US3507265A

US3507265A - Self-cleaning gas cooking oven

Info

Publication number: US3507265A
Application number: US755555A
Authority: US
Inventors: Bohdan Hurko; Raymond L Dills
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-08-27
Filing date: 1968-08-27
Publication date: 1970-04-21
Anticipated expiration: 1987-04-21
Also published as: ES172985Y; GB1256948A; DE1942965A1; FR2016461A1; ES172985U

Description

April 21, 1970 B. HURKO ETAL 3,507,265

SELF-CLEANING GAS COOKING Filed Aug. 27, 1968 5 Sheets-Sheet 1 99 64 INVENIORS BOHDAN HURKO BY&RAYMOND DlLLS THEIR ATTORNEY April 21, 1970 B. HURKO ETAL SELF-CLEANING GAS COOKING 5 Sheets-Sheef 2 Filed Aug. 27, 1968 FIGZ ' INVENTORS BOHDAIU HURKO 8RAVM0M0 L. DILLS W 4- THEIR ATTORNEY April 21, 1970 B. HURKO ETAL SELF-CLEANING GAS COOKING 5 Sheets-Sheet 3 Filed Aug. 27, 1968 INVENTORS BO HDAN HURKO 8 RAYMOND D\\ \.S

FLUE GAS FLow ROOM Am. now

, HEAT EXCHANGE; SURFACE THEIR ATTORNEY FIG.4

5 Sheds-Sheet &

Filed Aug. 27, 1968 OVEN WALL LATCH OPEN Ex HAUST H.D mm A c LO L o o O O o o o w M 6 4 2 12.0 MIN. INVENTORS BOHDAN HURKO BY 8 RAYMOND L. D\LLS FIGS MKM

THUR ATTORNEY April 21; 1970 B. HURKO EIAL I 3,507,265

SELF-CLEANING GAS COOKING Filed Aug. 27, 1968 5 Sheets-Sheet 5 ERAYMQND x... DlLLS THEIR ATTORNEY United States Patent U.S. Cl. 126-21 17 Claims ABSTRACT OF THE DISCLOSURE This invention pertains particularly to a gas cooking oven such as used in the home, although the inventive concept could easily be adapted for use in commercial or institutional cooking ovens. Self-cleaning ovens operating on the principle of pyrolysis or the chemical decomposition of food soil by the use of heat, have been available in electric ranges for over four years. The same principle is equally adaptable for gas ovens.

This gas oven design passes convection currents of the main portion of the combustion products or flue gases from the lower gas burner over the outside of the oven liner for heating the walls forming the cooking cavity. A stationary, vertically arranged heat exchanger is mounted in the walls of the oven body, and after the flue gases finish their pass over the oven liner they enter the bottom of the heat exchanger and rise to exit from the top thereof where they mix with room ambient air before they are returned to the kitchen atmosphere, at a relatively low exhaust temperature. Simultaneously, relatively cool room air enters the top of the heat exchanger flowing downwardly to exit at the bottom thereof after picking up thermal energy from the flue gases, and then the heated air is fed into the combustion chamber as preheated secondary air for the combustion process. This adoption and manner of utilizing a heat exchanger greatly increases the efliciency of combustion of a gas burner system from an efficiency that might be as low as 30% to an amount between about 70 and 90%, and likewise lowers the consumption of gas fuel, and both the amount and the heat of exhaust.

CROSS-REFERENCE TO RELATED PATENTS The present invention follows the general teachings of the basic patent of Bohdan Hurko No. 3,121,158 which explains the controlled use of heat for automatically cleaning the food soils from the inner walls forming an oven cooking cavity, where the cleaning cycle has a maximum oven wall temperature somewhere between about 750 F. and about 950 F. for a suflicient period of time for degrading the food soils into gaseous products which are then treated by an oxidation unit or smoke eliminator which oxidizes the gases before they are returned to the kitchen atmosphere.

This invention is also a later development of the initial invention of the present inventors, Raymond L. Dills and Bohdan Hurko, in Patent No. 3,364,912 which is specifically related to a self-cleaning gas cooking oven.

BACKGROUND OF THE INVENTION Popular models of standard gas coking ovens are provided with walls forming a cookinggcavity with a combustion chamber located beneath the oven that is provided with a gas burner for generating hot flue gases. Large apertures are usually provided in the bottom portion of the oven cavity walls so that the hot flue gases may sweep through the cooking cavity. An oven vent is provided adjacent the top of the oven liner. For a high temperature, self-cleaning pyrolytic operation in a gas oven built before the present invention it was necessary to utilize a larger 3,507,265 Patented Apr. 21, 1970 ICC output gas burner means than is necessary during normal cooking operations, on the order of between 25,000 and 32,000 B.t.u.s per hour.

The heating value of natural gas is about 30,000 B.t.u.s per pound. Theoretically, combustion requires only 15.3 pounds of air per pound of gas, but practically complete combustion requires an excess amount of air up to a maximum of about 300%, which is as much as 61.2 pounds of air per pound of gas so as to prevent the occurrence of excessive amounts of carbon monoxide. The American Gas Association specifies that the upper limit of acceptability is .01% of CO for a kitchen size of 1,000 cu. ft.

