Technical Field

[0001] The present invention relates to production of spiral heat exchangers for hot water tanks in a tube profile mill that is adapted to produce seam welded stainless tube lengths with various lengths, i.e., between 3 and 60 meters. More specifically the invention relates to a method for producing stainless seam welded tubes of various lengths for applications in spiral heat exchangers.

Background Art

[0002] Current tube rolling mill for seam welded stainless tubing are arranged to produce tubes with standard lengths at e.g., up to 12.5 meters.

Very few rolling mills produce tubing lengths with length beyond that. The tube lengths are usually transported in containers with the lengths of 40 feet, in other words 12.19 meters. Already at the transportation of 12.50 meters there will be problems to fit the tube lengths in a 40-foot container, and tube lengths beyond that will not be possible to transport in similar fashion.

[0003] Another problem is that since the tube lengths are already provided in standard lengths, it would most likely occur that excessive lengths from the lengths that are required, will have to be cut off. E.g., if the client requires a tube length of 9 meters, over 3 three meters of the original length will be disposed of. If the client requires longer lengths, one will often have to join lengths by welding. In addition to that it is time consuming to make a circular weld, the weld forms a weakened area in a tube and the weld is more exposed to i.e., corrosion, and such welded tubing must therefore be monitored closely. However, this is not possible when the tube is located in a hot water tank.

[0004] Stainless steel that are used for the production of heating spirals should not be bent and later be straightened out. Internal stress will occur in the material which may lead to stress corrosion. It is therefore not appropriate to bend the tubes for transport. [0005] Ideally one should produce the tubes and coil them to heating spirals in one continuous process. This is currently not possible, since the seam welded tubes must undergo a heat treatment with a following controlled cooling to remove the stresses after welding. If the tubes are subjected to bending before the stresses are removed, the weld will be weakened.

[0006] Specific prior art will hereby be mentioned:

[0007] CN1 11922115, which describes a production facility for extruding aluminum tubes. It comprise an extruding machine, a pulling machine and rails for handling of the finished tubes. The publication does not specify the lengths of the tubes, but the figures show tubes where all are of the same lengths. Such extruded aluminum tubes are not suitable for use in spiral heat exchangers.

[0008] US2015/0307328, which describes a lifting yoke used together with a crane. The yoke is intended to lift extended objects, the Yoke is equipped with sensors and can be adjusted to keep the object in balance. The objects are lifted only at two points, and the yoke is therefore only suitable to lift solid and rigid objects.

[0009] CN1 12125122, which describes a lifting yoke for lifting parts of a wind generator. The yoke is designed to counteract deformation during the lifting. However, the yoke only lifts the object in to points, and hence the objects to be lifted must have a certain stiffness.

[0010] CN107377666, which describes a method for producing a coiled seamless stainless steel tube with a length of 50 meters. Since the tube is not seam welded it is possible to coil it straight after profile rolling and pulling.

[0011 ] CN101797602, which describes the production of a coiled stainless seam welded steel tube with a length of between 15 and 133 meters. Here the coiling of the tube is done immediately after the profile milling and seam welding. This will introduce additional stress in the tube. When the tube is later used for production of products it has to be uncoiled and coiled again. This will introduce even more stress in the tube. The tube will then not be suitable for use as a heating spiral in a hot water tank. The only way to reduce stress is to provide heat treatment of the tube. The tube could still have acquired microscopic cracks at in the weld and will therefore be permanently structurally weakened.

Summary of invention

[0012] The object of the present invention is to solve the challenges of the production excessive tube cuttings when tubings are adapted to a preferred length, and remove or significantly reduce the need of joining tubing, also when there is need for long tubing lengths of e.g. up to 60 meters.

[0013] The object of the present invention is to also provide an appropriate handling of long tubing from the metalworks to the bending machine and the latter transportation.

[0014] Said objects are achieved in the following description and claims.

Brief description of drawings

[0015] Figures 1 a - d shows a tube rolling mill with accompanying rolling path, wherein

[0016] Figure 1 a shows the tube rolling mill and a part of the rolling path in view,

[0017] Figure 1 b shows the same as in figure 1 a but in planar view,

[0018] Figure 1 c shows the rest of the rolling path in view and figure 1 d shows the same as in figure 1 c but in planar view,

[0019] Figure 2 shows several lifting yokes about to lift a bundle of tubing.

Detailed description of the invention

[0020] Figures 1 a and 1 b shows a standard tube rolling mill for production of tubes. Such a standard tube rolling mill is capable of producing tubes with a length of up to 12 meters.

