Figure 1 shows a typical convection oven for cooking/baking food products such as breads, cakes, biscuits, pizza bases, dairy products, pies and meats. Such convection ovens are typically divided into zones (of which one is shown), each zone having an independent heating system. The product passes through each zone in turn on a conveyor.

In each zone, a circulation fan passes air from within the bake chamber over a heating system. The heating system may, for example, comprise a gas flame, a gas or oil fired heat exchanger or an electric heater bank.

The heated air is then blown into a plenum chamber. It is blown out of the plenum chamber through a series of nozzles. The nozzles are laterally disposed to the direction of movement of the conveyor and the product.

It is usual for there to be plenum chambers both above and below the conveyor carrying the product.

The main mode of heat transfer is accordingly by way of forced convection of the air coming out of the nozzles over the product, by impinging on the product and giving up part of its heat. By choosing the size of the plenum chamber and the size and distribution of the nozzles it is possible, for those skilled in the art, to obtain a sufficiently uniform

distribution of airflow from the plenum onto the product. In a classical convection system while it is straightforward to obtain a sufficiently uniform airflow onto the product, it has been difficult to obtain a sufficiently uniform airflow away from the product. This has been for two reasons.

Firstly, with reference to Figure 2, all the air has to return to a circulation fan inlet, which is necessarily only on one side of the conveyor, either above or below. Therefore either the top air or bottom air has been forced to go past the edge of the conveyor either wholly (if the conveyor is solid) or in part (if the conveyor is permeable). This has the effect of heating the products at the edge of the conveyor preferentially.

Secondly, with reference to Figure 3, there is very little return air near the centre of the conveyor, since the system is symmetrical, but there is a substantial amount of return air near the edge of the conveyor, since half of the plenum's airflow has to come past the edge products. This also has the effect of heating the products at the edge of conveyor preferentially.

As is evident from Figures 2 and 3, on reaching the conveyor the product items thereon, air, particularly towards the centre of the conveyor, follows an indirect path to the (upper) circulation fan inlet, in which air is drawn across the conveyor in a generally lateral direction and then on reaching the edge of the conveyor is drawn into a circulation fan inlet.

Attempts have been made to overcome these shortcomings. These attempts have been of two forms. One is to cut away part of the plenum, to provide a return air path vertically. However this has had the effect of making the radiation from the plenum non-uniform. Secondly to

provide some form of restriction local to the circulation fan inlet, to reduce the return air velocities local to the circulation fan inlet. This has been ineffective because it has not been implemented over the whole length of the zone and has not dealt with both the top and bottom return air.

According to a first aspect of the invention there is provided a return gas arrangement for food production apparatus, the return gas arrangement comprising upper return gas inlet means for drawing gas issued from upper gas outlet means to gas recirculation means, which upper return gas inlet means is located above the support surface, and/or lower return gas inlet means for drawing gas issued from lower gas outlet means to the gas recirculation means, which lower return gas inlet means is located below the support surface, the upper return gas means and/or the lower return gas means comprises a plurality of longitudinally-spaced apertures, wherein the upper return gas inlet means and/or the lower return gas inlet means are/is provided laterally outermost and to the side of the upper gas outlet means and/or the lower gas outlet means respectively, and gas issued from each gas outlet means on reaching the support surface and/or product items thereon is drawn to the respective return gas inlet means through a substantially direct path.

According to a second aspect of the invention there is provided a convection oven comprising the return gas arrangement in accordance with the first aspect of the invention.

According to a third aspect of the invention there is provided a dryer for food production apparatus comprising the return air arrangement of the first aspect of the invention.

According to a fourth aspect of the invention there is provided a method of returning gas in a food production apparatus to gas recirculation means, comprising drawing gas issued from upper gas outlet means into upper return gas inlet means and/or drawing gas issued from lower gas outlet means into lower return gas inlet means, the upper return gas inlet means and the lower return gas inlet means being located above and below the support surface respectively, the upper return gas means and/or the lower return gas means comprising a plurality of longitudinally-spaced apertures, and channelling gas drawn into said return gas inlet means to gas recirculation means, wherein the upper return gas inlet means and/or the lower return gas inlet means are/is provided laterally outermost and to the side of the upper gas outlet means and/or the lower gas outlet means respectively, and gas issued from each gas outlet means on reaching the support surface and/or product items thereon is drawn to the respective return gas inlet means through a substantially direct path.

According to a fifth aspect of the invention there is provided return gas arrangement for apparatus of the kind hereinbefore described, the return gas arrangement comprising upper return gas inlet means for drawing gas issued from upper gas outlet means to gas recirculation means, which upper return gas inlet means is located above the support surface, and/or lower return gas inlet means for drawing gas issued from lower gas outlet means to the gas recirculation means, which lower return gas means is located below the support surface, wherein the upper return gas inlet means and/or the lower return gas inlet means are/is generally laterally extending and at least one of the upper return gas inlet means and the lower return gas inlet means being provided interposed between respective adjacent gas outlet means, and gas issued from each gas outlet means on reaching the support surface and/or product items thereon is

drawn to the respective return gas inlet means through a substantially direct path.

