Self-cleaning Gas Oven With Heat Exchanger

Abstract

A heat exchanger system for a self-cleaning gas cooking oven where the oven is operated as a muffle oven for both baking and heat-cleaning operations. A heat exchanger is employed for cooling the flue gases as well as preheating the secondary air for the bake burner. There is an auxiliary secondary air supply for the bake burner, and it includes a normally open temperature-responsive damper means that serves to close the auxiliary air supply when the firebox temperature reaches a predetermined amount. This same auxiliary air supply is used when an upper broiler burner is operated during closed door broiling, and it cooperates with a normally closed oven vent that is opened for use during the broiling operation.

Description

BACKGROUND OF THE INVENTION

A great amount of testing has led to the conclusion that the most efficient self-cleaning gas cooking oven design is one using a muffle oven principle where the oven liner is heated externally by convection currents of hot flue gases from the lower firebox or combustion chamber. Both the baking and heat-cleaning operations would be performed using such a heating method. However, the bake burner is unable to draw sufficient ambient air through the heat exchanger when the burner first starts burning with ambient air temperature conditions in the firebox. Hence, it is well to provide other means for supplying secondary air to the bake burner during the preheat portion of either the baking or heat-cleaning operations. Moreover, it is well to provide a secondary air supply for an upper broiler burner so as to be able to perform a closed door broiling 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 hot flue gases is reduced in a heat exchanger before the gases are released to the atmosphere, as well as having provision for an auxiliary air supply to the bake burner until the heat exchanger reaches an operating temperature.

A further object of the present invention is to provide a gas oven of the class described with a temperature-responsive damper means for the auxiliary air supply so as to discontinue the auxiliary air supply once the heat exchanger is up to operating temperature.

A further object of the present invention is to provide a self-cleaning gas cooking oven of the class described with a closed door broiling cycle that utilizes the auxiliary air supply means for the bake burner as the secondary air supply for the broiler burner.

SUMMARY OF THE INVENTION

The present invention, in accordance with one form thereof, relates to a self-cleaning gas cooking oven with an insulated outer structure and an oven liner with outer heating channels associated with at least a major portion of the walls thereof. These heating channels communicate with an underlying firebox whereby hot flue gases from a bake burner in the firebox pass through the heating channels in the manner of a muffle oven for raising the temperature of the walls of the oven liner for normal baking operations as well as for high temperature, heat-cleaning operations. A heat exchanger means is associated with the oven for receiving the flue gases and cooling them down before the gases are returned to the atmosphere. This heat exchanger uses ambient air as a coolant and this air becomes heated in the process and is drawn into the firebox as preheated secondary air for the bake burner. An auxiliary secondary air supply means is available for use during the preheat portion of the baking or heat-cleaning cycle. This auxiliary air supply is furnished with a normally open temperature-responsive damper means for closing the auxiliary air supply once the heat exchanger reaches its operating temperature. Moreover, this auxiliary air supply means is used to advantage as the air supply means of secondary air for an upper broiler burner when a normally closed oven vent means is opened for a closed door broiling operation.

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.

FIG. 1 is a left side-elevational view of a free-standing gas range with some parts broken away and others in cross section to show a single oven cooking cavity for both baking and broiling operations, and a heat exchanger extending across the back of the range, and a layer of thermal insulating material on the inner surface of the outer cabinet structure for the oven.

FIG. 2 is a fragmentary front elevational view taken on the line 2--2 of FIG. 1 and showing the lower portion of the oven construction with a bake burner in the firebox beneath the oven liner, and cooperating heating channels for heating the walls of the oven liner, as well as an auxiliary secondary air supply means in the bottom of the firebox with its normally open damper means for use during the preheat portions of the baking and heat-cleaning operations.

FIG. 3 is a fragmentary plan view looking down into the heat exchanger and taken on the lines 3--3 of FIG. 1 to show the compressed corrugated sheet metal nature of the vertical pipes of the heat exchanger for the upward passage of the hot flue gases and the downward passage of the ambient air.

