GB2140258A

GB2140258A - Microwave heating apparatus

Info

Publication number: GB2140258A
Application number: GB08313191A
Authority: GB
Inventors: Kenneth George Barnes
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-04-06
Filing date: 1983-05-13
Publication date: 1984-11-21
Also published as: GB8313191D0; GB8314875D0; GB8317283D0

Abstract

Heating apparatus comprises a vessel (8) containing a substance (12), the properties of which are such that the substance (12) is heated under the action of microwave energy. A microwave emitter (16) having a horn (18) is positioned to direct microwaves into the liquid (12). A water jacket (20) surrounds the vessel (8), and has an inlet (22) and an outlet (24). Water flowing through the inlet (22) follows a helical path around the outer surface of the vessel (8), during which its temperature is raised by heat transfer form the substance (12). The apparatus may comprise a continuous water heater for washing purposes, or it may replace a central heating boiler. A similar principle may be applied to a hot water cylinder and a space heating radiator. <IMAGE>

Description

SPECIFICATION Heating Apparatus This invention relates to heating apparatus, particularly, although not exclusively, to space heaters and water heaters, possibly as part of a central heating system.

According to the present invention there is provided heating apparatus comprising a vessel and microwave emitting means for directing microwaves into the vessel, the vessel containing a substance which is heated when subjected to microwave energy, whereby heat generated in the substance is transferred to the ambient medium surrounding the vessel.

The substance is preferably one which experiences a greater rise in temperature than water when subjected to the same quantity of microwave energy. it is believed that the properties of the substance which are important for this purpose are its boiling point, its thermal conductivity, its electrical resistivity and its specific heat. It is presently believed that, for best results, desirable values of these properties are as given in the following Table.

TABLE

Property Desirable Preferred Boiling point > 1 50 C > 2500C Thermal conductivity > 3x 10-4 cal/s.cm. C Electrical resistivity > 107 ohm-cm > 1012 ohm-cm Specific heat < 0.8 < 0.5 Good results have been achieved using vegetable oils, such as vegetable oil compositions including additives to reduce the boiling point, such as are used in hydraulic brake systems of motor vehicles.

It is preferred, but not essential, for the substance to be a liquid, but satisfactory results have been obtained using powders or granular materials.

It is believed that the mechanism by which the temperature of a liquid is raised by microwave energy is as follows. The microwaves upon entering the liquid, are absorbed in a thin layer or skin of the liquid, generating heat in this skin. The remainder of the body of the liquid is heated by conduction or convection. If the liquid has a low boiling point, the heated skin will vaporize, the vapour then dispersing before the heat can be conducted into the body of the liquid, and so the process will be repeated on the next surface layer. Some of the energy of the microwaves will be required to provide the latent heat of vaporization, so that not all of the energy will be available to raise the temperature of the liquid. Clearly, the rate of heating of the interior of the body of liquid will be greater as the thermal conductivity of the liquid is greater.

Where a liquid is used, it is preferable in most cases for the microwave emitter to be beneath the vessel, in which case the microwave emitter may be separated from the liquid by a wall made of a material which is transparent to microwaves, for example a ceramic material. Heat from the heated skin of the liquid is then transferred to the body of the liquid not only by conduction, but also by convection.

The heated substance may be passed through a heat exchanger element which may, for example, be a space heating radiator or a heat exchanger for transferring the heat of the substance to another substance, such as water. The rate of transfer of heat from the substance to the surrounding air or the water is a function of the temperature difference, and so it is desirable, at least in water heaters, for the substance to be heated to as high a temperature as possible. This is one reason why the boiling point of the substance should be high, since this avoids the need for pressurization of the chamber.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view of a hot water cylinder; Figure 2 is a diagrammatic sectional view of a continuous-flow water heater; and Figure 3 is a diagrammatic sectional view of a space heating radiator.

Referring first to Figure 1, the hot water cylinder 2 has a cold water feed 4 and a hot water outlet 6. Within the cylinder, there is an elongate vessel 8 made of thermally conductive material. The interior of the vessel is provided with a partition 10, for example of the material available under the trade name "Pyrex", which is transparent to microwaves. The part of the vessel 8 above the partition 10 is partially filled with a liquid 12, which may be, for example, a commercially available hydraulic brake fluid or a similar vegetable oil based composition. An expansion space 14 is left above the surface of the liquid 1 2. At the bottom of the vessel 8 there is a microwave emitter 1 6 having a horn 1 8 which is oriented to direct microwaves into the liquid 12.

In use, the cylinder 2 is filled with water, entering through the feed 4. Microwaves emitted form the horn 1 8 of the microwave emitter 1 6 cross the partition 10 and are absorbed in a thin layer of the liquid 1 2 adjacent the partition 10, where they generate heat by creating molecular vibrations in the liquid 12. The remainder of the liquid 12 is heated by conduction and convection to form the thin layer adjacent the partition 1 0. The temperature of the liquid 1 2 may, for example, reach approximately 2600C. Heat from the liquid 1 2 is conducted across the material of the vessel 8 to the water in the cylinder 2.There may, for example, be a thermostat which is responsive to the temperature in the cylinder 2 to interrupt the emission of microwaves when the temperature of the water has reached the required value.

To improve heat exchange from the liquid 1 2 to the water in the cylinder 2, fins may be provided on the internal and/or external surfaces of the vessel 8.

