EP1888829B1

EP1888829B1 - Collecting chamber and method in the manufacture of mineral fibres

Info

Publication number: EP1888829B1
Application number: EP06755393.3A
Authority: EP
Inventors: Erik ÅSTRAND; Bjarne Walli
Original assignee: Paroc Oy AB
Current assignee: Paroc Hold Oy AB
Priority date: 2005-06-01
Filing date: 2006-05-31
Publication date: 2013-05-01
Anticipated expiration: 2026-05-31
Also published as: UA91365C2; FI20050582A0; RU2378205C2; WO2006128955A3; CN101189379A; FI120735B; RU2007149326A; EP1888829A2; SI1888829T1; CN101189379B; WO2006128955A2; FI20050582A; DK1888829T3; PL1888829T3

Description

The present invention relates to a collecting chamber and a method in the manufacture of mineral fibres according to the preambles of the claims below.
Mineral wool, such as stone wool, is made by melting suitable raw materials, for example diabase, limestone or slag in a melting furnace. The mineral melt obtained is discharged from the melting furnace in the form of a melt jet to a fiberising apparatus, where the melt is formed into mineral fibres. Normally a fiberising apparatus of the spinning machine type is used, which comprises a series of rotating fiberising rotors or spinning rotors, typically 3 to 4 rotors. The mineral melt from the melting furnace is directed toward the mantle surface of the first rotor where it gets hold of the mantle surface of the rotor to a certain extent before it is thrown out as a cascade of drops against the mantle surface of the adjacent second rotor in the series. A part of the mineral melt gets then sufficient hold of the mantle surface of the second rotor in order to be formed into fibres due to the effect of the centrifugal force. Another part of the mineral melt is thrown further against the mantle surface of the third rotor. In this way the mineral melt is "transported" as a jet of mineral melt drops or a drop cascade, successively from one rotor to the subsequent one through the whole fiberising apparatus, while a part of the mineral melt is formed into mineral fibres. A binder may be applied on the formed mineral fibres, either during the fibre formation or after it.
The mineral fibres formed at the fiberising rotors are transported away from the fiberising apparatus by means of blowing off. The blowing off of mineral fibres can be arranged by so-called primary blow-off means, which have been placed at the peripheries of the rotors or with secondary blow-off means, which have been arranged at a distance from the fiberising apparatus. The mineral fibres are transported from the fiberising apparatus through a collecting chamber towards a collecting member, which is arranged in front of the fiberising apparatus. The collecting member can be, for example, a belt conveyor or a rotating drum.
The mineral fibres are usually collected as a thin fibre web, a so-called primary fibre web or a primary web. The primary fibre web is normally collected by a travelling perforated surface forming the collecting surface of the collecting member. The velocity at which the collecting surface proceeds defines the surface weight of the collected primary fibre web if the fibre mass flow from the fiberising apparatus is constant. The higher the velocity of the collecting surface is, the thinner the collected primary web becomes and the lower its surface weight is. Generally, the aim is to collect as thin primary mineral fibre webs as possible. Snarls and tufts of mineral fibre are undesirable in the collected fibre web, as they lower the quality of the end product. It is naturally also an aim to avoid holes in the collected primary mineral fibre web.
When the amount of mineral fibres produced by the fiberising apparatus increases, the collecting velocity of the collecting member must be raised in the same extent, so that the surface weight of the collected primary web is kept constant and that the surface weight of the web does not become too high. The velocity of the collecting member is, however, restricted by other apparatuses later on in the process and can therefore not be raised at will. The post-processing of both a thin as well as a fast primary mineral fibre web has proven to be complicated among other things because the thin web breaks easily in transport and because of possible overlapping.
The structure of the collected primary web is greatly influenced by the conditions in the collecting chamber during the travel of the mineral fibres from the fiberising apparatus to the collecting member. If the flow conditions in the collecting chamber are turbulent and back-eddies exist, the collected primary web easily becomes nodular and inhomogeneous. The fiberising and collecting processes also become difficult to control when the produced amount of fibre increases.
It is important to optimise each subprocess in the manufacture of mineral wool so that a desired end product is achieved. It has proven difficult to manufacture and to collect large amounts of fibre from the same fiberising apparatus without deteriorating the structure of the collected primary web and the resulting mineral wool.
There is number of different uses for mineral wool, for example as insulating material in various constructions. Depending on the use different requirements are made on the properties of the manufactured mineral wool, on its strength, compressibility etc. The properties of mineral wool are influenced by the properties of the individual mineral fibres, in other words by the thickness, length, as well as space orientation of individual fibres in the mineral wool. These properties are, among other things, influenced by the conditions that prevail when the mineral fibres are transported by the gas flows through the collecting chamber to the collecting member. The gas flows are formed by blowing off by the blow-off means at the fiberising apparatus as well as through suction through the surface of the collecting member. Back-eddies and gas vortexes formed in the collecting chamber are particularly problematic, among other things, as the rotating fiberising rotors cause some of the air adjacent to the rotors to rotate with it.
An increased production of mineral fibres creates a need to increase the amount of air required for transporting mineral fibres from the fiberising apparatus to the collecting chamber. The increased amount of air in the collecting chamber increases the risk of problematic gas vortexes around the fibre cloud that is transported from the fiberising apparatus towards the collecting member. At the same time the suction has to be increased as an increased amount of gas has to be removed from the collecting chamber, usually through the surface of the collecting member.
Traditionally collecting chambers have been formed as big "boxes", the first end of which is placed in the fiberising apparatus and the second end in the collecting member. The cross section of the first end of the collecting chamber is traditionally arranged to be smaller than the cross section of the second end.
WO87/0663 describes an apparatus and a method for making a mineral wool web using a collecting chamber having a neck.
Therefore, the object of this invention is to provide a method and an apparatus for the manufacture of mineral wool fibres, where the above-mentioned disadvantages are minimised.
The object is thus to achieve a collecting chamber having a structure that facilitates a turbulence free fibre transport through the collecting chamber towards the collecting member.
Another object of the present invention is to achieve a method by means of which the formation of back-eddies can be minimised or prevented.
These objects are attained with a method and an apparatus having the characteristics presented in the characterising part of the independent claims below.
A typical collecting chamber according to the present invention comprises a first end towards a fiberising apparatus which produces mineral fibres from mineral melt, a second end towards a collecting member to which mineral fibres are transported by gas flows from the fiberising apparatus, between which first and second ends a ceiling, and a first and a second side wall are arranged. The cross section of the first end of the collecting chamber has a first area, and the cross section of the second end has a second area, which cross sections are defined by the ceiling, the first and second side walls as well as the cross sectional plane that horizontally touches the lowest rotor of the fiberising apparatus at its lower edge. The collecting chamber comprises a waist arranged between the first and the second end, the length of which waist is at least 100 mm and the cross section of which having a third area which is at the most 10 % larger than the first area and smaller than the second area.
A typical method according to the invention typically comprises the following steps