Turning now to a consideration of the drawings and in particular to FIG. 1, there is shown a free-standing domestic gas range 10 in left 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 and its operation. Arranged along the back of the cook top 12 is a backsplash 17 which supports a control panel in the front face thereof which would have a complete array of oven burner control components 18, including an oven thermostat, oven timer, convenience outlets, and the like; generally of the same nature as is found on a standard gas range. As shown, the controls 19 for the surface burners are located toward the front of the cooktop.

The oven cavity 13 is formed by a boxlike oven liner 20 in combination with a front-opening access door 21. The oven liner 20 has a bottom wall 23, opposite vertical sidewalls 24, a top wall 26 and a rear wall 27. The oven racks and their support means are not shown for the sake of simplicity. The bottom wall 23 of the oven liner is formed with a large generally rectangular opening 30, as is best seen in FIG. 1, which is adapted to be covered by a removable cover plate 31. The reason for the removable cover plate 31 is to make it possible to gain easy access to a bake burner 33 which is located in a combustion chamber or firebox 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. Fastening 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 body 11 until the flange 36 bears against a cooperating collar or door frame 38. Sandwiched between the flange 36 and the door frame 38 is a thermal break 40 such as a continuous asbestos or fiber glass gasket which serves to break the conductive path of the heat flow from the oven liner to the front of the oven body, as well as providing a sealing means between these two elements and preventing flue gases from escaping at the front of the oven.

The oven door 21 is fabricated of sheet metal panels and it is substantially filled with heavy thermal insulation (not shown) so as to retain most of the heat within the oven cavity so that the temperature of the outer surface of the door will be within a comfortable temperature range so as not to cause discomfort or injury to those who might touch it during a high-temperature heat-cleaning operation, 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 is thermally isolated therefrom by means of a high-temperature gasket material 45 such as woven asbestos or the like. This gasket 45 also serves to bear against the front flange 36 of the oven liner 20 and also engages the front wall of the oven body 11 to substantially seal the front opening of the oven when the door is closed and locked (by locking means not shown) as it would be during a heat-cleaning cycle.

Located within the oven body 11, and surrounding the oven liner 20, is an outer oven liner 47 that is spaced outwardly from the inner oven liner 20 to form heating channels 50 therebetween. These channels surround the oven liner and communicate with the firebox 34, as is best seen in the front elevational view of FIG. 2. The hot flue gases from the bake burner 33 pass through the heating channels 50 for raising the temperature of the walls of the inner oven liner 20 for normal baking operations as well as for a high-temperature heat-cleaning operation. This outer oven liner 47 has a bottom wall 52, rear wall 53, a top wall 54 and opposite sidewalls 56. A layer 58 of thermal insulating material such as fiber glass or the like is disposed outside of the outer oven liner 47 and inside of the oven body or cabinet structure 11 for retaining the heat within the oven structure.

The direction of flow of the flue gases from the bake burner 33 is generally up the two heating channels 50 at the two sides of the inner oven liner 20 and back through the heating channel 50 over the top of the inner oven liner and then down the back heating channel 50 to exit through a flue gas exhaust opening 60 adjacent the bottom portion of the back wall 53 of the outer oven liner 47, as is best seen in FIG. 1. For a more detailed explanation of the actual flue gas flow pattern for heating the walls of the inner oven liner 20 and the manner in which it is directed, reference may be made to U.S. Pat. No. 3,507,265 of Bohdan Hurko and Raymond L. Dills, which first taught of this construction. Another patent showing a muffle oven design is U.S. Pat. No. 3,364,912 of the same inventors Hurko and Dills.

In the oven configuration shown, the brake burner 33 is of a size and a heat rating somewhere between 16,000 and 30,000 B.T.U.'s an hour for a standard discharge nozzle and gas pressure. It is optional whether one large burner or two small burners should be used. The rear portion of the bake burner 33 has a venturi tube throat 62 connected to a gasline connector 64 through the back wall of the oven. A shroud 66 encircles the venturi and the gasline connector 64. Air openings 67 are formed in the back wall of the oven body so that the shroud is open to the atmosphere, and ambient air may be introduced into the venturi tube throat 62 as primary air for combustion. An adjustable air mixer head (not shown) is combined with the venturi tube as is standard in this art in order to provide the best air-to-gas mixture ratio. The means for physically supporting the burner within the firebox and the necessary pilot and gas line control valves are not shown, nor are the temperature-sensing and control systems since they may be of standard construction and do not form part of the present invention.

