Microwave air heating system

Abstract

The present invention discloses a microwave air heating system comprising a chamber (3) which in turn comprises an access (1) for microwaves generated by a microwave emitter, a susceptor (5) and an enveloping element (4) with two ends, wherein the enveloping element (4) has an elongated configuration, is located in the chamber (3), is made of a microwave-permeable material, houses the susceptor (5), and comprises a grid (2) at each end, the susceptor (5) being homogeneously illuminated by the microwaves and having a geometry that depends on the power of the radiation intended to be applied.

Description

[0001] MICROWAVE AIR HEATING SYSTEM

[0002] OBJECT OF THE INVENTION

[0003] The present invention relates to a microwave air heating system and, more specifically, to a microwave system that heats an electromagnetic susceptor with excellent thermal properties for heat transfer, thus allowing efficient heating of air to a temperature greater than 700 e C.

[0004] Throughout the report, reference will be made to the terms illumination and radiation, in both cases referring to electromagnetic radiation.

[0005] BACKGROUND OF THE INVENTION

[0006] Air heating systems are used in numerous industrial processes that process a wide variety of materials, including ceramics, glass, metals, chemicals, and food, among others. They are also used for thermal conditioning of warehouses and buildings, and for waste gas treatment.

[0007] There are numerous air heating systems based on electric resistors, burners and compression / expansion cycles.

[0008] Systems based on electrical resistors are highly efficient in converting electrical energy into heat energy. However, these systems consume a lot of energy due to the small surface area of ​​the resistors, which limits heat transfer to the air.

[0009] Burner-based systems generate combustion products and consume oxygen, which limits their use in many applications.

[0010] Heat pumps are very efficient, but the maximum temperature reached is very low. An alternative method is to heat air using microwaves. Air does not absorb microwave energy, so microwaves cannot be used for direct heating. However, microwave energy can be transferred through an intermediate element, called a susceptor, and then transmitted to the air.

[0011] Microwave heating systems offer several advantages over the previously mentioned heating systems, such as the high temperature reached, the speed of heating, the safety they present due to the absence of risks from combustion, and the quality of the air, which is not contaminated by any type of product during the process.

[0012] Likewise, the electrical energy used by the microwave generation system can be of renewable origin, which represents an additional advantage over other types of systems.

[0013] In the state of the art, a series of documents are known that refer to this technology.

[0014] Document CN1746588A uses a microwave system to heat a solid contained in a chamber that transfers heat to the circulating air. The area available for heat transfer is limited to the chamber walls, so it implements a fin system to increase the heat exchange surface. These documents do not provide information regarding the output air temperature or the characteristic parameters related to the thermal process. In this configuration, the air circulating through the microwave oven comes into contact with the susceptor on the outside, so the contact surface for heat transfer is very small and, consequently, its efficiency will also be reduced.The configuration described herein allows for more efficient heat exchange by having direct contact between the air and the susceptor and by forcing the air to circulate through a system of open pores with a large effective surface area.

[0015] Document CN2355263Y discloses a device in which air flows transversely through a grid of susceptor tubes heated by microwaves. This document refers to the magnetron's efficiency in converting electrical energy to radiant energy, but says nothing about the efficiency of energy transfer to the air, which is one of the key aspects. As in the previous system, the small contact area between the susceptor and the air prevents high efficiency.

[0016] Document US2022353961A1 discloses a system in which a flow of air is heated by forcing its passage over a microwave-heated liquid contained in airtight containers. The system described does not have high efficiency, like the rest of the systems in the documents mentioned above, due to the small contact surface, the thermal resistance generated by the walls of the devices containing the liquid, and the fact that the temperature of the liquid is not very high.

[0017] Document W02004068902A2 discloses a gas heating system using a susceptor that receives electromagnetic waves. The chamber where the gases circulate can have different geometries and is filled with a susceptor composed of silicon carbide-aluminosilicate-air. The development of said susceptor stems from the need to reduce radiation absorption in the chamber to increase wave penetration and achieve homogeneous heating. However, this system has several significant drawbacks, such as the fact that the reduction in the volumetric fraction of silicon carbide in the chamber limits the maximum power used, the loss of direct contact between the silicon carbide and the air considerably reduces heat transfer to the air, and the reduced air permeability severely limits the maximum air flow rate that can be used.

