WO2013107450A1

WO2013107450A1 - A counterflow mixer for mixing concrete

Info

Publication number: WO2013107450A1
Application number: PCT/DK2012/050024
Authority: WO
Inventors: Ole Langkjær Christensen
Original assignee: Haarup Maskinfabrik A/S
Priority date: 2012-01-20
Filing date: 2012-01-20
Publication date: 2013-07-25

Abstract

Disclosed is a counterflow mixer (1) for mixing concrete. The mixer (1) comprises a stationary mixing pan (2), including one or more discharge openings (3) at the bottom (4) of the mixing pan (2). The mixer (1) further includes a mixer unit (5) comprising a mixer unit drive (6) and one or more mixing means (7), and two or more suspension arms (8) extending between the mixing pan (2) and the mixing unit hereby suspending the mixing unit (5) above the mixing pan (2). At least one of the suspension arms (8) is provided with at least one cement inlet (9) adapted for leading cement into the mixing pan (2) through the suspension arm (8) during a concrete mixing process.

Description

A COUNTERFLOW MIXER FOR MIXING CONCRETE

Field of the invention

The invention relates to a counterflow mixer for mixing concrete.

Background of the invention

Horizontal axis mixers where the mixing drum rotates are well known for mixing small batches of concrete onsite but for mixing large concrete batches e.g. at a concrete precast production plant, a masonry production plant or for filling up ready mix trucks it is known to use vertical axis counterflow mixers - also known as counter current mixers.

Vertical axis counterflow mixers can be designed in many ways but the most common design is where the mixing pan is stationary and a mixer unit is suspended above the mixing pan. The mixer unit then comprises some rotating mixing means which are rotated around the mixing pan.

To ensure the life of the mixer and to ensure a high quality of the concrete being mixed in the mixer it is highly important that small lumps of concrete does not harden in the mixer and it is therefore important that easy and sufficient access to the mixing pan is ensured, so that the pan may regularly be inspected and cleaned. Furthermore, all the parts of the mixer that comes into direct contact with the concrete during mixing will at least to some extent be wear parts and it is therefore important that the mixing means and other devices inside the mixing pan is easily accessible for maintenance.

From WO 2011/128727 Al it is therefore known to suspend the mixing unit above the mixing pan by means of four suspension arms and then provide the space between the suspension arms with doors all the way around the mixing pan so that substantially all parts of the inside of the mixer can easily be accessed. However, during the concrete mixing process large quantities of ingredients such as cement, aggregate and water have to be introduced into the mixer and to ensure that the connection pipes or similar for introducing these ingredients does not hinder access to the mixing pan all the ingredients are introduced through coaxially arranged feeding pipes placed centrally in the middle of the mixing unit. However, a centre- less mixing unit requires a complex gear design or several individual mixing drives.

Furthermore, when the cement weight hopper, the aggregate supply, the exhaust device and/or other equipment all have to be arranged around this centrally arranged feeding pipe, the space above the mixing unit gets very crowded, hereby making it difficult to fit and access this equipment.

And even further, if water and cement are entering the mixer through the same inlet the risk of unwanted clogging and material build-up is severely increased in that the water will make some of the cement stick to the inlet and gradually build up. And even if the water and cement inlets are formed concentrically as in WO 2011/128727 the risk of unwanted material build up, especially at the bottom edge of the cement inlet is increased. An object of the invention is therefore to provide for an advantageous counterflow mixer design which does not present the above mentioned drawbacks.

The invention

The invention provides for a counterflow mixer for mixing concrete. The mixer comprises a stationary mixing pan, including one or more discharge openings at the bottom of the mixing pan. The mixer further includes a mixer unit comprising a mixer unit drive and one or more mixing means, and two or more suspension arms extending between the mixing pan and the mixing unit hereby suspending the mixing unit above the mixing pan. At least one of the suspension arms is provided with at least one cement inlet adapted for leading cement into the mixing pan through the suspension arm during a concrete mixing process.

It is advantageous to lead the cement into the mixing pan through the suspension arms in that the cement weight hopper (the cement weight and container) can be moved away from the centre of the mixer hereby making it easier to access the cement weight and container.

Furthermore, placing the cement inlet in a suspension arm enables that the mixer unit can now comprise a centrally arranged mixer unit drive which provides for a simpler and more durable mixer unit drive design since the mixer unit now can be dedicated to mixing.

