ES2965960B2 - MICROWAVE AIR HEATING SYSTEM - Google Patents

Description

DESCRIPTION

MICROWAVE AIR HEATING SYSTEM

OBJECT OF THE INVENTION

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 above 200° C.

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

BACKGROUND OF THE INVENTION

Air heating systems are used in many industrial processes where materials of very diverse nature are treated, such as ceramics, glass, metals, chemicals, food, among others. They are also used for thermal conditioning of warehouses and buildings, and in the treatment of waste gases.

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

Systems based on electrical resistors have a high efficiency 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.

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

Heat pumps are very efficient, but the maximum temperature reached is very low.

An alternative method is to heat the air with microwaves. Microwave energy is not absorbed by air, 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.

Microwave heating systems offer a number of advantages over the previously mentioned heating systems, such as the high temperature reached, the speed of heating, the safety they present due to the lack of risk derived from combustion and the quality of the air, which is not contaminated with any type of product during the process.

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

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

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 walls of the chamber, so it implements a fin system to increase the exchange surface. These documents do not provide information on the temperature reached by the outlet air or the characteristic parameters related to the thermal process. Under this configuration, the air circulating through the microwave oven comes into contact with the susceptor on the outside, so the contact surface for heat transmission is very small and, consequently, so will its efficiency. The configuration described in this document 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.

Document CN2355263Y discloses a device where air flows transversely through a grid of susceptor tubes that have been heated by microwaves. This document refers to the efficiency of the magnetron in converting electrical energy to radiant energy, but says nothing about the efficiency in transferring energy to the air, which is one of the key aspects. As in the previous system, the reduced contact area between the susceptor and the air means that the efficiency cannot be raised.

Document US2022353961A1 discloses a system in which an air flow is heated by forcing its passage over a microwave-heated liquid contained in airtight containers. The system described does not present a 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.

Document WO2004068902A2 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 comes from the need to reduce the absorption of radiation in the chamber to increase the penetration of the waves and achieve homogeneous heating. However, this system has a series of important drawbacks, such as the fact that the reduction of the volumetric fraction of silicon carbide in the chamber limits the maximum power used, that the loss of direct contact between the silicon carbide and the air considerably reduces the transfer of heat to the air and that the reduced air permeability severely limits the maximum air flow that can be used.

The above-mentioned drawbacks are overcome in the present invention. On the one hand, the three illumination systems used allow for homogeneous heating. On the other hand, the susceptor system used in the present invention, consisting of a porous bed of silicon carbide/air with a custom microstructure (porosity and pore size) to maximize the contact surface, maintains a high permeability and heat transfer capacity.

There are also documents such as CN210152864U or US2020147600A1 that use beds of silicon carbide balls, and take advantage of different properties of silicon carbide, such as its resistance to abrasion and its reactivity with oxygen, respectively, although they are not used in heating applications.

DESCRIPTION OF THE INVENTION

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 the efficient heating of the air to a temperature greater than 200°C. This temperature is useful for numerous industrial processes.

The system of the invention describes different configurations of high efficiency that allow achieving very high air outlet temperatures. These new configurations achieve a more effective air-solid heat exchange due to the direct air-susceptor contact and the large contact surface obtained.

On the one hand, the direct contact between the susceptor and the air allows the overall heat transmission coefficient of the system to be very high, since there are no intermediate resistances due to the use of other elements.

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.

The invention has two critical technical aspects, namely:

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

- the transfer of thermal energy from the susceptor to the air.

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 adequately to achieve high efficiency and adequate homogeneity.

There are various materials that could be used as susceptors, although only those of a ceramic nature, such as silicon carbide or ferrite, could be viable for high-temperature applications. Among these, the material that best meets the requirements outlined is silicon carbide. On the one hand, the dielectric constants of silicon carbide are very high (sr’=30, sr”=11, tan 8=0.37), not varying excessively with temperature, which guarantees high absorption in the temperature range used. On the other hand, the thermal conductivity is 100 W/(m K), higher than that of many metals, which also gives it, along with its good mechanical properties, 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°C continuously, which guarantees the stability of the system.

Furthermore, the use of electromagnetic radiation allows energy to be transported without physical contact and, as a consequence, to heat a large volume of susceptor, since the heat is generated directly in the material itself. This transmission of energy is conditioned by the high energy absorption of the susceptor, which limits the maximum penetration of radiation into a porous bed to about 2-3 cm deep.

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

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

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.

In the third configuration, a sandwich-type structure is used, composed of alternating layers of air and susceptor. The air layers are used to distribute the radiant energy evenly.

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 very 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 porous, sponge-like walls.

All the proposed configurations are microwave-tight. For this purpose, metallic materials that reflect microwaves have been used. At the air inlets and outlets, metal mesh has been used with a mesh size sufficiently fine to prevent the passage of microwaves, but with a high free passage surface, to allow the air to flow easily.

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

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

- Maintain direct contact between silicon carbide and air. Silicon carbide has a very high thermal effusivity (19400 W s05m-2K-1, higher than that of most metals (iron's is 15924 W s05m-2K-1), which indicates a very high rate of energy transfer. The use of any intermediate material, especially of a ceramic nature, would reduce this capacity by about ten times (porcelain: 2314 W s05m-2K-1) and, consequently, the efficiency.

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

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

In addition, to optimise the permitted air flow, the air permeability of the filling must be maximised. This depends on the porosity and pore size of the system. In the case of fillings made of balls of the same size, the porosity depends on the packing and can hardly be changed (approx. 35% vol.), which is why relatively large balls (4-5 mm diameter) are preferably chosen, which generate relatively large pores (approx. 1-2 mm). In the case of fillings with a sponge-like structure, both properties can be varied within wide ranges.

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

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.