A typical gas surface burner has a very low heating efliciency, on the order of 48% at a rating of 14,500 B.t.u.s. Such a gas burner has the same heat output as a 4,250 watt electric heater, and out of this 2,200 watts isheating the kitchen air. Larger burners have even lower efliciency. In a gas oven only about one-half of the heating value of the gas consumed is used for heating the oven, and the remainder is represented by both the heat of exhaust and the heat losses radiating from the walls of the oven insulation and the body or cabinet. Hence, in order to raise the temperature of the walls forming the oven cooking cavity to a maximum of between about 750 F. and about 950 F., it is highly desirable to avoid pumping a large amount of hot flue gases back into the kitchen atmosphere, and it is also important to process the gaseous degradation food soil products from the cooking cavity to eliminate carbon monoxide, smoke, odors, vapors and other undesirable constituents of the exhaust before the exhaust gases reach the kitchen atmosphere.

We have concluded that the best self-cleaning gas cooking oven design is one with means for excluding the major portion of the flue gases from the oven cavity during the cleaning operation. Small amounts of the flue gases passing through the oven cavity during the cleaning cycle are not objectionable, and they actually serve to improve the uniform temperature distribution at the front of the oven during cleaning as well as the performance of the oven during normal cooking operations. In any event, it is most desirable from a personal safety point of view to hold down the exhaust temperature of the flue gases below a maximum of about 550 F. for any cooking operation. This same temperature limit is also desirable for the automatic oven cleaning operation.

The principal object of the present invention is to provide a gas cooking oven with the capability of a high temperature, pyrolytic, self-cleaning oven cycle of operation wherein the temperature of the flue gases which are returned to the kitchen atmosphere is held below a maximum of about 550 F., and the oven exhaust is properly treated so that it does not contain harmful carbon monoxide or disagreeable smoke, odors, vapors and the like when it is returned to the kitchen atmosphere.

A further object of the present invention is to preheat the secondary air for use in the lower combustion chamber of a gas cooking oven so as to vastly improve the efiiciency of combustion and reduce the fuel consumption with a consequent reduction in the amount of flue gases to be handled.

A further object of the present invention is to provide a high temperature gas cooking oven with a heat exchanger whereby the flue gases passing therethrough preheat the secondary air for the combustion chamber before the gases are discharged from the oven thereby improving the efiiciency of combustion of the gas burner as well as holding down the exhaust temperature of the flue gases within a reasonable limit.

A further object of the present invention is to provide a gas cooking oven where the flue gases in the combustion chamber are separated into a major flow around the out- SUMMARY OF THE INVENTION The present invention, in accordance with one form thereof, relates to a self-cleaning gas cooking oven having an oven body supporting an oven liner and a front access door which in unison define an oven cooking cavity. A fire box surrounds the oven liner and is spaced outwardly therefrom to form external heating channels. A gas burner is located in the bottom of the fire box beneath the oven liner, and the main portion of the flue gases transfer heat to the oven liner by convection currents and by radiation from the hot gases which flow through the heating channels and into contact with the outside of the oven liner in a predetermined path. A stationary heat exchanger is mounted in the walls of the oven body and after the flue gases leave the oven liner they enter the bottom of the heat exchanger and exit from the top thereof for return to the kitchen atmosphere. Simultaneously, room ambient air is drawn into the top of the heat exchanger and flows out the bottom thereof, and in so doing extracts heat from the flue gases to lower the exhaust temperature of the flue gases into a comfortable temperature range, and likewise becomes heated thereby, and is finally supplied to the lower combustion chamber as pre-heated secondary air.

BRIEF DESCRIPTION OF THE DRAWINGS Our invention will be better understood from the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.

FIGURE 1 is a right side elevational view of a freestanding gas range with parts broken away and others in cross-section to show a single oven cooking cavity for both baking and broiling, and a fire box surrounding the oven liner, a heat exchanger mounted in the back wall of the range, a layer of thermal insulating material surrounding the fire box, as well as a drawer space beneath the combustion chamber of the fire box.

FIGURE 2 is a right side elevational view similar to that of FIGURE 1, but with the oven liner left intact, and showing the convection currents of flue gases rising from the combustion chamber in the bottom of the fire box and passing up the opposite side walls of the oven liner, more toward the front thereof, and then passing over the top wall of the oven liner toward the back of the oven as well as down the opposite sides of the oven liner and down the rear wall of the oven liner for passage into a flue gas inlet in the bottom of the heat exchanger so that the flue gases may rise in the heat exchanger and exit at the top thereof and be discharged into the kitchen over the cooktop of the range.

FIGURE 3 is a top cross-sectional plan view ostensibly taken through the heating channel located above the top wall of the oven liner generally on the line 33 of FIG- URE 1, while the front portion of the top wall of the oven liner is broken away for viewing into the oven cavity, as well as one corner of a removable panel resting in the bottom wall of the oven liner is broken away to uncover the combustion chamber, and the gas burner has been left out of the view to show a series of slotted openings in the bottom of the fire box, which openings substantial- 1y underline the gas burner and serve to admit the preheated secondary air to the combustion chamber.

FIGURE 4 is a graphical representation of the variations of the flue gas temperatures and the room air temperatures due to their passage in a counterfiow direction over a heat exchange surface, as for example would be 4 experienced in the heat exchanger shown mounted in the back wall of the oven body, as is best seen in the plan view of FIGURE 3. Notice in this example that the room air can be preheated to a temperature above the exhaust temperature of the flue gases.

FIGURE 5 is a graphic illustration of the temperaturetime relationship of the walls of the oven liner and the exhaust gases involved in a heat-cleaning cycle for a gas oven in accordance with an example of the present method.

