JPH0335924Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a radiant tube used for indirectly heating steel materials such as steel strips and steel pipes in a non-oxidizing atmosphere or a reducing atmosphere.

(Prior art) Radiant tubes generally raise the temperature of the tube itself to 800 to 900 during the process in which exhaust gas combusted in a burner passes through the tube and is discharged from the outlet.
℃ and further heats the object by radiation.

The lifespan of a radiant tube depends on the conditions of use, but in radiant tube heating and soaking furnaces that heat and soak steel materials, the lifespan of a radiant tube can last up to 2 hours until it is seriously damaged due to cracking or deformation and must be replaced.
~4 years. In addition, radiant tubes are generally used in large numbers, and replacement of the tube body requires a long furnace shutdown, so it is desirable to extend the life of the radiant tube body in order to improve the operating rate of the furnace. Ta.

Conventionally, the cause of radiant tube failure has been thought to be that the bending stress due to its own weight exceeds the creep strength of the material. Based on this judgment, efforts have been made to improve creep strength by upgrading materials, reduce stress values by increasing wall thickness, and change support shapes (for example, Utility Model Application No. 54-2712). However, even if these modifications were able to achieve a slight improvement in service life, the reality was that they did not lead to a major improvement.

This suggests that the cause of the radiant tube's failure is dominated by thermal stress rather than bending stress due to its own weight. This point will be explained in detail.

First, FIG. 1 shows an example of a conventional radiant tube. The radiant tube consists of four parts: a straight pipe part, a bent part, a mounting flange part, and a support part. In Fig. 1, 1 is the first straight pipe part, 2 is the second straight pipe part, 3 is the third straight pipe part, and 4 is the fourth straight pipe part.
A straight pipe part, 5 a first bend part, 6 a second bend part,
7 is the third bend part, 8 is the burner side flange, 9 is the exhaust gas outlet side flange, 10 is the intermediate support, 1
1 is a tip support, 12 is a combustion burner, 13 is an iron skin, and 14 is a brick.

Figure 2 shows an example of actual measurements of the exhaust gas flow direction and surface temperature of a radiant tube in an actual furnace. As shown in this figure, as the combustion exhaust gas passes through the radiant tube, heat is absorbed by the object to be heated, and the gas temperature gradually decreases. Comparing the first straight pipe section 1 and the fourth straight pipe section 4, there is a temperature difference of 100 to 200°C.

Next, in most of the W-shaped and U-shaped radiant tubes used in special atmospheres, the burner side flange 8 and the exhaust gas outlet side flange 9 are attached to the steel shell 14 with bolts or Joined by welding. If a temperature difference occurs between the burner side straight pipe section 1 and the outlet side straight pipe section 4 under such constraint conditions, the amount of thermal expansion of the first straight pipe section will be larger than that of the first straight pipe section 4 and will be proportional to the temperature difference. thermal stress is generated. In other words, the first straight pipe section 1 generates compressive stress because its own thermal expansion is hindered by the fourth straight pipe section 4 on the low temperature side. The fourth straight pipe section 4 is stretched by the first straight pipe section at a high temperature, so it becomes tensile stress. According to actual measurements by the inventor, the maximum thermal stress level is 10Kg/mm ²
This is a high value.

On the other hand, the simple bending stress due to its own weight that occurs when a radiant tube is supported and fixed in an actual furnace as shown in Figure 1 depends on the furnace width and cross-sectional performance of the radiant tube, but the In a continuous heating furnace for wide steel plates, the actual measurement result was 0.5 Kg/mm ² or less, which is a sufficiently low value compared to the creep strength of the material.

Furthermore, even when observing the failure patterns of actually damaged radiant tubes, there are many points that cannot be explained by bending stress alone, such as the tube bending upward, and deformation and cracking occurring in the vicinity of the bend where the moment is small. Judging from the above, it is very likely that thermal stress is the main cause of radiant tube damage.

Thermal stress is generally a product of temperature difference, longitudinal elastic modulus, and linear expansion coefficient, but methods for reducing the temperature difference are problematic because they result in a decrease in thermal efficiency. The most effective method seems to be to loosen the constraints on the radiant tube.

(Purpose of the invention) Therefore, the present invention provides a practical radiant tube that can alleviate thermal stress and significantly extend its life by changing the constraint conditions even if a temperature difference occurs in the flow direction of the exhaust gas of the radiant tube. It is something.

(Structure of the invention) The gist of the invention is that the straight pipe part of the W-shaped or U-shaped radiant tube, which indirectly heats the steel material in the heating furnace, has a flexible structure that absorbs the thermal stress caused by the expansion and contraction of the radiant tube. The pipes are directly joined, and the flexible pipe is made of stainless steel or heat-resistant steel with a spring constant of 500 kg/mm or less.

