Abstract:
A method of operating a steam food cooking device having a cooking cavity, a pool disposed below the cooking cavity; and a heat source for heating water in the pool so at to generate steam includes: a) in response to turning on the cooking device, initiating a water flow into the pool and substantially simultaneously activating the heat source to generate heat; and b) thereafter, controlling the on/off state of the heat source based on a thermal sensor and circulating steam within the device by unforced natural convection. The pool may be thereafter automatically drained in response to turning off the cooking device prior to repeating steps a and b. The water inflow into the pool may be via an inlet disposed proximate a lower side of the bottom. Steam cooking devices suitable for such method(s) are also described.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the field of commercial steam cooking devices.  
         [0002]     Modern steam ovens operate by heating water to generate steam and then supplying the steam to a cooking cavity containing food. Some of these steam ovens operate at low pressures, slightly above atmospheric, while others are openly vented to atmosphere. For either arrangement, most commercially available steam ovens require a substantial amount of time, on the order of 15-20 minutes, in order to start generating steam once the steam oven is turned on. Obviously, shortening this delay between turning on the steam oven and steam generation would lead to better energy efficiency and increased user satisfaction. However, efforts directed to shortening the delay have not yet resulted in significant success. Further, the control systems for steam ovens can sometimes be overly complicated, leading to increased costs.  
       SUMMARY OF THE INVENTION  
       [0003]     In one embodiment, the present invention provides a method of operating a steam food cooking device having a cooking cavity, a pool disposed below the cooking cavity; and a heat source for heating water in the pool so at to generate steam, the method comprising: a) in response to turning on the cooking device, initiating a water flow into the pool and substantially simultaneously activating the heat source to generate heat; and b) thereafter, controlling the on/off state of the heat source based on a thermal sensor and circulating steam within the device by unforced natural convection. The method may further comprise thereafter automatically draining the pool in response to turning off the cooking device; and thereafter repeating steps a and b. The method may further comprise feeding water to the pool at a uniform rate during step b. The controlling the heat source may comprise turning the heat source off in response to the thermal sensor sensing a temperature of at least a predetermined temperature and thereafter automatically turning the heat source back on in response to the thermal sensor sensing a temperature below the predetermined temperature. The pool may have a bottom disposed at an angle relative to horizontal, and the initiating a water flow into the pool may comprise initiating a water flow into the pool via an inlet disposed proximate a lower side of the bottom. The heat source may comprise one or more electrical heating elements disposed in thermal communication with the pool. The method may further comprise limiting pressures in the cooking chamber to not more than slightly above atmosphere by connecting the cooking cavity to atmosphere via exhaust duct and a cap that moveably engages the exhaust duct. The cooking device may comprise a steam trap assembly disposed adjacent the cooking cavity and having downwardly extending baffle; and the method may further comprise, after step a, feeding water from the pool to the steam trap assembly.  
         [0004]     In another aspect, the present invention may provide a natural convection steam cooking device, comprising: a cooking cavity; a steam chamber disposed below the cooking cavity, having a pool, and operatively connected to the cooking cavity to supply steam thereto; a heat source for heating water in the pool so as to generate steam in the steam chamber; wherein, in response to turning on the cooking device, water feed into the pool begins and the heat source is substantially simultaneously activated. The heat source may comprise one or more electrical heating elements. The bottom of the pool may be disposed at an angle to horizontal, with a water inlet for the pool disposed on a lower side of the bottom, wherein the water feed is via the water inlet. The device may further comprise an automatically resetting thermal sensor connected to control the heat source such that heat source is turned off in response to the thermal sensor sensing a temperature of at least a predetermined temperature, but is turned on in response to the thermal sensor sensing a temperature below the predetermined temperature. The cooking cavity may have a shape of a parallelepiped, with at least two walls defining the cooking cavity, not including a door, immovably fixed to the device and/or being non-porous. The cooking cavity advantageously has a volume of about two cubic feet or more, and is advantageously adapted to support a plurality of trays for holding food to be cooked. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0005]      FIG. 1  is a perspective view of one embodiment of a steamer according to the present invention.  
