The invention relates to a discharge pipe, consisting of a length of pipe for discharging hot substances, which length of pipe consists of an outer pipe, and an inner pipe placed coaxially inside the outer pipe, the inner pipe and the outer pipe enclosing a space, which space is filled up with insulating means.

Pipes serving to discharge hot substances are generally known. In order to ensure that the outside of such a di¬ scharge pipe does not become extremely hot, such pipes are provided with a layer of thermally insulating material, preferably ceramic wool, surrounding the pipe. However, the thickness of the heat-insulating material to be used de¬ pends on the temperature which the substances to be di¬ scharged can reach and the maximum permissible outer wall temperature of the discharge pipe. It has been found, however, that such discharge pipes must be surrounded by a very thick layer of insulating wool, in order to meet the desired standards. This makes such pipes very expensive, and they have an undesirably large external diameter.

If such discharge pipes are used for chimneys, the dischar- ge pipes must meet all kinds of regulations. For example, in some areas there are standards in which the temperature at the outside of a chimney flue must not rise more than 75°C if hot combustion gases at a temperature of about 1000°C are discharged for a certain short period through the discharge pipe or chimney, or if hot combustion gases at a temperature of about 600°C are discharged for a period of three hours. If insulating materials such as ceramic wool are used, the external dimensions of the discharge pipe or chimney then become too great. However, if insula- ting materials such as insulating concrete, which have much better heat-insulating properties, are used, with the result that the external dimensions of a discharge pipe can

be reduced to within desired dimensions, yet other problems then emerge. The inside of the discharge pipe must be gastight and must therefore be, for example, a smooth, thin metal pipe. However, such a metal inner pipe has a diffe- rent coefficient of expansion from the layer of insulating concrete, with the result that the inner pipe cannot expand and can consequently buckle, bend and even crack.

The object of the invention is a discharge pipe for hot substances such as combustion gases, in which the outer wall of the discharge pipe will not rise above a desired temperature if very hot substances are conveyed through the discharge pipe, and in which the external dimensions of the pipe are much smaller than has been found until now.

This object of the invention is achieved with a discharge pipe through the fact that the insulating means consist of a first layer of compressible material surrounding the outside of the inner pipe, and a second layer of material with very good thermal insulating properties which, on the one hand, surrounds the first layer and, on the other hand, is surrounded on the outside by the outer pipe. These measures ensure that the external diameter of the discharge pipe is as small as possible and is much smaller than was possible until now, and that the inner surface is formed by an inner pipe or wall which can expand freely in the radial direction. It is of no importance here whether the inner and outer pipes are circular. Of course, it is possible to use square, elliptical or cylindrical shapes, or a combina- tion of these shapes. This makes it possible to use, for example, a metal inner wall, with the result that the substances to be conveyed cannot penetrate through the wall of the discharge pipe.

The second layer of the insulating means is preferably of concrete with very good heat-insulating properties. It has been found that very good types of concrete, with very good heat-insulating properties, can currently be manufactured. This means that the insulating layer can be kept very thin.

In a preferred embodiment both the inner pipe and the outer pipe are cylindrical, thereby enclosing a cylindrical space. This is the most economic design of such a discharge pipe.

It has been found that it is preferable to use as the first or inner layer insulating means which have high thermal insulating properties, such as ceramic wool, as a result of which the heat insulation of the pipe can be improved and the external dimensions can consequently be smaller than if a compressible material with low heat-insulating proper¬ ties, but a lower purchase price is used.

In a preferred embodiment the discharge pipe is made up of several lengths of pipe placed on each other, the successi¬ ve lengths of pipe being fixed to each other by means of fixing means, and a layer of compressible material with good thermal insulating properties, such as ceramic wool, being placed between the contact faces of two successive lengths of pipe. This makes it possible to transport the pipe in parts and then assemble it on site, and it has been found that the coupling faces of two successive lengths of pipe can be fixed to each other so well that, through the use of a layer of ceramic wool between the contact faces, very little or no heat leakage occurs. Heat leakage can be prevented even better if the insulating layer consists of a compound which is sprayed between the contact faces after the making of the connection between the lengths of pipe.

The invention will be explained in greater detail with reference to the drawing, in which:

Figure 1: shows a length of pipe according to the in¬ vention in longitudinal section along the line I-I of Figure 2; Figure 2: shows a top view of a length of pipe accor¬ ding to Figure 1; Figure 3: shows a connection between two lengths of pipe according to Figures 1 and 2.

Figure 1 shows a longitudinal section of a length of pipe 1. The length of pipe 1 is made up of an outer pipe 2 and an inner pipe 3 placed therein. An annular space 4 is situated between the outer pipe 2 and the inner pipe 3. An insulating layer 5 is placed in the annular space. The insulating layer 5 consists of an inner layer of compressi¬ ble insulating material 6, such as ceramic wool or a cera¬ mic blanket, and an outer layer of rigid insulating materi¬ al 7, such as insulating concrete. The layer of compressi- ble insulating material 6 is placed around the outside of the inner pipe 3. The layer 7 of rigid insulating material is placed on the inside of the outer pipe 2. The inner pipe 3 encloses a flue 8, through which hot gases can flow. At one of the two ends the insulating layer 5 runs through to the end 9 of the length of pipe 1, and at the other side the internal space between the two pipes, the inner pipe 2 and the outer pipe 3, is not completely filled with insula¬ ting material, so that an empty annular space 10 is left. At the other end 11 of the length of pipe 1 the ends of the inner pipe and of the outer pipe slant slightly towards each other, so that the wall of the end 11 is slightly narrower than the wall of the remainder of the length of pipe 1. Figure 2 shows a top view of the same length of pipe 1.

Figure 3 shows how two lengths of pipe 1 are coupled to each other. An annular layer of compressible insulating material 12 is first placed in the annular space 10 of the end 9 of a first length of pipe, before the two pipes are placed on each other and fixed to each other. The end 11 with the narrowed wall of a second length of pipe is then pushed into the empty annular space 10. The end 11 of the top pipe presses on the annular insulating layer 12 of ceramic wool, with the result that the latter is compres- sed. The compressible and thermally insulating properties of the annular insulating layer 12 mean that no heat leaka¬ ge will occur between the two lengths of pipe. Clamping means 13, in the form of a generally known clamping strip connection, are placed around the join. The annular insula-

ting layer 12 can be, for example, insulating wool which is cut into an annular shape beforehand. However, the layer 12 can also be an injectable insulating and heatproof com¬ pound. Such an injectable compound seals better and pre¬ vents heat leakages more effectively.