[0018] The aforementioned drawbacks are overcome by the present invention. On the one hand, the three lighting systems employed allow for homogeneous heating. On the other hand, the susceptor system used in the present invention, consisting of a porous silicon carbide / air bed with a customized microstructure (porosity and pore size) to maximize the contact surface, maintains high permeability and heat transfer capacity.

[0019] There are also documents such as CN210152864U or US2020147600A1 that use silicon carbide ball beds and take advantage of different properties of silicon carbide, such as its abrasion resistance and its reactivity with oxygen, respectively, although they are not used in heating applications. DESCRIPTION OF THE INVENTION

[0020] As indicated, the present invention describes a microwave air heating system and, more specifically, a system that incorporates a microwave emitter and an electromagnetic susceptor with excellent thermal properties for the transfer of heat which, once heated by the electromagnetic radiation emitted by the microwave emitter, allows efficient heating of the air to a temperature greater than 700 e C. This temperature is useful for numerous industrial processes.

[0021] The system of the invention describes various highly efficient configurations that allow for very high air outlet temperatures. These new configurations achieve more effective air-solid heat exchange due to the direct air-susceptor contact and the large contact surface area obtained.

[0022] On the one hand, the direct susceptor-air contact allows the overall heat transfer coefficient of the system to be very high, since there are no intermediate resistances due to the use of other elements.

[0023] Furthermore, the use of a susceptor incorporating a porous surface allows the contact area (effective surface) to be much higher than that of any compact object.

[0024] The invention has two critical technical aspects, namely:

[0025] - the conversion of electromagnetic energy into thermal energy, by means of the susceptor, and

[0026] - the transfer of thermal energy from the susceptor to the air.

[0027] As for the first stage, to convert electromagnetic energy into thermal energy, it is essential to select the susceptor of appropriate composition, size and geometry and to illuminate it appropriately to achieve high efficiency and adequate homogeneity.

[0028] There are several materials that could be used as susceptors, although only those of a ceramic nature, such as silicon carbide or ferhta, could be viable for high-temperature applications. Among these, the material that best meets the requirements stated is silicon carbide. On the one hand, the dielectric constants of silicon carbide are very high (s r '=30, 8 r”=11 , tan 8=0.37), not varying excessively with temperature, which guarantees high absorption in the temperature range used. Furthermore, the thermal conductivity is 100 W / (nrK), higher than that of many metals, which also gives it, together with its good mechanical properties, a high resistance to thermal shock and a certain capacity to avoid excessive heating points. Silicon carbide is also a refractory material, capable of withstanding temperatures of up to 1,600 e C continuously, ensuring system stability.

[0029] Furthermore, the use of electromagnetic radiation allows energy to be transported without physical contact and, consequently, allows a large volume of susceptor to be heated, since the heat is generated directly within the material itself. This energy transmission is determined by the susceptor's high energy absorption, which limits the maximum penetration of radiation into a porous bed to a depth of approximately 2–3 cm.

[0030] In the present invention, three basic configurations are proposed to optimize this stage.

[0031] In the first, the behavior of waves in a resonant chamber is used to distribute electromagnetic energy and homogeneously heat the entire susceptor.

[0032] In the second configuration, a cylindrical waveguide with controlled losses along its axis is used to transmit and distribute energy homogeneously throughout the susceptor volume.

[0033] The third configuration uses a sandwich structure composed of alternating layers of air and susceptor. The air layers are used to distribute radiant energy evenly.

[0034] It should be noted that, in any of the three configurations described, the small differences in illumination / heating are compensated by a thermal conduction mechanism, due to the extremely high conductivity of silicon carbide and the direct contact between the individual elements that make up the susceptor, whether they are balls, irregular particles or the porous, sponge-like walls.

