GB2534242A

GB2534242A - Fire protection barrier

Info

Publication number: GB2534242A
Application number: GB1507028.7A
Authority: GB
Inventors: Cooper Anthony; Ricketts Neill; Preston Michael
Original assignee: Versaries Technologies Ltd
Current assignee: Versaries Technologies Ltd
Priority date: 2015-04-24
Filing date: 2015-04-24
Publication date: 2016-07-20
Anticipated expiration: 2035-04-24
Also published as: GB2534242B; GB201507028D0

Abstract

A fire protection barrier comprises a first layer and a second layer, the second layer being made form a sintered metal foam. In a preferred embodiment, the sintered metal foam may be copper foam. The first layer may be, for example, solid copper. The first layer may be bonded to the second layer by means of sintering, or alternatively, the layers may be held together by secondary fixing means such as screws or clips. A thermal interface material may be provided between the first and second layers which decomposes at a temperature below 1000°C to leave a thermal break between layers. The fire protection barrier which includes a metal foam layer is suitable for protecting components from fire, where the components generate heat and need to be passively cooled by convection in normal use. The barrier protects the component from large heat fluxes, whilst allowing small heat fluxes away from the component to prevent overheating.

Description

FIRE PROTECTION BARRIER

The present invention relates to protecting vulnerable components from fire, and in particular to protecting components using a barrier or enclosure which includes sintered metal foam.

BACKGROUND TO THE INVENTION

In various scenarios, it is important to protect certain components from fire. For example, on an aircraft or ship, it is vital that certain safety-related systems continue to work even in the event of a fire, and that they survive to continue working after a fire has been extinguished. For this reason, fire resistant barriers are provided to form a casing around the components to be protected.

Various types of fire resistant barriers are known. For example, extra materials having insulating properties can be added to enclosure walls to improve temperature lag and provide a sufficient delay to "burn through" to meet certain requirements. For example, a given enclosure might prevent a temperature rise inside the enclosure of, for example, more than 30 °C for at least thirty minutes, given an outside temperature of no more than 1000 °C. The insulating effect can come from the properties of the material used in the enclosure walls, for example silica is a good insulator which is used in this application. Alternatively, the structure of the enclosure wall might trap a layer of air to provide an insulating barrier.

Intumescent and/or ablative coatings are also known. These coatings can be applied to the outside surface of enclosures, and in the event of fire will blister or release a micro-encapsulated foaming agent, forming a protective layer to reduce heat transfer into the enclosure.

The problem with all of these known techniques is that adding insulation to an enclosure prevents heat from being transferred out of the enclosure in normal operation. All electronic components generate some heat in operation, and if this heat is allowed to build up inside an insulated container the temperature can rise to the extent that the electronics are no longer able to correctly function. Generally therefore, allowance is made for this in the design of enclosures, providing for heat to transfer out of the enclosure either by passive convection through vents, conduction through heat sinks, or active air flow driven by fans.

There can therefore be a difficult trade-off between providing enough insulation to meet fire protection requirements, and ensuring that electronics do not malfunction due to a build-up of internally-generated heat in normal operation.

Another problem with some existing insulating materials is that they add a significant amount of weight to the enclosure. On aircraft especially, this can lead to significant increases in running costs over a period of time. One way of controlling this is to make the enclosures as small as possible, by using complex geometries to provide the internal volume required to enclose the components being protected, whilst reducing the amount of insulating wall material and therefore the mass of the enclosure. However, producing insulating enclosures with complex geometries is generally expensive, since the fabrication techniques required are more time-consuming than for simple geometries, e.g. to produce a simple cuboidal box.

The above factors in combination mean that providing suitable heat shielding for components, especially in aerospace applications, is a difficult trade-off between cost, weight, fire resistance, and heat transfer performance for cooling in normal operation. It is an object of the invention to provide heat shielding which costs less, weighs less, provides good resistance to fire and yet allows for heat transfer out of an enclosure for cooling.

STATEMENT OF INVENTION

According to the present invention, there is provided a fire resistant enclosure, the enclosure having a wall and the wall comprising first and second layers, the second layer of the wall being made from a sintered metal foam.

Sintered metal foam is an open cell formation of fused metal particles, which typically has a porosity in the range from 40% to 95%. The thermal conductivity of this type of material typically ranges from 0.5W/mK to 20W/mK, and this property can be controlled by varying the porosity and sintering time during the manufacture of the metal foam material.

The first layer of the wall may be made from a metal, and this may be a similar metal to the metal foam. In such a case, the second layer may be bonded to the first layer by sintering.

