JPS5885008A - Radiant tube - Google Patents

Abstract

PURPOSE:To lower the temperature in a heat-treating furnace, and to decrease NOX in exhaust gas, during combustion, by providing an inner tube which has a gas-permeable tubular wall, to the inside of a metallic tube, and by forming an annular combustion chamber in a gap between the tubular wall of an inner tube and the metallic tube, where combustion is taken place dispersedly around the external periphery of a tubular wall. CONSTITUTION:When a radiant tube 2, provided to a heat-treating furnace by piercing through a furnace wall 1, 1, is operated, fuel gas is fed into a gas inlet port 12, and combustion air is fed into an inlet port 6, respectively. The fuel gas, passing through a gas feed tube 7 and an inner tube 8, is fed into a combustion chamber 13, nearly in the diametral direction, permeating through a tubular wall 9. On the other hand, the combustion air is fed into the combustion chamber 13 in the axial-flow direction, passing through an air box 5 and the end part of a metallic tube 3. The combustion air is mixed with the above-mentioned fuel gas, and is ignited, and then, combustion is started. Combustion can be taken place widely dispersed over the whole area of external surface of a tubular wall 9, because fuel gas is fed into the combustion chamber 13, being straightened and dispersed over the full length of an inner tube 8 when it passes through the tubular wall 9.

Description

[Detailed description of the invention] This invention relates to a radiant tube.

Radiant tubes generally used in heat treatment furnaces, etc.
A burner is installed at the end of the metal tube, and the metal tube is heated by the high-temperature combustion gas obtained by burning fuel within the metal tube, so NOx is generated in the high temperature part of the flame of the burner. Moreover, the metal tube was locally heated to a high temperature, resulting in uneven temperature distribution in the axial direction, and the service life was short. Furthermore, the metal tube is heated by convection heat transfer and radiation heat transfer from the combustion gas, but since gases generally have a smaller radiation injection capacity than solids, the combustion gas does not heat the metal tube sufficiently and reaches a high temperature. The heating efficiency was low.

The present invention solves the above-mentioned conventional drawbacks, and aims to provide a radiant tube that generates less NOx, has a uniform axial temperature distribution of the metal tube, and has excellent heating efficiency.

A first embodiment of the present invention will be described below with reference to FIG.

In Fig. 1(a), l is the furnace wall of the heat treatment furnace, 2 is a straight radiant tube installed through the furnace wall, 3 is the main body of the heat-resistant cast steel metal tube, and 4 is the exhaust gas. It is the mouth. 5 is a short cylindrical wind box attached to the combustion side end of the metal tube 3, and 6 is an air inlet to this wind box, which is connected to a combustion air supply source such as a blower (not shown). 7 is a gas supply pipe that passes through the wind box 5 and has one end fixed to the wind box; 8 is a cylinder connected and fixed to the other end of the gas supply pipe; 9 is a cylindrical wall having air permeability; 10 is an annular end plate, and 11 is a disc-shaped end plate. In this embodiment, the material of the cylinder wall 9 is stainless wire mesh (wire diameter 0).
．． Although 20 sheets (7 mm, 14 mesh) were laminated to form a plate shape, metals other than stainless steel or heat-resistant solid materials such as ceramics may also be used. An appropriate shape having air permeability, such as a porous shape, may be used. Reference numeral 12 denotes a gas inlet of the gas supply pipe 7, which is connected to a fuel gas supply source (not shown). Further, 13 is an annular combustion chamber formed between the metal tube 3 and the inner cylinder 8.

In the radiant tube 2 having the above configuration, if fuel gas is supplied to the gas inlet 12 and combustion air is supplied to the air inlet 6, the fuel gas passes through the gas supply pipe 7, the inside of the cylinder 8, and the cylinder wall 9. and flows into the fuel chamber 13 approximately diametrically. On the other hand, the combustion air flows axially into the combustion chamber 13 through the wind box 5 and the end of the metal tube 3, and mixes with the fuel gas flowing in from the cylinder wall 9, so that it can be ignited by an ignition burner (not shown) or the like. For example, the fuel gas is then combusted within the combustion chamber 13. This combustion is carried out in the form of distributed combustion over a wide area of the entire outer peripheral surface of the cylinder wall 9 because the fuel gas is rectified by the cylinder wall 9 and flows into the combustion chamber 13 while being dispersed over the entire length of the cylinder. Therefore, the maximum value of the combustion temperature is lowered and the amount of NOx generated is suppressed, and the metal tube 3 is heated by radiant heat transfer and convection heat transfer of the combustion gas, and combustion occurs internally.

This occurs near the outer circumferential surface of the cylindrical wall 9 of the tube, increasing the surface temperature of the cylindrical wall 9, and the metal tube 3 also increases due to radiant heat transfer from the cylindrical wall 9, which is made of a solid material that has a higher radiation ejection capacity than gas. Since it is heated, the amount of heat transferred to the metal tube 3 increases, and a radiant tube with high heating efficiency can be obtained.

Radiant tube with the above configuration (outer diameter 170 m).

