The present invention relates to a method of manufacturing a rotor for a screening apparatus and a rotor structure for a screening apparatus. The rotor structure of the invention is particularly suitable for screening fibre suspensions of the pulp and paper industry. The apparatus according to the invention relates to a novel rotor construction, and especially to a novel means of fastening a turbulence element on the rotor surface.
The screening apparatus used nowadays in the pulp and paper industry is almost without exception a pressurized screening device i.e. a so-called pressure screen into which the pulp to be screened is introduced in a pressurized state. The most popular pressure screens comprise a stationary screen cylinder and a rotating rotor in cooperation therewith. The purpose of the screen cylinder is to divide the fresh pulp or the fibre suspension entering into the screening cavity where the rotor rotates into an acceptable fibre fraction called the accepts, and a rejectable fibre fraction called the rejects. The screen cylinder as well as, naturally, the rotor are located inside a screen housing having ducts for both the fresh fibre suspension, the accepts, and the rejects. Normally, the inlet duct or inlet for the fibre suspension is at one end of the screen housing, whereby the rejects outlet is at the opposite end of the housing. The accepts outlet is in communication with the accepts cavity, which is positioned at the opposite side of the screen cylinder in relation to the screening cavity. The purpose of the rotor is to create turbulence, and positive and negative pressure pulses in the fibre suspension to be screened. This purpose is achieved by providing the rotor with specific turbulence elements.
At this stage it should be understood that screening devices whose screen cylinder is rotary, and the means creating turbulence and pressure pulses is stationary, are also known, though more seldom used. The word ‘rotor’ is supposed to cover also this kind of turbulence creating means, as they can be said to rotate in relation to the screen cylinder. Also it should be understood that the term ‘screen cylinder’ covers all screening means having openings, i.e. holes or slots, for instance, and having a rotationally symmetric shape. Thus also conical or frusto-conical shapes are covered, and also known from prior art.
The pressure screen is most often positioned such that its shaft is in an upright position. However, the pressurization of the fibre suspension makes it possible to position the shaft of a pressure screen in any direction including a horizontal direction. Due to the pressurized feed of the fibre suspension, it may be introduced into a pressure screen to the top, to the bottom or to the centre region thereof.
The pressure screens may also be divided into two groups based on the direction of the accepts flow through the screen cylinder. When the accepts flow is radially outwardly, the screen is called an outflow screen, and when the accepts flow is radially inwardly, the screen is called an inflow screen.
In accordance with the prior art there are, in principle, two different types of rotors, which are commonly used in the pulp and paper industry and the intention of which, as known, is to maintain the screen surface clean, in other words to prevent blockages of perforations in the screen surface, and to maintain sufficient turbulence in the screening cavity containing fresh i.e. non-screened fibre suspension. The rotor types may be called an open rotor and a closed rotor. An example of an open rotor is disclosed in U.S. Pat. No. 4,193,865 in which the rotor is arranged inside a cylindrical, stationary screen cylinder. The rotor comprises a concentric shaft and a number of turbulence elements in the form of foils extending close to the surface of the screen cylinder. Each foil is supported on the shaft by means of a pair of arms extending through the cavity, which contains fresh pulp when the screening apparatus is in operation. The foils of the above-mentioned patent form an angle with the shaft of the rotor and the axis of the screen cylinder. However, the foils may also be arranged parallel to the axis. While the foil, or the fibre suspension in relation to the foil, is moving, the leading surface of the foil subjects the screen surface to a positive pressure pulse, which pushes acceptable fibres through the screening openings, and the trailing surface of the foil subjects the screen surface to a negative pressure pulse for opening the perforations of the screen surface or, rather, for preventing the fibres from accumulating on the screen surface and from blocking the screening openings by means of creating a back flow from the accepts cavity to the screening cavity.