Even further, by moving the cement inlet to a suspension arm it can be separated from the water inlet and located at a safe distance from the water inlet so that unwanted material build-up can be avoided.

In an aspect of the invention one or more of said suspension arms are also provided with at least one water inlet adapted for leading water into said mixing pan through said suspension arm during a concrete mixing process.

A considerable amount of water is used for mixing a full batch of concrete in the mixer and it is therefore advantageous to also lead the water into the mixing pan through the suspension arms.

In an aspect of the invention one or more of said suspension arms are also provided with at least one aggregate inlet adapted for leading aggregate into said mixing pan through said suspension arm during a concrete mixing process. Concrete mainly consists of aggregate and the aggregate inlet in any kind of concrete mixer is therefore by far the largest inlet. It is therefore advantageous to also lead aggregate into the mixing pan through the suspension arms. By the term "aggregate" is to be understood any kind of sand, gravel, rubble, grit, stones or any combination thereof suitable for making concrete.

In an aspect of the invention the diameter of the inscribed circle of said cement inlet is between 50 mm and 1200 mm, preferably between 100 mm and 1000 mm, and most preferred between 120 mm and 800 mm.

If the diameter of the cement inlet becomes too big the strength of the suspension arm is reduced and the suspension arm becomes too wide. If the cement inlet becomes too small it takes too long to lead the cement through the inlet - hereby reducing the mixer's capacity - and the risk of inlet clogging is increased. The present diameter ranges therefore provides for an advantageous relationship between functionality and efficiency.

In an aspect of the invention the diameter of the inscribed circle of said aggregate inlet is between 50 mm and 1500 mm, preferably between 100 mm and 1200 mm, and most preferred between 150 mm and 1000 mm.

If the diameter of the aggregate inlet becomes too big the strength of the suspension arm is reduced and the suspension arm becomes too wide. If the aggregate inlet becomes too small it takes too long to lead the aggregate through the inlet - hereby reducing the mixer's capacity - and the risk of inlet clogging is increased. The present diameter ranges therefore provides for an advantageous relationship between functionality and efficiency. In an aspect of the invention said cement inlet, said water inlet and/or said aggregate inlet is formed as a cylindrical pipe.

All things being equal, holes in the suspension arms will inevitably reduce the strength of the suspension arms. However, by forming the inlets as cylindrical pipes the pipes can form an integral part of the suspension arm hereby reinforcing it instead of reducing the suspension arm's strength.

In an aspect of the invention said suspension arms are wider at the end connected to the mixing pan than at the end connected to the mixer unit.

The further the suspension arms have to reach the higher they are strained. By making the suspension arms wider the further they extend from the mixer unit it is easier to make the suspension arms cope with the strain substantially without additional reinforcement.

Furthermore, making the suspension arms wider at the end connected to the mixing pan than at the end connected to the mixer unit is advantageous in that it enables that, if the mixer is provided with more than one door between two neighbouring suspension arms, it is possible to form the doors with substantially identical shape.

In an aspect of the invention said suspension arms are evenly distributed around said mixer unit. By distributing the suspension arms evenly around the mixer unit a more advantageous load distribution is achieved as well as a less complex mixer design.

In an aspect of the invention the said counterflow mixer comprises exactly three suspension arms. Like a three legged chair never rocks it is simpler to mount a mixer unit on the mixing pan if it is supported by exactly three suspension arms. Furthermore, three suspension arms provides for a advantageous load distribution. In an aspect of the invention the said counterflow mixer comprises exactly four suspension arms.

A mixer provided with exactly four suspension arms provides for a symmetrically mixer design with an advantageous load distribution regarding loads from the mixer unit.

In an aspect of the invention suspension arms are connected to an upper edge of said mixing pan. Connecting the suspension arms to the upper edge of the mixing pan is advantageous in that load-wise this provides for an advantageous connection of the suspension arms. Furthermore, this location ensures that the suspension arms will not reduce the capacity of the mixing pan while at the same time ensuring that the suspension arms will not be in the way for the mixing means.

In an aspect of the invention the openings between said suspension arms are covered by two or more doors.

