JPH10110914A - Heating furnace with radiant tube as heat source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize uniform heating not only in the length direction of a radiant tube but only in the direction between the tubes in a heating furnace using the radiant tube as a heat source. SOLUTION: The heating furnace includes on opposite ends of a radiant tube 5 a pair of regenerative burners 6, 6 for alternately supplying exhaust of waste gas and supply of combustion through a heat storage structure, and uses as a heating source the radiant tube 5 including a regenerative burner system 2 for alternately combusting the pair of the burners 6, 6. In this case, the radiant tube 5 is disposed in a furnace in a range where a ratio Lw of a pitch Lp between the radiant tubes and a distance from the tube to a work is at most 2, preferably 0.75<Lp/Lw<1.75.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heating furnace having a pair of regenerative burners at both ends of a radiant tube, which are alternately burned to heat the tube to use as a heat source. More specifically, the present invention relates to improving the placement of radiant tubes in a furnace.

[0002]

2. Description of the Related Art As a conventional heat source in a heating furnace which refuses to contaminate the atmosphere, for example, in an atmosphere furnace, a radiant tube and one radiant tube burner which causes long flame combustion in the tube are generally used. The heating by the radiant tube is useful as a heat source because it does not pollute the atmosphere similarly to the heating by the electric heater and the cost can be remarkably reduced as compared with the electric heater and the like.

However, in this radiant tube burner, combustion proceeds in one direction from one end of the radiant tube toward the exhaust port at the other end, and therefore, as shown in FIG. If the surface temperature of the tube becomes the highest and the amount of combustion is excessively increased, the vicinity of the heating port is burned due to overheating. In order to prevent burning, it is necessary to lower the temperature of the tube in the vicinity of the heating port to a temperature lower than the use limit temperature of the tube material. Therefore, the amount of combustion must be reduced, and the amount of heat transfer per tube surface area is limited.

Further, the temperature of the combustion gas in the tube decreases toward the exhaust port, and the amount of heat transfer decreases in the length direction of the tube. This results in a large temperature gradient along the length of the tube, shortening the life of the tube.

Further, since the amount of heat transfer is proportional to the logarithmic average temperature difference between the combustion gas temperature and the work temperature, an enormous heat transfer area (tube surface area) is required to reduce the exhaust temperature and save energy. So, install a heat exchanger,
Although it is conceivable to recover the exhaust heat and preheat the combustion air, even in this case, the temperature of the tube in the vicinity of the opening must be lower than the limit temperature, and the heat transfer amount cannot be increased. Furthermore, even if a recuperator is provided at the tube exhaust port / other end side to perform heat exchange between the exhaust gas and the combustion air, although some energy saving is achieved, the heat exchange type heat exchanger used is used. Temperature efficiency of 60%
The effect is low due to the following low rates.

Therefore, the present inventors have provided a heat storage type burner at each end of a radiant tube as a heat source of an atmosphere furnace. The pair of burners are alternately burned to stop the combustion of the exhaust gas after heating the tube. While exhausting through the regenerator of the burner, the burner during combustion was supplied with combustion air preheated to a high temperature via the regenerator and burned with high-temperature combustion air close to the exhaust gas temperature.

In this case, since the combustion direction in the tube alternates in a short cycle, the combustion gas temperature has no peak as shown in FIG. 10 (B), becomes uniform in the length direction of the tube, and A uniform flame without a peak realizes a low NOx. Therefore, the amount of heat transfer from the tube to the work is uniform in the length direction of the tube. In addition, in the heat storage type heat exchange, a temperature efficiency of 90% or more can be easily obtained, and high-temperature air close to the exhaust temperature at the tube outlet can be obtained, so that a uniform and high temperature drop can be obtained between the combustion gas and the work. And high energy saving is achieved. Therefore, when the tube is uniformly heated at the limited temperature, the heat transfer amount can be approximately doubled with the same surface area as the conventional type, and the surface area can be reduced by about half when the heat transfer amount is the same as the conventional type. With the same surface area and the same heat transfer amount as the conventional type, the peak temperature of the tube is lowered, so that an inexpensive low-grade material having poor heat resistance can be used.

[0008]

However, there has been a problem that the above effects cannot always be obtained in an actual atmosphere furnace. That is, conventionally, uniform heat transfer from the radiant tube to the work is better as the pitch of the radiant tubes is reduced and densely arranged, and as the pitch is increased and the radiant tubes are separated, the heat transfer at an intermediate point between the tubes is improved. It was thought that the heat transfer of the work was hindered. Therefore, the pitch of the tubes has been determined mainly from the viewpoint of economy, on the premise that the tubes are arranged as densely as possible.

However, in practice, it is difficult to uniformly heat the tubes of the work, and even if the pitch of the tubes is reduced ignoring the economic requirement, uniformity of the heat transfer cannot be maintained. Was found.

[0010] It is an object of the present invention to provide a heating furnace using a radiant tube as a heat source capable of realizing uniform heating not only in the tube length direction but also in the inter-tube direction.

[0011]

As a result of the present inventors' various studies and studies, it was found that the arrangement pitch of the radiant tubes and the distance to the work had a correlation. In other words, as a result of a detailed study of the heat transfer from the tube to the work, the smaller the pitch of the tube is, the more uniform the heat transfer is not maintained, as has been said so far. It has been found that there is a region where the heat transfer is maintained, and even if the pitch is smaller or larger than that, the uniformity of heat transfer is impaired.

