Clair-thompson St., Stephen John (21 Crocket Lane, Empingham Oakham, Rutland LE15 8PQ, GB)
| 1. | An oven (1) comprising a fan heater assembly (13,29) comprising a fan (13) in series with a heater (29) having a heater assembly inlet (20) and a heater assembly outlet (10), the outlet (10) of the fan heater assembly being connected to the inlet (12) of a plenum chamber (6), a wall (8) of which bounds an oven space (22) in which products to be heated are positioned in use, the plenum chamber wall (8) being provided with a plurality of nozzles (9), or being capable of carrying a plurality of nozzles (9), the nozzles being directed, in use, into the oven space (22), for directing jets of hot air from the plenum chamber (6) into the oven space (22), a plenum chamber outlet (17) and an oven space outlet (21) from said oven space both being connected to the fan heater assembly inlet (20), and closure means (32) for closing either the plenum chamber outlet (18) or the oven space outlet (21). |
| 2. | An oven as claimed in claim 1 in which the fan (13) is positioned upstream of the heater (29) in the fan heater assembly. |
| 3. | An oven as claimed in claim 1 or claim 2 in which the closure means (32) is capable of being adjusted to a condition in which said plenum chamber outlet (18) and said oven space outlet (21) are both partially open (Fig 4) by controlled amounts to allow the oven to be operated in a mixed mode. |
| 4. | An oven as claimed in any of the preceding claims in which the closure means comprises a single damper (32) which is capable of being positioned either to substantially block the oven space outlet (21) or to substantially block the plenum chamber outlet (18), thereby providing said two possible modes of operation of the oven. |
| 5. | An oven as claimed an any of the claims 1 to 3 in which the closure means comprises two dampers one of the dampers being closable to close the plenum chamber outlet, and the other damper being closable to close the oven space outlet. |
| 6. | An oven as claimed in any of the preceding claims in which the nozzles are arranged to be readily removable and comprising blanking means (23) which can be substituted for the nozzles. |
Classical convection ovens are well known in industry. In this type of oven, air from within the bake chamber (oven space) is passed by a fan over a heating system, and then blown into a plenum chamber. It is blown out of the plenum chamber into the oven space through a series of nozzles.
Accordingly the main mode of heat transfer is by forced convection of the air coming out of the nozzles over the process product in the oven space.
The heating system may, for example, consist of a gas flame, a gas or oil fired heat exchanger, or an electric heater bank.
Classical cyclotherm ovens are well known in industry. In this type of oven, air from within a substantially closed system is passed by a fan over a heating system, and is then blown through tubes or a plenum making them very hot so that they radiate heat into the oven space, returning to the fan and heating system.
Accordingly the main mode of heat transfer in a cyclotherm oven is by radiation from the tubes or plenum onto the process product.
In a cyclotherm system the fan is usually upstream of the heater, with an exhaust duct of appropriate size between the fan and the heater. This arrangement puts the radiating tubes or plenum under suction. The heating system may, for example, consist of a gas flame, a gas or oil fired heat exchanger or an electric heater bank.
There has been perceived throughout industry a need for an oven able to be changed from one mode of heat transfer to the other, or able to work partly in each mode.
Versions of this dual-mode system exist. These consist of a convection system with a damper at the end of the plenum. In the convection mode the damper is closed, so that the air is forced to come out of the nozzles.
In the cyclotherm mode the damper is open allowing the air to spill out of the end of the plenum, back to the fan, with a minimal amount coming out of the nozzles.
The shortcoming of existing systems is that they lack sufficient power in the cyclotherm mode and are liable to give uneven heating across the product conveyor.
According to the invention an oven comprises a fan heater assembly comprising a fan in series with a heater having a heater assembly inlet and a heater assembly outlet, the outlet of the fan heater assembly being connected to the inlet of a plenum chamber, a wall of which bounds an oven space in which products to be heated are positioned in use, the plenum chamber wall being provided with a plurality of nozzles, or being capable of carrying a plurality of nozzles, the nozzles being directed, in use, into the oven space, for directing jets of hot air from the plenum chamber into the oven space, a plenum chamber outlet and an oven space outlet from said oven space both being connected to the fan heater assembly inlet, and closure means for closing either the plenum chamber outlet or the oven space outlet.
Preferably the fan is positioned upstream of the heater in the fan heater assembly, but it may be possible to position the fan downstream of the heater in some cases.
Such an oven can either be used in a convection mode in which the plenum chamber outlet is closed, or in a cyclotherm mode in which the oven space outlet is closed.
The closure means is preferably capable of being adjusted to a condition in which said plenum chamber outlet and said oven space outlet are both partially open by controlled amounts to allow the oven to be operated in a mixed mode.
