JPH10103624A - Radiant tube of heating furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiant tube of a heating furnace in which a thermal stress generated by thermal expansion in a direction perpendicular to an axial direction under applied temperature can be restricted and occurrence of bending or crack of the tube can be prevented. SOLUTION: One end of a radiant tube where two straight tubes 1a, 1d are fixed is expanded and fixed in such a way that an axial-central distance at this one end is elongated only by the factor of a thermal expansion under an applied temperature than an axial-central distance at an opposite end part. In addition, the radius of curvature r0 each of bent tubes 2a, 2b and 2c under a normal temperature is set in advance in such a way that it may become a predetermined radius of curvature r1 through thermal expansion under an applied temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a radiant tube used in a heat treatment furnace such as a continuous annealing furnace.

[0002]

2. Description of the Related Art In a steel or other heat treatment furnace, when fuel cannot be burned in the furnace, a radiant tube made of a heat-resistant steel tube is installed in the furnace, and a high temperature heated by a burner or the like from outside the furnace is used. By supplying gas into the radiant tube, the inside of the furnace is heated by radiant heat from the outer periphery of the radiant tube in the furnace while keeping the atmosphere in the furnace constant.

The radiant tube has a straight end consisting of a straight pipe only, and a U-shaped or W-shaped pipe in which a part of the pipe is curved and a curved pipe part is provided in the middle, and a straight pipe is arranged in a double end. There are shapes. For example, FIG. 1 shows a state in which a W-shaped radiant tube is installed vertically in a furnace. That is, in the W-shaped radiant tube, two long and short straight tubes 1a, 1b, 1c, and 1d are arranged in parallel with a short straight tube 1b and 1c at an interval, and 180 ° is provided on these tubes.
The three curved tubes 2a, 2b, 2c are welded and connected in a W shape.

[0004] There are also vertical, horizontal and vertical installation methods for installing these radiant tubes in a furnace. For example, as shown in FIG. 1, when a W-shaped radiant tube is installed vertically in a furnace, a long straight tube 1a, 1
The ends (d) are aligned in the vertical direction and fixed to the burner bank 3 on one side of the furnace wall, and a support (not shown) welded to the curved pipes 2a, 2b, 2c is supported on the furnace wall, so that the inside of the furnace is To be installed in The U-shaped radiant tubes are installed in the same manner, and when these radiant tubes are installed horizontally, they are supported on the furnace wall in the same manner as described above except that the ends are aligned in the horizontal direction. .

[0005] Since the radiant tube installed in the furnace is usually used at a high temperature exceeding 800 ° C, deformation due to thermal expansion occurs during use at the high temperature. The dotted line shown in FIG. 1 schematically shows the state of deformation of the radiant tube due to thermal expansion. Therefore, when the deformation due to the thermal expansion cannot be smoothly and easily absorbed and alleviated, and as a result, the expansion of the radiant tube is restricted, the thermal stress is reduced to the straight pipes 1a, 1b, 1c, 1d and Acting on the curved pipes 2a, 2b, 2c, bending or cracking is likely to occur, and the radiant tube has a short life.

Therefore, among the thermal expansions of the radiant tube, the thermal expansion δ _{H in} the axial direction is conventionally improved by improving the support structure for the furnace wall (for example, Japanese Patent Application Laid-Open No. Sho 61-25241).
No. 6 and Japanese Utility Model Application Laid-Open No. 63-12474), or a method in which bellows are provided at the end of the straight tube at the outside of the furnace to relax the restraint and control the generation of thermal stress. However, as for the thermal expansion δ _{V in the} direction perpendicular to the axial direction, the ends of the straight pipes 1 a and 1 d are fixed to the furnace wall by the burner bank 3, so that the thermal expansion is constrained. Will occur.

As a method for relaxing or suppressing the thermal stress acting due to the thermal expansion δ _{V in} a direction perpendicular to the axial direction, Japanese Patent Application Laid-Open No. 1-150705 discloses a method in which a fixed end of a radiant tube is perpendicular to the axial direction. A radiant tube provided with a flexible member that easily deforms in a direction has been proposed.
However, in the method using the flexible member, there is a problem that the thermal stress cannot be sufficiently suppressed, so that the radiant tube is liable to bend, crack, or the like, and also leads to an increase in cost.

[0008]

SUMMARY OF THE INVENTION The present invention eliminates such conventional problems and suppresses thermal stress generated by thermal expansion in a direction perpendicular to the axial direction in a use temperature range, and bends, cracks, and the like. It is an object of the present invention to provide a radiant tube for a heating furnace, which can prevent generation of cracks.

