The present invention relates to a process and a device for heating materials using microwaves.

Technical background.

The use of microwaves as energy source for heating or cooking has been known for some time and is based on the property of the microwaves to excite the molecular movements of some materials, including fluids such as water and air, causing them to heat. The use of microwaves to dry materials and products in general is known, too. In particular, the use of microwaves for heating domestic hot water has been proposed.

Patent application GB-A-2354688 describes a heating device wherein water is flowed through a transparent silica coil inside a sealed chamber provided with two magnetrons. However, this device has proved to be costly and of little efficiency.

Patent US-B-4956533 relates to ceramic compositions that can be used in disposable packagings for pre-cooked foods to be heated in microwave ovens.

According to this patent, alumina (Al203), sodium metasilicate, kaolin, talc or analogous hydrated ceramic materials are used as microwave radiation absorbing compounds, individually or as a mixture. These materials are used together with a variety of binders, ranging from PVC to chalk, with which they are mixed in damp form and then dried until they reach a water content ranging between 2.5% and 10%. The disadvantages of this embodiment are that heating is essentially based on the presence of water in the mixture of absorbing compounds and that the materials are not able to withstand prolonged or repeated heating cycles.

Summary of the invention It is an aim of the present invention to solve the aforesaid problems and to provide a process and a device that allow the use of microwaves as an energy source for heating, even continuously or repeatedly, in a simple, efficient and

inexpensive way.

This aim is reached by means of the present invention, which relates to a process for heating a material, characterized in that it comprises the steps of placing said material in a heat exchange relation with an inorganic compound selected from tectosilicates, phyllosilicates and mixtures thereof and applying microwave radiation to said inorganic compound to heat it. The heated inorganic compound then releases its heat to the material with which it is in contact and therefore functions, together with the microwave source, as a heat source, as well as a heat exchange means.

In an embodiment of the process, the material to be heated is a fluid that is flowed through a heating element comprising a heat exchanger, for example a coil or another type of heat exchanger, such as a tube bundle or a plate heat exchanger, said heat exchanger being covered by, or embedded in, a mixture containing the inorganic compound and a heat-resistant binder; alternatively, the fluid is made to flow directly through a heating element composed of said inorganic compound and if necessary by a binder.

According to another embodiment of the invention, the material to be heated is a thermoplastic material and the process comprises the steps of mixing this thermoplastic material with the microwave absorbing inorganic material, irradiating the mixture until plasticization of the thermoplastic material and subjecting the plasticized mixture to processing suc : h as injection or compression molding, extrusion, blow molding and the like. According to this aspect of the invention the inorganic compound is at least in part used as a filler for the thermoplastic material and it is molded or processed together with it.

The invention provides many advantages over the prior art. The microwave absorbing inorganic compounds selected according to the invention have extremely low costs because they are widely available minerals. The fact that these compounds are essentially anhydrous makes it even more surprising that they can be used according to the invention.

Moreover, the invention provides to irradiate, and thereby heat, these

compounds instead of the material itself ; the compounds in, turn release the heat to the material and thus make it possible to treat any material, irrespective of the water content, or humidity, of the material to be heated. This aspect is particularly advantageous as it allows the use of microwaves to heat materials such as thermoplastic materials, which can otherwise only be heated with traditional methods.

These materials, which have a thermal performance following irradiation, are used alone or mixed with one another in order to reach the desired temperature profile.

Brief description of the drawings.

These and other advantages are disclosed hereunder with reference to the accompanying drawings provided purely as a non-limiting example, wherein: - Figure 1 is a schematic view of a device for heating a fluid according to the invention; - Figure 2 is a schematic view, partially sectional, of a heating element to use in the device in Figure 1; and - Figure 3 is a block diagram of the device shown in Figure 1.

Detailed description of the invention.

As above mentioned, according to the invention a material is heated indirectly, by placing it in direct or indirect contact, that is in a heat exchange relation, with microwave absorbing inorganic compounds selected from-minerals--in the classes of tectosilicates, phyllosilicates and mixtures thereof.

With regard to tectosilicates, suitable compounds are feldspars which are aluminosilicates of K, Na and Ca, and in particular: - Orthoclase and Microcline - albite - Anorthite -Plagioclase - Zeolites With regard to phyllosilicates, suitable compounds are minerals belonging to the

Mica group such as Biotite, Muscovite, Paragonite, Phlogopite, Lepidolite and Zinnwaldite.

The inorganic compound (s) that absorb the microwaves and heat up are generally mixed with one or more binders. The term binders is intended as any type of matrix containing microwave absorbing inorganic compounds. An example of a natural binder is the matrices of the rocks containing these compounds.

These mineral compounds can in fact be found individually inside the earth's crust or, more commonly, inside rocks, in particular inside magmatic rocks such as: <BR> <BR> -Persilic<BR> - Mesosilicic - Basic - Ultrabasic A mineralogical composition of the aforesaid rocks is indicated below, with percentages indicated in volume.

