WO1993000781A1

WO1993000781A1 - A microwave heating method and a relative microwave heating device

Info

Publication number: WO1993000781A1
Application number: PCT/IT1992/000065
Authority: WO
Inventors: Alberto Breccia Fratadocchi
Original assignee: Ipctisa S.R.L.
Priority date: 1991-06-21
Filing date: 1992-06-19
Publication date: 1993-01-07
Also published as: EP0593606A1; JPH06511547A; AU2263292A; ITBO910224A1; ITBO910224A0

Abstract

In an energy-saving method of producing hot air or hot water for space heating, hot water for domestic and industrial consumption, or high pressure heated steam for operating turbines and pistons, use is made of microwaves and of materials having a high dielectric constant which are able to absorb high frequency radiation (300 - 300 000 MHz); a device enabling the implementation of the method comprises a container (2) into which to direct a fluid, such as air, and a set of plates (20) fashioned from the dielectic material which are directly exposed to the microwaves. Warmed by contact with the plates, the air can be used for heating purposes.

Description

A microwave heating method and a relative microwave heating device.

Technical field

The present invention relates to microwave heating, and to microwave heating devices. More particularly the invention relates to a method and to a relative device wherein use is made of microwave radiation, and of a microwave absorbing material capable of being heated and of transmitting heat in turn to a fluid with which it is in contact. A fluid heated in this manner can be used for space heating, or in the case of hot water, distributed for domestic and industrial consumption; in addition, high pressure steam generated by the method disclosed can be used to operate turbines or pistons.

Background art

It is known that electromagnetic waves in the range of frequencies between 300 and 300,000 megahertz, commonly referred to as microwaves, are capable of inducing a rotational and to an extent vibrational effect in the molecules of materials possessing a high dielectric constant. Among the characteristics of such microwave absorbing materials, referred to also as dielectrics in the following specification, is that they excite when exposed to radiation and undergo a rise in temperature.

Certain materials are transparent to microwaves; these undergo no rise in temperature when exposed to radiation and are considered good insulators. Finally, there is also a third group of materials which reflect microwave energy without heating and are considered good conductors. On the basis of their respective properties, the different materials mentioned above can be used in conjunction with microwaves for different purposes: transparent materials for inert containers, such as crucibles; reflective materials for the external claddings of microwave systems; and dielectrics as heat sources by virtue of their capacity to excite when invested with microwave radiation. According to the International Radio Regulations, the frequencies that can be utilized for microwaves are 915 ±25, 2450 +13, 5800 ±75 and 22125 ±125 MHz. Of these the most widely used is 2450 MHz, enabling as it does a good penetration of materials combined with high surface intensity.

Notwithstanding microwave devices have been used in conjunction with microwave absorbing materials, for instance to analyze samples for moisture content and the presence of volatiles, to digest a variety of materials and to promote chemical reactions, the applicant is unaware of any prior art disclosing or suggesting the application of microwave apparatus or devices as means of heating fluids destined for use in space heating systems, or of producing hot water for domestic or industrial consumption, or of generating high pressure steam for driving turbines or pistons. Accordingly, the object of the present invention is to provide a method and a device such as will bring about heat exchange between a fluid and a source of heat consisting in a microwave absorbing material. A further object of the invention is to provide a method and a device whereby microwave energy can be used to produce hot air or hot water as a means of space heating or as a utility for domestic and/or industrial use.

Yet another object of the invention is to provide a method and a device whereby microwave energy can be used to generate high pressure steam for operating turbines or pistons.

Disclosure of the invention

The stated objects are realized in a method and a device, as characterized in the appended claims, of which the function is to bring about heat exchange between a heat source and a fluid, wherein the heat source consists in materials with a high dielectric constant exposed to the action of microwaves. One of the advantages gained by way of the present invention is essentially that it provides a system, comprising microwave generator, dielectric material and heated fluid, characterized by high energetic efficiency. A preferred embodiment of the invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: -fig 1 is a perspective of the device according to the present invention, viewed from the front and in an embodiment suitable for producing hot air as a means of space heating;

-fig 2 is a further front perspective view showing a device according to the invention suitable for producing hot water; -fig 3 is a further front perspective view showing a device according to the invention suitable for producing hot water or generating high pressure steam. To the end of identifying the microwave absorbing materials most suitable for use in the context of the method disclosed, a test was conducted in which samples of dielectrics were heated for 3 minutes in a 750 w microwave furnace, alone and with 100 ml water, and the resulting temperatures then compared one with another and with that of the 100 ml water when heated alone. The results are shown in table 1.

