WO2005079116A2

WO2005079116A2 - Mehod and apparatus for heating a fluidic load using radio frequency energy

Info

Publication number: WO2005079116A2
Application number: PCT/GB2005/000479
Authority: WO
Inventors: Michael Kiernan; Nigel Jones
Original assignee: Micro Heat Limited
Priority date: 2004-02-11
Filing date: 2005-02-10
Publication date: 2005-08-25
Also published as: WO2005079116A3

Abstract

Apparatus for heating water (12) in a tank (10) comprising a radio frequency energy source, such as a magnetron (14). Radio frequency energy is supplied from the source (14) to the water (12) in the tank (10) via a substantially conical waveguide. The radius of the waveguide (16) at the point (18) at which the radio frequency energy is first incident from the magnetron (14) is beneficially greater than λ/4, where λ is the wavelength of the radio frequency energy. A PTFE plate or membrane (20) is provided between the distal end of the waveguide (16) and the body of water (12), at least portion of the upper surface plate (20) being substantially conical so as to provide a non-uniform interface between the water (12) to be heated and the radio frequency energy. The plate (20) is substantially transparent to the radio frequency energy, i.e. it does not absorb or reflect such energy. In use, RF energy emitted perpendicularly from the magnetron (14) strikes the conical waveguide (16) at points (18), and is reflected upwards towards the water (12). The energy passes through the first surface of the PTFE plate (20) with minimal absorption and strikes the interface between the plate (20) and the water (12). As a result, the angle of reflection of the RF energy is such that little, if any, thereof is reflected back to the magnetron (14). Instead, it simply ‘bounces’ around the inside of the waveguide (16) until it is returned to the water (12). An angled, microwave reflective plate (30) may be provided on an upper portion of the tank (10).

Description

Method and Apparatus for Heating a Flnidic Load Using Radio Frequency Energy

This invention relates to a method and apparatus for heating a fluidic load using radio frequency energy and, more particularly, but not necessarily exclusively, to a method and apparatus for heating a body of water or the like using microwave energy, so as to provide a highly efficient, relatively safe, relatively low maintenance water heater for commercial, domestic or industrial environments.

Electric water heating devices are well known. Such devices generally comprise a water tank for holding a predetermined volume of cold water for heating, and the water tank is conventionally lagged or surrounded by a thermally insulating outer casing. Within the tank is provided an electric resistance or element which becomes hot when electrical current is passed therethrough, and heats the water within the tank to a predetermined desired temperature.

Such electric water heating devices are conventionally used in environments where space and ventilation are minimal because they are generally considered relatively safe in operation. However, there are a number of disadvantages associated with this type of water heater, as follows. Firstly, electricity is a relatively expensive power source; and secondly, the electric element quickly becomes covered with limescale thereby greatly reducing its heating efficiency, which drawback can only be alleviated by frequent maintenance operations to clean or replace the element. As a result of both of these issues, the operation and maintenance costs of an electric water heater or boiler are comparatively high.

In environments where space is not necessarily limited, and where sufficient ventilation can be readily provided, it is more common to employ gas water heaters or boilers, largely because gas is a relatively low-cost power source in comparison to electricity. Again, many such devices comprise a large water tank for holding a predetermined amount of water to be heated, the tank being surrounded by some form of thermally insulating material, as in the case of an electric water heater or boiler. However, another type of gas boiler, namely the combination boiler, also exists, which heats water on demand and does not require a ater tank as such. Although such devices are smaller than the traditional water tank arrangements, they are still relatively large and cumbersome, making them unsuitable for some smaller environments.

Although gas water heaters tend to be much cheaper to operate than their electric equivalents, they still have a number of disadvantages associated with them. Firstly, the installation of a gas water heater or boiler requires adherence to very strict ventilation and other safety regulations. In particular, the device must be provided within an environment which is suitably ventilated to prevent the build up of dangerous combustion products resulting from the burning of gas. Secondly, this type of water heater or boiler, especially the combination type gas boiler, still suffers from the problems associated with a large build-up of limescale which greatly reduces its heating efficiency and can only be overcome by regular maintenance and replacement of components.

