A method of and an arrangement for agitating fluid in a vessel

Abstract

 The present invention relates to a method of and an arrangement for agitating fluid in a vessel in the manner described in the preambles of claims 1 and 15. The method and the arrangement of the present invention discuss a vessel (22) having an upright side wall (28), a bottom (24), a bottom fillet (26) and at least two agitators (30) supported to the side wall. The arrangement functions such that, when the agitators are arranged within the fluid in the vessel (22), a fluid flow is created by rotating each agitator (30), each fluid flow being directed towards the receiving edges (261, 264) of the fillet (26). The receiving edges divide each flow into two partial flows and the side faces (262, 263, 265, 266) of the receiving edges guide the partial flows to circulate in smaller volumes, i.e. in four smaller vortices ensuring efficient agitation in the entire area of the zone of agitation, i.e. the bottom area of the vessel round the fillet (26).

Description

Technical field

[0001] The present invention relates to a method of and an arrangement for agitating fluid in a vessel in the manner described in the preambles of claims 1 and 5. The method and the arrangement of the present invention are, for instance, especially suitable for treating fiber suspensions of pulp and paper making industry, i.e. both for diluting the suspensions and for agitating such for some other purpose, for instance for mixing a chemical with the suspension. The present invention is, naturally, applicable in other branches of industry, too, where agitation of fluid in a large sized vessel is desired.

Background art

[0002] Agitating of fluid in a vessel is a well-known basic process in various branches of industry. Such agitation may be performed in smaller or larger vessels depending on the branch of industry. Chemical, especially petro-chemical industry as well as pulp and paper making industry represent processes where the size of the mixing vessels or storage towers is relatively large, i.e. the diameter of the vessel may exceed 10 meters, and the height 30 meters. All mixing vessels, reactors, storage towers etc. require, in the least, that the fluid that is to be discharged from the vessel is homogenous, i.e. its concentration or composition remains substantially constant over time. Also, if the vessel is used for mixing a chemical or some other substance with the fluid, it is of utmost importance that the concentration of the chemical or other substance is, practically seen, constant in the fluid to be discharged from the vessel. Naturally, the larger the vessel is the more difficult it is to ensure the homogeneity of the fluid to be discharged.

[0003] In recent years a few patent documents have come up discussing the agitation of fluid in the bottom zone of a large sized vessel, i.e. in the so called agitation zone or zone of agitation.

[0004] US-A-5711600 discusses a vessel used in pulp and paper industry, i.e. pulp tower to which fiber suspension is introduced in a consistency of 0 - 40%, typically the consistency is 10 - 30% and from which the suspension is discharged in a consistency of 0 - 5%, typically 4 - 5%. However, if the vessel is used, for instance, for treating fillers of paper making industry the dry matter content may be up to 80%. Thus, the fiber suspension or pulp is diluted at the bottom part of the vessel by feeding and mixing the required amount of dilution liquid in the pulp. In accordance with the patent the bottom of the vessel is provided with a central rotationally symmetrical fillet. The fillet may be mainly cylindrical provided with a cone shaped top or it may have a cylindrical bottom part and a top part formed of a truncated cone inserted on the cylindrical part such that its smaller diameter cross section is against the top of the cylinder and of a cone positioned on the top of the larger diameter cross section of the truncated cone, whereby the fillet may be considered mushroom-shaped. The wall of the vessel round the bottom fillet is provided with a number of agitators positioned or directed such that the flow the agitators create is directed past the bottom fillet, i.e. the agitators make the fluid rotate at the bottom part of the vessel round the fillet. Naturally the purpose of the rotating fluid is to agitate the pulp and to mix the dilution liquid efficiently with the pulp.

