Patent Publication Number: US-2006000821-A1

Title: Oven with a system for generating steam

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to an oven with a system for generating steam.  
      2. Description of the Related Art  
      Both from literature and from tests, it is known that the use of steam in an oven during cooking positively influences the quality of the prepared food. For this reason, ovens have been constructed comprising in their cooking chamber the usual heating elements (for browning and heating with or without forced air circulation) and provided additionally with means for generating steam or for combining steam generation with hot air circulation. The steam can be fed to the cooking chamber either with or without activating the heating elements.  
      In the known art, devices generating steam for domestic use in ovens comprise a boiler with a heating element which, when immersed in water, heats the water to generate steam, which is then fed into the cooking chamber and used to cook food. These known systems suffer from the problem of water depletion in the boiler, which can lead to overheating and damaging the heating element. On the other hand, when there is an excessive amount of water in the boiler, energy is wasted by having to heat a large quantity of water when used to feed a relatively small amount of steam into the cooking chamber.  
     SUMMARY OF THE INVENTION  
      An oven according to one embodiment of the invention comprises a housing defining a cooking cavity, a steam generator defining a chamber and having an evaporation element located in the chamber for generating steam that is introduced into the cooking cavity, and a water reservoir in fluid communication with the chamber and positioned relative to the chamber to supply water to the chamber to at least a minimum level at which the water completely immerses the evaporation element.  
      The water reservoir can further comprise a water level sensor to sense a level of water in the water reservoir. The water level sensor can comprise a first water level sensor that senses a maximum level of water in the water reservoir and a second water level sensor that senses a minimum level of water in the water reservoir. The oven can further comprise a signal generator operable to communicate the user the sensed level of water in the water reservoir. The water level sensor can be operably coupled to the evaporation element to control activation and deactivation of the heating element in response to the sensed level of water in the water reservoir.  
      The water reservoir can comprise a base and can be positioned relative to the chamber with at least a portion of the base vertically higher than the evaporation element. The entire base can be vertically higher than the evaporation element. The water reservoir can comprise an outlet in the base, and the outlet can be vertically higher than the evaporation element.  
      A capacity of the water reservoir can be greater than a capacity of the chamber.  
      A level of water in the chamber can be the same as a level of water in the water reservoir. A minimum level of water in the water reservoir can be the same as or higher than the minimum level of water in the steam generator. A maximum level of water in the water reservoir can be the same as or less than a maximum level of water in the steam generator.  
      The oven can further comprise a temperature sensor in the steam generator to sense a parameter representative of a temperature of the water in chamber. The parameter can be the temperature of the water in the chamber.  
      The oven can further comprise a discharge valve near an outlet of the steam generator to drain water from the chamber.  
      The oven can further comprise an inlet valve near an inlet to the water reservoir to control the supply of water to the water reservoir.  
      The oven according can further comprise a heating system to heat the cooking cavity.  
      The steam generator can have a top defining an upper extent of the chamber, and the water reservoir can have a top positioned relative to the chamber such that the water reservoir top is vertically below the steam generator top.  
      An oven according to another embodiment of the invention comprises a housing defining a cooking cavity, a steam generator having a top defining an upper extent of a chamber, and a water reservoir having a top and in fluid communication with the chamber to supply water to the chamber and positioned relative to the chamber such that the water reservoir top is vertically below the steam generator top.  
      A level of water in the chamber can be the same as a level of water in the water reservoir. A capacity of the water reservoir can be greater than a capacity of the chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      In the drawings:  
       FIG. 1  is a perspective view of an exemplary automatic household oven.  
       FIG. 2  is a schematic view of the oven of  FIG. 1 .  
       FIG. 3  is a schematic diagram illustrating a control system of the oven of  FIG. 1 .  
       FIG. 4  is a schematic view of a system according to one embodiment of the invention for generating steam for use with the oven of  FIG. 1 . 
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
      Referring now to the figures,  FIG. 1  illustrates an exemplary automatic household oven  10  that can be equipped with a steam system according to one embodiment of the invention. The oven  10  comprises a cabinet  12  with an open-face housing  13  having a pair of spaced side walls  16 ,  18  joined by a top wall  20 , a bottom wall  22 , and a rear wall  23  ( FIG. 2 ) to define an open-face cooking cavity  14 . A door  24  pivotable at a hinge  27  selectively closes the cavity  14 , and a sensor  26  detects an open position of the door  24  and a closed position of the door  24 . When the door  24  is in the open position, a user can access the cavity  14 , while the door  24  in the closed position prevents access to the cavity  14  and seals the cavity  14  from the external environment.  
