GB1594434A

GB1594434A - Cooking oven

Info

Publication number: GB1594434A
Application number: GB5053/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-02-11
Filing date: 1978-02-08
Publication date: 1981-07-30
Also published as: SE7801437L; FR2380500B1; FR2380500A1; JPS53101580A; DE2803910A1

Description

(54) COOKING OVEN (71) We, N.V. PHILIPS' GLOEILAMPENFABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a cooking oven with a forced gas circulation system for agitating hot gases in a muffle interior, the oven comprising a centrifugal fan rotor and heating means, and being provided with a catalytic gas filter element for oxidising combustible components in said hot gases.

A cooking oven of this type is known comprising electric heating means, in which a catalytic afterburner, constituted by a filter element and an auxiliary electrical resistance heater, is located in a fume discharge path from the interior of the muffle to the outside kitchen atmosphere. This oven is of the so-called self-cleaning or pyrolytic type, in that it is capable of being heated above a maximum food cooking temperature up to a temperature of about 500"C. At this temperature, any food residues which may have been deposited and remain as a crust on an inner surface of the muffle during previous cooking operations become pyrolytically decomposed and gasified. The hot gas resulting from the gasification process passes through said catalytic afterburner, the purpose of which is to oxidise any combustible components in the hot gas.In practice hydrogen is oxidised within and downstream of the afterburner, with the production of water vapour, and there is also oxidation of carbon monoxide to carbon dioxide. However, this result is obtained by using an afterburner, that is the auxiliary heater, and thus the total electrical consumption of the oven is greater than that for an oven with only a forced hot gas circulation heating system but without catalytic filter elements.

A still more serious disadvantage of this kind of oven is that the hot gas which may comprise toxic components is circulated continuously within the muffle. This latter, for convenience of manufacture, is never completely airtight, and it has constructional gaps, for example at the door, through which a certain amount of the hot gas circulating inside the muffle can leak to the surrounding atmosphere. As the duration of the pyrolytic self-cleaning operation lasts generally from one to three hours, the total quantity of toxic components leaking through these gaps into the kitchen may be such as to cause disturbance or discomfort to a person in the kitchen at that time.

The object of the present invention is to provide an oven in which the combustible components in the hot gas are completely oxidised so that no unpleasant or harmful pollutants are likely to be produced, even when the exhaust gases are to be discharged directly from the muffle into the kitchen atmosphere. A further object of the invention is to provide an oven in which this oxidation is obtained not by using an afterburner but by the employment of a particularly effective catalytic filter element.

According to the invention, there is provided a cooking oven with a forced gas circulation system for agitating hot gases in a muffle interior, the oven comprising a centrifugal fan rotor and heating means, and being provided with a catalytic gas filter element for oxidising combustible components in said hot gases, wherein said filter element is disposed in a path between the fan rotor and the muffle interior, said path being defined by a plate located parallel to the plane of rotation of the fan rotor.

One advantage of the invention is that because of the forced circulation effect created by the centrifugal fan rotor, the hot gas is forced is traverse the catalytic filter elements many times before being discharged outside the oven muffle. Complete oxidation of practically all the combustible components produced in the muffle is therefore ensured.

In a preferred embodiment of the oven, said filter element has a composition such that it begins to exert its catalytic action on the hot gases at a minimum temperature of 1800.

There is thus the further advantage that the hot gas generated during an ordinary cooking operation at medium-high temperatures will also be purified, that is oxidised, whilst this operation is in progress. A cooking operation at medium-high temperatures has been found to comprise the majority of operations which take place in a cooking oven.

By way of example, particular embodiments of the invention will now be described with reference to the accompanying drawing in which: Figure 1 is a horizontal cross-sectional view through a first embodiment of the oven, taken at half muffle height, that is on the line I-I of Figure 2, Figure 2 is a cross-sectional view through the same oven on the broken line II-II of Figure 1, Figure 3 is a section, analogous to that of Figure 1, through a second embodiment of the oven, Figure 4 is a section on the line IV-IV of Figure 3 showing some details of the second embodiment, Figure 5 is analogous to Figure 4, but relates to a third embodiment of the oven, and Figure 6 is analogous to Figures 4 and 5, but relates to a further embodiment of the oven.

