WO1998035916A1

WO1998035916A1 - Method and apparatus for producing man-made vitreous fibres

Info

Publication number: WO1998035916A1
Application number: PCT/EP1998/000274
Authority: WO
Inventors: Leif Trier Jensen; Thomas Heldgaard
Original assignee: Rockwool International AS
Current assignee: Rockwool AS
Priority date: 1997-02-12
Filing date: 1998-01-20
Publication date: 1998-08-20
Anticipated expiration: 1999-08-12
Also published as: GB9702803D0; AU6211298A

Abstract

An assembly and process for producing MMV fibres are provided in which a furnace (1) is used which maintains during melting a superatmospheric pressure and has in its wall (2) at least two outlets (3, 4), of which at least one is an adjustable outlet (4) which comprises flow control means (17) capable of modifying the flow rate of melt (19) through the adjustable outlet. At least two channels (9, 10) are provided each leading from an outlet to a spinning apparatus (7, 8) so that melt is led from one outlet and along one channel to one spinning apparatus.

Description

METHOD AND APPARATUS FOR PRODUCING MAN-MADE VITREOUS FIBRES

The invention relates to processes of producing man- made vitreous fibres (MMVF) and to an assembly for carrying out these processes.

It is known to produce man-made vitreous fibres such as glass wool and stone wool by forming a melt having suitable composition for glass or stone wool (as desired) , sending this to a spinner apparatus, and pouring melt onto or into a spinning rotor from which it is flung centrifugally as fibres. Often the spinner apparatus comprises more than one spinning rotor, often 3 or 4. Each has a solid periphery onto which melt is poured. Melt is flung from the first rotor to the second rotor, from which it is flung partially as fibres and partially onto the third rotor and then onto the fourth rotor, if used. Manufacturers are constantly searching for ways to improve the quality of the wool produced.

It is known, for instance from our publications 096/38391, EP-A-567,480 and EP-A-551 , 334 , that improved wool quality can be obtained in certain processes on a single spinner but at the expense of a reduction in throughput and capacity. One way to regain the lost throughput is to use two (or more) parallel spinners each operating at lowered throughput but giving improved wool quality.

In standard systems for MMVF production the melt is formed in a furnace. The furnace is usually charged with solid mineral materials, which are then heated to form a melt. The melt then exits the furnace and is led to the spinner. Normally a single furnace provides the melt for a single spinner. However, in dual or multiple spinner systems the provision of an additional furnace for each additional spinner results in a large increase in the amount of energy required for the process and would require investment in large amounts of new equipment at huge cost. Further, however carefully the charges are regulated, compositions of melts exiting the furnaces may not be identical and as a result the compositions of fibres formed on different spinners may not be the same.

We have therefore considered recently the possibility of providing a production line in which a single furnace provides the melt for more than one spinner.

However, standard furnaces have a single melt outlet. We have provided systems in which we divide the melt stream exiting the furnace by directing it first to a balance trough, which divides the melt and forms two streams which can be sent to two spinners. Such a system is described in US 3,709,670. Unfortunately, the use of such a balance trough necessitates an increase in the length of the pathway travelled by the melt from the furnace to each spinner. Increased length of pathway increases the time during which the melt can cool before reaching the spinner. This is undesirable because it can prevent the melt having a high enough temperature when it is poured into or onto the spinning rotor, as well as leading to a melt viscosity which is too high and to melt inhomogeneity . Systems which include a balance trough are consequently really only appropriate for spinners which are very close together, and in particular those which are in the same spinning chamber.

It is known to use a furnace, for instance a cupola furnace, having two outlets, as described in DE 2,729,167. In the described system the two outlets feed onto two top spinning wheels of a single ^' spinner apparatus.

A particular disadvantage of this known furnace is its tendency to give uneven melt flow rates, in particular widely differing melt flow rates from the two outlets. In the cupola furnace the pressure is greater than atmospheric pressure. The excess pressure forces the melt out of the furnace. If one of the outlets should become slightly blocked and suffer a decrease in the flow rate of melt through that outlet, the pressure in the furnace does not diminish. This results in flooding of melt out of the other outlet. Highly uneven flow of melts from the two outlets arises.

