1. Field of the Invention
The invention concerns a device for producing steam as well as a cooking appliance with such a device.
2. Related Technology
Conventional steam generators, as they are used especially in cooking appliances, usually have a boiler, which is tilled partially with water and which is brought to boiling with a heating element. The space requirement of such a steam generator is determined mainly by two factors, namely on the one hand by the fraction of volume that is filled with liquid water and, on the other hand, by the fraction of volume of the gas space located above the water.
In this case the first volume fraction is delimited by the size of the heating element and by the space needed between the heating elements for circulation around them and for removal of steam bubbles. In turn, the size of the heating element is determined by the Leidenfrost effect for a given heat output power, according to which, upon heating of a vaporizable liquid, a certain surface power density of a heating body cannot be exceeded, since otherwise a closed steam layer develops between the surface of the heating element and the liquid that prevents heat transfer. The Leidenfrost effect is demonstrated nicely by the phenomenon that the water droplets do not immediately evaporate on a glowing plate, but move almost in a dancing manner, since they move irregularly on the plate supported by a developing steam layer.
The second volume fraction is needed in order to separate steam from the entrained water droplets. The size and the shape of the second volume fraction, that is, of the gas space above the liquid, are decisive for the steam quality. The steam quality is determined especially by the size of the water surface over which the flow of steam must be passed. The larger the steam flow per water surface, the more water will be entrained by the exiting steam.
Overall, based on the above considerations, in conventional steam generators a larger space is required when a high steam quality is to be generated. However, it should be pointed out additionally that the time needed to make a water-filled steam generator ready for operation also depends on the amount of water introduced that is necessary to cover the heating element safely, which again establishes requirements with regard to the size of the steam generator.
In the area of power generators, especially nuclear power generators, steam generators are known in which a rotational flow is induced in a steam-producing fluid by injection of another fluid. The centrifugal forces that are involved in the rotational flow are used for the separation of contaminants, as described in U.S. Pat. No. 4,972,804 and DE 690 13 906 T2.
Furthermore, steam generators are known in which the entire tubular or barrel-shaped steam generator container is set into rotation, which is associated with high energy expenditure and cost, and, moreover, requires high constructional complexity, which makes necessary to have a rotating support for the heat generator container, as a result of which necessarily the introduction and discharge of fluid can only be practicable through the axle. For example, such a rotating steam generator is described in DT 2 214 566, designed for the evaporation of organic drive fluids in a closed circuit of a Rankine engine. The steam generator known from DE Patent 904 653 operates similarly, but in addition has a control for the fluid feed in order to adjust a fluid ring of specific thickness.
A rotating steam generator with a rotatably supported boiler is disclosed in DE 27 57 913 A1, which in its construction does not have rotatable feeds for the introduction or discharge of a fluid working medium.
A tandem separator for a steam-water mixture is known from DE 37 83 361 T2 around which baffles are arranged in a screw shape, which set the steam-water mixture into rotation.
DE 692 07 830 T2 discloses a steam generator arrangement for ovens, in which thin water jets or small water droplets set into rotation fall onto a heating device in order to be converted to steam.
However, the previously-described steam generators, especially in a cooking appliance, do not operate satisfactorily for the evaporation of tap water, but they have the severe disadvantage that the minerals dissolved in the tap water, especially lime, are deposited onto the walls and built-in parts of the steam generator. This can lead to the failure of or damage to the steam generator, which can be prevented, as a rule, only by regular chemical removal of the lime. The necessary maintenance work after usage caused thereby is, again, cost-intensive.
In order to overcome this disadvantage, in WO 02/12790 A1, a method is described for producing steam, especially for a cooking appliance, in which a liquid is brought to boiling within a steam generator container by heating of at least one heatable wall surface of the heat generator container, and is brought into rotation during heating by at least one rotatably supported first rotor in the steam generator container, as well as being pressed against the heatable wall surface by the centrifugal forces due to the rotation, and the steam produced by evaporation of at least a part of the escapes from the steam generator container through a steam outlet while droplets of the liquid entrained by the steam are being separated. Furthermore, WO 02/12790 A1 discloses a device for producing steam, especially for a cooking appliance, comprising a steam generator container, which can be filled at least partly with a liquid through an inlet and from which steam can arrive to a steam outlet, and a heating device for heating the fluid in the steam generator container for the purpose of producing steam, which exhibits a heatable housing, whereby the device has a first rotor in the steam generator container through which at least the liquid can be set into rotation. Through this method or through the device in WO 02/12790 A1, especially small steam generators are supposedly provided for producing steam of high quality which, in addition, can be made ready for operation quickly, are cost-effective, flexibly heatable and reliable, without tending to superheating and without requiring regular lime removal.
The method described in WO 02/12790 A1 as well as the device described in this document have been found to be fundamentally useful, since especially the laborious and cost-intensive maintenance work for removal of the lime in the steam generator becomes superfluous, as well as providing maximum heat transfer from the walls to the liquid during operation and not reduced by lime deposits, with the inner rotating components acting so to speak as a scraping device, with which the deposits on the walls are removed instantaneously during the operation, that is, during the rotation.
However, the device for producing steam described in WO 02/12790 A1, cannot remain under a certain construction size due to the use of thick-layer heaters as heating elements for the heating of the liquid in the steam generator container, and due to a limited heat transfer to the steam generator container.
Finally, WO 96/41099 A1 and WO 98/45647 A1 each disclose a generic device for producing steam in the form of a steam generator. The steam generators disclosed in these two documents have a metal block with a central bore for holding a liquid and a multiple number of heating element bores surrounding the central bore. In order to heat the liquid in the central bore, heating elements are introduced into the heating element bores. However, a disadvantage of these steam generators is that efficient heat transfer from the heating elements to the liquid located in the central bore is not ensured. Therefore, the heat input into the liquid is unsatisfactory.