[0005] A problem with the above discussed construction is that the larger the vessel is the greater is the number of agitators needed in ensuring the efficient rotation of the fluid round the bottom fillet. Another problem is the orientation of the agitator shaft, as, for ensuring the rotation of fluid, the shaft has to be inclined with respect to the radius of the vessel, i.e. the direction of the shaft is not radially towards the bottom fillet, but inclined to the side thereof. This brings two types of problems. Firstly, the diameter of the propeller of the agitator is limited due to the inclination of the agitator shaft. And secondly, the fastening of the agitators to the wall of the vessel in inclined direction subjects the fastening elements to significant bending forces. Simultaneously also the mechanical components of the agitator are subjected to stresses due to non-symmetrical orientation of the agitator. And further due to the circulation of the fluid in an annular cavity, the turbulence within the fluid is relatively weak. Only the agitators and the circular surfaces of both the fillet and the vessel subject the fluid to turbulence. And only when the velocity of the circulating annulus of fluid is high enough the surface friction between the fluid and both the vessel and the fillet is able to create high enough turbulence for mixing the layers close to the surfaces with the rest of the circulating fluid. The sufficient velocity of the circulating annulus of fluid may be ensured only by forcing the agitators to rotate at a high speed, which results in both high energy consumption and strong forces subjected to the support structures or elements of the agitator and of the wall of the vessel.

[0006] US-A-5746890 discusses mixing devices for mixing a processing agent with a pulp suspension. The mixing devices include a mixing vessel, an agitator mounted on the upright side wall of the mixing vessel to create a flow of the pulp suspension across the mixing vessel, a processing agent supply for supplying processing agent to the flow of pulp suspension, and a flow divider mounted on the inner wall surface of the mixing vessel opposite the agitator. The flow divider includes a substantially vertical front distribution edge and a pair of arcuate or concave front side surfaces extending from the distribution edge to the inner wall surface of the mixing vessel on opposite sides of the vertical distribution edge.

[0007] A problem with the vessel provided with such a flow divider is that the larger the vessel is the larger the agitator has to be in order to be able to keep the fluid within the entire bottom area of the tower in movement. However, the power demand of a large-sized agitator is huge and also the construction of the agitator is so rugged that it may be necessary to provide the vessel wall with additional reinforcement for absorbing the forces subjected thereto, on the one hand, from the weight of the agitator, and on the other hand, from the working of the agitator while it is forcing the fluid to circulate within the bottom part of the vessel.

Brief summary of the Invention

[0008] Thus, an object of the present invention is to raise the state of the art in the area of agitated vessels by introducing a method and an arrangement for agitating fluid in a vessel capable of minimizing at least some problems and drawbacks of prior art.

[0009] Another more detailed object of the present invention is to design the bottom part of a vessel such that an efficient turbulence for agitating the fluid in the zone of agitation is created with smaller agitators, with lower energy consumption and with lower stress to the vessel wall and the support elements of the agitator.

[0010] At least some of the above and other objects of the present invention are reached by a method of agitating fluid in a vessel as discussed in the characterizing part of claim 1.

[0011] At least some of the above and other objects of the present invention are reached by an arrangement for agitating fluid in a vessel as discussed in the characterizing part of claim 5.

[0012] Other characteristic features of the present invention become evident from the appended dependent claims and the following description of the embodiments of the present invention.

[0013] By applying the present invention at least some of the following advantages are gained:

• smaller-sized agitators may be used for agitating the fluid in the vessel,
• the construction of the agitators may be standardized,
• lower energy consumption of the agitators,
• radial direction of the agitators optimizes the direction of the forces and reduces the forces subjected to the wall of the vessel,
• lighter agitators, lighter support elements in connection with the agitator and the vessel wall,
• higher homogeneity of the fluid being discharged from the vessel, and
• possibility to continue running the vessel, i.e. discharging fluid from the vessel even if one of the agitators is, for some reason, out of use.