For the provision of two (or more) very similar parallel production lines it is highly undesirable to obtain these fluctuating flow rates. In particular, if the lines are designed to be of very similar capacity and quality, widely differing flow rates from the two outlets are most inconvenient.

It would be desirable to be able to provide an assembly in which two or more spinners can be operated at carefully controlled relative throughput and giving carefully controlled MMVF quality. It would be particularly desirable to be able to achieve this in a system in which a single furnace provides the melt stream for two or more spinners. It would be desirable to be able to do this in particular for spinners which are not close together, especially those in separate chambers, without the necessity to have the melt travel over a relatively long distance to reach the spinners. According to the invention there is provided an assembly for the production of man-made vitreous fibres comprising

(1) a furnace for melting mineral material which is designed to maintain in the furnace during melting a pressure greater than atmospheric pressure and which comprises a furnace wall which defines the space in which a melt is formed from charged mineral material and at least two outlets in the furnace wall through which melt can pass to the exterior, and in which at least one of the outlets is an adjustable outlet which comprises flow control means capable of modifying the flow rate of melt through that outlet, (2) at least two spinning apparatuses which each comprise a rotor which is capable of spinning rapidly so as to fling melt poured onto or into the rotor from it centrifugally as fibres, and

(3) at least two channels each leading from an outlet to a spinning apparatus and by which melt can be led from an outlet to a spinning apparatus in which assembly the number of spinning apparatuses is equal to the number of outlets so that each outlet feeds a single spinning apparatus.

This assembly has the advantage that it provides two or more spinning apparatuses (spinners) which are fed from the same furnace. Consequently, melt composition can be consistent on all spinners. Because each spinner is fed from a single furnace outlet, the pathway from the furnace to the spinner is direct and can be made as short as possible. For instance the furnace can be located between the spinners. This alleviates the problem of cooling of the melt after it exits the furnace and allows the required high temperature to be maintained at the spinner. Further, the disadvantages of known multiple-outlet furnaces are also alleviated by the provision of the at least one adjustable outlet. If flow out of one outlet is in some way reduced, the flow control means of the adjustable outlet can be used to modify the flow rate of melt through that outlet instantaneously, so as to avoid or reduce flooding until the flow of melt through the other outlet is adjusted. Consequently the flow rates of melt to the spinners can be maintained constant relative to one another throughout the process of producing the fibres. The fibres produced on the spinners can therefore be controlled to provide mineral fibre of substantially identical quality and in substantially identical quantities.

In the furnace, an outlet can be any arrangement which allows melt to pass to the exterior. For instance it may be a simple orifice in the furnace wall or a pipe. A preferred outlet is one which comprises a lip over which melt flows to reach the exterior. This lip can for instance be provided by a lower outlet wall which is angled with respect to the furnace wall and protrudes therefrom. The lip is provided by the upper, free end of this angled lower wall, the lower end being connected with the furnace. A preferred outlet type is of the type known as a siphon-type outlet. In such an outlet the lip height is above the level of the surface of the melt in the furnace. In use, the excess pressure in the furnace provides a "head" which forces the melt up to the height of the lip. The melt overflows the lip and passes to the exterior of the furnace. Such outlets are known and are described in for instance US 4,057,231 for use in steel making furnaces, for instance in Figure 4 of that document.

In such a siphon outlet the pressure variables are expressed as follows: Δp = p.g.h + k.v + (pv ) in which Δp is the excess pressure, p is the density of the melt, g is acceleration due to gravity, h is the difference in height between the surface of the melt in the furnace and the lip, v is the rate of flow of melt over the lip and k is a constant. Thus the total excess pressure is made up of the pressure due to the "head", the viscous resistance k.v and the dynamic pressure (pv ) . The excess pressure Δp is constant, thus if any of the variables changes, for instance the constant k relating to the viscous resistance, the flow velocity v will tend to change to compensate. The height h of the lip can thus be adjusted to rectify the situation.

