The present invention relates to microwave apparatus used for heating liquids and associated methods. The invention finds particular, but not exclusive, utility in a domestic setting and may be used to instantaneously heat water for use in a variety of situations, such as single-point water heaters, showers or the like.

Presently, showers and baths account for approximately 45% of the water used in most households. Showers have, in recent years, become increasingly attractive alternatives to baths and this can be very useful since they tend to use less water. However, people tend to use them more frequently, resulting in the same or greater use of water overall. This problem of water usage has been exacerbated in recent times through an increased use of power showers and showers fed by mains pressure water. Indeed, even a relatively short shower using a power shower can use more water than a typical bath.

However, showers are still popular since, in principle, a typical shower can use about a third of the water of a typical bath.

The means by which hot water is provided to either a shower of a bath have also changed significantly in recent years. A few years ago, most houses were equipped with a water tank fitted with an immersion heater which was the sole source of hot water for the house. Such a system was acceptable to heat the water for a bath but was not typically suitable to provide the required pressure for a shower. As such, when separate shower units first appeared, they tended to incorporate a single point electrical heater so that the shower unit was fed only from a cold supply and the incoming cold water was heated to the required temperature for a comfortable shower. However, a problem with such electric shower units is that the maximum electrical rating of them tends to limit the flow rate that can be achieved, i.e. it is not generally possible to heat the incoming water quickly enough to provide a powerful spray at the required temperature. Typical electrical shower units are rated at 8 or 9 kilowatts.

Many homes are now fitted with so called combi-boilers which heat water on demand in response to the operation of a hot tap. Such combi-boilers are able to provide sufficient pressure to be useful in providing a powerful shower. However, many houses are not so equipped with combi-boilers and, the cost of retro-fitting them can be prohibitive. There therefore exists a requirement to provide an economic and easily fitted alternative to prior art electric shower units. In particular, it would be advantageous if such units could provide a similar or better performance to existing electrical shower units but use less energy to produce the hot water. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Figure 1 shows a schematic view of an embodiment of the present invention;

Figure 2 shows a cross-section of a coil forming part of an embodiment of the present invention;

Figure 3 shows a modified coil forming part of an embodiment of the present invention;

An embodiment of the present invention heats water from an incoming cold water supply by use of a microwave source 1 and gold plated FEP (fluorinated ethylene propylene) coil 10, located in a cavity 2. This configuration is shown in Figure 1. Such an arrangement can be incorporated into, for instance, a domestic shower unit or domestic single-point water heater to allow cold water to be heated to a desired temperature as needed.

Relatively cold water flows into the apparatus via inlet 11 and leaves the apparatus at outlet 12, having been heated.

It is known that water strongly absorbs microwaves and this forms the basis for domestic microwave ovens. When a foodstuff is introduced into such a microwave oven, the microwaves which are produced by the cavity resonator excite the water molecules in the food causing rapid heating of the contents of the oven

In spite of such heating performance, it has been found that it is not generally possible to directly heat a flow of water by use of a microwave cavity resonator since the transfer of energy required to heat the water to a comfortable level is not easily attainable.

Embodiments of the present invention utilise a microwave plasma flow reactor which is able to more efficiently raise the temperature of incoming water from room temperature (e.g. 25 ⁰ C) or cooler in winter (e.g. 5 ⁰ C) to 40-60 ⁰ C, which is suitable for use in a shower. It is found that the energy required to achieve this temperature increase is only about 10% of the microwave energy used in a conventional household microwave oven The presence of the gold coated FEP coil 10 within the cavity 2 is one factor which assists in enabling embodiments of the invention to operate in this efficient manner to heat water. The action of the microwaves upon the gold coating of the coil 10 produces a plasma. Such a plasma is defined as being a partially ionised gas in which a certain proportion of electrons are free, rather than being bound to an atom or molecule. The plasma itself is able to achieve a high temperature in the presence of microwaves. The passage of water through the coil 10 inside the cavity 2 causes the water to rapidly heat and so reach the desired temperature suitable for showering.

Figure 2 shows a cross-section through the coil 10. It is found that the presence of the gold coating 13 on the coil increases the efficiency of the heating process significantly. Other coatings on the coil can be used with similar effects. Other possible coatings include other metals, such as copper or silver, or other materials, such as carbon.

Further, other materials for the coil can be used, including PTFE. The chief requirement for any such material is that it is able to withstand the heat of the process and be substantially transparent to microwave energy.

The coil is a helical coil, which is easily produced and, in its basic form, is readily available. Other coil shapes and configurations are possible.

In a further enhancement, a further material can be added to the interior of the coil 10, which can serve to produce a further plasma, constrained inside the coil, which can further enhance the heating effect on the water.

Figure 3 shows such a configuration. In order to ensure that any such material is retained within the coil 10, first and second filters 14 are required at each end of the coil after the material has been introduced a the manufacturing stage, to trap the material within the coil. The filters 14 allow water to flow through, but bar the passage to the material particles.

Alternatively, it is possible to coat an anterior surface of the coil 10 with a suitable plasma- producing material, but this is a relatively complex operation and is, therefore, more costly.

One material that is found to be suitable for addition to the interior of the coil is carbon, in the form of a fine powder. Carbon powder is not affected by water and is retained in the coil at all times, but will react to incident microwave energy to form a plasma inside the coil. Another suitable material is boron. Other materials, including various metals are also suitable in principle, but some may degrade over time in the presence of water. The microwave source is a standard magnetron, as may currently be used in a microwave oven. It typically operates at a fixed frequency of 2.45GHz and at a power in the range 700W - 1 kW. Using a standard, readily-obtainable source, such as this, ensures that the water heater according to an embodiment of the present invention is economic.

The microwave source 1 is coupled to the cavity 2 containing the helical coil 10 by a direct coupling method i.e. the magnetron feeds directly into the cavity. Other coupling methods, including waveguides, cables and antennas can also be used.

In operation, water flows through the coil at inlet 11 and, once the microwave source 1 is activated, the water in the coil is heated and exits from the outlet 12 at the end of the coil at a higher temperature than when it entered the coil. The water absorbs some energy directly from the microwaves in the cavity, but the main temperature-raising effect is derived from the presence of the plasma.

Advantageously, the production of the plasma does not require significantly more energy from the source and the heating effect it produces is therefore a convenient and advantageous byproduct of the plasma production process.

In the embodiment where particles are constrained within the coil itself, then the heating effects are further enhanced.

The water heating apparatus described has a number of different applications. In domestic situations, the water heater can be used as an on-demand water heater in the style of a combi- boiler, and could feed an entire domestic setting. Alternatively, it could be used at a single point, such as a wash-basin, or it could form the basis of a single-point, electrically-powered shower.

Showers which utilise embodiments of the invention are particularly beneficial, due to the problems mentioned previously in heating water to a sufficient temperature while maintaining a desired flow-rate. For the same or less electrical input power as would be used by a conventional electric shower, a shower according to an embodiment of the present invention can yield a far higher flow rate, while still heating the water to a desired temperature. Such an effect mimics so-called power showers, which normally require complex and costly water heating arrangements.

Since the design is scalable, it is possible to adapt it to operate on a larger scale by adjusting the microwave source, cavity size and flow-rate to operate in a more demanding setting, such as an industrial process. Further, the liquid intended to be heated by the apparatus need not be water and, in an industrial process, a range of different liquids could be heated using the apparatus, or variants thereof. In such cases, it is advantageous to provide a tunable microwave source so that the frequency of radiation is selected according to the liquid to be heated.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.