Steam generator having a hot surface and means for spraying water on the hot surface

FIELD OF THE INVENTION

The present invention relates to a device for heating a fluid such as water, comprising: a heat emitting body; heating means which serve for supplying heat to the heat emitting body during operation of the device; and spraying means adapted to supplying fluid as a haze to a surface of the heat emitting body.

The present invention also relates to a domestic appliance such as a beverage maker or a steam iron, comprising the device for heating fluid.

BACKGROUND OF THE INVENTION Various embodiments of a device for heating water are known. In many cases, the device is suitable for heating water to such an extent that the water is evaporated and steam is generated. Examples of generally used steam generators are the thermo block and the boiler.

A thermo block comprises a block-shaped body, which is often made of aluminium, and heating means for heating the body. Inside the body, at least one hollow space is present for forming at least one duct for conveying water through the body. During operation of the thermo block, the heating means are activated, so that the body is heated. Furthermore, water is made to flow through the duct. In the process, heat is transferred from the body to the water, as a result of which the water is heated. When the thermo block is used for steam generation, the required heat flux, i.e. the required heat transfer rate per unit area, is not homogeneously distributed over the length of the duct extending through the body. In a first part of the duct, water is heated up, and therefore, the required heat flux is relatively low. In a second part of the duct, an evaporation process of the water takes place, and therefore, the required heat flux is relatively high, because the heat required to evaporate the water is very high compared to the heat required to heat up the water from the ambient temperature to the boiling temperature. A third part of the duct serves for conveying the steam which is obtained as a result of the evaporation process. Therefore, in this part of the duct, the required heat flux is relatively low again, because the steam will only be superheated.

Due to the large differences between the required heat fluxes, it is difficult to transfer thermal energy from the heating means of the thermo block to the required positions along the duct. In order to prevent high temperature gradients in the thermo block, a large part of the body of the thermo block, i.e. a large amount of aluminium, is arranged between the heating means and the duct. This large amount of aluminium is also used to store thermal energy when no steam is generated. The thermal energy is used to support the heating element when steam is generated, resulting in a higher maximum steam flow rate during a certain time. However, as a consequence of the presence of the aluminium, the thermo block has a high thermal capacity resulting in a long heating up time. If the thermo block is applied both for heating water to a temperature below the boiling point and generating steam, a switching time between these two functions is long, because the body of the thermo block needs to be heated up when changing the function from heating water without steam generation into generating steam, and the body needs to be cooled down when changing the function from generating steam into heating water without steam generation. Another known steam generator, namely a boiler filled with water, also has a rather high thermal capacity, due to the relatively high specific heat of the water. Therefore, heating up the boiler takes a relatively long time. It is noted that the boiler can not be completely filled with water, as there needs to be space above the water level for accommodating the steam. For that reason, a boiler which is intended to be used for steam generation needs to be equipped with a water level sensor, which can be used to control a volume of the water in the boiler.

Normally, when a boiler is used for heating water to a temperature below the boiling point, it is completely filled with water. Because of the difference in water levels associated with an application for heating water without steam generation on the one hand and an application for generating steam on the other hand, switching between the functions of heating water without steam generation and generating steam is rather complicated with a boiler. In particular, changing the function from heating water without steam generation into generating steam means that the boiler needs to be partially emptied and to be heated up, and changing the function from generating steam into heating water without steam generation means that the water needs to be cooled down to an appropriate temperature. It is possible to simply let the water in the boiler cool down naturally, but it is also possible to perform an action such as flushing the boiler with cold water in order to speed up the cooling-down process.

EP 0 395 843 discloses a steam generator of another type than the above- described thermo block and boiler. In particular, this steam generator comprises a steam chamber, a heater, and a heatable heat accumulator. During operation of the steam generator, the heater is activated, and the heat accumulator is heated under the influence of the heater. The steam chamber is located inside the heat accumulator, and therefore, the steam chamber is delimited by a surface of the heat accumulator, which is a hot surface during operation of the steam generator. The steam generator further comprises a spraying device for spraying water on the hot surface. Steam is generated when water droplets are evaporated when getting in the vicinity of the hot surface or contacting the hot surface. Like the thermo block and the boiler filled with water, the steam generator known from EP 0 395 843 has a relatively high thermal capacity, due to the fact that the steam generator comprises the heat accumulator. Among other things, this causes a heating- up time of the steam generator to be relatively long. Another disadvantage associated with the steam generator is the fact that the steam generation can only take place in a discontinuous fashion, because a batch of water which has been supplied by the spraying device first needs some time to evaporate and to leave the area of the hot surface where the water is sprayed before a next batch of water can be supplied.