According to a sixth aspect of the invention there is provided a method of returning gas in an apparatus of the kind hereinbefore described to gas recirculation means comprising drawing gas issued from upper gas outlet means into upper return gas inlet means and/or drawing gas issued from lower gas outlet means into lower return gas inlet means, the upper return gas inlet means and the lower return gas inlet means being located above and below the support surface respectively, and channelling gas drawn into said return gas inlet means to gas recirculation means, wherein the upper return gas inlet means and/or the lower return gas inlet means are/is generally laterally extending and at least one of the upper return gas inlet means and the lower return gas inlet means being provided interposed between respective adjacent gas outlet means, and gas issued from each gas outlet means on reaching the support surface and/or product items thereon is drawn to the respective return gas inlet means through a substantially direct path.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 4 is a schematic perspective view of a convection oven provided with return air apparatus in accordance with the invention; Figure 5 is an enlarged schematic perspective view of part of the return air apparatus of the convection oven of Figure 4; Figure 6 is a schematic end-on view of the paths of air flow issuing from the top nozzles;

Figure 7 is a schematic end-on view of the paths of air flow issuing from both the top and bottom nozzles; Figure 8 is a schematic partial perspective view of an alternative return air arrangement of the oven of Figure 4, Figure 9 is a schematic cross-section on A-A of the return air arrangement of Figure 8, Figure 10 is a schematic plan view of a second alternative return air arrangement of the oven of Figure 4, Figure 11 is a schematic plan view of a third alternative return air arrangement of the oven of Figure 4, and Figure 12 is a schematic plan view of a fourth alternative return air arrangement of the oven of Figure 4.

With reference to Figure 4 a convection oven assembly 1 comprises a circulation fan which is operative to force air through a heating system (shown generally at 12) and towards a channelling plate 11. The channelling plate 11 divides the heated air into a proportion 8 which is forced through top feed ducting 13 to a top plenum 3a and the remainder 9 which is forced through bottom feed ducting 14 to a bottom plenum 3b. The heated air in each then issues from rows of transversely extending and longitudinally-spaced nozzles 4 towards a conveyor 7 which carries a product.

Top and bottom nozzles 4 are provided in respective plates 18a and 18b, which plates are provided at sides thereof with a plurality of longitudinally-spaced apertures 6a and 6b respectively so as to flank the

rows of nozzles. For each row of convection nozzles there is provided a pair of return air holes. The ratio of the total of the areas of two return air holes to the total of the areas of a row of nozzles is approximately between two and four inclusive.

The apertures 6a and 6b are located intermediate of the rows of nozzles but laterally outermost and to the sides of the nozzles, as is best seen in Figure 5. The top and bottom return air holes 6a and 6b communicate with top return ducting (shown generally at 18) and bottom return ducting (shown generally at 19) on respective sides of the oven assembly 1. Said return ducts 18 and 19 lead to top return aperture 16 and bottom return aperture 15 (in each side of the oven assembly) respectively, which apertures communicate with return air ducting 17.

More particularly, from Figure 5 it may be seen that three enclosures are effectively defined, by the top and bottom plenums 3a and 3b and return air plates 18a and 18b. The return air plates 18a and 18b fully circumscribe the respective plenums. The top enclosure 5a is under suction caused by the circulation fan inlet. The value of the suction is chosen by the designer, by choosing the correct locations, size and number of return air holes. The bottom enclosure 5b is also under suction caused by the return air ducting 17 but additional ducts are provided to channel the bottom return air back to the return air ducting, remote from the product on the conveyor 7. Again, the designer determines the value of the suction. The top and bottom suction enclosures 5a and 5b define longitudinal channels 5c and 5d which lie above and below each line of return air holes.

A central enclosure accommodates the product and conveyor enclosure and is substantially at ambient pressure since it has open ends, through which the conveyor passes.

The, designer arranges for there to be substantially a pair of return air holes between each pair of adjacent rows of nozzles; one hole at each side of the respective plenum (Figure 5). In this way there is effectively formed a channel perpendicular to the direction of movement of the conveyor for the return air travelling from the nozzles towards a respective return air hole. The conveyor and a plate 18a or 18b form two sides of the channel and two curtains of air issuing from adjacent nozzles form the other two sides.

It is necessary that the return air on its journey along this channel does not have a substantial velocity close to the conveyor and product. This is achieved (Figure 6) because importantly air moving towards a hole with a relatively high suction will tend to take the shortest path (ie the most direct path) to that hole, and in this way the return air is drawn away from the product (Figure 7).

There are several important advantages of the inventive apparatus described above.

Firstly, referring to Figure 7, by keeping return air away from the products near the edge of the conveyor the heat transfer to the edge products is maintained substantially the same as the heat transfer to the products near the middle of the conveyor.