Brief Description of Drawling

[0014] In the following, the prior art method and an arrangement as well as the method of and an arrangement for agitating fluid in a vessel of the present invention are discussed in more detail by referring to the appended drawings, of which

Figure 1 illustrates schematically a prior art vessel having a central fillet at the bottom of the vessel and one or more agitators circulating the fluid in the vessel round the fillet,

Figure 2 illustrates schematically another prior art vessel having a single agitator supported on the side wall of the vessel, and a flow divider arranged in connection with the vessel wall opposite to the agitator,

Figure 3 illustrates schematically a novel fillet construction and a novel way of treating fluid in the vessel in accordance with a first preferred embodiment of the present invention,

Figure 4 illustrates schematically a novel fillet construction and a novel way of treating fluid in the vessel in accordance with a second preferred embodiment of the present invention,

Figure 5 illustrates schematically a novel way of arranging agitators round the fillet of Figure 3,

Figure 6 illustrates schematically another novel way of arranging agitators round the fillet of Figure 3, and

Figure 7 illustrates schematically a novel fillet construction and a novel way of treating fluid in the vessel in accordance with a third preferred embodiment of the present invention.

Detailed Description of Drawings

[0015] Prior art Figure 1 illustrates schematically a vessel 2 discussed in more detail in US-A-5,711,600, the vessel 2 being used in pulp and paper industry. Such a vessel 2 may be called a pulp tower to which fiber suspension is introduced in a consistency of 0 - 40%, typically the consistency is 10 - 30%, and from which the suspension is discharged in a consistency of 0 - 10%, typically 4 - 5%. However, if the vessel is used, for instance, for treating fillers of paper making industry the dry matter content may be up to 80%. The fiber suspension or pulp is diluted at the bottom part of the vessel 2 by feeding and mixing required amount of dilution liquid in the pulp. In accordance with the US- patent the bottom 4 of the vessel 2 is provided with a central rotationally symmetrical upright fillet 6. The fillet 6 may be mainly cylindrical provided with a conically tapering top or it may have, as shown in Figure 1, a cylindrical bottom part 61 and a top part formed of a truncated cone 62 inserted on the cylindrical part 61 such that its smaller diameter cross section is against the top of the cylindrical part 61 and of a cone 63 positioned on the top of the larger diameter cross section of the truncated cone 62 whereby the fillet 6 may be considered having a mushroom shape. The wall 8 of the vessel 2 round the bottom fillet 6 is provided with a number of agitators 10 positioned or directed such that the agitators 10 direct the fluid flow past the bottom fillet 6 and the agitators 10, thus, make the fluid or pulp circulate at the bottom part of the vessel 2 round the fillet 6. Naturally the purpose of the circulating fluid is to agitate the pulp and to mix the dilution liquid efficiently with the pulp.

[0016] One of the problems of the vessel-agitator-combination of the prior art Figure 1 is the need of high velocity in the fluid circulating round the fillet. As the zone of agitation is annular in Figure 1, the side walls of the turbulence chamber, i.e. the walls limiting the annular chamber at the bottom part of the vessel, are smooth, whereby at a slow flow velocity round the fillet the flow is easily substantially laminar close to the surfaces of the fillet and the vessel wall preventing efficient agitation. The only way to ensure proper agitation would be to increase the number of agitators and/or increase the rotational speed of the agitators. This adds both to the investment and running costs of the vessel-agitator-combination. It also increases the stresses subjected from the agitators to the vessel wall and to the support structures of the agitators themselves.

[0017] Prior art Figure 2 illustrates schematically a vessel 12 discussed in more detail in US-A-5746890. The US- patent discusses mixing devices for mixing a processing agent with a pulp suspension. The mixing devices include a mixing vessel 12, an agitator 20 mounted on the side wall 18 of the mixing vessel 12 to create a flow of the pulp suspension across the mixing vessel 12, a processing agent supply for supplying processing agent to the flow of pulp suspension, and a flow divider 16 mounted on the inner surface of the wall 18 of the mixing vessel 12 opposite to the agitator 20. The flow divider 16 includes a substantially vertical front distribution edge 161 and a pair of arcuate or concave front side surfaces 162 and 163 extending from the distribution edge 161 to the inner surface of the wall 18 of the mixing vessel 12 on opposite sides of the distribution edge 161.

