US3667447A

US3667447A - Gas-fired domestic ovens

Info

Publication number: US3667447A
Application number: US47798A
Authority: US
Inventors: Bela M Toth; Ian Thow
Original assignee: Parkinson Cowan Appliances Ltd
Current assignee: Parkinson Cowan Ltd; Parkinson Cowan Appliances Ltd
Priority date: 1969-06-21
Filing date: 1970-06-19
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06
Also published as: FR2052815A5; GB1239497A; JPS4928427B1; DE2030407A1

Abstract

In a domestic gas-fired oven of the pyrolytic self-cleaning type with a forced-draught fan re-circulating a portion of the combustion products back through the oven during self-cleaning the re-circulation ratio, that is to say, the ratio of the total mass circulating to the net mass entering and leaving the system, is between three to one and five to one, being preferably four to one.

Description

United States Patent Toth et al.

[54] GAS-FIRED DOMESTIC OVENS [72] Inventors: Bela M. Toth; Ian Thow, both of Solihull,

England Parkinson Cowan Limited, London, England [22] Filed: June 19,1970

[21] App]. No.: 47,798

[73] Assignee:

| '11)] Foreign Application Priority Data June 2|, I969 Great Britain ..3l,45l/69 521 u.s.c|. ..l26/21A 1 [58] Field ofSearch ..l26/21,2l A, 19,39,273

[56] References Cited UNITED STATES PATENTS 3,437,085 4/1969 Perry 126/21 A [451 June 6,1972

3,384,068 5/1968 Perry et al l 26/21 A FOREIGN PATENTS OR APPLICATIONS 742,248 l2/l955 Great Britain ..l26/2I A 1,142,804 H1963 Germany I 26/21 A Primary Examiner-Charles J. Myhre Attorney-Scrivener. Parker, Scrivener & Clarke ABSTRACT In a domestic gas-fired oven of the pyrolytic self-cleaning type with a forced-draught fan roe-circulating a portion of the combustion products back through the oven during self-cleaning the re-circulation ratio, that is to say, the ratio of the total mass circulating to the net mass entering and leaving the system, is between three to one and five to one, being preferably four to one.

3 Claims, 2 Drawing Figures GAS-FIRED DOMESTIC OVENS This invention relates to self-cleaning gas-fired domestic ovens, that is to say, ovens in which it is possible to raise the temperature of the oven walls substantially above the normal range of cooking temperatures to a temperature level at which any particles of grease or food adhering to the oven walls are burner and also for re-circulating the gases within the oven.

Other proposals have involved the provision of an additional burner or burners to be brought into action for the self-cleanin g function. I

A problem with such arrangements is that of controlling the temperature which the oven is to reach during self-cleaning and this is normally dealt with by the provision of a high-temperature thermostat, which switches the burner, or one of the burners, on and off in a cyclic manner to maintain the temperature within the desired range. However, such thermostats capable of operating at a high temperature, well above the normal cooking temperature range, are expensive and they add appreciably to the cost of the system.

A further problem in self-cleaning gas-fired ovens lies in the large heat output which must somehow be disposed of without overheating the kitchen or other room in which the cooker is situated, and without damaging its paintwork or giving rise to the danger offire.

The aim of the present invention is to provide a self-cleaning system which overcomes these problems by arranging the conditions of operation during self-cleaning such that the desired temperature is reached with the minimum of control equipment and with the minimum of heat loss to the surroundings.

According to the invention we now propose an oven-cleaning system comprising a single forced-draught ventilation system arranged to re-circulate the gases within the oven with a re-circulation ratio (as defined below) lying between three to one and five to one, preferably substantially four to one.

In this way it is possible to obtain effective self-cleaning with only a singe burner, preferably mounted at the bottom of the oven, and the choice of the re-circulation ratio lying within the limits set out above makes it possible to attain equilibrium conditions in the oven at a pre-arranged temperature level, thereby eliminating the need for thermostatic control; it is only advisable for safety reasons to provide a cheap form of thermal cut-out to shut down the burner in the event of a fault giving rise to dangerous temperature levels.

The re-circulation within the limits defined allows the required temperature to be attained with a burner of only modest size and it has the added advantage of reducing the heat escaping to the surroundings during self-cleaning.

The re-circulation ratio, referred to above, is defined as the ratio of the total mass flow of gases through the oven at equilibrium conditions to the total mass input of air and gas.

Preferably the gases for re-circulation and the gases to be discharged to the atmosphere are extracted from the oven through a common flue, placed low down in the rear wall of the oven.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical cross-section through an oven according to the invention, looking from one side; and

FIG. 2 is an isometric view of the burner and re-circulating fan assembly.