An upper gas burner or broil burner 70 is assembled in the top of the oven cavity 13 for use during normal broiling operations, but it may be disregarded during the baking operations or during the heat-cleaning cycle. This broil burner 70 and a venturi tube throat 72 that terminates within the oven cavity, and it connects with a gasline 74 that extends out through the back of the oven.

The bake burner 33 will be used during baking operations for raising the temperature within the oven cavity 13 to various levels between 150.degree. F. and 550.degree. F. It only takes a short time to reach the preset temperature such that the burner will only operate for a short period of time at full rating, as for example 10-15 minutes. However, during a heat-cleaning cycle where the maximum oven air temperature should reach somewhere between 750 .degree. F. and 950.degree. F., the preheat period of the cycle might approximate 40-60 minutes, and the oven may be maintained at maximum oven temperatures for up to 3 hours depending upon the amount of food soils lodged on the walls of the oven liner and inner surface of the door 21. The rate of "heat to maintain" the oven at the maximum heat-cleaning temperature for a period of from 1-3 hours is about 75 percent of the preheat rate or about 10,000-16,000 B.T.U.'s per hour.

A gas burner produces very little radiant heat since the flames radiate only about 10-15 percent of the heat of the burner, while the remainder of the heating is accomplished by convection currents of flue gases. In a standard gas oven large amounts of flue gases and hot air pass through the oven. The heating value of 1 pound of natural gas is about 30,000 B.T.U.'s per hour. For ideal combustion it is necessary to have at least 15.3 pounds of air for each pound of gas. However, an excess amount of air is needed in heating up the oven, up to 300 percent of excess air. This means that for each pound of gas up to about 61.2 pounds of air (15.3 .times.4 ) may be needed to provide complete combustion while holding the amount of carbon monoxide to a minimum.

If, for example, the flue gases are at the necessary maximum heat-cleaning temperature of about 900 .degree. F. above the room ambient temperature (.DELTA.t) when the gases break contact with the walls of the inner oven liner 20 during the cleaning cycle, and the heating rate is 1 pound of gas burner per hour, then the heat of the exhaust may be calculated as follows:

Q=C.sub.p .DELTA.t (m.sub. a +m.sub. g )

=0.26 .times.900.times.(61.2+1 )= 14,650 B.T.U.'s per hour

m.sub.g =pounds of gas

m.sub.a =pounds of air

C.sub.p =specific heat of flue gas

.DELTA.t =temperature differential

The exhaust heat of 14,650 B.T.U.'s per hour is equivalent to heating a kitchen with an electric heater of 4,300 watts rating. In addition to this amount of heat being returned to the kitchen, there will be the usual heat losses from the oven body which will be of the same order of magnitude as the heat losses in a self-cleaning electric oven and will depend upon the maximum surface temperature of the oven body or cabinet 11.

In a self-cleaning electric oven the amount of circulated air through the oven is very low, on the order of less than 3 cubic feet per minute, as compared with a gas oven with an air movement of about 38.6 cubic feet per minute. Hence, the heat exhausted by an electric oven will be less than 8 percent of the heat exhausted by a gas oven during the heat-cleaning cycle.

The high heat losses and the inherently low efficiency of a gas burner system for a self-cleaning oven have led to the discovery of significant improvements in the design; namely, the incorporation of a heat exchanger 80 shown in FIG. 1 between the insulation layer 58 and the rear wall 53 of the outer oven liner 47. The heat exchanger 80 substantially covers the back wall 53 of the outer oven liner 47. The purpose of this heat exchanger 80 is to recover much of the heat from the flue gases that leave the heating channels 50 by way of the opening 60 before the gases are returned to the kitchen atmosphere. This is accomplished by passing room ambient air in a counterflow direction down through the heat exchanger 80 which extracts heat from the flue gases. This heated air is fed through openings 82 in the lower portion of the rear wall of the outer oven liner 47 into the firebox 34 to serve as a source of preheated secondary air for the bake burner 33.