[0035] All the proposed configurations are microwave-tight. To achieve this, metallic materials that reflect microwaves have been used. Metal meshes with a mesh size fine enough to prevent microwaves from passing through, but with a large free surface area to allow air to flow freely, have been used at the air inlets and outlets.

[0036] To separate the air circuit, which contains the susceptor, from the radiant chamber or guide, a microwave-permeable (transparent) material is used. Selectable materials include Teflon, glass, quartz, alumina, or other ceramic materials with a tan 8 < 0.001.

[0037] As for the second stage, relating to the transfer of thermal energy from the susceptor to the air, to achieve high efficiency in energy transfer it is essential:

[0038] - Maintain direct contact between silicon carbide and air. Silicon carbide has a very high thermal effusivity (19,400 Ws°- 5 -nr 2 -K- 1 , higher than that of most metals (iron is 15924 Ws°- 5 -nr 2 -K' 1 ), which indicates a very high energy transfer rate. The use of any intermediate material, especially ceramic, would reduce this capacity by about tenfold (porcelain: 2314 Ws 0 5 -nr 2 -K' 1 ) and, as a consequence, efficiency.

[0039] - Use a high contact surface, since the heat flow between the silicon carbide and the air is directly proportional to the surface area between them.

[0040] - Cause turbulent flow between the internal channels of the system. The individual heat transport coefficient at the interface is much higher in turbulent flow than in laminar flow. The use of an irregular pore bed, either a bed of beads or a spongy bed, allows the indicated internal circulation regime to be achieved.

[0041] Additionally, to optimize the permitted air flow rate, air permeability must be maximized. This depends on the porosity and pore size of the system. In ball packings of the same size, porosity depends on packing and can hardly be changed (approximately 35% vol.), so relatively coarse balls (4-5 mm diameter) that generate relatively large pores (approximately 1-2 mm) are preferably chosen. In sponge-like packings, both properties can be varied over wide ranges.

[0042] In summary, the present invention describes a microwave air heating system comprising a chamber with an access for microwaves generated by a microwave emitter, a susceptor and a surrounding element.

[0043] In one embodiment, the enclosing element has an elongated configuration, is located in the chamber, which acts as a resonance chamber, is made of a microwave-permeable (transparent) material, houses the susceptor and comprises a grid at each end of the enclosing element itself.

[0044] Preferably, the susceptor is ceramic in nature or, more preferably, silicon carbide and is configured by elements that can be spheres, irregular particles or an air-permeable porous structure.

[0045] In the case of spheres or irregular particles, they preferably have a maximum size of between 1 mm and 20 mm and more preferably between 2 mm and 10 mm.

[0046] In the case of a rigid and porous structure, the air porosity is preferably between 70% and 90%.

[0047] Also preferably, the porous structure has pores with a maximum size of between 1 mm and 20 mm.

[0048] On the other hand, also preferably, the surrounding element is made of alumina and the grid is metallic.

[0049] Furthermore, in another embodiment, the chamber and the enclosing element are configured in the form of two coaxial tubes, where the chamber is an inner tube permeable to microwaves, acting as a waveguide, and the enclosing element is an outer metal tube coaxial with the inner tube, such that the grids close an annular zone located between the inner tube and the outer tube. The system can also be configured with a plurality of enclosing elements instead of a single one, all of them positioned with their longitudinal axes parallel. In this case, the microwaves are not emitted through a single access, which would not have sufficient power to heat all the enclosing elements to the desired level. Instead, the system comprises two microwave accesses located opposite each other and transversely to the longitudinal axis of the enclosing elements.

[0050] In another embodiment, the system comprises two susceptors of rigid and porous structural configuration housed in the chamber, each susceptor being located between the microwave access and one of the grids at each end of the chamber.

[0051] Furthermore, in this last embodiment the chamber is stackable longitudinally by simply removing the grid located between two chambers, leaving the grids located only at the ends of the stack.

[0052] Finally, it should be noted that any of the enclosing elements incorporated in the system may have a circular cross-section or may also be polygonal.

