Furnace for Continuous Gas Curing, Especially of Rubber Products
A continuous gas baking furnace includes elements for heating a baking area through which the product which is to be treated passes, formed by at least one module including a heat exchanger with a gas burner and elements for propelling and directing heated air towards a baking area, wherein the gas combustion circuit of the heat exchanger is separate from the circuit of hot air flowing through the baking area, the elements for directing the hot air include paths for directing hot air on both sides of the baking area and a path for evacuating air via the middle of the baking area. The furnace includes members for ensuring pyrolysis of gaseous effluents, the members treating a fraction of the gas circulating in a given section of the furnace, and at least one catalytic bed which is arranged on the hot air circuit.
The present invention concerns curing furnaces and relates, in particular, to furnaces for curing rubber products.
Known electric curing furnaces have reasonable construction costs but high operating costs.
Also known are gas curing furnaces which consume a very large amount of power and have the following technical drawbacks:
poor insulation resulting in needless consumption,
evacuation of the combustion gases released by the gas burner through the heating stream, entailing risks of pollution of the products to be cured,
simultaneous evacuation by mechanical extraction of the curing vapour released by the product and the combustion gases released by the heating means resulting in excess power consumption; it is therefore necessary to evacuate a large amount of hot air to extract the burnt gases even if the product does not release curing steam,
non-uniform air stream over the length of the furnace resulting in curing distributed non-uniformly over the product; this can cause heat shocks,
mechanical extraction which is dependent simultaneously or separately on the curing vapour and the combustion gases, thus rendering it difficult to adjust and optimise the curing.
In order to overcome these drawbacks there has been proposed a furnace configuration which is suitable, in particular but not exclusively, for the curing of rubber for the vulcanisation thereof, which configuration, despite having lower operating costs than known furnaces, has improved output and facilities for adjusting the curing parameters.
This furnace for the curing of rubber products comprises means for the gas heating of a curing area through which the product to be treated passes. The furnace is formed by juxtaposed modules integrated in a production line and each comprising a suitably insulated housing, a heat exchanger with a gas burner and means for propelling and directing air heated by the heat exchanger toward an area for curing the product, the gas combustion circuit of the heat exchanger being separate from the circuit of hot air passing through the curing area. The means for directing hot air toward the curing area comprise paths for directing hot air on both sides of the curing area and a path for evacuating air through the middle of the curing area.
This furnace is described in document EP-A-1 216 129.
During the vulcanisation of rubber there are produced polluting and toxic gaseous effluents such as nitrosamines, carbon monoxide and nitrogen oxides. Obviously, it is not desirable to release these effluents into the atmosphere either during or after vulcanisation of the products to be treated. These effluents have to be treated in advance in an installation dedicated to this operation. This treatment installation substantially increases the overall size and the cost of the furnace and its attachments.
The object of the invention is to propose a modification to the above-described curing furnace which, while preserving its optimum characteristics from the point of view of the curing of the products to be treated, also allows it to treat the polluting emissions under excellent technical and economic conditions.
The invention accordingly relates to a furnace for continuous gas curing, especially of rubber products, comprising means for heating a curing area through which the product to be treated passes, formed by a module or a plurality of juxtaposed modules, integrated within a production line and each comprising a suitably insulated housing, a heat exchanger with a gas burner and means for propelling and directing the heated air through the heat exchanger toward an area for curing the product, the gas combustion circuit of the heat exchanger being separate from the circuit of hot air flowing through the curing area, the means for directing the hot air also being distributed and set apart by grids along the curing area comprising paths for directing hot air on both sides of the curing area and a path for evacuating the air via the middle of the curing area, characterised in that it also comprises:
means for pyrolysing gaseous effluents present in the air circuit and resulting from the curing of the product, said means treating a fraction of the gases travelling in a given portion of the furnace;
and at least one catalytic bed interposed on the hot air circuit.
