GB2264849A - Resistance heating pipe for an electric industrial furnace - Google Patents

Description

2264849 -1 Heating pipe for an industrial furnace, industrial furnace and

method for heating the furnace The invention relates to a ceramic, porous heating pipe for electric heating of an industrial furnace, an industrial furnace equipped with such heating pipes as well as a method for electric heating of the furnace.

The electric heating of industrial furnaces 3.0 in the form of a so-called resistance heating has been known for a long time. The resistance heating is based on the principle that the electric current when flowing through the corresponding heating elements develops a resistance, whereby heat is released.

Metallic as well as ceramic materials can be considered as heat conducting materials.

In particular with higher temperatures ceramic heating rods and pipes are used which are produced, for example, from silicon carbide. All these require relatively high current flow, necessitating the use of transformers.

Such ceramic heating pipes can in principle be used in all types of industrial furnaces, but are particularly problematic if "extreme" furnace atmospheres exist. So, for example, when burning ferrites, in particular soft ferrites, there is a requirement for having a furnace atmosphere which is almost 100% oxidising, in order to obtain optimum material properties of the material to be burnt.

Conversely cases of application are also found in which the furnace atmosphere should be almost completely reducing.

When using ceramic heating rods (heating pipes) in these cases a drastic reduction of the life of the heating pipes is observed. This is a result of the aggressive attack from the furnace atmosphere, which is favoured by the fact that the ceramic heating pipes mentioned have a porousness (also open), which can be 10 to 20 % by volume. In this the result is partly also a burning of the actual heat conductor material, therefore a destruction of the heating pipe.

Attempts have been made to lengthen the life of such ceramic heating pipes, in that these are glazed. But even in this case residue porousness remains so that the described effects can at all events be slowed down, but not prevented.

With the destruction of the heat conductor material at the same time an increase in the resistance occurs. It is possible to compensate partly for this through a controlling of the associated transformer in order to adapt the exiting voltage (secondary voltage) correspondingly, finally the destruction can develop such that no heat output or at least no sufficient heat output is achieved any more.

In this respect the underlying object of the invention is to provide a ceramic, porous heating pipe for the electric heating of an industrial furnace, which also has a higher stability even over longer periods (therefore a reduced ageing) under extreme furnace atmospheres, in particular extreme oxidising and extreme reducing conditions. A correspondingly equipped industrial furnace as well as a method for its heating should be provided at the same time.

The invention is based on the finding that the above-described destruction of the heating rods (heating pipes) is caused by a type of "infiltration" of the aggressive furnace atmosphere which can penetrate by way of the pores into the material. The invention proceeds in this respect from the consideration that such an "atmosphere infiltration" is to be prevented as far as possible. In this context it has been recognised that this aim can be achieved in that in the corresponding heating pipes an excess pressure is generated, which is formed with the aid of the inert gas so that because of the partial pressure differences formed the inert gas penetrates the heating pipes from the inside outwards, thus preventing an inlet of the furnace atmosphere into the heating pipes.

In this manner the heating pipes are protected relative to an attack from the furnace atmosphere and at the same time a type of "inert gas veil" is formed around the heating pipes.

It is sufficient thereby to place the excess pressure of the inert gas atmosphere inside the heating pipes only slightly above the gas pressure of the furnace atmosphere. Theoretically it would even be sufficient to place the inert gas pressure and the pressure of the remaining furnace atmosphere at the same level. Since the furnace atmosphere can however vary and as much safety as possible relative to the atmospheric attacks should be created, the inert gas pressure lies preferably somewhat (for example 1 to 5%) above the atmospheric pressure within the furnace.

With this in mind the invention relates in its most general embodiment to a porous ceramic heating pipe for electric heating of an industrial furnace, which is distinguished by the following features.

- the heating pipe has at least at one first end an adapter for connecting an inert gas supply.

- the heating pipe is, moreover, formed so that the inert gas current supplied by way of the adapter can exit exclusively by way of the surface area of the heating pipe.

The two features together result in the (tubular) inner chamber of the heating pipe being able to be filled with inert gas and the inert gas there being able to be placed under a fixed excess pressure, wherein the inert gas current can exit exclusively by way of the surface area of the heating pipe and, for example, not at the opposite second end.

In order to ensure this exclusive inert gas current by way of the surface area of the heating pipe the invention provides various embodiments.

In the simplest case the heating pipe on the free second end opposite the first end on the side of the gas connection is formed in a closed manner so that the inert gas current cannot exit here. Because of the inert gas current constantly supplied under pressure at the first end a return flow of the gas is prevented at the same time.

According to an alternative embodiment the heating pipe is formed on the second end opposing the first end on the gas connection side with a further adapter for connecting an inert gas supply. In this connection the inert gas flows therefore from two sides into the heating pipe, wherein the inert gas currents meet in the centre and can also only exit in this respect by way of the porous heating pipe jacket.

Even with this embodiment it is possible to close the free pipe crosssection at a point between the gas connection-side ends, so that the two partial gas currents, directed in opposing directions, do not directly touch.

Because of the electrical resistance properties the heating pipe consists preferably of silicon carbide, reaction-siliconized silicon carbide or silicon-infiltrated silicon carbide (SiSiC). The heating pipe can, however, also consist of other ceramic, porous materials, which fulfil the function of a resistance heating.

Also it is possible to form the heating pipe with a glazed surface. In this case the surface of the heating pipe is additionally protected (made denser), although a complete density cannot be achieved here either and is not necessary. Because of the partial "blocking action" of the glazed surface in this embodiment, however, smaller inert gas currents exit.

The electrically heated industrial furnace according to the invention is equipped with several heating pipes of the described type and comprises at least one inert gas station for supplying inert gas by way of the adapters, connected to the heating pipes on the end side, into the interior of the heating pipes.

