Abstract:
A sub-cavity with an integral steam heater is provided in a ceiling portion of a heating chamber. A gas in the heating chamber is sucked into a blower through a gas suction opening and sent into an external circulation path. The gas passing through the external circulation path sucks steam from a steam producing device by using steam suction ejector. The gas that took the steam in enters into the sub-cavity, as the gas return opening, from the external circulation path. The steam contained in the gas is heated by the steam heater in the sub-cavity to become an overheated state, spouted out downward from spouting holes in the bottom face of the sub-cavity, and collides with an object to be heated. The external circulation path is constructed from a pipe with a circular cross-section.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a steam cooker. 
   2. Description of the Related Art 
   Conventionally, there have been proposed various steam cookers for performing cooking by using steam. Examples of the steam cookers can be seen in Patent Publication 1 and Patent Publication 2. For example, Patent Publication 1 discloses a steam cooking apparatus that blows steam into a food tray. Patent Publication 2 discloses a heat cooking apparatus that supplies superheated steam into a cooking chamber or makes steam in the cooking chamber overheated by radiation heating. 
   Patent Publication 1: JP-U 03-67902 (1991) (p. 6 to 11, FIGS. 1 to 3) 
   Patent Publication 2: JP-A 08-49854 (1996) (p. 3 to 4, FIGS. 1 to 7) 
   The steam cooking apparatus disclosed in Patent Publication 1 is an apparatus for professional use in which steam is supplied to a plurality of food trays from a steam supply pipe. The configuration is not preferable from the aesthetic viewpoint since the steam supply pipe is provided uncovered in the cooking chamber, and is not suitable for a cooker for home use. The applicable range of steam jet is limited due to the shape of the steam supply pipe and it is difficult to uniformly jet steam to foods (i.e., object to be heated) in the cooking chamber. Since steam, which has heated food, is exhausted via a steam exhaust port straightaway or condensed to be drained through a drain pipe, in order to continue cooking, a large quantity of steam has to be continuously supplied from the outside, and a boiler of high steam generating capability is required. Necessitating such large amount of energy, this kind of steam cooking apparatus is not suitable for home use. 
   In the heat cooking apparatus disclosed in Patent Publication 2, steam is not blown toward foods whereas it is generated in the apparatus itself. Since an external boiler is not used, the apparatus can be used in a home. Steam is circulated and used in the cooking chamber, so that energy efficiency is high. However, foods are cooked with steam in a manner such that the object to be heated is cooked by being surrounded by steam which is introduced into the cooking chamber or generated in the cooking chamber. The ability of the cooking apparatus is insufficient in terms of promptly transmitting large quantity of heat to foods to cook them quickly. 
   The present invention has been made in consideration of the above points, and provides a steam cooker of high heating efficiency. The present invention also reduces the quantity of dissipated heat without being used for cooking. 
   SUMMARY OF THE INVENTION 
   A steam cooker according to the present invention comprises: a cooking chamber in which foods are put; an external circulation path for blowing gas sucked from the cooking chamber via a suction port into the cooking chamber via a blowhole so that the gas strikes the foods in the cooking chamber; a blower for generating gas current traveling from the suction port to the blowhole in the external circulation path; and a steam generator for supplying steam to the gas passing through the external circulation path. Herein, the external circulation path is provided with a steam ejector in a position downstream of the blower for sucking steam generated by the steam generator. 
   With this configuration, the gas in the cooking chamber circulates via the external circulation path and steam is supplied in the external circulation path in the circulation process. Consequently, different from the case of continuously blowing steam in one-way, without requiring the steam generator of high performance, the steam cooker having excellent energy efficiency and suitable for home use can be achieved. Since forced circulation gas current which is sucked from the cooking chamber into the suction port and then blown into the cooking chamber via the blowhole is generated, heating of the object to be heated is accelerated. Further, since the external circulation path sucks steam generated by the steam generator by an ejector provided in a position downstream of the blower, the steam can be promptly sucked without applying pressure on the steam generator, and the steam can mingle with gas current efficiently. 
