The present invention generally relates to a steam generating apparatus and to a method of controlling the pressure of steam in a steam generating apparatus. In particular, the invention relates to a steam generating apparatus having improved heat transfer properties and to a method of controlling the pressure of steam in a steam generating device on the basis of these heat transfer properties.
The heating of water, e.g. for generating steam, may be performed in water heating apparatuses or boilers. In these systems, the temperature of the water can be controlled within a certain temperature range by means of a heating device and a temperature sensor as follows: When the temperature signal of the temperature sensor indicates, that the temperature of the water falls below a certain level, the heating device is activated and the water is heated. If the temperature signal indicates, that the water temperature rises above a certain level, the heating device is deactivated.
Heating the water for the generation of steam requires water heating means under pressure and a control of the pressure of the steam. The controlling of the steam pressure can be performed directly by the use of a pressure sensor or indirectly by the use of a temperature sensor. Controlling the pressure by sensing the water temperature makes use of the correlation of the steam pressure and the temperature in the boiler, since during a heating of the water the steam pressure rises, and it decreases, when the water in the boiler is cooling down.
For controlling the pressure in the boiler on the basis of the measured temperature, the temperature of the water needs to be sensed accurately. In particular, the arrangement of the temperature sensor is critical. The sensor may be attached to the side walls of the boiler shell or to the bottom of the boiler shell.
Arranging the temperature sensor at the side walls requires a flat portion for a proper mounting of the sensor, which in turn complicates the forming of the shell. In some of these arrangements a heat conductive paste is applied between the temperature sensor and the boiler shell. This makes additional mounting processes necessary.
Attaching the temperature sensor at the bottom of the boiler shell also is disadvantageously. Some boilers comprise a heating plate with an embedded heating element. The heating plate usually is mounted to the bottom of the boiler shell by means of bolts or screws. A layer of thermal conducting material, e.g. graphite, may be arranged between the boiler and the heating plate to fill the air gap and to improve the heat transfer. However, the heat transfer between the boiler shell and the heating plate is not optimal. Especially during power up the water temperature and the temperature of the heating plate differ considerably. This causes a time delay in the temperature-time curve at the sensing location compared with the temperature-time curve of the water, since the heat transfer from the heating element into the water is considerably delayed. Furthermore, the spatial and temporal temperature distribution in the boiler is not even. For example, water within the sensing area of a sensor attached remotely from the heating device may be heated up later than water within the region of the heating device. This tends to cause either an overshooting of the steam pressure or the opposite.
It is an object of the invention to provide an apparatus and a method of generating steam providing an improved capability of controlling the steam pressure.
This object is solved by the features of the independent claims. Further developments and preferred embodiments of the invention are outlined in the dependent claims.
In accordance with a first aspect of the invention, there is provided a steam generating apparatus, comprising a body for receiving water to be heated and comprising a first portion comprising a first metal, and a heating device comprising a second portion comprising a second metal, wherein the heating device comprises a heating plate connected with the body by forming an intermetallic layer between the first portion and the second portion, and a temperature sensor for measuring a temperature that is indicative of a pressure inside the body is arranged in thermal contact to the heating device outside the body. The intermetallic layer provides both a mechanical and a thermal connection between the first and second portions of the heating device and the body of the steam generating apparatus. This ensures a rigid mechanical attachment of the heating device to the body and, at the same time, a good heat transfer capability between the two portions on the basis of a single process step. The intermetallic layer may comprise parts of the first metal, the second metal, and/or a third metal, e.g. a soldering metal. Conventional attaching methods like bolting or screwing create an unevenly distributed, mostly spot-like, contact surface. The intermetallic layer provides a large and contiguous contact surface allowing a higher and more uniform heat transfer. The properties of the two metals can be chosen according to the needs of the body and the heating element, respectively. The first metal and the second metal may be each mixture containing two or more metallic elements or metallic and non-metallic elements and may be optimized independently regarding their heat transfer properties. Therefore, the metal of the first portion comprised by the body may be designed to meet the water heating and steam storing requirements, whereas the second metal may be optimized regarding heat generating and transferring requirements. There are several methods of forming the intermetallic layer, which will be discussed below. The temperature sensor may be a thermistor or another sensor producing a signal associated with a sensed temperature. Due to the improved thermal conductivity the temperature sensor may be arranged adjacent to the heating device or may be directly attached to or integrated in the heating device. As a quick heat transfer takes place between the body, the heating device, and the sensing point of the temperature sensor, hence the development of the temperature can be measured by the temperature sensor without much delay.