Providing the openings between the suspension arms with doors is advantageous in that it is hereby possible to protect an operator from getting in contact with the mixer means inside the mixing pan during mixing, it is possible to ensure that unwanted foreign objects is not introduces into the concrete during mixing and since the doors can open it is still possible to provide easy access to the inside of the mixer. In an aspect of the invention said two or more doors have a substantially identical shape.

Forming the doors with substantially identical shape is advantageous in that the doors can be mass-produced hereby reducing the cost of each door.

Figures

The invention will be described in the following with reference to the figures in which fig. 1. illustrates a cross section through the middle of a counterflow mixer according to the invention, as seen from the front, fig. 2 illustrates the counterflow mixer shown in fig. 1, as seen from the top, fig. 3a illustrates a suspension arm comprising a cement inlet, as seen from the front, fig. 3b illustrates the suspension arm shown in fig. 3a, as seen from the top, fig. 3c illustrates the suspension arm shown in fig. 3a, as seen from the side, fig. 4a illustrates a suspension arm comprising an inspection door, as seen from the front, fig. 4b illustrates the suspension arm shown in fig. 4a, as seen from the top, fig. 4c illustrates the suspension arm shown in fig. 4a, as seen from the side, fig. 5a illustrates a suspension arm comprising an aggregate inlet and a connection pipe for an air-bag, as seen from the front, fig. 5b illustrates the suspension arm shown in fig. 5a, as seen from the top, fig. 5c illustrates the suspension arm shown in fig. 5a, as seen from the side, and fig. 6 illustrates a counterflow mixer with doors, as seen in perspective. Detailed description

Fig. 1 illustrates a cross section through the middle of a counterflow mixer 1 according to the invention, as seen from the front.

In this embodiment of the invention the counterflow mixer 1 comprise a mixer unit 5 including mixing means 7, wherein the mixer unit 5 is suspended above a mixing pan 2. The mixing means 7 comprises one or more mixing stars 15 extending down into the mixing pan 2 and each of the mixing stars 15 are rotated around a substantially vertical centre axis 16 of each star 15 and all the centre axis 16 of all the mixing stars 15 are also rotated around a centre axis 17 of the mixer unit 5. This mixing means design entails that the outer periphery of the mixing stars 15 at the outer edge of the mixing pan 2 is rotating with the direction of the rotation of all the stars 15 around the mixer unit's center axis 17 and the inner periphery of the mixing stars 15 is rotating against the direction of the rotation of all the stars 15 around the mixer unit's center axis 17 - hence the name "counterflow mixer". However, it should be noted that mixers of this particular type are also know under other names such as countercurrent mixers. Since the mixer unit 5 is usually coaxially arranged in a cylindrical mixing pan 2 the centre axis of the mixer unit 5 is usually also coaxial with the centre axis of the mixing pan 2. In this embodiment all the mixing stars 15 are rotated in the same direction at the same speed but in another embodiment one or more of the stars 15 could rotate in an opposite direction and/or one or more of the mixing stars 15 could rotate at a different speed. The mixing stars 15 can be designed in a multitude of ways but in this embodiment each mixing star 15 comprises a number of mixing arms 19 extending down towards the bottom 4 of the mixing pan 2. The bottom end of each arm 19 is provided with a mixing shovel 20 designed to lift the material in the mixing pan 2 of the bottom 4 of the pan 2 and mix it around.

Furthermore, to ensure that the ingredients in the mixing pan 2 are mixed properly the mixer unit 5 of a counterflow mixer 1 usually also comprises one or more side scrapers 18. The side scrapers 18 only rotate around the centre axis 17 of the mixing unit to scrape the side 21 and the corner 22 between the side 21 and the bottom 4 of the mixing pan 2 and deliver the scraped-off material in front of a rotating mixing star 15 so that the material at the sides of the pan 2 can also be thoroughly mixed into the batch.

In this embodiment of the invention all the rotating parts 7 of the mixer unit 5 are driven by a mixer unit drive 6 which in this embodiment comprises a centrally arranged motor 23 - which in this case is electrical - connected to a gear

arrangement 24 - which in this case is not illustrated in any details. Different parts of the gear 24 is in turn connected to the mixing stars 15, the side scrapers 18 and possibly other mixing devices so that the direction and speed of the motion of the mixing means 7 is synchronized by the gear 24 and controlled by the gear 24 and the motor 23.