[0018] Both above discussed prior art vessel and mixing/agitating structures suffer the same disadvantage. Now that the size or volume of the mixing/agitating vessels or storage towers etc. grows gradually the diameter of the vessels grows, too, naturally. This means that in order to work in the desired manner the agitators have to be large-sized whereby their power demand is high and the stresses the agitators subject to the wall of the vessel are high, too.

[0019] Figure 3 illustrates schematically a novel construction of a fillet 26 and a novel way of treating fluid in the vessel 22 in accordance with a first preferred embodiment of the present invention. In this embodiment the bottom 24 of the vessel 22 is provided with a substantially central upright fillet 26 and at least two agitators 30 supported radially to the upright side wall 28 of the vessel 22 on opposite sides of the fillet 26. The upright side wall is understood as a wall surrounding the zone of agitation of the vessel and having an inclination (either inwards or outwards) of less than 45 degrees, preferably less than 30 degrees from the vertical direction. Most preferably the side wall is vertical. The agitators 30 are fastened or supported to the side wall 28 such that the direction of the flow they create is inwardly in the vessel 22, i.e. towards the fillet 26. The agitators are normally arranged such that their drive motor and gearbox are outside the vessel and only the shaft with possible support structures extends into the vessel, the shaft carrying at its end the agitator propeller. In some specific cases other parts of the agitators may be installed within the vessel wall, too. The agitators 30 are supported to the vessel side wall 28 such that the shaft of the agitator points towards the fillet, which direction is here considered to be within the term "radial". Preferably, the shaft is arranged in a radial plane, i.e. the axis of the shaft runs in a plane in which also the axis of the vessel runs. However, the shaft of the agitator need not necessarily be horizontal, but it may be inclined in either direction by at most 30 degrees, which direction is also considered to be encompassed by the definition "radial".

[0020] The fillet 26 has, in this embodiment, a square, in broader terms quadrangular, cross section such that two opposite edges (261 and 264) of the cross section face the agitators 30 and act as the flow receiving edges. The fillet 26, thus, has four side faces 262, 263, 265 and 266 such that in operation, firstly, the receiving edges 261, 264 facing the agitators 30 divide the flow coming from the agitators 30 to the sides of the edges and the side faces 262, 263 and 265, 266 guide the flow to the sides of the fillet 26 as shown by arrows in the upper projection such that four separate vortices are formed at the zone of agitation. The internal angle α between the side faces 262 and 263, and 265 and 266 at the receiving edges 261 and 264 is preferably 90 degrees whereby also the other internal angles at the flow departing or trailing edges of the square cross section are 90 degrees. However, the internal angle α at the receiving edges 261 and 264 may vary between 60 and 120 degrees whereby the other two internal angles at the trailing edges are varying between 120 and 60 degrees.

[0021] The top of the fillet 26 may be designed freely whereby, for instance, the way the fluid is introduced into the vessel may have an influence on the top design. Figure 3 shows a top design where the top is in fact a kind of a roof formed of two side planes 267 and 268 or slopes facing the agitators 30. However, the roof may also be arranged in another direction i.e. the ridge being arranged on the same diametral plane with the agitators 30. As another alternative the top of the fillet may be formed of four side planes or slopes, whereby the top is pyramid-shaped. Or, if desired, cone-shaped. This kind of a top design is preferable if the vessel is filled from the top, whereby the fluid flow hits the fillet from above. Naturally, the top may also be horizontal if there is no need to protect the top of the fillet from fluid hitting it from above. Also, the fillet may be open from inside if the fluid is introduced into the vessel via the fillet.

