WO2009087126A2

WO2009087126A2 - Plate-shaped ceramic heat radiating element of a planar infrared radiator

Info

Publication number: WO2009087126A2
Application number: PCT/EP2009/050033
Authority: WO
Inventors: Jörg ADLER; Wieland Beckert
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2008-01-07
Filing date: 2009-01-05
Publication date: 2009-07-16
Also published as: DE102008000010A1; DE102008000010B4; WO2009087126A3

Abstract

The invention relates to the field of material science, more specifically a plate-shaped ceramic heat radiating element of a planar infrared radiator which can be used in drying systems for drying paper and cardboard webs or for heating buildings or halls, for example. The aim of the invention is to design a plate-shaped ceramic heat radiating element of a planar infrared radiator which has high emissivity and is very resistant to crack formation. Said aim is achieved by a plate-shaped ceramic heat radiating element of a planar infrared radiator in which at least the peripheral sides of the plate that are subjected to greater tensile stress during the heating-up process have a macroscopic structure with a height of more than 0.5 mm, said macroscopic structure increasing the external circumference of the plate by at least 25 percent compared with the circumference of previously known plate-shaped ceramic heat radiating elements.

Description

Plate-shaped ceramic heat radiating body of an infrared surface radiator

The invention relates to the field of materials science and relates to a plate-shaped ceramic heat radiating body of an infrared panel radiator, as it can be used for example in dry systems for drying paper or board webs or for the heating of buildings or halls.

Are known infrared panel radiators, which consist essentially of a burner system with at least one burner plate which burns a fluid-air mixture with a variety of flames and thus heats a previously arranged heat radiating body, the energy as infrared radiation to the opposite Page yields. This infrared radiation then serves for drying or heating. According to WO 0042356 a designed as a surface radiator infrared radiator with a heat radiating body is known, which is heated at its rear by a burning fluid-air mixture from a burner plate and the front emits the infrared radiation, the heat radiating body a variety of contains continuous, acting as a cavity radiator channels, in which the ratio wall surface / cross-sectional area in the flame-free area greater than 10, preferably greater than / equal to 20, is. The heat radiating body is advantageously formed of ceramic.

It has also been known for a long time that ceramic plates provided with a plurality of holes are passed through as burner plates by a gas-air mixture and the gas mixture burns on the surface (DE 46 46 92 C, US Pat. No. 2,103,365 A). By heating the ceramic plate is emitted from this infrared radiation, which is particularly desirable because infrared radiation most effectively leads to the heating of the opposite environment. Special designs of the holes and the surface of the burner plate, as described in US 4,340,357 A, to produce a high anchorage of the flames on the surface and an intense heat transfer to the plate, whereby this can give off a higher yield of infrared radiation.

To increase the efficiency of such heat radiators various mechanisms are used. For example, By using burner plates with a high emission coefficient, the yield of infrared radiation is increased. Or the absolute temperature of the plates is increased by increasing the energy density, whereby the infrared radiation can be increased.

In other known embodiments, e.g. DE 46 65 86 C or GB1082823 A, a plate is mounted in front of the combustion zone and heated by the rear side, wherein the exhaust gases are passed through the plate. Overall, this results in a very strong warming of this heat radiation body, which in turn can give off a high thermal radiation to the environment.

For such a heat radiating plate different structured materials can be used, for example wire mesh, fiber felts or open-celled Foam ceramic, as described for example in US 3,912,443 A or EP 04 15 008 A1. In some cases, the combustion may still take place in the heat radiating body, as described in principle in EP 06 57 011 A1.

Or it is, as described in DE 199 01 145 A1, a high emission of infrared radiation achieved in that heat radiating bodies are used, which are formed as a channel-shaped cavity radiators, i. plates are used which have a plurality of channels with a certain length to diameter ratio. The channels are aligned parallel to the direction of radiation, i. perpendicular to the plate surface, thus acting as a nearly black radiator, i. with a very high emission coefficient.

Due to the preferably occurring high temperatures, ceramics are particularly suitable for these applications since others, e.g. Metallic materials do not have sufficient high temperature stability and durability.