However, in another embodiment of the invention the mixer unit 5 could be formed differently i.e. it could comprise more than one motor 23, it could comprise none or more than one gear arrangement 24, each mixing star could be provided with their own individual gear 24 and motor 23 or the mixing means 7 could be driven by another power source such as a combustion engine, a hydraulic or pneumatic motor or the mixing means 7 could be driven by drive means arranged externally to the counterflow mixer 1.

Also, in this embodiment of the invention the mixing pan 2 is formed with vertically cylindrical sides 21 but in another embodiment of the invention it would be feasible that the mixing pan 2 was shaped differently such as square, rectangular, oval or other and the sides walls could be formed other than vertical such as sloping or none- linear.

In this embodiment the mixing pan 2 and the mixer unit 5 are formed as two separate parts. However, they are connected by means of a number of suspension arms 8 extending between the mixer unit 5 and the upper edge 12 of the mixing pan 2. The main function of the suspension arms 8 is to carry and suspend the mixer unit 5 above the stationary mixing pan 2. However, when the mixer unit 5 is operating and the mixing shovels 20 and side scrapers 18 are being dragged through the material in the mixing pan 2 the suspension arms 8 also have to transfer a substantial torque from the mixer unit 5 to the mixing pan 2. Fig. 2 illustrates the counterflow mixer 1 shown in fig. 1, as seen from the top.

In this embodiment of the invention the mixer unit 5 is suspended above the mixing pan 2 by means of four substantially evenly spaced suspension arms 8. However in another embodiment of the invention the mixer 1 could comprise two, three, five, six or more suspension arms 8 and the suspension arms 8 could be spaced differently.

The reason why the suspension arms 8 are not entirely evenly spaced in this embodiment is that the suspension arm 8 comprising the aggregate inlet 11 is wider than the remaining three suspension arms 8 hereby making the opening 13 in the counter clockwise direction in relation to the aggregate inlet 11 smaller than the remaining three openings 13. However, in this embodiment the strength giving reinforcement 25 of the suspension arm 8 comprising the aggregate inlet 11 is placed symmetrically with the strength giving reinforcements 25 of the other suspension arms 8.

A counterflow mixer 1 is provided with one or more discharge openings 3 at the bottom 4 of the mixing pan 2. In this embodiment the mixing pan 2 is provided with two discharge openings 3 arranged at either sides of the pan 2. Each discharge opening 3 is provided with a sliding door so that when the concrete in the mixing pan 2 has been mixed sufficiently, one or more of the doors are opened and the concrete is emptied out of the mixer 1 through the discharge opening 3.

The concrete is now ready for use and could be emptied directly down into a waiting concrete lorry or it could be emptied down onto some kind of conveyer system which will transport the concrete to another location e.g. in a concrete panel manufacturing site or into a concrete chute, a concrete holding hopper or a similar device.

Fig. 3a-3c illustrates a suspension arm 8 comprising a cement inlet 9, as seen from the front, the top and the side. In this embodiment of the invention the suspension arm 8 is provided with a cement inlet 9 approximately at the middle of the arm 8. The cement inlet 9 could in one embodiment of the invention merely be a hole in the suspension arm 8 but in this embodiment the cement inlet 9 is formed as a cylindrical pipe. This is advantageous in that the cement inlet 9 in the form of the cylindrical pipe hereby can form part of the strength giving reinforcement 25 of the suspension arm 8 and thus adding strength to the suspension arm 8 instead of making the arm 8 weaker.

At the end of the arm 8 adapted to be connected to the upper edge of the mixing pan 2, the suspension arm 8 is provided with two additional inlets which could be used for exhausting air to compensate for the cement being entered through the cement inlet 9.

In this embodiment of the invention the connection pipe for the cement inlet 9 is approximately 360 mm in diameter whereby ensuring that a large quantity of cement can enter the mixing pan fast. However in another embodiment the connection pipe could be formed with a larger or a smaller diameter but ideally the diameter should be within a range of 100 mm to 700 mm depending on the capacity of the mixer 1. In this embodiment the cement inlet 9 is formed as a cylindrical pipe which is advantageous regarding the strength of the suspension arm 4 but in another embodiment of the invention the cement inlet 9 could be formed differently i.e. it could be formed oval to not increase the width of the suspension arms 8, it could be rectangular, square or shaped as a trapezium to fit the shape of the suspension arm 8 or it could be formed as some kind of more or less complex curve.