         [0006]      FIG. 2  is a sectional view of the cooking device of  FIG. 1  illustrating the steam flow paths.  
         [0007]      FIG. 3  is a view of the floor of the pool.  
         [0008]      FIG. 4  is a view of the floor of the cooking cavity.  
         [0009]      FIG. 5  is a view of the right sidewall of the cooking cavity.  
         [0010]      FIG. 6  is a partial side view showing the exhaust chamber.  
         [0011]      FIG. 7  is a simplified representation of the steamer of  FIG. 1  showing control of water supply, water draining, and heater energization.  
         [0012]      FIG. 8  is a simplified schematic of electronics suitable for use in the steamer of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     One embodiment of a cooking device according to the present invention, sometimes referred to herein as a steamer, is generally indicated at  10 . From outward appearance, the steamer  10  may appear similar to steamers currently available on the market. Indeed, as is typical for such devices, the steamer  10  includes a latched door  12  for providing access to the cooking cavity  30  and controls  16  to control the operation of the steamer  10 . The controls  16  may take any form known in the art, and typically include an on/off switch  17 , indicator lights, a timer  18 , and other suitable electronics, such as a door-open sensor  14  for sensing when the door  12  is open. The electronics of the controls  16  may be generally segregated from cooking cavity  30  and may be advantageously vented to the ambient atmosphere for cooling.  
         [0014]     The cooking cavity  30  typically takes the form of a substantially parallelepiped chamber, preferably substantially rectangular chamber, with racks (not shown) for supporting food trays  5  as is known in the art. The cooking cavity  30  is bounded by suitable sidewalls  42 , 44 , 48 , a floor  50 , a ceiling  46 , and the inside of the door  12 . The sidewalls  42 , 44  and the ceiling  46  are advantageously immovably fixed to the device  10  and non-porous. In most embodiments, the cooking cavity  30  that has a volume of about two cubic feet or more. In the present invention, the cooking cavity  30  may be vented to the atmosphere via an exhaust system  32  that typically includes an exhaust duct  34  and associated cap  36 . The cap  36  acts as a slight damper on the venting of gases from the cooking cavity  30 . However, once the pressure in the cooking cavity  30  reaches a sufficient level to overcome the weight of the cap  36 , the cap  36  is displaced in a “burping” action that vents some gases from the cooking cavity  30 , thereby preventing significant build-up of pressure therein. In preferred embodiments, the cap  36  is designed to prevent the build-up of pressures more than slightly above atmospheric in the cooking cavity  30 , such as of not more than about five inches of water column. As such, no special pressure vessel structure or certification should be required for the steamer  10 .  
         [0015]     In the illustrative embodiment, the ceiling  46 , rear sidewall  44 , and left sidewall  42  are solid, while the floor  50  and the right sidewall  48  have a plurality of holes  52 , 54  therein. In particular, the floor  50  has a plurality of holes, referred to herein as the primary holes  52 , that connect to the steam chamber  60  as discussed below. The primary holes  52  may advantageously be arranged in two arrays of similarly sized holes. For example, there may be sixteen rows of six holes  52  each in a middle portion of the floor  50 , with the holes  52  having a ½ inch diameter, and eight slots  52  of ½ inch by two inches arranged around the peripherally of the floor  50 . Of course, other hole arrangements may be used in other embodiments. The floor  50  may advantageously be readily removable from the cooking device  10  to allow access by a user from the cooking cavity  30  to at least a portion of the steam chamber  60  for cleaning. If so, it may be advantageous to enlarge one hole  52  on each end to a larger size, such as one inch, to provide a clear finger hole to aid in removing the floor  50 . The right sidewall  48  likewise includes a plurality of holes, referred to herein as secondary holes  54 , that connect the cooking cavity  30  to the steam chamber  60  as discussed further below. These secondary holes  54  may advantageously take the form of an array of slots, such at the ¼ inch wide slots shown in  FIG. 5 , but this is not required by all embodiments.  
