Abstract:
An automatic oven cooks or bakes a food product over a timed period of time during which a burner delivers heat to the oven cavity. Thermocouple sensors in the cavity control a modulator which regulates the amount of gas flowing to the burner to maintain a uniform temperature in the oven. A conveyor carries the food product in order to time the baking period.

Description:
This application claims the benefit of Provisional application Ser. No. 60/249,685, filed Nov. 17, 2000. 
    
    
     This invention relates to conveyor ovens having reduced fuel consumption and quieter operation, and more particularly to such ovens having a modulated gas flow. 
     BACKGROUND 
     Prior art conveyor ovens are shown in U.S. Pat. Nos. 4,964,392 and 5,277,105 owned by the assignee of this invention and in the references cited on the cover pages of these patents. These and other similar patents may be consulted in order to learn details of how conveyor ovens are constructed and operate. Often—but not always—this type of oven is used to cook or bake pizzas, bread, or the like. 
     Conveyor ovens are devices for automatically baking or cooking food products over timed periods. Normally, they have a conveyor belt which travels through an elongated oven cavity having open ends and at a speed which times the exposure of the food product to the heat of the oven. A food product, such as a pizza, for example, is placed on one end of the conveyor at the entry to the oven cavity and delivered from the oven at the opposite end of the cavity. The heat in the oven and the speed of the conveyor are coordinated so that the food product is fully and correctly cooked or baked by the time when the conveyor delivers it at the exit end. 
     The conventional method of delivering controlled heat has been to switch burners off and on in order to hold the resulting temperature in the oven cavity within a relatively narrow range. This process has functioned very well in the past. However, anything can always be improved and, therefore, it is always possible to do a better job heating and cooking or the food product. 
     Also, the cost of the fuel (natural or propane gas) for the burners is increasing sharply. Thus, an important goal is to reduce the fuel consumption, which the invention has done by approximately 30%. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Accordingly, an object of the invention is to cook or bake a better food product by maintaining a closer control over the uniformity of the heat in the oven cavity. Here, an object is to maintain a substantially smooth level of heat after the oven is switched on and continuing throughout the oven operation. In particular, an object is to avoid the peaks and valleys of heat swings as the burner switches on and off as it hunts for the targeted temperature. 
     Another object is to provide a quieter operation by eliminating a blower-like noise which occurred heretofore as the burner switched on. 
     Still another object is to provide a universal heat controller which can control either a modulating valve or an on/off valve, thereby eliminating a need for many controllers individually dedicated to specific ovens. 
     In keeping with an aspect of the invention, these and other objects are accomplished by a use of a modulating valve which increases or decreases the amount of the gas flow to a burner without fully switching the burner off or on during a bake cycle. The modulating valve is controlled responsive to temperatures sensed by thermocouple sensors located in the oven. Furthermore, the controller is also able to control an on/off gas valve in response to the same sensor signals, so that the same controller may be used universally for both the modern oven using the modulating valve and the older ovens using on/off valves. 
     The advantages of the invention are many. There is an improved reliability. There is a higher quality bake at a shorter bake time and at a lower temperature. The ovens operate at a lower temperature; therefore, the components are in a cooler environment which extends the life of all components. The digital speed control is more reliable than the older speed controlled by non-digital means. The oven is quieter and the energy management system is more efficient due to a use of the modulating gas valve and to a two-way air return, with less turbulence, creating lower DB levels. There is an increased flexibility making it easier to rearrange the fingers for delivering heated air to the food product. There is an ability to add a deck, as volume increases, or to remove a deck if volume falls off. All decks are the same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following specification taken with the accompanying drawings, in which: 
     FIG. 1A is a perspective view which shows a plurality of ovens stacked one upon the other in order to increase baking capacity without increasing the oven footprint; 
     FIG. 1B is a perspective view of a hot air delivery finger being removed from the oven cavity; 
     FIG. 1C is a perspective view of a disassembled finger; 
     FIG. 2 is a perspective view of equipment which is seen if a side panel (not shown) is removed from conveyor oven  20   a  depicted in FIG. 1A; 
     FIG. 3 is a view, partly in cross section, of an automatic safety on/off switch and a modulating switch coupled into in a gas line; 
     FIG. 4 is a line drawing of the parts seen in FIG. 2, together with labels identifying the various items shown in the drawing; 
     FIG. 5 is a graph which illustrates the peaks and valleys of the burner duty cycle of the prior conveyor ovens using on/off gas valves; and 
     FIG. 6 is a graph which illustrates the continuous burner operation responsive to the inventive modulated gas flow. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A is a perspective view of four conveyor ovens  20   a - 20   d  each of which receives a conveyor  21  extending from an input end  21   a  through a cavity to an output end  21   b  of the oven. The motor for driving the conveyor is in the housing  22 . FIG. 2 shows equipment which is seen if a side panel (not shown) has been removed from the housing of the oven in order to show the parts  26  which are material to the invention. 