A better understanding of the nature of the heat exchanger 80 may be obtained by reference to the fragmentary cross-sectional plan view of FIG. 3 taken on the line 3--3 of FIG. 1. The heat exchanger 80 has a series of individual vertical pipes where one set of alternate pipes 86 handle the upward passAge of the hot flue gases, while the alternate set of pipes 84 are for the downward passage of the room ambient air. The actual construction of the heat exchanger is very simple and ingenious in that it incorporates a thin corrugated sheet metal configuration where the corrugated sheet is compressed in its plane between its two sides such that the corrugation close and engage each other along both the front side 87 and the back side 88, and it is best shown and described in a copending application that was filed concurrently herewith, Ser. No. 46,114, filed June 15, 1970. Notice that the pipes 84 and 86 have a transverse cross section of an isosceles triangle. Thus, the heat exchanger 80 has a large surface area and it is an efficient heat exchange medium. Notice also that these triangular pipes 84 and 86 reduce heat losses toward the outside because the entire back wall 88 is cooled by ambient air. Thus, less insulation 58 is needed to hold down the exterior temperatures of the range to allowable limits. A louvered opening 90 is formed in the upper portion of the back wall of the oven body as an ambient air inlet opening. This air enters into a channel-shaped manifold 92 which extends across the back of the oven and from which the air enters the air pipes 84 through individual slots 94 in the air pipes.

Another set of louvered openings 96 are formed above the louvered air inlet openings 90, and this is for exhausting the flue gases that rise up the flue gas pipes 86 and empty in the rear area beneath the cooktop 12 where they are dispersed either rearwardly through the exhaust openings 96 or up through an opening 98 beneath the backsplash panel.

It has been found best to provide some auxiliary means for supplying secondary air to the bake burner 33 during the preheat portion of a baking or heat-cleaning cycle when the heat exchanger 80 is relatively cold and it is not possible to create a sufficient downdraft to draw the ambient air down the air pipes 84, 84. Such an auxiliary air supply is furnished by providing a pair of parallel openings 100 through the insulated bottom wall 52 of the outer oven liner 47 such that room air may first enter the lower portion of the oven body, as for example through the openings 102 in the back of the oven, and pass into the firebox through the openings 100. Actually there is another plate 101 above the bottom wall 52 and it has a series of spaced slots 103 positioned directly under the bake burner 33.

Once the temperatures in the firebox 34, in the heating channels 50 and in the heat exchanger 80 have risen to a predetermined level, sufficient air would then be drawn down through the heat exchanger and introduced into the firebox as preheated secondary air. Then it would be preferable to discontinue the auxiliary air supply through the bottom openings 100. Hence, a normally open temperature-responsive damper mechanism 104 is installed on the bottom wall 52 of the outer oven liner 47, and it has a center support member 105 that carries across its top a bimetallic strap 107 that extends over the sides of the support and is furnished at each end with a hinged damper 109, such that when the temperature within the firebox 34 reaches a predetermined level the bimetal will flex downward causing the dampers 109, 109 to clamp down over the openings 100 and substantially seal these openings, thereby discontinuing the auxiliary secondary air supply.

In the oven as described above, no airflow has been described as being capable of moving through the oven cavity 13. Such movement would not be necessary during the baking operations or the heat-cleaning operations. Now we come to a consideration of the upper broiler burner 70. It, like all gas burners, requires a large amount of oxygen for mixture with the gas to obtain the proper combustion. Moreover, it is preferable to be able to operate the broiler with the oven door closed such that the heat and spatter of the oven will not enter the kitchen.

Closed door broiling has been furnished in this design by first taking advantage of the presence of the bottom air openings 100 in the bottom wall of the outer oven liner 47. Moreover, additional air openings 110 are provided in the removable plate 31 forming a portion of the bottom wall 23 of the inner oven liner 20. Of course, air would not flow continuously into the oven cavity unless there were also an exhaust vent for the air. This is provided by an oven vent opening 115 in the top wall of the oven adjacent the rear thereof. This oven vent 115 is provided with a normally closed damper 117, and this damper is manually operated by linkage 119 which extends forwardly to the front edge of the cooktop 12 and is shown with an external handle 121.