Said means for pyrolysing the gaseous effluents resulting from the product may comprise means for adjusting said fraction of the gases that is treated by pyrolysis.
The heat exchanger of each furnace module may be an exchanger with a hairpin tube which is internally heated using a corresponding gas burner, the means for propelling and directing hot air toward the curing area of each module may comprise at least one ventilation turbine arranged in proximity to the bottom of the housing of the module facing paths for directing hot air which are delimited by metal sheets for deflecting hot air to direct it either side of the curing area, and grids for distributing hot air along the curing area, and said means for pyrolysing said gaseous effluents may comprise a casing which is open at its two ends surrounding said hairpin tube and is positioned in the vicinity of said turbine, separated from said tube by a gap and terminated facing said turbine.
Said casing may be provided with means for varying its distance from the turbine.
Said casing may be provided with means for varying its length.
Said catalytic bed may be based on one or more precious metals deposited on an inorganic support.
As will be clear, the invention consists in interposing on the path itself of the gases travelling in the furnace of the type described hereinbefore:
on the one hand, means for pyrolysing the gaseous effluents which are capable of being treated in this way, said pyrolysis using as a heat source the heat exchanger of the furnace itself;
on the other hand, at least one catalytic bed capable of catalysing the gaseous effluents which can be treated in this way and were produced by the vulcanisation or pyrolysis referred to hereinbefore.
In a favoured variation of the invention, the gaseous effluents are treated by pyrolysis as a result of the fact that the heat exchanger is a hairpin exchanger and that, in proximity to the ventilator causing the gases to travel in the furnace, a branch of the hairpin is surrounded by a casing which is open at its two ends and assists the passage of a portion of the gas travelling in immediate proximity to the exchanger. The pyrolysis is thus carried out using extremely simple means which may easily be adapted to an existing furnace.
The invention will be better understood on reading the following description given with reference to the attached figures. In the drawings:
The following description relates to an embodiment of the invention based on a development of the furnace described in the above-cited document EP-A-1 216 129. For further details concerning the characteristics and the operation of this furnace, reference may usefully be made to said document. Nevertheless, it will be understood that this specific embodiment of the invention does not entail any limitation.
The furnace shown schematically in
The modules 1, 2, 3, 4 are joined together by means for assembling the housings thereof, appropriate heat seals 64, 65, 66 being interposed.
Associated with each module 1 to 4 is a gas burner 5, 6, 7, 8 which is supplied with gas via a supply conduit 9, 10, 11, 12 and internally heats a stainless steel heat exchanger 13, 14, 15, 16 formed by a hairpin tube. Each burner produces, from a mixture of air and gas, a flame which extends inside the tube and then conveys the combustion products without any contact with the exterior of the tube. The combustion products are drawn in by an extraction ventilator at the output of the tube.
In order to simplify the construction of the furnace, the heat exchangers are symmetrically inverted in the region of the junctions between the modules, thus allowing assembly of the gas supply conduits 9 and 10 and the conduits 17, 18, 19 and 20 for extraction of burnt gases, the extraction conduits being connected to evacuation chimneys 22, 23 rising above the roof.
As may be seen more clearly from
Each housing comprises, like the housing 25 shown in greater detail in
Each module also comprises an upper door 28 for accessing a curing tunnel in which there is arranged a roller conveyor 29 carried by two rails 30 and arranged above the region of the heat exchanger 15.
Opening the doors 28 allows complete access over the entire length of the furnace, facilitating maintenance of the curing area (cleaning or replacing a mat or rollers of the conveyor) and guiding of the product to be cured during start-up of a production operation.
Instead of a roller conveyor, it is possible to use a perforated or meshed mat (not shown).
In the wall of the housing 25 that is remote from the maintenance access door 27 there is arranged a ventilation turbine 32 which is driven in rotation by an electric motor 33 equipped with a cooling turbine 34 intended to dissipate the heat transmitted by the transmission shaft connecting the ventilation turbine and the electric motor and having one end inside the furnace and one end outside the furnace.
