The present invention relates to a method in manufacturing a heat insulated tube comprising a tube for conducting fluid and a sheath enclosing said tube, a heat insulating material being disposed between the fluid tube and the sheath, wherein the sheath is being extruded in an angle extruder die while the fluid tube and the heat insulating material enclosing said tube are being fed axially through the angle extruder die as the sheath is being extruded around the heat insulating material.

The invention has been developed particularly for manufacturing heat culvert tubes which are used as secondary culvert conduits from a sub-station in a residential area to individual consumers in a remote heating system, the fluid tube carrying water at a temperature of 70 to 80 C, and for manuf cturing heat insulated tapwater tubes. At present there are used in this connection mostly fluid tubes comprising copper or steel tubes, which means that the heat culvert tube or the tapwater tube is stiff and therefore is cumbersome to handle, the laying of heat culvert tubes moreover being expensive because it is necessary to provide expansion means in the heat culvert conduit made up of such tubes.

It is known to apply the heat insulating material on the fluid tube in different ways. This material can be foamed in situ around the fluid tube, or heat insulating material comprising a tape can be wound or wrapped around the fluid tube.

In a prior art embodiment of the method mentioned above in manufacturing a heat insulated tube a smooth sheath is extruded around the heat

insulating material the sheath which is sticky when leaving the angle extruder die, adhering to the surface of the heat insulating material. Such a smooth sheath has a low circumferential stiffness, and it had been better if the sheath could be made corrugated in the same manner as applied to drain and sewage tubes, but since the sheath adheres to the heat insulating material filling the interior of the sheath it is not possible to corrugate the sheath by shaping against chains of chill molds under the influence of positive or negative pressure. Therefore, it is customary to use a heat insulating material having high density in order to reduce the risk of flattening of the heat insulated tube. The invention solves the related problem by the method having obtained the characteristics appearing from claim 1.

The corrugated sheath provides a greater circumferential stiffness than a smooth sheath. Owing to the increased circumferential stiffness a heat insulating material having a lower density can be used without the risk of flattening, which means a lower price and an improved heat insulation. Moreover, the corrugated sheath and the softer heat insulation imparts to the tube increased flexibility which is particularly advantageous if the fluid tube is flexible and consists for example of plastic material because in that case the tube can be wound up so as to be stored and distributed as a coil. Accordingly, the handling and laying of the tubes will be facilitated and cheapened.

The invention also relates to a device in extruders having an angle extruder die, for working the method according to claim 1, having the characteristics appearing from claim 4.

In order to explain the invention in more detail it will be described below with reference to the accompanying drawing which discloses diagramatically the method and device of the invention. In the drawing an angle extruder die 10 is shown which is connected to a conventional extruder 11. From the angle extruder die a sheath 12 of suitable plastic material, for example polyetylene or PVC is extruded. Then, a helical or annular corrugation is imparted to said sheath by means of two chains of chill molds 13 in a manner which is known per se by applying a negative pressure in the molds. From a coil 14 a tube 15 of heat insulating material is led through the angle extruder die 10. This tube can consist of foamed polyuretane. It forms an axial slot 16 in order to be passed onto a fluid tube 17 at the introduction. The fluid tube can consist of crosslinked polyethylene (PEX) or polybutene and is supplied from a coil 18. Before the tube 15 passes into the angle extruder die the surfaces of the slot 16 are coated or sprayed with an adhesive by means of an adhesive applicator 19. The tube 15 can be coated on the outside surface thereof with some release agent such as a silicon composition. The tube can be produced by extrusion, in a separate operation of course, and then be slotted.

As the sheath 12 is being extruded the fluid tube 17 thus is introduced therein enclosed by the heat insulating tube 15 but the sheath should not be allowed to contact the tube 15 immediately after the emergence from the angle extruder head 10. It should be kept spaced from the tube 15 up to the chains 13 of chill molds in order not to stick to and/or tightly enclose the tube. For this purpose a metal sheet sleeve 20 is provided inside the angle extruder

head 10 to define a passage for the tube 15 of heat insulating material, which is thus fed into and through the sleeve 20 together with the fluid tube 17 enclosed by the heat insulating material. The sleeve 20 is burnished on the outside surface thereof and is coated with polytetrafluoroetylene on the inside surface thereof. Between the outside surface of the sleeve and the angle extruder die a gap 21 is provided of the order of 20 to 25 mm such that the sleeve will not be heated by the hot angle extruder die. The gap may be filled with a heat insulating material. The downstream end of the sleeve, i.e. the lefthand end as seen in the Figure, forms a conical end portion 22 so that the tube 15 passing through the sleeve will be somewhat compressed just as it leaves the sleeve. This end should be placed close to the chains of chill molds 13, and in order to make possible that the position of the sleeve is adjusted set screws 23 are provided between the angle extruder die and a flange 24 at the other end of the sleeve 20 for adjustment of the sleeve axially. The sheath 12 emerging from the angular extruder head thus passes on the outside surface of the sleeve right up to the chains of chill molds and only there the sheath is brought into contact with the tube 15 of heat insulating material so as to make possible that the sheath is corrugated by being pressed against the chains of chill molds under the influence of negative pressure said molds being shaped so as to provide the desired corrugation either a helical corrugation or an annular corrugation.

In order to glue together the tube 15 around the fluid tube 17 press means such as press rollers or sliding jaws can be provided between the adhesive applicator 19 and the extruder die 10 to press

together the surfaces coated with adhesive before the tube 15 passes into the extruder die. It should be mentioned that the slot 16 can be closed in a way different from glueing for example by heat welding the surfaces of the slot to each other.

Owing to the fact that the sheath 12 is corrugated bending of the finished heat insulated tube will be facilitated without the heat insulating material, i.e. the tube 15 being compressed due to the fact that the sheath 12 is folded at bending of the heat insulated tube as in case of a smooth sheath tube. Since the corrugated sheath is stiffer than a smooth sheath and the risk of the heat insulated tube being flattened therefore is considerably less than in case of a smooth sheath, a heat insulating material of lower density can be used; the density can be as low as 30 kg/m as compared with 80 kg/m which is common for tubes having a smooth sheath. At this density about 1/3 of material will be required the heat losses at the same time being reduced. By applying the invention a cheaper tube thus will be obtained which causes lower heat losses.

The chains of chill molds also function as a feeder unit which contributes to the discharge of the finished heat insulated tube from the extruder die.