mineral melt is led to a first rotating fiberising rotor in a fiberising apparatus comprising at least two fiberising rotors,
mineral melt is thrown from the mantle surface of the first rotating rotor to the mantle surface of a second rotating rotor, and from there successively further to the mantle surfaces of possible following rotors,
mineral melt is formed to mineral fibres at the rotating fiberising rotors,
the formed mineral fibres are blown off from the fiberising rotor of the fiberising apparatus with a fibre blow-off means towards a collecting member through a collecting chamber, comprising a first end towards the fiberising apparatus, which first end has a first area, a second end towards the collecting member which second end has a second area, between the first and the second end a ceiling, a first and a second side wall which with a cross sectional plane that horizontally touches the lowest rotor of the fiberising apparatus at its lowest edge define the first and second areas, and
the formation of back-eddies in the collecting chamber is minimised by means of a waist arranged between the first and the second end, the length of which waist is at least 100 mm and the cross section of which has a third area which is at the most 10 % larger than the first area and smaller than the second area.

Now it has surprisingly been found that by forming the collecting chamber between the fiberising apparatus and collecting member, so that the collecting chamber has a waist, the back-eddies formed between the collecting member and fiberising apparatus can considerably be reduced. The waist forms a mechanical restraint reducing the formation of back-eddies between the collecting member and the fiberising apparatus. The conditions for fibre transport from the fiberising apparatus are in this manner made more stabile and homogenous than before as the flow conditions at the fiberising end of the collecting chamber are made more laminar and less turbulent.
The collecting chamber is usually arranged approximately 300 - 700 mm, preferably 350 - 450 mm in front of the fiberising apparatus, as measured from the nearest edge of the fiberising rotor. The first area, i.e. the cross section area of that end of the collecting chamber facing the fiberising apparatus is usually measured at this point. According to an embodiment the distance between the first end of the collecting chamber and the upper fiberising rotor of the fiberising apparatus is arranged to be shorter than the distance between the first end of the collecting chamber and the lower fiberising rotor of the fiberising apparatus, i.e. the fiberising apparatus is located at an angle in relation to the first end of the collecting chamber.
According to the present invention a waist is arranged in the collecting chamber between the first and second end thereof, the length of which waist is at least 100 mm and the cross section of which has a third area which is at the most 10 % larger than the first area and smaller than the second area. The waist can be arranged as a channel or a passage located in that end of the collecting chamber that opens up towards the fiberising apparatus, i.e. in the fiberising end of the collecting chamber. The waist forms thus a fibre controlling part of the collecting chamber. The part of the collecting chamber that is located between the waist and the second end can be called the fibre forming part of the collecting chamber.
The present invention is advantageously combined with a fiberising apparatus of the cascade type described above. Such an apparatus usually comprises from three to four rotating fiberising rotors and the fibres are blown off from the rotating rotors in a horizontal direction towards the collecting member.
According to the invention the waist has a certain length, which is at least 100 mm, and its cross section has a certain area, which is always smaller than the second area. It can be thought that the waist forms a pipe-like duct, such as an air channel, through which mineral fibres and gas flows flow from the fiberising apparatus towards the collecting member. The upper part of the waist can be rounded, whereby it resembles a half of a pipe. The cross sectional area of the waist can grow slowly towards the collecting member, the inclination of the ceiling of the waist and/or side walls is then usually less than 5°. The inclination of the ceiling of the collecting chamber can become a great deal steeper at the point where the waist ends, in other words, the inclination of the ceiling is typically at least 20°. According to one embodiment of the invention the discontinuity point of the derivative exists at the point where the waist ends and the fibre forming part continues.
The cross section of the waist can be smaller than or as large as the first area. The cross section of the waist can in other words also decrease towards the point where the waist ends and where it is united with the fibre forming part of the collecting chamber. The reduction is usually relatively small, the inclination of the ceiling of the waist is usually less than 5°. According to a preferred embodiment the cross section of the waist is kept constant over the entire length of the waist.