The closure means may comprise a single damper which is capable of being positioned either to substantially block the oven space outlet or to substantially block the plenum chamber outlet, thereby providing said two possible modes of operation of the oven.
The nozzles may be arranged to be readily removable and replaceable by blanking means when it is desired to ensure that substantially no forced convective heating takes place, and the sole form of heating is to be cyclotherm heating.
Alternatively, the nozzles may be left open when the plenum chamber outlet is open (and the oven space outlet closed) and some inflow and/or outflow of air through the nozzles is tolerated.
A dual mode baking oven in accordance with the invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic longitudinal cross-section of the oven showing the damper closing the plenum chamber outlet, so as to cause the oven to operate in a convection mode,
Figure 2 is a view similar to Figure 1 but showing the damper closing the oven space outlet to cause the oven to operate in a cyclotherm mode, Figure 3 is a view similar to Figure 1 but showing the nozzles of the plenum chamber replaced by blanking plates, and showing the oven operating in a cyclotherm mode.
Figure 4 is a partial view similar to Figure 1 showing the damper in an intermediate position.
With reference to Figure 1, an oven 1 comprises an oven casing 2 provided with a conveyor inlet 3 and a conveyor outlet 4 through which the top run of a product conveyor 5 extends. A plenum chamber 6 is defined by an upper wall 7 and a lower wall 8 which is spaced vertically above the conveyor 5 with sufficient clearance for products to be baked to pass beneath the lower wall 8. Lower wall 8 is provided with an array of downwardly directed nozzles 9 which may, as discussed hereafter, be mounted on removable plenum base plates.
A heater 29 is mounted in a convenient position above, or below, the oven casing 2, and the heater outlet 10 is connected by ducting 11 to an inlet 12 to the plenum chamber 6. The heater 29 can be of any convenient type, as will be appreciated by those skilled in the art.
A fan 13 has the fan outlet 14 thereof connected to the inlet 15 of the heater, and the fan outlet 14 is also provided with an exhaust vent 16 to atmosphere in well known manner.
The plenum chamber 6 is provided at the opposite end to the plenum chamber inlet 12 with a plenum chamber outlet 17 connected to a vertically extending duct 18 which leads to the lower end of a downwardly directed horn 19 connected with the fan inlet 20.
A vertically directed outlet 21 from the oven space 22 is positioned alongside the upper end of the duct 18 and is connected to the lower end of horn 19 in a symmetrical position to that of the upper end of duct 18, whereby a slideable damper 32 can be slid from the position shown in Figure 1 to the position shown in Figure 2.
When the damper 32 is positioned as in Figure 1, the outlet from duct 18, and thus from duct 6, to the fan inlet 20 is closed, but the outlet 21 from the oven space 22 is open, thereby causing hot air in plenum chamber 6 to be blown downwardly through nozzles 9 to provide forced convection heating of products carried by conveyor 5. The air received in oven space 22 is then sucked from space 22 by the fan, passing through the open oven space outlet 21.
In the condition of the damper 32 shown in Figure 2 the oven acts primarily as a cyclotherm oven. As before, hot air from the heater outlet 10 passes to the plenum chamber inlet 12 by way of duct 11, but since the outlet 21 from the oven space 22 is closed by damper 32, and the upper end of duct 18 is open, only a little of the air in plenum chamber 6 will tend to inflow and/or outflow through nozzles 9.
Accordingly, the main mechanism for heating products in oven space 22 is by way of radiation from the hot base of the plenum chamber 6. Thus, the oven operates in a cyclotherm mode when the damper 32 is positioned as in Figure 2.
The damper 32 may alternatively be positioned at an intermediate position, as shown in Figure 4, in which the upper part of duct 18 is partially open, and the outlet 21 is also partially open, in order to provide a combination of convective and cyclotherm heating, ie mixed mode heating. The proportions of such heating modes can be varied by altering the position of the damper 32.
Instead of a single slidable damper 32, the closure means may, for example, comprise two dampers, which may be pivotable or slidable or iris type dampers, and which respectively control the upper part of duct 18 and the outlet 21.
Figure 3 shows the oven when the base plates of the plenum chamber 6 of Figure 2 have been replaced by a series of blanking plates 23 that are not provided with nozzles. This measure minimises convective heating, but it will generally not be necessary to use such blanking plates since the presence of the damper 32 in the position shown in Figure 2 effectively minimises flow through the nozzles 9.
Although the drawings show the plenum chamber 6 positioned above the conveyor band, it will be appreciated that the plenum chamber may be positioned beneath the band, and there will often be plenum chambers positioned above and below the products to be heated.
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