Alternatively, the first and second layers may be held together by screws, clips, or any suitable fixing means. In this case, providing a thermal interface material between the first and second walls is preferable. A thermal interface material increases conductivity between the layers, and allows for similar heat transfer performance to the embodiment where the layers are bonded by sintering. Preferably, the thermal interface material is a material that decomposes at a certain temperature. In such a case, fire conditions will cause the material to decompose and leave a thermal break (i.e. trapped air or other gas) in its place, further increasing the insulating properties of the enclosure.

In one embodiment, the sintered metal foam is copper foam. However, other metals are suitable in different applications. For example, steel could be used in a marine application where weight is generally not a critical issue, whereas aluminium may be suitable for many aerospace applications. Other suitable metals include titanium.

Metal foam may be produced by pressing a mixture of small metal (e.g. copper) grains, larger filler particles and a binding agent, and then sintering at around 950 °C so that the metal particles fuse together. The filler particles may then be dissolved out, leaving a metal foam with an open cell formation. The filler particles may be, for example, potassium carbonate. Where the first layer is bonded to the second layer by sintering, the two layers may be created in a single sintering process, a layer of metal powder being provided for the first layer, and the mixture as described above being used for the second layer. The two layers can then be sintered together to create a solid metal first layer and a metal foam second layer.

Preferably, the second layer of the wall is the external layer of the enclosure, that is, the first layer is provided internally of the second layer. Metal foam on the exterior of an enclosure allows heat which has been conducted through the enclosure wall to be convected away from the enclosure very efficiently, by providing a larger surface area over which air may flow. Typically, the surface area enhancement factor of suitable metal foams is between 1.5 and 2.5, depending on the porosity of the foam which can be controlled by using different proportions of metal and filler material in manufacture.

The enclosure of the invention is found to be particularly suitable in passive heat management applications, i.e. where air is allowed to flow over the exterior of the enclosure but there are no fans or other devices which force air. The additional external surface area provided by the metal foam allows much more efficient cooling by convection, and more than compensates for the additional thermal impedance added to the enclosure walls by the foam, even for foams manufactured with conductivity at the lower end of the range (e.g. around 0.5 W/mK). However, when the heat flux is much greater, for example as a result of a flame impinging on the external wall of the enclosure, the additional thermal impedance becomes the significant factor, and the foam layer can reduce the heat flux into the enclosure by up to 60%.

Although the melting point of copper is 1085 °C, it has been found that copper-based enclosures according to the invention are resistant to flames burning in the range 1000 °C to 1500 °C, as is typically of a flame from burning propane, other gases, and kerosene. In practice, in fire protection applications flame impingement is transient and heat is conducted away from the impinged surface during application. Peak temperatures in the copper material therefore rarely approach 1000 °C, and the structural integrity of copper-based embodiments is found to be satisfactory for many requirements.

The second layer is produced by sintering, and in some embodiments the first layer is also produced by sintering, as described above. One advantage of this is that complex shapes can be produced at low cost, compared with the fabrication techniques which apply to working with most solid materials.

The enclosure of the invention is lightweight, due to the fact that the metal foam is porous.

For example, although copper is a relatively dense material, at a porosity of 70% the weight of the enclosure wall is similar to a wall of equivalent thickness made from solid aluminium. An even more lightweight embodiment can be made with aluminium foam. Nevertheless, the fire resistance and heat transfer performance is very good.

According to a second aspect of the invention, there is provided a method of protecting a component from a possible source of heat, the method comprising the step of providing a barrier between the component and the possible source of heat, the barrier having first and second layers, the second layer of the barrier being made from a sintered metal foam.

Preferable and/or optional features of the second aspect of the invention are set out in claims 18 to 28.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first embodiment, a barrier or a wall of an enclosure comprises first and second layers. The first layer is made from solid copper and the second layer is made from a porous copper foam. The copper is produced by taking a mixture of small copper grains, filler particles and a binding agent, pressing the mixture, and then sintering it at approximately 950 °C so that the copper particles fuse together. The filler particles are then dissolved out and the remainder is a copper foam with an open cell formation of fused copper particles, with porosity in the range from 40% to 95%.

EP1755809 discloses techniques for producing metal foams in more detail, and is incorporated herein by reference. Note that alternative embodiments can comprise foams made from, for example, aluminium, titanium and steel.

The first layer can be made at the same time as the second layer, by including a layer of copper powder (without any filler particles) which is then pressed and sintered during the manufacture of the copper foam. Alternatively, the copper foam can be produced, and then later sintered onto a copper plate. Either way, the result is a barrier or wall in which the first and second layers are bonded to each other by sintering.

Because of the manufacturing process, especially in the case where the two layers are sintered at the same time, it is relatively easy to produce a barrier or enclosure having a complex geometry. The powder mixture can be pressed into a mould of substantially any 20 shape.