   In the above steam cooker according to the present invention, the external circulation path is provided with a bypass for the gas to bypass the steam ejector. 
   With this configuration, pressure loss of the circulation system is reduced, so that the fan can be driven efficiently and the jet force of steam also is strengthened. 
   In the above steam cooker according to the present invention, the external circulation path is constructed with pipes with circular cross section. 
   Being constructed with pipes of circular cross-section, the surface area of the external circulation path is smaller than a duct having a rectangular cross-section. Consequently, heat dissipation from the external circulation path is reduced, and the energy efficiency improves. 
   In the above steam cooker according to the present invention, the blower includes a centrifugal fan. 
   With this configuration, as compared with the case of using a propeller fan, the flow velocity of gas current blown from the blower can be made higher. Thus, the diameter of the pipe constructing the external circulation path can be decreased. By decreasing the diameter of the pipe, the surface area of the external circulation path is narrowed so that the heat dissipation can be reduced and energy efficiency can be further improved. Since the centrifugal fan can generate higher pressure as compared with a propeller fan, the force of jet from the blowhole can be increased. As a result, the steam jet is elongated, and the foods can be heated intensely. 
   In the above steam cooker according to the present invention, a motor for driving the centrifugal fan is a direct current motor. 
   With this configuration, the centrifugal fan can be rotated at high speed and gas current of extremely high flow velocity can be obtained. 
   The steam cooker according to the present invention comprises: a cooking chamber in which foods are put; an external circulation path for blowing gas sucked from the cooking chamber via a suction port into the cooking chamber via a blowhole so that the gas strikes the foods in the cooking chamber; a blower for generating gas current traveling from the suction port to the blowhole in the external circulation path; and a steam generator for supplying steam to the gas passing through the external circulation path, wherein the external circulation path is provided with a steam ejector in a position downstream of the blower for sucking steam generated by the steam generator, and the gas in the cooking chamber circulates via the external circulation path and steam is supplied in the external circulation path in the circulation process. Therefore, different from the case of continuously blowing steam in one-way, without requiring the steam generator of high performance, the steam cooker having excellent energy efficiency and suitable for home use can be achieved. Since forced circulation gas current which is sucked by a suction port from the cooking chamber and then blown into the cooking chamber via the blowhole is generated, heating of the object to be heated is accelerated. Further, since the external circulation path sucks steam generated by the steam generator by an ejector provided in a position downstream of the blower, the steam can be promptly sucked without applying pressure on the steam generator, and the steam can mingle with gas current efficiently. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a steam cooker according to an embodiment of the present invention. 
       FIG. 2  is a perspective view of the steam cooker in a state where a door of a cooking chamber is open. 
       FIG. 3  is a basic structural diagram of an internal mechanism. 
       FIG. 4  is a top view of the cooking chamber. 
       FIG. 5  is a vertical sectional view of a steam generator. 
       FIG. 6  is a horizontal sectional view of the steam generator. 
       FIG. 7  is a control block diagram. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the present invention will be described below with reference to the drawings. 
   A steam cooker  1  comprises a cabinet  10  of a rectangular parallelepiped shape. In a front face of the cabinet  10 , an operation panel  11  is provided on an upper part and a door  12  is provided below the operation panel  11 . The door  12  swings around its lower end as a center in a vertical plane. A user grips a handle  13  provided on an upper part of the door  12  and pulls to the front, thereby enabling a posture of the door  12  to be changed by 90° from a perpendicular closed state shown in  FIG. 1  to a horizontal open state shown in  FIG. 2 . Most part of the door  12  is a window  14  in which a pane of heat-resistive glass is fit. 
   When the door  12  is opened, two compartments appear as shown in  FIG. 2 . A large compartment on the left side is a cooking chamber  20 , and a small compartment on the right side is a water tank chamber  70 . The structures and accompanying components of the cooking chamber  20  and the water tank chamber  70  will be described below with reference to  FIG. 3  and subsequent drawings. 
