The present invention relates to an arrangement according to the preamble of claim 1 for protecting the bearings of two concentrically mounted rotors in, e.g., a mill, mixer or other similar equipment.

The industry uses a variety of different mills, mixers, gas scrubbers and other apparatuses incorporating two concentrically mounted elements rotating in opposite directions to perform a milling, mixing or other similar function. These elements typically comprise interplaying- ly arranged rotors whose perimeters are provided with rings carrying impact stop elements. The purpose of the rotors is to subject the material to be milled or mixed to high acceleration imparted by maximally large speed differences between the rotors, or alternatively, to a great number of local impacts of high momentum. If such an apparatus, conventionally called a pin mill, is used for gas scrubbing, for this purpose water or other scrubbing liquid is atomized into small mist particles thus generating large liquid surface areas for mass transfer, whereby the binding of small dust particles in water for instance is promoted.

As noted above, such pin mills can be used for a variety of different applications, whereby their interior spaces are almost invariably filled with such materials to be processed that contain abrading particles. Obviously, this causes wear on the impact surfaces of the rotors in a manner that is considered normal, and even if the rotors are made from wear-resistant materials as is cus¬ tomary, they still remain the most inexpensive components of the apparatus. Components more expensive than rotors are in fact the bearings and electric drive of the apparatus.

Concentric rotors running in different directions are conventionally rotated on shafts of which one is con¬ nected through one side of the apparatus to one of the rotors, while the other rotor is driven through the oppo- site side of the apparatus. In such an arrangement, it is problematic to adapt the infeed flow into the apparatus to occur through the center of the rotor system so that the infeed flow would be distributed symmetrically in all radial directions and the apparatus would run dynamically balanced under all conditions.

To overcome the above-described difficulty of feed arrangement, apparatuses have been manufactured having the shafts of their rotors taken into the apparatus from the same side of the apparatus with the help of a coaxial arrangement of the shafts. Thus, a free location can be found for the inlet in a manner unobstructed by the shafts. Furthermore, such a single-sided location of the drive shafts makes the use, maintenance and service of the apparatus easy. However, a number of other problems are involved with the coaxially on bearings mounted shafts. The greatest disadvantage in these constructions is related to the access of wear-causing matter into the bearings resulting in rapid failure of the bearings and frequent need of service. Obviously, the bearings can be protected against abrading particles as well as dirt and water using different kinds of seals. However, it is con¬ ventionally known that lip seals and similar seal elements contacting moving elements such as rotating shafts are deteriorated rapidly under wearing conditions already after a few hours of use even if a moderate amount of small particles has access to the seals.

It is an object of the present invention to achieve an arrangement which is in an extremely reliable and uncom¬ plicated manner capable of keeping the seal, and thus, also the bearings, clean under conditions associated with

the processing of dust, slurry or other abrading materi¬ als by means of the above-mentioned pin mill.

The goal of the invention is achieved by virtue of adapting, close to the plate located in the rotor housing of the apparatus on the bearing side, that is, the rotor housing bottom plate, a centrifugally acting scraper vane plate that blows material landing on the bottom plate radially outward and additionally adapting close to the scraper vane plate a labyrinth seal via which an air flow induced by the vacuum generated by the scraper vane plate is passed to the scraper vane plate.

More specifically, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits.

To implement the method, no major need is found to per¬ form essential modifications in existing equipment as the scraper vane plates and the labyrinth seal can be adapted with relative ease in conventional pin mills and similar apparatuses. The service life of the bearings is improved significantly, because the material flows can be directed away from the bearings and seals thus permitting their operation under clean and well-lubricated conditions. Owing to the extended seal life and resulting cleanliness of bearings, whereby also the service life thereof is ex- tended, it will now be possible to construct pin-mill- type equipment for continuous use in the process indus¬ tries. Due to the simple and inexpensive concept of the protection arrangement, it will supplant the use of more expensive seal embodiments. By virtue of the present in- vention, the operating costs of pin mills will be lowered drastically as the replacement interval of bearings will be extended. Thus, the operating economy of the entire

process served by the pin mill will be improved due to the reduced number of process shut-downs.

In the following the invention will be examined in greater detail with reference to the appended drawings in which

Figure 1 is a partially sectional side view of an appa¬ ratus equipped with the arrangement according to the invention; and

Figure 2 is a more detailed view of the arrangement illustrated in Fig. 1.

Referring to Figs. 1 and 2, the apparatus shown therein is a two-rotor pin-mill-type mixer having five inter- playing vane rows 3 - 7. The rotors 1, 2 are mounted on coaxially adapted shafts 8, 9, respectively, and the feed opening 10 of the mixer is disposed to the opposite side of the apparatus relative to the shafts 8, 9. The shafts 8, 9 are mounted rotatable on bearings with respect to a shaft support housing 11 and each other. As the construc¬ tion of the bearing arrangement is not related to the invention, its closer description will be omitted herein. The shafts 8, 9 and the rotors 1, 2 are rotated by sheaves 12, 13, which are mounted on the shafts and are driven by a V-belt drive. In the illustrated apparatus the lower rotor 2 is mounted on the outer drive shaft 8, while the drive shaft 9 of the upper rotor 1 is passed through the outer shaft 8 and the center of the lower rotor 2 to the upper rotor 1. Of the vane rows, the innermost row 2, the center row 5 and the outermost row 7 are connected to the upper rotor 1, while to the lower rotor 2 are connected the second row 4 and the fourth row 6, which are adapted to run interplayingly in the inter- row spaces remaining between the vane rows of the upper rotor. The rotors 1, 2 are enclosed in a rotor housing

comprising a bottom plate 14, through which the shafts 8, 9 are passed, and a top plate 15, to which an infeed opening 10 is attached, and a side wall 16 encircling the rotors 8, 9 at a distance from their perimeter.

