Abstract:
A steam oven includes a heating chamber or boiler in which steam is generated. Water fill and refill operations of the heating chamber are controlled in attempt to account for water expansion and/or to enable substantially regular production of steam.

Description:
TECHNICAL FIELD 
   The present application relates generally to steam ovens used to steam food products, and more particularly to a steam oven including a low pressure steam generator with a controlled fill process. 
   BACKGROUND 
   Steam cookers have been successfully employed by restaurants, hospitals and other food service operations to prepare quickly and conveniently large quantities of food. Many such cookers are used in high volume situations. It is desirable to provide steamers with reduced maintenance requirements. It is also desirable to provide steamers that have the ability to produce steam on a relatively uninterrupted basis. 
   SUMMARY 
   In one aspect, a steam cooker includes a heating chamber defining a volume for holding water, the heating chamber including a water inlet and a steam outlet, a heating unit associated with the heating chamber in order to heat water in the heating chamber so as to generate steam and a steam path from the steam outlet to a steam cooking chamber. A heating chamber control system is associated with the heating chamber for controlling water filling and heating operations of the heating chamber. During an initial fill operation of the heating chamber the heating chamber control system operates such that water flow into the heating chamber is at least temporarily stopped or restricted based at least in part upon temperature of the water in the chamber. 
   In another aspect, a steam cooker includes a heating chamber control system associated with the heating chamber for controlling water filling and heating operations of the heating chamber. During normal steaming operations water level varies between a higher level and a lower level in accordance with steam exiting the heating chamber and replacement water being added back to the heating chamber. During an initial fill operation of the heating chamber the heating chamber control system operates to control water flow into the tank and heating of the water so that a substantial majority of water volume expansion due to temperature increase occurs prior to water level in the tank reaching the higher level. 
   In a further aspect, a steam cooker includes a heating chamber control system associated with the heating chamber for controlling water filling and heating operations of the heating chamber. During normal steaming operations water level varies between a higher level and a lower level in accordance with steam exiting the heating chamber and replacement water being added back to the heating chamber. During an initial fill operation of the heating chamber the heating chamber control system operates to control water flow into the tank and heating of the water so that when the water level initially reaches the higher level a temperature of the water is at least 190° F. 
   In another aspect, a steam cooker includes a heating chamber defining a volume for holding water, the heating chamber including a water inlet and a steam outlet. A heating unit is associated with the heating chamber in order to heat water in the heating chamber so as to generate steam. A steam path leads from the steam outlet to a steam cooking chamber. A heating chamber control system is associated with the heating chamber for controlling water filling and heating operations of the heating chamber in reference to a first water level, a second water level and a third water level, the second water level above the first water level and below the third water level. During an initial fill operation of the heating chamber the heating chamber control system operates such that water flow into the heating chamber is at least temporarily stopped or restricted when the water reaches the first water level. During regular steam production steam exits the heating chamber through the steam outlet and water level in the heating chamber is varied between the third water level and the second water level, when water level drops to the second water level, the heating chamber control system operates such that water flows into the heating chamber through the water inlet at a refill flow rate slow enough to enable continued generation of steam until water level again rises to the third water level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is side schematic of one embodiment of a steam cooker; 
       FIG. 2  is a schematic view of one embodiment of a heating chamber/boiler and associated control arrangement; and 
       FIG. 3  is a schematic view of one embodiment of a flow control device. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a low-pressure steam cooker  10  includes a steam generator  12  for generating steam and a cooking chamber  14  that is in communication with the steam generator. The cooking chamber  14  may be formed by an insulated housing and includes a door  15  movable between open and closed conditions. The steam generator  12  includes a heating chamber  18  where water is heated to generate steam and a steam superheater  16  capable of superheating the steam generated in the heating chamber. 
   Disposed within the heating chamber or boiler  18  of the illustrated embodiment is a gas heat exchanger  20  in the form of a submerged heat exchange tube having a helical portion  22 . The heat exchanger  20  is connected to a burner unit  24  (e.g., a metal fiber, fan-driven burner having a stainless steel mesh and stainless steel tube, such as a Model BCT0027, available from N.V. Acotech S.A., Kennesaw, Ga.) that is capable of generating hot gases for delivery to the heat exchanger. Heat exchanger  20  is located in the heating chamber  18  such that it can be in a heat exchange relationship with water contained therein. While the illustrated heat exchange relationship with the water is via submersion of the heat exchanger, it is possible that hot gas could pass through ducts that are not submerged, such as ducts that run along the exterior wall of the heating chamber  18 . Further another heating arrangement could be provided, such as an electrical resistance heater within or on the side(s) of the tank. The heating chamber  18  includes an inlet  26  for ingress of water into the heating chamber from a water source and an outlet  28  for egress of water from the heating chamber (as when the chamber is to be drained). 