SUMMARY OF THE INVENTION It is an objective of the present invention to provide a device for heating water, which is capable of heating water to steam, and which is capable of switching relatively fast between the function of heating water without steam generation and the function of generating steam. It is a further objective of the present invention to provide a device for heating water which does not require incorporation of rather complicated additional functions such as water level sensing for ensuring proper operation.

The objectives are achieved by a device for heating a fluid, comprising:

- a heat emitting body;

- heating means which serve for supplying heat to the heat emitting body during operation of the device; and - spraying means adapted to supplying fluid as a haze to a surface of the heat emitting body; wherein the heat emitting body is in the form of a plate.

In the following description, it is assumed that the fluid which is heated by means of the device according to the present invention is water. Nevertheless, the device is just as well suitable to be used for the purpose of heating other fluids.

The device according to the present invention, like the steam generator known from EP 0 395 843, comprises spraying means for supplying the water to be heated and/or evaporated to steam, wherein the water is sprayed on a surface of a heat emitting body which is heated under the influence of heating means during operation of the device. A major difference between the device according to the present invention and the steam generator known from EP 0 395 843 is that the first device comprises a heat emitting body in the form of a plate, while the latter device comprises a heat accumulator. In fact, in the device according to the present invention, heat storage does not take place, as a result of which heating up or cooling down the device takes much less time. Despite of an absence of energy storing mass in the device according to the present invention, it is possible to avoid the occurrence of large temperature gradients within the device, even when the device is used for the purpose of generating steam. The reason for this is that it is possible to operate the heating means of the device in such a way that on every location of the surface on which water is sprayed, the heat flux supplied to the surface area is in equilibrium with the heat flux dissipated by the water which heats up and evaporates. Therefore, there is no need for extra material for better heat distribution and/or storage of thermal energy.

For sake of completeness, it is noted that the term "plate" is used in a usual sense, i.e. in the meaning of a sheet which is relatively thin. Within the scope of the present invention, the heat emitting body, which is in the form of a plate, may be planar, but may also be curved in any suitable way. For example, in an advantageous compact embodiment, the heat emitting body is bent to form a tube. Preferably, in such an embodiment, the heating means comprise at least two heating elements which are arranged at an outside of the heat emitting body. Furthermore, in such an embodiment, it is preferred if the spraying means are arranged inside the heat emitting body. For example, it is possible that the spraying means comprise a tube extending inside the heat emitting body, which tube is provided with a plurality of openings for spraying water. Advantageously, an arrangement in which the openings are present along a length of the tube is realized, so that the process of heating water may take place along a length of the heat emitting body. It is noted that in a practical embodiment, valves or the like may be applied for opening and closing the openings of the spraying means, wherein the valves may be controlled in such a manner that opening and closing of different openings takes place at different times.

In a device according to the present invention in which the heat emitting body is bent to form a tube, it is also possible that the heating means comprise at least one heating

element which is arranged inside the heat emitting body, and that the spraying means comprise a tube which is provided with a plurality of openings for spraying water at the heat emitting body, wherein the heat emitting body is extending inside the spraying means. According to this possibility, the heat emitting body is in a centre of the device, and the water is sprayed on the heat emitting body in a direction from the outside of the device to the inside.

The above-described and other aspects of the present invention will be apparent from and elucidated with reference to the following description of two embodiments of a device which is suitable to be used for the purpose of heating water to a temperature below the boiling point or to steam, dependent on which application of the device is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in greater detail with reference to the figures, in which equal or similar parts are indicated by the same reference signs, and in which:

Fig. 1 diagrammatically shows a number of components of a beverage maker, including a device for heating water according to a first preferred embodiment of the present invention;

Fig. 2 diagrammatically shows a cross-section of the device for heating water as shown in figure 1; and

Fig. 3 diagrammatically shows a number of components of a beverage maker, including a device for heating water according to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 diagrammatically shows a number of components of a beverage maker, including a device 10 for heating water according to a first preferred embodiment of the present invention, and figure 2 diagrammatically shows a cross-section of this device 10. Among other things, the device 10 for heating water is suitable to be used as a steam generator. In view of this, for sake of simplicity, the device 10 for heating water will be denoted as steam generator 10 in the following description.