According to an embodiment of the present invention the length of the waist can vary within the range of approximately 150 mm to 1150 mm, typically 200 - 1100 mm, more typically 200 - 800 mm, preferably 150 - 600 mm. According to another embodiment the length of the waist varies within the range of 400 - 1000 mm, typically 500 - 1100 mm, more typically 500 - 800 mm, preferably 550 - 700 mm.
According to an embodiment of the present invention the waist may comprise an adjustment seam and an adjustment member for changing the length of the waist. The adjustment member can, for example, be an overlapping seam and it can be operated manually or it can be automated. In changing the length of the waist, naturally the length of the collecting chamber is changed, whereby also the distance of the fiberising apparatus from the collecting apparatus is changed accordingly. According to an embodiment of the present invention the distance between the fiberising apparatus and the collecting member is usually 0.75 - 5.5 m, preferably 1.5 - 2.75 m. The distance can also be between 1.5 - 5 m, typically 2.5 - 3.5 m.
The fibre forming part of the collecting chamber is usually streamlined and its ceiling can be formed in the second end arched down towards the collecting member. The distance between the collecting member and the end of the ceiling of the fibre forming part, i.e. that part of the ceiling which is located closest to the surface of the collecting member, can be arranged adjustable by means of a separate movable ceiling part, which is arranged in contact with the actual ceiling. By means of the movable ceiling part the distance between it and the collecting member is usually varied between 40 - 400 mm, typically between 100 - 300 mm.
The distance between the first and the second wall of the collecting chamber forms the inner width of the fibre forming part of the collecting chamber. This width is often equal to the collecting width of the collecting member.
According to an embodiment the waist can be formed of two sides inclined towards each other and a ceiling. The sides and the ceiling can be arched in order to make the waist more streamlined. The arching of the sides and the ceiling follows advantageously the outer profile of the fiberising apparatus, i.e. the form of the cross section of the fiberising apparatus at the fiberising rotors. In this manner the amount of air sucked into the collecting chamber from the sides of the fiberising apparatus can be reduced. As this amount of air can be minimised there is no need to suck it out from the collecting chamber.
According to a preferred embodiment of the invention the total cross section area of the fiberising rotors of the fiberising apparatus is at least 10 %, preferably between 15 - 35 % of the cross section of the waist, i.e. of the third area, when the third area is defined by the first and second side walls, ceiling and cross sectional plane. If, for example, the fiberising rotor of the fiberising apparatus has a total cross sectional area of 0.33 m² the cross sectional area of the waist is 2.2 m².
According to a preferred embodiment of the invention a shot separator can be connected to the fiberising end of the collecting chamber. The shot separator can be furnished with a cavity in which a transport means, such as a screw, can be arranged. With the transport means collected shot can be removed from the collecting chamber. The possible shot separator is placed in the space between the fiberising apparatus and the first end of the collecting chamber.
According to an embodiment of the invention the shot separator can be connected to an air inlet channel, the cross section of which mainly follows the profile of the waist at the first end. This air inlet channel extends typically 50 - 600 mm away from the first end of the collecting chamber measured from the fiberising rotors, from the edge which is located furthest away from the collecting chamber.
To the fiberising end of the collecting chamber a connecting channel can further be connected, which channel is formed by the area under the fiberising apparatus itself. The connecting channel and the collecting chamber are advantageously approximately of the same width, and the height of the connecting channel can vary between 0.5 - 1.0 times the inner width of the collecting chamber. The connecting channel is advantageously arranged in such a manner that its ceiling forms the platform, upon which the fiberising apparatus is placed during production conditions. The air, which was sucked from the collecting chamber through the collecting apparatus, can be returned to the connecting channel after filtering and cooling.
According to an embodiment of the invention a number of rolls can be additionally arranged in the collecting chamber. In the collecting chamber can, for example, a first threshold roller be arranged at the seam between the floor and the collecting member, as well as a ceiling roller at the seam between the fibre forming part of the collecting chamber and the collecting member. If the collecting member comprises two or more collecting surfaces, e.g. collecting drums, one or more rolls can be arranged between the collecting surfaces.
Some embodiments of the present invention are described in more detail below with reference to the enclosed figures, in which

