The present invention relates to a furnace insulation in¬ tended for a furnace that is heated with the aid of electri- cal resistance elements.

In the cases of furnaces heated to very high temperatures, high requirements are placed on the material used to insulate the volume heated in the furnace. Because the insulating ma- terial surrounds the volume heated in the furnace, the insu¬ lating material will also become very hot.

The temperature of a given type of electrically heated fur¬ nace will, under typical conditions, reach 1700°C for a pe- riod of several hours. The insulating material used may, for instance, be comprised of insulating fibre or high grade brick.

One known problem existing in the technology applied in re- spect of such furnaces is that there are not many known mate¬ rials that are able to withstand these high temperature stresses over a period of time sufficiently long for them to be used effectively. The known materials normally shrink at high temperatures, resulting in sealing problems with regard ■ to those furnaces in which these known materials are used.

A suitable material is one which consists essentially of alu¬ minium oxide (AI2O3) and silicon dioxide (Siθ2) . Although this material is effectively resistant to heat, it shrinks at high temperatures. This shrinkage increases with high tempera¬ tures. Shrinkage is due to the fact that the material sinters as it is heated. This results in the material successively shrinking over a number of hours in operation, such over 1 - 10 hours. In Applicant' s so-called superthal modules there are used fi¬ bre modules designed as two semi-cylindrical elements which are placed againsttogether each other to form a cylinder that constitutes the furnace space. Each semi-cylindrical element will often comprise two layers that lie radially outwards of each other, an inner layer and an outer layer.

The inner layer consists, for instance, of vacuum-formed fi¬ bres for a maximum use temperature of 17000C and has a den- sity of 400 kg/m3. This inner layer may consist of 80% Al2O3 and 20% SiO2. The outer layer consists, for instance, of fi¬ bres for a maximum working temperature of 1600°C and has a density of 300 kg/m3. The outer layer may consist of 50% Al2O3 and 50% SiO2.

In the case of modules of the SMU-type (Superthal muffle unit) the inner layer will most often have a thickness of 25 mm and the outer layer a thickness of 75 mm. The inner layer includes on its inner surface grooves for accommodating heli- cal electric resistance elements. In the case of an internal diameter of the inner layer of 150 and 200 mm, the position of the resistance element in the groove is secured with the aid of fasteners.

In the case of modules of the SHC-type (Superthal half cylin- • der) the inner layer will most often have a thickness of 75 mm and the outer layer a thickness of 25 mm. The resis¬ tance element is mounted on the insulation with the aid of fasteners .

When furnaces that include the aforesaid type of insulation are heated, the insulation will thus shrink at high tempera¬ tures. Cracks in the insulation can be observed when the fur- nace is switched-off. In the worst case, whole pieces of the insulation are liable to loosen.

The problem is more of an aesthetic nature in the case of furnace spaces having inner diameters in the orders of magni¬ tude of 100 - 125 mm. The problem increases with larger diameters, resulting in wide cracks and deformation of the inner insulation, and also with the risk that pieces of the insulation will loosen.

The insulation thus becomes less effective due to the forma¬ tion of cracks.

One serious problem resulting from the shrinkage of the insu- lation is that the resistance element will tend to be pulled away owing to the fact that it is fastened to the inner sur¬ face of the insulation, as before mentioned. As the insula¬ tion shrinks cracks form which, in turn, displace different parts of the insulation relative to one another. Because the resistance element is fastened to the insulation punctiline- arly, the fastening points will be displaced relative to one another, therewith subjecting the resistance element to ten¬ sion stresses and bending stresses of a magnitude such as to cause the resistance element to be pulled away.

These problems are solved by means of the present invention.

The present invention thus relates to a furnace insulation comprising fibre modules that are designed as at least two cylindrical segments which are placed against one another so as to form a cylinder whose inner volume constitutes the fur¬ nace space, wherein the modules are adapted to enable an electric resistance element to lie against and be fastened to the inner surface of the cylinder, and wherein the invention is characterized in that one or more radially extending or generally radially extending openings is/are disposed on the inner part of the cylinder.