   Outlet  37  leads to steam superheater  16 , which includes an outer tube  34  and an inner tube  36  disposed within the outer tube. Outer tube  34  includes an inlet coupling  38  associated with the steam outlet  37  of the heating chamber  18  and an outlet coupling  39  associated with the cooking chamber  14 . Inner tube  36  includes a gas inlet  40  connected to the heat exchanger  20  and an exhaust outlet  42  for the venting of combustion gases. As steam travels along the annular space between outer tube  34  and inner tube  36 , heat from the combustion gases traveling through the inner tube  36  is transferred through the surface of the tube to the steam in order to superheat the steam. Disposed between the steam superheater  16  and cooking chamber  14  is a valve  32  that controls the flow rate of superheated steam into the cooking chamber (in some embodiments, the flow rate of superheated steam from steam superheater  16  into the cooking chamber is between about 35 and about 90 pounds per hour, such as about 50 pounds per hour where the volume of the cooking chamber is between about 164 and 245 cubic inches). 
   Referring now to  FIG. 2 , one embodiment of a water fill process for heating chamber  18  is now described. A flow control mechanism  70  is shown in association with the inlet  26 . Three water level sensors  72 ,  74  and  76  are also provided in the illustrated embodiment, along with a water temperature sensor  78 . The sensors may be within the main heating chamber as shown, or could be contained in a housing unit that is placed alongside the main heating chamber and includes a path of fluid communication with the heating chamber (as might be provided by tubing). Mechanism  70 , level sensors  72 ,  74 , and  76  and temperature sensor  78  are connected with a controller  80 , and the controller  80  is also connected to control the operation of the burner unit  24  or other heating unit to provide a heating chamber control system that, in one implementation, operates as follows. During an initial fill operation of the heating chamber  18  (such as when the steamer is turned on in the morning), device  70  is controlled such that water initially flows into the heating chamber through water inlet  26  at a first flow rate until water level in the heating chamber rises to a first set water level at which point water flow into the heating chamber is stopped. When water temperature reaches a set temperature, as indicated by temperature sensor  78 , water again flows into the heating chamber through the water inlet  26  at a second flow rate that is lower than the first flow rate until a second, higher set water level is reached. Use of the second, slower flow rate can help reduce the impact that incoming water has on the temperature of the water that has already been heated to the set temperature. 
   Where the first set water level and second set water level are sufficiently close, the water level may reach the second set water level merely by expansion of the water volume due to heating to the set temperature, in which case additional water flow after the water reaches the set temperature might not necessary. In such an embodiment the technique basically involves the control of water flow during initial fill based upon the temperature of the water (e.g, in the specific embodiment when the water temperature is below the set temperature, incoming water flow is controlled (e.g., stopped or restricted) to prevent the water from rising all the way to the second, higher water level until the temperature of the water rises to or approaches the set temperature). 
   In one implementation: (i) the first set water level is between about 0.5 inches and 1.0 inches below; the second set water level; (ii) the second flow rate is no more than about ⅕ the first flow rate, or no more than about 1/10 the first flow rate; and (iii) the set temperature is at least 190° F., or at least 198° F. 
   In one example, the first set water level is a water level indicated by level senor  72  and the second set water level is indicated by level sensor  76 . In this manner, undesired water volume expansion to levels too far above the high level sensor  76  may be avoided by assuring that a substantial majority (i.e., at least 75%) of water volume expansion due to temperature increase occurs before the water level ever reaches the high level sensor  76 . This result may be particularly useful in heating chambers where the high level sensor  76 , which represents an operating water level, is located close to the steam outlet  37  because it can help to reduce or avoid liquid water entering the steam outlet  37 , either as droplets carried with the steam or as a result of the water expansion itself, as liquid water entering the steam path can result in increased maintenance requirements. 
   In one example, the controller  80  maintains the burner  24  or other heating unit off until the first set water level is reached, but it is possible some heating could occur before the first set water level is reached. Where the low level sensor  72  acts as a safety, the controller  80  will typically prevent any operation of the burner unit  24  or other heating unit whenever the water level is below sensor  72 . 
   During normal steaming operations, as steam exits the heating chamber through the steam outlet  37 , the water level in the heating chamber drops. When the water level drops to a third set water level, which is between the first set water level and the second set water level, as reflected by middle level senor  74 , the device  70  is controlled such that water again flows into the heating chamber through the water inlet  26  at a refill flow rate slow enough to enable continued generation of steam until the water level again rises to the second set water level reflected by high level sensor  76 . In one example the refill flow rate is the same as the second flow rate mentioned above. 
   In one implementation, flow control device  70  may be a valve that can be energized at different levels in order to provide for different water flow rates. In another implementation, as shown in  FIG. 3 , the flow control device  70  may include multiple flow paths  82 ,  84  and multiple valves  86 ,  88  for enabling selective control of each flow path. The valves  86  and  88  can be sized to provide different flow rates when opened. Assuming valve  86  provides a lower flow rate than valve  88 , such an arrangement potentially enables four different flow rates, namely (1) no flow when both valves  86  and  88  are closed, (2) a lowest flow rate when only valve  86  is opened, (3) a middle flow rate when only valve  88  is opened, and (4) a highest flow rate when both valves  86  and  88  are opened. 
   It is to be clearly understood that the above description is intended by way of illustration and example only and is not intended to be taken by way of limitation. For example, while the illustrated steam cooker includes a superheater along the path from the heating chamber to the steam cooking chamber, it is recognized that steam cookers without superheating technology could utilize the heating chamber/boiler fill process described herein. Further, while certain embodiments are described, it is recognized that other variations on the control of water inflow to the heating chamber based at least in part upon temperature are possible. Other changes and modifications could be made.