Various arrangements have been proposed in which radio frequency energy, and more particularly, microwave energy, is used to heat water (or other fluid). For example, European Patent Application No. EP0849546 describes a water heater comprising a water tank for holding a predetermined volume of water to be heated, and a microwave heating source for heating the water in the tank. The microwave heating source is coupled to the base of the water tank (which is substantially transparent to microwave energy) via a waveguide.

One of the main advantages of employing a microwave heating source for this purpose is the substantial reduction in the build-up of mineral deposits within the equipment, thereby substantially reducing the maintenance costs associated therewith in this regard. As such, in theory, the water heating equipment described in the above-mentioned document is intended to provide a water heating capability which is relatively efficient and remains so over long periods of time. However, in practice, a significant amount of microwave energy incident on the fluidic load will be reflected by the load surface back toward the microwave heating source.

The primary factors which influence the level of reflected energy are the physical properties of the load and the angle at which the energy impinges on the load. Thus, 5

if the load is metallic, such as brass or aluminium, and the incident RF energy is applied perpendicular to the load surface, approximately 100% of the incident energy is reflected. For a fluid load such as water, again applying the incident RF energy perpendicularly, the reflected energy is approximately 50 - 70% of the applied energy. As a result of excessive reflection , the efficiency of the heating process is significantly compromised, and such reflected energy quickly results in the microwave heating source (i.e. the magnetron) becoming overheated which has an adverse impact on the performance of the microwave, source, and will eventually result in failure thereof.

In more detail, a typical RF source is a magnetron, such as that shown schematically in Figure 1 of the drawings. A primary component within the magnetron is the filament. The filament acts as a source of electrons required to produce the RF field. The electrons are emitted in response to an applied electric current, which elevates the temperature of the filament. In perfect operation, all of the RF energy is conducted away from the magnetron and hence the filament temperature remains constant. However, as explained above, in prior art arrangements, reflected RF energy impinging on the magnetron results in the filament temperature being increased, often leading to catastrophic failure. In typical operation, the reflected energy should be less than 2-3% of the total emitted energy.

Referring to Figure 2 of the drawings, a conventional microwave oven comprises an oven chamber 100 for receiving the material to be heated, the chamber having a relatively small opening 102 in a side wall thereof, to which is coupled a microwave heating source (i.e. magnetron) 104 via a substantially rectangular waveguide 106. Microwave energy enters the heating cavity, i.e. the oven chamber 100, through the distal end of the waveguide 106 via the opening 102 and 'bounces' around the chamber 100 as it is repeatedly reflected by the inner side walls thereof, and the energy is absorbed by the object to be heated. The opening 102 of the waveguide 106 is relatively small in relation to the internal surface area of the cavity 100, such that the proportion of reflected energy returning to the magnetron 102 minimal. However, the material within the chamber 100 is primarily heated by reflected energy, as opposed to direct energy, which results in a significant reduction in efficiency. In order to increase the efficiency of the heating process, it is preferable to apply the radio frequency energy directly to the object to be heated, either by locating the radio frequency source 104 in close proximity to the load 108, as illustrated schematically in Figure 3 of the drawings, or by using a waveguide 106, where the source 104 is located at one end of the waveguide 106 and the load 108 is located at the other end, as illustrated schematically in Figure 4 of the drawings. However, direct application of RF energy of the load will result in excessive reflection of energy from the load surface, as explained above.

We have now devised an improved arrangement.

Thus, in accordance with the present invention, there is provided apparatus for heating a body of fluid, the apparatus comprising a radio frequency energy source arranged to supply radio frequency energy to said body of fluid, the apparatus further comprising means for defining a non-uniform interface between said radio frequency energy and said body of fluid.

The present invention also extends to a method of heating a body of fluid, the method comprising supplying radio frequency energy to said body of fluid, and providing means for defining a non-uniform interface between said radio frequency energy and said body of fluid.

In a preferred embodiment, the fluid is a liquid, such as water or the like, disposed in a container which preferably comprises a tank, which in one embodiment may have an inlet and an outlet. Alternatively, an (at least partially) sealed tank with a fixed fluid load may be provided, in the form of a stand-alone heater. The upper surface of the container is preferably microwave-reflective, and is more preferably angled. In one embodiment, an angled plate, which may be substantially conical, having a microwave-reflective surface may be provided in the upper portion of the container. The angled plate may extend into the fluid load.

