Patent Publication Number: US-2005139202-A1

Title: Oven with forced air circulation

Description:
The present invention refers to an oven with forced air circulation comprising an oven chamber and a fan for circulation of heated air in the oven chamber.  
      In ovens for baking and cooking purposes it is sometimes desirable to be able to place dishes and baking-plates at different levels for simultaneous treatment and so increasing the capacity of the oven. In a traditional oven having heating elements disposed at the top and bottom parts of the oven it may be hard for heat from the lower heating element to reach dishes, cakes, pastry and the like placed on an upper one of, for example, two treatment planes in the oven while, in the same way, it will be hard for heat from the upper heating element to reach articles of food placed on the lower plane. In case of baking-plates, for example, these will effectively screen one another from the heating elements with the result that the treatment time will be very long if at all it would be possible to get a satisfactory result under the conditions described. Hence, in a traditional oven with heating elements placed at the top and bottom, respectively, of the oven, for practical reasons dishes or articles to be baked can be placed in one treatment plane only if a satisfactory result is to be obtained.  
      In these circumstances it has been suggested that the traditional oven be provided with a fan for circulating heated air in the oven chamber. By this circulation of air it would be possible to distribute the heat in the oven to all places where dishes or articles to be baked are present irrespective of the position of the same in a single or in any of plurality of planes. In a commonly used embodiment of a forced air circulation oven the fan has the form of a centrifugal blower placed adjacent to the rear wall of an oven chamber. The blower sucks air in the centre and blows it radially in a vertical plane at the rear wall. The air is forced past an annular heating element and via adjacent oven walls conveyed out into the oven chamber.  
      It has turned out that even if a relatively satisfactory result can be obtained as far as the heat distribution is concerned a completely even distribution of heat is out of reach. This is true in particular in domestic ovens where for cost reasons necessary additional arrangements, such as additional fans and special air guides, are not an option.  
      In the publication Eck B.: Ventilatoren, 5. Aufl., Springer-Verlag Berlin, Heidelberg 1991, p. 194-197 a tangential fan, also called cross flow fan, is described. The fan is of a special design in which a drum-shaped impeller is allowed to rotate freely, i.e. without any guidance of the air by a surrounding fan housing neither at the inlet nor at the outlet side. The impeller is provided with a plurality of fan blades evenly distributed along the periphery of the impeller. When the impeller is rotating air is driven into the interior of the impeller and out therefrom via openings between the fan blades. In a fan so designed a phenomenon appears in the form of a strongly pronounced jet which by itself rotates slowly about the axis of rotation of the fan. The rotational speed of the jet depends on the angular setting of the fan blades and the jet can rotate in the same direction as the impeller or in the opposite direction.  
      From tests carried out with a cross flow fan of the design just described and applied in a forced air circulation oven for circulation of heated air therein it has turned out that it is possible to obtain an improved distribution of the heated air in the oven chamber. This is achieved by the slowly rotating jet, which can also be seen as a slowly rotating big air whirl, generated by the fan. Thereby the formation of areas of elevated temperature in the oven chamber is reduced or even eliminated.  
      Accordingly, the object of the invention is to obtain an improved distribution of heated air in an oven with forced air circulation by making use of the particular characteristics of the cross flow fan described above. The object is achieved in an oven which has obtained the characteristic features indicated in the appending claim  1 . Preferred embodiments have been included in the associated sub-claims. 
    