FIGURE 6 is a top cross-sectional plan view similar to that of FIGURE 3, but of a modification of the present invention where the heat exchanger is divided into two segments, and one segment is located in each opposite side wall of the oven body.

FIGURE 7 is a right side elevational view, similar to that of FIGURE 2, but showing the modification of FIG- URE 6, but with parts of the heat exchanger broken away to show the room air flowing down the alternate room air columns of the heat exchanger.

FIGURE 8 is a right side elevational view, similar to that of FIGURE 7, except parts of the heat exchanger are broken away to show the flue gas after it finishes its path over the oven liner to feed into the bottom of the heat exchanger section at the side of the oven to rise up the alternate flue gas columns and exit from the backsplash of the range.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to a consideration of the drawings and in particular to FIGURE 1, there is shown a free-standing domestic gas range 10 in right side elevational view comprising a sheet metal outer cabinet or body structure 11 supporting a horizontal cooktop 12 and an underlying baking and broiling oven cooking cavity 13. The cooktop 12 supports a plurality of surface burners 15, which are not illustrated in detail since the present invention centers around the oven structure. Arranged along the back of the cooktop 12 is a backsplash 17 which supports a control panel in the front face thereof having a complete array of surface and oven burner control components 18, and it may include an automatic surface burner control, an oven timer, a convenience outlet and the like, generally of the same nature as is found on a standard electric range.

The oven 13 is formed by a box-like oven liner 20 in combination with a front-opening access door 21. The oven liner 20 has a bottom wall 23, opposite vertical side walls 24, a top wall 26 and a rear wall 27. The bottom wall 23 of the oven liner is formed with a large rectangular opening 30, as is best seen in FIGURE 3, which is adapted to be substantially closed by a removable cover plate 31. The reason for the removable cover plate 31 is to provide access to a gas burner tube 33 which is located in a combustion chamber 34 beneath the bottom wall 23 of the oven liner. The front of the oven liner 20 is open, and it has an outwardly turned flange 36 on its peripheral edge as part of the supporting means for the oven liner. Means (not shown) such as adjustable J-bolts or the like are used at the rear of the oven to pull the oven liner 20 back into the oven body 11 until the flange 36 bears against a cooperating collar 38 that serves as a front frame for the oven liner. Sandwiched between the flange 36 and the front frame 38 is a thermal breaker 40 such as a continuous asbestos or fiberglass gasket which serves to break the path of the heat flow by conduction from the oven liner to the front of the oven body, as well as to provide a sealing means between these two elements and prevent flue gases from escaping at the front of the oven.

The oven door 21 may be a standard door that is used for any self-cleaning oven, whether it be electric or gas. By that is meant that the door must be heavily insulated so as to retain most of the heat within the oven cavity 13 so that the temperature of the outer surface of the door will be within a comfortable temperature range so as not to cause injury to those in the household, as well as to prevent the wasteful loss of heat into the kitchen. The door 21 is shown with an outer door panel 42 of shallow pan shape and an inner door panel 44 which is adapted to be carried thereby but thermally isolated therefrom by period of from 1 to 3 hours is about 75% of the preheat rate or about 21,000 B.t.u.s per hour, which when com pared with electric heating is about 6,150 Watts.