In a siphon outlet the part of the furnace wall above the outlet forms an upper limit on the outlet. The lip may be provided higher than or lower than or at the same height as this upper limit. The pressure forcing the melt over the lip is high enough to ensure that the melt stream contacts the upper limit of the outlet, so that there is no air gap through which gases may flow into and out of the furnace. Thus the lip, if it is below the upper limit of the outlet formed by the furnace wall, must not be so far below it that such a gap exists. A particular advantage of the invention lies in the fact that certain standard outlets can be modified to form adjustable outlets without major remodelling of the outlet. For instance a simple sliding plate may be used in combination with a simple outlet such as a pipe or orifice. This is arranged to be able to change the cross-sectional area of the outlet so as to modify the flow rate of melt passing through the outlet. For instance reducing the cross-sectional area of an orifice in a furnace wall will tend to reduce the rate of flow of melt through that orifice. Various systems for adjusting outlets have been described. For instance US 1,090,101 describes a moveable metal plate which changes the size of an orifice through which molten glass is flowing. Plates of this type adapted for use on a furnace outlet can be used in the invention.

In preferred adjustable outlets the melt flows through the outlet to the exterior over a lip. Modification of the flow rate of the melt is carried out by adjustment of the height of this lip. If a siphon-type outlet is used having an angled lower wall of which the upper end forms the lip, this may be designed so that the entire outlet, or at least the lower wall, is pivotable. As the outlet or lower wall pivots the vertical height of the upper end of the lower outlet wall is varied. Thus for instance if flooding through an adjustable outlet occurs, the outlet can be pivoted so as to increase the height of the upper end of the lower outlet wall.^' Flow is thus reduced. Alternatively, if it is desired to increase flow through such an adjustable outlet, pivoting can be induced so as to reduce the vertical height of the upper end of the lower outlet wall. Such an adjustable outlet is described in the context of a single-outlet furnace for purposes other than production of MMVF in DE 2,610,333. Pivotable outlets of this type can be used in the present invention. Alternative outlets which rely on adjustment of the vertical height of the overflow lip and which can be used in the invention are described in US 3,348,937 (for use with molten glass) . Such outlet configurations can be effective within the assembly of the invention.

A preferred adjustable outlet comprises a weir gate which lies in contact with the lower outlet wall along its length and is moveable parallel to the wall towards and away from the upper end. This system is particularly convenient, at least in part because existing furnace outlets of the siphon type can be modified by the provision of a single weir gate. A weir gate is particularly useful because it is simple and economical to provide and to maintain and can give rapid reaction to changes in flow rate.

The weir gate may be provided on the side of the lower outlet wall on the exterior or the interior of the furnace. Preferably it is provided on the exterior. In initial operation the top edge of the weir gate is at a height lower than that of the upper end of the lower outlet wall. If flooding through the adjustable outlet occurs, the weir gate can be moved, parallel to the lower outlet wall and in contact with it in a sliding fashion towards the upper end of the lower outlet wall. With sufficient movement the top edge of the weir gate reaches a vertical height greater than that of the upper end of the lower outlet wall. This top edge then forms the lip over which the melt flows. This top edge can be raised to whichever height is necessary to reduce the flow of melt from that outlet to a sufficient degree.

Conversely, if it is desired to increase the rate of flow of melt from the adjustable outlet, the weir gate can be slid in the opposite direction, away from the upper end of the lower outlet wall, so as to reduce the height of the lip over which the melt flows.

The weir gate and any plates or other devices used as flow control means for modifying the flow rate of melt through the adjustable outlet may be made of any suitable material, for instance iron, refractories, steel and steel alloys. If necessary, these parts of the apparatus, together with any other elements forming the outlet, adjustable or otherwise, may be cooled in any suitable manner. For instance they may be water-cooled. In the assembly of the invention the furnace may possess outlets of which all are adjustable. Alternatively specific outlets may be chosen to be adjustable. In preferred systems not more than one outlet is non- adjustable, and more preferably all outlets are adjustable. Preferably each adjustable outlet is adjustable independently of all other outlets.