Secondly, referring to Figure 7, air from the top nozzles returns upwards away from the product towards the top return air system leading back to the fan inlet, while the air from the bottom nozzles returns downwards away from the product towards the bottom return air system leading back to the main return air ducting 17. In this way neither the top nor bottom return air needs to pass the edge of the conveyor to return to the circulation fan 2.

Thirdly, since the top air returns to the top return air holes and the bottom air returns to the bottom air holes (Figure 7) and the top and bottom nozzles are disposed opposite each other, the quantity of, air passing through a permeable conveyor will be minimised. This is an advantage where it is desired not to heat the edges of the product preferentially.

Fourthly, referring to Figure 4 a substantial proportion of the body of the oven assembly is kept under suction. This is useful to minimise heat leakage through, for example, the explosion relief panels on a direct- fired oven. It will be appreciated that the enclosures under suction may be extended into the central/product enclosure by extending ducts, for example, from the enclosures under suction into the product enclosure.

Fifthly, by controlling the return air by inserting the circumscribing return air plates it is made easier for the oven designer to limit the flow of air through the oven (drift) which can be an unwanted feature of convection ovens.

It will be appreciated that any suitable gas, or combination of suitable gases, could be urged towards the conveyor and recirculated.

In a modified embodiment, suction creation means are provided downstream of the top and bottom return air holes to create additional suction, if required, and so not relying solely on suction created by the circulation fan 2, and the configuration of the return air holes.

Although the return air holes are located intermediate of the rows of convection nozzles it may be that the return air holes are disposed in alignment with each respective row of nozzles

With reference to Figure 8 there is shown an alternative return air arrangement (of which only part of the bottom air return arrangement is shown) of the oven of Figure 4, which alternative arrangement comprises a transverse pipe 20. The pipe 20 comprises a main portion which extends between and runs parallel to rows of convection nozzles 4 and is provided on each side with a plurality of spaced-apart return air holes 21.

At each end of the main portion the pipe 20 comprises vertical portions 20a which are in communication with the bottom suction enclosure 5b.

As can be seen from Figure 8 each nozzle 4 has associated with it a respective return air hole 21. On reaching the conveyor/product air is then drawn directly to a respective return air hole 21. It will be appreciated that although only two rows of nozzles 4 and one return air pipe 20 are shown, typically more rows of nozzles and more return air pipes would be provided.

Three further alternative return air arrangements will now be described, with reference to Figures 10,11 and 12. Each alternative embodiment is similar to that shown in Figures 8 and 9, in that a plurality of laterally- extending conduits is provided above and below the conveyor, each being provided with a series of apertures spaced along the lengths thereof.

With reference to Figure 10, there is shown a plan view of part of the upper surface 24 of the bottom plenum enclosure 3b which is provided with laterally-extending rows of spaced convection nozzles 4. Located between rows of nozzles 4 there are provided longitudinally-spaced conduits 30 (of which only one is shown) each having a series of return air holes 31 formed therein and being spaced lengthwise with respect to each conduit 30. The return air holes are positioned so as to be in register with the respective heated air nozzles 4. The distal ends 26 are

connected to the bottom suction enclosure for communication of the return air ducting and onwards to the inlet of the circulation fan.

In use, on reaching the product and/or conveyor heated air is drawn downwardly and longitudinally towards a respective return air hole 31 and is then (as shown generally at 23) drawn towards the bottom suction enclosure.

Figure 11 shows a very similar arrangement to that of Figure 10 and like reference numerals are used to denote like features. A plurality of longitudinally-spaced tubes 40 is provided between rows of the heated air outlets 22, and each tube being provided with a lengthwise series of return air holes 41 which are offset with regard to the convection nozzles 4. More particularly the return air holes are provided aligned with the spaces between the convection nozzles 4. In use, on reaching the product and/or the conveyor the heated air is drawn downwardly, longitudinally and laterally towards a respective return air hole 4.

With regard to Figure 12 there is shown a further embodiment in which each of a plurality of conduits 50 is disposed between rows of the convection nozzles 4. Each hole 50 is provided with a plurality of extended portions 52 which extend from opposite sides of the conduit 50 and between adjacent convection nozzles 4. Each extended portion is provided with a return air hole 51. In use, on reaching the product and/or the conveyor heated air is drawn downwardly and generally laterally towards a respective return air hole 51.

It will be appreciated both upper and lower return air means are provided in the embodiments of Figures 10,11 and 12. However it may be that where only upper or lower convection nozzles are provided then only upper or lower return air holes respectively may be provided.

By providing a plurality of upper and lower longitudinally-spaced and laterally-extending return air holes, issued heated air is caused to take a substantially direct path on reaching the conveyor/product to a respective return air hole. Furthermore, the various advantages stated in respect of the first embodiment of Figure 4 generally result from the embodiments of Figures 8 to 12.

In all of the embodiments herein described it is preferred that the total of the areas of the return air holes is greater than the total of the areas of the convection nozzles. In particular it is highly preferred that the ratio of the aforementioned areas is between two and four inclusive.

Further embodiments of the invention may combine some or all of the various return air arrangements disclosed herein.