Table 1 - Temperature of samples measured after 3 minutes in 750 W microwave furnace

* after 2 minutes.

With a quantity of BN equivalent to 1/20 of the quantity of other dielectrics tested, a violent evaporation of the water occurs at the surface of the material after only 2 minutes. Fig 1 illustrates a microwave device for producing hot air. The device comprises a microwave generator unit 1 consisting essentially in a magnetron 10 rated 700 W - 10 kW, operated off 220/380 V a.c. and producing frequencies of 2455 MHz and 915 MHz, a waveguide 11 and a waveguide resonator 12, which combine to direct a flow of microwaves, denoted by the arrows, onto a plurality of silicon carbide or boron carbide plates 20. The plates 20 in question are spaced apart one from another and mounted on a support 21 fashioned in a material capable of refracting microwaves, such as quartz, internally of a container 2 constructed in material capable of reflecting microwaves, such as steel.

The container 2, of substantially parallelepiped embodiment, is associated both with the microwave generator unit 1 and with means by which to effect a forced induction of air, consisting for example in a fan unit 30 and a duct 31.

Invested by the microwaves, the plates 20 undergo a rise in temperature and give off heat into the air to which they are directly exposed; the air is then expelled from the container 2 at a temperature of between 60 and 100 °C or higher, by way of suitable vents 22 fitted with grilles 23, in such a way as to warm the surrounding ambient.

The device thus described can be embodied adopting compact dimensions, for example a few decimetres per side, which will be variable according to the quantity of hot air it is wished to produce. As to the plates 20, these would be fashioned from materials combining advantages of economy (i.e. low cost) with good heat dissipation; boron carbide is preferred by virtue of its greater resistance to oxidation at high temperature and its ability to heat at a faster rate.

A system as described above is able to achieve an output of some 400 kcal in 10 minutes; experiments have shown that when heating a room of 75 m³, the energy consumed by the microwave system operating at 700 W power output is between 25 and 50% of that consumed by a conventional 1-5 k warm air electric fan heater. Fig 2 shows a microwave device for producing hot water. In like manner to the embodiment described above, microwaves emitted by a generator unit l¹ are directed at a dielectric, in this instance a coiled tube 24 fashioned from or faced externally with a microwave absorbing material.

The coil 24 is housed internally of a container 2' of microwave reflecting material, and connected to a heating circuit or to a hot water distribution system. More exactly, the coil 24 is fashioned from tube of 10-15 mm diameter, some 20-30 cm in length, in a material selected from carbides of silicon, boron, titanium or tungsten, or borides of silicon, titanium or aluminium, etc., which might be pure or combined with nitrides of the same elements; the option also exists of using ferroelectric ceramics containing titanates of barium or lead or other metallic elements (zinc, manganese, copper, etc.). To advantage, the coil 24 is coated with a material transparent to microwaves such as will give greater mechanical strength and more readily dissipate excess temperature. Preferably, the water flowing through the coil 24 will be deionized. Fig 3 illustrates a microwave device for producing hot water or generating steam at high temperature and high pressure. In this embodiment, microwaves emitted by a generator unit 1" are directed onto a plurality of plates 25 fashioned from material able to absorb high frequency radiations and housed in a first container 26 embodied in material transparent to microwaves.

The first container 26, mounted on a support 27 of material refractive to radiation, is positioned internally of a second container 2" embodied in microwave reflecting material and associated with the generator unit 1".