In a second embodiment, the second layer is produced substantially as described above, possibly to a complex geometry, and is then bonded to the first layer using screws, clips, or other secondary fixings. Before the layers are fixed together, a thermal interface material is introduced between the layers. The thermal interface material is highly conductive at low temperatures, but at a certain temperature (for example, a few hundred degrees) the thermal interface material decomposes. The result of this is that, at low temperatures, there is good conductivity between the layers, allowing heat to conduct through the first layer and then be drawn away from the second layer by convection. However, when the temperature reaches the decomposition temperature of the thermal interface material the material will decompose, leaving a gap between the first and second layers. The gap acts as a thermal break, substantially reducing the thermal conductivity of the barrier or enclosure.

In both embodiments, in use, the second layer of the barrier is used as an external surface (i.e. facing away from the component to be protected) and the first layer of the barrier is used as an internal surface (i.e. facing towards the component to be protected).

Both embodiments protect components from fire, whilst allowing passive convective cooling during normal operation. In addition, due to the manufacturing process very complex geometries can be created without significantly increased cost. As such, the space taken up by enclosures can be kept to an absolute minimum, whilst keeping expense low and ensuring good performance both in normal conditions and during a fire.

Embodiments of the invention are suitable for use in safety critical applications where components vulnerable to fire attack must be protected and where space and weight are critical issues. For example, the invention may be used to protect component on board aircraft, in military installations, in nuclear sites, and in any other scenario where components need to be protected from fire.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. A fire resistant enclosure, the enclosure having a wall and the wall comprising first and second layers, the second layer of the wall being made from a sintered metal foam.
2. A fire resistant enclosure as claimed in claim 1, in which the metal foam has porosity in the range from 40% to 95%.
3. A fire resistant enclosure as claimed in claim 1 or claim 2, in which the thermal conductivity of the metal foam is in the range from 0.5W/m1( to 20W/mK.
4. A fire resistant enclosure as claimed in any of the preceding claims, in which the first layer of the wall is made from metal.
5. A fire resistant enclosure as claimed in claim 4, in which the first layer of the wall is made from the same metal as the metal foam in the second layer.
6. A fire resistant enclosure as claimed in claim 4 or claim 5, in which the second layer is bonded to the first layer by sintering.
7. A fire resistant enclosure as claimed in any of claims 1 to 5, in which the first and second layers are held together by secondary fixing means.
8. A fire resistant enclosure as claimed in claim 7, in which the secondary fixing means are screw or clips.
9. A fire resistant enclosure as claimed in claim 7 or claim 8, in which a thermal interface material is provided between the first and second layers.
10. A fire resistant enclosure as claimed in claim 9, in which the thermal interface material is a material which decomposes at a particular temperature.
11. A fire resistant enclosure as claimed in claim 10, in which the thermal interface material is a material which decomposes at a temperature below 1000 °C.
12. A fire resistant enclosure as claimed in any of the preceding claims, in which the metal foam is copper foam.
13. A fire resistant enclosure as claimed in any of the preceding claims, in which the metal foam is produced by mixing metal particles with filler particles and a binding agent, pressing the mixture, sintering so that the metal particles fuse together, and then dissolving the filler particles.
14. A fire resistant enclosure as claimed in claim 13, when dependent on claim 6, in which the first and second layers are bonded together in the same sintering step as the metal particles are fused together to produce the metal foam.
15. A fire resistant enclosure as claimed in any of the preceding claims, in which the first layer is provided internally of the second layer.
16. A passively-cooled electronic device including an electronic circuit which emits heat in normal operation, and a fire resistant enclosure as claimed in any of the preceding claims which substantially surrounds the electronic circuit.
17. A method of protecting a component from a possible source of heat, the method comprising the step of providing a barrier between the component and the possible source of heat, the barrier having first and second layers, the second layer of the barrier being made from a sintered metal foam.
18. A method as claimed in claim 17, in which the metal foam has porosity in the range from 40% to 95%
19. A method as claimed in claim 17 or claim 18, in which the thermal conductivity of the metal foam is in the range from 0.5W/mK to 20W/mK.
20. A method as claimed in any of claims 17 to 19, in which the first layer of the wall is made from metal.
21. A method as claimed in claim 20, in which the first layer of the wall is made from the same metal as the metal foam in the second layer.
22. A method as claimed in claim 20 or claim 21, in which the step of providing the barrier includes the sub-steps of providing the first layer and the second layer, and bonding the first layer to the second layer by sintering.
23. A method as claimed in claim 22, in which the step of providing the second layer includes the sub-steps of: mixing metal particles with filler particles and a binding agent; pressing the mixture; sintering so that the metal particles fuse together; and dissolving the filler particles.
