Patent Publication Number: US-2007108180-A1

Title: Cookers

Description:
The invention relates to heat-storage type domestic cookers.  
      Domestic cookers fall into two main categories. In the first, most common category, the oven compartment is made from pressed steel and an exposed element (gas or electrically powered) located within the walls of the oven compartment with or without further concealed elements, heats food put into the compartment. Fans are often provided in such ovens for speed and evenness of cooking. These ovens are turned off when not in use and take a short time (about 10 minutes) to warm up when they are required for cooking. They have a low thermal mass.  
      The second category of domestic cookers comprises the heat-storage type of cooker typified by the well known Aga (RTM). In this type, the or each oven compartment is slowly heated over a long period (6 hours or so) from a heat source (oil, gas, electric) located centrally within the cooker outside the walls of the or each oven compartment. The walls of the oven compartment are formed from cast iron usually about 5-6 mm thick. This is not only heavy but has a high thermal mass. These ovens are not turned off between uses but run continuously so that they are always ready for immediate use. In cooking, heat stored in the cast iron walls is transferred into the food. These ovens run at a constant temperature which is maintained by thermostatic control of the heat source.  
      Heat-storage cookers do not have any powered heating elements within the walls of the oven compartment. Rather, they operate by radiation of heat stored in the oven compartment walls which has been received from heating elements located outside those walls. Cookers of this type cook quite differently to cookers in which a heating element is located within the oven compartment. Typically, food cooked in this way retains much more moisture.  
      The present invention provides an improved heat-storage cooker by replacing the cast iron walls with walls of a ceramic material formed from silicon carbide.  
      This has a number of advantages. Although ceramic materials are generally less dense than cast iron they have a higher specific heat. Thus an oven compartment wall mainly comprising silicon carbide having a nominal thickness of about 30 mm will be no heavier than a conventional cast iron wall and will provide a very good thermal mass which also has extremely good heat conducting properties, essential for spreading heat from the heat source. Further, silicon carbide provides a smooth, very hard, non-scratch surface with extremely good cleaning properties. Silicon carbide forms an attractive pale grey/blue coloured surface which can be wiped clean with a damp cloth without staining. 
    
    
      Embodiments of the invention are described below with reference to the accompanying drawings, in which:  
       FIG. 1  is a perspective view of a domestic cooker;  
       FIG. 2  is a perspective view of the cooker of  FIG. 1  with two doors removed to reveal two oven compartments;  
       FIG. 3  is a perspective view of an oven compartment shown removed from the cooker; and  
       FIG. 4  is another perspective view of the oven compartment of  FIG. 3 , showing a heating element. 
    
    
      The heat storage cooker  1 , shown in  FIG. 1  and  FIG. 2  comprises a frame  2  supporting four heated compartments closed by hinged doors  3 , two gas rings  4  and an electric hot-plate  5 . The right hand side of the cooker has two oven compartments  6  mounted one above the other as seen in  FIG. 2 .  
      As shown best in  FIGS. 3 and 4 , the compartments  6  comprise a rear wall  7 , a bottom wall  8 , a top wall  9  and opposite side walls  10  fitted together to form a cooking space having an open front face opposite the rear wall  7 . In use the front face is closed by a hinge mounted door  3 . An electric heating element  11  having a length of about  5  metres is mounted on the frame  2  outside and beneath each of the oven compartments  6 . The element  11  is relatively low-powered (about 900 W).  
      The walls  7  to  10  of the oven compartment comprise silicon carbide (SiC). In a preferred embodiment, the material of the wall consists of silicon carbide combined with fillers and binders as required. The silicon carbide will comprise at least 50% of the mixture and in a preferred example, comprises about 80% of the mixture.  
      Each wall is formed by a rectangular plate. The bottom and top walls  8 ,  9  are identical to each other as are the side walls  10 . Further, the walls are all symmetrical about both their major axes so that their front edges are identical to their rear edges and their side edges are identical to each other. This assists in manufacture in that only three different plates need to be made and in assembly. The plates interlock at their adjacent edges and are additionally held together by adhesive, this makes the compartment air-tight when the door is closed. The walls are moulded from ceramic material such as by pressing powdered silicon carbide into a mould to form a self supporting structure which is subsequently fired over several days to form a strong, hard wall.  
      A plurality of parallel grooves  12  are formed in the inner faces of the side walls for supporting oven shelves or cooking vessels directly. Since the grooves are formed in the moulding process, no additional fittings are required. The grooves slightly reduce the mass of the side walls but this is compensated for by them being slightly thicker overall than the other walls.  
      In operation, the element  11  heats the oven compartment walls over a long period (about 6 to 8 hours) to a desired temperature. Heat from the element is spread evenly around the compartment walls and stored for cooking. Once the desired temperature is reached it is maintained by thermostatic control of the element.  
      In the cooker shown, the two oven compartments  6  are provided with heating elements. Two warming compartments are mounted one above the other next to the oven compartments and receive heat by conduction through the cooker from the oven compartments. The two gas rings  4  on the top of the cooker are powered and controlled separately from the oven compartments as is the hot plate  5 .