This rules out the risk of an excessive rise in the temperature of the electric motor.
The ventilation turbine 32, which is advantageously made of stainless steel, is arranged in the lower portion of the wall of the housing 26 that is remote from the door 27.
The number and the position of the ventilation turbines 32 may vary as required and in accordance with the overall design of the furnace. There may be one or more turbines per module 1, 2, 3, 4.
Owing to the central extraction of steam by the device, to which reference will be made hereinafter, and to these turbines 32, there is drawn in outside air which penetrates the furnace via the orifice for feeding the product to be cured into the first module 1 and via the orifice in the last module 4 via which the cured product leaves the furnace; this drawn-in air also progresses in this way toward the central area of the furnace.
Between the turbine 32 and the area of the roller conveyor there are arranged metal deflection sheets 35, 36, 37, made for example of stainless steel, which define paths 38, 39 of heating air indicated by corresponding arrows, said paths 38, 39 ending either side of the roller conveyor 29 in order uniformly to heat the product conveyed by the conveyor and to provide a return path 40 toward the bottom of the housing.
The heating air paths 38, 39 open onto lateral grids 41, 42 arranged either side of the rollers 29, whereas the return path 40 is defined by a series of air inlets 44 located below the rollers 29.
The air inlets 44 pass through the path 39 and end at the base of the housing between the metal deflection sheets 36 and 37. Owing to the advancement of the hot air brought about by the turbines 32, there is thus obtained a helical configuration of the path of the air within the furnace. As it advances, this air becomes filled with a broad range of gases resulting from the curing of the product. These are gases to be treated within the scope of the invention.
The furnace may be equipped on one of its lateral walls with an orifice 54 opening into a conduit 55 for evacuating the curing gases travelling therein. This orifice 54 and the corresponding evacuation conduit 55 are highly preferably arranged in the central position relative to the furnace as a whole. If the furnace comprises an odd number of modules, the orifice 54 and the conduit 55 are formed in the middle of the central module. If the furnace comprises an even number of modules (as shown in the figures), the orifice 54 and the conduit 55 are (as shown in
This central arrangement of the evacuation of the gases provides a high degree of symmetry and a high degree of uniformity of the flow of curing area within the furnace, and also satisfactory drawing ensuring an effective exchange of heat, on the one hand, between the air intended for curing and the exchangers and, on the other hand, between the air intended for curing and the product to be cured. The helical configuration of the path of curing air that is provided by the metal deflection sheets 35, 36, 37 and the air inlets 44 and the drawing assisted by the turbine or turbines 32, causing the curing air to follow this helical path, help to obtain this result.
It may therefore be seen that the circuit for directing and evacuating the heating air from the product displaced on the roller conveyor 29 is independent of the gas combustion circuit in the burners.
The walls and the doors of the housing 25, like those of the other housings, are in the form of an insulating material formed from ceramic fibres with crosswise layers arranged between an inner coating 31a in the form of a stainless steel sheet and an outer coating 31b made of painted steel.
The doors, such as the lateral door 27 and the upper door 28 for access to the curing tunnel, are equipped, as are the frames thereof, with respective seals 48, 49 and 50, 51 in the form of refractory braids.
As shown in
Depending on the nature and the characteristics of the product to be cured, a device for regulating the humidity of the hot air required for curing may be installed to prevent any drying-out of the product to be cured or to modify the surface appearance of the product after curing (brightness, orange peel effect, etc.). As soon as it is detected that the moisture content of the curing air is too low, an adjustable amount of water will be poured into a tank 63 positioned on the bottom of the furnace. Once this water has evaporated, the amount of water vapour contained in the curing air will return to an adjustable humidity level.
The furnace operates as follows.
When a product to be heated is placed at the input of the furnace, the operator opens the upper doors 28 for access to the curing tunnel and the front end of a product to be continuously cured is positioned in the curing tunnel. The operator then closes the doors 28.