Fig. 4a-4c illustrates a suspension arm 8 comprising an inspection door 27, as seen from the front, the top and the side. In this embodiment the suspension arm 8 is provided with an inspection door 27 for use when inspecting the concrete mixing processes either manually or by camera. In this embodiment the door 27 is operated by a pneumatic cylinder but in another embodiment the door 27 could be manually operated.

In this embodiment of the invention the suspension arm is also provided with a water inlet 10 for adding water to the mixture in the mixing pan 2 during the concrete mixing process. In this embodiment both the inspection door 27 and the water inlet 10 are formed as cylindrical pipes but in another embodiment they could be formed differently as discussed in relation to fig. 3a-3c.

Fig. 5a-5c illustrates a suspension arm 8 comprising an aggregate inlet 11 and a connection pipe 26 for an air-bag, as seen from the front, the top and the side.

When aggregate is loaded into the mixer 1 through the aggregate inlet 11 it takes up a substantial part of the volume in the mixer 1. The air in the mixer 1 can comprise cement dust and other particles and it is therefore not desirable that this air is pushed out of the mixer 1 into the surrounding environment. So to control where the air - that the volume of the aggregate displaces - is exiting the mixer 1, the mixer 1 can be provided with an air-bag which in principle is a large bag into which the displaced air is pushed. In this embodiment of the invention the connection pipe 26 for the air-bag is forming part of the strength giving reinforcement 25 of the suspension arm 8 but in another embodiment it could be placed elsewhere on the suspension arm 8.

In this embodiment the aggregate inlet 11 is placed besides the strength giving reinforcement 25 because the connection pipe for the aggregate inlet 11 is so large that the pipe would have to be made with very thick walls to not weaken the suspension arm 8 if it was integrated in the strength giving reinforcement 25.

In this embodiment of the invention the connection pipe for the aggregate inlet 11 is approximately 645 mm in diameter whereby ensuring that a large quantity of aggregate can enter the mixing pan fast. However in another embodiment the connection pipe could be formed with a larger or a smaller diameter but ideally the diameter should be within a range of 200 mm to 900 mm depending on the capacity of the mixer 1.

As explained above the connection pipe for the aggregate inlet 11 could be formed different the circular in another embodiment of the invention.

Fig. 6 illustrates a counterflow mixer 1 with doors 14, as seen in perspective.

To protect an operator of the mixer 1 from coming into contact with the concrete in the mixer, from getting in contact with the mixing means 7, to protect the concrete in the mixer from foreign object, to hinder dust or concrete splashes for exiting the mixer and for other reasons the openings 13 between the suspension arms 8 are in this embodiment of the invention provided with doors 14 that can be closed during the concrete mixing process. In fact in an embodiment the doors 14 would be provided with safety switches (not shown) so that the mixer 1 can only operate if all the doors 14 are closed. In this embodiment of the invention the mixer 1 comprises four suspension arms 8 and thus also four openings 13. Three of these openings 13 are each provided with three doors 14 and the last opening 13 is only provided with two doors 14 in that the suspension arm 8 comprising the aggregate inlet 11 is so wide to one side that it takes up the space of exactly one door 14. However, in another embodiment each opening 13 could be provided with a different number of doors 14 such as one, four, five or more and/or all the openings 13 could be identical and thus be provided with an identical number of doors 14. In this embodiment of the invention the suspension arms 8 are formed so that they are wider at the end connected to the upper edge 12 of the mixing pan 2 that at the end connected to the mixer unit 5. This is advantageous in that it enables that the arms 8 are stronger especially regarding torque at the end connected to the mixing pan 2 i.e. at the end that is strained the most when the mixer 1 is operating.

Furthermore, the arm design enables that when each opening 13 is provided with more than one door 14 each of these doors 14 can be formed with an identical outer contour i.e. in principle all the doors 14 are identical except for the location of the hinges, which provides for a simpler manufacturing process and simpler logistic. In this illustration of the invention a cement weight hopper 28 is suspended above the cement inlet 9, a water weight hopper 30 is suspended above the water inlet 10 and a dust extraction filter 29 is suspended above a dust extraction outlet (not shown). The hoppers 28, 30 are prepared to be connected to the respective inlets 9, 10 by means of flexible couplers allowing for small misalignments between the hoppers 28, 30 and the respective inlets 9, 10.