The steam generator 10 comprises an inner tube 20 and an outer tube 30, wherein the inner tube 20 is extending inside the outer tube 30. Cross-sectional areas of the tubes 20, 30 may have any suitable shape. The inner tube 20 comprises a plurality of

openings 21 along its length. At one side, the inner tube 20 is connected to a system 40 for supplying water, which is part of the beverage maker, and at another side, the inner tube 20 is closed. In particular, the inner tube 20 is suitable to be used for spraying water from the openings 21 and thereby supplying water as a haze to an inner surface 31 of the outer tube 30. In view of this, the inner tube 20 will hereinafter be referred to as haze injector 20.

For the purpose of pressurizing the water that is supplied to the haze injector 20 during operation of the steam generator 10, a pump 41 is arranged in the water supplying system 40. In the shown example, the water supplying system 40 comprises a water tank 42 for containing a certain quantity of water, but it is also possible that the system 40 is connected to the water mains through a tap or the like. The haze injector 20 may be provided with any suitable kind of components for realizing a good spray of water from the openings 21. For example, the haze injector 20 may comprise any suitable type of spray nozzle (not shown). Furthermore, means may be provided for temporarily opening and closing the openings 21. A main function of the outer tube 30 of the steam generator 10 is providing a hot surface 31 for supplying heat to the water that is supplied by the haze injector 20. Therefore, in the following, the outer tube 30 will be referred to as heat emitting tube 30, and its inner surface 31 will be referred to as hot surface 31.

For the purpose of heating the heat emitting tube 30, heating elements 32 are provided, which are brazed to the tube 30. In the shown example, the heating elements 32 are extending along an outer surface 33 of the heat emitting tube 30, in a longitudinal direction of the tube 30, and have a triangular cross-section area. For sake of completeness, it is noted that within the scope of the present invention, any suitable heating means 32 may be applied for heating the component 30 of the steam generator 10 that has the hot surface 31. Furthermore, in case the heating means 32 are attached to that component 30, it is possible that the connection between the heating means 32 and the component 30 is established in another way than through brazing.

During operation of the steam generator 10, the heating elements 32 are activated such as to heat the heat emitting tube 30, and the pump 41 is activated such as to supply water to the haze injector 20. Consequently, the hot surface 31 of the heat emitting tube 30 is at a certain temperature, and the haze injector 20 sprays water through the openings 21, wherein water droplets fly in the direction of the hot surface 31. Under the influence of the heat emitted by the hot surface 31, the water droplets are evaporated and steam is generated. The steam moves upwards, towards an outlet side of the steam generator

10, where the interior 34 of the heat emitting tube 30 is in communication with a system 45 for conveying steam. By means of the steam conveying system 45, the steam is supplied to an outlet unit 50 having flow resistance. This outlet unit 50 may be a fluid pump of the beverage maker, for example, for pumping and heating a beverage ingredient under the influence of the steam.

By supplying water as a haze to the hot surface 31 , a homogeneous distribution of the water over a heated surface area is obtained. On every location of the heated surface area, a heat flux supplied to the heated surface area is in equilibrium with a heat flux dissipated by the haze that heats up and evaporates. As a consequence, there are no large temperature gradients within the steam generator 10. Furthermore, because of this equilibrium between supplied heat flux and dissipated heat flux, it is not necessary to have extra material for improving heat distribution or storing thermal energy. Therefore, it is sufficient for a heat emitting body having the hot surface 31, such as the heat emitting tube 30 as shown, to be relatively thin. In general, it is sufficient for the heat emitting body to be in the form of a plate, wherein it does not matter whether the plate is planar or curved, as it does not matter whether the hot surface 31 is planar or curved. Other consequences of the fact that it is not necessary to have extra material is that the steam generator 10 is capable of heating up and cooling down very quickly, and that it is easy to control the steam generator 10. When a process of generating steam is started and the steam generator 10 is activated, the heating elements 32 are switched on first. When the temperature of the heat emitting tube 30 reaches a predetermined value, the pump 41 is switched on, and water is pumped through the holes 21 of the haze injector 20 on the hot surface 31 of the heat emitting tube 30, where it evaporates. During the process of generating steam, a temperature of the heat emitting tube 30 is monitored at one or more appropriate positions on the tube 30, and controlled by adjusting the power of the heating elements 32, if necessary. The process of generating steam is terminated by switching off the pump 41.

It will be clear to a person skilled in the art that within the scope of the present invention, suitable components may be applied for performing the functions of monitoring a temperature, processing data and controlling a power supply to the heating elements 32, etc. Furthermore, commonly known safety measures may be taken, especially for preventing a situation in which the heating elements 32 are operating at full power while there is no supply of water. In this respect, it is noted that a level sensor or the like may be provided for monitoring a water level in the water tank 42.

