Method and apparatus for pumping high consistency medium

Abstract: The present invention relates to a centrifugal pump for pumping of stiff and air containing liquids or various suspensions, and is characterized by considerable extension of rotor (13) of the impeller (10) of the pump so that rotor (13) extends far into the space (4) containing the material being pumped.

Description: The present invention relates to an improved method and apparatus for pumping media which are stiff and contain air, i.e. fluids or liquids or various kinds of suspensions. The method and apparatus in accordance with the present invention are especially suitable for pumping medium consistency (8 to 20 %) or high consistency (over 20 %) fiber suspensions in the pulp and paper industry. The method part of the invention especially relates to methods of eliminating or minimizing the disadvantages brought about by the air and/or gases in the medium and which are absorbed by the medium. The apparatus part of the present invention especially relates to the structure of an impeller being used in a centrifugal pump. There are several known centrifugal pumps that have been used and are still used in the wood processing industry for pumping fiber suspensions. The largest group is formed by basically conventional centrifugal pumps, which are modified by some non-essential changes, in order to enable them to pump pulp. An example of this kind of changes is the installation of inducers in front of the actual impeller to facilitate the flow of pulp to the impeller of the pump itself. In spite of a number of attempts and slight structural changes it is not possible to use the above described pumps for pumping pulp having a consistency which exceeds 6-8 %. This is, on one hand, because of the increase of the gas content of the pulp simultaneously with an increase of the consistency, whereby the air or gas bubble which accumulates in the center of the impeller prevents the pulp from entering the impeller area, and, on the other hand, because of the poor flowing abilities of the high consistency pulp in the suction duct of the pump or from the space containing pulp to the suction duct of the pump. A development in the late 1970s resulted in the so called MC™-pump (MC = medium consistency), which is characterized in that a rotor is mounted in the suction opening of the pump most commonly extends through the suction duct and extends to some extent inside the mass tower, drop leg or the like. The rotor is used for loosening the bonds between fibers of the suspension by feeding energy in the form of a field of shear forces to the pulp, whereby the pulp more easily flows towards the impeller of the pump. The aim with these pumps was to enable pulps to be pumped with a consistency of 8 to 15 %. The main problem appeared to be the poor flowing abilities of pulp of said consistency in the suction duct of the pump and consequently, the invention was aimed at how to make the pulp flow in the suction duct towards the impeller. Various embodiments of such a pump are illustrated, for example, in the specifications of US Patents 4.410.337, 4.435.193 and 4.637.779. All said embodiments are characterized in that they both fluidize the pulp being pumped and that gas, mostly air, which is harmful in the further processing of the pulp and in the pumping is discharged. The fluidization is carried out by blades of rotor inside a relatively long suction duct of the pump, the blades being substantially located in radial planes and mainly axially, although in some embodiments also helically wound rotor blades are used to some extent. The separation of gas to the hollow center of the rotor takes place in all illustrated embodiments in front of the impeller due to the centrifugal force, from which gas is further discharged through the openings in the rear plate of the impeller most commonly by means of the suction caused by a vacuum pump. It may be pointed out that of the structural details of the MC-pumps in the prior art, the rotor in all said publications extends to some extent to the space containing pulp. The most detailed description is given in the most recent publication, US 4.637.779, in which the rotor is said to extend about 3 inches, in other words about 75 mm, into said space. This measure is really considered to be the maximal dimension, as the production mainly includes pumps, the rotors of which do not extend even that far in the suction chamber. The maximal dimension may be said to be about 0.5 x diameter of the suction duct, which ratio in reality reduces as the diameter of the suction duct increases. In practice the suction duct of even the smallest MC-pump has a diameter of about 150 mm, whereby said ratio is reached. When the diameter of the suction duct increases therefrom the actual extension of the rotor to the pulp chamber remains basically the same. Now that a lot of practical experience has been gained with the MC-pumps it has been noted that the pumps which operate really well even at the consistency of about 15 %, may be further developed. Originally, in the development of the MC-pumps, the basic assumption was that the most significant hindrance in pumping high consistency pulp was the friction between the pulp and the wall of the suction duct, and attempts were made to eliminate the friction by fluidizing the pulp in the suction duct. A second problem appeared to be the discharge of pulp from the suction chamber or from the drop leg to the suction duct, because high consistency pulp tends gradually to fill the openings surrounded by sharp edges, in other words also the suction opening. Consequently, this resulted in the extension