[0053] DESCRIPTION OF THE DRAWINGS

[0054] For a better understanding of the present invention, the figures listed below are introduced in this specification.

[0055] Figure 1 represents a perspective view of the system of the invention in a first embodiment where a chamber is shown, with two microwave accesses, in which a surrounding element filled with a susceptor is located.

[0056] Figure 2 represents a perspective view of the system of the invention represented in Figure 1 but with a plurality of surrounding elements in the chamber.

[0057] Figure 3 represents a section of a casing element of those represented in Figures 1 and 2. Figure 4 represents a perspective view of the system of the invention in a second embodiment.

[0058] Figure 5 represents a section of an enclosing element of those represented in Figure 4.

[0059] Figure 6 represents a perspective view of the system of the invention represented in Figure 4 but with a plurality of enveloping elements.

[0060] Figure 7 represents a perspective view of the system of the invention in a third embodiment.

[0061] Below is a list of the references used in the figures to facilitate their follow-up:

[0062] 1. Microwave access.

[0063] 2. Grilles.

[0064] 3. Camera.

[0065] 4. Surrounding element.

[0066] 5. Susceptor.

[0067] 6. Inner tube.

[0068] 7. Outer tube.

[0069] 8. Annular zone.

[0070] PREFERRED EMBODIMENT OF THE INVENTION

[0071] The present invention relates to a microwave air heating system that incorporates a microwave emitter and an electromagnetic susceptor that, once heated by the radiation emitted by the microwave emitter, allows the heating of the air that comes into direct contact with its structure.

[0072] Figure 1 represents a first embodiment where the system comprises a chamber (3), with a microwave access (1) that can be located in an upper or lower position. The figure shows both accesses (1), upper and lower, but only with the intention of showing that they can be located in either of the two positions and not in both at the same time. In this embodiment, the chamber (3) is a resonance chamber. The microwaves are generated by a microwave emitter, not shown in the figures. Inside the chamber (3) there is a surrounding element (4) permeable to microwaves and capable of resisting the high temperatures caused by the reception of microwaves.The surrounding element (4) is made of alumina and is filled with a susceptor (5) which, in turn, is made of silicon carbide and has a configuration that can be in the form of spheres, irregular particles or a rigid porous structure.

[0073] Each end of the enclosing element (4) is closed with a metal grid (2) incorporating holes of an appropriate size to prevent microwaves from leaving the enclosing element (4). Preferably, the holes have a dimension of 1 mm. 2 .

[0074] At each end of the enclosing element (4) pipes are connected for the entry and exit of air from the system so that a current of air can be forced to circulate through the interior of the enclosing element (4) and absorb the thermal energy of the susceptor (5).

[0075] In this configuration, a microwave emitter introduces microwaves into the chamber (3), which is metallic, to confine the electromagnetic radiation. The electromagnetic radiation is dispersed by reflections on the walls of the chamber (3) and homogenized before its energy is absorbed by a susceptor (5) housed inside a microwave-permeable enclosing element (4). This homogenization allows the heating of the susceptor (5) to be more uniform and is directly linked to both the dimensions of the chamber and the size of the enclosing element (4), since the heating is limited by the maximum penetration of the energy of the electromagnetic waves into the susceptor (5).

[0076] Figure 2 represents the system of the invention with two main differences with respect to the first embodiment of Figure 1.

[0077] The first is that the chamber (3) incorporates a plurality of surrounding elements (4) instead of just one, all of them located in parallel.

[0078] The second consists of the chamber (3) incorporating two microwave accesses (1), one in the lower area and another in the upper area of ​​the chamber (3), perpendicular to the longitudinal axis of the surrounding elements (4). In this way, the system has a greater capacity to heat all the surrounding elements (4) that are in the chamber (3).

[0079] Figure 3 shows a sectional view of the susceptor (5) wrapped by the wrapping element (4) and closed at the ends with grids (2). It should be noted that, as described above, although the appearance of the susceptor (5) in the figures may be that of a rigid porous structure, the susceptor (5) may also be configured in the form of spheres or irregular particles.