Thus, in order to initiate the closed door broiling operation it would first be necessary to open the damper 117 and then ignite the broiler burner 70 and air would flow through the oven cavity 13 by first entering the openings 100 in the bottom of the oven and then passing through the firebox and the openings 110 in the bottom of the inner oven liner, and then after sweeping over the broiler burner 70 would exhaust through the oven vent 115 and pass either out through the backsplash opening 98 or back through the rear openings 96, as best shown in FIG. 1. Only the broiler burner 70 would be operating during a broiling operation. Hence, the firebox 34 would be relatively cool and the normally open temperature-responsive damper means 104 would remain open during the entire broiling operation so as not to shut off the air supply through the openings 100 to the broiler burner 70. The manually operated damper 117 would be closed during both the baking and the heat-cleaning operations and would be used only during the broiling operations.

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.

Claims

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

1. A gas oven comprising an oven body supporting a boxlike oven liner and a front-opening access door which define an oven cooking cavity, a firebox disposed beneath the bottom wall of the oven liner, a bake burner means located within the firebox, heating channels extending from the firebox around the outside of the oven liner so that convection currents of flue gases from the bake burner means may pass through the channels so as to heat the oven liner, a heat exchanger mounted in the oven body and having adjacent one end a flue gas inlet from the heating channels and a flue gas outlet adjacent the other end as well as an ambient air inlet adjacent the flue gas outlet and an ambient air outlet communicating with the firebox, said ambient air outlet supplying heated air to the firebox as preheated secondary air for the bake burner means, additional ambient air openings in the walls of the firebox for supplying secondary air to the bake burner means during a preheat period of the heating cycle while the heat exchanger is operating relatively cold and the burner means is incapable of drawing the ambient air through the heat exchanger, and normally open temperature-responsive closure means cooperating with the said ambient air openings to close said openings after the firebox reaches a predetermined temperature whereupon preheated secondary air is furnished to the bake burner means by the heat exchanger.

2. A gas oven as recited in claim 1 including a broiler burner means located adjacent the top portion of the oven liner as the sole heating means during a broiling operation whereby the said firebox remains relatively cool so as not to operate the said temperature-responsive closure means and thereby allowing the said ambient air openings to remain open, second air openings in the bottom portion of the oven liner, an oven vent means in the top portion of the oven liner, and second closure means for closing the oven vent means during normal baking and high-temperature heat-cleaning operations while being held open during broiling operations, so that broiling may be accomplished with the oven door closed, whereby secondary air for the broiler burner means enters the firebox through the ambient air openings and enters the oven liner through the said second air openings and after passing over the broiler burner means exits through the said oven vent means.

3. A self-cleaning gas-fired cooking oven comprising an oven body supporting a boxlike inner oven liner and a front-opening access door which define an oven cooking cavity, and an outer oven liner surrounding the inner oven liner and spaced therefrom to form a firebox beneath the inner oven liner as well as heating channels around the inner oven liner, and bake burner means within the firebox to cause convection current of flue gases to pass around the inner oven liner for heating the walls thereof during both baking and heat-cleaning operations, and a vertically arranged stationary heat exchanger located adjacent at least one wall of the outer oven liner, and flue gas inlet openings in the bottom of the heat exchanger communicating with the said heating channels, and flue gas outlet means in the top portion of the heat exchanger, and ambient air inlet means adjacent the top portion of the heat exchanger and heated air outlet means adjacent the bottom portion of the heat exchanger communicating with the firebox as a source of preheated secondary air for the bake burner means, second ambient air openings in the firebox, and normally open temperature-responsive closure means associated with the said second ambient air openings and adapted to close said openings after the temperature within the firebox reaches a predetermined temperature thereby furnishing an initial source of secondary air for the bake burner means while the heat exchanger is in a relatively cool condition when the bake burner means is incapable of drawing the ambient air supply down through the heat exchanger, the heated air supply from the heat exchanger being established prior to the closure of the second ambient air openings.

