydrofoils of FIG. 1 known in the art produce pulses illustrated in FIG. 2. As shown in the figure, the leading surface or nose on the hydrofoil develops a positive pulse with the maximum value or amplitude just about at the minimum clearance point between the foil and the screen. The positive pulse contributes to induce the flow in the screening zone through the apertures within the screen because the flow will always be from a higher pressure zone to a lower pressure zone. However, after the minimum foil clearance point a maximum negative pulse is developed due to the change in slope of the foil blade from its minimum clearance position. As shown in FIG. 1, the angle of this slope for most foil blades would be a maximum of approximately 30.degree. or even less. After the maximum negative pulse point the pressure increases again up to the reference O point, The negative pulses induced by the rotating hydrofil cause a momentary reverse or back-flow through the apertures within the screen thus dislodging any build-up of fibers at the apertures. This self-induced backwashing is greater, the greater is the negative pulse.

The foils or lobe-shaped surfaces or protrusions of rotors and blade type segments known in the art have leading surfaces curved towards the screen cylinder until the minimum rotor clearance point is reached followed by a trailing surface curving away from the screen. At the minimum rotor clearance point, the change in slope of the curved surface is O. Shortly after this point, the magnitude of the negative pulses starts to increase to a maximum negative value after which the pulses start decreasing again, thus repeating the cycle, as shown in FIG. 2.

The crux of the present invention resides in providing a rotor and blade type segments with grooves, the grooves having a contour surface being formed of a first plane parallel to the envelope surface, an inclined plane, an upper plane and a side plane, the side plane being substantially perpendicular to the first plane and the inclined plane of the grooves forming a 30.degree.-60.degree. angle against the first plane, the upper plane being parallel to the first plane. According to a preferred embodiment this angle is about 45.degree..

By reference to FIGS. 3 and 3a, numeral 1 designates the first plane and numeral 2 designates the inclined plane. Numeral 3 designates the upper plane and numeral 4 designates the side plane perpendicular to the first plane.

By reference to FIG. 5, the same numerals 1, 2, 3 and 4 are used to designate the first plane, the inclined plane, the upper plane and the side plane which is perpendicular to the first plane.

As shown in FIG. 3a, starting from reference point A, the leading surface has a first plane 1 parallel to the envelope surface. It then slopes forming inclined plane 2 until it reaches the upper plane 3. The length of the upper plane 3 should be between 1/4 and 3 inches. After the upper plane, the groove comprises a side plane 4 perpendicular to the first plane 1. The first plane 1 may vary from a length of 0 up to 3 inches.

This feature of the side plane 4 being substantially perpendicular to the bottom plane is essential in order to produce pulses which are more effective in keeping the screen from plugging, as compared with rotors which have a curved surface.

FIG. 4 illustrates the pulses from a rotating element having the contour profile according to the present invention. A very sharp negative pulse is produced by the 90.degree. angle in the rotor surface or in the blade type segments. This is due to the rapid change in the flow path of the fluid coupled with a Bernouilli effect caused by the increasing velocity of the fluid within the minimum rotor or blade type segment clearance point. This severe or rapid change in pulsation makes the rotor or the blade type segment more effective in back-flushing the screen cylinder. The explanation for this effect is that the maximum velocity and acceleration of the fluid which goes through the apertures during back-flushing must be greater than with conventional pulse producing rotating elements because the change in pulses or amplitude with time is much greater with a contour surface (see FIG. 4a)

The term "intensity of the pulses" within the scope of the present invention is intended to mean the change of amplitude of the pulses per unit time. To state the matter in different words, the intensity is not merely the result of the amplitude but also depends on the fast and sharp change. The intensity factor, IF, is the ratio of the change of amplitude over the unit of time: .DELTA.amplitude/.DELTA.time

(FIGS. 7 and 7a).

FIG. 4 when compared with FIG. 2 shows the superiority of the rotating element of the present invention with the contour surface with respect to conventional rotors or conventional blade type elements. Further, the slope and leading surfaces of the contour rotating elements produce positive pulses which help in inducing the flow of the fibrous suspension through the apertures within the screen. The upper surface 3 is needed just ahead of the side plane 4 to cause the rapid change in pulsation.

FIG. 5 illustrates the embodiment in which a large screening zone with a substantial open area is desirable to allow inlet stock to enter the full screening zone thus minimizing the concentration of the long fibers in the reject and minimizing the consistency of the reject, so that more long fibers are in the accept. In this embodiment, instead of a rotor, blade type segments are used with a contour surface.

The contour surface of the rotating element, a rotor or blade type segments according to the present invention, provides high frequency pulsations. For instance, with a rotating speed of 600 RPM, it is possible to achieve a pulsation frequency of 200 Hz and even higher. The high frequency is advantageous because there is less tendency for the fibers to plug the openings and due to the fact that the pulses also exhibit sufficient amplitude, back-flushing occurs. The higher frequency permits the screen to handle stock of higher concentration.

Comparison of FIGS. 7 and 7a shows the superiority in intensity factor achieved by the rotating element according to the present invention as compared with a conventional rotating element. In FIG. 8 the pulse profile of a conventional hydrofoil is shown in broken line and the pulse profile of a rotor with the contour surface according to the present invention is shown in solid line.

Other advantages of the rotor and blade type segments according to the present invention are that it is possible to reduce substantially the size of the apertures of the screen without any appreciable increase in power consumption, nor with any substantial loss in long fibers and with good yield of the accept portion. The consistency of the accept may be kept essentially the same as the consistency of the stock at the inlet.

The rotor as shown in FIG. 3 and the blade type segment shown in FIG. 5 may be used with conventional screen plates and also in conjunction with the screen plate having a contour profile according to U.S. Ser. No. 472,742 as shown in FIG. 6.