      The oven  10  further comprises a console  29  with a control panel  28  having a user interface accessible to the user for inputting desired cooking parameters, such as temperature and time, of manual cooking cycles or for selecting automated cooking cycles. The user interface can comprise, for example, a push button, a rotatable knob, a touch pad, a touch screen, or a voice command unit. The control panel  28  communicates with a controller  30  located in the cabinet  12 , as shown in  FIG. 2 . The controller  30  can be a proportional-integral-derivative (PID) controller or any other suitable controller, as is well-known in the automatic oven art. The controller  30  stores data, such as default cooking parameters, the manually input cooking parameters, and programs for the automated cooking cycles, receives input from the control panel  28 , and sends output to the control panel  28  for displaying a status of the oven  10  or otherwise communicating with the user. Additionally, the controller  30  includes a timer  32  for tracking time during the manual and automated cooking cycles.  
      With continued reference to  FIG. 2 , the oven  10  further comprises a heating system  34  having an upper heating element  36 , commonly referred to as a broiler, and a lower heating element  38 . The schematic illustration of  FIG. 2  shows the lower heating element  38  as being hidden or mounted beneath the cooking cavity bottom wall  22  in a heating element housing  40 . Heat from the lower heating element  38  conducts through the bottom wall  22  and into the cavity  14 . Alternatively, the lower heating element  38  can be mounted inside the cavity  14 , as is well-known in the oven art. Further, the upper and lower heating elements  36 ,  38  can be mounted at the side walls  16 ,  18  of the cavity  14 , as disclosed in U.S. Pat. No. 6,545,251 to Allera et al., which is incorporated herein by reference in its entirety. The heating system  34  according to the illustrated embodiment further comprises a convection fan  42  that circulates air and steam, when present, within the cavity  14 . The convection fan  42  can be any suitable fan and can be mounted in any suitable location of the cavity  14 , such as in the rear wall  23 . The particular type of heating system is not germane to the invention; the heating system  34  shown and described herein is for illustrative purposes only and is not meant to limit the invention in any manner.  
       FIG. 3  is a block diagram that schematically illustrates a control system of the oven  10 . The control system comprises the controller  30 , which operably communicates with the control panel  28 , as described above, the heating system  34 , and a steam system  44 . The controller  30  instructs the heating system  34  to activate or deactivate the upper heating element  36 , the lower heating element  38 , and the convection fan  42 , either all together, individually, or in groups, and provides instructions regarding the desired temperature of the cavity  14  and the rate at which the heating system  34  heats the cavity  14 . Similarly, the controller  30  instructs the steam system  44  to activate or deactivate to provide to the cavity  14  a desired amount of steam at a desired steam generation rate corresponding to a desired relative humidity in the cavity  14 .  
      Ovens having a heating system  34  and a steam system  44  are commonly referred to as combi ovens. The heating system  34  can be employed alone, as in a traditional oven, or in combination with the steam system  44 . When both the heating system  34  and the steam system  44  are utilized, the steam system  44  functions as a support or accessory for the heating system  34 . Alternatively, the steam system  44  can be used alone for cooking with steam only.  
      The steam system  44  according to one embodiment of the invention is illustrated schematically in  FIG. 4  and comprises a boiler or steam generator  46  in which an evaporation element  48 , such as a resistive heating element, is disposed. The steam generator has a peripheral wall  50  closed at an upper end by a top  52  and at lower end by a bottom or base  54 , and the peripheral wall  50 , the top  52 , and the base  54  define a chamber  56 . In the top  52  of the steam generator  46 , a steam exit aperture or outlet  58  is provided, to which a conduit  60  is connected to transfer the steam generated in the steam generator  46  to the cooking chamber  14 . A water reservoir  62  is connected to the steam generator  46  via a pipe or conduit  64  to supply water to the steam generator  46  between an outlet  66  of the water reservoir  62  and an inlet  68  of the steam generator  46 , and a discharge valve  70  is connected near an outlet  72  of the steam generator  46  for emptying the water from the steam generator  46  through the outlet  72 , for example when the steam system  44  is to remain unused for lengthy periods.  