of the fluidizing rotor to some extent into the inside of said chamber in order to allow the rotor to remove the fibers and fiber flocs possibly stuck on the edges of the suction duct and to prevent the clogging of the suction opening. However, the researchers of the MC-pump were afraid to give up the old self-evident guidelines that the flow of the material being pumped should be as laminar as possible when reaching the pump so as to avoid losses in flow, e.g. pressure loss. References to this thinking may still be found, for example, in the above mentioned US Patent specification 4.637.779, in which in column 2 lines 24-30 it is stated that an apparatus in accordance with the prior art technique generates in front of the suction opening of the pump and around it a "doughnut"-shaped, turbulent and at least partly fluidized zone, which is really located in close proximity of the edges of the suction opening of the pump. The teaching of said US Patent specification is based on the old guidelines of the pump research and development and comes to the conclusion that said phenomenon disturbs the pumping and therefore the tips of the rotor blades of the MC-pump extending to the inside of the mass tower or the like of the MC-pump are bent so as to subject the pulp to a force component directed towards the suction opening of the pump. The use of said solution is based in the specification on the belief that by doing so it is possible to create pressure on the inflowing pulp, which pressure again facilitates the discharge of gas from the front side of the impeller of the pump. There is, however, a good reason to doubt the operation of the apparatus and the exactness of the above mentioned conclusions at least when higher consistencies within the range 6 to 20 % mentioned in the specification are concerned, because pumps of the described type have not been brought onto the market. On the contrary it may be maintained that in the embodiment of said US patent a hollow pulp arching is easily formed at higher pulp consistencies of pulp in front of the tips of the rotor blades, because the object is especially to try to prevent the circulation of the pulp in the pulp vessel, in other words the pulp is drawn "as discreetly as possible" directly from the vessel to the pump. This problem is, however, encountered only with pulp having the consistency of 10 to 15% depending on the physical and chemical qualities of the pulp. While the consistency grew higher a problem in the discharge of pulp from the mass tower or the like to the suction duct was encountered. When pumping medium consistency pulp said problem had already earlier been experienced even though the pump and its rotor were able to treat pulp in the suction duct and further therefrom with sufficient efficiency. The cause of this problem is both arching of the pulp in the pulp space, in other words the formation of an empty arch-like space in front of the suction opening of the pump, and the friction between the pulp and the walls of said space, which slows down the downward flow of the pulp. It has been noted in the experiments performed that an especially efficient method of preventing the arching of the pulp and reducing the friction between the pulp and the wall of the pulp space is to ensure circulation of the pulp in the pulp vessel. This kind of circulating movement may be considered to generate enough turbulence to the pulp layer close to the wall of the pulp space so that small pulp flocs are generated when larger pulp flocs are broken, which smaller flocs operate in a way as bearing balls between the pulp and the wall, whereby the friction between the pulp and the wall reduces and the pulp flows downwards faster and more easily. When pumping experiments have been performed with an MC-pump in accordance with the prior art simulating the mill conditions it has been noted that gas flows through the lumpy pulp in the pulp space into the suction opening of the pump, which has, as it was already mentioned above, a "doughnut-shaped" fluidized ring along the edges of the suction opening and which has in a way an open center part, whereby the lumpy pulp and even the air space in the upper part of the pulp vessel may be directly subjected to the suction of the pump and the gas discharge system and even to the air space in the upper part of the pulp space. Thereby gas flows both from the spaces between the pulp lumps and when pulp with a very high consistency is involved, from the upper part of the pulp space, the air space, to the pump. The fact that this phenomenon has not been observed before is partly due to the fact that in the test pumping situations, on one hand, there is only little air in the pulp and, on the other hand, there are only small flocs in the pulp up to a consistency of 15 %, whereby the air spaces between the pulp flocs are small and they do not reach the surface of the pulp space. This problem occurs only at the stage when the amount of free water (water not stuck in the fibers) in the pulp reduces so much that it does not have enough time to filter to the bottom part of the mass tower or the like to form a layer of water and pulp lumps there. No exact consistency limit or the like may be given to the appearance of said problem, because it depends on very many factors. For example, the consistency of the pulp itself, the length of the fibers of the pulp, the speed, at which the pulp flows downwards in the tower, etc. The problem, however, appears soon after the consistency of 10 % is exceeded. When experiments simulating the mill conditions have been performed with the pump in accordance with the present invention, it has been