Due to the special nature of ceramic materials, i. high rigidity / low ductility in conjunction with low mechanical stability under tensile stresses, but there are a number of problems with the use of such heat radiation body plates, especially when heating and cooling of the plates. Due to the occurrence of high thermal gradients during heating / cooling due to the different thermal expansion in the plates mechanical stresses arise that exceed the inherent strength of the plates and then lead to breakage and failure of the plates. In this respect, ceramics with high strength, low thermal expansion and good thermal conductivity are preferred, because there the effects of these effects are lower than in ceramics, which have correspondingly poorer properties. As a ceramic with the preferred properties, as described in DE 199 01 145 A1, silicon carbide ceramic is suitable.

In DE 199 01 145 A1 various embodiments of heat radiation body plates are described, which differ in particular in the shape and dimensions of the channels within the heat radiation body plates. However, the use of the plates described there has shown problems, in that the plates at high loads during heating and cooling due to high thermal gradient rupture and fail. The described reinforcement of silicon carbide ceramic with carbon fibers provides relief only for a short time, since the reinforcing effect due to oxidation of the carbon fibers at the high application temperatures decreases very quickly and also cracks occur in a renewed heating / cooling cycle.

In addition, it is known that non-oxide ceramics, such as silicon nitride or silicon carbide ceramics, have high oxidation stability at high temperatures by forming a surface passivation layer of SiO 2. However, dense monolithic ceramics possess the highest rigidity (modulus of elasticity), which results in high thermal stresses as a result of locally different thermal expansions. To reduce the stresses, it is therefore advantageous to reduce the rigidity, which is known to be possible through fine pores in the structure. Fine porosity here means pore sizes which are significantly smaller than the channel diameter of the heat radiating bodies. The channel diameters of known heat radiating plates are in the mm range, while the fine porosity of ceramic materials is typically below 0.1 mm average pore size.

Carbon fiber reinforced SiC materials would have reduced strength after firing of the carbon fibers, but also reduced stiffness, as the burned fibers leave pore channels. Or, special porous nonoxide ceramics containing sufficiently fine porosity, such as recrystallized silicon carbide, could be used.

However, it has been shown that porous non-oxide ceramics also oxidize in the pores and the long-term formation of the crystalline modification of the SiO 2, cristobalite, then leads to unfavorable thermal expansion effects to a disruption of the structure and a complete destruction of the plates during prolonged use. In contrast, various countermeasures have been investigated, such as doping, which should delay the crystallization of SiO ₂ , or special protective layers, which should prevent the penetration of oxygen into the pores. However, these measures also do not work permanently or are complex to manufacture and prone to damage. The object of the invention is therefore to provide a plate-shaped ceramic heat radiating body of an infrared panel radiator, which has a high emissivity and at the same time has a high resistance to cracking.

The object is achieved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims.

In the inventive plate-shaped ceramic heat radiating body of an infrared panel radiator, at least the edge sides of the plate, which are subject to increased tension during heating, a macroscopic structure with structurings> 0.5 mm, through which the outer edge circumference of the plate relative to the edge circumference of the known plate-shaped ceramic heat radiating body is increased by at least 25%.

Advantageously, all four edge sides of the heat radiation body have a macroscopic structure.

Likewise advantageously, the edge sides have macroscopic structuring over their entire surface.

Further advantageously, the macroscopic structuring partially correspond to the shape and size of the channels in longitudinal section in the heat radiation body.

It is also advantageous if the outer edge circumference of the plate is increased by 25 to 300%, more advantageously by 90 to 200%.

And it is also advantageous if the plate consists of silicon carbide and / or silicon nitride ceramic.

It is furthermore advantageous if the ceramic material of the heat radiating body has an overall porosity of 3-15% and an open porosity of <10%. And it is also advantageous when the plate-shaped body is segmented, even more advantageously when the plate-shaped body is divided into half segments, in quarter or third segments.

In the context of the invention, the entire system, consisting of gas supply / air mixture, burner plate, radiant heat body, frame / fasteners / brackets is to be understood by an infrared panel radiator.

Furthermore, should be understood in the context of the invention under a burner plate, a perforated plate with fine channels or a nozzle plate or a plate of porous material, at the back of the gas / air mixture is supplied, and at the front of the gas burns flat.