The cooking oven of Figures 1 and 2 comprises a muffle 1 suitable surrounded by thermal insulation material 2 and closed at its front by a door 3 comprising three glass panels 4, 5, 6 containing two air cushions therebetween.

A seal gasket 7 is disposed along at least a substantial portion of an inner periphery of the door 3. At a rear region of the muffle 1 there is a vertical metal plate 8 fixed by bolt 9 to a rear wall 10 of the muffle 1. In the space between the plate 8 and rear wall 10 there is a centrifugal fan rotor 11 surrounded by an electrical resistance heater 12. The fan 11 (which is operated by an electric motor 14 by way of a motor shaft 15 passing through the thickness of the rear wall 10) is used to maintain a forced hot gas circulation in the muffle 1 through a central grid aperture 16 (constituting a suction port for the fan rotor 11) in the plate 8 and through the peripheral spaces between the two vertical edges 17 and 18 of the plate 8 and rear wall 10 of the muffle 1. The said peripheral spaces thus form the delivery ports for the fan rotor 11.

The arrows in Figure 1 show approximately the direction of flow of this hot gas circulation in the muffle 1. In'the present embodiment, said peripheral spaces include two catalytic filter elements 19 and 20. The elements 19 and 20 are of parallelepiped form and they are kept in a position by the same bolts 9 that fix the plate 8 to the rear wall 10.

A third catalytic filter element 21 is fixed by screws 22 to a roof 23 of the muffle 1 behind the plate 8 at a duct 24 which connects the interior of the muffle 1 to the surrounding atmosphere. This duct serves for the controlled discharge of the hot gas from the inside to the outside of the muffle. The filter elements 19, 20 and 21 are of a known type in which the catalytic action begins at a minimum hot gas temperature of about 1800C, but is at its maximum level when the gas temperature is approximately within the range 450-500"C, that is at the temperature at which the pyrolytic self-cleaning operation of the inner surface of the oven muffle is carried out.One example of a suitable catalyst for this purpose is ceramic based and has the following composition: A1203 86.30% SiO2 10.20% Cr2O3 1.87% CoO 0.66% Foe203 0.29% Pd 0.022% With this material, using methods known to any expert of the art, it is possible to manufacture self-supporting filter elements of any shape as the presence of alumina (Al2O3) and silica (SiO2) gives the ceramic base adequate mechanical characteristics. The filter elements are made in a form which comprises a very large number of perforations or passages which greatly increase the reactive surface area of the elements in relation to their volume. In contrast with known ovens in which catalyst filter elements are used only in the fume afterburners and where sometimes the inner surface of the muffle is lined with a catalytic enamel, in the oven of the invention catalytic filter elements are mainly disposed at the outlet sides of the centrifugal fan rotor 11. As the fan is kept in rotation continuously both during cooking and during the pyrolytic self-cleaning operation, the hot gas under forced circulation in the muffle is effectively oxidised by the catalytic filter elements which are traversed by the gas an indefinite number of times.The catalytic action conveniently commences when the hot gas reaches a minimum temperature of about 1800C, and is therefore effective during a substantial part of the normal cooking usage, that is that part of the cooking effected at a temperature of 1800C or above and up to 2800C. The catalytic action of the filters elements is most necessary during the pyrolytic self-cleaning operation, to prevent discharge into the surrounding atmosphere of hot gas containing toxic components, in particular carbon monoxide (CO). The applicant has carried out a series of comparative tests on an electric oven with forced gas circulation, and designed for pyrolytic self-cleaning of its muffle.The oven was firstly provided, in accordance with the known art, with catalytic filter elements connected to an auxiliary electrical resistance heater in an afterburner disposed only in the hot gas discharge duct from the muffle, and the test was carried out under the conditions described hereinafter. The same over was then provided, in accordance with the present invention, with catalytic filter elements on the discharge side of the centrifugal fan using the construction shown in Figures 1 and 2, and the test was carried out under the same conditions. In both cases the catalyst composition was as indicated heretofore and the construction of the catalytic filter elements was identical.