   The cooking chamber  20  has a rectangular parallelepiped shape and its whole front side facing the door  12  is an opening. The other faces of the cooking chamber  20  and the inner face of the door  12  are formed with stainless steel plates. The periphery of the cooking chamber  20  and the inside of the door  12  are thermal-insulated. A pan  21  made of a stainless steel plate is disposed on the floor face of the cooking chamber  20 , and a rack  22  made of a stainless steel wire, on which foods  90  are placed, is disposed on the pan  21 . 
   Gas in the cooking chamber  20  (Usually, the gas in the cooking chamber  20  is air. When steam cooking starts, the air is replaced by steam. In the specification, therefore, it is expressed as “gas” not “air”.) is circulated via an external circulation path  30 . Beside a side wall of the cooking chamber  20 , an gas current control plate  23  (also made of a stainless steel plate) extending downward from the ceiling to a position near the floor face is disposed in parallel with the side wall. A gap between the lower end of the gas current control plate  23  and the side wall at the back serves as a suction port  24  facing downward, which leads gas into the external circulation path  30 . 
   Gas sucked from the suction port  24  travels behind the gas current control plate  23  toward a blower  25  provided in an upper part on the outside of the cooking chamber  20 . The blower  25  includes a centrifugal fan  26 , a fan casing  27  for housing the centrifugal fan  26 , and a motor (not shown) for rotating the centrifugal fan  26 . A sirocco fan is used as the centrifugal fan  26 . As the motor for rotating the centrifugal fan  26 , a direct current motor, which is capable of rotating at high speed, is used. 
   The external circulation path  30  connected to the discharge port of the fan casing  27  is constructed by combining pipes each having a circular shape in its cross section. A first pipe  31  projects in the horizontal direction from the fan casing  27 . An exhaust port  32  is provided at the end of the first pipe  31 . An elbow-shaped second pipe  33  is connected a little downstream of the exhaust port  32  of the first pipe  31 . A horizontal part of the second pipe  33  extends in an upper part of a steam generator  50  (which will be described in detail later) and forms a steam ejector  34 . The discharge end of the second pipe  33  is tapered, thereby becoming an inner nozzle of the steam ejector  34 . An outer nozzle  35  of the steam ejector  34  extends from the side of the steam generator  50  to the downstream, and the discharge end of the outer nozzle  35  is tapered into a nozzle shape. 
   The nozzle-shaped discharge end of the outer nozzle  35  is fit in a third pipe  36  of the external circulation path  30  on the downstream of the steam ejector  34 . An end of the third pipe  36  is bulged so as to surround the outer nozzle  35 , and a rear ejector  37  is formed in this portion. The nozzle-shaped discharge end of the outer nozzle  35  of the steam ejector  34  plays the role of an inner nozzle in the rear ejector  37 . A bypass  38  branched from the first pipe  31  is connected to the rear ejector  37 . The bypass  38  is also formed with pipes with circular cross section. As shown in  FIG. 4 , two bypasses  38  are provided and gas flows into the rear ejector  37  symmetrically in a horizontal plane. 
   The other end of the third pipe  36  is connected to a sub-cavity  40  serving as a gas return port  39  for returning gas into the cooking chamber  20 . The sub-cavity  40  is provided above the ceiling of the cooking chamber  20  and in a portion corresponding to the center of the ceiling. The sub-cavity  40  has a circular shape in plan view, and a steam heater  41  as gas heating means is disposed in the sub-cavity  40 . The steam heater  41  takes the form of a sheathed heater. The bottom of the sub-cavity  40  is formed with a metal panel (which also serves as the top plate of the cooking chamber  20 ) as a part of the ceiling wall of the cooking chamber  20 . Plurality of perforations, which collectively forms a blowhole  43 , are disposed on a bottom panel  42  of the sub-cavity  40  so as to spread two-dimensionally or three-dimensionally in the almost whole panel. The upper and lower faces of the bottom panel  42  are finished in dark color by a surface processing such as coating. The bottom panel  42  may be made of a metal that changes into dark color with use. Alternatively, the bottom panel  42  may be constructed with a ceramic component of dark color. 