Figure 2 shows a continuous water heater comprising a microwave emitter 1 6 having a horn 1 8, and a vessel 8, which are similar to those of the embodiment of Figure 1, except that, in the case of Figure 2, the microwave emitter 1 6 is positioned at the top, so that the partition 10 is not required, although it may be included in order to prevent fouling of the emitter horn 1 8 by the liquid 12, for example during transit. Most of the vessel 8 is surrounded by a water jacket 20 which may, for example, be surrounded by insulating material (not shown). The water jacket 20 has an inlet 22 and an outlet 24. The outer surface of the vessel 8 has a helical baffle 26 which defines a helical path for water passing through the water jacket 20 from the inlet 22 to the outlet 24.In use, the liquid 12 is heated, as in the embodiment of Figure 1, by the action of microwaves emitted from the emitter horn 1 8, and the heat of the liquid 1 2 is transmitted to the water flowing in the water jacket 20. The helical path provided by the baffle 26 ensures that the water takes a circuitous path through the water jacket 20, ensuring maximum contact with the hot vessel 8.

Figure 3 shows a space heating radiator 28 which communicates through circulation ducts 30 with a vessel 8 corresponding to that of Figure 1. The vessel 8 has the partition 10, the microwave emitter 1 6 and the horn 18. in use, the heated liquid 12 in the vessel 8 passes into the radiator 28 through the upper circulation passage 30. As the heat of the liquid 1 2 is transferred to the ambient air from the radiator 28, it falls to the bottom of the radiator 28, and returns to the vessel 8 through the lower circulation passage 30. It is then reheated by the microwave energy emitted from the horn 1 8, and returned to the radiator 28.

In all cases, appropriate controls, such as thermostats and time switches, may be provided. The continuous water heater of Figure 2 may be used to provide domestic hot water, but the same principle can be applied to a central heating boiler heating water for radiators and indirect immersion cylinders.

The emitters 1 6 of all three embodiments may include individual cavity magnetrons for generating the microwaves, but alternatively they could be connected to an external magnetron by suitable waveguides, in which case a single magnetron could be used to supply microwave energy to, for example, several radiators as shown in Figure 3, a hot water cylinder as shown in Figure 1, and a continuous water heater as shown in Figure 2.

It will be necessary for suitable shielding to be provided to prevent microwaves from reaching the outside of the system. For example, the vessel 8 could incorporate such shielding.

The advantages of apparatus in accordance with the present invention are that space heating and water heating can be achieved more economically than is currently possible using electric heaters. The flues which are normally required with combustion systems are not required, nor will dangerous flames, vapours or gases be emitted. There is no need to store hazardous fuel.

Claims

1. Heating apparatus comprising a vessel and microwave emitting means for directing microwaves into the vessel, the vessel containing a substance which is heated when subjected to microwave energy, whereby heat generated in the substance is transferred to the ambient medium surrounding the vessel.

2. Heating apparatus as claimed in claim 1, in which the substance is a liquid.

3. Heating apparatus as claimed in claim 1 or 2, in which boiling point of the substance is higher than 250"C.

4. Heating apparatus as claimed in claim 3, in which the boiling point of the substance is higher than 4000 C.

5. Heating apparatus as claimed in any one of the preceding claims, in which the specific heat of the substance is less than 0.8.

6. Heating apparatus as claimed in claim 5, in which the specific heat of the substance is less than 0.5.

7. Heating apparatus as claimed in any one of the preceding claims, in which the thermal conductivity of the substance is greater than 3 x 10-4 calories/second centimetre "C.

8. Heating apparatus as claimed in any one of the preceding claims, in which the resistivity of the substance is greater than 107 ohm-centimetres.

9. Heating apparatus as claimed in claim 8, in which the resistivity of the substance is greater than 10-12 ohm-centimetres.

10. Heating apparatus as claimed in any one of the preceding claims, in which the substance comprises vegetable oil.

11. Heating apparatus as claimed in any one of the preceding claims, in which the substance comprises commercially available hydraulic brake fluid.

12. Heating apparatus as claimed in any one of the preceding claims, in which the microwave emitter is situated beneath the substance.

13. Heating apparatus as claimed in any one of the preceding claims in which a partition is disposed within the vessel between the substance and the microwave emitter, the partition being transparent to microwaves.

14. Heating apparatus substantially as described herein with reference to, and as shown in, the accompanying drawings.

1 5. A hot water cylinder comprising inlet and outlet means for water, the cylinder containing heating apparatus as claimed in any one of the preceding claims, for heating water within the cylinder.

1 6. A continuous water heater comprising heating apparatus as claimed in any one of claims 1 to 14, means being provided for causing water to flow frdm an inlet, over the outer surface of the vessel, to an outlet.

1 7. A continuous water heater as claimed in claim 16, in which the heating apparatus is surrounded by a water jacket, one or more baffles being provided for causing water to flow through the water jacket in a helical path around the vessel.

1 8. A space heating radiator comprising a radiator element and heating apparatus as claimed in any one of claims 1 to 14, the heating apparatus being connected to the radiator element by circulation passages, whereby the substance in the vessel of the heating element circulates through the radiator element.

1 9. A domestic central heating system comprising a hot water cylinder as claimed in claim 1 5, and/or a continuous water heater as claimed in claim 1 6 or 17, and/or one or more radiators as claimed in claim 1 8, and microwave generating means which is connected to the microwave emitter of the or each component by a waveguide.