[0022] Figure 4 illustrates schematically a novel construction of a fillet 36 and a novel way of treating fluid in the vessel 32 in accordance with a second preferred embodiment of the present invention. In this embodiment the bottom 34 of the vessel 32 is provided with a central upright fillet 36 and at least two agitators 30 (here four agitators are shown) supported radially to the side wall 38 of the vessel 32 on opposite sides of the fillet 36. The upright side wall 38 is understood as a wall surrounding the zone of agitation of the vessel and having an inclination of less than 45 degrees, preferably less than 30 degrees from the vertical direction. Most preferably the side wall is vertical. The agitators 30 are fastened to the wall 38 such that the direction of the flow they create is radially inwardly in the vessel 32, i.e. towards the fillet 36. The agitators are normally arranged such that their drive motor and gearbox are outside the vessel and only the shaft with possible support structures extends into the vessel, the shaft carrying at its end the agitator propeller. In some specific cases other parts of the agitators may be installed within the vessel wall, too. The agitators are supported to the vessel side wall such that the shaft of the agitator points towards the fillet, preferably towards the edge of the fillet, which direction is here considered to be within the term "radial". Preferably, the shaft is arranged in a radial plane, i.e. the axis of the shaft runs in a plane in which also the axis of the vessel runs. However, the shaft of the agitator need not necessarily be horizontal, but it may be inclined in either direction by at most 30 degrees, which direction is also considered to be encompassed by the definition "radial". This kind of an agitator arrangement may be applied either with the quadrangular fillet of Figure 3 or with a star-shaped fillet 36 of Figure 4.

[0023] The fillet 36 has, in this embodiment of the present invention, a somewhat star-shaped cross section such that two opposite receiving tips or edges 361 and 364 of the cross section face the agitators 30. The fillet 36, thus, has four arcuate or concave side faces 362, 363, 365 and 366 such that in operation, firstly, the receiving edges 361 and 364 of the fillet 36 facing the agitators 30 divide the flow coming from the agitators 30 to the sides of the receiving edges and the side faces 362, 363 and 365, 366 guide the flow to the sides of the fillet 36 as shown by arrows in the upper projection in Figure 4 such that four separate vortices are formed at the zone of agitation. The side faces 362 and 363, and 365 and 366 are arranged to adjoin at the receiving edges 361 and 364 such that the tangents (in a radial plane) of the side faces at the receiving edges form an internal angle of less than 90 degrees, preferably less than 60 degrees, and more preferably less than 45 degrees. Preferably the side faces 362 and 365, and side faces 363 and 366 adjoin at the flow departing or trailing edges of the fillet 36 such that their tangents (in a radial plane) form an internal angle of less than 90 degrees, preferably less than 45 degrees, i.e. all internal angles are sharp.

[0024] The top of the fillet of Figure 4 may be of any appropriate design. The discussion in connection with the fillet of Figure 3 concerning the shape of the top is here referred to as an example of the variations in the top design.

[0025] As to the overall shape and size of the fillet of both above discussed embodiments, there are several applicable options. The receiving and the trailing edges of the fillet may be vertical (as shown in the Figures) or slightly inclined. The same applies to the side faces, too. Thus an option is that the entire fillet may be pyramid shaped. And, as shown in the Figures 3 and 4, the fillet extends at least to the operative height of the agitators, i.e. to the height or level of the shaft of the uppermost agitator plus the length of the agitator blade.

[0026] The arrangement in accordance with the above embodiments of the present invention (Figures 3 and 4) functions such that, when the agitators are arranged within the fluid in the vessel, a fluid flow is created by rotating each agitator, each fluid flow being directed towards the receiving edge of the fillet, in broader terms, towards the fillet. The receiving edges divide each flow into two partial flows and the side faces of the receiving edges guide the partial flows to circulate in smaller volumes, i.e. in four smaller vortices ensuring efficient agitation in the entire area of the zone of agitation, i.e. the bottom area of the vessel round the fillet. This kind of agitation may be maintained such that the agitators and their fastening elements or support structures are subjected to significantly lighter or lower stress than those in prior art vessels of the same size. Also, the efficiency of the agitation or mixing is better due to smaller size of each individual vortex or turbulence field.

[0027] In the above embodiments of the present invention the number of agitators in a certain level is a half of the number of edges in the fillet, or equal with the number of receiving edges in the fillet. When taking into account that there may be agitators in more than one level, the number of agitators is a half of the number of edges in the fillet or multiple thereof, or equal with the number of receiving edges or a multiple thereof.