Figure 1: schematically shows an embodiment of the collecting chamber according to the present invention as seen from the side,
Figure 2A - 2C: schematically show different embodiments of the collecting chamber according to the present invention as seen from the side, and
Figure 3: schematically shows an embodiment of the collecting chamber according to the present invention as seen from the side,
Figure 4: schematically shows in perspective view an embodiment of the collecting chamber according to the present invention.

In figure 1 an embodiment of the collecting chamber according to the present invention is schematically shown. A collecting chamber 1 is arranged between a fiberising apparatus 2 and a collecting member 3. A shot separator 5 is connected to the collecting chamber 1 for collecting possible shot formed during the manufacture of mineral wool. An air inlet channel 5' is connected to the shot separator 5. A first end 1' of the collecting chamber 1 is located at a distance A in front of the fiberising apparatus 2. In figure 1 the first end 1' of the collecting chamber 1 is visualised with a line 10 the cross section of which first end has a first area. A line 11 defines a boundary level which horizontally touches the lowest rotor 2' of the fiberising apparatus 2 at its lower edge. A second end 1" of the collecting chamber 1 is visualised with a line 12, the cross section of which second end has a second area. Line 12 touches the collecting member 3 at a point which is located closest to the fiberising apparatus 2. Between the first end 1' and the second end 1" of the collecting chamber 1 a waist 4 is arranged, the cross section of which has a third area. This third area is smaller than the area of the second end 1", and as large as or smaller than the area of the first end 1'. The line 13 visualises the point where the waist 4 ends and the fibre forming part of the collecting chamber 1 begins.
In figure 1 also a first threshold roller 7 is shown at the seam between a floor 8 and the collecting member 3. A second threshold roller 7' is arranged at the seam between the fibre forming part 6 of the collecting chamber and the collecting member 3. To the first end 1' of the collecting chamber also a connecting channel 8' is connected, the ceiling 8 of which forms the platform upon which a fiberising apparatus 2 is placed.
In figure 2A an embodiment of the collecting chamber according to the present invention is schematically shown. A collecting chamber 21 is arranged between a fiberising apparatus 22 and a collecting member 23. To the collecting chamber 21 a shot separator 25 is arranged. A first end 21' of the colleting chamber 21 is located at a distance A in front of the fiberising apparatus 22. The first end 21' of the collecting chamber is visualised with a line 20 the first cross section of which end has a first area, and a second end 21" of the collecting chamber 21 is visualised with a line 21', the cross section of which end has a second area. Line 20' touches the collecting member 23 at a point P which is located closest to the fiberising apparatus 22. Between the first end 21' and the second end 21" of the collecting chamber 21 a waist 24 is arranged, the cross section of which has a third area. This third area is smaller than the area of the second end 21", and as large as or smaller than the area of the first end 21'. The point where the waist 24 ends and the fibre forming part 26 of the collecting chamber 21 begins is visualised with the line 20". In figure 2A the cross section of the waist 4 is kept constant over the entire length X of the waist 4.
In figure 2B another embodiment of the collecting chamber according to the present invention is schematically shown. The numbering and parts shown in figure 2B correspond to those shown in figure 2A. The cross section of the waist 24 is smaller than the area of the second end 21", and as large as or smaller than the area of the first end 21'. In figure 2B the cross section of the waist 24 decreases towards the fibre forming part 26.
In figure 2C an embodiment of the collecting chamber according to the present invention is schematically shown. The numbering and parts shown in figure 2B correspond to those shown in figures 2A and 2B. The cross section of the waist 24 is smaller than the area of the second end 21", and as large as or smaller than the area of the first end 21'. In figure 2C, however, the cross section of the waist 24 grows towards the fibre forming part 26, in other words the ceiling 24' of the waist 24 shows an inclination and forms an angle α with an imaginary horizontal line.