Once the furnace has been adjusted in advance as a function of the curing parameters of the product to be treated, it is started up and the product is driven in the roller conveyor 29 at the desired speed which is compatible with the residence time of the product in the furnace.
The gas burners 5, 6, 7, 8 heat the air entering the heat exchangers 13, 14, 15, 16. The flames of the burners spread in the hairpin tubes of the exchanger and the burnt gases are evacuated via the chimneys 22, 23 (
For each module 1 to 4, a turbine 32 draws in the hot air preceding the heat exchanger 15 via its centre and dispels the air over its entire periphery. A plurality of turbines 32 may be provided instead of just one. In this case, the deflector 35 separates the flow of air propelled in two air streams 38 and 39 having identical speeds.
The air stream 38 guided by the walls 31a and 35 then passes through the lateral grids 41 in order to spread its heat uniformly and moderately in the curing area over the entire length of the right-hand side of the furnace.
Similarly, the air stream 39 guided by the walls 35 and 36 then passes through the lateral grids 42 in order to spread its heat uniformly and moderately in the curing area over the entire length of the left-hand side of the furnace.
A large number of air inlets 44 located below the conveyor 29 then combine the air streams 38 and 39 into a single return air stream 40 toward the heat exchanger 15 in order to be reheated.
If the furnace is equipped with a fume burning device, a portion of the gas which is caused to move by the turbines is evacuated from the housing of the furnace via the conduit 54 (
According to the invention, the above-described furnace also comprises the following elements (not shown in
In the areas adjacent to a turbine 32, the branch 15a of the hairpin 15 of the heat exchanger that is closest to the turbine 32 is surrounded by a casing 67 which terminates facing the turbine 32.
The purpose of this casing 67 (which in the illustrated example extends either side of the turbine 32 and terminates facing said turbine 32) is to allow a fraction of the gases travelling in the furnace to be drawn into the gap 68, which may have a width of for example from about 5 to 50 mm, separating the casing 67 and the branch 15a of the hairpin 15. The arrows 69, 70, 71, 72, 73 in
In this way, said fraction of the travelling gases filled with curing vapour passes closest to the exchanger 15 and is subjected, in a confined space, to the intense heat radiation released by the exchanger, especially as the branch 15a which is closest to the turbine 32 is (in the construction of the furnace provided by way of example) the branch closest to the burner 7 and therefore the hottest branch.
As the turbine 32 creates a reduction in pressure in the area which faces it and in which the casing 67 terminates, there is therefore generated a natural draught between the branch 15a of the exchanger 15 and the casing 67, allowing a significant fraction of the gas flow travelling in the relevant portion of the furnace to pass through the gap 68. The amount of this fraction may be adjusted if appropriate, by varying the rotational speed of the turbine 32 and/or the distances between the turbine 32 and the ends of the two portions of the casing 67 if displacement of these portions is controllable and/or of the length of the casing 67 if this is variable during use of the furnace (for example, by making the casing telescopic and by associating therewith appropriate control means).
The remainder of the gas flow travels in the furnace and passes above and below the casing 67. As the casing is made of a heat-conducting material, it is brought to a temperature similar to that of the exchanger 15 and is also involved in the heating of the gases which lick the casing in the directions indicated by the arrows 74, 75 in
The heat conditions prevailing in the gap 68 promote pyrolysis of the vapour released by curing. This pyrolysis requires temperatures of from about 350 to 800° C., more generally from 400 to 700° C. and more specifically from 500 to 600° C., in the case of the vapour resulting from the curing of the rubber.