Referring first to FIG. 1, the oven 1 can be of orthodox construction except that it is much more heavily lagged against heat loss than a normal non-self-cleaning oven. A trough 2 near the rear edge of the floor of the oven contains an elongated burner 3 having, in the example shown, a maximum heat output of 13,000 B.Th.U. (3,250 Kilocals) per hour. Gas is supplied to the burner as indicated at G and primary air P for combustion is entrained by the usual mixing tube 4.

A fan 5 of the tangential type driven by an electric motor 6 is divided into two

unequal sections

7 and 8, of which the smaller section 7 draws in air S from the atmosphere to form secondary air for combustion. The outputs of both sections of the fan lead to a slot in the bottom of the trough so that the output flows upwards past and around the burner, and assists combustion as well as causing a circulating flow of the products of combustion around the oven as indicated by the arrow C.

Combustion products escape through an outlet 9 and are divided, a minor part passing upwards as indicated by the arrow F through nozzles 10 to atmosphere, where they entrain outside air, as indicated by arrows A, and then pass upwards, cooled and diluted by this air, to a flue 11.

The major part of the combustion products entering the outlet 9 is drawn through a duct 12 by the larger section 8 of the fan 5 as indicated by the arrow R and re-circulated through the oven.

Thus the total gases circulating in the oven 1 are formed by the total mass flow C of the gases represented by the arrows G, P, S and R. The total mass entering the system is represented by G, P and S. The total mass F leaving the system must necessarily be equal to this.

The re-circulation ratio is the ratio (G P S R) to (G P S). This is equivalent to the ratio (F R) to F. Thus if one quarter of the combustion products entering the outlet 9 pass to the flue and three-quarters are re-circulated through the duct 12, this means R is three times F (which equals G P S) and the re-circulation ratio is four.

Tests have shown that with a typical domestic oven with a volume of 2.5 cubic feet (71 liters), having the burner mentioned above, with a heat output of 13,000 B.Th.U (3,250 Kilocals) per hour, and consuming a total of 15.5 lb. (7 Kg.) per hour of gas and air, while maintaining a re-circulation R of 46.7 lb. (21.1 Kg.) of combustion products per hour (giving a re-circulation ratio of 4), the oven attains an equilibrium temperature of 522 C. This is adequate for burning off food and grease particles and leaving the walls of the oven completely clean.

The primary air P is arranged to provide about 60 percent of the air required for complete combustion of the gas according to stoichiometric principles. The secondary air S is arranged not only to provide the remaining 40 percent for combustion of the gas but also a further 70 percent excess air, making a total of 170 percent of the quantity needed for burning the gas, and this excess air is present to oxidize and burn away the waste products on the oven walls. I

With the equilibrium conditions mentioned above the amount of heat that is recirculated through the duct 12, as opposed to the mass re-circulated, is between 50 and 70 percent of the net heat output of the burner, i.e., between 6,000 and 9,000 B.Th.U. 1,500 and 2,250 Kilocals) per hour where the burner output can vary between 1 1,500 and 16,000 (2,875 and 4,000 Kilocals) per hour. With the burner giving 13,000 B.Th.U per hour, 62 percent of the heat input is being re-circulated (8,000 B.Th.U. or 2,000 Kilocals per hour).

It will be understood that the-equilibrium temperature attained can be varied by varying the heat input from the burner. In practice it should be at least 425 C. in order to ensure adequate cleaning of the oven walls and if it is allowed to rise above 650 C there is danger of damage to the enamel coating on these walls.

The value of four to one for the re-circulation ratio has been found to provide the optimum balance between efficient combustion and minimum unnecessary heating of the surroundings, as well as achieving maximum economy. If the ratio is substantially increased, i.e. if a smaller proportion of the combustion products are allowed to escape, the flame stability is adversely affected by the high proportions of combustion products around the burner, incomplete combustion occurs and the system is no longer able to continue the cleaning process. In practice the re-circulation ratio can be increased a little above four, but cannot be more than five to one without inefficiency.

Likewise, if the re-circulation ratio is reduced appreciably below four to one resulting in a higher proportion of the combustion products being allowed to escape to the atmosphere, not only does this increase the undesirable discharge of large quantities of hot products to be disposed of, but also it results in a reduction in oven temperature due to dilution of the cir culating gases with cold incoming air. In practice worthwhile results are not obtainable with a ratio of less than three to one.

It will be understood that for normal operation of the oven for cooking or baking, the same burner can be used, but at a reduced heat output. The fan 5 can be kept running to circulate the combustion products and-ensure even temperature conditions in the oven. The burner may be of the duplex type, that is to say, of the type in which the gas flow to one group of the ports is controlled separately from that to another group, and one group may be cut off altogether during cooking. Under these conditions the oven temperature can be controlled by the normaloven thermostat acting on the burner.