[0028] Figure 5 illustrates schematically a further agitator arrangement in a vessel 42 provided with a fillet 46 in accordance with Figure 3, though also the fillet of Figure 4 could as well be applied. In this embodiment the vessel wall is provided with an agitator 30, or a plurality of agitators one above the other, for each edge of the fillet 46. Thus, each edge of the fillet operates as the receiving edge. This means in practice that the agitators 30 make the fluid flow against each edge of the fillet 46 for creating partial flows whereafter the partial flows advance along the side faces of the fillet 46 until two partial flows originating from the operation of two neighboring agitators 30 and two adjacent receiving edges collide on the side face. The collision of the partial flows forces the flows to turn towards the wall of the vessel 42, whereby, in a cross section of the vessel 42 at the level of the agitators eight vortices, more generally speaking, twice the amount of agitators, may be seen. It also a possible option that there may not be seen any vortices but the collision of the flows result in a chaos- type efficient turbulence field. The agitators 30 of the arrangement of Figure 5 are preferably arranged on substantially the same horizontal level. However, it is also possible to arrange the agitators of Figure 5 on two horizontal levels either such that there are four agitators on both levels, whereby the agitators of the different levels are one above the other, or such that in total of four agitators are arranged such that a pair of opposite agitators are located on a lower horizontal level and another opposite pair on an upper horizontal level.

[0029] In this variation of the present invention the number of agitators in a certain level is the number of edges in the fillet. When taking into account that there may be agitators in more than one level, the number of agitators is the number of edges in the fillet or a multiple thereof.

[0030] Figure 6 illustrates schematically a yet further agitator arrangement in a vessel 52 provided with a fillet 56 in accordance with Figure 3, though also the fillet of Figure 4 could as well be applied. In this embodiment each single agitator on the vessel wall, i.e. as shown in Figure 5, is replaced with a pair of agitators 50. Now the agitators 50 of each pair of agitators are standard agitators, whereby they rotate in the same direction. However, the agitators may be arranged, if desired, to rotate in opposite directions. In both cases, the two agitators create a powerful flow towards an edge or the side faces of the fillet 56 depending on the orientation of the agitators. Now all edges of the fillet 56 operate as flow receiving edges. This means in practice that the agitator pairs make the fluid flow against the receiving edges or the side faces of the fillet 56 for creating partial flows whereafter the partial flows advance along the side faces of the fillet 56 until two partial flows originating from the operation of two neighboring agitators 50 of different pairs of agitators collide. The collision of the partial flows force the flows to turn towards the wall of the vessel 52, whereby, in a cross section of the vessel 52 either eight vortices, more generally speaking, the same number with the agitators, or only a chaotic turbulent zone, may be seen. The agitators 50 of the arrangement of Figure 6 are preferably arranged on substantially the same horizontal level. However, it is also possible to arrange the agitators of Figure 6 on two horizontal levels either such that there are eight agitators on both levels, whereby the agitators of the different levels are one above the other, or such that in total of eight agitators are arranged such that a pair of opposite pairs agitators are located on a lower horizontal level and another pair of opposite pairs on an upper horizontal level.

[0031] In this variation of the present invention the number of agitators in a certain level is may start from the number of edges of the fillet, whereby there are flow receiving edges and trailing edges, i.e. not all edges of the fillet receives the flow. However, a preferred number of agitators in a certain level is twice the number of edges in the fillet. When taking into account that there may be agitators in more than one level, the number of agitators is twice the number of edges in the fillet or a multiple thereof.

[0032] Naturally, it is also possible, for instance, to replace the single agitators of the embodiment of Figures 3 or 4 with pairs of agitators, whereby the operation of the vessel - fillet - agitator combination is substantially the same as discussed in connection with Figures 3 and 4. Also, it is obvious that the agitators of a pair of agitators need not necessarily have their shafts parallel with each other, but the shafts may be arranged slightly inclined such that the centerlines of the shafts of each pair of agitators cross in the centerline plane of the agitator pair, the centerline plane running via the axis of the fillet somewhere within the vessel or behind the fillet.

