GB2069306A - Travelling baking oven - Google Patents

Abstract

In order to improve the thermal efficiency of a multi-zone, indirectly- heated, baking oven, with conveyor (1) and a closed-circuit gas heating system (circulation shown by single- headed arrows) which heats at least the major part of the length of the oven and which includes at least two heaters (3-7) which are spaced apart in the direction of gas flow in the heating system, the gas flow is passed along the bottom of the oven to adjacent the delivery end of the oven and then back along the top of the oven, the temperature generally falling throughout the circuit so that the maximum bottom heat is adjacent the input end of the oven and the maximum top heat is adjacent the delivery end of the oven. <IMAGE>

Description

SPECIFICATION Travelling baking oven Background of the Invention The present invention relates to a multi-zone indirect-heated, travelling baking oven, which can be for bakery products such as bread, cake or confectionery (i.e. small cakes, tarts, etc.) or for biscuits.

The term "multi-zone" means that there is not a uniform temperature or heat input throughout the length of the oven. For instance in bread making, a high heat input is used on the bottom in the first zone of the oven, for the development or rising of the bread, while a very low or zero heat input is required on the top, to keep the surface temperature as low as possible and prevent a skin forming so that expansion can take place at a high rate without splitting the bread - the baking tins or the conveyor prevent the bottom splitting; in the second zone a somewhat lower heat input is used on the bottom and the top heat input is less than that on the bottom but greater than the top input in the first zone, for drying the bread; in the third or last zone, a yet lower bottom heat input is required while the top input is somewhat higher, about equal to the bottom input, for caramelising or browning the crust of the bread. Very roughly, each zone can extend for a third of the length of the oven, but there is overlap of the zones and of the specific processes occurring in them. The same sort of heat or temperature control is required for the majority of bakery products, but not for all products. For instance, in making some biscuits, high heat is applied to the top and low heat to the bottom in the first zone.

In indirect heating, there is usually a closedcircuit gas heating system heated by a heater which in general terms can be of any suitable type, including electrical heaters or a combination of electrical heaters and burners, but in practice is usually gas or oil fired or gas/oil dual purpose; with such a burner, the combustion gases pass into the circuit and, usually immediately before the gas is returned to the burner, there is a bleed off of say 10% (by mass) of the gas flow - the system is known as the "cyclotherm" system.

A travelling oven has some way of passing products to be baked through the oven from the input end to the delivery end, usually in the form of a travelling conveyor for carrying the products either directly or in baking tins. The oven chamber can be very large, for instance about 30 metres long and 4 metres wide.

The Invention The present invention provides an oven as set forth in Claim 1 and a method of baking as set forth in Claim 11. Claims 2 to 9 set forth preferred features of the invention.

The oven of the invention can have the following advantages: (a) the oven provides the heating pattern normally required for bread and many other bakery products, namely a falling input along the bottom of the oven and a rising input along the top. As the single gas heating system is operated with a generally falling temperature throughout the circuit, and as the output end of the heating system is at a zone of the oven where a low temperature is required, the returning gases will be at a low temperature and the gas bleed will not deprive the system of too much energy. Thus the oven can have good energy utilisation and can provide an energy running cost saving of around 10% relative to a comparable modern threesection "Turbo-radiant" oven (generally as in GB 1 212 525) and of up to around 20% relative to a comparable oven of GB 382 266.Virtually all the gas flow in the heating system can be longitudinal, significantly reducing the system resistance, and it is found that for an oven about 30 metres long, the gas heating system can be operated with a pressure drop of about 25 millibars.

(b) a single basic heating system can be provided for the whole oven, replacing two, three or more systems, and reducing manufacturing costs. In fact, it is found desirable to have a relatively large first heater so as to provide higher temperature gases at the input end of the circulatory heating system, and considering an oven of the present invention against a comparable modern three-section "Turboradiant" oven, the first heater can have almost double the capacity of each of the "Turboradiant" heaters and the further heaters, four in number, can have a capacity of about half of that of each "Turboradiant" heater. In general, it is found desirable to operate the heaters at 6570% of their maximum output, so that their maximum output can be used for rapid heat-up.This can increase the manufacturing costs of the heaters, but the oven as a whole can be cheaper due to the simplified gas flow system.

(c) the gas circulation system can be arranged to circulate low volumes of gases and thus require significantly less fan power against a comparable modern three-section "Turboradiant" oven; for instance an oven according to the invention having a length of about 30 metres can have a gas circulation of only about 500 m3/min compared to about 2400 m3/min for the "Turboradiant" oven.