According to the above analysis, if we compare heat 'output between a gas and an electric oven during the means of a high temperature gasket 45 such as woven 5 heat cleaning cycle, the gas oven will produce approxiasbestos or the like. This gasket also serves to bear mately four times as much heat in the kitchen as an elecagainst the flange 36 at the front of the oven liner and also tric oven will, on the assumption that the heat absorbed against the front wall of the oven body 11 to substantially in the oven system during cleaning is the same for both close the front opening of the oven. For more detailed ingas and electric; namely, about 5,800 B.t.u.s per hour. formation on a suitable design of a door for the oven of The reason for this is due to the method of heat transfer the present invention, reference may be made to the patent in a gas oven. A gas burner produces very little radiant of Clarence Getman No. 3,189,020 which is entitled heat since the flames radiate only about 10 to 15% of the Oven Door with Floating Inner Panel, which is likewise heat of the burner, while the remainder of the heating is assigned to the assignee of the present invention. 15 accomplished by convection currents of flue gases. In a Located within the oven body 11, and surrounding the standard gas oven large amounts of flue gases and hot oven liner 20, is a fire box 47 which is spaced outwardly air pass through the oven. The heating value of one pound from the oven liner to form narrow heating channels of natural gas is about 30,000 B.t.u.s per hour. For ideal around the oven liner for the passage of convection curcombustion it is necessary to have at least 15.3 pounds of rents of hot flue gases therethrough. This fire box 47 is air for each pound of gas. However, an excess amount of deepened at the bottom to form the combustion chamber air is needed in heating up the oven, up to 300% of excess 34 for accommodating the gas burner 33. The fire box has air, which means that for each pound of gas up to about a bottom wall 49, opposite side walls 51, a top wall 53 61.2 pounds of air (l5.3 4) may be needed to provide and a rear wall 54. Looking at the top plan, cross-seccomplete combustion while holding the amount of carbon tional view of FIGURE 3, these heating channels may be monoxide to a minimum. identified as th t Side Channels a P channel If the flue gases are at the necessary temperature of 57 and a rear channel 58- about 900 F. above the room ambient temperature (At) A gas humhr tube 33 is shown f a $116 and ratmg When the gases break contact with the walls of the oven somewhere w n 18,000 Bill-,5 P hour liner during the cleaning cycle, and the heating rate is one f a Standard g discharge nozzle and gas pressure- 30 pound of gas burned per hour, then the heat of exhaust There may be one burner or two burners as a matter of may be calculated as f choice. The burner is supported at its front end on an angle bracket 60, and it has an angularly arranged Venturi tube Q=C r( throat 62 which extends out through an opening 63 in the back wall of the fire box and down toward the 35 =026X900X (612+1):14650 sper hour bottom of the range where a connection is made to a fuel mg=pounds f gas gas line 64. Primary air at generally room ambient temmazpounds f i perature is fed into an adjustable mixer head of the Ven- Cp=specific heat f fl gas turi tube in order to provide the best performance. Atztemperature diff ti l The necessary pilot and main gas lines and control 40 valves are not shown nor are the te pe Sensing and The exhaust heat of 14,650 B.t.u.s per hour is equivalent control systems since they may be of standa d nstru to heating a kitchen with an electric heater of 4,300 watts. tion and do not form part of the present invention. In addition to this amount of heat there will be the usual An upper gas burner or broil burner 65 is assembled heat losses from the oven body which will be of the in the p of the Oven Cavity 13 for use during normal same order of magnitude as the heat losses in an electric broiling operations, but it is not used at all during the oven and will depend upon the maximum surface temheat-cleaning cycle. Since the oven cavity 13 is supplied rat e of th ov n body or abinet, with both a lower baking burner 33 and an upper broilirlg In an electric oven, the amount of circulated air through burner 65 in a manner similar to electric ovens, there is th oven i very l on th order f l h 3 bi room beneath the oven for a utensil drawer 67 in place feet per minute, as compared with a gas oven with about of the small r il r Oven Of a Standard gas range- 38.6 cubic feet per minute. Hence, the heat exhausted The lower gas burner in a Standard baking Oven has a by an electric oven would be less than 8% of the heat exrating Of about 20,000 B.t.llS per 110111 which iS equivalent hausted by a gas, oven during the heat-cleaning cycle It in electrical terms to 5,850 watts of electrical power. In has been calculated that the efliciency of the heating order to reach the proper heat-cleaning temperature, which system of the gas self-cleaning oven would be very low, is somewhere between about 750 F- and ab ut 950 F-, on the order less than 20%. Our present invention, which it WOUld be necessary to increase the size Of the burner includes a heat exchanger for a gas self-cleaning ven, to about 28,000 B.t.u.s or to an electrical equivalent of h th effect of substantially increasing the efliciency about 8, Watts- During a standard baking p at n, of the heating system of the oven. Theoretically, the the burner operates only for a short time at full rating, efficiency of the gas heating system could be increased as for example 10 to 15 minutes. However, during a heatf o b ut 283% up to abo t 85% wh using th v cleaning cycle the preheat period would be approxim e y with heat exchanger of the present invention. This has 50 to 60 minutes and the oven iS maintained at cleaning the advantage of a saving in fuel, but the saving in the temperatures for up to 3 hours or l ng r epe ing p n cost of fuel will not be the only factor. With the heating the amount of food soil lodged on the walls of the oven ystem using mu h l ss fuel, the amount of heat in h liner and inner door. The rate of heat to maintain the exhaust will be proportionately smaller. As one example oven at the maximum heat cleaning temperature for a see the table below:

Heat absorbed in system dur- Total heat Heat in Efficiency, ing cleaning, input, Burned fuel, Burned air, Exhaust, exhaust, percent B.t.u./hr. B.t.u./hr. lb./hr. lb./hr. lb./hr. B.t.u./hr. 2s. a 5, s00 20, 450 1. 0 e1. 2 62. 2 14, 300 85.0 5,800 6 850 0.315 19.4 19. 7 1, 050

The high heat losses and the inherently low efficiency of a gas burner system have led to the discovery of significant improvements in the design; namely, the incorporation of a heat exchanger 66, shown for example at the back of the oven body, for recovering much of the heat from the flue gases before the gases are exhausted to the kitchen atmosphere, and at the same time preheating a stream of room ambient air while it passes through the heat exchanger and before it is supplied to the combustion chamber 34 as pre-heated secondary air. First, will be explained the construction of the heat exchanger 66, and then attention will be given to explaining the results obtained from its use. Notice in the preferred embodiment of FIGURES 13 that the heat exchanger 66 is located across the back of the oven just outside the rear heating channel 58, and that it substantially extends from top to bottom of the oven, as well as from side-to-side of the oven, as is clear from looking at the top plan view of FIGURE 3. The heat exchanger 66 is a fabricated structure having a heat exchange surface area of about 18 sq. ft. that is constructed of a series of rearwardly facing channel members 68 which extend from top to bottom of the oven and are provided at their back edges with outwardly extending angular flanges 69.

Hence, when the channels are stacked side-by-side these flanges bear back-to-back against each other as spacers and are welded in place, and the resulting structure is a deeply corrugated heat exchange surface 72. On the front side of the heat exchange surface 72 flue gases are caused to rise, and on the back side of the heat exchange surface 72 room ambient air is caused to flow in a. counter flow direction to the flue gases because of the heat developed in the combustion chamber 34. The corrugated heat exchange surface 72 is closed at the front side by a large sheet metal panel 74 and across the back side by a large sheet metal panel 76, thus there is formed a plurality of flue gas columns 78 and a plurality of associated ambient air columns 80 interspersed therebetween.