Adjustment is usually done manually, as a result of observations by the operator of the furnace and spinners. However, in the invention the adjustment of one or more outlets may be controlled automatically, in response to increase or decrease in flow through any of the outlets.

Control of the outlets may be designed so that each outlet is controlled individually with a local control loop. Thus the flow from each outlet is monitored individually so that on increase or reduction of flow from that outlet adjustment of the outlet can be carried out so as to increase or decrease the flow through that outlet as required. Thus the time lag between any change in the flow through an individual outlet and correction of that change is reduced and flow through other outlets is not interfered with. The flow through an outlet may be measured by any suitable means, for instance by measuring the consumption of electrical energy on the corresponding spinner.

The furnace has at least two outlets. Each outlet feeds a single spinning apparatus. In the invention it is possible to use a furnace having 3, 4 or more outlets, feeding a corresponding number of spinners, but it is not usually necessary to use more than two outlets and two spinners. In preferred assemblies of the invention the furnace has two outlets of which one is adjustable, although it is possible to use a furnace in which both outlets are adjustable.

The combination of types of outlet may be chosen as required. It is possible to use furnaces having outlets of different types. Usually however all outlets are of basically the same construction, with the adjustable outlets having modifications to provide flow control means which enable variation of the flow rate of melt through these. The furnace used in the assembly of the invention comprises a wall which defines the space in which the melt is produced. Raw materials are charged to the furnace through a charge inlet, normally positioned in the upper half of the furnace wall. As the charge is heated a melt forms and molten material trickles down through the charge to form a pool of melt in the lower half of the furnace in the interstices between the pieces of coke carrying the charge. This melt can then exit the furnace through the outlets. In the invention the outlets are normally provided in the same position as in standard furnaces.

The temperature in the bottom of the furnace is normally from 1,300 to 2,3O0°C, to provide a melt exiting the outlet having a temperature of from 1,300 to 1,700°C. Pressure in the furnace is above atmospheric pressure. Often it is at a pressure of at least 1.03, often at least 1.05 times atmospheric pressure. Suitable furnaces often operate at pressure up to 1.1 times atmospheric pressure. Particularly suitable furnaces are cupola furnaces. In a cupola furnace increased pressure is supplied by air blasted in at the base of the furnace through tuyeres. The air, together with some waste products, passes out through an exhaust outlet at the top of the furnace.

Although not included within the scope of the present invention, the system may also be applied to furnaces which are not designed to maintain a pressure greater than atmospheric pressure. For instance it can be applied to tank furnaces.

In a second aspect of the invention we also provide a process of producing man-made vitreous fibre comprising (1) providing a furnace which comprises a furnace wall which defines the space in which a melt is formed from charged mineral material and at least two outlets in the furnace wall

(2) providing at least two spinning apparatuses which each comprise a rotor,

(3) providing at least two channels which each lead from one outlet to one spinning apparatus,

(4) charging the furnace with mineral material and melting it under a pressure which is greater than atmospheric pressure to form a melt,

(5) allowing the melt to flow out of the furnace through the outlets and along the channels to the spinning apparatuses,

(6) pouring the melt onto or into the spinning rotors and allowing it to be flung centrifugally from the rotors as fibres, in which process the flow rate of the melt through at least one outlet is modified independently of the flow rate through other outlets. By this process we achieve the various benefits of the assembly of the invention.

In the process the assembly of the invention is used.

In particular, any of the furnaces described for use in the assembly are suitable. In the process and assembly of the invention melt flows through the outlets to the exterior of the furnace.

From an outlet it then passes to a channel which leads from the outlet to one spinning apparatus. This channel may be a suitable means of conveying melt, such as a trough or pipe and may be made of any suitable material, for instance iron and/or refractories, which may each be cooled with water. The route taken by the channel along which the melt is led to the spinning apparatus should be as direct as possible between the outlet and the spinning apparatus. Preferably it is substantially straight.