      The steam system  44  further comprises a temperature sensor  74  for sensing a parameter representative of a temperature of the water in the steam generator  46 , which can include a direct or indirect sensing of the temperature of the water in the chamber  56  of the steam generator  46 . Direct sensing includes directly sensing the water. Indirect sensing includes sensing the temperature of an item, such as the peripheral wall  50 , having a known or determinable relationship with the temperature of the water. The temperature sensor  74  can be positioned in any suitable location to sense the parameter. For example, the temperature sensor  74  can be mounted to the steam generator  46  in direct contact with the water in the chamber  56  of the steam generator  46 , embedded in one of the walls  50 ,  52 ,  54  of the steam generator  46  to encase and protect the temperature sensor  74 , or attached to an exterior surface of the steam generator  46 . According to the illustrated embodiment, the temperature sensor  74  is vertically positioned at or below a level H, or a minimum level of water acceptable in the steam generator  46 , to ensure that the temperature sensor  74  is able to sense the temperature of the water. The temperature sensor  74  can be any suitable type of sensor, including, but not limited to, thermocouples, ceramic thermistors, metallic resistance temperature devices (RTDs), and infrared temperature measurement devices.  
      As stated above, the reference letter H indicates the minimum level of water acceptable in the steam generator  46 , and the reference letter K indicates a height of the evaporation element  48  measured from the base  54  of the steam generator  46 . In order for the system to function, the pressure drop through the exit aperture  52  must not exceed the pressure produced by a column of water of height H-K, which corresponds to the minimum height of water present at the top of the evaporation element  48 . This can be achieved by connecting a known nozzle of suitable dimensions to the exit aperture  58 .  
      The pipe  64  connects the steam generator  46  to the reservoir  62 , which has much larger dimensions and hence volume or capacity than the steam generator  46 . The reservoir  62  comprises a peripheral wall  76  closed at an upper end by a top  78  and at a bottom end by a bottom or base  80 . An access or inlet  82  for feed water entry into the reservoir  62  is provided in the top  78  of the reservoir  62  or in any other suitable location of the reservoir  62 . An inlet valve  84  upstream from the access  82  is operable to close feed water entry when the reservoir  62  is sufficiently full. The reservoir  62  includes a reservoir water level sensor, which, according to the illustrated embodiment, comprises a first level sensor  86  and a second level sensor  88 . The first level sensor  86  detects the attainment of a maximum water level in the reservoir  62 , and the second level sensor  88  detects the attainment of a minimum water level in the reservoir  62 . However, it is within the scope of the invention for the water reservoir  62  to include one or more than two water level sensors. For example, the water reservoir  62  can comprise three water level sensors: a low water level sensor to sense when the water level is too low, a high water level sensor to sense when the water level is sufficiently high, and a water level sensor between the low and high water level sensors that senses when the water level is approaching the lower water level sensor to warn the user that the reservoir  62  will need to be filled soon.  
      As stated previously, the steam system  44  operably communicates with the controller  30 . Data from the level sensors  86 ,  88  and the temperature signal generated by the temperature sensor  74  are fed to the controller  30 . Additionally, the controller  30  communicates with the evaporation element  48  to activate or deactivate the evaporation element  48  to achieve or maintain a desired temperature of the water in the steam generator  46  according to a manual or automatic cooking cycle. The controller  30  is also operably coupled with the control panel  28  for activating or deactivating a suitable signal generator  90 , such as a light and/or acoustic signal generator, associated with the control panel  28  to communicate to the user the level of water in the reservoir  62 . For the illustrated embodiment, the signal generator  90  can indicate that the water level in the reservoir  62  is too low or sufficiently high.  
      By the principle of communicating vessels, the water level in the reservoir  62  and in the chamber  56  of the steam generator  46  is substantially the same. For example, as shown in  FIG. 4 , the level of water in the reservoir  62  is indicated by the letter J, and the water level J is the same as the level of water in the chamber  56 . As used herein, the “level” of water corresponds to a height of water relative to a common reference. Thus, two vessels having the same level of water, such as the water level J, need not hold the same volume of water.  