It is important to retain the heat within the oven so that the heat losses from the oven body to the kitchen will be at a minimum in order to conserve fuel and to maintain a safe operating temperature of the outer walls of the oven. A heavy layer 107 of thermal insulating material such as fiber glass or the like is placed under the bottom wall of the fire box 99, up the rear of the oven behind the heat exchanger 66, over the top wall 53 of the tire box as well as up the two sides 51 of the fire box, as is best seen in FIGURE 3. Since the fiber glass batting is not self-supporting, metal sheets or insulation guards 109 support the fiber glass in place, except this is not needed at the back of the oven because the range body includes a back wall 110 and this serves to support the fiber glass. Along both sides of the oven, outside of the fiber glass insulation 107, are cooling wall channels 92 which have a natural draft to assist in maintaining the outer surface of the sides of the oven body at a temperature below about 194 F. As an alternative, a fan (not shown) may be installed for a forced draft of cooling air.

First, looking at FIGURE 2, the path of the convection currents of flue gases from the burner tube 33 in the combustion chamber 34' starts up the opposite side heating channels 56, more toward the front of the oven by virtue of the existence of an inclined bafile or partition 82 which is fastened on the outside of the side wall 24 of the oven liner and extends generally from the lower rear corner to a point spaced slightly rearwardly of the upper front corner. Thus the hot flue gases are directed up toward the front of the oven liner until they reach the front portion of the top heating channel 57 where they sweep rearwardly of the oven and wash over the outside of the two back corners of the oven liner and also pass down the rear heating channel 58 and down the side channels 56 behind the partition 82 until they gather along the rear edge of the bottom of the oven liner. There is a flue gas inlet opening 84' shown in FIGURE 2 in the bottom portion of the heat exchanger 66 for each of the flue gas columns 78 so that the flue gas is distributed throughout the columns 78. Then the gas rises rapidly toward the top of the heat exchanger Where it exits through openings 85 into a header 86 which is connected into the bottom of the backsplash 17 and joins with a vent opening 88. The header 86 is open at the back side of the range by means of louvers 87 so that room air may enter and mix with the flue gases before the flue gases leave the vent opening 88 for the kitchen atmosphere at a temperature between 300 F. and 550 F.

New for a discussion of the flow path of room air down through the heat exchanger 66 wherein it may pick up enough heat to develop an air temperature between about 300 F. and 800 F. as is seen in FIGURE 4, and later is supplied to the combustion chamber 34 and serves as pre-heated secondary air for the gas combination process. Looking first at the rear of the oven in the top plan view of FIGURE 3, there is shown a header or conduit 90 which extends from side to side of the oven and is open on each end 91 into

cooling wall channels

92, 92 which are'formed in the opposite side walls of the oven. Each cooling wall channel 92 is open at the bottom of the range to receive room air freely in a natural draft. Thus room air is available in the header 90. As seen in FIGURE 1, this header is located at the top of the heat exchanger 66 along the back side thereof. This header 90 is provided with a series of openings 93 which are in alignment with the ambient air columns 80 in the heat exchanger so that room air is drawn down over the corrugated heat exchanger surface 72 at the same time the flue gases are passing up the flue gas columns 78.

There is a second but larger header 94 located at the bottom of the heat exchanger along the back side thereof, and it has a series of openings 95 also in alignment with the ambient air columns 80 for receiving the nowheated air into the header 94. This header 94 is of extended height, and it has an outlet opening 96 which opens into a small compartment 97 located beneath the bottom wall 49 of the fire box and hence beneath the combustion chamber 34. This heated air is to be supplied to the combustion chamber 34, and thus a series of elongated slots 98 are formed in the bottom wall 49 of the fire box generally in alignment with each other and underlying the burner 33 as can be seen in the top plan view of FIGURE 3. The use of pre-heated secondary air for the combustion process vastly reduces the amount of gas used during the heating cycle of the gas oven 13 especially during the high temperature heat cleaning cycle, but also during normal cooking operations.

Most of the hot flue gases from the combustion chamber 34 pass around the outside of the oven liner 20, but a small portion of the gases is allowed to flow through the cooking cavity. As mentioned previously, the bottom wall 23 of the oven liner is provided with a large rectangular opening 30 which is adapted to be covered by the removable cover plate'31. The front edge of the panel 31 is formed with a series of elongated slots 101 which are best visualized in the top plan view of FIG- URE 3. It is deemed well to supplement the heat lost through and around the oven door 21 so as to insure that the temperature of the inner surface of the oven door that is within the mouth of the oven liner has substantially the same operating temperature as the walls of the oven liner so as to obtain generally uniform wall cleaning characteristics. One problem area in many selfcleaning ovens has been a failure to clean properly the lower edge of the door and the front edge of the bottom Wall of the oven liner. This does not occur in the design of the present invention where more heat is directed toward the front of the oven.

An oven vent opening 103 is formed in the top wall 26 of the oven liner to serve as an exhaust means, and it is shown furnished with a catalytic oxidation unit 105 which is represented by a block of corrugated ceramic such as is sold by the Coming Glass Company under the trade name Cercor. This block has a plurality of elongated perforations, and the ceramic surface is coated with a thin coating of catalytic material such as patinum or the like. Hence, when the perforated ceramic block is heated by the flue gases exhausting up through the cooking cavity or passing over the top of the block in the top heating channel 57, the smoke, odors, vapors and any objectionable gases are further degraded. These undesirable products in the oven effluent undergo an exothermic reaction in the oxidation unit such that the unit is self-heating and self-sustaining. Such an oxidation unit is described and claimed in a copending application Ser. No. 396,551 of the present applicants which was filed on Sept. 15, 1964, now Patent No. 3,428,435, and assigned to the General Electric Company, the assignee of the present invention. Moreover, the minor flow of flue gases through the oven cavity serves to supply necessary oxygen for the secondary degradation in the oxidation unit 105 of the primary degradation products produced by pyrolysis.