Said test consisted of preparing a simulated soiling mixture of animal and vegetable fats which had the following composition: Butter 45 g Margarine 45 g Olive Oil 45 g Sieved skinless tomatoes 90 g, and the inner surface of the oven muffle was coated with 50 g of this basic soiling mixture.

Starting from ambient temperature (20"C) the oven was heated to 2750C (temperature measured at the centre of the muffle) while constantly rotating the centrifugal fan and with the auxiliary electrical resistance heater continuously working under the control of a heater thermostat. This operation lasted ninety minutes from the moment of switching on to the moment when the fan stopped and the resistance heater was finally switched off. The soiling mixture was caused to become baked on the muffle inner surface by this treatment. The oven was then allowed to cool naturally to ambient temperature, upon cooling the soiling mixture was found to have formed a crust strongly adhering to the inner muffle surface.A pyrolytic self-cleaning operation was then carried out for a duration of ninety minutes, the operation consisting of raising the oven temperature from 20 C to 480"C and then maintaining this temperature under the control of the oven thermostat. The centrifugal fan was continuously rotated during this operation. The incrustation formed by the soiling mixture was thus pyrolytically decomposed and gasified.

In the first case (state of the art) only one cylindrical filter element was used, this element being disposed in the hot gas discharge port between the muffle interior and the outside.

This filter element had a frontal area (in the direction of gas flow) of 45.34 square centimetres, a height of 2.32 centimetres and a density of 12.7 perforations per square centimetre of frontal area. As it was intended to be used as an afterburner, the filter element was connected to the auxiliary resistance heater, which had an electrical power consumption of 150 watts, and was disposed immediately upstream of the first filter element encountered by the hot gas during its discharge from the muffle interior to the outside of the oven through the discharge duct.

The efficiency of the catalytic filter elements was evaluated by determining during said pyrolytic self-cleaning operation the percentage of carbon monoxide (CO) in the total quantity of hot gas circulating inside the muffle and immediately downstream of the catalytic filter element, using a gas analyser. The said auxiliary resistance heater was energised only during the initial stage of the pyrolytic self-cleaning operation, that is up to a time when the temperature in the muffle reached 370"C. Above this temperature the post-combustion of the gaseous incrustation residues is maintained automatically.In this first case a carbon monoxide content greater than 1% from the 25th minute to the 26th minute was determined inside the muffle, while downstream of the catalytic filter element the carbon monoxide content was greater than 0.02% over the same time interval, with a peak of 0.05% at the 40th minute. These data show that an oven in accordance with the state of the art is very satisfactory with regard to the hot gas discharged through the discharge duct after passing through the catalytic filter elements in that it may be considered completely oxidised, whereas the hot gas as yet unoxidised and passing from the muffle directly into the surrounding atmosphere through the constructional gaps as stated heretofore may create disturbance. This part of the hot gas contains a relatively large percentage of toxic gas in the form of carbon monoxide, over a long period.In the second case (in accordance with the invention) investigated by the applicant, two parallelepiped catalytic filter elements were used on the centrifugal fan discharge side and a third parallelepiped element (but without the post-combustion auxiliary resistance heater) before the entry to the hot gas discharge duct from the muffle, as shown in Figures 1 and 2. The first two elements had a frontal area of 63.25 square centimetres each and the third element 33.60 square centimetres. The height of all elements was 2.32 centimetres with a density of 12.7 perforations per square centimetre of frontal area. The efficiency of the catalytic filter elements was evaluated using the same tests as in the first case.A maximum carbon monoxide content inside the muffle of 0.018% was determined from the 25th to the 30th minute of the pyrolytic self-cleaning operation, which stabilised around 0.010%. In the discharge duct a peak value of0.005% was determined at the 26th minute, and only traces afterwards. This data shows that, according to the invention, that proportion of hot gas which emerges directly into the surrounding atmosphere through constructional gaps has only an insignificant carbon monoxide content, because of which there is likely to be no danger of disturbance or discomfort to persons in the kitchen during the pyrolytic self-cleaning operation.This is obtained according to the present invention by the continuous oxidising action on the gaseous residues from the soiling incrustation, effected by the catlytic filter elements disposed on the delivery side of the fan which maintains forced circulation of the hot gas in the muffle.