Under the heat radiating body is to be understood in the context of the invention, a ceramic plate with channels or of a highly porous material which is heated on the back of the burner plate and emits heat radiation (infrared radiation) to the opposite environment on the front.

In the plate-shaped ceramic heat radiation bodies according to the invention the high resistance to cracking is achieved by the edge sides of the plate, at least on the sides, which are subject to an increased tensile stress during heating, provided with a macroscopic structure that the outer edge circumference compared to a plate according to the state of the art with smooth edge increase by at least 25%. It should be understood under the edge sides of the plate, the four in terms of area smallest sides of the plate.

In the context of the invention, macroscopic structuring should be understood to mean a geometric design of the edge surfaces which has at least feature sizes of> 0.5 mm and thus differs from the surface enlargement due to roughness increase. The outer peripheral edge of the plate is understood to be the length of the delimiting outer lines of the projection of the plate in the direction of radiation, that is to say the outer delimiting line of the plate. The structuring can be parallel or run obliquely or tapering to the direction of radiation, that is, the extent of different plate sections may also be different.

A particular advantage of the solution according to the invention is that the heat radiation body according to the invention in addition to a high resistance to cracking also has a high oxidation stability.

This is achieved in particular when the plates are made of non-oxide ceramic, the pores (open and closed pores) are contained in a total amount of 3-15% and the proportion of open pores is less than 10%, advantageously less than 8%. Under total porosity is understood to mean the total volume of pores, which is determined from the ratio of the bulk density and the true density of the ceramic. By open porosity is meant the volume of pores accessible from the outside and e.g. is determined by weighing the water absorption. With this small amount of pores, it is ensured that the oxygen transport is so severely limited that the internal oxidation of the plates is very low and the plates achieve a long service life. At the same time, due to the low content, the strength of the plates is only slightly reduced while the elasticity is increased, ie the modulus of elasticity is reduced.

With a total porosity of> 15% and open porosity of> 10%, the number of interconnected pores increases greatly, so that the internal oxidation increases greatly and the lifetime is limited. At pore levels <3%, on the other hand, the modulus of elasticity is so high that the stresses increase with thermal gradients, so that the susceptibility of the plates to cracking becomes too high. Silicon carbide and silicon nitride ceramics, in particular, can be used to advantage as non-oxide ceramics, ie ceramics which consist predominantly of silicon carbide or silicon nitride.

As is known, heat radiating plates have lateral dimensions of about 150x200 mm or 130x180 mm and thicknesses of 5-25 mm. These are, as described in DE 199 01 145 A1, mounted in front of the burner plate or special burner nozzle assemblies and held at the edges of a metal frame, with intermediate strips of a thermal insulation material. Of the Frame is gas-tight connected to the back of the burner, which contains fasteners, gas mixture and gas supply connections.

For technical tasks, such as drying of paper webs or hall heating, then several heat radiators are arranged side by side to allow a uniform heat radiation in a large width. The dimensions of the heat radiating surface can in principle be varied; however, the radiation yield due to the edge losses increases with smaller radiators and the expense for the production of the entire radiator grows disproportionately. With an increase in the area of the radiators, the problems associated with the thermal expansion of the materials increase very strongly.

To reduce the stresses, the heat radiation body plates can therefore be segmented and the individual heat radiating body plate segments are used in a radiator. However, too many and small partitions are counterproductive because they are more difficult to enclose in the support frame. It has proven advantageous in a typical lateral radiating surface of e.g. a total of 130x180 mm, half segments with dimensions of e.g. 90x130 mm or third segments with dimensions of 60x130 mm or quarter segments with dimensions of 45x130 mm, i. E. bisect the area on the long side, or cut in half or quarter it. The plates are then placed side by side in the frame. The segmentation reduces the thermo-mechanical stresses in the plates and reduces the susceptibility to cracking. However, it is also evident here that in the case of half-finished panels, these often tear the two outer panels on the inner sides during heating or cooling on the inner sides and on the one-third or quarter panels.

For this reason, an attempt was made to increase the strength of the perforated plates by a thickening of the edge on these insides, but without success.