In this regard, it is advantageous that the first metal is stainless steel. Stainless steel and the like complies with the requirements of low corrosion under a damp heat environment.
Similarly, the second metal is aluminum or an aluminum alloy. These materials combine a good thermal conductivity with good processing properties.
According to a particular embodiment of the present invention, the intermetallic layer is formed by soldering and/or brazing and/or welding. These alternative or combined processing steps create an intermetallic layer between the first portion and the second portion as described above and are well proven methods of joining different metals. Furthermore, metal filled adhesives may also be used to provide a joint showing a high thermal conductivity and a good mechanical connection.
In accordance with an embodiment of the invention, the heating plate comprises a heating element. The heating element may be attached to the heating plate by casting-in, soldering, brazing, welding or similar techniques.
According to a preferred embodiment of the present invention, the heating device comprises control means for controlling the temperature of the water. The generation of steam requires an accurate control of the steam pressure, as discussed above. By utilizing the improved heat transfer capabilities from the body to the heating device and vice versa, an accurate controlling of the water temperature and, in consequence, of the steam pressure may be obtained. Further, the improved heat transfer capability of the intermetallic joint reduces the feedback time in the system and allows for a faster and more accurate control of the water temperature.
In accordance with a second aspect of the invention, there is provided a method of controlling the pressure of steam in a steam generating apparatus comprising a body for receiving water to be heated and comprising a first portion comprising a first metal, a heating device comprising a second portion comprising a second metal, the body being connected with a heating plate of the heating device by forming an intermetallic layer between the first portion and the second portion, and a temperature sensor for measuring a temperature that is indicative of a pressure inside the body, the temperature sensor being arranged in thermal contact to the heating device outside the body, the method comprising the steps of setting the target water temperature for a first time period to a first set temperature, setting the target water temperature for a second time period to a second set temperature higher than the first set temperature, and setting the target water temperature for a third time period to a third set temperature lower than the second set temperature. Adjusting the target temperature of the water to be heated to different temperature levels during several time periods provides a flexible method of controlling the steam pressure of a steam generating device by measuring the water temperature. For example, the steam pressure level may be set to a nominal pressure, corresponding to the first set temperature. During the second time period, a higher temperature setting and therefore also a higher steam pressure level is set. This may be utilized to temporarily raise the steam pressure for providing a steam output at a higher rate without the need to design the components involved for higher pressure. This may be performed at predetermined time periods or in response to a signal or event. Another example is the possibility to compensate for a reduction in the steam pressure that is predictable at a certain time point by respective signals, but not yet detectable via the temperature sensor, as will be discussed later in detail.
According to a preferred embodiment of the invention, the beginning of the second time period and/or the duration of the second time period and/or the second set temperature is at least one of the following: predetermined; a function of the steam output of the steam generating device, and a function of the water input into the steam generating device. Adjusting the target water temperature to a higher level compared to an initial nominal set temperature during a predetermined time period allows for the compensation of regularly appearing steam demands in advance. The beginning of the second time period and its duration may be adjusted in a flexible way to correspond to the expected steam rate output. Further, the configuration of the second time period and a corresponding set temperature may be correlated to the current steam output. For example, the second time period may reflect the current output steam rate and its duration. Accordingly, the same holds for the amount of water input into the steam generating device. Appropriate signals communicating the triggering of the steam output or the water input may be a switch actuated by the user or an electrical signal activating a water pump.