[0080] Figures 4 and 5 show the system of the invention in a second embodiment formed by two coaxial tubes, an inner tube (6) and an outer tube (7) which, when coaxially coupled, form an annular area (8) that is closed at the ends by metal grids (2). The radiation emission is carried out by one end of the inner tube (6) as a cylindrical waveguide with controlled losses along its axis, to transmit and distribute the energy homogeneously throughout the volume of the susceptor (5), acting as a chamber (3). The susceptor (5) is located in the annular area (8), where the microwaves reach by passing through the wall of the inner tube (6), which is permeable to microwaves.

[0081] In this case, the inner tube (6) is ceramic in nature, permeable to microwaves and temperature resistant and may be partially covered with metal bands / rings to promote axial transmission of radiation through its interior. In this way it is achieved that the area closest to the end where the access (1) is located for the emission of microwaves does not receive much greater illumination than the furthest area, but rather the aforementioned losses are generated along the axis to provide homogeneity in the radiation received by the susceptor (5) when passing through the inner tube (6) along its entire length.

[0082] For its part, the external tube (7) is metallic in nature, to prevent electromagnetic radiation from escaping, as are the ends of the internal tube (6).

[0083] The air inlet and outlet of the system is made through the ends of the annular zone (8) formed between the two tubes (6, 7) and where the grids (2) are located. In this configuration, the radiant energy is radiated homogeneously along the inner tube (6), with the thickness of the annular zone (8) being limited by the penetration of the radiation. The main advantage of this configuration with respect to the previous ones is its compactness.

[0084] Figure 6 represents the second embodiment with the difference that it comprises a plurality of coaxial tubes.

[0085] Figure 7 represents a third embodiment in which the chamber (3) has an elongated configuration and incorporates grids (2) at the ends and a microwave access (1) that is flanked by two susceptors (5) that are located transversely in intermediate positions between the microwave access (1) and each of the grids (2). The susceptors (5) are configured as porous and rigid structures. Furthermore, this configuration is scalable, simply by joining several chambers at the ends linearly, eliminating the grids (2) located between two chambers (3) to leave only the grids (2) at the ends of the final stacked system. Thus, the remaining configuration is formed by different chambers (3) with access (1) for the emission of microwaves alternating with chambers (3) where microwaves are not emitted, each chamber (5) being flanked by two susceptors (5).

[0086] In this figure 7 it can be seen that the cross section of the chamber (3), as it could be for any of the previous embodiments, is square in configuration, although it could also be polygonal or circular.

[0087] This configuration is especially suitable for systems with sponge-type susceptors (5) due to the rigidity it provides, since it does not require the spatial confinement of the balls by means of a radiation-permeable material. Each of the modules between the porous plates of the susceptors (5) is used to transmit the waves and achieve more homogeneous illumination. This is the system with the simplest assembly and is very easily scalable. The surrounding element (4) covers the susceptor (5) to isolate it from the walls of the chamber (3) so that it can heat them and lose heat through that area. In a different way with respect to the rest of the configurations mentioned above, in this case the susceptors (5) receive the microwave radiation directly and not through the surrounding element (4).The wavelength of the electromagnetic radiation used (X«12 cm) and the maximum penetration of the radiation into the material (2-3 cm) restricts the dimensions allowed in each individual module and, consequently, its power, the air flow rate used and the temperature increase achieved, which is why the possibility of using it in parallel (to increase the air flow rate) and in series (to increase the temperature increase) is included.

[0088] A first practical example of implementation, carried out in a laboratory, following the first embodiment is described below.

[0089] Thus, a microwave oven with a resonant chamber (3) of 18 L and dimensions of 31 cm wide, 19 cm high and 31 cm deep was used. The oven used, with Inverter technology, had selectable output powers between 100 W and 1,000 W.