The above-described apparatus is operated by rotating the rotors 8, 9 either in the same direction with a speed differential between the rotors or in opposite direc¬ tions. The materials to be mixed are passed via the infeed opening 10 to the center of the rotors, wherefrom the materials are accelerated radially by virtue of the rotational movement of the rotors. when the materials to be mixed undergo collisions with the fast moving mixer vanes, a strong mixing action is achieved and simulta- neously the mixture is blown by the vanes against the side wall 16 and will subsequently be removed from the apparatus via a discharge opening made to the side wall 16. As the vanes can generate an extremely strong blower effect, a vacuum is generated at the infeed opening.

Although the particles of materials being mixed are extremely vigorously ejected radially outward, a portion thereof may fall on the bottom plate 14 of the mixer chamber and thus find a way to the seals and bearings of the drive shafts 8 and 9. By virtue of the present invention, this shortcoming can be prevented. Referring to Fig. 2, the entry of the shafts 8, 9 on the bottom plate 14 into the mixing chamber is shown. To the lower rotor 2 is attached a scraper vane plate 19 carrying scraper vanes 18 attached thereto with the vanes aligned parallel to the radius of the plate. Between the lower surface of the scraper vane plate 19 and the bottom plate 14, in the radial direction between the row of the scraper vanes 19 and the shaft entry point on the bottom plate 14, is adapted an annular labyrinth seal 20 with one end of the labyrinth opening in a slot 21 provided at the shaft entry point and the other end just behind the

scraper vanes. Thus, the labyrinth seal 20 forms a path for an air flow from the ambient atmosphere to the mixer chamber, behind the scraper vanes 18. To the stationary shaft support housing 11 is formed a compressed air channel 23 exiting in an interrotor space 24 remaining between the rotors 1 and 2, said space being sealed by means of two labyrinth seals. Said interrotor space 24 exits in the slit formed between the rotors l and 2 enclosed by the rotor housing 15 - 17. Further along the air path, the upper surface of the scraper vane plate 19 is provided with channels 22 passing from the interrotor space 25 to the space remaining between the rotor 2 and the rotor housing bottom plate 14, in front of the scraper vanes 18. Facing the bottom plate, scraper vanes 17 of the upper rotor are respectively adapted on the perimeter of the upper rotor 1.

Accordingly, the rotor wheels of the apparatus have both the lower rotor 2 and the upper rotor 1 equipped with scraper vanes 17, 18 dimensioned to keep the rotor housing bottom plate 14 clean during the operation of the apparatus. Simultaneously, compressed air is supplied via the compressed air channel 23 into the apparatus for the purpose of providing indirect cooling of the seals and bearings. The entering compressed air is partially discharged via the radial channels 22 made to the upper surface of the scraper vane plate 19 into the space 26 between the rotor housing bottom plate 14 and the lower rotor 2. This flow of compressed air sucks ambient air by its ejector effect from the slit 21 via the labyrinth seal 20. Furthermore, the flow generated by the rotation¬ al movement of the rotors 1 and 2 generates an additional air flow via the same path through the labyrinth seal 20 due to the blower effect of the vanes rows 3 - 7 of the rotors.

In the above-described embodiment, the two scraper vane rows 17 and 18 provide continuous evacuation of material leaked into the lower space 26 of the pin mill, and the flushing air flow induced by two different mechanisms continuously expels the leaked material along with the flushing air flow toward the rotor housing perimeter on which the discharge opening for processed material is disposed. Thus, the access of abrading materials to the seals and bearings is prevented by means of the two scraper vane rows 17 and 18 complemented with the radially passing flow of flushing air.

The improvement in the service life of the bearings has been investigated experimentally in an apparatus of the above-described kind. An apparatus equipped only with the outer row 17 of scraper vanes, but lacking the inner row 18 of scraper vanes on the lower rotor 2 as well as having no flushing air flow through the labyrinth seal 20 nor ejected air via the channels 22 - 25, achieved a bearing service life of only 400 h in a typical applica¬ tion for the dispersion of coating mix pigments.

After modifying the bearing/seal protection arrangement as described above, the service life of the bearing/seal combination was extended to better than 16,000 h.

Obviously, the air channels of the protection arrangement can be implemented in a number of different ways. Further, although the number of scraper vanes may be selected freely, obviously at least two of them should be disposed in any vane row to keep the system dynamically balanced during running. While the mounting of the scraper vanes on the rotors may be varied, a small gap must be provided between the scraper vane edge and the rotor housing bottom plate to avoid unnecessary mechani¬ cal wear. The row of scraper vanes 18 mounted on the lower rotor 2 is advantageously adapted to run as close

as possible to the center of the apparatus so that the vanes run essentially near the smaller radius of the space remaining between the rotor housing bottom plate 14 and the lower rotor 2. In practice this means that the row of scraper vanes 18 is then radially aligned to run approximately at the first vane row 4 of the lower rotor 2. Obviously, the flushing air flow to the row of the inner scraper vanes, which here is described to occur as suction-induced ambient air flow, can be replaced by a compressed air flow routed to the vanes.