As the actual curing of the rubber has to be carried out at a temperature of about merely 300° C., it will be understood that it would not be desirable simultaneously to treat by pyrolysis all of the gas flow travelling in the furnace. Such treatment would rapidly bring all of the gas flow to a temperature which exceeds this order of magnitude and is therefore unsuitable for curing the products. This is why a substantial portion of the gas flow is left free to travel outside the casing 67 in order to be maintained at the normal temperature for curing the products. In practice, the installation is configured and operatively regulated so that approximately, for example, 20% of the overall gas flow stirred in the corresponding area of the furnace passes through the gap 68 and is subjected to pyrolysis before being rediluted in the main flow.
As the furnace operates in a closed circuit, as the gaseous effluents pass repeatedly before one of the exchangers 15, both outside and inside the casing 67, all of the gases which result from the curing of the products and are capable of being pyrolysed end up being treated, although this does not result in excessive heating of the gases involved in curing.
More specifically, it is the nitrosamines derived from the vulcanisation that have to be destroyed by the pyrolysis.
This pyrolysis, but also the curing of the products, generates polluting effluents which have to be destroyed before the gases travelling in the furnace are dispelled into the atmosphere. In order to carry out this destruction within the actual interior of the furnace, as these effluents are produced, the invention provides for the gases travelling in the furnace also to pass through at least one catalytic bed 76, 76′. This catalytic bed must, in particular, be suitable for treating the CO and nitrogen oxides produced by the vulcanisation of the rubber products and by the above-described pyrolysis.
This catalytic bed 76, 76′ may be implanted at any desired location of the path of the gaseous effluents travelling in the furnace, for example with reference to the type of furnace illustrated in the figures:
above the turbine 32 and below the deflector 35 so as to treat the rising gas flow before it separates into two streams 38, 39 oriented either side of the roller conveyor 29; this is the case for the bed 76 from
or on the return path 40 toward the bottom of the housing of the gases which have passed through the roller conveyor 29; this is the case for the bed 76′ from
or immediately upstream of the turbine 32;
or at any other locations in the furnace through which there passes a flow of travelling gases;
or at a plurality of these locations simultaneously.
Typically, the catalysts of the catalytic bed or beds 76, 76′ are based on one or more precious metals deposited on various inorganic supports, typically caesium, rhodium, platinum, palladium. Platinum and palladium, and preferably palladium, are the preferred examples thereof. Treated or non-treated aluminas, various kieselguhrs, ceramics or any available inorganic chemical forms may be used as the supports.
These supported catalysts may be in all the various possible geometrical forms, for example in the form of perforated or non-perforated hollow tubes of all sizes, all diameters and all lengths with perforations of all possible dimensions and all possible shapes.
The catalysts are typically in the form of powders and preferably in the form of spherical balls of all diameters and all specific characteristics in terms of porosity, specific surface area, density, wear, and all of the other characteristics of the supported catalysts that enable them to transform by catalysis the vapour emitted during curing the products which the furnace has to treat and during pyrolysis of this vapour. The content of the above-cited precious metals may typically be between 0.01 and 5% by weight, generally between 0.1 and 2% by weight and preferably between 0.3 and 1% by weight.
The furnace which has been described and illustrated hereinbefore is merely one possible embodiment of the invention. It will be understood, in particular, that the gaseous effluents could be pyrolysed by means other than the casing 67 surrounding the branch 15a of the exchanger 15, especially if use is made of a type of heat exchanger other than a hairpin exchanger.
It is preferable for each module 1, 2, 3, 4 of the furnace to be equipped with pyrolysis means and catalysis means even if, strictly speaking, it might be sufficient to provide merely a single module owing to the fact that the gases travel within the furnace.
It will be understood that the presence 54 of the evacuation conduit 55 and the members associated therewith is not compulsory.
Obviously, the invention would also apply to a furnace composed merely of a single module.