      For the evaporation element  48  to always remain completely immersed in or covered with water during operation to prevent overheating with consequent possible burning of the evaporation element  48 , the water level in the steam generator  46  must be sufficiently high. For this reason, at least a portion of the base or bottom wall  80  of the reservoir  62  is positioned at a higher level than the level K attained by the evaporation element  48 , i.e. at the aforesaid level H in  FIG. 4 . Additionally, the outlet  66  of the water reservoir  62  is preferably vertically above the inlet  68  to the steam generator  46 . The level H must also not be too high to prevent water filling the entire volume of the steam generator  46  and entering the cooking chamber  14  together with the generated steam. As used herein, “completely immerse” refers to a condition wherein substantially all exterior surfaces of the evaporation element  48  in the steam generator  46  are covered by or in contact with water most of the time. It is within the scope of the invention for some exterior surfaces of the immersed evaporation element  48  to intermittently be exposed, such as a result of turbulence caused by boiling water in the steam generator  46 . Thus, “completely immerse” is not limited to all exterior surfaces of the evaporation element  48  being covered by or in contact with water all of the time.  
      Because of the communicating vessels, the water level in the reservoir  62  when the reservoir  62  is substantially full (i.e., water level in the reservoir  62  is at a maximum water level), determines a maximum water level L in the chamber  56 . Thus, the water reservoir  62  is preferably positioned and sized so that an upper extent of the reservoir  62 , which is defined by the top  78  in the illustrated embodiment, corresponds to or is lower than a maximum water level L acceptable in the steam generator  46 . Furthermore, the maximum water level in the reservoir  62  and, thus, the maximum water level L acceptable in the steam generator  46 , preferably correspond to a relatively small volume of water in the chamber  56 . As a result, the evaporation element  48  can efficiently and quickly heat the relatively small volume of water in the chamber  56 .  
      In operation, the feed water is supplied to the steam system  44  through the inlet valve  84  and the access  82  of the reservoir  62 . A system can be provided by which water is manually added to the reservoir  62 , or, alternatively, water can be supplied by an automated system directly connected to the water main. In the latter case, the controller  30  can control the flow of water from the water main into the reservoir  62  by opening and closing the inlet valve  84 . Water from the reservoir  62  is then fed through the pipe  64  into the chamber  56  of the steam generator  46  at least until the water completely immerses or covers the evaporation element  48 . By the principle of communicating vessels, the water level in the reservoir  62  and in the steam generator  46  is the same, as shown by the water level J in  FIG. 4 ; hence, there is no need for pumps for transferring water from the reservoir  62  to the steam generator  46 .  
      The relatively small quantity of water present in the steam generator  46  enables the evaporation element  48  to rapidly heat the water and produce steam. Additionally, the temperature sensor  74  feeds to the controller  30  a temperature signal, which is useful, for example, if the water in the steam generator  46  is to be preheated so that steam can be introduced rapidly into the cooking cavity  14  when required by the manual or automatic cooking cycle. Moreover, as the water levels in the chamber  56  of the steam generator  46  and the water reservoir  62  are substantially the same, as shown in  FIG. 4 , and the base  80  of the reservoir  62  is at a higher level than the evaporation element  48 , the evaporation element  48  always remains covered with water in the steam generator  46 , even with a small water quantity in the reservoir  62 .  
      When the water level falls to the minimum allowable level in the reservoir  62 , such as due to steam generation or water evaporation in the steam generator  46 , the second level sensor  88  feeds a signal to the controller  30 , which deactivates the evaporation element  48  and, depending on preference, either emits a signal via the signal generator  90  so that the user is aware to fill the water reservoir  62  and/or automatically opens the inlet valve  84  to again feed water into the steam system  44 . By eliminating power to the evaporation element  48 , the evaporation element  48  does not remain in an active condition while being exposed (i.e., not completely immersed in the water), which prevents overheating of the evaporation element  48 .  
      When the water level reaches maximum level in the water reservoir  62  during filling of the water reservoir  62 , the first level sensor  86  feeds a signal to the controller  30  which, depending on preference, either emits a signal via the signal generator  90  so that the user is aware that water reservoir  62  is full and/or instructs the inlet valve  84  to close and prevent water flow to the water reservoir  62 .  
      Because the water reservoir  62  is separate from but connected to the steam generator  46 , the second level sensor  88  can be disposed in the water reservoir  62  instead of in the steam generator  46 . If such a level sensor were disposed in the steam generator  46 , the sensor would not give reliable results because, for example, the sensor would be subjected to turbulence of the boiling water.  
      While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.