noted that when the rotor of the pump has been extended far enough in the suction chamber and especially when the end of the rotor has been provided with foils intensifying the circulating flow which is heavily criticized, for example in US Patent specification 4.637.779, the amount of gas removed by the pump from the medium has reduced substantially. In order to eliminate or minimize the disadvantages of the prior art MC-pumps a new type of rotor arrangement for the centrifugal pump has been developed, which fluidizes the pulp as the earlier MC-pumps, but which extends the fluidization field far in to the suction chamber, and an embodiment of which is further characterized in that the flow cross-sectional area of the suction opening of the pump remains as open as possible thus allowing the pulp to flow also through the center part of the suction opening towards the impeller, whereby maximal efficiency is gained of said flow cross-sectional area. In order to bring about said operation, an embodiment of a rotor in accordance with the present invention is provided with special blades which throw the pulp radially outwards which blades, when located far enough from the inlet end of the suction duct do not substantially reduce the amount of pulp flowing into the suction duct, but only ensure that the supply of pulp coming to the rotor for fluidization remains continuous and sufficient. The pulp in the chamber is subjected to such a powerful and widely extending field of shear forces by the rotor in accordance with the present invention, which rotor is possibly provided with auxiliary blades extending far into the suctioning chamber, that the entire portion of pulp in the chamber surrounding the tip of the rotor and the suction opening becomes fluidized, whereby the suction of both the pump and also the gas discharge system is directed only to the fluidized pulp and not through the air cavities between the pulp lumps upto the air space in the upper part of the pulp chamber. The form of the fluidized area might be described as apple-shaped, whereby the suction opening of the pump is surrounded by a large fluidized pulp zone, which is completely closed from gas. The operation of the method in accordance with the present invention is thus based on the fact that pulp flow is circulated in the pulp vessel around the suction opening by the rotor extending far enough inside the suction chamber in such a way that a large pulp zone practically acting like water is formed preventing exterior gas which is not bound with fibers from entering the suction duct of the pump. The circulating pulp flow breaks pulp lumps, whereby the gas as lighter flows upwards from the caveties between the lumps and is discharged to the upper part of the chamber. Thus the only gas, which flows into the pump is the gas stuck to the fibers as micro bubbles, a gas which is separated by utilizing the centrifugal force in the suction duct of the pump in front of the impeller. The method and apparatus in accordance with the present invention are characterized by the features as disclosed in claims 1 and 5. The method and apparatus in accordance with the present invention is described more in detail below, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a sectional side view of a prior art MC-pump and a fluidized pulp zone generated by it; Fig. 2 is a sectional side view of an embodiment in accordance with the present invention together with an alternative auxiliary device and a fluidized pulp zone generated by them; Fig. 3 is a sectional side view of a second embodiment of the apparatus in accordance with the present invention; and Fig. 4 is a sectional side view of a third embodiment of the apparatus in accordance with the present invention. Fig. 1 shows that a zone 2 of fluidized pulp generated by a prior art MC-pump 1 extends in a very restricted annular area surrounding the tips of blades 3 of the rotor. Thus, there is lumpy pulp 5 in the outer region of the pulp space 4 or immediately in front of the rotor, which pulp has air/gas spaces 6, wherefrom the suction of the pump draws gas 7 directly to the pump in front of the impeller of which a gas bubble 8 is generated in a known way. The use of this particular arrangement is based on the fact that previously the only problem was considered to be the transfer of the pulp from the suction duct to the pump, and thus it was a mere waste of energy and against the earlier designing guidelines of the centrifugal pumps to extend the rotor far into the suction chamber in order to fluidize pulp without any particular explicable reason. By extending the rotor from the suction opening slightly into the pulp chamber it was ensured that fibers or pulp flocs were not able to attach to the edges of the suction opening and gradually to clog it. Fig. 2 illustrates an impeller 10 of a centrifugal pump in accordance with an embodiment of the present invention, which mainly comprises a rear plate 11 with pumping vanes 12 and a rotor 13 mounted on it. As may be seen in the drawing, the impeller 10 is located in the housing 14 of the centrifugal pump in such a way that rotor 13 extends substantially outside the suction opening of the pump inside the pulp space 4. It has been discovered in the performed experiments that the distance the rotor extends inside space 4 containing pulp should at least correspond to the diameter of suction opening 15 before the above described advantages are gained by the present invention. One advantageous alternative to arrange rotor 13 extending further to the space containing the material being pumped is to shorten suction duct 15 of the pump. The suction duct in the