[0033] Figure 7 illustrates schematically a novel fillet construction and a novel way of treating fluid in the vessel in accordance with a third preferred embodiment of the present invention. The fillet 66 of Figure 7 is formed of two substantially equal parts, i.e. a lower part 66' and an upper part 66" having either a square (shown in Figure 7) or a star-shaped cross section, or one having a square cross section and the other a star-shaped cross section. The upper part 66" of the fillet 66 is arranged co-axially with the lower one 66', but, however, such that the angular position of the upper part 66" of the fillet 55 is turned 45 degrees round its axis compared to the lower one 66'. This results in that the edges of the upper part 66" of the fillet 66 are located in the same vertical plane with the vertical centerline of the side faces of the lower part 66' of the fillet 66. Now that this kind a fillet structure needs agitators 60' and 60" on two levels, the agitators 60' and 60" are, naturally, again positioned such that the flows of fluid they create are directed towards the edges of the fillet 66. In this embodiment the operation of the agitation at each level is comparable to the teaching of Figure 5.

[0034] In this embodiment of the present invention the number of agitators in a certain level is the number of edges in the fillet. When taking into account that there may be agitators in more than one level, the number of agitators is the number of edges in the fillet or a multiple thereof.

[0035] It is also possible, naturally, to have only two agitators on one or both levels of agitators, whereby the operation of the agitation at such a level is comparable to the teachings of Figures 3 and 4.

[0036] It should also be understood that the fillet may have more than four edges. Also the number of edges may be an odd number, whereby the agitators may either direct the flow towards each edge of the fillet, or towards each side face between the edges. In the former option the flows along the side faces of the fillet collide and are either directed to form vortices or form a chaotic turbulence field. In the latter option the flows follow the side faces to the end thereof and then turn to form vortices. Especially if the side faces of the fillet are arcuate and divert the flow in an appropriate manner to counter rotating vortices the number of which is the same as that of the edges of the fillet.

[0037] In view of the above by using the central fillet of the present invention the size of agitators may be maintained relatively small or at least reasonable, as there is no need for a single agitator to be able to keep the entire bottom area of the vessel in movement but the vessel bottom volume is divided between at least two agitators. Also, when the number of agitators increase, there is a point where taking one agitator out of use, for instance for service, does not result in a considerable weakening of the agitation in the vessel. This means in practice that the agitators may be serviced without interrupting the process the vessel is arranged in.

[0038] It is to be noted that above only a few most preferred embodiments or variations of the present invention have been discussed. Thus, it is obvious that the invention is not restricted to the above described embodiments and variations, but it may be applied in many different ways within the scope of the appended claims. The features of the present invention described in relation to a certain embodiment are within the basic concept of the invention, whereby they may be used in connection with another embodiment of the invention. Thereby also different features of the invention may be used in combination provided that such is desirable and that the technical possibilities for such are available.

Claims

1. A method of agitating fluid in a vessel (22; 32; 42; 52; 62), the vessel (22; 32; 42; 52; 62) having an upright side wall (28; 38), a bottom (24; 34), a bottom fillet (26; 36; 46; 56; 66) and at least two agitators (30; 50; 60', 60"), the method being characterized by the steps of

a) Providing the fillet (26; 36; 46; 56; 66) with a cross section having a plurality of side faces (262, 263, 265, 266; 362, 363, 365, 366) and a plurality of edges (261,264;361,364),

b) Supporting the at least two agitators (30; 50; 60', 60") to the side wall (28; 38) of the vessel (22; 32; 42; 52; 62) by orienting the at least two agitators (30; 50; 60', 60") towards the fillet (26; 36; 46; 56; 66),

c) Rotating each agitator within the fluid to create a fluid flow inwardly towards the fillet (26; 36; 46; 56; 66),

d) Guiding each fluid flow by means of at least one side face (262, 263, 265, 266; 362, 363, 365, 366) of the fillet (26; 36; 46; 56; 66) to at least one side of the fillet to increase turbulence in the vessel (22; 32; 42; 52; 62).