Claims
1. Furnace for continuous gas curing, especially of rubber products, comprising means for heating a curing area (29) through which the product to be treated passes, characterised in that it is formed by a module or a plurality of juxtaposed modules (1, 2, 3, 4), integrated within a production line and each comprising a suitably insulated housing (24, 25, 26), a heat exchanger (13, 14, 15, 16) with a gas burner (5, 6, 7, 8) and means (32, 33, 35, 36, 41, 42) for propelling and directing the heated air through the heat exchanger toward an area (29) for curing of the product, the gas combustion circuit of the heat exchanger being separate from the circuit of hot air flowing through the curing area, the means for directing the hot air also being distributed and set apart by grids (41, 42) along the curing area comprising paths (38, 39) for directing hot air on both sides of the curing area (29) and a path (40) for evacuating the air via the middle of the curing area, characterised in that it also comprises:
- means for pyrolysing gaseous effluents present in the air circuit and resulting from the curing of the product, said means treating a fraction of the gases travelling in a given portion of the furnace;
- and at least one catalytic bed (76, 76′) interposed on the hot air circuit.
2. Gas furnace according to claim 1, characterised in that said means for pyrolysing the gaseous effluents resulting from the product comprise means for adjusting said fraction of the gases that is treated by pyrolysis.
3. Curing furnace according to claim 1, characterised in that the heat exchanger (13, 14, 15, 16) of each furnace module is an exchanger with a hairpin tube which is internally heated using a corresponding gas burner (4, 5, 6, 7), in that the means for propelling and directing hot air toward the curing area of each module comprise at least one ventilation turbine (32) arranged in proximity to the bottom of the housing of the module facing paths for directing hot air which are delimited by metal sheets (35, 36) for deflecting hot air to direct it either side of the curing area, and grids (42, 42) for distributing hot air along the curing area, and in that said means for pyrolysing said gaseous effluents comprise a casing (67) which is open at its two ends surrounding said hairpin tube (15a) and is positioned in the vicinity of said turbine, separated from said tube by a gap (68) and terminated facing said turbine (32).
4. Furnace according to claim 3, characterised in that said casing (67) is provided with means for varying its distance from the turbine (32).
5. Furnace according to claim 3, characterised in that said casing (67) is provided with means for varying its length.
6. Furnace according to claim 1, characterised in that said catalytic bed (76, 76′) is based on one or more precious metals deposited on an inorganic support.
7. Curing furnace according to claim 2, characterised in that the heat exchanger (13, 14, 15, 16) of each furnace module is an exchanger with a hairpin tube which is internally heated using a corresponding gas burner (4, 5, 6, 7), in that the means for propelling and directing hot air toward the curing area of each module comprise at least one ventilation turbine (32) arranged in proximity to the bottom of the housing of the module facing paths for directing hot air which are delimited by metal sheets (35, 36) for deflecting hot air to direct it either side of the curing area, and grids (42, 42) for distributing hot air along the curing area, and in that said means for pyrolysing said gaseous effluents comprise a casing (67) which is open at its two ends surrounding said hairpin tube (15a) and is positioned in the vicinity of said turbine, separated from said tube by a gap (68) and terminated facing said turbine (32).
8. Furnace according to claim 7, characterised in that said casing (67) is provided with means for varying its distance from the turbine (32).
9. Furnace according to claim 4, characterised in that said casing (67) is provided with means for varying its length.
10. Furnace according to claim 2, characterised in that said catalytic bed (76, 76′) is based on one or more precious metals deposited on an inorganic support.
11. Furnace according to claim 3, characterised in that said catalytic bed (76, 76′) is based on one or more precious metals deposited on an inorganic support.
12. Furnace according to claim 4, characterised in that said catalytic bed (76, 76′) is based on one or more precious metals deposited on an inorganic support.
13. Furnace according to claim 5, characterised in that said catalytic bed (76, 76′) is based on one or more precious metals deposited on an inorganic support.
Type: Application
Filed: Jan 9, 2006
Publication Date: May 22, 2008
Applicant: ESTABLISSEMENTS PAUMELLE (BOLBEC)
Inventor: Dominique Paumelle (Eustache La Foret)
Application Number: 11/795,261
International Classification: F23G 7/06 (20060101);