(Example) The details of the present invention will be explained below based on examples.

FIG. 3 is an overall view showing a first embodiment of a radiant tube in which the wall thickness near the root of the fourth straight pipe portion is reduced to form a wave shape. In the figure, 15 is a flexible pipe, 16 is a joint, 17 is a short exhaust gas outlet pipe,
18 indicates a support cylinder.

The role of the flexible pipe 15 is as the first straight pipe 1
Thermal stress generated due to the temperature difference between the straight pipe portion 4 and the fourth straight pipe portion 4 is absorbed by elastic deformation of the thin corrugated portion. The flexible pipe 15 expands and contracts in response to the axial displacement of the tube due to thermal stress, and generates a reaction force proportional to the amount of expansion and contraction, using its own spring constant as a proportionality constant. In this case, the thermal stress in the axial direction of the radiant tube is the value obtained by dividing the reaction force of the flexible pipe 15 by the cross-sectional area, but it is relaxed by the amount that the flexible pipe 15 is elastically deformed.

The joints 16, 16' are welded.
Since the straight pipe portion of the radiant tube is generally manufactured by centrifugal casting, it is difficult to integrally mold the flexible pipe 15. The flexible pipe 15 is formed by processing a thin plate by press forming or welding, and welding this to the fourth straight pipe part 4 of the radiant tube. Also, the short pipe 1 on the exhaust gas outlet side
7 connects the flexible pipe 15 and the flange 9 on the outlet side of the furnace, and is not necessary when the flexible pipe is directly connected to the flange 9 on the outlet side.

The support cylinder 18 supports the radiant tube against deformation in the axis-perpendicular direction. The structure of the support cylinder 18 is such that one end (the upper side in the drawing) is the iron skin 13
At the other end (lower side in the drawing), the difference t between the outer diameter of the fourth straight pipe section 4 of the radiant tube and the inner diameter of the support cylinder 18 is set within 2 mm in radius.

Next, the material of the flexible pipe 15 is stainless steel, heat-resistant steel, or the like. Also, the spring constant of the flexible pipe 15 in the axial direction is 500 kg/mm or less.
If it exceeds this value, the elastic reaction force will be large and the thermal stress relaxation effect will be small. Also, if the spring constant is too low,
Since the holding function of the flexible pipe main body is deteriorated, it is preferably in the range of 200 to 500 kg/mm. The position of the flexible pipe 15 is as close as possible to the iron shell 1.
3, and the radiant heat of the radiant tube is shielded by the brick 14.

Furthermore, with the radiant tube of the present invention, if a part of the existing iron skin 13 is modified, the flexible pipe 1
5 can also be exposed outside the iron skin 14.

(Effect of the invention) As mentioned above, the radiant tube of the invention absorbs and alleviates the stress by elastic deformation of the expandable part when there is a temperature gradient in the flow direction of exhaust gas and damage occurs due to thermal stress. It is. Conventionally, materials have been made of higher quality and wall thickness has been increased in order to cope with the bending stress due to its own weight, but the present invention makes it possible to prevent thermal stress damage and significantly extend the life of the product. Furthermore, the radiant tube according to the present invention has a simple structure and does not require modification of the existing furnace body, and can be implemented by modifying the tube itself. Also, the price increase due to radiant tube modification is extremely low at around 20%.

[Brief explanation of the drawing]

Figure 1 is an overall view of the conventional radiant tube, Figure 2
The figure shows the surface temperature of a conventional radiant tube, and Figure 3 shows an embodiment of the present invention, where A is an overall view and B is an enlarged view of part A. DESCRIPTION OF SYMBOLS 1...First straight pipe part, 2...Second straight pipe part, 3...Third straight pipe part, 4...Fourth straight pipe part, 5...First bend part, 6...
2nd bend part, 7... Third bend part, 8... Burner side flange, 9... Exhaust gas outlet side flange, 10... Intermediate support, 11... Tip support, 12... Combustion burner, 13... Iron shell, 14... Brick, 15 ...Flexible pipe, 16...Joint part, 17...Exhaust gas outlet side short pipe, 18...Support cylinder.

Claims (1)

[Scope of utility model registration request] W type or U type that indirectly heats steel materials in a heating furnace
A flexible pipe that absorbs thermal stress due to expansion and contraction of the radiant tube is directly connected to the straight pipe part of the radiant tube, and the flexible pipe is made of stainless steel or heat-resistant steel and has a spring constant of 500 kg/mm or less. Features a radiant tube.