   At an upper corner of the cooking chamber  20 , a steam bleeding port  44  is formed. A motor-operated damper  45  is disposed at the end of the first pipe  31 . The damper  45  selectively closes the exhaust port  32  and the inlet of the second pipe  33 . 
   The structure of the steam generator  50  will now be described with reference to  FIGS. 5 and 6 . The steam generator  50  has a cylindrical pot  51  with its center line disposed perpendicularly. The top portion of the pot  51  is closed and the steam ejector  34  is formed thereon as described above. 
   The bottom portion of the pot  51  is formed in a funnel shape, and a drain pipe  52  extends downward therefrom. The lower end of the drain pipe  52  is connected to a drain pipe  53  disposed so as to be slightly inclined from the horizontal line. An end of the drain pipe  53  extends through the side wall of the cooking chamber  20  to a position above the pan  21 . A drain valve  54  and a water-level sensor  55  are provided on some midpoints of the drain pipe  52 . 
   The water in the pot  51  is heated by a steam generation heater  56 , which is closely attached to the outer surface of the pot  51 . The steam generation heater  56  takes the form of an annular sheathed heater. A heat transfer unit  60  is disposed inside the pot  51  at a height almost the same as the steam generation heater  56 . 
   The heat transfer unit  60  is constructed with a ring  61  closely attached to the inner surface of the side wall of the pot  51 , and a plurality of fins  62  disposed radially in the ring  61 . The ring  61  and the fins  62  are integrated by a method such as extrusion, welding or soldering. Each of the ring  61  and the fins  62  has a predetermined length in the axial direction of the pot  51 . 
   Water is supplied into the pot  51  via a water supply pipe  63 . The water supply pipe  63  enters the pot  51  from a portion near the bottom portion of the pot  51  and extends upward between the fins  62 . The upper end of the water supply pipe  63  is slightly higher than that of each of the fins  62 . As shown in  FIG. 6 , when the fins  62  are regarded as spokes of a wheel, the water supply pipe  63  is disposed at the position of a hub. An end face of each of the fins  62  is brought into contact with the external surface of the water supply pipe  63  to thereby transfer heat to the water supply pipe  63  via the fins  62 . 
   The pot  51 , the heat transfer unit  60  and the water supply pipe  63  are each made of a metal having high heat conductivity. Copper or aluminum is recommendable for their high heat conductivity. Copper or its alloy, however, is susceptible to patina. Consequently, although the heat conductivity is lower than that of copper or copper alloy, stainless steel, which is free from patina, can be an alternative. 
   A funnel-shaped inlet  64  is formed at an end of the water supply pipe  63 . A flushing pipe  65  is connected in a position a little downstream of the inlet  64 . The flushing pipe  65  is connected to the drain pipe  53  via a flushing valve  66 . 
   To the water supply pipe  63 , not only the flushing pipe  65  but also a water head pipe  67  having an inverted J shape are connected. The other end of the water head pipe  67  is connected to the drain pipe  53 . 
   A water tank  71  having a thin rectangular parallelepiped shape is inserted into the water tank chamber  70 . A water supply pipe  72  having an elbow shape, extended from the water tank  71 , is connected to the inlet  64  of the water supply pipe  63 . A pump  73  pumps water in the water tank  71  via the water supply pipe  72 . The pump  73  is constructed with a pump casing  74  formed in the base portion of the water supply pipe  72 , an impeller  75  housed in the pump casing  74 , and a motor  76  for transmitting power to the impeller  75 . The motor  76  is fixed to the side of the cabinet  10 . When the water tank  71  is set in an appropriate position, the motor  76  is electromagnetically connected to the impeller  75 . 
