Pressure screen

A pressure screen for cleaning a fibrous stock including a screen element which has a rotationally symmetrical configuration about a screen axis. The screen element divides the pressure screen into an inlet chamber and an accepted stock chamber. The inlet chamber is connected at a first axial end with a suspension inlet and at a second axial end with a reject outlet. The accepted stock chamber is connected with an accepted stock outlet. The pressure screen also includes a rotor situated in the inlet chamber and including a plurality of rotor blades. The rotor rotates about the rotational axis relative to the screen element. The plurality of rotor blades is arranged on a plurality of circumferential planes of the rotor. The width of an annular gap between the rotor and the screen element decreases from the suspension inlet to the reject outlet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pressure screen for cleaning a fibrous stock, and, more particularly, to a pressure screen having a screen element which is of rotationally symmetrical configuration about a screen axis and divides the pressure screen into an inlet chamber and an accepted stock chamber. The inlet chamber is connected at one axial end with a suspension inlet and at the opposite axial end with a reject outlet; and the accepted stock chamber is connected with an accepted stock outlet, and a rotor with rotor vanes is situated in the inlet chamber. The rotational axis of which rotor corresponds to the screen axis and which rotor rotates relative to the screen element. The rotor vanes are arranged distributed over a plurality of circumferential planes of the rotor which run perpendicular with respect to the rotational axis.

2. Description of the Related Art

Pressure screens are used in the preparation of fibrous stock suspensions, particularly in order to process the fibrous stock suspension in a wet screening process. For this purpose a pressure screen of this type includes a screen element that is provided with a multitude of apertures. The fibers contained in the suspension are intended to pass through the apertures in the form of accepted stock, whereas the undesirable solid components are repelled and are again directed out of the screen in the form of rejects.

It can also conceivably be used to separate different fiber components, in other words the shorter from the longer fibers.

Round apertures or slots are generally used as screening apertures. Generally, pressure screens of the type discussed herein are equipped with screen scrapers that have scraping surfaces moving along the screen. This prevents clogging of the screen apertures.

A screen scraper is known from WO 98/53135 that is equipped with blade elements for cleaning of the screen. These blade elements have a hydrodynamic profile that extends over the entire length of the screen element of a screen basket. Due to the relative movement relative to the surrounding suspension, the blade element exerts a pressure at the front and a suction impulse on the back upon the screen that is to be cleaned. Consequently a portion of the suspension that was rejected on the screen or has already passed the screen as accepted stock is sucked back, thereby keeping clear or clearing the screen apertures.

In contrast thereto, the rotor blades are formed for example by elevations in DE-OS 3701669.

What is needed in the art is a pressure screen to reduce the energy intake and the danger of clogging.

SUMMARY OF THE INVENTION

The present invention provides a pressure screen in that the width of the annular gap between the rotor and the screen element decreases from the suspension inlet to the reject outlet.

The through-flow that increases with the smaller annular gap very effectively counteracts clogging.

Depending on the type of fibrous stock and/or structure of the pressure screen, the width of the annular gap may decrease in steps or else continuously.

Since the fibrous stock suspension is increasingly being rotated by the rotor with increasing distance from the suspension inlet, the radial extension of the rotor blades may decrease from the suspension inlet to the reject outlet, which has a corresponding positive effect on the energy requirement.

The radial extension of the rotor blades can decrease here too in steps or continuously. Whereas the stepped decrease may be easier to manufacture, the continuous decrease can provide a more balanced effect.

For an additional counteraction to clogging, the rotor should at least include one recess between the rotor blades in at least one circumferential plane. Moreover, the rotor may have at least three circumferential planes with rotor blades and/or each circumferential plane may include two rotor blades respectively and/or the rotor blades may have the same shape.

The rotor blades of adjacent circumferential planes can moreover be arranged offset to one another in circumferential direction. This too counteracts clogging.

In order to achieve a cross flow in axial direction over the screen element, the rotational axis can be positioned diagonally or perpendicular to the machine plane.

The rotor can be positioned inside the screen element, the screen element can be cylindrical and the rotor can be in the embodiment of a cylindrical drum.

The suspension inlet can hereby be arranged below the reject outlet.

An advantage of the present invention is that it reduces energy intake and it helps to counteract clogging.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly toFIG. 1, there is shown a pressure screen which includes a screen element2—here in the embodiment of a cylindrical screen basket with vertical screen axis1—which divides the interior space of the pressure screen into an inlet chamber3and an accepted stock chamber4.

Via a suspension inlet5the fibrous stock suspension is fed at the lower end of the screen basket into inlet chamber3.

With the herein utilized pressure screen the fibrous stock suspension receives an angular momentum that moves it into a circumferential motion. In addition hereto and as a result of the prevailing pressure gradient between lower suspension inlet5and reject outlet6of inlet chamber3that is located at the upper end of the screen basket a transport flow is produced.

Over the path of this transport flow a large portion of the fibrous stock suspension is diverted as intended through screen element2as accepted stock into accepted stock chamber4and from there is discharged via upper accepted stock outlet7. During this process a large portion of fibers that are contained in the fibrous stock suspension also pass into accepted stock chamber4.

The portion of fibrous stock suspension that is rejected by screen element2is removed as reject from inlet chamber3via reject outlet6.

In order to avoid that the apertures of screen element2become clogged, a screen scraper that moves relative to screen element2can be used.

According to the invention this screen scraper can also include a rotor8that is rotating in screen element2and has rotor blades9attached to it. Rotor8herein has the shape of a cylindrical drum, whereby rotational axis10corresponds to screen axis1.

All rotor blades9herein have the same shape, resulting in a uniform effect on the fibrous stock suspension and screen element2.

Rotor blades9can furthermore be arranged distributed over several—in this case three—circumferential planes11of rotor8that extend perpendicular to rotational axis10, whereby each circumferential plane11comprises several—in this case two—rotor blades9.

The width of annular gap12between rotor8and screen element2from suspension inlet5to reject outlet6can decrease in steps. The stepped transitions can be located between two adjacent circumferential planes11. Since screen element2has a cylindrical shape this means that the diameter of rotor8enlarges intermittently from the bottom to the top. This has a very positive effect on the energy requirement of the pressure screen.

In order to further support this, the radial extension of rotor blades9also decreases from suspension inlet5to reject outlet6.

The invention may also include other shapes of rotor blades9. However, manufacture is especially simple, if rotor blade9is formed by elevations on rotor8.

For the same reason, the radial extension of rotor blades9can decrease in steps, in other words, the height of the elevations on rotor8can decrease in steps from suspension inlet5to reject outlet6, whereby also in this case the steps are positioned between circumferential planes11.

The cross sectional area of rotor blades9extending in rotational direction of rotor8is rectangular.

Rotor blades9herein not only have the same shape, but also the same dimensions, resulting in a reduction of manufacturing costs. Moreover, rotor blades9completely overlap each other in rotational direction in one circumferential plane11.

In order to reduce clogging danger even further, rotor8can include in each circumferential plane11at least one recess13between rotor blades9, which can however extend only over a part of the axial extension of respective circumferential plane11.

Rotor blades9of adjacent circumferential planes11are moreover arranged offset to one another in the circumferential direction.