According to a further embodiment of the present invention, the duration of the second time period equals the duration of the steam output or the duration of the water input. In addition, the beginning of the second time period may coincide with the beginning of the steam output and the beginning of the water input, respectively. This is a simple way of improving the controlling of the steam pressure by adding additional heat at appropriate time periods.
Particularly, the second time period is elongated by a time period being a function of at least one of the following: the duration of the steam output, and the duration of the water input. According to the amount of heat power being transferred into the water and according to other aspects of the steam generating device, appropriate heating periods can be chosen to compensate for the heat loss caused by a steam output and a water input, respectively.
It is also preferred, that the step of controlling the water temperature at the second temperature comprises the step of activating the heating device in the case of at least one of the following: the current water temperature is lower than the second temperature; a steam output is requested; and a water input is performed. During the second time period the heating device transfers heat into the water, whenever one of the mentioned events takes place. Even if the current water temperature is still higher than the second temperature, the heating device is activated for preventing or mitigating a future pressure drop.
According to a particular embodiment of the present invention, the step of controlling the water temperature at the second temperature comprises the step of deactivating the heating device, if the current water temperature is higher than a maximum temperature. In order to prevent an excessive increase in steam pressure, the current water temperature is limited to a maximum temperature.
Particularly, the step of controlling the water temperature at the second temperature comprises the step of deactivating the heating device after a time period being a function of at least one of the following: the duration of the steam output; and the duration of the water input.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The steam generating device 10 is suitable for use in a domestic appliance comprising, besides the steam ironing device shown as a preferred embodiment, a steamer, a steam cleaner, an active ironing board, a facial sauna, a steam cooking device, a coffee making machine and the like. The water level sensor 30 is used to detect changes in the water level of the boiler 12. When the water level is lower than a certain level or the boiler 12 is empty, the water level sensor 30 sends a signal to the electronic control unit 26. The electronic control unit 26 activates the pump 38 to feed water into the boiler 12 for raising the water level. When the water level in the boiler 12 is higher than the certain level, the water level sensor 30 sends an appropriate signal to the electronic control unit 26. The electronic control unit 26 deactivates a pump 38 to stop pumping. In this way, the water level of the boiler 12 is maintained within a certain range. The de-airing valve 42 provides a connection of the boiler 12 with the atmosphere to prevent the boiler 12 from being overfilled with water, if during cooling down after use a vacuum is formed inside the boiler 12. The water level sensor 30 may be mounted on the heating plate 15 (as shown) or alternatively on the boiler shell, on the side walls of the boiler 12 or even inside the boiler 12 depending on the sensing method used. If the water level sensing is done based on the temperature from the temperature sensor 24, the temperature sensor 24 can be used as the water level sensor.
The temperature sensor 24 is mounted on the heating plate 15. In this way, the temperature sensor 24 is located adjacent to an area being in good thermal contact with the water inside the boiler 12 in order to properly sense the water temperature. Since the steam pressure of the water inside the boiler 12 is directly related to the water temperature, the temperature sensor 24 is used to control the pressure of the water. If the sensed temperature is lower than a preset temperature value, the pressure is also lower than the required level. In this case, the electronic control unit 26 activates the heating element 12. If the temperature sensor 24 signals a water temperature reaching or exceeding the preset temperature value, the heating element 22 is turned off by the electronic control unit 26. This is a simple way of controlling the steam pressure inside the boiler 12. More sophisticated methods are described in relation to
A further reduced embodiment of the invention comprises a simple boiler system, for example a boiler 12 without the water tank 40, the electrical pump 38, the de-airing valve 42, and the feed water inlet 36. As a temperature sensor 24 a thermostatic switch can be used. The power control of the heating device 22 can be performed by the thermostatic switch directly without the need for an additional electronic control unit 26. Thus, the pressure is controlled at one level, if the thermostatic switch only works at one temperature level.
Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
Number | Date | Country | Kind |
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05112354 | Dec 2005 | EP | regional |
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PCT/IB2006/054676 | 12/8/2006 | WO | 00 | 6/19/2008 |
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WO2007/072271 | 6/28/2007 | WO | A |
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