prior art arrangements always consists of two parts, namely of a suction opening, which is a member of the pump and which is conventionally surrounded by a cylindrical part with a flange, a so called suction neck, and a suction pipe 19 attachable, for example, to the wall of the mass tower, to which pipe the pump is mounted with the flange. It has been proved possible to remove the cylindrical part surrounding the suction opening of the pump almost completely so that mounting holes are arranged to housing 14 of the pump for suction pipe 19 to be mounted to the wall of the mass tower. Thus, the production of the pump is also simplified, because there is no more need for the provision of a cylindrical suction duct projecting from the housing of the pump, and terminating in a flange. Further, in some embodiments it is also possible to exclude the suction pipe being mounted to the wall of the mass tower, for example, when the pump is mounted to the bottom of the mass tower or the drop leg and the discharge opening pointing to the side. The pump is then mounted directly from the housing to the mass tower, whereby one, in practice irrelevant, connecting member has been managed to eliminate. Rotor 13 of the impeller of the centrifugal pump in accordance with the present invention comprises blades 16 projecting from rear plate 11 of the impeller to the suction duct of the pump, which blades are advantageously, but not necessarily, extensions of the actual pumping vanes 12, for example, in such a way that if the impeller has six vanes altogether, three of them extend as blades 16 of rotor 13 through suction duct 15 to pulp space 4. Blades 16 of the rotor are advantageously and substantially axial and additionally in radial planes. In some cases it is, however, necessary to deviate from the above described arrangement, if it is desired that pulp is gently fed towards the impeller, whereby the blades are deviated from the axial direction to the desired extent. Respectively, the blades may also be deviated from the radial direction, for example, when the blades are desired to feed pulp to the space inwardly thereof. According to the drawing, blades 16 of the rotor are connected from their tips to each other by a connecting member far in the pulp space so that the head part of the rotor when rotating forms a conical or rotationally paraboloidal or the like surface which is advantageous relative to the flow. The purpose of this is to prevent the attachment of pulp particles to the tips of the rotor blades. The head part, in other words point 17 of rotor 13, of which point the blades are joined with each other, may be closed, as shown in Fig. 2, in other words axial flow may be prevented within the radius shorter than the rotational radius of the blades of rotor 13. On the other hand, blades 16 of rotor 13 may be joined also in such a way that the end view is generally star-like in shape (Fig.3), which allows axial flow excluding the area closest to the axis of the rotor. In some cases it is possible to join the blades of the rotor with each other over a longer part or even for the whole length and either directly or by means of an axial connecting member. Fig. 4 illustrates as a third alternative embodiment an arrangement, in which blades 16 of rotor 13 are connected to each other by an annular or the like member forming a continuous rim 20 which may naturally be located on any diameter defined by blades 16 of rotor 13 or also even arranged by means of an interim member completely on the outer or inner side of blades 16 if so desired. Thereby the flow towards the suction duct of the pump becomes possible also along the center line of the rotor. These connecting members may also be several in number, if rotor 13 is especially long. Similarly, the connecting member does not necessarily have to be located in the area near the head of the rotor, but it might be more advantageous, according to the strength of materials, if the connecting member were located at certain distance from the tips of the blades and towards the impeller. Yet, it must be maintained that when the rotor extends far enough into the pulp space the shape or connecting method of the head part of the blades lose their meaning. This is explained by the fact that even downstream of the head part of the rotor, the pulp has good space and time to flow through the openings between blades 16 of rotor 13 also to the inside of the rotor, whereby the whole cross-sectional area available in the suction duct becomes efficiently utilized, which cannot be said of the prior art MC-pump arrangements. This utilization or more exactly the transfer of pulp to the middle part of the rotor may be intensified by bending the blades of the rotor slightly inwards thus creating a drawing effect in the part between the head of the rotor and the front part of the suction opening, whereby the filling of the middle part of the rotor with pulp may be ensured. In this kind of embodiment the outer edge of the rotor blades has been slightly bent towards the rotational direction of the rotor, whereby the material being pumped is subjected by the blades to an inwardly directed radial force component. Fig. 2 also illustrates special auxiliary blades 18 for circulating pulp, which in the embodiment of the drawing are mounted on the connecting member joining blades 16 of rotor 13 together. The direction of the auxiliary blades 18 is arranged so that they generate a strong radial motion component in the pulp, by which pulp is fed along the walls of the mass tower (4'), as shown in said drawing by arrows A. The non-fluidized pulp respectively flows in the middle part of the tower rapidly