Water directed through an inlet 28 into the first container 26 is heated rapidly on making contact with the plates 25 exposed to microwave radiation, and directed through an outlet 29 into the heating circuit or the hot water distribution system. This type of device is capable of discharging water or water vapour at temperatures between 60 and 80 °C, according to the quantity of water introduced and to the number of plates installed. The plates 25 might be between 10 and 25 in number, spaced apart 5...10 mm one from the next, and with typical dimensions of 30 x 15 x 1 cm. To advantage, the plates 25 will be of boron nitride, exhibiting a honeycomb structure and immersed three quarters in deionized water.

In another possible embodiment of this same device, the first container 26 might be embodied in boron carbide or nitride reinforced with steel or other materials able to withstand high pressures. Experiment has shown that steam can be generated at pressures of 30 to 40 bar and temperatures of the order of 230 to 350 °C, in approximately one third of the time taken where use is made of materials unable to absorb high frequency radiations. Steam generated in this way can be used to advantage in driving turbines, for example, with at least a part of the heat being recovered by means of exchangers. The device of fig 3 is again compact, measuring substantially less than one metre on any side, the dimensions of the first container 26 being, by way of example, 30 x 15 x 40 cm.

Observing fig 1 in particular, it is clear that the magnetron 10 might be set at a certain distance from the container 2, and the microwaves directed into the container through a waveguide of suitable length. In this instance, the microwave generator unit 1 or 1* or 1" is detached from the relative container 2 or 2' or 2", rather than attached as shown in the drawings.

Claims

1) An energy-saving method of producing hot air or hot water for space heating, or hot water for domestic and industrial consumption, and of generating steam at high pressure and high temperature for operating turbines and pistons, characterized in that it utilizes microwaves, and materials with a high dielectric constant capable of absorbing high frequency. radiation (300 - 300 000 MHz).

2) A method as in claim 1 by means of which to bring about heat exchange between a heat source and a fluid, comprising the steps of:

-directing a fluid into at least one container of which the internal walls are embodied in material capable of reflecting microwaves;

-directing microwaves onto a material possessing a high dielectric constant capable of absorbing high frequency radiation (300 - 300 000 MHz), placed internally of the container, in such a way that the irradiated material provides the heat source;

-bringing the fluid and the irradiated material into contact in such a way as to heat the fluid; -drawing off the heated fluid from the container.

3) A method as in claim 2, wherein the fluid is air, directed into the container at a first temperature and returned to the ambient at. a second temperature higher than the first. 4) A method as in claim 2, wherein the fluid is water, directed into the container from the space heating circuit or from the water supply system, heated, and returned to the circuit or system.

5) A method as in claim 2, wherein the heat exchange occurs internally of a second container embodied in or at least lined internally with a material having a high dielectric constant, and the heated fluid consists in high pressure steam 10 suitable for the operation of a turbine or a piston.

6) A microwave device by means of which to bring about heat exchange between a heat source and a fluid, characterized in that it comprises:

-a microwave generator unit;

-means embodied in or faced with material having a high dielectric constant capable of absorbing high frequency radiation (300 - 300 000 MHz), placed in contact with the fluid and invested with microwaves emitted by the generator unit in such a way as to provide the heat source;

-at least one container fashioned from a material capable of reflecting microwaves, internally of which to bring about the heat exchange between the means embodied in material having a high dielectric constant and the fluid.

7) A device as in claim 6, wherein the means capable of absorbing high frequency radiation consist in a plurality of plates exhibiting either a plain or a honeycomb structure, or in a coiled tube, fashioned in a material selected from carbides of silicon, boron, titanium or tungsten, or borides of silicon, titanium or aluminium, utilized either pure or in combination with nitrides of the selfsame elements, or ferroelectric ceramics containing titanates of barium or lead or other metallic elements such as zinc, manganese, copper and the like.

8) A device as in claim 7, comprising means by which fluid in the form of air is directed forcibly into the container, and means by which heated air is released into the surrounding ambient.

9) A device as in claim 7, comprising means by which to draw fluid in the form of water from a heating circuit or from a supply system, and means by which to return heated water to the selfsame circuit or system.

10) A device as in claim 7, further comprising a second container embodied in or at least lined internally with a material having a high dielectric constant, internally of which heat exchange occurs, wherein the heated fluid consists in high pressure steam suitable for operating a turbine or a piston.