When operation of the steam generator 10 is initiated, and the pump 41 is started, formation of steam does not immediately take place. During the period in which no steam is generated yet, a backpressure for the pump 41 is determined by a pressure drop of a water flow through the openings 21 of the haze injector 20, which is rather low. Consequently, during that period, a water flow rate is high. As soon as steam is generated, a steam flow through the outlet unit 50 will be high, leading to a high pressure drop across the outlet unit 50. This high pressure drop is much higher than the pressure drop across the openings 21 of the haze injector 20. An advantageous consequence of this fact is that the performance of the steam generator 10 is not very sensitive to tolerances on the dimensions of the openings 21 of the haze injector 20.

The steam generator 10 may also be applied for the purpose of heating water to a temperature below the boiling point. In such a case, in order to prevent steam generation, less heat needs to be supplied by the heating elements 32, and a temperature of the hot surface 31 needs to be lower. Water to be heated can be supplied through the haze injector 20, but it is also possible that it is supplied in the annular space 34 that is present between the haze injector 20 and the inner surface 31 of the heat emitting tube 30. When the latter option is realized, switching the function of the steam generator 10 from heating water without steam generation to generating steam involves a step of emptying the steam generator 10. Apart from that, a time needed for switching functions may be relatively short, as the thermal capacity of the steam generator 10 is relatively low, due to the fact that only a tube 30 is heated.

Figure 3 diagrammatically shows a number of components of a beverage maker, including a device 11 for heating water according to a second preferred embodiment of the present invention. The device 11 according to the second preferred embodiment of the present invention functions in substantially the same manner as the device 10 according to the first preferred embodiment of the present invention. A difference between the two devices 10, 11 relates to the mutual positioning of the haze injector 20 and the heat emitting tube 30. Both devices 10, 11 comprise an inner tube and an outer tube, but the functions which are assigned to the tubes are exchanged. Therefore, in the device 11 according to the second preferred embodiment of the present invention, the haze injector 20 constitutes an outer tube, wherein the heat emitting tube 30 is extending inside the haze injector 20. The heat emitting tube 30 is heated by a heating element 32 which is extending inside the heat emitting tube 30. Thus, in the device 11 according to the second preferred embodiment of the present invention, during

operation, water is sprayed from openings 21 in an inner surface 22 of an outer tube 20 towards a centrally positioned inner tube 30, and the inner tube 30 is heated.

It is noted that besides the two shown embodiments, more embodiments are feasible within the scope of the present invention. It will be clear to a person skilled in the art that the scope of the present invention is not limited to the examples discussed in the foregoing, but that several amendments and modifications thereof are possible without deviating from the scope of the present invention as defined in the attached claims. While the present invention has been illustrated and described in detail in the figures and the description, such illustration and description are to be considered illustrative or exemplary only, and not restrictive. The present invention is not limited to the disclosed embodiments.

In the foregoing, an application of the device 10 according to the present invention in a beverage maker is disclosed. That does not mean that the application of the device 10 is restricted to such an application. On the contrary, the device 10 according to the present invention may be applied in any situation in which a supply of steam or hot water is required.

Variations to the disclosed embodiments can be understood and effected by a person skilled in the art in practicing the claimed invention, from a study of the figures, the description and the attached claims. In the claims, the word "comprising" does not exclude other steps or elements, and the indefinite article "a" or "an" does not exclude a plurality. 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. Any reference signs in the claims should not be construed as limiting the scope of the present invention.

In the foregoing, a steam generator 10, 11 comprising an assembly of an outer tube and an inner tube is disclosed. In one embodiment, the inner tube 20 serves for spraying water on an inner surface 31 of the outer tube 30, and the outer tube 30 is heatable by means of heating elements 32. The inner tube 20 comprises a number of openings 21 which are arranged along the length of the tube 20. When water is sprayed from these openings 21 during operation of the steam generator 10, evaporation of the water to steam takes place as soon as the water reaches the inner surface 31 of the outer tube 30. Due to the fact that only a tube 30, i.e. a component having a relatively thin wall is heated during operation of the steam generator 10, heating up and cooling down of the steam generator 10 does not take much time, and controlling a temperature of the inner surface 31 of the outer tube 30 is relatively easy.

In another embodiment, the inner tube 30 is heatable by means of a heating element 32, and the outer tube 20 serves for spraying water on the inner tube 30.

Within the scope of the present invention, it is not necessary that the device 10, 11 comprises a heatable tube 30. It is important to have a hot surface 31 for supplying heat to a haze of water which is sprayed towards the surface 31, wherein a shape of the surface 31 may be chosen freely.