WO1991005208A1

WO1991005208A1 - Cooking oven

Info

Publication number: WO1991005208A1
Application number: PCT/GB1990/001507
Authority: WO
Inventors: Brian William Harris
Original assignee: Brian William Harris
Priority date: 1989-10-03
Filing date: 1990-10-02
Publication date: 1991-04-18

Abstract

A cooking oven of the continuous kind is described in which the base and/or the enclosing walls are fabricated from a plastics laminate having a high degree of heat resistance and good insulation properties. The laminate has a sandwich structure and comprises a first layer of thermoset resin impregnated with carbon and/or ceramic fibres, a foamed plastics core and a second layer of thermoset resin impregnated with carbon and/or ceramic fibres.

Description

C00KIV OVEN

This invention relates to cooking ovens of the kind used on a commercial or industrial scale. The invention is particularly concerned with ovens of the continuous type in which an endless belt carries the product to be cooked through an elongated oven. Ovens of this kind are described, for example, in U.K. Patent No. 1 439 971.

Commercial and industrial food ovens are conventionally constructed from a metal framework onto which are hung a base and wall members, including a hood and all these members are normally constructed from stainless steel. Stainless steel is the material of choice because of the ease of cleaning and resistance to corrosion and because such material is associated in the industry with hygiene and a low maintenance costs. One of the disadvantages of constructing ovens using stainless steel panels is the high weight of the resulting equipment which is also made worse by in the relatively heavy gauge framework necessary to support the wall panels. Furthermore, because of the high heat conductivity of stainless steel, it is necessary to construct some of the wall or base panels of the oven as a double skin. This is required in some parts of the oven in order to protect operatives from contact with very hot surfaces and in other parts of the oven to provide a jacket through v/hich cooling water or other suitable fluid may be circulated in order to maintain the surface temperature at a desired level. All these factors militate against a simplified structure and necessarily involve a structure which is heavy, difficult and expensive to produce.

The present invention is based on the realisation that materials which have not heretofore been used in oven construction can be substituted for stainless steel and give rise to pronounced advantages.

According to one aspect of the present invention there is provided a cooking oven, particularly of the continuous type having a base and enclosing walls, wherein at least part of the said base and enclosing walls are formed from a carbon fibre laminate.

The materials which it is intended to use in the construction of the oven are broadly carbon fibres embedded in a thermoset resin matrix. Typical thermoset resins are epoxy resins, e.g. those based on glycidyl ethers of a poly phenol, e.g. bis-phenol A. Conventional hardeners may be employed such as a compound containing free amino groups, for example, 4, 4-diamino-diphenyl sulphone. A typical epoxy resin formulation contains 36 parts by weight of the hardener per 100 parts of the resin.

The carbon fibres are manufactured by conventional procedures, e.g. by spinning an organic polymer fibre and subjecting this to heating to carbonise the fibre. Typical polymers which are readily carbonised in this way are polyacrylate nitrile fibres.

Preferably, the carbon fibre is mixed with a ceramic fibre such as those formed from metal oxides. Ceramic fibres may be produced by forming a paste of appropriate metal salts and converting these to fibres by extrusion or spinning and heating to convert the salts to their respective metal oxides. Typical metal oxides are alumina, boron oxide and silica. Preferred ceramic fibres comprise about 60% alumina, about 25% of silica, the balance being boron oxide.

The carbon fibre laminate is preferably formed as a sandwich material comprising, for example, two layers of fibre impregnated resin sandwiching a layer of insulating material. The insulating material may be a high temperature resistant foam, such as an acrylic foam or a glass fibre impregnated resinous material. The laminate is preferably constructed by foaming an insulating material between two layers of carbon fibre impregnated resin to form a sandwich structure. The thickness of the components of the structure may vary, but Figure 1A below in the attached drawings indicates typical thicknesses of the components of the sandwich structure. The inner surface layer of the carbon fibre impregnated resin preferably incorporates ceramic fibres because these impart higher heat resistance to the laminate. In some cases it may be possible to dispense with the carbon fibre reinforcement and to use essentially only ceramic fibres to reinforce the inner resin layer. Inner surface layers having a thickness of from about 1 to about 4 mms thick are preferred.