From fig. 6 it is obvious that providing e.g. the cement inlet 9 in a suspension arm 8 is advantageous in that it frees the space at the centre of the mixer 1 and it makes it much easier to access the cement inlet 9 and particularly the cement weight hopper 28 while still ensuring that all the doors 14 can be opened and that sufficient access to the inside of the mixer 1 is provided, which is important particularly for inspection, maintenance and cleaning purposes.

The invention has been exemplified above with reference to specific examples of counterflow mixers 1, mixer units 5, suspension arms 8 and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

List

1. Counterflow mixer

2. Mixing pan

3. Discharge opening

4. Bottom of mixing pan

5. Mixer unit

6. Mixer unit drive

7. Mixing means

8. Suspension arms

9. Cement inlet

10. Water inlet

11. Aggregate inlet

12. Upper edge of mixing pan

13. Opening between suspension arms

14. Door

15. Mixing star

16. Centre axis of mixing star

17. Centre axis of mixer unit

18. Side scraper

19. Mixing arm

20. Mixing shovel

21. Side of mixing pan

22. Corner between side and bottom of mixing pan

23. Motor

24. Gear arrangement

25. Strength giving reinforcement

26. Air-bag connection pipe

27. Inspection door

28. Cement weight hopper 29. Dust extraction filter

30. Water weight hopper

Dc. Diameter of inscribed circle of cement inlet

Da. Diameter of inscribed circle of aggregate inlet

Claims

1. A counterflow mixer (1) for mixing concrete, said mixer (1) comprising a stationary mixing pan (2), comprising one or more discharge openings (3) at the bottom (4) of said mixing pan (2), a mixer unit (5) comprising a mixer unit drive (6) and one or more mixing means (7), and two or more suspension arms (8) extending between said mixing pan (2) and said mixing unit (5) hereby suspending said mixing unit (5) above said mixing pan (2), wherein at least one of said suspension arms (8) is provided with at least one cement inlet (9) adapted for leading cement into said mixing pan (2) through said suspension arm (8) during a concrete mixing process.

2. A counterflow mixer (1) according to claim 1, wherein one or more of said suspension arms (8) are also provided with at least one water inlet (10) adapted for leading water into said mixing pan (2) through said suspension arm (8) during a concrete mixing process.

3. A counterflow mixer (1) according to claim 1 or 2, wherein one or more of said suspension arms (8) are also provided with at least one aggregate inlet (11) adapted for leading aggregate into said mixing pan (2) through said suspension arm (8) during a concrete mixing process.

4. A counterflow mixer (1) according to any of the preceding claims, wherein the diameter (Dc) of the inscribed circle of said cement inlet (9) is between 50 mm and 1200 mm, preferably between 100 mm and 1000 mm, and most preferred between 120 mm and 800 mm.

5. A counterflow mixer (1) according to any of the preceding claims, wherein the diameter (Da) of the inscribed circle of said aggregate inlet (11) is between 50 mm and 1500 mm, preferably between 100 mm and 1200 mm, and most preferred between 150 mm and 1000 mm.

6. A counterflow mixer (1) according to any of the preceding claims, wherein said cement inlet (9), said water inlet (10) and/or said aggregate inlet (11) is formed as a cylindrical pipe.

7. A counterflow mixer (1) according to any of the preceding claims, wherein said suspension arms (8) are wider at the end connected to the mixing pan (2) than at the end connected to the mixer unit (5).

8. A counterflow mixer (1) according to any of the preceding claims, wherein said suspension arms (8) are evenly distributed around said mixer unit (5).

9. A counterflow mixer (1) according to any of the preceding claims, wherein the said counterflow mixer (1) comprises exactly three suspension arms (8).

10. A counterflow mixer (1) according to any of the preceding claims, wherein the said counterflow mixer (1) comprises exactly four suspension arms (8).

11. A counterflow mixer (1) according to any of the preceding claims, wherein suspension arms (8) are connected to an upper edge (12) of said mixing pan (2).

12. A counterflow mixer (1) according to any of the preceding claims, wherein the openings (13) between said suspension arms (8) are covered by two or more doors (14).

13. A counterflow mixer (1) according to claim 12, wherein said two or more doors (14) have a substantially identical shape.