The radio frequency energy is beneficially supplied to the body of fluid via a waveguide, which is preferably substantially conical. The radio frequency energy source is preferably located adjacent the narrower, proximal end of the waveguide, the radius of which is beneficially greater than λ/4, where λ is substantially equal to the wavelength of the radio frequency energy. The wider, distal end of the waveguide is beneficially communicably coupled to the body of fluid via a substantially cylindrical portion.

The means for defining a non-uniform interface between the radio frequency energy and the body of fluid preferably comprises a plate or membrane having a non-uniform cross-section. Preferably, at least a portion of one surface of the plate or membrane is non-uniform and may, for example, be generally conical. The plate or membrane is beneficially formed of a material which is substantially transparent to the radio frequency energy, which is preferably microwave energy.

Thus, the conical waveguide reflects radio frequency energy into the fluid load, and the irregular or angled boundary at the fluid interface leads to irregular reflection of energy, such that reflected energy re-enters the fluid after traversing the conical waveguide again. With the correct geometry, substantially zero reflected RF energy reaches the RF energy source.

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiment described herein.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a magnetron;

Figure 2 is a schematic illustration of the structure of a conventional microwave oven;

Figure 3 is a schematic diagram illustrating a first manner of direct heating of a load according to the prior art;

Figure 4 is a schematic diagram illustrating a second manner of direct heating of a load according to the prior art; Figure 5 is a schematic diagram illustrating heating apparatus according to an exemplary embodiment of the present invention;

Figure 6 is a schematic illustration of a waveguide for use in apparatus according to an exemplary embodiment of the present invention;

Figure 7 is a schematic illustration of a membrane or plate for providing a non- uniform load surface in an exemplary embodiment of the present invention;

Figure, 8 is a schematic diagram illustrating heating apparatus according to an exemplary embodiment of the present invention, as used to obtain experimental data;

Figures 9a and 9b are schematic cross-sectional representatives of respectively plano- plano and plano-convex membranes as used in the apparatus of Figure 8 to obtain experimental data;

Figure 10 is a schematic diagram illustrating heating apparatus according to an exemplary embodiment of the invention; and

Figure 11 is a partial schematic diagram illustrating heating apparatus according to an exemplary embodiment of the invention.

Referring to Figure 5 of the drawings, apparatus for heating a body of fluid according to an exemplary embodiment of the present invention, comprises a tank 10 for holding a predetermined quantity of fluid 12, such as water, to be heated, and a radio frequency energy source, such as a magnetron 14. The tank 10 may be lagged and/or provided with fins (not shown) for insulation purposes. Radio frequency energy is supplied from the source 14 to the fluid 12 in the tank 10 via a substantially conical waveguide 16, such as that illustrated in Figure 6 of the drawings. The radius of the waveguide 16 at the point 18 at which the radio frequency energy is first incident from the magnetron 14 is beneficially greater than λ/4, where λ is the wavelength of the radio frequency energy. A PTFE plate or membrane 20, such as that illustrated in Figure 7 of the drawings, is provided between the distal end of the waveguide 16 and the fluidic load 12, at least a portion of the upper surface plate 20 being substantially conical so as to provide a non-uniform interface between the fluid 12 to be heated and the radio frequency energy. The plate 20 is substantially transparent to the radio frequency energy, i.e. it does not absorb or reflect such energy.

In use, RF energy emitted perpendicularly from the magnetron 14 strikes the conical waveguide 16 at points 18, and is reflected upwards towards the fluid load 12. The energy passes through the first surface of the PTFE plate 20 with minimal absorption and strikes the interface between the plate 20 and the fluid load 12. As a result, the angle of reflection of the RF energy is such that little, if any, thereof is reflected back to the magnetron 14. Instead, it simply 'bounces' around the inside of the waveguide 16 until it is returned to the load 12. In a preferred embodiment, the distal end of the waveguide 16 is coupled to the load 12 via a relatively short, cylindrical portion 22.

Experimental investigations have shown that the use of a non-uniform fluidic interface results in improved coupling of the incident microwave energy to the load (say, water).

An experimental configuration may be as shown in Figure 8. A cylindrical tank 10 is attached to a conical waveguide 16 containing the microwave source 14. At the bottom of the waveguide 16 is located a membrane 20 to contain the fluid (in the case of the experiments the fluid was water). Two types of membranes 20 were tested in these experiments, the first being planar-planar, i.e. both surfaces of the membrane are flat, as shown in Figure 9a, and secondly a planar-convex type where the surface nearest the magnetron is flat and the surface in contact with the fluid is conical in nature, as shown in Figure 9b.