FIGURE 4 is a diagrammatic showing of one example the nature of the heat exchange surface 72 with respect to the variation of temperature of the room air and the flue gases from the time they both enter until the time they both leave the heat exchanger, and it illustrates rather well the relative changes that take place in both the flue gas and the room air as a result of the presence of the heat exchanger in the system. Notice particularly that the room air can be pre-heated to a temperature above the exhaust temperature of the flue gases.

It will be well recognized by those skilled in this art that it is not critical that the heat exchanger 66 be located at the back of the oven. FIGURES 6-8 are presented to show another modification of the present invention where the heat exchanger is divided into two

parts

112, 112 and each part is mounted in one of the opposite side walls of the oven body. The nature of the heat exchange surface is substantially the same as before wherein the preferred embodimnt of this invention the heat exchanger is arranged in a vertical position with the flue gases rising from bottom to top and the room ambient air flowing from top to bottom. The same elements in both the preferred embodiment of FIGURES 1-3 and the modification of FIGURES 6-8 will have the same reference numerals. The flue gas flow path over the oven liner 20 is the same for both modifications. Each heat exchanger 112 has a header 114 along the inner surface of the exchanger at the bottom thereof as is seen in FIGURES 6 r and 8. Each header 114 has a flue gas inlet opening 115 located at the back lower corner of the oven liner for supplying the flue gas columns 117 of FIGURE 8 through inlet openings 118. A second header 120 joins the tops of the flue gas columns 117 and exhausts by suitable duct work 122 through the backsplash 17 for returning to the kitchen atmosphere.

Looking at FIGURES 6 and 7, the room air enters at the top of each heat exchanger 112 through a header 124 which i open at 130 to the air channels 92 at the front of the oven and has spaced openings 125 aligned with the ambient air columns 127. These openings 130 at the front serve to pull cooling air up the sides of the oven to cool the outer surfaces of the even before the air enters the heat exchanger header 124. Each air column 127 is open at its bottom end into the compartment 97 beneath the bottom wall 49 of the fire box for furnishing pro-heated secondary air to the combustion chamber. Notice in both heat exchangers of FIGURE 3 and FIGURE 6 the cool channels are on the outside and the hot channels are on the inside toward the oven.

Modifications of this invention will occur to those skilled in this art; therefore, it is to be understood that this invention is not limited to the particular embodiments disclosed but that it is intended to cover all modifications which are within the true spirit and scope of this invention as claimed.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. The method of cleaning from the interior surfaces of both an inner oven liner and access door defining an oven cooking cavity the food soils accumulated thereupon during the previous carrying out in said oven cavity of normal food cooking operation in the normal food cooking temperature range extending from about F. to about 550 F., wherein said oven liner is surrounded by an outer oven liner defining a fire box encompassing the oven liner; said method comprising supplying heat from a gas burner in the fire box beneath the inner oven liner, directing the flue gases up around the opposite sides of the inner oven liner and across the top of the inner oven liner and down the back portion of the inner oven liner so as to raise the temperature of the walls forming the oven cooking cavity into a pyrolytic temperature range above about 750 F., introducing a layer of thermal insulating material around the sides of the outer oven liner to retain the heat of the fire box therein, providing a heat exchange surface between the outer oven liner and the layer of thermal insulating material, introducing relatively cool air and the flue gases across opposite sides of the heat exchange surface so as to recover heat from the flue gases and lower the temperature of the flue gases exhausting from the oven to between about 300 F. and 55 0 F. while raising the temperature of the cooling air to between 300 F. and 800 F., and then supplying this heated air to the gas burner as pre-heated secondary air, and supplying cooling air over the outside of the layer of thermal insulating material at the bottom, the opposite sides and the top of the oven so as to hold down the temperature of the outer side of the oven.

2. The method set forth in claim 1, wherein a small portion of the flue gases from the gas burner is fed into the oven cavity adjacent the access door so as to assist in obtaining uniform temperature distribution across the walls forming the oven cooking cavity, as well as to supply oxygen whereby the accumulated food soils mentioned are degraded with the production of corresponding gaseous degradation products, subjecting the degradation products to catalytic oxidation and mixing the degradation products with the flue gases of the fire box before returning the flue gases to the atmosphere.

3. The method of heating the walls of a baking oven for the heat cleaning of soils from the interior wall surfaces of both an oven liner and an access door defining an oven cooking cavity which includes soils accumulated thereupon during the previous carrying out in said oven cavity of normal food cooking operations in the normal food cooking temperature range extending from about 150 F. to about 550 F.; said method comprising generating flue gases by a gas burner located in a fire box beneath the bottom of the oven liner, directing the flue gases up around the opposite sides of the oven liner and across the top of the oven liner and then down the back portion of the oven liner, passing the flue gases over a heat exchanger surface and then exhausting the flue gases to the atmosphere, while at the same time passing relatively cool ambient air over the exchanger surface so as to use the heat from the flue gases to heat the ambient air, then supplying the heated air as pre-heated secondary air for the gas burner, whereby the heat recovered from the flue gases lowers the exhaust temperature of the flue gases to the temperature range between about 300 F. and 550 F. and also increases the efliciency of combustion of the gas burner, so as to reduce the consumption of fuel and hence the amount of the exhaust gases and the total heat in the exhaust gases.