    
      The invention will now be described more in detail in connection with an embodiment with reference to the appending drawings.  
       FIGS. 1 and 2  show an oven for domestic use having forced air circulation. The oven comprises a cross-flow fan adapted to generate a hot air stream in the oven. The oven  10  comprises an oven chamber  12  defined by an upper wall  14 , a lower wall  15 , side walls  16 ,  18  and a rear wall  20 . The oven chamber  12  has an opening, at the front side thereof, which is closable by means of a door  22 . Customary rails  24 ,  26  are provided in the oven chamber  12  on which baking plates  28 ,  30  can be supported at predetermined levels. In the oven chamber the air is heated in a traditional way by means of electric elements the nature and positioning of which will be described more in detail below.  
      At the rear wall  20  a fan  32  of the cross-flow type is provided which comprises an impeller  34  driven by an electric motor  36 . The impeller has the shape of a short cylinder with side walls  38 ,  40  and a plurality of fan blades  42  distributed along the pheriphery  44  of the cylinder, said fan blades  42  being curved in the direction of rotation of the impeller, see  FIG. 3   a ,  3   b.    
      As indicated, the fan is of the cross-flow type in which air is led into and out from the impeller  34  via openings  48  formed between the fan blades  42 . As shown in  FIG. 4   a , when the impeller rotates air escaping through one of said openings  48  will leak back into the interior of the impeller via an adjacent opening along the periphery of the impeller thereby generating a whirl around each fan blade  42 , as indicated by the arrow  43 . These air whirls are combined into a whirl  45  around two adjacent fan blades  42 , as shown in  FIG. 4   b , and the formation of whirls goes on, as indicated by the arrow  47  in  FIG. 4   c  for four fan blades, until one single large whirl  49  ( FIG. 5   a - d ) has been formed which extends along the whole of the periphery of the impeller  34 . Due to the fact that no fan housing or other obstacle is present adjacent to the impeller  34  the large whirl  49  will not be bound to any fixed direction and in the typical case it will rotate at a speed considerably lower than that of the impeller. The rotation of the whirl or air jet  49  is shown in the series of  FIGS. 5   a - 5   d  and the ratio between the rotational speed of the jet and that of the impeller is about 1:20. The rotational speed of the air jet expressed as a pulse frequency is about 2 Hz in the case referred to. By variation of the blade angle of the fan blades  42  with respect to the direction of the tangent of the pheripheral surface of the impeller the pulse frequency can be changed both upwards and downwards and even take negative values, which means that the air jet  49  rotates in the opposite direction as the impeller  34 . A preferred value of the pulse frequency is about 1 Hz which in the application indicated has proved to provide effective stirring and distribution of the warm air circulating in the oven.  
      A common problem in a classical oven with forced air circulation is associated with the simultaneous baking of buns placed on two baking plates disposed at different levels in the oven.  FIG. 1  shows the two plates  28  and  30  supporting buns, not shown, which are evenly distributed on the repective plate. By means of the cross-flow fan  32 , described above, a turbulent air flow is generated which sweeps along the plates  28 ,  30 , both on the underside and on the overside where the buns are placed, from the rear wall  20  of the oven chamber in a direction towards the oven door  22 . The turbulent flow, which is created by the slowly rotating air jet  49  from the fan  32  causes the warm air to approach the buns on the plates  28 ,  30  from different directions and as a result the buns will be evenly baked without the creation of burnt or too light areas.  
      In  FIG. 6  the oven chamber  12  is shown in a front view. The air from the fan  32  is being thrown out radially in a vertical plane parallel to the rear wall  20  and there is a tendency that, to a high extent, the air is deviated towards the corners formed by the rear wall  20 , the upper wall  14 , the lower wall  15  and the adjacent side walls  14 ,  16 . This means a more concentrated air flow past the plates  28 ,  30  in areas of the corners which may cause the buns most close to said corner areas to reach a too high temperature as compared to the remining buns on the plates so that there is a risk for the corner buns to be burnt. In order to solve the problem a plurality of guide plates  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64  are provided, suitably on the rear wall  20 , the purpose of which is to divide the flow into different streams  51 ,  53 ,  55 ,  57 ,  59 ,  61 ,  63 ,  65  so that a reduced part of the flow is led towards the corners in favour of intermediate areas along the limiting walls of the oven chamber.  
      As shown in  FIGS. 1 and 2 , the transitions between the walls at the rear part of the oven adjacent the fan  32  have been modified in order to achieve a softer deviation of the air flow in the direction towards the oven door  22 . This has been achieved by guide plates  66 ,  68 ,  70 ,  72  provided in the transitions between adjacent walls at the rear part of the oven chamber. As a result the air jet  49  from the impeller  34  will rotate more evenly and the risk for the air jet  49  to be locked in a any specific direction will be reduced. In the practical case the transitions between adjacent walls in the rear part of the oven chamber can be brought about by giving the transitions between the rear wall  20  and the adjacent walls a suitable curved shape during the manufacturing process.  
      As indicated above heating of the air in the oven  10  takes place by means of electrical heating elements and in  FIG. 1  an annular heating element  19  is shown to surround the impeller  34 . In order to prevent direct radiation from the annular heating element  19  onto food on the plates  28 ,  30  a shield plate  25  is provided.  
      In case the oven air is to be heated by means of the element  19  alone this element should have a rated power of 2500 W for the temperature levels and loads to be handled by the oven. In order to achieve a more favourable heat distribution with a reduced risk of areas of elevated temperature to appear the oven is provided with an upper heating element  21  and a lower heating element  23 . If the power of the element  19  is reduced to about 1500 W and the rating of the elements  21  and  23  is chosen to about 500 W the available power will remain unchanged while the temperature of the air in the rear part of the oven will decrease. Another way of obtaining a reduction of the temperature of the air in the rear part of the oven is to use an annular element  19  having a reduced diameter so that the outlet opening of the air stream between the annular element  19  and the adjacent oven walls increases. This gives a lower air velocity which causes a lower heat transfer to the load most close to the fan  32 . A reduction of the diameter of the annular element  19  may need to be accompanied by other modifications due to the fact that, most likely, the pulse frequency will be affected by this modification of the annular element.  
      As a summary, it can be pointed out that the application of a cross-flow fan in an oven in the way described will give a considerably more even temperature distribution in the oven wherein a spread load, for example in the form of buns disposed on plates at different levels in the oven chamber, will be exposed to an essentially uniform heating effect in all points.