     In greater detail, FIG. 1A shows a plurality of the ovens  20   a - 20   b  (sometimes called “decks”) stacked one on the other to increase the baking capacity without increasing the footprint dimensions. FIG. 1B is a perspective view of a person removing an upper hot air delivery finger  24  from an oven by sliding it in direction A along side rail tracks. FIG. 1C is a perspective view of the finger  24  construction where two perforated plates  24   a ,  24   b  direct streams of hot air downwardly and onto the upper surface of a food product. Lower fingers  25  direct hot air upwardly through perforated plate  25   b  and onto the lower surface of a food product. The hot air recycles by flowing from a plenum  10 , through the fingers  24 ,  25  and returning by upper and lower air return flow paths  12 ,  14  between the top and bottom fingers  24 ,  25  to the plenum  10 . 
     The material parts  26  are shown in greater detail in FIGS. 2 and 4. Natural or propane gas is fed from a source to the burner via a line  28  and a modulating valve  30  under the control of a signal conditioner  31  and a temperature controller  32 . Both the preferred modulating valve  30  and signal conditioner  31  are products of the Maxitrol Company and are sold under the trademark “Selectra”. The Maxitrol Company has a business address at 23555 Telegraph Rd (P.O. Box 2230), Southfield, Mich., U.S.A. 48037-2230. 
     FIG. 3 shows gas line  28  extending from any suitable source of natural or propane gas on the left to a burner on the right. Interposed in the gas line between the source and the burner are two valves  29  and  30 . Valve  29  is any suitable on/off valve prescribed by a regulatory agency for safety purposes. For example, a conventional valve  29  might be adapted to shut down the gas delivery responsive to excessive pressure appearing in the gas supply line. 
     The modulating valve  30  means is shown in cross section. A main spring  33  biases a main valve  34  into a position either to close or open the gas line  28  in order to prevent or enable a flow of gas to the burner. A by-pass line  35  is provided for enabling gas to flow around the main valve  34  and through a pressure regulator  36  even when valve  34  is closed. A manual valve  37  in the by-pass line may cut-off or allow the by-pass gas to flow, as a safety or shut down procedure. Midway between regulator  36  and the manual by-pass valve  37 , a tap line  38  allows the by-pass gas to flow through modulator  39  in order to enable the gas to flow from the source into an upper chamber  40  which is closed by a diaphragm  41 . Modulator  39  is controlled responsive to signals from thermocouple sensors  42  in the oven. As the oven becomes colder, the diaphragm moves down, and as it becomes hotter, the diaphragm moves up. Hence, the diaphragm  41  moves up or down as a function of the instantaneous oven temperatures. 
     As the diaphragm  41  moves down, it overcomes the bias of spring  33  and opens main valve  34  by a distance which enables a volume of gas to flow in line  28  depending on the distance that valve  34  has moved. 
     If the oven temperature sensed at  42  goes down, the modulator  39  enables more gas flow from the by-pass line  35  to increase pressure in upper chamber  40 , thereby deflecting the diaphragm  41 , pushing valve  34  against the bias of spring  33  and opening the main valve  34  by a discrete distance. If the oven temperature sensed at  42  goes up, modulator  39  restricts the flow of by-pass gas, the pressure in upper chamber  40  reduces, the diaphragm  41  returns somewhat from its deflected condition, and spring  33  pushes the valve  34  to a more closed position. 