   A rail  77  having a trough shape, which supports the water tank  71 , is fixed on the floor of the water tank chamber  70  (see  FIG. 2 ). The tank supporting plane of the rail  77  is at the same level as the inner face of the door  12  opened horizontally. Consequently, the user puts the water tank  71  on the door  12  in the horizontal posture and pushes it along the rail  77 , thereby enabling the water tank  71  to be smoothly set in a predetermined position in the water tank chamber  70 . When the user opens the door  12  horizontally and pulls the water tank  71  out, the water tank  71  pulled out of the water tank chamber  70  is supported by the door  12 . Therefore, the user does not have to support the water tank  71  with his/her hand for pulling it out. 
   A controller  80  shown in  FIG. 7  controls the operation of the steam cooker  1 . The controller  80  includes a microprocessor and a memory and controls the steam cooker  1  in accordance with an installed program. A control state is displayed on a display device in the operation panel  11 . An operation instruction is input to the controller  80  via various operation keys disposed on the operation panel  11 . A sound generator for generating sound signals is also disposed on the operation panel  11 . 
   To the controller  80 , not only the operation panel  11 , but also the blower  25 , the steam heater  41 , the damper  45 , the drain valve  54 , the water-level sensor  55 , the steam generation heater  56 , the flushing valve  65  and the pump  73  are connected. In addition, a water-amount sensor  81  for measuring the amount of water in the water tank  71 , a temperature sensor  82  for measuring the temperature in the cooking chamber  20 , and a moisture sensor  83  for measuring moisture in the cooking chamber  20  are connected to the controller  80 . 
   The steam cooker  1  operates as follows. First, the user pulls the water tank  71  out of the water tank chamber  70 , and supplies water into the tank via a water supply port (not shown). The user pushes the water tank  71  fully filled with water back into the water tank chamber  70  and sets it in the predetermined position. Confirming that the tip of the water supply pipe  72  is firmly connected to the inlet  64  of the water supply pipe  63 , the user presses the power key on the operation panel  11  to turn the power on. The motor  76  of the pump  73  rotates and water supply to the steam generator  50  starts. At this time, the drain valve  54  and flushing valve  65  are closed. 
   Water spouts from the tip of the water supply pipe  63  like a fountain and drops to the bottom portion of the pot  51  while wetting the fins  62  of the heat transfer unit  60 . The water accumulates from the bottom of the pot  51 . When the water-level sensor  55  senses that the water level reaches the half of the length of the heat transfer unit  60 , the water supply is temporarily stopped. The water level in the pipe on the inlet side of the water head pipe  67  reaches the same level as that in the pot  51 . 
   After the above-mentioned amount of water is supplied into the pot  51 , the steam generation heater  56  is energized. The steam generation heater  56  heats water in the pot  51  from outside the sidewall of the pot  51 . When the side wall of the pot  51  is heated, the heat is conducted to the heat transfer unit  60  and transferred from the heat transfer unit  60  to the water. Since the height at which the steam generation heater  56  is disposed and the height at which the heat transfer unit  60  is disposed almost coincide with each other, heat is directly transferred from the steam generation heater  56  to the heat transfer unit  60  and efficiency of heat transfer is excellent. 
   The heat transfer unit  60  in which the plurality of fins  62  are disposed radially has a wide heat transfer area, so that water in the pot  51  is quickly heated. The fins  62  disposed radially support the pot  51  from the inside like the spokes of a wheel and the strength of the steam generator  50  is increased. 
   Concurrently with energization of the steam generation heater  56 , energization of the blower  25  and the steam heater  41  is started. The blower  25  sucks gas in the cooking chamber  20  from the suction port  24  and blows the gas into the external circulation path  30 . Since the centrifugal fan  26  is used to blow the gas, the flow velocity of gas current is higher as compared with a propeller fan. Moreover, the centrifugal fan  26  is rotated at high speed by the DC motor, the flow velocity of gas current is considerably high. Consequently, the pipes of the external circulation path  30  may have a circular cross section and, moreover, a small diameter. As compared with the case of forming the external circulation path  30  by a duct having a rectangular cross section, the surface area of the external circulation path  30  can be made smaller. Therefore, irrespective of the fact that hot gas passes the inside, heat dissipation from the external circulation path  30  is reduced, the proportion of the quantity of heat dissipated without being used for cooking is lowered, and the energy efficiency of the steam cooker  1  is improved. Also in the case where the external circulation path  30  is surrounded with a heat insulating material, a smaller amount of the heat insulating material is sufficient. 