(d) the use of a single heating system enables the flow system to be simplified so that less maintenance is required and fewer fans and motors need be used; the system can be designed to have no dampers and no balancing vanes, so that there is a very low system resistance and gas flow can be achieved with low pressure drops.

(e) the oven of the invention enables very flexible heat control to be performed because it is possible to have independent top and bottom heat and independent top and bottom zone lengths.

The heating gases are fed from one zone to the next zone and their temperature can be monitored and used to control the further heater(s) automatically. Thus the further heater(s) can be modulated (preferably all the heaters are modulating), or switched off, or cold air can be blown in at the position of the heater; as a further possibility, a zone of the oven, such as the delivery end, can be partially or completely by-passed.

The oven of Claim 2 can have general applicability for bakery products, and the first burner can have 75% of the heating capacity of the whole oven. However, for special purposes, other arrangements can be used.

The oven of Claim 6 enables a single flue to be used for the whole oven heating system, facilitating the use of heat recovery schemes.

The oven of Claim 7 provides even heating across the oven -- without the distribution cylinders, it is hard to provide even distribution of heat across the oven.

Description of Preferred Embodiments of the Invention The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of an oven in accordance with the invention; Figures 2a and 2b show two alternative constructions for the first or feed end zone of the oven, in side elevation on a larger scale; Figure 3 shows a construction for the second zone of the oven, in side elevation; Figure 4 shows a construction for the third or delivery end zone of the oven, in side elevation; and Figures 5 to 8 are vertical sections along the lines V-V in Figure 2a and VI--VI, VIl-VIl and Vill-VIll in Figure 4 (the lines are also indicated in Figure 1).

Figure 3 overlaps Figure 2a or 2b and Figure 4 to a certain extent. Throughout the drawings, the single-headed arrows indicate the heating (combustion) gas flow, the double-headed arrows indicate the circulatory air flow and the trebleheaded arrows indicate the movement of the products; the arrows indicating the gas and air flows are only rough indications of the direction of flow.

The oven is a multi-zone, indirect-fired, travelling baking oven of the tunnel type, having a single circulating wire band conveyor 1, the bottom of the baking chamber being defined by a floor 2a and the top of the oven chamber being defined by a roof 2b. The oven has gas duct means defining a single closed-circuit gas heating system which heats substantially the whole length of the oven and which is fired by a first or main heater 3 and a number of further heaters 4, 5, 6 and 7 downstream of the first heater in the direction of gas flow in the heating system.The general nature of the circulatory gas system can be seen in Figure 1, and it will be observed that all the gas flows are substantially longitudinal for the whole length of the oven, apart from returns at the ends of the oven and divergencies adjacent the heaters 3-7, the gas flow being along the bottom of the oven and then back along the top. The first heater 3 is of much larger capacity than the further heaters 4-7, and to give an example, the first heater 3 can have a capacity of 0.725 x 106 K.cals/hour whilst the further heaters 4-7 can each have the same capacity of 0.2 x 1 06 K.cals/hour. There is a single bleed 8 at the downstream end of the heating system, which can be connected to a suitable flue and heat recovery device.

Details of the main heater 3 can best be seen in Figure 2a. The main heater 3 has a burner 9 which discharges hot combustion gases into an unperforated heater tube 1 0. A roof-mounted recirculating fan 11 draws the return gases from the heating system and discharges them into the annular space around the heater tube 10, the return gases mixing with the hot burner gases before entering an inlet duct 1 2 which feeds a bottom header 13.

In general, the radiators in the oven are in the form of parallel longitudinal tubes 14 which can be seen for instance in Figures 5, 7 and 8, the tubes 14 being close together but with slight air gaps therebetween. However, the first radiator 1 5 in the alternative of Figure 2a may be either a box type or tubular type, dependent on the type of conveyor 1 used and the product being baked.

Generally a box-type radiator 1 5 would be used with a wire band for "oven bottom" products which rest directly on the conveyor 1, whilst the tubular-type radiator would be used with a grid sole type conveyor (not as illustrated) on which tinned products are conveyed.

Specifically referring to Figure 2a, part of the hot gases from the bottom header 13 are passed rearwards into the first radiator 1 5 and a bottom pre-heat radiator 16 for pre-heating the conveyor 1.

The arrangement is different in Figure 2b, where all the gases pass forwards from the bottom header 1 3 into a first radiator 1 7.