A fixed volume of water was placed in the tank, which had been insulated with heat absorbing material on the outer surface. A suitable magnetron was attached to the bottom of the conical waveguide and supplied with electrical power. At the start of the experiment the temperature of the water was measured and recorded. The magnetron was switched on for a fixed time period and the induced temperature rise in the fluid was measured after the magnetron was switched off. With the thermal properties of water know, along with the volume, start and end temperature, the level of absorbed energy was extrapolated using the formula:

E = ΔTpvc

Where E - Absorbed Energy (Joules) ΔT = Temperature Change (End temperature - start temperature) (centigrade) p = Density (Kg/m ) v = Volume (m³) Specific Heat Capacity (J Kg M³)

By calculating the delivered energy from the magnetron and the duration of the heating phase, the total delivered energy E_Tot was calculated. By dividing the calculated energy by the delivered energy, the efficiency of the heating process was determined.

Example of Results

If microwave energy passes through the entire volume of water 12, it is desired for this to be reflected back into the water to enhance the heating process. Accordingly, it is preferred that the upper surface of the tank 10 be made of a microwave-reflective material (e.g. metal or the like). Referring to Figure 10 of the drawings, in a more preferred embodiment, this reflective surface 30 is angled, such that any microwaves incident on this surface return at a different angle so as to prevent destructive interference thereof with the microwaves propagating upwards through the water. Referring to Figure 11 of the drawings in one embodiment, it may be an advantage for the reflective plate 30 to be in contact with the water 12.

It should be noted that the above-mentioned embodiment illustrates rather than limits the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice- versa. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

ιυCLAIMS:

1. Apparatus for heating a body of fluid, the apparatus comprising a radio frequency energy source arranged to supply radio frequency energy to said body of fluid, the apparatus further comprising means for defining a non- uniform interface between said radio frequency energy and said body of fluid.

2. Apparatus according to claim 1, wherein said fluid is a liquid.

3. Apparatus according to claim 2 wherein said body of fluid is disposed within a tank.

4. Apparatus according to claim 3, wherein said tank is at least partially sealed and contains a substantially fixed fluid load.

5. Apparatus according to claim 3, wherein said tank has an inlet and an outlet.

6. Apparatus according to claim 3, wherein an upper surface of said tank is radio frequency reflective.

7. Apparatus according to claim 6, wherein said upper surface is angled.

8. Apparatus according to claim 6 or claim 7, wherein an angled plate, having a radio frequency reflective surface is provided in an upper portion of said tank.

9. Apparatus according to claim 8, wherein said angled plate extends, at least partially, into said body of fluid.

10. Apparatus according to any one of the preceding claims, wherein said radio frequency energy is supplied to said body of fluid via a waveguide.

11. Apparatus according to claim 10, wherein said waveguide is substantially conical.

12. Apparatus according to claim 11, wherein said radio frequency energy source is located adjacent the narrower, proximal end of said waveguide.

13. Apparatus according to claim 12, wherein the radius of said waveguide at said proximal end is greater than λ/4, where λ is substantially equal to the wavelength of said radio frequency energy.

14. Apparatus according to claim 12 or claim 13, wherein the wider, distal end of said waveguide is communicably coupled to said body fluid via a substantially cylindrical portion.

15. Apparatus according to any one of the preceding claims, wherein said means for defining a non-uniform interface between said radio frequency energy and said body of fluid comprises a plate having a non-uniform cross-section.

16. Apparatus according to claim 15, wherein at least a portion of one surface of said plate is non-uniform.

17. Apparatus according to claim 16, wherein at least a portion of a surface of said plate is substantially conical.

18. Apparatus according to any one of claims 15 to 17, wherein said plate is formed of a material which is substantially transparent to said radio frequency energy.

19. Apparatus according to any one of the preceding claims, wherein said radio frequency energy comprises microwave energy.

20. A method of heating a body of fluid, the method comprising supplying radio frequency energy to said body of fluid, and providing means for defining a non-uniform interface between said radio frequency energy and said body of fluid.