         [0016]     The steamer  10  includes a steam chamber  60  located below and along at least one side of the cooking cavity  30  (see  FIG. 2 ). The steam chamber  60  includes a pool area  62 , a first steam compartment  76 , and a second steam compartment  80 . The pool area  62  (or “pool”) is disposed in a lower portion of steam chamber  60 , beneath the first steam compartment  76  and the lower portion of second steam compartment  80 . The pool  62  holds the liquid (typically “tap” water) that is heated to generate steam. The pool  62  is filled via a water inlet port  64  on the right side and drained via a pool drain  66  located on the right side that connects to the main drain line  99  for the steamer via a drain valve  68 . The floor of the pool  62  is advantageously slightly angled to the right so that the water is directed to the pool drain  66  and is shallower on the left than on the right.  
         [0017]     One or more electric heating elements  20  are provided to supply sufficient heat to the pool  62  so as generate steam. These electrical resistance heaters  20  are advantageously positioned directly below the pool  62  and may come in a variety of shapes and sizes. In an advantageous embodiment, each heating element  20  is a rectangular shaped block of aluminum with an embedded electrical resistance core and a thermal fuse  24 . A plurality of these heating elements  20  are attached to the underside of the pool  62  of the steam chamber  60 . Optionally, a compressible heat transfer layer (not shown) may be used, as disclosed in U.S. Pat. No. 5,968,388, which is incorporated herein by reference. An automatically resetting thermal sensor  22  is advantageously associated with the heating elements  20 , such as being mounted to the side of one of the heating elements  20 . While not strictly required for all embodiments, the thermal sensor  22  is advantageously disposed in a location that is external to all moisture (steam/water) contact areas of the cooking device  10 . It should be noted that other heating element arrangements are encompassed by the present invention, including but not limited to common electrical resistance heaters, film resistance heaters, induction heaters, and gas heaters. It should be noted, that while not preferred, the heating elements  20  may alternatively be located in the pool  62 , rather than underneath the pool  62  if desired. The operation of the heating element(s)  20  may be controlled as discussed further below.  
         [0018]     The first steam compartment  76  is disposed between the pool  62  and the floor  50 . Steam generated by the heated water in the pool  62  naturally rises into the first steam compartment  76 . It is intended that there will be unforced flow through the first steam compartment  76 , as described further below. The first steam compartment  76 , and thus the steam chamber  60 , is separated from the cooking cavity  30  by the floor  50  of the cooking cavity  30 , meaning the cooking cavity&#39;s floor  50  may also form the “ceiling” of the first steam compartment  76  in some embodiments.  
         [0019]     Second steam compartment  80  is disposed generally vertically along a selected side of the cooking cavity  30 . The lower portion of second steam compartment  80  is defined by the water in pool  62  (or the floor of pool  62  in the absence of water). Right sidewall  48  helps define the boundary between second steam compartment  80  and cooking cavity  30 . The secondary holes  54  in sidewall  48  allow steam from second steam compartment  80  to enter cooking cavity  30  via natural convection. Thus, steam entering the second steam compartment  80  flows upward by natural convection through the second steam compartment  80  and out the secondary holes  54  into the cooking cavity  30 . Exhaust port  94  is disposed in a rear portion of second steam chamber  80 , on a wall opposite right sidewall  48 . Exhaust port  94  connects to the exhaust control system  90  as discussed further below. The exhaust port  94  is located at a height above the inlet port  64 , and acts as a water overflow port to prevent overfilling of the pool  62 .  