The invention will now be described in more detail, partly with reference to an exemplifying embodiment of the invention illustrated in the accompanying drawings, in which - Figure 1 is a perspective illustration of an inventive fur¬ nace and - Figures 2 - 5 illustrate different embodiments of a fibre module according to the invention.

The figures illustrate SMU-type furnaces, although they are also applicable to SHC-type furnaces.

Figure 1 illustrates a furnace 1 that includes a furnace in¬ sulation which comprises fibre modules 2, 3. The fibre mod¬ ules 2, 3 are formed as semi-cylindrical elements, where one semi-cylindrical element is shown in Figure 2. At least two semi-cylindrical elements 2, 3 are placed against each other so as to form a cylinder whose internal volume 4 constitutes the furnace space.

Only one internal fibre module is shown in Figures 2 -5, this fibre module being intended to be placed against a further corresponding fibre module so as to form a cylinder, as il¬ lustrated in Figure 1. Fibre modules in the form of further semi-cylindrical elements are placed on the cylinder, so as to obtain a furnace that includes two mutually concentrical layers.

The insulation is comprised generally of aluminium oxide and silicon dioxide. The furnace includes an electric resistance element 5 which lies against and/or is fastened in the inner surface 11 of the cylinder. A power connection element 6 is also provided for delivering electric power to the resistance element.

Figure 1 shows an embodiment in which two outer semi- cylindrical elements 2, 3 surround two mutually facing inner semi-cylindrical elements I1 8. The resistance element may have a helical configuration or some other configuration, and is fastened in the inner surface of the cylinder by means of fasteners 9. The resistance element preferably extends in grooves 10 formed in the inside 11 of the cylinder, as shown in Figure 2.

According to the invention, one or more radially directed openings 12 is/are provided in the inner part of the cylinder 7, 8, as shown in Figure 2.

According to one preferred embodiment of the invention, the radial opening or openings may consist of a notch-like crack indicator or notch-like crack indicators 14, see Figure 5.

According to an alternative and preferred embodiment, the ra¬ dial opening or openings consist of radially directed grooves or slots 13, as shown in Figure 4 for instance. These grooves 13 extend down slightly into the semi-cylindrical fibre mod¬ ules 2, 3, 7, 8.

The opening or openings may have other configurations, such as conical or round configurations.

In the case of the Figure 2 embodiment, the openings 12 ex¬ tend through roughly half of the inner semi-cylindrical fibre module 8. In a preferred embodiment, the radial openings extend through roughly half .the thickness of the inner layer of said mutu¬ ally concentric layers.

The radial openings function as an expansion joint that con¬ tributes towards preventing the actual formation of cracks or in at least reducing crack formation. In the event of cracks forming, these cracks will form in a controlled manner due to the presence of the radial opening or openings.

In the case of SHC-furnaces, these furnaces are equipped with meandering elements, wherewith the radial openings are dis¬ posed at those positions where the meandering element bends or curves.

It is preferred that the radial opening or openings ex¬ tends/extend axially along the cylinder, ■ as shown in Figure 2 among other figures.

According to another preferred embodiment the furnace insula¬ tion of fibre modules comprises three or more cylinder seg¬ ments 15, 16 which are placed against one another so as to form a cylinder; see Figure 4.

According to a highly preferred embodiment of the invention, the insulation includes two mutually concentrical layers 1, 2; 7, 8 of fibre modules.

It is highly essential that the openings are placed so as to be generally uniformly distributed circumferentially within each cylinder half or cylinder segment. The openings 12 or the notch-like crack indicators 14 may, however, define an angle Vl, V2 or V3 with the inner surface of the cylinder; see Figure 3.

Moreover, the openings 12 or the crack indicating notches 14 may define axially an angle V4 with the longitudinal axis of the cylinder, as shown by the chain line 17 in Figure 4.

Although the invention has been described above with refer- ence to a number of exemplifying embodiments, it will be un¬ derstood that the shape and dimensions of the furnace space can be varied and that the furnace insulation may consist of one layer or several mutually concentrical layers.

The present invention shall therefore not be considered lim¬ ited to the aforedescribed embodiments, since variations can be made within the scope of the accompanying claims.