Surprisingly, however, according to the invention, it has been shown that the susceptibility to cracking completely disappears if at least one of the outer edge sides of the plates is geometrically designed such that they have a resistance to a smooth edge of the plate has at least 25% increased circumference and this edge side is used on the most crack-prone side.

Advantageously, all sides of the plates can be provided with a structured edge, since this avoids the risk of a wrong position when inserting the plates in the frame.

It is particularly advantageous if the structuring leads to a significantly further increase in the circumference, than 25%; e.g. has a 30, 50, 90%, up to 300% larger circumference than straight, unstructured plate sides.

It is also advantageous if the macroscopic structures are produced by guiding the channels or large pores of the heat radiation body beyond the edge of the plate, whereby, for example, channels with round cross sections and uniform arrangement, the channels each have semicircular recesses in cross section. If the channels have a different cross-sectional shape or arrangement, the recesses of the structuring also each have a different shape, wherein in these cases, the recesses of the structuring in cross-section always partially have the cross-sectional shape of the channels. When using a large pore ceramic material for the heat radiation body, the edges are completely irregular in cross-sectional and surface shape.

But it is also possible to produce the edges by special edge processing in each desired manner, on the one hand allows the largest possible increase in the edge circumference, on the other hand makes the plate still implementable in the existing brackets or devices.

The invention will be explained in more detail with reference to an exemplary embodiment.

Show

Fig. 1: a heat radiation body plate (1) according to the prior art with 180x130x10 mm in plan view or as a projection in the emission direction of Surface 180x130 made of a ceramic (2) with continuous cylindrical channels (3) with diameters of 4 mm and lengths of 10 mm parallel to the direction of radiation.

Figure 2: in the projection of three identical heat radiating body panel segments (4) according to the prior art with dimensions of 60x130x10 mm and with straight edge circumference, which side by side arranged a heat radiating body plate 180x130x10 mm. (27) shows areas on the inner sides of the segments where cracking occurs during rapid heating.

Fig. 3: in the projection of an inventive heat radiating plate segment (5) with the dimensions 60x130x10 mm, in which all 4 sides (6) have a structured edge circumference, which is a total of 38% greater than that of the segments (4).

Fig. 4: in the projection of an inventive heat radiating plate segment (7) with the dimensions 60x130x10 mm, in which one side (9) has a structured edge circumference, which is compared to the unstructured edge periphery (8) increased by 30%.

Fig. 5: in the projection another possibility of embodiment of a heat radiating body panel segment (10) according to the invention with the dimensions 60x130x10 mm, in which one side (12) has a structured edge circumference, which is compared to the unstructured edge periphery (11) increased by 180%.

Fig. 6: in the projection another possibility of the embodiment of a Wärmestrahlkörperplattensegmentes (13) according to the invention with the dimensions 60x130x10 mm, in which one side (15) has a structured edge circumference, which is compared to the unstructured edge periphery (14) enlarged by 50%.

Fig. 7: in the projection another possibility of the embodiment of a heat radiating body panel segment (16) according to the invention with the Dimensions 60x130x10 mm, which contains prismatic channels, which form hexagons in cross section and in which one side (18) has a structured edge circumference, which is compared to the unstructured edge circumference increased by 28%.

Fig. 8: in the projection another possibility of the embodiment of a heat radiating body plate segment (19) according to the invention with the dimensions 60x130x10 mm, which contains prismatic channels which form in cross-section squares 4x4 mm and in which one side (21) has a structured edge circumference, the compared to the unstructured edge circumference (20) is increased by 95%.

9: a perspective view of a heat radiating body panel segment (22) according to the invention with the dimensions 60x130x10 mm, in which one side (23) has a structured edge circumference. The indicated surface section A-A results in the area gem. Picture 4.

Fig. 10: in the projection of a three segments (24) composite heat radiating plate according to the invention with the dimensions 180x130x10mm. The segments (24) correspond to the variant (5) described in FIG.