According to the inventor's experimental results using a furnace with an internal length of 1300 mm, the combustion amount was 50 mm using butane air dilute gas as the fuel. When combustion was performed at OOOkcal/hour, the heating efficiency of the radiant tube was 67% when the furnace temperature was 800°C, which was 60% of the heating efficiency of a conventional radiant tube (same dimensions) using a normal burner.
This is an improvement of about 7 inches compared to the previous year.

Further, as shown in FIG. 1(b), the surface temperature T of the metal tube 3 during the above experiment was lower than the surface temperature l1lo of the conventional radiant tube shown by the chain line in the figure, and the maximum temperature was lower. Uniformity has been achieved.

Next, FIGS. 2 to 6 show a second embodiment of this invention,
The same or equivalent parts as in FIG. 1 are given the same reference numerals.

In the figure, reference numeral 21 denotes an inner cylinder whose one end is fixed to the wind box 5, and is composed of a cylindrical cylinder wall 22 having air permeability, an annular end plate 23, and a disc-shaped end plate 24. 25 is the inner cylinder 21
A partition provided inside divides the inside of the inner cylinder 21 into a fuel passage 26 with which the gas inlet 12 communicates and an air passage 27 with which the air inlet 6 communicates. 28 is a ring having a U-shaped cross section and is a component of the partition 25, and both flange surfaces 29.
30 has two ventilation holes 31 and 32 formed therein with a phase shift of 90 degrees. The partition 25 is formed by arranging seven rings 28 with their phases shifted by 90 degrees, and connecting the ventilation holes 31 and 32 of adjacent rings with a rectangular ventilation pipe 33. That is, the air flow path 27 is an annular chamber 27a surrounded by each ring 28 and the cylindrical wall 22, which is connected by a ventilation pipe 33.

Further, the fuel flow path 26 is composed of the inner diameter portion of each ring 28 and a disc-shaped chamber 26B formed between adjacent rings 28, 28 and surrounded by the cylindrical wall 22.

When fuel gas and combustion air are supplied to the radiant tube 34 configured as described above, the fuel gas flows through the fuel flow path 26 and flows from each disk-shaped chamber 26B through the cylindrical wall 22 into the combustion chamber 13. Further, the combustion air flows through the air passage 27, passes through the cylindrical wall 22 from each annular chamber 2, 7a, flows into the combustion chamber 13, and mixes with the inflowing fuel gas from the adjacent disc-shaped chamber 26a. Combustion takes place. That is, this embodiment differs from the first embodiment in that combustion air is also supplied into the combustion chamber 13 through the inner cylinder 2, but the air is distributed over the entire outer peripheral surface of the cylinder wall 22 of the inner cylinder 21. Since combustion takes place, the same effects as in the first embodiment can be obtained. In this embodiment, the cylindrical wall 22 of the portion of the fuel flow path surrounding the disk-shaped chamber 26a may be removed to allow the fuel gas to flow directly into the combustion chamber 13 from the disk-shaped chamber 26a.

The present invention is not limited to the above embodiments; for example, the fuel gas flow path and the combustion air flow path may be interchanged in each of the above embodiments.

As explained above, according to the present invention, an inner cylinder having an air-permeable cylinder wall is provided inside the metal tube, and an annular combustion chamber is formed between the metal tube and the cylinder wall of the inner cylinder. Since decentralized combustion is performed near the outer peripheral surface of the cylinder wall of the inner cylinder, the amount of NOx generated is reduced by lowering the maximum combustion temperature, and the temperature distribution of the metal tube is made uniform. The heating efficiency of the radiant tube is improved by adding radiant heat transfer from the radiant tube.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are diagrams showing a longitudinal section and surface temperature distribution of a radiant tube showing a first embodiment of the invention, and FIG. 2 is a longitudinal section of a radiant tube showing a second embodiment of the invention. 3 is an enlarged vertical sectional view of the inner cylinder in FIG. 2, FIG. 4 is a sectional view taken along line A-A in FIG. 3, and FIG.
-B sectional view and FIG. 6 are the same (C-C line sectional view. 2...Radiant tube, 3...Metal tube,
5... Wind box, 6... Air inlet, 7.
... Gas supply pipe, 8 ... Inner cylinder, 9 ... Cylinder wall, 12.
...Gas inlet, 13...Combustion chamber, 21...Inner cylinder,
22... Cylinder wall, 25... Partition, 26... Fuel flow path, 27... Air flow path, 34... Radiant tube. Mouth surface 1 Mouth area ? [

Claims (1)

[Claims] A cylinder having an air-permeable cylinder wall is provided inside the metal tube, an annular combustion chamber is formed between the metal tube and the cylinder wall of the inner cylinder, and the inner cylinder is connected to the cylinder from the first inlet. A first flow path passing through the cylinder wall and reaching the combustion chamber, and a second flow path leading from a second inlet to the combustion chamber are provided, and the first flow path and the second flow path are provided. A radiant tube characterized in that the fuel gas is caused to flow in one of the channels and the combustion air is caused to flow through the other channel so that the fuel gas is combusted in the combustion chamber.