Considering now the second embodiment (Figures 3 and 4) of the present invention, it can be seen that this differs only slightly from the first embodiment, and the same reference numerals have in general been retained. The main difference is that the vertical plate 28 has been pressed to assume a trapezoidal cross-section. The delivery apertures for the centrifugal fan 11 are now constituted by two vertical rows of holes 29 and 30 provided in the plate 28 on the inclined sides of the trapezium. There are now two catalytic elements 31 and 32 for each row of holes, each of these being of somewhat elongated form and being positioned over only some holes in the row. The filter elements 31 and 32 are shown in Figures 3 and 4 supported by the vertical plate 28 via clips 33 and 34 and screws 35 which keep them pressed against the surface of the plate 28 facing the centrifugal fan 11.Any permanent or temporary fixing means may however be used. The operation of the oven is the same as that described for the first embodiment, with the hot gas passing an indefinite number of times through the catalytic filter elements disposed on the delivery side of the centrifugal fan.

In the third embodiment (Figure 5) the only difference is that the delivery apertures for the centrifugal fan, again provided in the vertical plate 38, are in the form of vertical slots 39. Catalytic filter elements 40 of parallelepiped shape are disposed over the slots and are fixed to the vertical plate 38 by through screws 41. Finally in the fourth embodiment (Figure 6) the discharge apertures for the centrifugal fan, again provided in the vertical plate 48, are constituted by two vertical rows of rectangular holes 49. A catalytic filter element 50 in the form of a disc is disposed above each hole and is fixed to the vertical plate 48 by through screws 51. The furnace operation is similar to that of the first embodiment of the invention.

The foregoing descriptions of embodiments of the invention have been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims.

For instance, the gas discharge duct from the muffle interior may have its inlet in the muffle roof in front of the rear plate, and downstream of the catalytic filter elements disposed on the delivery side of the centrifugal fan. The inventive principles described may also be applied to electric ovens in which the centrifugal fan and resistance heaters are differently located (for example under the muffle roof) and to gas ovens.

WHAT WE CLAIM IS: 1. A cooking oven with a forced gas circulation system for agitating hot gases in a muffle interior, the oven comprising a centrifugal fan rotor and heating means, and being provided with a catalytic gas filter element for oxidising combustible components in said hot gases, wherein said filter element is disposed in a path between the fan rotor and the muffle interior, said path being defined by a plate located parallel to the plane of rotation of the fan rotor.

2. A cooking oven as claimed in Claim 1, wherein said filter element has a composition such that it begins to exert its catalytic action on the hot gases at a minimum temperture of 1800C.

3. A cooking oven as claimed in Claim 1 or 2, wherein said filter element has a composition such that it exerts its maximum catalytic action on the hot gases at a temperature of 450-500"C.

4. A cooking oven with a forced gas circulation system for agitating hot gases in a muffle interior, the oven comprising a centrifugal fan rotor surrounded by electrical resistance heating elements in a rear muffle region and a plate or the like disposed in front