4. The method set forth in claim 3 wherein a small portion of the flue gases from gas burner is supplied into the oven cavity adjacent the access door to assist in obtaining uniform temperature distribution across the walls of the oven cooking cavity as well as creating convection currents of air, whereby the accumulated food soils mentioned are degraded with the production of corresponding gaseous degradation products, then exhausting from the oven the gaseous products into the flue gases prior to the passage over the heat exchanger surface.

5. The method of cooking in an oven having both an oven liner and an access door defining an oven cook ing cavity; the method comprising the use of gas burner for generating flue gases beneath the oven liner, directing most of the flue gases up around the opposite sides of the oven liner and across the top of the oven liner and then down the back portion of the oven liner, passing the flue gases through a heat exchanger to recover some of the heat therefrom and reduce the temperature of the flue gases prior to the return of said gases to the atmosphere, a small portion of the flue gases from the gas burner passing directly into the cooking cavity and exiting from the cooking cavity adjacent the top thereof and mixing with the main portion of the flue gases before the gases reach the heat exchanger, and introducing relatively cool air into the heat exchanger in a counter-flow direction to the flue gases whereby the cool air becomes heated and then is fed to the gas burner as pro-heated secondary air to im prove the efiiciency of the combustion process.

6. A gas cooking apparatus comprising an outer supporting structure, box-like walls forming an oven cavity, one wall of the cavity including a door for gaining access thereto, a layer of heat-insulating material surrounding the oven cavity for retaining the heat therein, heating means for the cavity for supplying heat for cooking food placed therein, said heating means including a gas burner in a fire box beneath the walls of the oven cavity, said fire box extending up around the sides and back and over the top of the oven walls, said fire box being located inside the layer of insulating material, the gas burner generating flue gases which flow through the fire box in a direction up the opposite side walls of the oven, then over the top wall and finally down the back wall, the bottom wall of the oven having an opening to the fire box so that a small portion of the flue gases may enter the oven cavity to create a convection current, a vent opening in the oven walls so the flue gas may exit from the oven cavity and mix with the main flow of flue gases, a heat exchanger located behind the back wall of the oven and between the fire box and the layer of insulation, the flue gases entering the heat exchanger at the bottom thereof and passing upwardly and exhausting to the atmosphere, relatively cool air entering the heat exchanger at the top thereof and passing downwardly therethrough and becoming heated thereby, this heated air being fed beneath the fire box, and an opening in the fire box, so that the heated air passes over the gas burner as pre-heated secondary air to improve the efficiency of combustion, said heat exchanger having its cool side to the outside of the oven and its hot side toward the walls of the oven' cavity.

7. A gas oven comprising a substantially box-like oven liner having a front opening, and an oven door cooperating with the opening for defining an oven cooking cavity and allowing access thereto, a fire box located beneath the inner oven liner, gas burner means positioned in the fire box, flue channels completely surrounding the vertical walls and the top wall of the oven liner, the gas burner means generating flue gases which flow through the flue channels for heating the walls of the oven liner, the gas burner means being adapted to receive a supply of gas fuel and a supply of primary air for admixture therewith, means for introducing a supply of secondary air to flow over the gas burner means, said means causing said secondary air to recover some of the heat from the flue gases before the flue gases are expelled from the oven so that the secondary air is first heated before reaching the gas burner means and thereby increasing the efliciency of combustion of the gas burner means.

8, A self-cleaning gas oven comprising an outer supporting structure, a layer of thermal insulating material, a box-like fire box, gas burner means in the bottom of the fire box, the fire box being open at the front thereof and above the burner means, a box-like oven liner positioned behind the front opening and within the fire box and above the burner means, the oven liner also having a front opening substantially coinciding with the from opening of the fire box, an access door fitting over the opening in the oven liner thereby forming an oven cooking cavity, the gas burner means being adapted to receive therein a supply of fuel gas and a first supply of ambient primary air for admixture therewith, openings formed in the bottom portion of the fire box adjacent the gas burner means for supplying a second supply of ambient secondary air over the gas burner means, exhaust vent means combined with the fire box so that the convection currents of flue gases generated by the gas burner means may return to the atmosphere, a heat exchange means interposed between the layer of thermal insulation and the walls forming the fire box, the heat exchange means being heated by the heat of the flue gases, the ambient secondary air flowing into contact with the heat exchange means and becoming heated thereby, whereby the secondary air supply for the gas burner means is pre-heated prior to its reaching the gas burner means so as to increase the efliciency of combustion of the gas burner means.

9. A self-cleaning gas oven as recited in claim 8 wherein the said heat exchange means has a counter fiow systern with the ambient secondary air entering the heat exchange means at the top of the oven supporting structure and flowing toward the underside of the fire box where the pre-heated secondary air passes up through the said openings in the bottom wall of the fire box, the flue gases enter the heat exchange means at the opposite end thereof, the heat exchange means also serving as a separator to prevent the mixture of the flue gas with the ambient secondary air, and means to separate the pre-heated secondary air from the ambient primary air, the heat exchange means serving to recover much of the heat from the flue gases thereby lowering the temperature thereof before the gases are returned to the atmosphere.