         [0020]     The exhaust control system  90  includes an exhaust chamber  92 , a spray nozzle  95 , and a drain port  98 . The exhaust chamber  92  is operatively connected to the second steam compartment  80  of steam chamber  60  via the exhaust port  94 . The exhaust chamber  92  includes a downwardly extending baffle  96  that divides the exhaust chamber  92  into front and rear portions. The rear portion includes the drain port  98 , which advantageously takes the form of a vertically extending open pipe structure. The drain port  98  provides a fluid flow path to the main drain line  99  of the steamer  10 . The upper end of the drain port  98  is higher than both the lowest part of exhaust port  94  and the lower end of the baffle  92 . It is intended that water will fill the exhaust chamber  92  to a level that is above the lower end of the baffle  92 , but at or below the upper end of the drain port  98 , so that a steam trap is formed in the front portion of the exhaust chamber  92 . The spray nozzle  95  is located above the drain port  98 , and sprays water into the rear portion of the exhaust chamber  92 . As the pressure rises in the cooking cavity  30  and steam chamber  60 , this pressure is communicated to the exhaust chamber  92  via the exhaust port  94 . The rise in pressure may cause steam to break the steam trap formed at the baffle  92 , but the spray from the spray nozzle  95  will act to cool the steam down to acceptable levels for draining into a municipal wastewater system. It should be noted that the exhaust port  94  may advantageously take the form of a cluster of slots with a smaller size, such as ¼ inch width, that are staggered in height so that one or more of the slots are lower than the others. The size of the drain port  98 , drain line  99 , and all other possible constrictions downstream from the exhaust port  94  are advantageously larger, such as ¾ inch minimum, so as to reduce the opportunity for clogging. In addition, the top of the exhaust chamber  92  may advantageously be removable, so as to allow access to the exhaust chamber  92  for cleaning, but should be suitably sealed against the anticipated steam pressures. Finally, the exhaust chamber  92  may advantageously be oriented relatively sideways, so as to project laterally outward from the area of the cooking cavity, so that the exhaust chamber  92  may reside within the lateral space for the controls  14  without increasing the overall width of the unit  10 .  
         [0021]     Steam is generated in the steam chamber  60  and flows to the cooking cavity  30  and back along two different paths, denoted as the primary path  110  and the secondary path  120  for convenience. The primary path  110  is from the first steam compartment  76  of steam chamber  60 , through the primary holes  52  in the floor  50  of the cooking cavity  30 , into the cooking cavity  30 , and then back to the steam chamber  60 . The secondary path  120  is from the second steam compartment  80  of the steam chamber  60 , out the secondary holes  54  in the right sidewall  48  into the cooking cavity  30 , and then back to the steam chamber  60 . The flow of steam along both paths  110 , 120  is via natural convection. That is, the flow along the paths  110 , 120  is not caused by any sort of fan or other means typically associated with forced flow. It should be understood that natural convection does not preclude slight pressure differentials along the path, but there is not any propulsion mechanism located along either flow path  110 , 120 .  
         [0022]     The control of water supply is simple in the steamer  10 . First, the on/off switch  17  is advantageously mechanically linked to the pool drain valve  66 , such as via a connecting rod, so that when on/off switch  17  is in the on position, the valve  66  is closed, but when the on/off switch  17  is turned to the off position, the valve  66  is opened. Further, both the on/off switch and the door-open sensor  14  are electrically linked to the water valve  74  so that the water valve  74  is open when the unit  10  is on and the door  12  is closed, otherwise, the water valve  74  is closed. See  FIGS. 7-8 . When open, the water valve  74  advantageously supplies water to the pool  62  and the sprayer  95  at a continuous uniform rate such as a constant 0.12 gallons per minute. As can be seen, no water level sensor per se, such as a float valve, is required. Indeed, no control sensors are exposed to the water and/or steam inside the unit  10 .  