In preferred embodiments of the process and assembly of the invention, each spinning apparatus is situated in a separate spinning chamber. Fibres produced for each spinning apparatus are collected, normally on a collecting belt, in the relevant chamber and taken to a transport system for cross-lapping. The several fibre fleeces thus produced are then sent to a single line in which they pass through a single curing oven, cooling zone and cutting stations etc as normal.

In the process of the invention the flow rate of melt out of at least one of the outlets is adjusted independently of the flow rate out of other outlets. This is done in any of the ways described for the assembly of the invention. By doing this it is possible to maintain the ratios of flow rate from the outlets at substantially constant values throughout the process of producing the mineral fibres. In the process it is inevitable that there will be very short periods of fluctuation in these ratios when for instance a blockage to one of the outlets comes about. However, in the invention the ratios of flow rates are modified rapidly using the adjustable outlets so that during substantially all of the operation of the process these ratios remain substantially constant. In this specification, by "substantially constant" we mean that the ratios remain within 5% of a predetermined value, that is an optimum ratio is determined between the rates of flow out of any pair of outlets at a value x, and the ratio is not less than 95% of x and not more than 105% of x. Preferably they each remain within 2% of their predetermined value.

The process and assembly of the invention may be used for producing mineral fibres in processes where different melt flow rates from the outlets are required, ie the ratio is not 1:1. For instance, if two outlets are provided, the melt flow rate from one outlet may be set at 80% of the melt flow rate from another, ie the ratio between the melt flow rates is 1.25. The ratio between the two outlets may vary from for instance 0.5 to 2. The invention is particularly useful in processes where it is required to maintain substantially identical flow rates of melt through all outlets in the furnace. Thus substantially identical rates of melt flow into the spinning apparatuses are maintained. In this specification, by "substantially identical", we mean that the flow rates of melt remain within 5% of each other. That is, if the rate of melt flow from one outlet is taken as reference, the rate of flow from each of the other outlets is not more than 105% of that rate and not less than 95% of that rate. Preferably they remain within 2% of each other. As discussed above, there will inevitably be instantaneous deviations from substantially identical flow rates of melt through all outlets. However, by the invention it is possible to correct these rapidly and thus to maintain substantially identical flow rates of melt throughout substantially the whole of the process of producing mineral fibre.

The spinning apparatuses may each be any known apparatus for the production of mineral fibre. For instance a suitable spinner is of the spinning cup type which is mounted for rotation about a substantially vertical axis and which has apertures in its walls through which fibres are extruded as the cup rotates rapidly. In the process a spinning cup type of spinning apparatus may be used to produce stone wool fibres or glass wool fibres. It is particularly suitable for production of glass wool fibres.

One or more of the spinners may alternatively be of the type which comprises a fiberising rotor mounted about a substantially horizontal axis. In the process the melt is poured onto the periphery of the rotor and flung from it centrifugally as fibres as the rotor rotates. There can be a single centrifugal fiberising rotor. Preferably however the fibres are formed using a cascade spinner comprising a first rotor onto which melt is poured and off which it is thrown centrifugally and at least one subsequent rotor onto which the melt is thrown from the preceding rotor and off which fibres are thrown and wherein each of the rotors rotates about a substantially horizontal axis. A cascade spinner comprising at least three, and particularly four rotors in total is preferred. Spinners of this type are particularly suitable for the production of stone wool. In preferred processes and assemblies of the invention the furnace has two outlets, each of which is connected by a channel to a cascade spinner to which the melt is led and the charge of mineral material provided to the furnace is such that the melt is suitable for production of stone wool fibres.

Particularly suitable spinners are described in our publications W096/38391, EP-A-567,480 and EP-A-551 , 334.

The MMVF materials produced can be used as, for instance, fire, heat or sound insulation or protection, as an agricultural growing medium, as a filler, as reinforcement or for other conventional MMVF purposes.

Embodiments of the invention will now be illustrated with reference to the accompanying drawings.