   At this time, the damper  45  opens the inlet of the second pipe  33  of the external circulation path  30  and closes the exhaust port  32 . Steam flows from the first pipe  31  to the second pipe  33 , the third pipe  36 , and then the sub-cavity  40  that serves as the gas return port  39 . The steam is heated in the sub-cavity  40  by the steam heater  41  and, after that, is blown downward from the blowhole  43 . 
   When the water in the pot  51  boils, saturated steam at a pressure of 1 atm at 100° C. is generated. The saturated steam is sucked into circulating gas current flowing in the external circulation path  30  at the steam ejector  34 . Since an ejector structure is employed, the saturated steam is promptly sucked in. Because of the ejector structure, the circulating gas current does not apply pressure to the steam generator  50  and does not hinder discharge of the saturated steam. 
   In the rear ejector  37 , gas is sucked from the bypass  38  by the gas current blown from the outer nozzle  35  of the steam ejector  34 . By the existence of the bypass  38  which bypasses the steam ejector  34  to directly introduce gas flow, pressure loss of a circulation system is reduced and the centrifugal fan  26  can be driven efficiently. The gas containing saturated steam flows from the rear ejector  37 , and then rushes into the sub-cavity  40  at high speed. 
   The gas containing saturated steam is heated to 300° C. by the steam heater  41  in the sub-cavity  40 , and becomes superheated steam at this point. The steam expands as the temperature rises and blows with force from the blowhole  43 . 
   A downward gas current formed in the center portion of the cooking chamber  20  (the place where foods are to be put) turns upward outside thereof at the bottom the cooking chamber  20  to form convection of gas. The gas is sucked into the suction port  24  again and returns from the external circulation path  30  to the sub-cavity  40 . In such a manner, circulation of the gas in the cooking chamber  20 , that goes out to the external circulation path  30  and returns to the cooking chamber  20 , is repeated. 
   As time elapses, the proportion of steam in the gas increases. Excessive gas is discharged from the steam bleeding port  44  to the outside of the cooking chamber  20 . If the gas containing steam is discharged directly into the cabinet  10 , dew condensation develops inside the cabinet  10  to cause undesirable results such as rusting or electric leakage. If the gas is discharged directly to the outside of the cabinet  10 , dew condensation spreads over a kitchen wall and the wall becomes susceptible to mold attack. To avoid this, the gas is introduced into a maze-like dew condensation path (not shown) provided in the cabinet  10  to condensate dew, and then discharged outside of the cabinet  10 . Water resulted from the dew condensation is collected in the pan  21  together with the water resulted from other causes, and dumped after cooking. 
   When the gas containing superheated steam starts blowing, the temperature in the cooking chamber  20  rapidly rises. When the temperature sensor  82  senses that the temperature in the cooking chamber  20  reaches cooking range, the controller  80  displays the message on the operation panel  11  and beeps a sound. Being informed by sound or display that cooking is enabled, the user opens the door  12  and puts foods  90  into the cooking chamber  20 . 
   When the user starts to open the door  12 , the controller  80  changes the state of the damper  45  to close the inlet of the second pipe  33  and to open the exhaust port  32 . The gas in the cooking chamber  20  is sucked by the blower  25  and, then, exhausted from the exhaust port  32 . When the inlet of the second pipe  33  is closed, the superheated steam from the blowhole  43  is not blown, so that the user does not burn his/her face or hands with the superheated steam. The damper  45  maintains its state of opening the exhaust port  32  and closing the inlet of the second pipe  33  while the door  12  is open. 