         [0023]     The control of the energization state (on or off) of the heating elements  20  is relatively simple in the steamer  10 . With reference to  FIG. 8 , the heating elements  20  are energized in normal operation whenever the four following conditions are met: on/off switch  17  indicates that the steamer  10  is to be on, door-open sensor  14  indicates the door  12  is closed, thermal fuses  24  have not tripped, and thermal sensor  22  indicates a temperature of the floor  70  below a predetermined threshold. If any of the four conditions are not met, then the heating elements  20  are not energized.  
         [0024]     In the prior art, steam generation from a cold start was fairly slow because a timer was engaged when the unit was turned on, and the timer was typically required to expire before the heating elements were energized. This timer controlled delay was for the purpose of allowing sufficient time to fill the pool to the desired level. In the preferred embodiments of the present invention, the initial steam generation from a cold start is much faster. In these embodiments, the heating elements  20  are energized immediately once the unit  10  is turned on (assuming the door  12  is shut) and water is simultaneously added to the pool  62 . Thus, both the heating elements  20  and the water in the pool  62  begin heating immediately once the unit  10  is turned on, helping to generate steam faster. As a result, the built-in delay of the timer is avoided. It should be noted that the use of the automatically resetting thermal sensor  22  allows undesirable degradation of the heating elements  20  due to overheating to be avoided, both during “normal” operation and during initial start-up.  
         [0025]     As discussed above, steam from the steam chamber  60  flows into the cooking cavity  30  via the primary holes  52  in the floor  50  and the secondary holes  54  in the right sidewall  48 . It is believed that substantial improvement in performance is achieved by directing steam into the cooking cavity  30  not only at the bottom of the cooking cavity  30 , but also directing steam into the cooking cavity  30  via at least one side of the cooking cavity  30 . In this manner, a portion of the steam enters low in the cooking cavity  30 , where it naturally rises to the top, while another portion of the steam is able to reach the middle of the cooking cavity  30  more directly. No known natural convection steamers were believed to provide such routing of the steam.  
         [0026]     However, the inventors also discovered that some tuning of the size and position of the relevant steam openings  52 ,  54  may also help improve the performance. To this end, experiments have been run, as described further below, that indicate routing the side-entry steam to the lower portion of the cooking cavity  30 , but not all the way to the bottom, yields the best results. To run the experiment, five trays were loaded into the cooking cavity  30 , each with a mass of a block of ice having a thermocouple embedded therein. The primary holes  52  in the floor were as shown in  FIG. 4 , with ninety-four ½ inch diameter holes  52 , two 1.0 inch diameter holes  52 , and six ½ inch by 2 inch obround holes  52  along the edge. The secondary holes  54  in the sidewall  48  were arranged in an array of three rows of slots, ¼ inch by 4 inches, with twenty-three slots  54  in each row. The elapsed time between the first tray to reach 180° and the last tray to reach 180° was then measured for various slot configurations. It is believed that users of the steamer  10  desire that there be as little variation in time between the trays as possible, meaning that the minimum elapsed is considered the best performance. The results of the experiments are presented below:  
                                                                                                                                   Top Row   Middle Row   Bottom Row                Test   upper   lower   upper   lower   upper   lower   Result                    A   ∘   ∘   ∘   ∘   ∘   ∘   12   min       B   x   ∘   ∘   x   ∘   ∘   9¾   min       C   x   x   ∘   ∘   ∘   ∘   4¼   min       D   x   x   ∘   ∘   x   x   5   min       E   x   x   ∘   x   ∘   ∘   5   min       F   x   x   x   ∘   ∘   ∘   4   min       G   x   ∘   x   ∘   ∘   ∘   7   min       H   x   x   ∘   ∘   ∘   x   4   min                 ∘ = upper/lower half of slot open            x = upper/lower half of slot closed             
 
         [0027]     As can be seen, the best results were achieved when the top row of slots  54  were closed off completely, the middle row were completely open, and the bottom row of slots  54  were half open (upper half) and half closed (lower half). This is the arrangement of slots  54  is shown in  FIG. 5 . It should be noted that the best results correspond to a ratio of cross-sectional area of the primary holes  52  to the cross-section of the secondary holes  54  of approximately 2:3.  