An example of the other rotor type i.e. the closed rotor has been discussed, for instance, in U.S. Pat. No. 3,437,204, in which the rotor is a substantially cylindrical closed body positioned inside a screen cylinder. The rotor surface is provided with turbulence elements, i.e. protrusions, which, in this example, are almost hemispherical in form. In this kind of an apparatus, the fresh fibre suspension is fed between the rotor and the screen cylinder, whereby the protrusions of the rotor, the so-called bumps, in this case, create turbulence and pressure pulses towards and away from the screen cylinder. In other words, the leading surface of each bump pushes the pulp towards the screen cylinder and the trailing surface of the bump induces a suction pulse that draws the fibre accumulations from the openings of the screen cylinder. Most often the closed rotor surface is cylindrical. In a broader sense, also rotationally symmetrical rotor surfaces may be discussed, as there are rotors having a frusto-conical shape or a dome shape. Additionally, there are also rotors not literally having a rotationally symmetrical shape. One such option is a so-called S-rotor, which is formed of two identical cylinder halves attached to each other such that two radially, or substantially radially, arranged surfaces join the half-cylindrical surfaces. Also, there are rotors formed of a number of planar, possibly rectangular, members arranged to form an annular surface. Further, there are rotors, which are formed of a number of discs attached one on top of the other. The discs have an ellipsoidal outer surface, and the discs are positioned such that the foci of two adjacent discs are not situated in the same plane running along the rotational axis of the rotor.
As to the shape of the turbulence elements arranged on the surface of a closed rotor there is a huge number of different alternatives. A first alternative is a turbulence element, which is a more or less hemispherical bump, as already discussed above. A second alternative is formed of an axially or spirally extending ridge, which still has a rounded top surface. A third alternative is formed of a grooved rotor surface where the groove is formed of a bottom surface, an inclined side surface and a side surface perpendicular to the envelope surface of the rotor. The groove is either axially oriented or spiral. Depending on the width of the bottom surface one could also call the rotor surface not grooved but ridged. A fourth alternative is formed of a protrusion, which, in a way, resembles the above ridged rotor except that the ridge is cut such that the length of a protrusion is of the order of 50-200 mm. This protrusion type has a number of variations. The leading surface of the protrusion may be perpendicular to the rotor surface or inclined; it may also be axially oriented or inclined in either direction. The protrusion may, or may not, have a top surface either parallel to the rotor envelope surface or inclined in either direction. The protrusion also has a trailing surface which is either inclined or perpendicular to the rotor surface. Thus one has four variables, each having several options, whereby the number of possible alternatives for the shape of a protrusion is very high. And finally, as the fifth alternative, where the surfaces (leading, top and trailing surfaces) of the protrusion may be arranged to be smoothly changing whereby they form a curved surface being formed of several sections each having (possibly) a different radius. In fact, the fifth alternative is formed by combining the foil of an open rotor with a closed rotor, as here the foil has been (with possibly minor modifications to the surface facing away from the screen surface) attached on the surface of the rotor. Thus, when taking into account the above mentioned surface options, though they may also comprise planar sections, the number of possible shapes of the turbulence elements grows even higher.
Yet one more rotor type may be mentioned. It is, in a way, a combination of an open rotor and a closed rotor, as the rotor has both types of turbulence elements i.e. both protrusions, which are fastened from their bottom on a closed rotor surface, and foils being attached by means of short arms on the rotor surface, or even by means of longer arms on the rotor shaft, whereby the rotor can be called either a partially closed or a partially open rotor.