   If gas exhaustion from the exhaust port  32  starts when the blower  25  is stopped, a time lag occurs until a stationary gas flow is obtained. In the case of the present invention, however, the blower  25  is already in operation, and no time lag takes place. Moreover, the current circulated in the cooking chamber  20  and the external circulation path  30  is directly turned into exhaust current from the exhaust port  32 , there is also no time lag for changing the direction of current. Thus, the steam in the cooking chamber  20  can be exhausted without delay and the time required to enable the door  12  to be opened can be shortened. 
   At the time of exhausting steam from the cooking chamber  20 , the second pipe  33  is closed and supply of steam to the cooking chamber  20  is stopped. Consequently, the pressure or quantity of the steam in the cooking chamber  20  decreases rapidly and the time required to enable the door  12  to be opened can further be shortened. 
   The signal that the user starts to open the door  12  can be transmitted to the controller  80 , for example, as follows. A latch for maintaining the door  12  in a closed state is provided between the cabinet  10  and the door  12 , and a latch lever for disengaging the latch is provided so as to be exposed from the handle  13 . A switch which opens/closes in response to the motion of the latch or latch lever is disposed inside the door  12  or the handle  13 . When the handle  13  and the latch lever are gripped together for unlocking, a signal is transmitted from the switch to the controller  80 . 
   Like the gas bled from the steam bleeding port  44 , the gas discharged from the exhaust port  32  also contains a large quantity of steam, and the gas discharged as it is may cause problems. Therefore, the gas exhausted from the exhaust port  32  is also passed through the maze-like dew condensation path provided in the cabinet  10  to remove moisture, and only the dried gas is discharged to the outside of the cabinet  10 . Water resulted from the dew condensation is collected in the pan  21  together with the water resulted from other causes, and dumped after cooking. 
   When the user sets foods  90  on the rack  22  and closes the door  12 , the damper  45  is reset to the state of opening the inlet to the second pipe  33  and closing the exhaust port  32 . Consequently, blowing of the superheated steam from the blowhole  43  is restarted and cooking of foods  90  starts. 
   The superheated steam heated to about 300° C. and blown downward from the blowhole  43  strikes foods  90  and transfers heat to foods  90 . Through this process, the steam temperature decreases to about 250° C. The superheated steam, which comes into contact with the surface of foods  90 , discharges latent heat resulted from dewing on the surface of foods  90 . Foods  90  are heated also by the latent heat. 
   Since foods  90  is heated while circulating the gas in the cooking chamber  20 , the energy efficiency of the steam cooker  1  is high. The gas containing superheated steam is blown downward from the plurality of blowhole  43  disposed so as to spread two-dimensionally or three-dimensionally in the almost whole surface of the bottom panel  42  of the sub-cavity  40 , so that the superheated steam strikes the whole top face of foods  90 . Because the steam that strikes foods  90  is superheated and its striking area is wide, the heat of the superheated steam is quickly, efficiently transmitted to foods  90 . Heating of the gas supplied into the sub-cavity  40  by the steam heater  41  makes the gas expand, and this expansion causes increased jet force and increased collision speed with foods  90 . Thus, foods  90  are heated more quickly. 
   Since the centrifugal fan  26  can generate higher pressure as compared with a propeller fan, the force of jet from the blowhole  43  can be increased. As a result, the jet of superheated steam becomes longer and foods  90  can be heated intensely. And the centrifugal fan  26  is rotated at high speed by the DC motor and gas is blown strongly, the effect appears more conspicuously. The strong blowing power of the blower  25  can also be exploited to promptly exhaust the gas from the exhaust port  32  at the time of opening the door  12 . 
   The superheated steam jet downward strikes foods  90  and, after that, changes its direction to the upward. Since the steam, particularly, superheated steam is lighter than air, the direction change occurs naturally and it causes convection current in the cooking chamber  20 . By this convection current, while maintaining the temperature in the cooking chamber  20 , the superheated steam just heated in the sub-cavity  40  can be continuously made strike foods  90 , and a large quantity of heat can be promptly transmitted to foods  90 . 