In figure 3 an embodiment of the collecting chamber according to the present invention is schematically shown. A collecting chamber 31 is arranged between a fiberising apparatus 32 and a collecting member 33, and between a first end 31' and second end 31" of the collecting chamber 31 a waist 34 is arranged, the cross section of which is essentially kept constant over the entire length of the waist. To the collecting chamber 31 a shot separator 35 is connected. A threshold roller 37 is arranged at the seam between a floor 38 of the collecting chamber 31 and the collecting member 33. By means of dashed lines the approximate length of the suction surface of the collecting member 33 is shown.
In figure 3 is shown how the waist 34 prevents the flowing of fibre gas flows 9 from the collecting member 33 towards the fiberising apparatus 32. The gas flows towards the fiberising apparatus 33 are prevented by the ceiling 36' of the fibre forming part 36 of the collecting chamber 31. The waist 34 forms a mechanical restraint, blocking the back flow of the streams 9 to the fiberising apparatus 32.
In the ceiling end 36" of the fibre forming part 36 of the collecting chamber 31 a separate movable ceiling portion 40 is arranged. With the ceiling portion 40 the distance between the ceiling end 36" and the collecting member 33 can be varied. The ceiling portion 40 can be arranged movable, for example by means of an overlapping. Behind the roof portion 40 a sealing means 41 is arranged. In an end 41' of the sealing means 41 a valve means 42 is arranged. The valve means 42 is usually of rubber or some corresponding material, and it has been arranged movable. The position of the valve means 42 is influenced by the pressure that prevails in the collecting chamber 31. If the pressure in the collecting chamber is optimal the position of the valve means is straight. If an underpressure prevails in the collecting chamber the valve means is sucked towards the collecting chamber and the fiberising apparatus. If an overpressure prevails in the collecting chamber the valve means is pushed away from the collecting chamber and the fiberising apparatus. The position of the valve means can therefore be used under certain conditions for indicating pressure conditions in the collecting chamber. The pressure conditions in a levelling chamber 43 are controlled by controlling the distance between the collecting member 33 and the ceiling end 36 ". By controlling the pressure conditions the structure of the collected primary web can be affected, while roll ups and nodules can be minimised. Then a primary web showing decreased variations in density is obtained.
In figure 4 an embodiment of the collecting chamber according to the present invention is schematically shown in a perspective view. In this embodiment the collecting member is formed by two collecting drums 53, 53'. The collecting chamber 51 is arranged between the fiberising apparatus 52 and the collecting drums 53, 53'. A waist 54, the cross section of which is essentially kept constant over the entire length of the waist, is arranged between the first end 51' and the second end 51" of the collecting chamber 51. A shot separator 55 is also connected to the collecting chamber 51. The mineral fibres are collected on the collecting surfaces of the rotating collecting drums 53, 53' and the fibres are removed from the collecting chamber 51 in the form of two primary fibre webs 56, 56' comprising mineral fibres and binder which is applied to the fibres at the fiberising apparatus 52 or in the collecting chamber 51.
Collecting chambers according to the present invention are well suited to be used with a fiberising apparatus having a high fibre producing capacity. Such fiberising apparatuses are disclosed for example in Finnish Patent Applications FI 20011561 and FI 20011562 . When a collecting chamber according to the present invention is used in conjunction with a fiberising apparatus having a high fiberising capacity turbulence free conditions can be guaranteed in the collecting chamber even if the produced amount of fibre is high. In this manner fibre orientation in the primary fibre web can be improved.