Figure 1 shows a schematic plan view of a reference assembly, not arranged according to the invention.

Figure 2 shows a schematic plan view of an assembly according to the invention.^'

Figure 3 shows a vertical cross-section through adjustable outlet 4 of Figure 2. Figure 1 shows a scheme which we have used in the past for producing mineral fibre, in which two spinners are used simultaneously. In this process mineral material having an overall composition suitable for the production of stone wool is charged to the cupola furnace 1. In the furnace the material is melted at very high temperatures to produce a melt having a temperature of from 1,400 to 1,600. This is contained by the furnace wall 2. The melt exits the furnace through outlet 3 and passes along a channel down the path 10 to a balance trough 11. The balance trough splits the stream of melt so that it passes along both paths 12 and 13. The two melt streams are then channelled to spinners 7 and 8 which are situated in a single spinning chamber 5. Both spinners 7 and 8 are of the cascade type. This system is suitable only for providing melt streams for two spinners which are very close together, for instance in the same spinning chamber. If these spinners are placed further apart it is necessary for the melt to travel a greater distance after it has been split into melt streams 12 and 13. Lengthening the path along which the melt must travel increases the time available for the melt to cool. If significant cooling occurs between the furnace and the spinner then the viscosity and other properties of the melt are no longer suitable for spinning.

Figure 2, on the other hand, shows an assembly according to the invention. In this system it is desired to feed spinners 7 and 8, in chambers 5 and 6 respectively. The diameter AB of the furnace is approximately 2 metres. The distance CD between the spinners 7 and 8 is approximately 3.5 metres. It can be seen that if a system such as that shown in Figure 1 were to be used, the melt would have to travel a considerable distance from the furnace to each of the spinners.

In the invention however the melt travels from outlets 3 and 4 along channels 10 and 9 respectively, directly to spinners 8 and 7 respectively. The direct, substantially straight pathways EC and FD have length only around 1.5 metres. The difference between this and the path which would be necessary if a system of Figure 1 were to be used is significant in reducing cooling of the melt between the furnace and the spinner.

In Figure 2 outlet 3 is a standard siphon-type outlet. Outlet 4 is an adjustable outlet. Detail of outlet 4 is given in Figure 3. Figure 3 shows a vertical cross-section through adjustable outlet 4. The outlet is formed from water- cooled housing 13, protected by refractories 14. The surface of the melt 19 is under the superatmospheric pressure in the furnace. This pressure forces the melt through the outlet and to the height of the lip 20. The melt flows over the lip 20 and downwards to the channel 9. During operation, if a blockage arises in outlet 3, so as to force reduction in the flow rate of melt through that outlet, then the melt level in the furnace will rise since the rate of flow of melt down through the charge will be unaffected. Consequently the flow rate of outlet 4 increases, and the flow rate over the lip 20 is increased. In response to this the weir gate 17 can be moved, in this case manually. The weir gate lies in contact with the lower wall 16 of the outlet and is moveable parallel to that wall in the directions X and Y, ie towards and away from the upper end 21 of the lower wall 16. As the weir gate 17 is moved in the direction X its top edge 22 eventually attains a greater vertical height than the lip 20. The top edge 22 of weir gate 17 then forms the lip over which melt flows. As the height of this lip is increased (as the weir gate is moved further in the direction X) the flow rate of melt through outlet 4 is reduced. As a result the flow rate of melt out of outlet

3 will be gradually increased to its former level, forcing removal of any blockage. Alternatively the blockage may be removed manually by the operator. The weir gate 17 can then be moved gradually in the direction Y to lower the height of the lip over which the melt flows in outlet 4. Consequently the rates of flow of melt out of outlets 3 and

4 can be maintained at substantially the same value throughout the process, after the sudden change due to blockage of outlet 3 has occurred. As a result the melt streams travelling to spinners 7 and 8 are substantially identical. The qualities and quantities of wool produced by the spinners are therefore substantially identical. It is thus possible to carry out a process in which two spinners operate at reduced capacity and improved quality without the necessity to provide either two furnaces or two spinners in the same chamber.