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. case (in accordance with the invention) investigated by the applicant, two parallelepiped catalytic filter elements were used on the centrifugal fan discharge side and a third parallelepiped element (but without the post-combustion auxiliary resistance heater) before the entry to the hot gas discharge duct from the muffle, as shown in Figures 1 and 2. The first two elements had a frontal area of 63.25 square centimetres each and the third element 33.60 square centimetres. The height of all elements was 2.32 centimetres with a density of 12.7 perforations per square centimetre of frontal area. The efficiency of the catalytic filter elements was evaluated using the same tests as in the first case.A maximum carbon monoxide content inside the muffle of 0.018% was determined from the 25th to the 30th minute of the pyrolytic self-cleaning operation, which stabilised around 0.010%. In the discharge duct a peak value of0.005% was determined at the 26th minute, and only traces afterwards. This data shows that, according to the invention, that proportion of hot gas which emerges directly into the surrounding atmosphere through constructional gaps has only an insignificant carbon monoxide content, because of which there is likely to be no danger of disturbance or discomfort to persons in the kitchen during the pyrolytic self-cleaning operation.This is obtained according to the present invention by the continuous oxidising action on the gaseous residues from the soiling incrustation, effected by the catlytic filter elements disposed on the delivery side of the fan which maintains forced circulation of the hot gas in the muffle. Considering now the second embodiment (Figures 3 and 4) of the present invention, it can be seen that this differs only slightly from the first embodiment, and the same reference numerals have in general been retained. The main difference is that the vertical plate 28 has been pressed to assume a trapezoidal cross-section. The delivery apertures for the centrifugal fan 11 are now constituted by two vertical rows of holes 29 and 30 provided in the plate 28 on the inclined sides of the trapezium. There are now two catalytic elements 31 and 32 for each row of holes, each of these being of somewhat elongated form and being positioned over only some holes in the row. The filter elements 31 and 32 are shown in Figures 3 and 4 supported by the vertical plate 28 via clips 33 and 34 and screws 35 which keep them pressed against the surface of the plate 28 facing the centrifugal fan 11.Any permanent or temporary fixing means may however be used. The operation of the oven is the same as that described for the first embodiment, with the hot gas passing an indefinite number of times through the catalytic filter elements disposed on the delivery side of the centrifugal fan. In the third embodiment (Figure 5) the only difference is that the delivery apertures for the centrifugal fan, again provided in the vertical plate 38, are in the form of vertical slots 39. Catalytic filter elements 40 of parallelepiped shape are disposed over the slots and are fixed to the vertical plate 38 by through screws 41. Finally in the fourth embodiment (Figure 6) the discharge apertures for the centrifugal fan, again provided in the vertical plate 48, are constituted by two vertical rows of rectangular holes 49. A catalytic filter element 50 in the form of a disc is disposed above each hole and is fixed to the vertical plate 48 by through screws 51. The furnace operation is similar to that of the first embodiment of the invention. The foregoing descriptions of embodiments of the invention have been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, the gas discharge duct from the muffle interior may have its inlet in the muffle roof in front of the rear plate, and downstream of the catalytic filter elements disposed on the delivery side of the centrifugal fan. The inventive principles described may also be applied to electric ovens in which the centrifugal fan and resistance heaters are differently located (for example under the muffle roof) and to gas ovens. WHAT WE CLAIM IS:

1. A cooking oven with a forced gas circulation system for agitating hot gases in a muffle interior, the oven comprising a centrifugal fan rotor and heating means, and being provided with a catalytic gas filter element for oxidising combustible components in said hot gases, wherein said filter element is disposed in a path between the fan rotor and the muffle interior, said path being defined by a plate located parallel to the plane of rotation of the fan rotor.

of the fan rotor with at least one central aperture for hot gas intake, wherein the hot gas is discharged by the fan to the muffle interior through a discharge port formed by a peripheral aperture also provided in said plate and defined by a gap between the plate edge and the inner muffle surface, the oven also comprising a catalytic gas filter element, wherein said filter element is disposed in the path between the fan rotor and the muffle interior.

5. A cooking oven as claimed in Claim 4, wherein the peripheral aperture is defined by at least one gap between the vertical edges of the plate and adjacent walls of the muffle, wherein said filter element is inserted in said gap.

6. A cooking oven as claimed in Claim 4 or 5, in which the peripheral aperture is provided in said plate, and wherein said filter element is fixed to that surface of said plate facing the centrifugal fan rotor over said peripheral aperture.

7. A cooking oven as claimed in Claim 6, in which the peripheral aperture is constituted by at least one row of vertical holes, wherein said filter element is of elongated shape and disposed over a plurality of the holes.

8. A cooking oven as claimed in Claim 6, in which the peripheral aperture is constituted by at least one slot parallel to a vertical wall of the muffle wherein said filter element is of elongated parallelepiped shape, and each element is disposed over one complete slot.

9. A cooking oven as claimed in Claim 6, in which the peripheral aperture is constituted by at least one vertical row of holes, wherein said filter element is of disc or plate shape, and each element is disposed over a single hole.

10. A cooking oven as claimed in any one of the preceding claims, wherein at least one further catalytic filter element is positioned in a path through which the gas is discharged to the outside of the muffle.

11. A cooking oven substantially as hereinbefore described with reference to the accompanying drawing.