The arrangement of the heating pipes can remain unchanged relative to the state of the art. Usually the heating pipes are arranged below the cover and transverse through the furnace channel (furnace chamber), wherein they project on the end side from the furnace walls, so that the corresponding adaptors or the gas connections can be mounted outside the furnace and therefore outside the temperature- charged zone.

The inert gas station can consist of a continual gas source; also however of a gas bottle. In order to operate the furnace the inert gas current is introduced into the heating pipes, so that the inert gas in the interior of the heating pipes is under excess pressure relative to the furnace atmosphere.

In principle each inert gas (protective gas) can be used, which does not influence in a negative manner the furnace atmosphere and therefore neither the quality of the material to be burnt. Nitrogen (N2) is a low-priced inert gas for this purpose. Argon can also be used for example.

If required the heating pipe can also be used for targeted supply of a gas into the furnace. The gas which flows through the heating pipe from inside outwards can also be an inert gas or any other gas. The heating pipe in this case serves to regulate a protective gas atmosphere in the furnace or maintain this or even as a "ventilator", in order to preheat, -6 for example, the material to be burnt in the preheating zone of the furnace and/or to drive off volatile components from the material to be burnt (for example: water, binding agent). The heating pipe can have additional holes in the coating surface area for this application so that more gas flows through.

The invention is illustrated in more detail in the following with the aid of an exemplary embodiment, wherein in each case in a very diagrammatic representation.

Figure 1 shows a section through a heating pipe according to the invention in a first embodiment, Figure 2 shows a section through a heating pipe according to the invention in a second embodiment, Figure 3 shows a section through an associated industrial furnace, In the Figures the identical components or components with identical action are shown with the same reference numerals.

The heating pipe represented in Figure 1 is denoted as a whole with the reference number 10. The heating pipe 10, the jacket of which has the reference number 12, consists of silicon carbide with a porousness of 15.

On one end 10a (left in the Figure) the heating pipe 10 is formed with an adapter 14, which has a smaller cross-section than the heating pipe 10 and is connected to this end in a gas-tight manner.

On the opposing (second) end 10b the heating pipe 10 is provided with a closed cover 16 which is also gas-tight.

The exemplary embodiment according to Figure 2 is distinguished from that in Figure 1 in that both ends of the heating pipe 10 are formed with the described adapters 14 and a closed wall 18 approximately in the centre of the heating pipe divides 2 the heating pipe 10 into two parts.

Figure 3 shows schematically an industrial furnace (here a bogie hearth furnace), which is formed below its cover 20 with several heating pipes 10 arranged in series, perpendicular to the drawing plane, according to Figure 1, wherein the heating pipes 10 have a greater width than the furnace, so that they penetrate the furnace walls 22a, b. In this way the closed end 10b of each heating rod 10 projects over the furnace wall 22a which is left in the Figure, whilst the first furnace end formed with the connection piece (adapter) 14 projects over the right wall 22b.

In operation an inert gas line, in this case a nitrogen line, is now connected to the adapters 14 of the diverse heating pipes, so that the nitrogen flows into the interior 10c of the heating pipes 10, wherein in the heating pipe 10 in each case an excess pressure relative to the furnace atmosphere prevailing in the furnace chamber 20a is formed, which is purely oxidising here.

The inert gas attempts to penetrate the heating pipe jacket 12 and this occurs finally by way of the open pore volume of the jacket 12 in the direction of the arrows P.

It is obvious that the inert gas current exits by way of the whole surface of the jacket 12. In this way it is ensured that in the reverse direction the furnace atmosphere cannot penetrate the jacket 12 and destroy the heating pipes 10.

The inert gas supply (arrow I) is not represented here in detail, it consists of a nitrogen station with a corresponding pressure device for conveying the gas into the heating pipes 10, which are, moreover, applied to a voltage source naturally in a known manner.

For reasons of improving clarity in Figure 3 the material for burning is not represented. Here is soft ferrites.

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Claims (12)

Claims

1. A ceramic porous heating pipe for electric heating of an industrial furnace, which is formed at least at one first end with an adapter for connecting a gas supply and moreover so that the gas current supplied by way of the adapter can exit exclusively by way of the surface area of the heating pipe.

2. A heating pipe according to claim 1, the second free end of which opposing the first end on the side of the gas connection is formed in a closed manner.

3. A heating pipe according to claim 1, the second end of which opposing the first end on the side of the gas connection is also formed with an adapter for connecting a gas supply.

4. A heating pipe according to claim 3, in which the free pipe crosssection is formed in a closed manner at a point between the end-side gas connection side ends with a dividing wall.

5. A heating pipe according to one of claims 1 to 4, consisting of a silicon carbide, re crystallised silicon carbide or reaction-siliconized silicon carbide.

6. A heating pipe according to one of claims 1 to 5, with a glazed surface.

7. A heating pipe according to one of claims 1 to 6, the surface area of which has bores for the passage of gas.

8. An electrically heated industrial furnace with several heating pipes according to one of claims 1 to 7, as well as at least one gas station for supplying gas, by way of the adapters connected to the heating pipes on the end side, into the interior of the heating pipes.

9. A method for electric heating of an industrial furnace according to claim 8, in which a gas current is introduced into the heating pipes so that the gas in the interior of the heating pipes is under excess pressure relative to the furnace atmosphere. 5

10. A method according to claim 9 in which an inert gas in the form of nitrogen or argon is used as gas.

11. A ceramic porous heating pipe, for electric heating of an industrial furnace, substantially as hereinbefore described with reference to Figure 1, or Figure 2, or Figure 3 of the accompanying drawing.

12. A method for electric heating of an industrial furnace, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawing.