WO1990014553A1

WO1990014553A1 - Thermal insulation

Info

Publication number: WO1990014553A1
Application number: PCT/EP1990/000830
Authority: WO
Inventors: Norbert Kuhn; Jürgen BLUMENBERG
Original assignee: F. Willich Gmbh & Co.
Priority date: 1989-05-23
Filing date: 1990-05-23
Publication date: 1990-11-29
Also published as: EP0473658A1; DE3916810A1

Abstract

Thermal insulation for large temperature differences between the hot and cold sides, to which pressures above atmospheric pressure can be applied without impairing its performance. This thermal insulation consists of at least two insulating layers (14, 16, 18); at least the two coldest insulating layers (16, 18) are interconnected by an intermediate layer (20, 22) consisting, at least partly, of a homogeneous, non-cellular elastomer.

Description

Thermal insulation

The invention relates to thermal insulation for large temperature differences between the warm and cold sides.

Such thermal insulation can be used, for example, in low-temperature containers or flow or wind tunnels which are operated at low temperatures, for example at the temperature of liquid nitrogen, to improve the imaging ratio. In this case, the heat insulation is arranged on the inside of the duct wall, and the cold inside of the heat insulation is at the temperature of liquid nitrogen, while the outside of the heat insulation connected to the concrete or steel wall of the wind tunnel is approximately at room temperature . In the case of such thermal insulation, stresses occur in the thermal insulation during cooling, in particular in the event of frequent thermal shock loads, which result from the thermal insulation contracting on cooling on the cold side, while the warm side essentially retains its volume. As a result, thermomechanical stresses occur in the system. If the stresses in the insulating material exceed the breaking stress of the material, cracks and tears occur in the Material. Undesired convection and diffusion flows also occur in these cracks.

DE-OS 25 55 618 discloses thermal insulation for low-temperature containers made of several heat-insulating foam layers, in which a flexible fabric layer is provided between two heat-insulating foam layers. By means of this fabric layer it is possible, if necessary, to prevent the spreading and transmission of a thermal expansion crack from a foam layer of the thermal insulation into the adjacent foam layer; however, the fabric layer can cause stress cracks in the foam layers glued to the fabric.

From DE-PS 30 42 374, thermal insulation for large temperature intervals at low temperatures is known, which consists of several layers of foam, at least the coldest of which is separated from one another by a plurality of joints filled with elastic insulating material Plates exists. The individual foam layers are integrally bonded to one another, an intermediate layer of flexible material integrally bonded to the two foam layers extending at least between the two coldest foam layers. The intermediate layer is soft to the touch, ie it should be so flexible that it essentially prevents the transfer of shear stress between the two adjacent foam layers. The minimum thickness of the intermediate layer must be 5% of the total thickness of the two foam layers connected by the intermediate layer in order to ensure that the two adjacent foam layers are largely decoupled from one another with regard to the transmission of shear stresses. The intermediate layer preferably also consists of a material with good insulating or insulating properties. Preferred materials for the intermediate layer are elasticized polystyrene foam or silicone foam. With the thermal insulation according to DE-PS 30 42 374, the adjacent foam layers are largely decoupled with regard to the transmission of shear stresses, the generation of stress accumulations in the intermediate layer is to be prevented and the formation of thermal expansion cracks is generally avoided.

The thermal insulation according to DE-PS 30 72 374 may prevent the formation of thermal expansion cracks even at large temperature intervals at low temperatures, but it is at most suitable for pressureless applications. A pressurization of the thermal insulation and the problems that arise are not addressed in DE-PS 30 42 374, and the materials mentioned for the intermediate layer, namely elasticized polystyrene foam or silicone foam, are also not suitable for pressurizing the thermal insulation they would either be compressed too much and / or undesired convection currents would occur. those that in the long term lead to moisture penetration and destruction of the thermal insulation. For many applications of thermal insulation or thermal insulation, for example for pressurized low-temperature containers or as inner lining for a wind tunnel, pressurization of the thermal insulation is necessary or worth considering. For example, larger objects can be simulated in a wind tunnel that is operated under increased pressure than in a wind tunnel operated at atmospheric pressure due to the more favorable imaging conditions. The heat insulation used as an inner lining for a wind tunnel should therefore be able to be subjected to pressures of up to, for example, about 10 to 12 bar, without losing its functionality in the long term.

The invention is therefore based on the object of providing thermal insulation which is designed for large temperature intervals at low temperatures and can be operated at pressures above atmospheric pressure, and in which thermal expansion cracks and the occurrence of undesired convection and / or diffusion flows are prevented under these conditions.

This problem is solved by the thermal insulation according to the claims. The invention is based on the basic idea of building up the thermal insulation from at least two layers of insulating layers, at least the two insulating layers which are coldest in operation being connected to one another by an intermediate layer which is made entirely or partially of a homogeneous, non-cellular elastomer exists.

By forming the intermediate layer from a homogeneous, non-cellular elastomer, mechanical stresses can be reduced and reduced on the one hand by targeted deformations of the intermediate layer, and on the other hand, the closed-cell structure without large open-cell layers can cause undesirable convection and Diffusion paths can be avoided.

The thermal insulation according to the invention is constructed in such a way that even at low temperatures and large temperature differences between the cold and warm side, for example room temperature on the one hand and the temperature of the liquid nitrogen on the other hand, pressures of up to approximately 10 to 12 bar are applied to it can, especially from the cold side, without their function being impaired in the long term.

The thermal insulation according to the invention preferably consists of two or three layers of foam layers, the individual foam layers being constructed from plate-shaped rigid polyurethane foam elements. The hard foam elements of at least the two coldest layers are preferably separated from one another by circumferential joints, which are preferably covered with an elastic insulating material a foamed material such as polyimide foam.

If the thermal insulation is arranged, for example, on the inside of a channel, for example a wind or flow channel, the outermost foam layer is bonded to the wall of the channel, preferably with an adhesive. At least the two coldest foam layers, preferably all the insulating layers, are each connected to one another by the homogeneous elastomer layer, which forms a type of intermediate layer between the insulating layers. In this way, all the insulating layers and the intermediate layers made of the homogeneous elastomer are integrally bonded to one another.

The homogeneous elastomer for the intermediate layer is preferably selected such that it remains essentially elastic and does not become brittle even at low temperatures down to about -100 ° C. and below. Any homogeneous, non-cellular elastomer which does not become brittle even at low temperatures down to about -100 ° C. and below is suitable according to the invention as a material for the intermediate layer. A silicone is advantageously used as the elastomer. Because of the excellent cold behavior, a phenylmethyldimethyl silicone or a phenylvinylmethyl silicone is particularly preferred, for example a silicone rubber of the type SILASTIC (trademark of Dow Corning) LT-50 or LT-70 TE 142/4 (red). This elastomer is completely homogeneous and has an embrittlement point according to ASTM D 2137-A of approximately -116 ° C. corresponding to 157 K.

At room temperature the phenylvinylmethyl silicone elastomer preferred according to the invention has a pressure modulus of approximately 7N / mm ² and a tensile modulus of approximately 1.5 N / mm ² , the shear modulus being approximately between these two values. With decreasing temperatures, the modulus of elasticity only slowly and in the vicinity of the embrittlement point of about -116 ° C quickly. At -100 ° C the pressure module is approximately

35 N / mm ² and the tensile modulus about 20 N / mm ² . These values are still in the range in which the elastomer layer can fulfill its function according to the invention, namely the controlled deformation to reduce thermomechanical stresses in the thermal insulation system, as will be explained in more detail below.

Due to the structure of the thermal insulation according to the invention, thermomechanical stresses in the system are reduced by targeted deformations via the elastomer layer and, if appropriate, the joint filling material made of polyi idschau, which acts as an elastic wedge. In this way, the thermomechanical stresses in the rigid polyurethane foam elements acting as insulation can be reduced to such an extent that they are well below the breaking limit of rigid polyurethane foam for realistic systems. In general, safety factors against breakage that are greater than 2 can be achieved.

At least the coldest of the elastomer layers is preferably laminated on its underside for reinforcement with a glass fiber fabric, which acts as a tear brake.

So that the elastomer layer can fulfill its function in the system, the coldest elastomer layer in particular should be arranged within the system so that the temperature of this elastomer layer during operation is above the brittleness temperature of the elastomer, ie above -116 in the preferred phenylvinylmethyl silicone ° C.

The elastomer material itself has a much higher thermal conductivity than the neighboring rigid polyurethane foam elements. The two boundary layers of the elastomer layer which are in contact with the two adjacent insulating layers are therefore essentially at the same temperature, ie within the elastomer layer itself there are no additional shear stresses due to heat differences. In contrast, there are large temperature gradients in the insulating layers between the warm and the cold side. The plate-shaped rigid polyurethane foam elements from which the insulating layers are constructed accordingly contract on cooling on the cold side, while essentially maintaining their volume on the warm side. As a result, the elements bend on the edge and deform like a bowl towards the cold side. This effect is also known as "bowls". Since practically no temperature gradient occurs within the elastomer layer due to its higher thermal conductivity, there are no shear stresses and no additional "bowl" effect within the elastomer layer. However, due to its properties, the elastomer material can adapt to the bowl-like deformation, in particular the colder insulation layer, in that it expands at the points adjacent to the edges of the plate-shaped insulation elements in the direction perpendicular to the plane of the plate-shaped elements. This can prevent thermal stresses and cracks in the system.

So that the coldest elastomer layer remains fully elastic at low operating temperatures and can thus accommodate the deformation of the hard foam elements of the insulating layer, the insulating layer pointing towards the cold side is dimensioned so strongly that the temperature of the coldest elastomer layer is still above the most brittle temperature of the elastomer. The elastomer layer itself must be so thick that it can deform sufficiently to practically compensate for the curling of the insulating layers and to prevent the occurrence of undesired joints and cracks. The thickness of the elastomer layer is typically about 2 to 6 mm, preferably about 3 to 5 mm, particularly preferably about 4 mm. As an additional safety measure to prevent undesired diffusion flows, the cold side of the coldest insulating layer and the two circumferential edges of the warmer insulating layers can preferably be laminated with an aluminum foil which is plastic-coated on both sides and is impervious to water vapor diffusion. This lamination prevents water vapor from escaping from the rigid polyurethane foam of the insulating layer towards the cold side of the thermal insulation. When using the thermal insulation according to the invention in one

This cold side of the wind or flow channel faces the interior of the channel. This measure prevents the diffusion of water vapor into the wind or flow channel.

The invention is explained below with reference to the drawing. Show it:

FIG. 1 shows a partially sectioned overview of the thermal insulation according to the invention,

Figure 2 shows the structure of the thermal insulation in detail and

Figure 3 shows a section of the substructure of the liner.

Figure 1 shows an application example of the thermal insulation according to the invention for a power or wind tunnel, which is operated under pressure.

The pressure shell or channel wall 10 has an outer side 11 and an inner side 12. The thermal insulation is applied to the inside 12 of the wall. The direction of flow in the flow or wind tunnel is indicated by an arrow.

The thermal insulation essentially consists of three layers 14, 16, 18 of plate-shaped elements made of insulating material, preferably rigid polyurethane foam with a bulk density of about 200 kg / m ³ . In the two inner, ie cold Other layers 16, 18 made of rigid polyurethane foam are formed with circumferential joints which are filled with polyimide foam. These joints are not shown in detail in FIG. 1. The outermost warm layer or layer 14 made of rigid polyurethane foam facing the channel wall 10 is glued there, preferably with the Henkel Makroplast adhesive. The layers 14, 16, 18 made of rigid polyurethane foam are each connected to one another with a layer 20, 22 made of a homogeneous, non-cellular, low-temperature-resistant elastomer which does not yet become brittle at the respective operating temperature. A phenylvinyl ethyl silicone or phenyl ethyl dimethyl silicone is preferably used as the elastomer. The underside of the elastomer layers can be laminated as a reinforcement with a glass fiber fabric, which acts as a tear brake.

The structure of the heat insulation according to FIG. 1 leads to a substantial reduction in the thermomechanical stresses in the system due to targeted deformations via the elastomer layers 20, 22 and the elastic wedges made of polyimide foam arranged in the joints of the rigid polyurethane foam elements. In this way, safety factors against breakage greater than 2 can be achieved.

The polyurethane rigid foam layer 18 facing the interior of the flow channel and the two circumferential edges of the two inner layers 16, 18 made of rigid polyurethane foam are preferably laminated with an aluminum foil which is plastic-coated and water-vapor diffusion-tight. This lamination prevents water vapor from diffusing out of the rigid polyurethane foam into the flow channel. The glass fiber reinforcements which may be present on the undersides of the two elastomer layers 20, 22 result in a further increase in security. With this system of thermal insulation, on the one hand thermal stresses in the system can be reduced through targeted deformations and on the other hand desired convection and diffusion paths can be prevented by the closed-cell structure of the insulation or insulation without large open-cell layers.

FIG. 1 also shows the substructure of the liner, which covers the thermal insulation towards the interior of the channel. This consists essentially of holders 30 and liner plates 32, which are held between inner and outer frames or rails 34, 36. The structure of the substructure is explained in more detail with reference to FIG. 3.

Figure 2 shows the structure of the insulation or insulation in detail. FIG. 2 shows the channel wall 10, on which the three insulating layers 14, 16, 18 made of rigid polyurethane foam are applied one after the other. Between the insulating layers, the homogeneous, non-cellular Ela¬ are stomerschichten 20, 22 from phenylvinyl ethylsilikon having a brittle temperature of -116 C ^β arranged. The individual layers are bonded to one another by an adhesive 40, preferably Henkel Makroplast. Continuous wedges 42 made of polyimide foam are arranged in the joints between the individual plates of the two inner, colder insulating layers 16, 18. An aluminum lamination 44 is provided on the inside of the coldest insulating layer 18 and on the two circumferential edges of the two colder insulating layers 16, 18. The wedges 42 are circumferentially arranged in the area of the two colder insulating layers 16, 18.

In a specific exemplary embodiment with an average diameter of the flow channel of approximately 7 m, the two inner (colder) insulating layers have a thickness of 45 and 60 mm, for example, the thickness of the elastomer layers 20, 22 is, for example, approximately 3 to 5 mm, preferably about 4 mm. The polyimide wedges 42 are, for example, about 40 mm wide. Depending on the application, only two insulating layers with one elastomer layer or more than three insulating layers can be provided. The substructure for the liner according to FIG. 3 is constructed from holders 30, on which the frames or rails 34, 36 are mounted, which support the liner plates 32.

The holders 30 consist essentially of glass fiber tubes or GRP tubes 50, on which ends of stainless steel shoes 54, 56 are arranged at both ends by means of an adhesive 52, 53. The one on the bottom, i.e. The shoe 54 arranged on the warm side of the thermal insulation facing the channel wall is provided with a threaded pin which is screwed into a corresponding nut 58 which is welded to the inside of the channel wall 10.

The shoe 56 of the holder 30 arranged on the upper, cold side of the heat insulation facing away from the channel wall 10 has a central bore with an internal thread for mounting the frames 34, 36 by means of countersunk screws 57.

The cavity of the GRP tube 50 is filled to suppress radiation and convection on site with foam 60 with a high bulk density or with pre-compressed polyimide.

The holders 30 are each attached to the corner points of the individual panels or plates. Their distance in the direction of the channel axis is approximately 1.0 m, their distance in the circumferential direction is calculated in accordance with the division of the thermal insulation into, for example, 24 plates along the circumference of the channel wall.

The holders 30 are located in a recess of the actual thermal insulation, which is formed by the insulating layers 14, 16, 18. The cavity between the holder 30 and the thermal insulation is filled with pre-compressed, hydrophobized glass wool 62 or with pre-compressed polyimide. There is an aluminum foil 63 between the actual heat insulation and the glass wool 62 and the inner frames 34 intended. Between the actual heat insulation and the frames 34, intermediate washers 64 made of Teflon are also provided, which largely prevent unwanted convection. The intermediate disks 64 can be prefabricated with the glass wool 62 and can therefore be easily processed. Because of the rapid pressure changes, the intermediate disks 64 have perforations 65, the holes of which preferably enlarge from the warm to the cold side.

Each holder 30 fixes the four panels or plates adjoining it via a pressure plate 66 made of glass fiber or GRP. This fixation is designed on the one hand so that the desired thermal deformations are not hindered, on the other hand, the fixation represents an additional mechanical security against lifting of the panels or panels from the channel wall 10 in addition to the adhesive.

The arrangement of the holder 30 according to the invention ensures that the heat insulation can deform in a targeted manner without being hindered by the substructure.

The rails or frames 34, 36 for receiving the liner plates 32 each run in the direction of the longitudinal axis of the flow or wind tunnel. The rails 34, 36 are, for example, about 2 m long and are each fastened to two holders 30, one fastening point being designed as a fixed point and the other as a loose point. As a result, the rails or frames 34, 36 can thermally contract or expand in accordance with the change in temperature of the channel.

The upper and lower rails 34 and 36 are connected via the actual fastening screws in the holder 30 and additionally by connecting screws. All screw connections in connection with the holder, the rails and the liner plates are secured in accordance with DIN 17440. The actual liner plates 32 are pre-curved so that they sit in the rails 34, 36 without preload and do not rattle. The liner plates 32 are fastened to the lower rail 34 with a screw on each side, so that a defined fit is ensured. The plate transition from one liner plate to another is designed such that, on the one hand, there is no impediment to the flow in the channel and, on the other hand, pressure equalization can take place between the channel and the area under the liner plates 32.

Both stainless steel and aluminum alloys are suitable as the material for the rails 34, 36 and the liner plates 32. With the same loads and permissible deformations, an aluminum and a steel construction are essentially equally safe. The steel liner plates, however, are only about 1/3 the thickness of aluminum liner plates. For example, the aluminum liner plates are about 10 mm and the steel liner plates are about 3.3 mm thick. Since the specific heat of steel and aluminum behaves approximately as 1: 2, a steel structure has the advantage that it only has approximately half the heat capacity of a corresponding aluminum structure.

The selection of suitable materials is essential for the function of the heat insulation according to the invention. For the insulating layers 14, 16, 18, plates made of rigid polyurethane foam are preferably used, the bulk density of which is adjusted according to the requirements. The bulk density is preferably about 180 to 220 kg / m ³ , particularly preferably about 200 kg / m ³ . The rigid polyurethane foam meets the requirements of fire class B2.

A homogeneous, elastic, non-cellular, rubber-like material is preferably used for the intermediate layers 20, 22, such as phenylvinyl ethyl silicone, which remains plastic up to a temperature of -116 ° C. and only at lower temperatures then quickly become brittle. For the present invention, this material has excellent properties with regard to the modulus of elasticity and water vapor diffusion. The processing of this material is unproblematic with a suitable adhesive in connection with rigid polyurethane foam.