In the case of the outer resin layer, very high heat resistance is not usually necessary since the layer is protected from the heat of the oven by the intervening insulation layer. Therefore, the outer resin layer need not contain ceramic fibre and may be thinner than the inner layer, e.g. from 0.75 to 3 mms thick.

The thickness of the insulating core of the composite laminate is not critical. Obviously, the thicker the core the higher the insulation factor. Suitable core thicknesses are in the range of 15 to 50 mms. Typical properties for the structure shown in Figure 1A are as follows:- PROPERTIES OF THE LAMINATE

Inner temperature (continuous) 305°C (572°F) outside temperature 30°C (86°F)

Inner temperature resistance = 1093°C (2000°F)

Flexural strength = 200,000 psi

Flexural modulos = 20 x 10° psi

Strength retention at 300°C = 70%

Stiffness retention at 300°C = 95% Percentage reflectance of radiation at 1093°C (2000°F) = 95%

Weight of panel structure = 1.75 lbs/ft 2

Thermal conductivity inner skin = 0.6 Btu/in ft² h°F.

Thermal conductivity core = 0.3 Btu/in ft² h°F.

Thermal conductivity outer skin = 6.0 Btu/in ft² h°F

The specific heat capacity of the panel is approximately 0.15 - 0.2 Btu/lb

The thermal expansion of the panel structure lies in the range 1 - 4 x 10⁶ in/in°C.

All the strength and physical properties mentioned can be varied by modifying the fibre/resin ratio, the orientation or lay up configuration in the laminate.

Carbon fibre/epoxy resin materials are non-toxic and have been used in surgical applications. References to their preparation, including various fibre/resin formulations, include the following:-

3.1) Hastings G.W. and N.G. Thanhthuy: Biochemically compatible materials:Carbon fibre reinforced plastics in Williams, D. Biocompatibility of implant materials London: Section Publishing Ltd. 1976, p. 208.

3.2) Hastings, G.W. Composites 7, 193 (1978).

3.3) Musikant, S : J. Bio ed. Mater. Res. Symposium 1, 225 (1871).

3.4) Cardiac Pacemakers: Leading arti^e, Brit. Med. J.l, 77 (1965).

3.5) Bloch, B. and G.W. Hastings: Plastics Materials in Surgery, 2nd Ed., 100 (Thomas, Springfield) 1972.

The invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 represents two half sections through a continuous oven constructed in accordance with the invention,

Figure 1A is a schematic section through a sandwich laminate material for use in the construction of ovens in accordance with the invention,

?i -ure 2 is a schematic longitudinal view of the oven, and

Figures 3 and 4 are enlarged views showing the way in which the conveyors are guided at the mid-point of the oven.

Figure 5 is a side elevation of a second embodiment of the invention.

Figure 6 is a plan view of the oven shown in Figure 5,

Figure 7 is a section taken on the line A-A in Figure 5,

Figure 8 is a section through the chamber of the oven of Figure 5 on an enlarged scale.

Figure 9 is a sectional view of the lower end of the oven of Figure 5,

Figure 10 is a partial sectional view similar to that of Figure 7 showing the way in which the upper section of the chamber can be lifted off the base section.

Referring to Figure 1, the left-hand side is" a section of the oven at the feed end, while the right-hand side is a section of the oven at the discharge end. In accordance with conventional practice, the oven comprises a framework 1 which supports the base and wall panels of a generally tunnel-shaped continuous enclosure 2. The enclosure 2 includes an upper hood part 3 which is rais-able en jacks 4 to facilitate servicing and cleaning of the internal parts of the oven. The main cooking chamber comprises a base 5 through which the operative run 6 of a conveyor belt is guided on supporting rods 7. The conveyor belt is preferably an open mesh stainless steel belt and its return run 8 extends beneath the base 5 and over a drip pan 9 arranged to collect fat and other juices which drip from the belt 8.