[0021] From left to right are the following components in the tube rolling mill shown: a roll of metal sheets, so called strips 1 , rolling mill stations 2, and 3 which in sequences shapes the sheet to a tube, a laser welding station 5, that welds the tube together along a longitudinal weld, a straightening station that straightens the tube after the welding, a heating station that heats the tube at a temperature at i.e. 860 °C, so that the material becomes homogenous, straightening stations 8, 9 that straightens the tube after heating, and sequentially a cutting station 10 that cuts the tube at a predetermined length. After the cutting station the tube is transported out to a rolling path 11 . When the tube is transported on the rolling path it will be transported in a sideway manner, in such a manner that more tube lengths can fit next to each other.

[0022] In a specific embodiment the invention comprise a tube rolling mill as described above, which is a conventional and well known way of producing seam welded tube lengths of stainless steel from tubing. The tubes are intended for use in heating spirals in a hot water tank, similar to a boiler for heating up water for domestic use or water for heating up buildings.

[0023] The tube lengths are transported from the tube rolling will to a rolling path 11 , that is extended with an additional rolling path 11 b, as shown in figures 1 c and 1 d. This combined rolling path 11 , 11 b is capable of to receive a continuous tube length up to 60 meters. The cutting station 10, that is arranged in connection to the rolling path 11 , 11 b, can cut the tube lengths at selectable lengths between 3 and 60 meters.

[0024] After the cutting the cut tubes 2 are transported in a sideways manner to a tubing collector 12b.

[0025] From the tubing collector 12b the tubes 2 are transported by one or more overhead cranes 21 , 22, 23 to a tubing storage. Each overhead crane 21 , 22, 23 is moveable on a rail (not shown) which extends transversely in relation to tubing collector’s 12b longitudinal direction. The rails are moveable in the tube’s longitudinal direction. From each overhead crane 21 , 22, 23 is a lifting yoke 24, 25, 26. The lifting yoke is attached to a hook that is connected to a winch (not shown) on the overhead crane 21 , 22, 23. The hook can preferably be moved above a height of 7 meters. The hook is attached in the middle of the lifting yoke 24, 25, 26, in such a manner that the lifting yoke is in balance and is kept horizontally. An array of hooks is attached beneath the lifting yoke to fasten straps which can be led around one or several tubes 27. The distance between the hooks can be e.g., 5 meters.

[0026] The overhead cranes 21 , 22, 23 can be controlled individually or jointly with a joint control in order to move in a synchronous manner. [0027] When the tubing collector 12b have received a number of tubes that is suitable for lifting by the overhead cranes 21 , 22, 23, the overhead cranes are positioned above the tubes 27. Here one can use one, two or several overhead cranes depending on the length of the tubes 27. By moving the rails in the tube’s 27 longitudinal direction the positioning of the yokes can be adapted in such a way that they are centered above the tubes 27. In this way the load is balanced when it is lifted. The yoke or yokes 24, 25, 26 are lowered towards the tubes 27, and the straps are led around the tubes 27. Afterwards the tubes are lifted hanging in the yokes 24, 25, 26. With a distance of 4 meters in between the straps, the bundle of tubes will not bend in any significant manner during the lift. The tubes can have a wall thickness as little 0.8 millimeters. It is therefore essential to a certain extent to avoid bending of the tubes during lifting.

[0028] The bundle of tubes 27 are transported by moving the rails in the tube’s 27 longitudinal direction and the winch transversely positioned in the tube’s longitudinal direction to a tubing storage (not shown). In the tubing storage the tubes 27 are sorted by length, so that tubes of same length are placed together.

[0029] From the tubing storage tubes are retrieved for the production of tubing spirals for spiral heat exchangers. The tube first receives a heat treatment to remove or reduce stress that occurred during the milling and the seam welding.

[0030] The spiral heat exchangers are produced mounting a connection at each end of the tube. These are welded to tube’s straight end. The welding of the end connection happens after coiling and subsequent cutting are completed. A completed spiral can e.g., have a length of 28 meters. At the ends, wherein the couplings are attached, the tube has straight section. After the coiling the tube undergoes a controlled cooling.

[0031] Such spiral can have different dimensions and tube lengths intended for producing a spiral heat exchanger can therefore vary depending on the type of spiral. When one knows which spiral heat exchanger that shall be produced, one can select a tube of a suitable length and according to the number of heat exchangers of the similar type to be produced. If, in a specific use case shall produce only one spiral heat exchanger, one intentionally selects the tube in the tubing storage that has a length that is at least as large as one need to produce the spiral. If one shall produce several spirals, one can select a longer tube that has a length that is at least similar to the sum of the length of several of such spirals, but with the least amount of excessive length. The tube length are cut prior the installation of the end couplings. The cutting happed preferably in the coiling machine straight after the tube has been coiled to a spiral. [0032] In this way one can produce spiral heat exchangers of various lengths without being forced to weld tubes together in order to achieve sufficient length. This can also be done with a minimum of cuts. When the tubes are coiled to spirals, they can be transported to a point of destination. The spirals have a length that can without a problem fit in a standard container.