As indicated above, the temperature during self-cleaning is set solely by the heat balance attained under equilibrium conditions and no high-temperature thermostat is used. However, there is preferably a simple thermal cut-out responding to severe excess temperature conditions, for reasons of safety.

In FIG. 2 is shown a practical layout for the burner and fan. The same reference numerals as in FIG. 1 have been used where applicable The oven itself has been omitted. Two outlets from the oven, shown at 9 and 9" enter a cylindrical manifold 13 from which a minor part of the combustion products escape through nozzles 10 to the flue 11, while the re-circulation duct 12 leads off at right angles from the middle .Of the manifold. The fan 5 is secured to the bottom of the burner trough 2. It will be noted, as indicated by the arrow A, that the fan rotates in the opposite direction to one which one would expect, and so its output is across rather than along its tangential output duct that leads into the trough 2. This helps to ensure good turbulence and mixing of the incoming secondary air and the re-circulated products as they both reach the burner.

It will be understood that the relative values of the different masses flowing can be controlled by appropriately selecting the relative cross-sections of the oven outlets 9 and 9", the nozzles 10, and the

sections

7 and 8 of the fan. The inlet to the section 7 is preferably made easily adjustable to control the quantity of secondary air.

In the preferred embodiment the outlets 9' and 9" are 1% inches above the floor of the oven; the nozzles 10 have a total cross-sectional area of not more than 5 square inches. The cross-sectional area, measured in a horizontal plane, of the trough 2 is up to thirty square inches, with about half of this cross-section occupied by the burner 3.

In a modification it may be possible to mount the burner in the top of the oven instead of the bottom, in which case the overall system could be similar to that illustrated in FIGS. 1 and 2 but inverted.

We claim:

1. A self-cleaning gas-fired domestic oven comprising an oven wall structure defining an oven cavity, a gas-fired burner disposed-in said wall structure and serving to heat said cavity, a gas inlet pipe supplying said burner, structure defining an air inlet opening to the neighborhood of said burner, structure defining a first exhaust opening for heated air and combustion products from said oven cavity, structure defining a second exhaust opening for heated air and combustion products from said oven cavity, a recirculation duct having an entry end and an exit end, said entry end communicating with said second exhaust opening, whilst said first exhaust opening communicates with atmosphere, the exit end of said duct communicating with said oven cavity in the neighbourhood of said burner, and a power-driven fan disposed in said duct, the dimensions and power of said fan and the dimensions of said duct and second exhaust opening being selected in relation to the dimensions of said first exhaust opening and of said gas inlet pipe and air inlet openin that under equilibrium conditrons and with said burner in ull operation the total mass of gases entering said cavity in the neighbourhood of said burner from said duct, said air inlet opening and said gas inlet pipe lies between three times and five times the mass of gas escaping to atmosphere through said first exhaust opening.

2. The oven set forth in claim 1 wherein said dimensions are such that said total mass of gases entering said cavity is substantially four times the mass of gases escaping to atmosphere through said first exhaust opening. I

3, The oven set forth in claim 1 including a trough disposed in the bottom of said wall structure, said burner being disposed in said trough and said duct exit end opening into the bottom of said trough.

Claims

1. A self-cleaning gas-fired domestic oven comprising an oven wall structure defining an oven cavity, a gas-fired burner disposed in said wall structure and serving to heat said cavity, a gas inlet pipe supplying said burner, structure defining an air inlet opening to the neighborhood of said burner, structure defining a first exhaust opening for heated air and combustion products from said oven cavity, structure defining a second exhaust opening for heated air and combustion products from said oven cavity, a recirculation duct having an entry end and an exit end, said entry end communicating with said second exhaust opening, whilst said first exhaust opening communicates with atmosphere, the exit end of said duct communicating with said oven cavity in the neighbourhood of said burner, and a powerdriven fan disposed in said duct, the dimensions and power of said fan and the dimensions of said duct and second exhaust opening being selected in relation to the dimensions of said first exhaust opening and of said gas inlet pipe and air inlet opening that under equilibrium conditions and with said burner in full operation the total mass of gases entering said cavity in the neighbourhood of said burner from said duct, said air inlet opening and said gas inlet pipe lies between three times and five times the mass of gas escaping to atmosphere through said first exhaust opening.

2. The oven set forth in claim 1 wherein said dimensions are such that said total mass of gases entering said cavity is substantially four times the mass of gases escaping to atmosphere through said first exhaust opening.

3. The oven set forth in claim 1 including a trough disposed in the bottom of said wall structure, said burner being disposed in said trough and said duct exit end opening into the bottom of said trough.