In accordance with the invention, the base 5 is manufactured from a carbon fibre sandwich laminate having the construction shown in Figure 1A. Base 5 may be fabricated as a single structural member by laying up carbon-fibre impregnated resin in a mould followed by the insulating foam or other material and finally with an interior layer of the glass fibre impregnated resin in a manner analogous to laying up of GRP moulded structures. A vacuum bag is placed over the la d up component and a vacuum is applied. The complete mould and contents are then autoclaved for an extended period, e.g. 2 to 4 hours. Alternatively, the base 5 may be constructed from preformed panels which are joined using high temperature resistant thermoset resin adhesive, e.g. epoxy resin adhesives and the corners may, if desired, be strengthened with appropriately angled reinforcing strips which may be fabricated from thermoset plastics or metals. Any necessary drain ports or input ports may be formed in the base 5 if desired using plastics or metallic flanges or other localised inserts which are preferably of stainless steel and are bonded or bolted to the base material. In contrast with conventional procedure in which a water- cooled jacket is provided between double skins forming the base 5, the use of the sandwich material in accordance with the invention means that the external surface of the base 5 can be maintained at a temperature low enough to be comfortably touched while the internal surface can withstand the temperatures prevailing during cooking of meats and other foods.

The top of the cooking chamber 10 is formed by a baffle 11 which may be, for example, a metallic mesh designed to limit the amount of fat or other materials that are spattered into the heating chamber 12 located above. Heating chamber 12 includes a circulating fan 13 for circulating heated air into and through the cooking chamber from heaters such as a gas burner 14, located in the discharge end of -he oven. The sides of the heating chamber 12 are formed from panels 15, which may be constructed integrally as a moulded hood or more conveniently by bolting together preformed carbon fibre sandwich laminates of the kind described above. Further details of the construction of continuous ovens may be found in U.K. Patent No. 1,439,971. Because the use of the carbon fibre sandwich laminates for construction of the base of the oven avoids the need to provide double- skinned water-cooled surfaces, ovens in accordance with the present invention can be readily constructed in two or more sections which are each subjected to different cooking or heating conditions and may Include conveyor belts operating at different speeds.

Referring to Figure 2, the oven may comprise two sections in which, for example, in the first section B the food is conveyed through a steam atmosphere for rapid internal cooking of a food product and a second in-line section C where the same product is subjected to heating in a dry atmosphere for rapid browning As can be seen, the oven is provided with two conveyors 6B and 6C which convey the food product first through the oven section B and then through the section C. Each conveyor 6B and 6C may be independently regulated so that the speeds through the different sections of the oven can be regulated to suit the product. The central section of the oven may include a washing unit 20 into which the belts are lead prior to or after passage through their respective oven sections. Each part of the oven B and C may be subjected to different heating levels as well as different atmospheres. The discharge end of belt 6B is located closely adjacent to the inlet end of belt 6C so that the products flow in an interrupted fashion from oven section B to oven section C. Figures 3 and 4 show details of the way in which the belts are guided from the cooking chamber through a belt guard which prevents loss of oven atmosphere into the water bath and thence through the water bath for cleaning the belts. In addition, if desired additional water baths or cleaning stations may be provided at other stages in the return run part of the belts.

Heating of the oven may be effected by any convenient fuel, e.g. gas or electricity, and may include steam in part or all of the cooking chamber.

One further advantage of the method of construction of the ovens of the present invention is that a variety of different configurations can be readily produced. According to one especially advantageous form of the invention, the cooking chamber is formed as a loop or series of loops. For example, the chamber may be formed as a helix or spiral. This has the advantage that the oven takes up much less floor space than a straight continuous oven of similar capacity and throughput. Because the chamber is constructed from plastics laminates having a low thermal expansion, a limited degree of distortion takes place during the heating and cooling phases of the operation of the oven.

Referring to Figures 5 to 10 of the accompanying drawings, the embodiment shown is a continuous oven having a generally helical configuration. The tunnel structure forming the oven is formed from moulded modular sections of ceramic and/or carbon fibre reinforced plastics mouldings. The sections are formed from convenient lengths, preferably moulded in upper and lower sections 51 and 52, the upper sections being attached to external supports 53 and the internal sections to central internal supports 54. A central supporting post 54 is provided with radiating arms 55, on which the lower parts of the tunnel sections of the oven are supported. The tunnel may be opened for cleaning and servicing by raising the supports 53 relative to the central supporting posts 54, as illustrated in Figure 10. Typically, the*^*chamber may be about 1 metre wide and 600 mms in height.