         [0028]     Merely by way of example, a steamer  10  of the present invention can be built using a generally rectangular cooking cavity  30  with dimensions of 13½ inches wide by twenty-four inches deep by 15½ inches high resulting in a volume of approximately three cubic feet; a pool  62  with dimensions of 16¼ inches wide by twenty-two inches deep by ⅝ inches high and a 2° sloping floor and holding approximately ⅞ of a gallon of water; a first steam compartment  76  of general dimensions 16¼ inches wide by twenty-two inches deep by 2½ inches high; a second steam compartment  80  of general dimensions of 2½ inches wide by 16¼ inches deep by sixteen inches high; primary and secondary holes  52 ,  54  as described above, six electrical heating elements  20  of 1850 watts; and a thermal sensor  22  designed to trip at a temperature of 420°. Such a unit is appropriate for holding five common trays of food.  
         [0029]     While not shown, the steamer  10  may optionally include a condensate tray (not shown) attached to the door  12  for catching condensate. Such a condensate tray may optionally include a drain hole disposed close to the door&#39;s pivot axis that feeds water therein into a drip tray attached to the main housing of the steamer  10 . A drain line may connect this drip tray to the drain line  99  if desired. Further, the condensate tray may be made reversible by having suitably mounted drain holes toward each end, with the drain hole located farthest from the door&#39;s pivot being suitably plugged.  
         [0030]     In some embodiments, the door-open sensor  14  may take the form of a conventional contact switch that is activated by an arm (not shown) that is pivotally mounted to the steamer&#39;s housing at a midpoint of the arm&#39;s length, above the cooking cavity. A pin attached to the arm may advantageously extend out the midpoint of the cooking cavity, so that the door pushes against the pin at the same point regardless of the mounting orientation of the door.  
         [0031]     Further, in some embodiments, the exhaust chamber  92  of the exhaust control system  90  may include backup overflow prevention in the form of an overflow port  97  placed high in the front portion of exhaust chamber  92 . This overflow port may be vented to atmosphere with the understanding that some steam may be vented to atmosphere via the overflow port  97 . The overflow port  97  may advantageously be located at a height above the highest expected water level in normal operation, but below the lowest level of the opening for the door  12 , so that water should not flow out the front of the unit  10  even if drain port  98  or drain line  99  somehow become clogged.  
         [0032]     The right sidewall  48  may be made to be removable from the unit  10 , if desired, such as by having the right sidewall hang on suitable mounting knobs. In addition, while the discussion above has been in terms of the secondary holes  54  being in the right sidewall  48 , this is not required by all embodiments. In some embodiments, the secondary holes  54  may instead be in the rear sidewall  44 , or the left sidewall  42 , or some combination of the sidewalls  42 , 44 , 48 . As such, references in the accompanying claims to the steam flowing into the cooking cavity  30  via a sidewall  48 , and the like, should be construed to cover steam flowing into the cooking cavity  30  via the appropriate holes in one or more of the sidewalls  42 , 44 , 48 , unless otherwise indicated. However, it is believed advantageous if the holes for routing steam to the cooking cavity are located only in the floor  50  and two or less sidewalls, preferably only one sidewall.  
         [0033]     The discussion above has used water as an illustrative fluid in the pool  62  for generating steam. It should be understood that pure water is not required; for instance, suitable agents may be added to the water to help prevent scaling, as is known in the art. As such, the term “water” as used herein is meant to encompass any fluid that may be used generate a hot vapor (called “steam” herein) suitable for contact with food.  
         [0034]     It should be understood that the discussion above has focused on those areas relevant to one of ordinary skill in the art to understand and practice the present invention, and some related features. However, the discussion above has omitted several details of the steamer not relevant to understanding the present invention, as these details are understood by one of ordinary skill in the art without explicit explanation thereof.  
         [0035]     Although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.