Claims

Collecting chamber in the manufacture of mineral fibres,
which collecting chamber comprises
- a first end towards a fiberising apparatus, which produces mineral fibres from mineral melt,

- a second end towards a collecting member, to which the mineral fibres are transported by gas flows from the fiberising apparatus,

- between the first and second end arranged

- a ceiling, and

- a first and a second side wall,
where the cross section of the first end of the collecting chamber has a first area, and the cross section of the second end has a second area, which cross sections are defined by the ceiling, the first and second side walls as well as a cross sectional plane that horizontally touches the lowest rotor of the fiberising apparatus at its lower edge, the first area being measured at the edge of the fiberising rotor nearest to the collecting member and the second area at a point of the collecting member located closest to the fiberising apparatus,characterised in that
the collecting chamber comprises a waist arranged between the first and second end, the length of which waist is at least 100 mm and the cross section of which has a third area being at the most 10 % larger than the first area and smaller than the second area.
Collecting chamber according to claim 1, characterised in that the third area is as large as or smaller than the first area.
Collecting chamber according to claim 1 or 2, characterised in that the cross section of the waist is constant over the length of the waist.
Collecting chamber according to claim 1, characterised in that total cross sectional area of the fiberising rotors of the fiberising apparatus is at least 10 %, advantageously between 15 - 35 % of the third area.
Collecting chamber according to claim 1, characterised in that the length of the waist is in the range of 150 mm to 1150 mm, preferably between 200 - 1100 mm.
Collecting chamber according to claim 1, characterised in that the first end of the collecting chamber is arranged approximately 300 - 550 mm, preferably 350 - 450 in front of the fiberising apparatus.
Collecting chamber according to claim 1, characterised in that the waist comprises an adjustment seam and an adjustment member for changing the length of the waist.
Collecting chamber according to claim 1, characterised in that the collecting chamber comprises a movable ceiling portion, by means of which the distance between the ceiling edge of the collecting chamber and the collecting member is varied between 40 - 400 mm.
Method in the manufacture of mineral fibres, in which method
- mineral melt is led to a first rotating fiberising rotor in a fiberising apparatus comprising at least two fiberising rotors,

- mineral melt is thrown from the mantle surface of the first rotating rotor to the mantle surface of a second rotating rotor, and from there successively further to the mantle surfaces of possible following rotors,

- mineral melt is formed to mineral fibres at the rotating fiberising rotors,

- the formed mineral fibres are blown off from the fiberising rotor of the fiberising apparatus by means of fibre blow-off means towards a collecting member through a collecting chamber, comprising

- a first end towards the fiberising apparatus, which first end has a first area,

- a second end towards the collecting member, which second end has a second area, the first area being measured at the edge of the fiberising rotor nearest to the collecting member and the second area at a point of the collecting member located closest to the fiberising apparatus,
a ceiling between the first and second end, and a first and a second side wall, which together with a cross sectional plane, which horizontally touches the lowest rotor of the fiberising apparatus at its lower edge, define the first and second areas,
characterised in that
the formation of back-eddies in the collecting chamber is minimised by means of a waist arranged between the first and the second end, the length of which waist is at least 100 mm and the cross section of which has a third area which is at the most 10 % larger than the first surface area and smaller than the second area.
Method according to claim 9, characterised in that the length of the waist is varied by means of an adjustment seam and an adjustment member.
Method according to claim 9, characterised in that the distance between the ceiling edge of the collecting chamber and the collecting member is varied between 100 - 300 mm by means of a movable ceiling panel.