   The suction port  24  is positioned at a lower part of the side wall of the cooking chamber  20  (at or lower than the height of foods  90 ). The steam jet from the blowhole  43  travels straight without being deflected, strikes foods  90  and is sucked into the suction port  24 . Consequently, the capability of heat transmission to foods  90  is maintained at a high level. Since the steam jet from above is sucked to the lower part of the side wall, when the door  12  is opened, the steam will not flow toward the user aggressively, making the safety level of the steam cooker  1  high. 
   As the suction port  24  faces downward, the lateral force acting on the steam jet becomes modest and the steam jet can be prevented from being deflected. Even if oil bubbles from the surface of foods  90 , the oil is not so easily sucked into the suction port  24 , that the inner surface of the blower  25  and the external circulation path  30  is prevented from oil accumulation. 
   The top face of the bottom panel  42  of the sub-cavity  40  wears dark color and absorbs radiation heat generated by the steam heater  41  well. The radiation heat absorbed by the bottom panel  42  is radiated from the bottom face of the bottom panel  42  of dark color into the cooking chamber  20 . Consequently, temperature rise of the sub-cavity  40  and the outer surface of the sub-cavity  40  is suppressed and the safety of the steam cooker  1  is improved. On the other hand, the radiation heat of the steam heater  41  is transmitted to the cooking chamber  20  via the bottom panel  42  and the cooking chamber  20  is heated more efficiently. The shape in plan view of the bottom panel  42  may be circular or rectangular similar to that of the cooking chamber  20 . The ceiling of the cooking chamber  20  may also serve as the bottom panel of the sub-cavity  40 . That is, the ceiling of the cooking chamber  20  and the bottom panel of the sub-cavity  40  may be common to each other. 
   In the case where foods  90  are meat, as the temperature rises, oil may drip. Also, if the food  90  is liquid in a vessel, it may spill during boiling. Dripped or spilt liquid is received by the pan  21  and dumped after cooking. 
   In accordance with steam generation by the steam generator  50 , the water level in the pot  51  lowers. When the water-level sensor  55  senses that the water level has lowered to a predetermined level, the controller  80  restarts operation of the pump  73 . The pump  73  pushes up the water in the water tank  71  and supplies water as a compensation for evaporated water. When replenishment water passes through the water supply pipe  63 , the heat of the steam generation heater  56  is transferred to the replenishment water via the fins  62  of the heat transfer unit  60 . The replenishment water is thereby preheated and time required for the replenishment water to reach the boiling point is shortened. 
   The replenishment water overflowing from the upper end of the water supply pipe  63  is sprinkled on the upper part of the fins  62  out of water. The temperature of the part of the fins  62  exposed out of water is higher than that of the part in the water, so that the water sprinkled on the fins  62  instantaneously boils and evaporates, thereby increasing the steam pressure in the pot  51 . Consequently, the steam blows forcefully from the outer nozzle  35 , flows into the sub-cavity  40 , and energizes jet of the superheated steam from the blowhole  43 . Therefore, strong jet of the superheated steam is generated each time water is supplied. 
   At the time when the water-level sensor  55  senses that the water level in the pot  51  rises to the predetermined level, the controller  80  stops driving of the pump  73 . In such a manner, the pump  73  intermittently performs water supply during cooking. The temperature of the part of the fins  62  exposed out of water temporarily drops each time water is sprinkled, and resumes its desired level soon after water sprinkling is stopped. Thereby, each time water is sprinkled, the water evaporates rapidly, and the jet power of the superheated steam is increased. 
   One embodiment of the present invention has been described above. The present invention can be further variously modified without departing from the gist of the present invention. 
   INDUSTRIAL APPLICABILITY 
   By carrying out the present invention, a steam cooker particularly suitable for home use can be obtained.