[0009]

To achieve the above object, a radiant tube for a heating furnace provided by the present invention comprises:
In a radiant tube having two or more straight pipes connected to each other by a curved pipe, two straight pipes each having one end fixed to the furnace wall are connected to a shaft at an end on the opposite end where the center distance between the shafts is opposite to each other. The one end side is widened and fixed to the furnace wall so that the length is longer than the center-to-center distance by the amount of thermal expansion at the operating temperature.

In a preferred embodiment of the radiant tube for a heating furnace according to the present invention, all straight tubes are connected to each other by a curved tube having a radius of curvature set so as to have a predetermined radius of curvature at an operating temperature. One end of the two straight pipes is fixed to the furnace wall such that the distance L between the axial centers at one end of the two straight pipes is L = 2 × the radius of curvature of the bent pipe at the operating temperature × the number of bent pipes. And a radiant tube.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS When a W-shaped radiant tube is installed in a furnace and used at a high temperature, for example, as shown in FIG. the thermal expansion [delta] _V appears mainly as a curved pipe 2a, 2b, 2c radius of curvature increased. The relationship between the radius of curvature r ₀ of the curved tubes 2a, 2b, and 2c at room temperature and the radius of curvature r ₁ when the tubes are heated to a use temperature can be expressed by Equation 1 below.

[0012]

R ₀ = r ₁ / (1 + αT) where r ₀ : radius of curvature at normal temperature (mm) r ₁ : radius of curvature at operating temperature (mm) α: coefficient of linear expansion at operating temperature T ° C. ℃) T: Operating temperature (℃)

In a preferred embodiment of the present invention, a radius of curvature r ₀ at room temperature, which is a predetermined radius of curvature r _{1 at} the operating temperature T of the heating furnace, is obtained by using the above equation (1), and as shown in FIG. The curved pipes 2a, 2b, 2c having a radius r ₀ are produced, and the curved pipes 2a, 2b, 2c and the straight pipes 1a, 1a, 1c are formed.
b, 1c and 1d are respectively joined by welding to assemble a W-shaped radiant tube. Therefore, the axial center distance L at one end of each of the straight tubes 1a and 1d before being fixed to the furnace wall of the W-shaped radiant tube is 6r ₀ as is clear from FIG.

Next, as shown in FIG. 3, one ends of the long straight tubes 1a and 1d of the W-shaped radiant tube of FIG.
It is fixed by passing through the burner bang 3 of the furnace wall. At this time,
The distance between the axial centers at the fixed ends of the straight pipe 1a and the straight pipe 1d is extended and fixed so as to be longer by the thermal expansion in the direction perpendicular to the axial direction at the operating temperature T. That is, at a predetermined use temperature T, the radius of curvature of each of the curved pipes 2a, 2b, 2c is r ₁
, The distance L between the centers of the axes at the fixed end is
The 6r _1, and the fixed straight tube 1a by a burner bank 3 separates the shaft center distance 6r _1, one end of 1d in the furnace wall.

However, the original center distance between the axial centers of the straight tubes 1a and 1d of the W-shaped radiant tube manufactured with the radius of curvature r _{0 as} described above is 6r ₀ , while the expanded center distance between the axial centers 6r is 6r. ₁ is 6 as can be seen from Equation 1 above.
Since it is equal to r ₀ + 6r ₀ αT, there is a difference of 6r ₀ αT corresponding to the thermal expansion in the direction perpendicular to the axial direction. Therefore, when the one ends of the straight pipes 1a and 1d are passed through the burner bang 3, the distance between the axial centers of the fixed one ends is increased by the thermal expansion amount 6ρ ₀ αT. As a result, bending stress acts on the radiant tube, but since such bending stress is sufficiently within the elastic limit at room temperature, plastic deformation or cracking does not occur.

The distance L between the axial centers at one end of the straight pipe to be fixed varies depending on the number of curved pipes.
It goes without saying that L = 2 × the radius of curvature of the curved pipe at the operating temperature × the number of curved pipes. Therefore, the W-shaped radiant tube has three curved tubes, and thus 6r ₁ as described above.
Or 6r ₀ , and in the case of U-shape, there is one curved tube, so 2
the r ₁ or 2r _0.

When the W-shaped radiant tube of the present invention installed in the furnace reaches the operating temperature T, the radii of curvature of the curved tubes 2a, 2b, 2c are changed from r ₀ to r ₁ as shown in FIG.
Thermal expansion. Along with this, the axial distance between the centers of the curved pipe end of the straight tube 1a and straight pipe 1d also changed from 6r ₀ to 6r _1, equal to the axial distance between the centers of the one end side fixed to the burner Bang 3 Become. Although thermal stress acts on the radiant tube due to this thermal expansion, the direction of the stress is opposite to the direction of the bending stress acting in a state of being fixed to the furnace wall at room temperature. Is almost zero. Therefore, it is possible to prevent the radiant tube from being bent or cracked during use at a high temperature.