Example 1 :

60x130x10 mm heat radiating plate segments with 372 continuous, uniformly arranged, parallel cylindrical channels with a diameter of 4 mm and a length of 10 mm, all perpendicular to the plate surface 60x130, are made by hot casting a suspension of silicon carbide ceramic powder and additives, with subsequent debinding and pressureless sintering. The SiC ceramic is sintered to have a 7% open porosity determined by water uptake, which is formed from small, isolated pores having a mean size of 8 μm (Determination by image evaluation on the ceramographic grinding). The total porosity is 13%.

The outer 4 narrow sides of the plates are processed after sintering so that no smooth outer edges arise, but that these edge sides are provided with many repeated structuring, which in detail in the form of cutouts of cylindrical channels with diameters of 4 mm; the longitudinal sides have 16 such halved cylindrical recesses, while the shorter end faces each have 12.5 of these semi-cylindrical recesses. In the plan view, that is, in the projection perpendicular to the surface 60x130 mm results in the image shown in Fig. 3, in which all 4 sides (6) have a structured edge circumference, which has a total circumference of 525 mm, which is 38% larger than a segment with smooth edges (circumference 380 mm).

Each of these three heat radiating plate segments according to the invention with the dimensions 60x130x10 mm are combined to a heat radiating plate 130x180x10 mm, as in the plan view, that is, as a projection on the surface 130x180 in Fig. 10. This heat radiating plate is installed in an infrared surface radiator, as described in WO 0042356, Figure 1.

This burner is experimentally heated with propane gas-air mixture at a gas pressure of 190000 Pa and a total power of 11 kW. After ignition, the plates are heated within a few seconds to a temperature of 1200 ⁰ C. By interrupting the gas supply, the burner extinguishes and the plates cool down within a few minutes. This start-stop cycle is repeated 10 times without cracks appearing on the heat radiating plate segments. In a continuous operation test of 1000 h at a constant temperature of 1200 ⁰ C, a mass gain due to oxidation of 1, 2% is recorded, after 10,000 h of 3.9%. A critical loss of strength due to oxidation is expected only at a mass increase of 5%. Comparative Example 2:

Heat radiating plate segments with the dimensions 60x130x10 are prepared analogously to Example 1, with the difference that they have a smooth edge without the structuring according to the invention and thus correspond to the heat radiating body plate segments according to the prior art. Three of these heat radiating plate segments with the dimensions 60x130x10 mm are combined to a heat radiating plate 130x180x10 mm, as in the plan view, that is, as a projection on the surface 130x180 shown in Figure 2. This heat radiating body plate is tested analogously as in Example 1. Even at the first heating crack on the inner side of the two outer segments ((27) in Figure 2) are recorded, which continue to grow in further cycling and lead to the rupture of individual parts of the plates to the point of complete breakage.

Claims

claims

1. A plate-shaped ceramic heat radiating body of an infrared surface radiator, in which at least the edge sides of the plate, which are subject to an increased tensile stress during heating, have a macroscopic structure with structurings> 0.5 mm, through which the outer edge circumference of the plate relative to the edge circumference of the known plate-shaped ceramic heat radiation body is increased by at least 25%.

2. Ceramic heat radiating body according to claim 1, wherein all four edge sides of the heat radiating body have a macroscopic structure.

3. Ceramic heat radiating body according to claim 1, wherein the edge sides have macroscopic structuring over their entire surface.

4. Ceramic heat radiating body according to claim 1, wherein the macroscopic structuring partially correspond to the shape and size of the channels in longitudinal section in the heat radiating body.

5. Ceramic heat radiating body according to claim 1, wherein the outer edge circumference of the plate is increased by 25 to 300%.

6. A ceramic heat radiating body according to claim 5, wherein the outer peripheral circumference of the plate is increased by 90 to 200%.

7. Ceramic heat radiating body according to claim 1, wherein the plate consists of silicon carbide and / or silicon nitride ceramic.

8. Ceramic heat radiating body according to claim 1, wherein the ceramic material of the heat radiating body has a total porosity of 3 - 15% and an open porosity of <10%.

9. Ceramic heat radiating body according to claim 1, wherein the plate-shaped body is segmented.

10. Ceramic heat radiating body according to claim 9, wherein the plate-shaped body is divided into half segments, in quarter or third segments.