The present invention relates, irrespective of the cross sectional shape of the turbulence element or of its length, to a turbulence element, which is attached on the surface of an at least partially closed rotor. As to closed, or partially closed, rotors the turbulence elements are fastened typically on the closed surface of the rotor by means of welding. This means that the turbulence elements must be manufactured such that their bottom surface facing the closed rotor surface has a curvature matching that of the rotor surface. At this stage we have to take into account the fact that when designing a pressure screen the designer cannot design a pressure screen for one customer and one production rate only, but he has to be able to fulfil the requirements of pulp or paper mills having production rates that differ a great deal from each other. The only way the designer is able to accomplish the above requirement is to design a series of pressure screens matching the varying production rates of different customers. Normally the way to alter the production rate of a pressure screen is to alter either the diameter or the height of the screen cylinder, or both. This means, in practice, that similar turbulence elements cannot be used for all rotors of a series of pressure screens, when the diameter of the rotor changes. Thereby, in principle, each rotor diameter requires specifically manufactured turbulence elements, which complicates the manufacturing process of the elements. Another disadvantage in fastening the turbulence elements by welding can be seen when the elements have worn to such a degree that they should be repaired. If it is decided that the elements have to be changed to totally new ones, opening the weld seams all around the element takes time and is a cumbersome task.
A turbulence element structure where the element is easily replaceable is known from prior art (See
An object of the method and rotor structure of the invention is to correct at least some of the deficiencies and/or disadvantages of prior art rotor structures and their manufacture. The basic problem the rotor of the present invention solves relates to the varying rotor diameters and the demands it sets for the fastening of a turbulence element on the rotor.
The present invention solves the above problem by providing the rotor with such a surface configuration that similar turbulence elements may be used in the entire series of screening apparatus having rotors of different diameters, or, in the least, in several rotors of different diameter.
In accordance with a first preferred embodiment of the present invention the rotor surface is manufactured flat at the positions where the turbulence elements are to be fastened. Thereby the surface of the turbulence elements facing the surface of the rotor may also be manufactured flat i.e. planar, whereby only one type of turbulence element is required for the entire screen series. Naturally, other factors may require other types of elements, but still their bottom surface needs no specific attention.
In accordance with a second preferred embodiment of the present invention the rotor surface is manufactured at the positions where the turbulence elements are to be fastened to a curvature that is preferably the same for all rotor sizes/diameters of a screen series. Preferably the surface of the turbulence elements facing the surface of the rotor is manufactured curved such that the curvature is the same as that of the rotor having the largest diameter of the screen series, whereby, at best, only one type of turbulence element is required for the entire screen series. However, smaller curvatures may be used, even as part of a complex surface shape, especially when the rotor is manufactured as a cast rotor. In other words, in this embodiment the surface of the turbulence element facing the surface of the rotor has a curvature different from the one resulting from the diameters of at least most of the rotors in a series of rotors on the surfaces of which the turbulence elements are meant to be attached.
In accordance with a third preferred embodiment of the present invention the rotor surface is provided with grooves and/or ridges, in more general terms depressions and/or projections, which, on the one hand, are designed such that they are alike in all rotors of a series of rotors irrespective of the rotor diameter, and, on the other hand, may be used, due to their shape, in positioning the turbulence element exactly where planned on the rotor surface.
Another problem the present invention solves relates to the wear of the turbulence element, and the support thereof discussed already above in connection with the prior art rotor shown in
In accordance with a further preferred embodiment of the invention the rotor surface at the positions where the turbulence elements are to be fastened is provided with anchoring means to which the turbulence element is fastened. Further, the turbulence element is provided with a cavity into which the anchoring means fits when the element is positioned on the rotor surface.
In accordance with a still further preferred embodiment of the invention the rotor surface at the positions where the turbulence elements are to be fastened is provided with at least one projection per each position to which the turbulence element is fastened. Naturally the turbulence element is, then, provided with a cavity into which the at least one projection fits when the element is positioned on the rotor surface.
These and other embodiments of the present invention are discussed in more detail in the following by referring to the appended drawing figures of which
a-9e illustrate a few preferred embodiments of the rotor surface configurations at the areas where the turbulence elements are to be fastened.