24. A method as claimed in claim 20 or claim 21, in which the step of providing the barrier includes the sub-steps of: providing a layer of metal particles; providing a layer of a mixture adjacent the layer of metal particles; pressing the mixture; and sintering so that the metal particles fuse together, in which the mixture comprises metal particles, filler particles and a binding agent, and in which the step of providing the barrier includes the further sub-step of dissolving the filler particles.
25. A method as claimed in any of claims 17 to 24, in which the metal foam material is a copper foam.
26. A method as claimed in any of claims 17 to 25, in which the barrier is oriented such that the first layer faces the component being protected, and the second layer faces the possible source of heat.
27. A fire resistant enclosure substantially as described herein.
28. A method of protecting a component from a possible source of heat substantially as described herein.Amendments to the claims have been made as followsCLAIMS1. A fire resistant enclosure, the enclosure having a wall and the wall comprising first and second layers, the second layer of the wall being made from a sintered metal foam.2. A fire resistant enclosure as claimed in claim 1, in which the metal foam has porosity in the range from 40% to 95%.3. A fire resistant enclosure as claimed in claim 1 or claim 2, in which the thermal conductivity of the metal foam is in the range from 0.5W/m1( to 20W/mK.4. A fire resistant enclosure as claimed in any of the preceding claims, in which the first layer of the wall is made from metal.5. A fire resistant enclosure as claimed in claim 4, in which the first layer of the wall is made from the same metal as the metal foam in the second layer.6. A fire resistant enclosure as claimed in claim 4 or claim 5, in which the second layer is bonded to the first layer by sintering.7. A fire resistant enclosure as claimed in any of claims 1 to 5, in which the first and second layers are held together by secondary fixing means.8. A fire resistant enclosure as claimed in claim 7, in which the secondary fixing means are screw or clips.9. A fire resistant enclosure as claimed in claim 7 or claim 8, in which a thermal interface material is provided between the first and second layers.10. A fire resistant enclosure as claimed in claim 9, in which the thermal interface material is a material which decomposes at a particular temperature.11. A fire resistant enclosure as claimed in claim 10, in which the thermal interface material is a material which decomposes at a temperature below 1000 °C.12. A fire resistant enclosure as claimed in any of the preceding claims, in which the metal foam is copper foam.13. A fire resistant enclosure as claimed in any of the preceding claims, in which the metal foam is produced by mixing metal particles with filler particles and a binding agent, pressing the mixture, sintering so that the metal particles fuse together, and then dissolving the filler particles.14. A fire resistant enclosure as claimed in claim 13, when dependent on claim 6, in which the first and second layers are bonded together in the same sintering step as the metal particles are fused together to produce the metal foam.15. A fire resistant enclosure as claimed in any of the preceding claims, in which the first layer is provided internally of the second layer.16. A passively-cooled electronic device including an electronic circuit which emits heat (.0 in normal operation, and a fire resistant enclosure as claimed in any of the precedingCDclaims which substantially surrounds the electronic circuit. C) 2017. A method of protecting a component from a possible external source of heat, the method comprising the step of providing a barrier between the component and the possible source of heat, the barrier having first and second layers, the second layer of the barrier being made from a sintered metal foam.18. A method as claimed in claim 17, in which the metal foam has porosity in the range from 40% to 95% 19. A method as claimed in claim 17 or claim 18, in which the thermal conductivity of the metal foam is in the range from 0.5W/mK to 20W/mK.20. A method as claimed in any of claims 17 to 19, in which the first layer of the wall is made from metal.21. A method as claimed in claim 20, in which the first layer of the wall is made from the same metal as the metal foam in the second layer.22. A method as claimed in claim 20 or claim 21, in which the step of providing the barrier includes the sub-steps of providing the first layer and the second layer, and bonding the first layer to the second layer by sintering.23. A method as claimed in claim 22, in which the step of providing the second layer includes the sub-steps of: mixing metal particles with filler particles and a binding agent; pressing the mixture; sintering so that the metal particles fuse together; and dissolving the filler particles.24. A method as claimed in claim 20 or claim 21, in which the step of providing the barrier includes the sub-steps of: providing a layer of metal particles; providing a layer of a mixture adjacent the layer of metal particles; pressing the mixture; and sintering so that the metal particles fuse together, in which the mixture comprises metal particles, filler particles and a binding agent, and in which the step of providing the barrier includes the further sub-step of dissolving the filler particles.25. A method as claimed in any of claims 17 to 24, in which the metal foam material is a copper foam.26. A method as claimed in any of claims 17 to 25, in which the barrier is oriented such that the first layer faces the component being protected, and the second layer faces the possible source of heat.27. A fire resistant enclosure substantially as described herein.28. A method of protecting a component from a possible source of heat substantially as described herein.