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

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

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

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

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

Furthermore, in another embodiment, the chamber and the enclosing element are configured in the form of two coaxial tubes where the chamber is an internal tube permeable to microwaves, as a wave guide, and the enclosing element is an external metal tube coaxial to the internal tube, such that the grids close an annular zone located between the internal tube and the external tube.

The system can also be configured with a plurality of enclosing elements instead of just one, all of them placed with the 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, but the system comprises two microwave accesses placed opposite each other and transversely to the longitudinal axis of the enclosing elements.

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.

Furthermore, in this last embodiment the chamber can be stacked longitudinally by simply removing the grid located between two chambers, leaving the grids located only at the ends of the stack.

Finally, it must be considered that any of the enclosing elements that the system incorporates can have a circular cross section or also be polygonal.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, the figures listed below are introduced in this report.

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.

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.

Figure 3 represents a section of an enclosing 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.

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

Figure 6 represents a perspective view of the system of the invention shown in Figure 4 but with a plurality of surrounding elements.

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

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

1. Microwave access.

2. Grids.

3. Camera.

4. Envelope element.

5. Susceptor.

6. Inner tube.

7. Outer tube.

8. Annular zone.

PREFERRED EMBODIMENT OF THE INVENTION

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.

Figure 1 shows 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 two accesses (1), upper and lower, have been represented in the figure, 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) that is permeable to the microwaves and capable of resisting the high temperatures caused by the reception of the 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.

Each end of the enclosing element (4) is closed with a metal grid (2) incorporating holes of an appropriate size to prevent the microwaves from leaving the enclosing element (4). Preferably, the holes have a dimension of 1 mm2.

At each end of the enclosing element (4) pipes are connected for the air inlet and outlet of the system so that a current of air can be forced to circulate inside the enclosing element (4) and absorb the thermal energy of the susceptor (5).

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 is homogenized before its energy is absorbed by a susceptor (5) housed inside a microwave-permeable enclosure (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 enclosure (4), the heating being limited by the maximum penetration of the energy of the electromagnetic waves into the susceptor (5).

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

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

The second consists in that the chamber (3) incorporates 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 greater capacity to heat all the surrounding elements (4) that are in the chamber (3).

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

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 zone (8) which is closed at the ends by metal grids (2). The radiation is emitted through one end of the inner tube (6) as a cylindrical waveguide with controlled losses along its axis, to transmit and distribute the energy evenly throughout the volume of the susceptor (5), acting as a chamber (3). The susceptor (5) is located in the annular zone (8), where the microwaves reach by passing through the wall of the inner tube (6), which is permeable to the microwaves.

In this case, the inner tube (6) is of a ceramic nature, permeable to microwaves and resistant to temperature and can 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) for the emission of microwaves is located 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.

The outer tube (7) is metallic in nature to prevent electromagnetic radiation from escaping, as are the ends of the inner tube (6).

The air inlet and outlet of the system is carried out through the ends of the annular zone (8) formed between the two tubes (6, 7) and where the grilles (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) limited by the penetration of the radiation. The main advantage of this configuration over the previous ones is its compactness.

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

Figure 7 shows a third embodiment in which the chamber (3) has an elongated configuration and incorporates two 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. In addition, 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 configuration that remains is made up of 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).

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.

This configuration is especially suitable for systems with sponge-type susceptors (5), due to the rigidity it provides, since it does not require 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 a more homogeneous illumination. This is the system with the simplest assembly and is very easily scalable. The enveloping element (4) is covering 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. Differently from the other configurations mentioned above, in this case the susceptors (5) receive the microwave radiation directly and not through the enveloping element (4).

The wavelength of the electromagnetic radiation used (^«12 cm) and the maximum penetration of the radiation into the material (2-3 cm) restricts the dimensions permitted in each individual module and, consequently, its power, the air flow used and the temperature increase achieved, so the possibility of using it in parallel (to increase the air flow) and in series (to increase the temperature increase) is included.

Below is a description of a practical example of implementation, carried out in a laboratory, following the first form of implementation.

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.

Inside the chamber (3) a hollow tubular alumina envelope (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 envelope (4) has been filled with silicon carbide (SiC) spheres of 5 mm diameter, configuring a packing factor of approximately 0.65. The envelope (4) has been covered with rock wool to prevent heat loss from the system.

The air inlet and outlet were made by means of copper tubes coupled directly to the alumina casing element (4). The air flow was 9.9 L/min.

The air inlet temperature was, in all cases, 21°C.

Under the conditions described, the air outlet temperature for a radiant power of 100 W was 326°C, which represents an overall efficiency of approximately 66%.

For a radiant power of 1,000 W, the outlet air temperature was 740°C, allowing the system to be used in high temperature applications.

Claims (19)

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), - It is made of a microwave-permeable material, - houses the susceptor (5), and - comprises a grid (2) at each end, where the susceptor (5) is homogeneously illuminated by the microwaves and has a geometry that depends on the power of the radiation intended to be applied. 2. The microwave air heating system of claim 1, wherein the susceptor (5) is ceramic in 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 selected from spheres and irregular particles. 5. The microwave air heating system of claim 4, wherein the elements that configure the susceptor (5) have a maximum size of between 1mm and 20mm. 6. The microwave air heating system of claim 5, wherein the elements that configure 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 as a rigid and porous structure permeable to air. 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 1mm and 20mm. 10. The microwave air heating system of claim 1, wherein the wrapping 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 internal tube (6) permeable to microwaves and the enclosing element (4) is an external tube (7) coaxial to the internal tube (6), the grids (2) closing an annular zone (8) located between the internal tube (6) and the external 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 accesses (1). 15. The microwave air heating system of claim 14, wherein the microwave accesses (1) are located facing 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 longitudinally stackable 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.