In accordance with
When considering the required configuration/curvature in view of manufacturing or forming of the areas 12 having the different configuration/curvature, it can be concluded that in such a case that the rotor is cylindrical, the radius of the area 12 should preferably be at least the radius of the largest rotor cylinder in the series of pressure screens. In that case all rotors except the largest one should be machined/formed at the positions where the turbulence elements are supposed to be located. If the radius of the area 12 is made larger, all rotors have to be machined/formed. Depending on the machinery used for the machining it may be easiest to machine the turbulence element seats i.e. the areas 12 flat or planar i.e. having an infinite diameter. However, it has to be understood also that, particularly, if the rotor is manufactured by casting, it is possible that the surface of the rotor is provided with projections having a curvature smaller than the rest of the rotor.
In accordance with the embodiment shown in
In accordance with
Preferably, but not necessarily, the side walls 36′ and 36″ of the turbulence element 30 are provided with holes 40 opening into the anchoring cavity 32. The anchoring means 20 are preferably provided with tapped holes 22 (see
Also other means for fastening the turbulence element than screws may be used. An example is a locking pin that is pushed through a hole in the side wall of the turbulence element in a blind hole or a through-bore in the anchoring means. The locking pin may extend at a certain distance (corresponding that of a screw) inside the anchoring means or it may extend through the anchoring means into a hole in the opposite side wall of the turbulence element. When using the locking pin the hole ends in the turbulence element side walls should preferably be closed by means of small threaded covers or by a small weld dot, which may be drilled open when the turbulence element needs to be replaced. Another option is to arrange the locking pin to extend from the first side wall of the turbulence element to the second side wall thereof, whereby a small weld dot at either end of the pin is sufficient to lock it in place.
Yet another means of fastening the turbulence element is to arrange a blind hole at an end of the anchoring means, and a corresponding stationary pin at an end of the anchoring cavity of the turbulence element. The other end of the turbulence element could be attached to the anchoring means by a removable pin or screw.
The second end wall 158 of the turbulence element (best shown in
It should be understood that also in this preferred embodiment of the invention an essential feature of the invention is that the bottom surface surrounds entirely the opening into the cavity thus forming a continuous rim for the opening.
The turbulence element 30′ is installed on the rotor as explained in the following by referring to
Referring to the embodiment of the present invention discussed in
It should also be understood that the embodiment discussed in
All the embodiments discussed above are based on providing the rotor surface at the positions where the turbulence elements are to be fastened with a smooth rotor surface area having a curvature similar to all rotor sizes. However, another option is to provide the rotor surface with a non-smooth surface configuration at the areas where the turbulence elements are to be attached. By arranging the configuration such that it is equal for all rotor sizes, only one type of turbulence element is needed for the entire rotor series. Thus the surface configuration may comprise machined or otherwise arranged grooves or depressions in the rotor surface which assist in positioning either the anchoring means or the turbulence element on the rotor surface. The surface configuration may also comprise ridges or protrusions, which are arranged on the rotor surface either alone or together with grooves or depressions. The advantage in arranging ridges or protrusions on the rotor surface is that the ridges or protrusions not only aid in positioning the turbulence element, or anchoring means, on the rotor surface but also may, if desired, facilitate in attaching the turbulence element or the anchoring means on the rotor surface, as the fastening may be done in non-radial direction, and directly between the turbulence element and/or anchoring means and the rotor surface i.e. the ridges or protrusions thereon. In other words, it is possible to arrange, or to machine, the ridges or protrusions to act as the anchoring means needed in the earlier embodiments of the invention. In fact, it is as simple as providing the protrusions with an appropriate shape matching the interior cavity of the turbulence elements, and means for attaching the turbulence elements thereto.
a-9f show a few preferred embodiments for the surface configuration options. The configuration of
In view of the above, it should be understood that the simplest embodiment of the present invention is a rotor having a different surface configuration at the areas where the turbulence elements are supposed to be attached than the rest of the rotor surface, and turbulence elements having a complementary surface configuration at the bottom surface thereof. The turbulence element may be fastened on the rotor surface by means of welding or by any other known means of fastening. Thus the various options for the different surface configuration start from a smooth or planar or flat surface, and end up to a surface having anchoring means i.e. means in which the turbulence element may be fastened. Thus also there are options for the bottom surface of the turbulence element i.e. the bottom surface may be smooth or planar or flat, it may be grooved, or it may have a cavity for the anchoring means of the rotor surface, just to name a few alternatives. In fact, a properly designed groove is considered a cavity for anchoring means. Thus the anchoring means may be either part or parts that is/are separately attachable on the rotor surface or material parts of the rotor shell extending radially outside the rest of the shell outer surface.