-quartz 5-50% - orthoclase 5-60% -plagioclase (isomorphous mixtures of albite and anorthite) 5-60% - mica group 4-90% A preferred rock is granite in all its types.

Therefore, the scope of protection of the patent also covers processes using rocks comprising phyllosilicate and tectosilicate compounds.

As well as natural binders, synthetic binders may also be used: the term "synthetic binders"is intended as comprising all binders obtained through the action of man, such as cement, in particular refractory cements, the use of which is illustrated hereunder.

In a preferred embodiment of the process according to the invention the material to be heated is a fluid and in particular air, water or a water-based fluid.

In fact, the invention is implemented with excellent results to produce heating

modules and to the heating of fluids by means of materials sensitive to microwaves, by natural or forced convection of fluids in stationary, laminar or turbulent motion, by conduction, both stationary and non-stationary, and by irradiation.

Air will be heated when, for example, house or buildings heating is required, while water will be heated to produce hot water, and a fluid containing water and if necessary additives is heated when house heating through a plumbing system is required.

In the aforesaid cases, in a preferred embodiment of the invention the fluid is flowed through a heat exchanger, such as a coil, or a tube bundle or a plate- type heat exchanger. Said heat exchanger element is covered by, or embedded in, a mixture comprising said inorganic compound (s) and a heat resistant binder, such as a cement binder; in another embodiment the fluid is made to flow through a heating element which releases the heat directly to the fluid. In the case of solid materials, the heating element according to the invention, having the required shape, for example a container, can contact directly the material to be heated, or it can be provided with a"liner"or other type of coating positioned between it and the material to be heated.

Figures 1-3 show an application of the invention for the production of hot water.

With reference to figures 1 and 2, a microwave generator 1 has been fitted, if necessary provided with'a waveguide (not shown) on a casing 2'not transparent to the microwaves and designed so as to allow maximum reflection of the microwaves inside it.

The power of the megatron is depending on the civil or industrial application for which it is designed and in general it is within the range of 0.3 and 9.0 kW; in the application for the production of hot water for domestic use the power is within the range of 300 MHz to 300 GHz, corresponding to a wavelength from 1. 0 m to 1. 0 mm.

Inside the casing 2 there is provided a heating element 3, embedded inside which is a coil 4 in copper or another material with high thermal conductivity

and/or material resistant to high temperatures.

The heating element where the coil 4 is embedded in, comprises a mix of the aforesaid inorganic compound with a binder resistant to high temperatures, for example a refractory cement; the coil 4 has an inlet 5 and an outlet 6 for the fluid and is provided, in a way known per se, with a valve 5a so as to allow both continuous and discontinuous flow of the fluid through it.

When the microwave generator 1 is started, the material of which the heating element 3 is composed receives energy in the form of microwaves and returns it in the form of heat to the coil 4, which in turn releases the heat to the fluid contained inside the coil.

Example.

Purely as a non-limiting example, a magnetron 1 of 1000W was used inside an steel irradiation chamber 2, inside which is fitted a heating element composed of 40% granite powder and 60% high temperatures resistant cement binder.

Seven meters of copper coil with an internal diameter of 12.5 mm (1/2") were embedded inside the heating element. By applying a continuous flow of water with a flow rate of 10-15 I/min depending on the mains pressure, with an input temperature of 15°C, it is obtained an output temperature of 20°C after one minute, of 25°C after two minutes, of 75°C after 5 minutes and with longer irradiation times the liquid was delivered in the form of steam.

Once the desired ternperature has 6een reached it-viiill only-be necessary to temporarily irradiate the heating element, which has a considerable thermal capacity. In this way hot water was obtained without irradiation of the water itself, without fume emissions and at a low cost.

Figure 3 shows a block diagram of a complete apparatus for heating water, comprising a flowmeter 7 at the inlet, a water temperature and flow rate sensor 8 at the inlet, a water temperature sensor 9 at the outlet and an electronic device 10 to control the temperatures and the power of the microwave source.

Panels or tiles produced with the materials indicated hereinbefore as microwave absorbers, generally mixed with binders, may advantageously be used as

elements for heating ovens and similar equipment.

As mentioned hereinbefore, the microwave absorbing material according to the invention is essentially anhydrous and may advantageously be used as at least part of the filler in plastic materials. For example, a possible such a use is in the extrusion of a plate of thermoplastic material containing a quantity of compound according to the invention that allows its subsequent heating by means of microwaves before a compression molding or thermoforming step. This technique is particularly useful in the production of panels for automobile interiors, refrigerator and freezer interiors and the like.

With regard to injection molding, thanks to the presence of the filler it is possible to substitute the traditional plasticizing screw with a mixer or with a screw of reduced length.