Food to be cooked in the oven is transported through the helical structure on a continuous belt conveyor 56, which enters the oven at one end, leaves from the other and during its return run is conveniently passed through a cleaning station 57. Preferably, the conveyor belt is guided through the oven in a direction which is counter to the flow of hot gases. Thus, for example, in the arrangement shown in Figure 5, hot gases may be generated or blown through the oven by a fan 8, operating together with heating elements not shown so that air flows upwardly through the helix and exits at the exhaust end of the oven 59. In such a case, belt 56 preferably flows in the opposite direction so that goods to be cooked enter at 59 and are unloaded from the oven at 60.

Alternatively, as illustrated in Figure 9, a main fan 68 may be mounted within the chamber close to the end 60 of the oven and be arranged to blow heated air through the helical chamber. Where a source of heat is located in the chamber at the end 60, a baffle 69 is conveniently located adjacent the fan 68 to assist the direction of hot gases along the helical chamber in the direction of the arrow F. A drain 62 for liquid by-products of the cooking process is provided in the base of the chamber in the vicinity of the chamber end 60.

The conveyor, which may, for example, be up to 1 metre in width, may be of stainless steel mesh or other food grade material and will be driven continuously. The drive system is preferably by means of electric motors, with a facility to take up any slackness in the belt automatical1y. Preferably, there are two fan-assisted exhausts from the oven, one at the inlet and the other at the outlet. Air through ports can be controlled by vane type vales.

Ovens constructed in accordance with this invention can be operated more efficiently than conventional ovens of similar capacity. For example, a conventional continuous oven of about 120 square feet cooking area constructed in accordance with U.K. Patent No. 1,439,972, would require a heat input of about 2,450,000 Btu/Hr, in order to run continuously at about 305°C. with a raw goods input temperature of 0 to 2°C. In contrast, using a spiral oven of the kind described in Figures 5 to 10 of the accompanying drawings, having a similar cooking area and operating at the same temperature rise, the heat requirement is only about 1,120,000 Btu/Hr. Typically, the air velocity in the oven will be about 5000 to 6000 feet per minute, e.g. 5,600 feet per minute.

Although cooking using an air stream heated with electricity or gas has been particularly mentioned, steam can be used as the heat source or an auxiliary heat source, e.g. at a pressure of 2 to 7.2 bar.

Claims

1. A cooking oven, particularly of the continuous type, having a base and enclosing walls, wherein at least part of the said base and enclosing walls are formed from a carbon and/or ceramic fibre plastics laminate.

2. An oven according to claim 1 in which the plastics laminate comprises a first layer of thermoset resin impregnated with carbon and/or ceramic fibres, a foamed plastics core and a second layer of thermoset resin impregnated with carbon and/or ceramic fibres.

3. An oven according to claim 2 in which the first layer forms the inner surface of the base or the walls of the oven and comprises a thermoset resin layer reinforced with ceramic and carbon fibres.

4. An oven according to claim 2 or 3 in which the core comprises a heat resistant foamed acrylic polymer.

5. A continuous oven according to any one of the preceding claims comprising a tunnel-like chamber through which the food to be cooked is conveyed, said chamber having been formed by assembly of preformed, moulded sections, said sections comprising sandwich laminates of carbon/ceramic fibre reinforced thermoset resin.

6. An oven according to claim 5 in which the chamber is separable longitudinally, and the upper section is raisable to allow access for cleaning and servicing.

7. An oven according to claim 5 or 6 in which the food to be cooked is supported on an endless conveyor having an upper run which passes through said chamber and a lower run which passes through a cleaning bath.

8. An oven according to claim 7 in which the food is carried on two endless conveyors whose upper runs are arranged end to end in the chamber and the lower runs pass through the same or different cleaning baths.

9. An oven according to any one of claims 5 to 8 in which the chamber extends through different cooking or treatment zones.

10. An oven according to any one of claims 5 to 9 in which the cooking chamber is formed as a loop or series of loops.

11. An oven according to claim 10 in which the chamber has a helical form.