10. A self-cleaning gas oven comprising an outer supporting cabinet structure enclosing a box-like oven liner and a front-opening access door which define an oven cooking cavity, a box-like fire box surrounding the oven liner and Spaced outwardly therefrom to form heating channels, gas burner means in the bottom of the fire box for generating fiue gases which pass through the heating channels and raise the temperature of the walls of the oven liner for both cooking of foods placed therein and the pyrolytic cleaning of the food soils from the inner surfaces of the walls forming the oven cooking cavity, baffie means in the heating channel for directing the flue gases up the side walls of the oven liner more toward the front of the oven and then across the top wall and down the rear of the side walls and also down the back wall, a counter flow heat exchanger mounted across the back wall of the fire box with a flue gas inlet at the bottom thereof and a cooling air inlet at the top thereof, the heat exchanger also having a flue gas outlet at the top thereof for exhausting to the atmosphere, and a cooling air outlet at the bottom thereof in communication with the gas burner means as a source of pro-heated secondary air, a layer of thermal insulating material surrounding the bottom, op-

posite side walls, top wall of the fire box and the heat exchanger at the back of the oven for retaining the heat of the oven.

11. A self-cleaning gas oven as recited in claim with the addition of cooling channels across the bottom and up the side walls of the oven between the layer of insulating material and the cabinet, said side cooling channels communicating with the said cooling air inlet at the top of the heat exchanger.

12. A self-cleaning gas oven comprising an outer supporting cabinet enclosing a box-like oven liner and a frontopening access door which defines an oven cooking cavity, a box-like fire box surrounding the oven liner and spaced outwardly therefrom to form heating channels therebetween, gas burner means in the bottom of the fire box for generating flue gases which pass through the heating channel and raise the temperature of the walls forming the oven cooking cavity for both the cooking of foods placed therein at temperatures between about 150 F. and 550 F. and for the pyrolytic cleaning of the food soils from the inner surfaces of the walls of the oven cavity at temperatures between about 750 F. and 950 F., a vertically arranged stationary heat exchange means located outside the fire box and having a flue gas inlet at the bottom thereof and an ambient air inlet header at the top thereof, the flue gas and ambient air flowing in opposite directions to each other, there being a flue gas exhaust header at the top of the heat exchange means and an ambient air exhaust header at the bottom of the heat exchange means, the flue gas exhaust header having an outlet to the atmosphere, conduit means beneath the bottom of the fire box communicating with the ambient air exhaust header, and openings in the bottom wall of the fire box whereby the ambient air becomes heated within the heat exchange means and passes through the conduit means and into the fire box as pre-heated secondary air for the gas burner means, a layer of thermal insulating material encircling the heat exchange means and the outer surfaces of the fire box for retaining the heat therein.

13. A self-cleaning gas oven as recited in claim 12 wherein openings are formed in the bottom portion of the oven liner so that a small portion of the flue gases enter the oven cooking cavity, an exhaust vent in the upper portion of the oven liner leading into the heating channels, an oxidation unit mounted in the exhaust vent for degrading the oven exhaust gases.

14. A self-cleaning gas oven as recited in claim 13 with the addition of baflle means in the heating channels at the opposite sides of the oven so that the main portion of the flue gas is directed over the opposite side walls of the oven liner generally toward the front portion of the oven and then rearwardly over the top wall and down over the rear portion of the side walls as well as down the back wall of the oven liner and then into the flue gas inlet of the heat exchange means.

15. A self-cleaning gas cooking oven comprising an oven body supporting a box-like oven liner and a frontopening access door which define an oven cooking cavity, a fire box disposed beneath the bottom wall of the oven liner, a gas burner located within the fire box, heating channels extending up the opposite side walls back across the top wall and down the rear wall of the oven liner and communicating with the fire box so that convection currents of flue gases from the burner may pass through the channels thereby heating the walls forming the oven cooking cavity, a vertically arranged stationary heat exchanger located adjacent at least one of the heating channels, a flue gas inlet at the bottom of the heat exchanger, and a flue gas outlet at the top of the heat exchanger, an ambient air inlet at the top of the heat exchanger and an ambient air outlet at the bottom of the heat exchanger, said ambient air outlet supplying heated air to the fire box as pre-heated secondary air for the burner, whereby the flue gases flowing through the heat exchanger give up much of their heat to the ambient air and thereby exhaust to the atmosphere at a temperature in the order of magnitude between one-third and two-thirds the temperature of the flue gases entering the flue gas inlet of the heat exchanger.

16. A self-cleaning gas oven as recited in claim 15 wherein the bottom portion of the oven liner has restricted openings into the fire box whereby a small portion of the flue gases may enter the oven cooking cavity, an exhaust opening in the upper portion of the oven liner, an oxidation unit located in the exhaust opening and being heated by both the flue gases in the heating channels and the oven exhaust gases flowing therethrough for degrading the oven exhaust gases of smoke, odors, vapors and carbon monoxide, said oxidation unit discharging into the heating channels.

17. A self-cleaning gas cooking oven as recited in claim 16 wherein the gas burner is adapted to be connected to a source of fuel gas and to have a source of room ambient air as the supply of primary air for the burner.

References Cited UNITED STATES PATENTS 3,364,912 1/1968 Dills et al. 3,416,509 12/1968 Huebler et a1. 3,417,742 12/1968 Perl. 3,422,809 1/1969' Perry.

CHARLES I. MYHRE, Primary Examiner