The wedges 42 between the individual insulating packets consisting of the rigid polyurethane foam sheets and the elastomer layer are preferably made of pre-compressed polyimide foam. An open-pore polyimide foam is preferred which remains elastic even at the temperature of liquid nitrogen. Because of the properties of this material with regard to water vapor diffusion and convective heat transfer, the precautionary measures explained above, in particular lamination with aluminum foil, should be taken for the water vapor diffusion barrier. As a water vapor diffusion barrier and to prevent increased convective heat transfer during pressure changes, the two insulating layers 16, 18 facing the cold side made of rigid polyurethane foam plates with the intermediate elastomer layer 22 lying horizontally, ie on the side facing the inside of the channel, and enclosed vertically with a double-sided coated aluminum foil 44 (or 63). The thickness of the aluminum foil is preferably approximately 10 to 15 μm, preferably approximately 12 μm in the vertical region and approximately 20 to 50 μm, preferably approximately 30 to 40 μm in the horizontal region. The heat flow due to the aluminum foil of this thickness perpendicular to the plate level is <10% of the permissible total heat flow through the heat insulation. The water vapor barrier is bonded to the rigid polyurethane foam with an adhesive, with a particularly good adhesive bond being achieved by corona pretreatment of the double-sided coated aluminum foil. Depending on the application, the structure of the thermal insulation can be model-based calculated with a three-dimensional, non-stationary FEM program and optimized with regard to the thickness of the insulating layers and the elastomer intermediate layers. The aim of this optimization is to keep the stresses in the polyurethane foam as low as possible in order to achieve a high level of security against breakage. With these optimizations it turns out that the polyurethane layer pointing towards the cold side should be dimensioned just enough that the coldest elastomer layer underneath remains fully elastic, ie not embrittled. Only when the elastomer layer is still elastic can it counteract the deformation of the polyurethane foam in such a way that stresses in the polyurethane are sufficiently reduced. The model calculations show maximum stresses in the rigid polyurethane foam of approximately 1.43 N / mm ² at normal pressure and 1.51 N / mm ² at 3.5 bar gauge pressure in the channel. These stresses are about a factor of 2 below the breaking stress of rigid polyurethane foam.

Claims

Patent claims

1. Thermal insulation from at least two layers of insulating layers (14, 16, 18), at least the two coldest insulating layers (16, 18) being connected to one another by an intermediate layer (20, 22), characterized by that the intermediate layer (10, 22) has a homogeneous, non-cellular elastomer.

2. Thermal insulation according to claim 1, characterized in that the elastomer is a silicone.

3. Thermal insulation according to claim 2, characterized in that the elastomer is a phenylvinylmethyl silicone or phenylmethyl dimethyl silicone.

4. Thermal insulation according to one of claims 1 to 3, da¬ characterized in that the elastomer does not embrittle significantly at temperatures down to about -100 ° C.

5. Thermal insulation according to one of claims 1 to 4, characterized in that the elastomer in the temperature range from room temperature to about -100 ° C has an elasticity module of about 1.5 N / mm ² to about 35 N / mm ² has.

6. Thermal insulation according to one of claims 1 to 5, characterized in that the elastomer is laminated on its underside with a glass fiber fabric.

7. Thermal insulation according to one of claims 1 to 6, characterized in that at least the coldest insulating layer (18) is laminated with an aluminum foil (44), which is preferably plastic-coated on both sides. 1 8. Thermal insulation according to one of claims 1 to 7, da¬ characterized in that the coldest intermediate layer (22) is arranged within the thermal insulation so that the temperature of the intermediate layer (22) in operation

5 is above the brittleness temperature of the elastomer.

9. Thermal insulation according to one of claims 1 to 8, da¬ characterized in that the thickness of the interlayer

IC ten (20, 22) is approximately 2 to 6 mm, preferably approximately 3 to 5 mm, particularly preferably approximately 4 mm.

10. Thermal insulation according to one of claims 1 to 9, da¬ characterized in that the insulating layers (14, 16, 5 18) are constructed from plate-shaped elements made of foam.

11. Thermal insulation according to claim 10, characterized gekennzeich¬ net that the joints (42) between the individual elements 0 of the insulating layers are filled with elastic foam, preferably polyimide foam.

12. Thermal insulation according to one of claims 1 to 11, further characterized by a liner (32). 5

0

5