In accordance with a yet one more preferred embodiment of the present invention the rotor surface is provided with either the grooves, depressions or protrusions discussed above in connection with
This far the manufacture of the rotor has not been discussed. However, the manufacture of the rotor relates to the invention, as different ways of manufacturing give different opportunities to manufacture the surface configuration of the areas where the turbulence elements are supposed to be arranged. There are in principle two options to manufacture the rotor. The first one is casting the rotor whereafter, depending at least on the quality of casting and on the position at a mill where the rotor is to be installed, the rotor surface may be machined more or less smooth. Now, the casting of the rotor makes it possible to provide the rotor surface with the surface configuration required by the areas where the turbulence elements are supposed to be arranged. Thus the normally round rotor surface may be provided with both depressions and protrusions, i.e. grooves, dents, ridges, bulbs etc. when casting the rotor. The casting makes it possible to arrange the areas to have a curvature smaller than that of the rest of the rotor surface i.e. to provide the rotor surface with a protrusion. After casting the rotor surface may again, and most often will, be machined to improve the surface quality.
The second option to manufacture the rotor is rolling the rotor from sheet metal having a desired thickness, and welding the ends of the rolled sheet together to form a rotor shell. Normally the rotor manufacture continues by welding end caps with bearing units to the axial ends of the rotor shell. However, there are some rotor types where one or both ends of the rotor are not closed, but the attachment of the rotor shell on its shaft is performed in some other appropriate way. Anyway, in view of the present invention, the attachment of the rotor on its shaft does not play any role. As to the surface configuration at the areas where the turbulence elements are to be arranged, a rolled rotor does not give as many opportunities as the cast rotor. In other words, there are only two further options, i.e. one is to machine one or more depressions of desired shape in the rotor surface, and the other is to press the depressions in the rotor surface. However, as the pressing may be done, in principle, from both sides of the rotor shell, it is possible to make protrusions extending radially outside the remaining rotor surface. However, the shapes of the protrusions made by pressing are more limited than that made by casting.
It should be understood that the above description discusses only a few preferred embodiments of the present invention without any purpose to limit the invention to the detailed structures disclosed above. Thus it is clear that, for instance, the shape, size and number of the turbulence elements on the rotor may be whatever the designer of the rotor sees practical. Also, the shape and size of the rotor may be whatever required by the specific application the rotor is designed for. Thus either the entire surface of the rotor or only a part (preferably, but not necessarily, in axial direction) of the rotor surface may be provided with areas having a certain surface configuration discussed in the present invention. In other words, for instance one longitudinal section of the rotor surface may be machined in the manner described above, whereas the other section/s is/are, if needed, provided with turbulence elements attached by some other means on the rotor surface. Further, it is clear that the rotor of the invention may be used in connection with either inflow, or outflow screens. And, finally it has to be pointed out that the word ‘rotor’ covers above and in the claims all such means arranged in a screening device of pulp and paper industry that, on the one hand, creates turbulence in the fibre suspension to be screened, and, on the other hand, subjects the screening means, like a screen cylinder, to pressure pulses. Thus, as long as the ‘rotor’ is in relative movement in relation to the screening means, the turbulence creating and pressure subjecting means are called by the word ‘rotor’. In other words, also stationary turbulence creating and pressure subjecting means arranged in cooperation with a rotating screening means are called ‘rotors’.
Number | Date | Country | Kind |
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20085967 | Oct 2008 | FI | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FI09/50647 | 8/5/2009 | WO | 00 | 8/3/2011 |