[0090] Inside the chamber (3) a hollow tubular alumina enclosing element (4) of 17 cm length, 4.5 cm external diameter and 3.2 cm internal diameter, with a thickness of 0.65 cm has been used. The hollow enclosing element (4) has been filled with silicon carbide (SiC) spheres of 5 mm diameter, configuring a packing factor of approximately 0.65. The enclosing element (4) has been covered with rock wool to prevent heat loss from the system.

[0091] The air inlet and outlet were provided by copper tubes connected directly to the alumina casing (4). The air flow rate was 9.9 L / min.

[0092] The air inlet temperature was, in all cases, 21 e C.

[0093] Under the conditions described, the air outlet temperature for a radiant power of 100 W was 326 e C, which represents an approximate overall efficiency of 66%.

[0094] For a radiant power of 1,000 W, the outlet air temperature was 740 e C, allowing the system to be used in high-temperature applications. A second practical implementation example, also carried out in a laboratory, is described below, following the third embodiment.

[0095] A device like the one in Figure 7, corresponding to the third embodiment, was used. The power applied, via a microwave generator, was 1500 W. The susceptor used was a silicon carbide sponge, with 80% porosity and dimensions of 14x14x4 cm3.

[0096] For an air flow rate of 300 L / min, the temperature increase observed in the module was 240 e C. The use of multiple modules in series would allow additional temperature increases to be achieved, limited by the thermal resistance of the materials used.

Claims

1. Microwave air heating system comprising a chamber (3) which in turn comprises an access (1) for microwaves generated by a microwave emitter, a susceptor (5) and a surrounding element (4), characterized in that the surrounding element (4): - has an elongated configuration with two ends, - is located in the chamber (3), - is made of a microwave-permeable material, - houses the susceptor (5), and - comprises a grid (2) at each end, where the susceptor (5) is illuminated homogeneously by the microwaves.

2. The microwave air heating system of claim 1, wherein the susceptor (5) is of a ceramic nature.

3. The microwave air heating system of claim 1, wherein the susceptor (5) is made of silicon carbide.

4. The microwave air heating system of claim 1, wherein the susceptor (5) is configured by elements to be selected between spheres and irregular particles.

5. The microwave air heating system of claim 4, wherein the elements that make up the susceptor (5) have a maximum size of between 1 mm and 20 mm.

6. The microwave air heating system of claim 5, wherein the elements that make up the susceptor (5) have a maximum size of between 2mm and 10mm.

7. The microwave air heating system of claim 1, wherein the susceptor (5) is configured in the form of a rigid, porous, air-permeable structure.

8. The microwave air heating system of claim 7, wherein the susceptor (5) has a porosity of between 70% and 90%.

9. The microwave air heating system of claim 7, wherein the susceptor (5) has a porosity with pores of a maximum size between 1 mm and 20 mm.

10. The microwave air heating system of claim 1, wherein the enclosing element (4) is made of alumina.

11. The microwave air heating system of claim 1, wherein the grid (2) is metallic.

12. The microwave air heating system of claim 1, wherein the chamber (3) is an inner tube (6) permeable to microwaves and the enclosing element (4) is an outer tube (7) coaxial to the inner tube (6), the grilles (2) closing an annular zone (8) located between the inner tube (6) and the outer tube (7).

13. The microwave air heating system of claim 1, comprising a plurality of enveloping elements (4), located with the longitudinal axes parallel, covering the respective susceptors (5).

14. The microwave air heating system of claim 13, comprising a plurality of microwave ports (1).

15. The microwave air heating system of claim 14, wherein the microwave ports (1) are located opposite and transversely to the axis of the tube (4).

16. The microwave air heating system of claim 1, wherein the chamber (3) houses two susceptors (5) of rigid and porous structural configuration, each susceptor (5) being located between the microwave access (1) and one of the grids (2).

17. The microwave air heating system of claim 16, wherein the chamber (3) is stackable longitudinally by simply removing the grid (2) located between two chambers (3).

18. The microwave air heating system of any of the preceding claims 1 to 17, wherein the enclosing element (4) has a circular cross-section.

19. The microwave air heating system of any of the preceding claims 1 to 18, wherein the enclosing element (4) has a polygonal cross-section.

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