     Hence, it should now be clear that the amount of gas delivered to the burner follows the instantaneous fluctuations of the oven temperature. With a need for a low fire, there is little or no pressure on the diaphragm  41  and gas flows only through a by-pass and at a very low rate. In between the high and low demands for a high level of fire and a low level of fire, the pressure in the upper chamber  40  will have an intermediate effect upon the deflection of diaphragm  41  and, therefore, on the position of main valve  34  and the amount of gas flowing to the burner. 
     The temperature controller  32  is a device which receives a signal from thermocouple sensors located in the baking cavity of the oven. The sensor may continuously supply any convenient signal indicating the instantaneous oven temperature. The signal conditioner  31  interfaces between the temperature controller  32  and the modulating valve  30  by converting the sensor signal into a signal which the modulating valve uses. 
     The details on the arrangement of the various parts described thus far are best seen in FIG.  4 . The gas is delivered from any convenient source through a line  44  to the various ovens via a pipe  28 . As here shown, it may be assumed that pipe  28   a  is in oven  20   a  (FIG. 1A) and pipe  28   b  is in oven  20   b . The remainder of the ovens  20   c ,  20   d  are served in a similar manner. The top oven has a pipe  28   a  which is closed by a cap  46 . 
     The manual shut-off valve  48  simply provides for a complete shut down of the system. Usually, this valve is left in an “on” position. 
     The automatic valve  29  is a conventional device which meets any local safety standards. While such safety valves tend to be fairly uniform, various locations may have their own, non-standard requirements. 
     Next, the modulating gas valve  30  is located to admit a regulated amount of gas into a burner  50 . While any suitable burner may be used, a high efficiency burner is preferred. These burners are found in many appliances from heavy duty home heating to relatively light duty in appliances. 
     A blower  52  is coupled to the burner  50  via a suitable duct  54  in order to supply combustion air to the burner. When there is a mixture of gas and force air as the burner first comes on, there is usually a very noisy roar; hence, a blower which switches off and on is noisy. The invention avoids this noise by modulating the flow of gas which never shuts off as demand increases and decreases when the oven temperature decreases or increases. 
     FIG. 5 is a graph which discloses at  56  the duty cycle of the prior art burner as it responds to the temperature sensed in the oven and at  58  the temperature fluctuations at various locations in the oven. The prior art burner duty cycle  56  is shown as having peaks  60   a ,  60   b ,  60   c , as the burner is generating maximum heat and valleys  62   a ,  60   b ,  60   c  when the burner is shut down. The food product may have a variegated cooking or baking depending upon the peaks and valleys. The peaks and valleys depend upon sensed oven temperatures. Those temperatures vary with ambient temperatures, drafts, frozen or thawed condition of the food product, etc. Hence, it is not possible to predict with any certainty as to the relationship between the appearances of the peaks and valleys relative to the excursion of the food product on the conveyor. 
     A second point indicated in FIG. 5 is that there is a considerable demand for fuel because the oven heats and cools depending upon the peaks and valleys. Hence, the burner has to work harder to repeatedly recover from a cool down in the off stage. 
     FIG. 6 is a graph similar to FIG. 5, but showing the operation responsive to the inventive use of the modulating valve  30 . The flow of gas to the burner is seen in the curve  70 . While the gas flow varies almost continuously, it is never off, so that the burner modulates its out put within a relatively narrow band but does not shut down. Since the burner does not come on suddenly, there is no blow torch-like roar at the ignition. Curve  72  shows the temperatures sensed at the front, exit and middle of the oven. While this curve shows that the sensor does track the instantaneous variations of the heat put out by the burner, the average temperature in the oven is much more uniform over time. 
     The most important feature is that the fuel required to maintain the burner operation represented by curve  56  (FIG. 5) is 30% greater than the fuel required to maintain the burner operation represented by curve  70  (FIG.  6 ). 
     Those who are skilled in the art will readily perceive various modifications that fall within the scope and spirit of the invention. Therefore, the appended claims are to be construed to cover all equivalents.