Claims

1. An assembly for the production of man-made vitreous fibres comprising

(I) a furnace (1) for melting mineral material which is designed to maintain in the furnace during melting a pressure greater than atmospheric pressure and which comprises a furnace wall (2) which defines the space in which a melt (19) is formed from charged mineral material and at least two outlets (3, 4) in the furnace wall through which melt can pass to the exterior, and in which at least one of the outlets (4) is an adjustable outlet which comprises flow control means (17) capable of modifying the flow rate of melt through that outlet,

(II) at least two spinning apparatuses (7, 8) which each comprise a rotor which is capable of spinning rapidly so as to fling melt poured onto or into the rotor from it centrifugally as fibres, and (HI) at least two channels (9, 10) each leading from an outlet to a spinning apparatus and by which melt can be led from an outlet to a spinning apparatus in which assembly the number of spinning apparatuses is equal to the number of outlets so that each outlet feeds a single spinning apparatus.

2. An assembly according to claim 1 in which the flow control means (17) is capable of modifying the flow rate of melt (19) through the adjustable outlet (4) independently of all other outlets.

3. An assembly according to claim 1 or claim 2 in which the furnace wall (2) has two outlets (3, 4), each outlet comprising flow control means (17) .

4. An assembly according to claim 1 or claim 2 in which in which the furnace wall (2) has two outlets (3, 4) and only one of the outlets (4) comprises flow control means (17).

5. An assembly according to claim 1 or claim 2 in which the furnace wall (2) has three outlets, each outlet comprising flow control means (17) .

6. An assembly according to any preceding claim in which all outlets (3, 4) comprise a lip (20) over which melt (19) flows to reach the exterior and the or each adjustable outlet (4) comprises flow control means (17) adapted to modify the flow rate of melt by adjusting the vertical height of the lip.

7. An assembly according to any preceding claim in which each outlet is a siphon-type outlet (3, 4) having an angled lower outlet wall (16) which has an upper end (21) and the or each adjustable outlet (4) is provided with a weir gate (17) which contacts the lower outlet wall and is moveable parallel to the wall towards and away from the upper end.

8. An assembly according to any preceding claim in which the furnace (1) is a cupola furnace.

9. A process of producing man-made vitreous fibres comprising (I) providing a furnace (1) which comprises a furnace wall (2) which defines the space in which a melt (19) is formed from charged mineral material and at least two outlets (3, 4) in the furnace wall

(II) providing at least two spinning apparatuses (7, 8) which each comprise a rotor,

(III) providing at least two channels (9, 10) which each lead from one outlet to one spinning apparatus,

(IV) charging the furnace with mineral material and melting it under a pressure which is greater than atmospheric pressure to form a melt,

(V) allowing the melt to flow out of the furnace through the outlets and along the channels to the spinning apparatuses ,

(VI) pouring the melt onto or into the spinning rotors and allowing it to be flung centrifugally from the rotors as fibres, in which process the flow rate of the melt through at least one outlet (4) is modified independently of the flow rate through other outlets (3).

10. A process according to claim 9 in which the spinning apparatuses (7, 8) are all in separate chambers (5, 6).

11. A process according to claim 9 or claim 10 in which the ratios of flow rates of melt (19) from the outlets (3, 4) are maintained substantially constant throughout substantially all of the process.

12. A process according to any of claims 9 to 11 in which the flow rates of melt (19) through all outlets (3, 4) are substantially the same as one another throughout substantially all of the process.

13. A process according to claim 11 in which melt (19) is allowed to flow out of the furnace (1) through at least two of the outlets (3, 4) in such a way that the ratio of flow rates at the two outlets is not 1:1.

14. A process according to any of claims 9 to 13 in which the pressure in the furnace (1) is at least 1.03 times atmospheric pressure.

15. A process according to any of claims 9 to 14 in which the furnace (1) has any of the additional features set out in claims 2 to 8.