(1) Field of the Invention
The present invention relates to apparatus for heating a fluid, (i.e. liquid or gas) and in particular to apparatus capable of heating a continuous stream of fluid with high efficiency, without the use of exposed heating elements or open flames.
The apparatus of the present invention is especially useful for commercial--or industrial--scale water-heating, and will be described with particular reference to that application. However, it will be appreciated that the apparatus is by no means limited to this application, but also may be used to heat any of a wide range of fluids.
At present, commercial and industrial scale water-heating is generally a batch process: water held in a storage tank is heated by an electric heating element or by gas burners, and is held in the storage tank until required. This process has several drawbacks: the storage tank is bulky, and needs to be located near the place of use if heat losses in the delivery pipes are to be avoided; if the rate of use of hot water is low, a great deal of energy is consumed in holding a large volume of water at a high temperature needlessly; or if the rate of use of the water is high, the supply from the storage tank may be inadequate. To overcome these drawbacks, several designs of `through-flow` water heaters have been marketed, but all such designs to date have been able to supply hot water only at relatively low flow-rates, and are expensive to install.
It is therefore an object of the present invention to provide a through flow (i.e. continuous) fluid heater which is relatively inexpensive to manufacture and install, but which is capable of operating efficiently at relatively high flow-rates.
In most commercial and domestic premises, mains electric power is available. It greatly reduces the expense of installing and operating electric fluid heaters if mains power can be used (i.e. an AC supply, with a frequency in the range 50-60 Hz) without the need to modify the type of supply or its frequency. It therefore is a further object of the invention to provide fluid heating apparatus capable of operating upon mains electric power.
(2) Description of the Prior Art
There have been many prior proposals to use an electric transformer to heat fluids, in particular, water.
For example, U.S. Pat. No. 1458634 (Alvin Waage, 1923) discloses a device consisting of a common core upon which primary and secondary coils are wound. The secondary coil is shorted, so that the induced voltage in the secondary causes a current to flow in the secondary coil, heating it. The secondary coil is tubular, and water to be heated is arranged to flow through it. The primary may also be tubular.
Heaters of this general type also are disclosed in U.S. Pat. Nos. 4602140 and 4791262.
A variant of this design is disclosed in U.S. Pat. No. 1656518, in which the fluid to be heated flows through a tank, which functions as a shorted secondary.
Another variant is disclosed in U.S. Pat. No. 2181274, in which the fluid to be heated flows through the core of the transformer; the primary and secondary coils are concentric about the core, the secondary coil effectively being a single shorted turn.
A further variant is disclosed in U.S. Pat. No. 1671839, in which the primary and secondary coils and the common core all may be hollow, and fluid to be heated is circulated through the core and (optionally) also through the primary and secondary coils. The secondary coil is shorted.
However, in all of the above-mentioned devices, the transformer has a core.
It is a well-established principle in electrical engineering practice that for mains-frequency devices, efficient magnetic flux linkage is achieved only if a transformer core is used. Coreless transformers have been known and used for many years, but only for high-frequency applications, (typically 50 kHz i.e. a thousand times greater than mains frequency) since in high-frequency applications, efficient flux linkage can be achieved without a core.
However, the design of the present invention has been found to possess an unexpected and surprising advantage, in that although the device of the present invention is coreless, it has been found to operate with very high efficiency at mains frequency.
Coreless transformers have a number of advantages over cored transformers: firstly, there is a significant cost saving because the core does not have to made or fitted. Secondly, coreless transformers typically exhibit a near-linear magnetization curve, in contrast to the plateaued magnetization curve exhibited by cored transformers. The near-linear magnetization curve means that the transformer can be operated efficiently over a much larger voltage range, and is therefore more controllable i.e. it is possible to vary the voltage over a much wider range without being effected by the plateau. A further advantage is that a coreless transformer is easier to cool simply because there is no core to offer impediment to cooling fluids; hence, the efficiency of the transformer is improved.
A further characteristic of all of the above-mentioned devices is that the fluid essentially is heated by a single method only i.e. by conduction from the shorted secondary. The secondary coil normally is made of low resistance material, because this is required for efficient power transfer. However, a low resistance material is not ideal for a resistance heating element, for which a high resistance material is preferable.
U.S. Pat. No. 4471191 discloses a fluid heating device which essentially incorporates a coreless transformer: a primary coil surrounds a container, the interior of which is subdivided by metallic cylinders, which create passages through which flows the fluid to be heated. Secondary coils in the form of metallic rings or helices are located within the container, spaced from the cylinders.
In use, the primary coil induces a voltage in the secondary coil or coils, which are shorted so that heat is generated therein by the induced current. The metallic cylinders also are inductively heated, and the heat from the secondary coil or coils and from the cylinders heats the fluid passing through the container.
However, in this design, energy is wasted: firstly, the primary is outside the container, and thus can contribute nothing to the heating of the fluid. Secondly, the concentric arrangement of the secondary coils and metallic cylinders means that the linkage of magnetic flux between primary and secondary coils is far from ideal, and flux leakage will occur, lowering the effectiveness of the device. Thirdly, the secondary coil or coils are shorted, rather than being connected to a ,load which is resistance-heated by the secondary voltage; this has the drawbacks discussed above.
It is therefor a further object of the present invention to provide a fluid heating device which overcomes at least the third of the above described disadvantages and which is capable of operating with high efficiency at mains frequency.