2. The method as recited in claim 1, characterized by, in step c), each fluid flow is directed towards a side face (262, 263, 265, 266; 362, 363, 365, 366) of the fillet (26; 36; 46; 56; 66).

3. The method as recited in claim 1, characterized by, in step a), providing the fillet (26; 36; 46; 56; 66) with receiving edges (261, 264; 361, 364); in step b) orienting the agitators (30; 60', 60") towards the receiving edges (261, 264; 361, 364) of the fillet (26; 36; 46; 66); in step c) rotating each agitator within the fluid to create a fluid flow inwardly towards the receiving edge (261, 264; 361, 364) of the fillet (26; 36; 46; 66); and in step (d) dividing each fluid flow by means of the receiving edge (261, 264; 361, 364) of the fillet (26; 36; 46; 66) into two partial flows.

4. The method as recited in claim 1 or 2, characterized by, in step b) arranging the agitators (50) in pairs and orienting the agitators (50) of each pair towards the side faces of the fillet (56), and in step c) rotating the agitators (50) of each pair to create a fluid flow towards the fillet (56).

5. An arrangement for agitating fluid in a vessel, the vessel (22; 32; 42; 52; 62) having an upright side wall (28; 38), a bottom (24; 34), a bottom fillet (26; 36; 46; 56; 66) and at least two agitators (30; 50; 60', 60"), characterized in that the fillet (26; 36; 46; 56; 66) is provided with a cross section having a plurality of side faces (262, 263, 265, 266; 362, 363, 365, 366) and a plurality of edges (261, 264; 361, 364).

6. The arrangement as recited in claim 5, characterized in that the agitators (30; 50; 60', 60") are supported to the side wall (28; 38) of the vessel (22; 32; 42; 52; 62) by orienting the agitators (30; 50; 60', 60") towards the fillet (26; 36; 46; 56).

7. The arrangement as recited in claim 6, characterized in that at least a part of the edges of the fillet (26; 36; 46; 56) are receiving edges (261, 264; 361, 364) and that the agitators (30; 50; 60', 60") are oriented towards one of the receiving edges (261, 264; 361, 364) and the side faces (262, 263, 265, 266; 362, 363, 365, 366) of the fillet (26; 36; 46; 56) .

8. The arrangement as recited in claim 6 or 7, characterized in that the agitators (50) are arranged in pairs such that each pair of agitators create a fluid flow towards the fillet (56).

9. The arrangement as recited in any one of the preceding claims 5 - 8, characterized in that the agitators (30; 50; 60', 60") are supported in at least one level to the wall (28; 38) of the vessel (22; 32; 42; 52; 62).

10. The arrangement as recited in any one of the preceding claims 5 - 9, characterized in that the fillet (26; 36; 46; 56; 66', 66") has at least two receiving edges (261, 264; 361, 364) and at least two departing or trailing edges.

11. The arrangement as recited in any one of the preceding claims 5 - 10, characterized in that the fillet has four edges and that the side faces (262, 263, 265, 266) are planar and have an internal angle α of between 60 and 120 degrees.

12. The arrangement as recited in any one of the preceding claims 5 - 10, characterized in that the side faces (362, 363, 365, 366) are curved planes, i.e. concave such that the internal angles formed at the edges between the tangents of the side faces are sharp.

13. The arrangement as recited in any one of the preceding claims 5 - 9 or 11 or 12, characterized in that all the edges of the fillet are flow receiving edges and that the fillet has at least three such flow receiving edges.

14. The arrangement as recited in any one of the preceding claims 5 - 13, characterized in that the number of agitators (30; 50; 60', 60") at each level is half of the number of edges in the fillet (26; 36; 46; 56; 66) or a multiple thereof.

Drawing