4. A domestic cooking oven as recited in claim 3 with a broiler burner means positioned adjacent the upper portion of the inner oven liner, and openings in the bottom portion of the oven liner communicating with the firebox, and a normally closed oven vent means for the cooking cavity adjacent the top portion of the inner oven liner, whereby closed door broiling may be performed in the oven by first opening the oven vent means and operating the broiler burner means such that secondary air for this burner means enters the firebox through the said second ambient air openings and passes into the cooking cavity through the openings in the bottom of the inner oven liner for sweeping over the broiler burner means and exhausting through the oven vent means.

5. A self-cleaning gas oven comprising an outer supporting structure, an inner layer of thermal insulating material protecting the supporting structure, a boxlike outer oven liner positioned within the thermal insulating material, an inner oven liner positioned within the outer oven liner and spaced therefrom to form a firebox beneath the inner oven liner and heating channels around at least a major portion of the inner oven liner, a bake burner located within the firebox, whereby hot flue gases from the burner means pass through the heating channels for raising the temperature of the walls of the inner oven liner for normal cooking operations as well as for high-temperature heat-cleaning operations, and heat exchanger means mounted across at least one vertical wall of the outer oven liner, said firebox having air inlet means communicating with the heat exchanger, the lower portion of said vertical wall having flue gas outlet means from the heating channels communicating with the heat exchanger, the upper portion of the heat exchanger having an ambient air inlet means and flue gas outlet means whereby the heat exchanger causes the flue gases to be cooled before they are released from the oven and the air supply to the firebox to become preheated secondary air for the bake burner means; the invention comprising an auxiliary secondary air supply means for the bake burner means for use when the temperature within the firebox is not high enough to create a draft strong enough to draw the ambient air down through the heat exchanger, said auxiliary air supply means comprising second ambient air openings in the firebox, and normally open temperature responsive damper means cooperating with the said second ambient air openings and serving to close said second openings when the firebox temperature reaches a predetermined level.

6. A self-cleaning gas oven as recited in claim 5 with a broiler burner means located in the upper portion of the inner oven liner, openings through the bottom portion of the inner oven liner communicating with the firebox, an oven vent means in the upper portion of the inner oven liner, and normally closed cover means cooperating with the oven vent means, whereby closed door broiling operations may be performed in the oven when the said cover means is opened and the broiler burner means is operated so that secondary air flow is created for the broiler burner means with air entering the said second ambient air openings and rising through the firebox and passing through the openings in the bottom portion of the inner oven liner and passing around the broiler burner means and exhausting through the said oven vent means.

7. A self-cleaning gas oven comprising an outer supporting structure, an inner layer of thermal insulating material within the supporting structure, a boxlike oven liner positioned within the insulated oven structure, a firebox positioned beneath the oven liner, bake burner means within the firebox, and heating channels associated with at least a major portion of the walls of the oven liner and communicating with the firebox whereby hot flue gases from the bake burner means pass through the heating channels in the manner of a muffle oven for raising the temperature of the walls of the oven liner for normal baking operations as well as for high-temperature heat-cleaning operations, and heat exchanger means associated with the oven for receiving the flue gases and cooling them down before the gases are exhausted from the oven, the heat exchanger means using ambient air as a coolant means, said air becoming heated in the process of cooling the flue gases and being fed into the firebox as preheated secondary air for the bake burner means, an auxiliary secondary air supply means for the bake burner means for use when the temperature within the firebox is below a predetermined temperature, said auxiliary supply means comprising ambient air openings in the firebox, and normally open temperature-responsive damper means cooperating with the said ambient air opening and serving to close said air openings when the firebox temperature reaches a predetermined level, and broiler burner means located in the upper portion of the oven liner, openings through the bottom portion of the oven liner, oven vent means in the upper portion of the oven liner, and normally closed cover means to cooperate with the oven vent means whereby closed door broiling operations may be performed in the oven when the said cover means is opened and the broiler burner means is operated so that a secondary airflow is created for the broiler burner means by air passing through the said ambient air openings in the firebox, then through the openings in the bottom portion of the oven liner, over the broiler burner means and exhausting through the oven vent means.