downwards thus reaching the fluidization zone of the rotor extending far to the pulp space, whereby a part of the pulp flows inside the rotor and that way into the suction duct of the pump and another part of the pulp flows back to the circulation. The circulation has an especially strong effect at the bottom part of the pulp space, to which part a zone of standing pulp tends to accumulate. The pulp circulating at the bottom part creates therein a stronger turbulence relatively speaking, than in other parts of the pulp space due to the smaller mass volume, to which the circulation is directed. It is also possible to provide the embodiments of Figs. 3 and 4 with radial pumping blades 18. For example, in the embodiment of Fig. 3 by bending the tips of the blades of the rotor radially to effect better pumping or by adding a separate blade to each blade tip for radial pumping. Respectively, in the arrangement in accordance with Fig. 4 it is possible to add the radial blades either to the rim 20 connecting the blade tips of the rotor or directly to the blade tips or by arrange already at the casting stage the required auxiliary blades 18 to the blade tips. It is naturally clear that blades 18 may also extend radially to a longer diameter than blades 16 of the rotor or they may also be located very close to the axial line of the rotor, if so desired. These facts are determined mostly on the basis how strong a circulation effect is desired to the pulp space. Another embodiment worth mentioning is an arrangement, in which the blades in the area of the head part of the rotor are substantially axial, in other words, they do not draw pulp at all inwards like the arrangement described in US Patent 4,637,779, mentioned already as prior art, does cause a hollow pulp arching to be formed in front of the rotor. When the tips of the blades are axial, they tend firstly to fluidize more effectively the lumpy pulp and, secondly, bring about an effect which is almost on its own sufficient to circulate the pulp. When approaching the suction duct the blades bend to an angled position with the axial direction in such a way that the pulp is subjected by them to an effect which gently feeds the pulp towards the pump. In the tests performed it has been observed that a centrifugal pump provided with an impeller in accordance with the present invention is able to pump pulp that has a 5 % higher consistency than the centrifugal pump in accordance with the prior art, the economic field of use of which has remained considerably below the consistency of 20 %, which may be exceeded by the pump in accordance with the present invention. On the other hand, if pulp with a consistency of below 20 % is being pumped, the energy consumption is lower, in other words the efficiency is considerably better due to a more open suction duct and due to the pulp being very efficiently fluidized already in the pulp space and also due to the fact that, practically speaking, hardly any gas may be absorbed through the caveties between the pulp lumps to the pump. It has also been discovered in testing the pump structures illustrated already above, that the amount of discharged gas is considerably and significantly lower compared with the prior art MC-pumps. Similarly, the reason therefor was already given above, which is the fact that such an intensive fluidization is carried out in the pulp space that no non-fluidized lumpy pulp remains in front of the center of the rotor in the pump, through the cavities of which pulp the gas would have been able to flow from the upper part of the pulp space to the pump. Thus it may be considered that a "liquid lock" is formed in front of the suction opening and to the sides thereof, which "liquid lock" does not allow gas to enter the pump. This corresponds in principle to the situation of pumping low consistency pulps when liquid has been allowed to filter to the bottom of the pulp space, so that when the surface of the liquid layer is higher than the suction opening of the pump no gas discharge problems occur. As may be observed from the above description a new method and apparatus has been developed for pumping medium and high consistency pulp. The apparatus is characterized by the rotor arrangement, which is already illustrated above in a number of embodiments and alternatives. The scope of invention is, however, not intended to be restricted by the above described most advantageous constructional solutions, the purpose of which is merely to show exemplary different alternatives for the realization of the present invention. Thus, the scope of the invention is restricted only by what is set forth in the accompanying claims. The present invention easily enables the provision of a degassing vacuum pump to the same shaft with the centrifugal pump or in advantageous conditions it is even possible to leave out the whole vacuum pump. Further, the arrangement in accordance with the invention enables the nowadays so desired lowering of the mass tower, because the discharge of the mass tower has become more reliable due to the fact that the method in accordance with the present invention enables the efficient prevention of pulp from arching in the tower in front of the suction opening. It is, yet, worth mentioning that, although the pump in each drawing has been installed having its shaft in a horizontal position, it is in some cases advantageous to arrange the pump to another angular position, whereby the shaft may be either in an inclined position or even vertically positioned. Also some special situations are possible, in which the pump is located above the pulp space in a hanging position relative to the motor.