[0018]

EXAMPLE As shown in FIG. 2, long straight tubes 1a and 1d having a length of 2500 mm and an outer diameter of 200 mm, and a length of 1500 mm
, A short straight pipe 1b, 1c having an outer diameter of 200 mm was welded with three curved pipes 2a, 2b, 2c to produce a W-shaped radiant tube. At this time, the bent pipes 2a, 2b and 2c are made of Fe-48Ni-28Cr having a thermal expansion coefficient of 16 × 10 ⁻⁶ / ° C.
-5W heat-resistant alloy, bent tube 2a at operating temperature (T) 1000 ° C. according to the above equation 1 in consideration of its thermal expansion,
2b, so that the radius of curvature r ₁ of 2c is 150mm,
The radius of curvature r ₀ at the normal temperature is calculated, and r ₀ = 147.6 m
m.

As shown in FIG. 3, one end of each of the long straight tubes 1a and 1d of the W-shaped radiant tube was passed through a burner bang 3 on the furnace wall and fixed. However, the straight pipes 1a, 1
The distance between the axial centers of the respective ends fixed to the burner bank 3 of d is the radius of curvature r _{0 of the} curved pipes 2a, 2b, 2c (147.
6 r ₁ (900 m) so as to be equal to the distance between the axial centers of the straight pipes 1 a and 1 d at the time when the radius of curvature becomes r ₁ (150 mm) due to thermal expansion at a use temperature of 1000 ° C.
m).

As a result, one end of each of the straight tubes 1a and 1d of the W-shaped radiant tube is forcibly expanded and fixed to the furnace wall, and a distance between the one end and the opposite end is about 800 kg.
Although a bending stress of fm was generated, no plastic deformation or cracking occurred in the W-shaped radiant tube.

Using a heating furnace having the W-shaped radiant tube installed therein and setting the operating temperature to 1000 ° C. and performing the heat treatment on the steel sheet as usual, the heat treatment can be carried out without any trouble. Heat treatment time 5.0
Even after the elapse of 00 hours, it could be used without any abnormality such as bending or cracking, and it could be confirmed that it could be used for a longer time than the conventional one.

In this embodiment, the radius of curvature r ₀ of the curved tube at normal temperature is set in advance to be the radius of curvature r _{1 at the} operating temperature T based on the above equation (1). The radius of curvature r ₀ may be any value. In that case,
The radius of curvature r ₁ at the operating temperature is determined from the radius of curvature r ₀ at the arbitrary normal temperature by the above equation 1, and the distance between the axial centers at one end of the two straight pipes to be fixed is set based on the radius of curvature r _1. I do.

[0023]

According to the present invention, the stress caused by thermal expansion in the direction perpendicular to the axial direction of the straight pipe can be absorbed and alleviated without using any special accessory member or equipment, so that the radiant tube can be used at a high temperature. Can be prevented, and the life of the radiant tube can be made longer than before.

[Brief description of the drawings]

FIG. 1 is a schematic front view schematically showing a conventional W-shaped radiant tube and a state of thermal expansion at a use temperature thereof.

FIG. 2 is a schematic front view showing a specific example of a W-shaped radiant tube of the present invention.

FIG. 3 is a schematic front view showing a state in which the W-shaped radiant tube of the present invention of FIG. 2 is fixed to a furnace wall.

[Explanation of symbols]

 1a, 1b, 1c, 1d Straight pipe 2a, 2b, 2c Curved pipe 3 Burner bank

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Satoshi Uchibayashi 1 3860 Shimotsuruma Jingu, Yamato-shi, Kanagawa Prefecture Taihei Metal Industry Co., Ltd.

Claims (2)

[Claims] 1. A radiant tube comprising two or more straight pipes connected to each other by a curved pipe, wherein two straight pipes each having one end fixed to a furnace wall are arranged such that the axial center distances at the one end are opposite to each other. A radiant tube for a heating furnace, characterized in that the one end side is widened and fixed to a furnace wall so as to be longer by a thermal expansion at an operating temperature than a distance between shaft centers at an end. 2. A straight pipe having a radius of curvature set so as to have a predetermined radius of curvature at an operating temperature, all the straight pipes are connected to each other, and an axial center distance L at one end of two straight pipes.
Is: L = 2 × radius of curvature of curved pipe at operating temperature × number of curved pipes,
2. The radiant tube for a heating furnace according to claim 1, wherein one end of each of the two straight tubes is fixed to a furnace wall such that