Figures 3 and 4 and 6 give good illustrations of further heaters 5, 7 which are immediately below and above the baking area and are fired by burners 18, 1 9 which are immediately to the side of and below and above the baking area, injecting transversely of the path of travel of the products through the oven into a respective perforated heater tube 20 which extends transversely across a path of gas flow in the heating system. Adjacent each heater tube 20, the individual tubes 14 of the, radiators connect with a header which extends for the whole width of the oven chamber and contains the respective heater tube 20.

At the delivery end of the oven (see Figure 4), the circulating gases are taken through ducts 21 at the sides of the oven chamber to the top of the oven, and then they travel back to the recirculating fan 11 through top radiators and passing over further heater tubes 20.

There are no dampers and no balancing vanes throughout the hot re-circulating gas system. The quantity of heat released is determined by regulating the temperature of the mass circulation (which mass is substantially constant), specifically by regulating the further burners 4-7 whilst keeping the heat output of the main burner 3 constant. In practice, very little heat is required from the further burners 4-7 for bakery products such as bread as they require a falling heat on the bottom and a rising heat on the top as they travel through the oven, and the system can utilise to the full, the heat remaining in the gases from the previous zone. Automatic temperature controls (not shown) are incorporated.

The burners and heaters can be as described in GB 707 750.

Convection fans 22 are provided for circulating the oven chamber atmosphere over the radiator tubes 14, as shown in Figure 8. The circulation can be upwards (as shown) or downwards, depending on the direction of rotation of the convection fan 22, and the convection fans 22 are positioned on alternate sides of the oven to obtain as even a flow of air as possible.

Alternatively convection or turbulence fans feeding a system of blowing tubes can be provided generally as described in GB 1 212 525 for circulating the oven chamber atmosphere over and between radiator tubes 14. By this method the application of convection can be both upwards and/or downwards depending on the damper settings in that system.

An extraction fan 23 (see Figure 4), connected to the centre part of the oven chamber by way of a mechanically-controlled damper plate 24 (Figure 7), is provided for extracting air and steam from the oven chamber, and is also connected to a withdrawal duct 25 for preventing excessive emission of fumes from the delivery end of the oven. A further extraction fan 26 and extraction duct 27 prevent excessive emission of fumes at the feed end of the oven, and steam for gelatinisation is provided by blowing steam in through perforated transverse tubes 28 at the feed end of the oven (see Figures 2a and 2b).

For a product such as bread, the gas in the gas circulating system can drop from a temperature of about 6900C to a temperature of about 2400C, the latter being the temperature at the bleed 8.

Claims (12)

1. A multi-zone, indirect-heated, travelling baking oven having a closed-circuit gas heating system which heats at least the major part of the length of the oven, which system includes at least two heaters which are spaced apart in the direction of gas flow in the heating system, the gas flow being along the bottom of the oven to adjacent the delivery end and then back along the top of the oven, the temperature generally falling throughout the circuit so that the maximum bottom heat is adjacent the input end of the oven and the maximum top heat is adjacent the delivery end of the oven.

2. The oven of Claim 1, wherein the first heater in the system is adjacent the input end of the oven and is of much larger capacity than the or each further heater.

3. The oven of Claim 2, wherein there is a plurality of said further heaters, all of which are of substantially the same capacity.

4. The oven of any one of the preceding Claims, wherein all gas flows in the heating system are substantially longitudinal for the whole length of the oven or of the part heated by the heating system, apart from returns at at least one end of the oven or part and divergencies adjacent the heaters.

5. The oven of any one of the preceding Claims, wherein the gas flow in the heating system is passed along longitudinal tubes over which circulating fans circulate the oven atmosphere.

6. The oven of any one of the preceding Claims, wherein there is a single bleed from the heating system.

7. The oven of any one of the preceding Claims, wherein there is a first heater, adjacent the input end of the oven, and at least one further heater spaced downstream of the first heater in the heating system, which further heater(s) is or are one or more burners positioned immediately to the side of and above or below the baking area, injecting transversely of the path of travel through the oven into the circulating gases of the heating system, the or each burner injecting into a perforated distribution cylinder extending transversely across the path of gas flow in the heating system.

8. The oven of any one of the preceding Claims, and arranged such that a zone of the oven can be partially or completely by-passed.

9. The oven of any one of the preceding Claims, wherein no dampers are incorporated for regulating the gas flow in the heating system.

1 0. A multi-zone, indirect-heated, travelling baking oven, substantially as herein described with reference to, and as shown in, Figure 1,2a and 3 to 8, or Figures 1,2b and 3 to 8 of the accompanying drawings.

11. A method of baking, comprising using the oven of any one of the preceding Claims.

12. A method of baking, comprising using the oven of Claim 1, and substantially as herein described.