Claims: A method of pumping high consistency pulp with a centrifugal pump from a space containing the pulp, characterized in subjecting the pulp to a sufficiently strong field of shear forces so as to fluidize the pulp and to form a continuous liquefied zone in front of and around the suction opening of the pump so that the suction of the pump acts merely on the fluidized pulp zone so that the flow of gas from cavities in the spaces between pulp lumps in the pulp space or from an upper part of the pulp space along or through cavities between the pulp lumps to the suction opening of the pump is prevented or minimized; and pumping the pulp onwards in a conventional manner. A method in accordance with claim 1, characterized in allowing the gas to be discharged along the cavities between the lumps upwards. A method in accordance with claim 1, characterized in forming a fluidized pulp zone by guiding the pulp to execute a strong circulatory movement in front of and on the sides of the suction opening of the pump so that the suction opening of the pump is from the side of the pulp space completely surrounded by the fluidized pulp substantially acting like liquid. A method in accordance with claims 1 - 3, characterized in pumping the pulp without the discharge of gas from the pulp being pumped, or by allowing the gas separating from the pulp in the suction duct to be discharged from the pump due to the pressure of the suction duct. A centrifugal pump for pumping high consistency pulp from a vessel (4') defining a space (4) for containing pulp, said pump comprising a housing (14), a suction opening (15) of circular or substantially circular cross-section and a discharge opening in the housing (14) and an impeller (10); said impeller including a back plate (11), one of more pumping vanes (12) on the back plate, and a rotor (13) having one or more blades (16) and projecting into the suction opening (15) of the pump; characterized in that the rotor (13) extends into said space (4) so that its distal end (17) is spaced from the suction opening in the inner wall of the vessel (4') by a distance which is at least equal to the diameter of said suction opening (15,19). A centrifugal pump in accordance with claim 5, characterized in that blades (16) of rotor (13) are joined by a connecting member at a point (17), the distance of which from the inner surface of the wall of space (4) containing material being pumped is at least as long as the diameter of suction opening (15) of the pump. A centrifugal pump in accordance with claim 5, characterized in that auxiliary blades (18) for forcing the material being pumped in a radial direction are arranged at the distal end of the rotor (13) to effect circulation of the material being pumped in a mass tower (4), which is provided as said vessel, and also to effect strong fluidization of the pulp in front of the head of rotor (13). A centrifugal pump in accordance with claim 5, characterized in that blades (16) of rotor (13) are interconnected by an annular or the like member (20) forming a continuous rim, whereby the center of the rotor is open at the connection point. A centrifugal pump in accordance with claim 5, characterized in that a connecting member is in the head part of blades (16) and the blades or likewise radially pumping members (18) are arranged into communication with it. A centrifugal pump in accordance with claim 5, characterized in that the connection point on the side of the suction opening (15) of the pump to the wall of mass tower (4) or the like is arranged to housing (14) of the pump without a suction pipe (19) arranged into mass tower (4) or a suction neck in housing (14) of the pump. A centrifugal pump in accordance with claim 5, characterized in that blades (16) of rotor (13) are joined with each other from the inner edges either directly or through a connecting member. A centrifugal pump in accordance with claim 5, characterized in that blades (16) of rotor (13) are in the head part substantially axial for bringing about an efficient fluidization and a circulation effect in the area of the mass tower, and towards the pump, blades (16) form an angle with the axial direction, which generates in the area of the suction duct an effect of feeding the pulp towards the pump.

Inventors: TIMPERI JUKKA, VESALA REIJO, VIKMAN VESA, TIMPERI, JUKKA, VESALA, REIJO, VIKMAN, VESA
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1989
Main classification: F04D7
IPC codes: F04D29, D21C9, F04D7