The present invention has been made based on this finding, and includes a heat storage type burner at both ends of a radiant tube which alternately supplies exhaust gas and feeds combustion air through a heat storage body. In an atmosphere furnace equipped with a radiant tube as a heating source equipped with a heat storage burner system for alternately burning burners, a radiant tube having a ratio Lp / Lw of a pitch Lp between radiant tubes and a distance Lw from a tube to a work of 2 or less is used as a furnace. To be placed inside. Here, the pitch L between the tubes
The ratio Lp / Lw of p to the distance Lw to the workpiece is preferably set to 0.75 <Lp / Lw <1.75.
This allows for uniform heating between tubes as well as the length of the tubes.

That is, heat transfer from the radiant tube to the work is given as shown in FIG. At this time, the amount of radiant heat per unit surface area of the radiant tube (heat flux of the tube) q is expressed as in the following Expression 1. Then, the amount of heat received per unit surface area (heat flux of the heat receiving surface) q ”at an arbitrary position of the work receiving the radiation.
Is represented by a basic expression (1) shown in the following Expression 2. Also,
The radiation angle α of the heat from the center of the tube is represented by the following basic formula (2) shown in Expression 3.

[0014]

(Equation 1)

[0015]

(Equation 2)

[0016]

(Equation 3)

Here, q ′ is a virtual heat receiving amount per unit surface area (heat flux of the virtual surface) on the virtual heat receiving surface perpendicular to the radiation from the tube on any arbitrary heat receiving surface, r is the radius of the radiant tube, and ds is A small surface area of a tube that radiates heat at an angle α at an arbitrary position on the work, ds "is a small surface area of the work heat receiving surface, ds' is a small surface area of the virtual surface,
L1 indicates the vertical distance from the tube to the work surface, and L2 indicates the horizontal distance from the tube to any work heat receiving surface.

Next, for a workpiece having a pitch between two radiant tubes, the amount of heat received at an arbitrary location is displayed by using a ratio with the amount of heat received at a location to which the most heat is given between the radiant tubes. It is possible to evaluate the distribution of the amount of heat received.

Therefore, 2
The heat transfer to the work between the tubes is considered as follows. That is, heat is radiated in all directions from the two radiant tubes, and is reflected directly from each tube to the work and to the furnace wall (radiation heat directed toward the furnace wall is absorbed by the furnace wall at a very small portion). Most of them are reflected) and indirectly give heat. On the other hand, for an arbitrary place n on the work between the two tubes, heat radiated directly from the tube to the point n and heat given to the work at the point n by reflecting on the furnace wall are shown. is there. The reflection from the furnace wall varies depending on the relative positional relationship between the furnace wall and the work. When this magnitude is represented as radiant heat reflected at an n-point on the work at an equal angle, n The magnitude of the radiant heat given to the point is given as a dimensionless evaluation value △ Kn by the basic formulas (1) and (2) as in the following Expression 4.

[0020]

(Equation 4)

Here, Lp is the pitch between tubes, Lp
w is the vertical distance from the center of the tube to the work surface, Lr
Is the vertical distance from the tube center to the furnace wall (reflection surface), D
p indicates the diameter of the tube.

This ΔKn and ΔKn between the tubes
Is defined as the heat flux ratio Kn at the point n, and the distribution of the Kn can evaluate an appropriate pitch of the radiant tube that enables uniform heating of the work.

The distribution of the heat flux ratio of the work between the two radiant tubes will be examined based on the above.

The results of examining the heat flux ratio distribution at each point when the space between the tubes is divided into six are shown below.

(1) Lp / at Lr / Lw = 0.5
Distribution of Lw to heat flux ratio With Lr / Lw kept constant at 0.5, Lp / Lw is changed to 0.5, 1.0, 1.5, 2.0, 2.5 to accompany it. The change of the heat flux ratio distribution was determined. The result is shown in FIG. From this, as Lp / Lw increases, the heat flux ratio near the center between the tubes (the division position 3 of the work) decreases, and the heat flux ratio at 2.0 is a sufficiently high value of 0.9 or more. However, the heat flux ratio at 2.5 was less than 0.85, which is not high.

(2) Lp / Lw / Lw = 1.0
Distribution of Lw to heat flux ratio With Lr / Lw kept constant at 1.0, Lp / Lw is changed to 0.5, 1.0, 1.5, 2.0, 2.5 to accompany it. The change of the heat flux ratio distribution was determined. The result is shown in FIG. From this, as Lp / Lw increases, the heat flux ratio near the center between the tubes (the division position 3 of the work) decreases, and the heat flux ratio at 2.0 can be said to be a sufficiently high value of 0.9 or more. However, the heat flux ratio at 2.5 was close to 0.8 at less than 0.9, which cannot be said to be high.

(3) Lp / Lp at Lr / Lw = 1.5
Distribution of Lw to heat flux ratio With Lr / Lw kept constant at 1.5, Lp / Lw was changed to 0.5, 1.0, 1.5, 2.0, 2.5 to accompany it. The change of the heat flux ratio distribution was determined. The result is shown in FIG. From this, as Lp / Lw increases, the heat flux ratio near the center between the tubes (the division position 3 of the work) decreases, and the heat flux ratio at 2.0 is slightly higher than 0.9, which can be said to be a sufficiently high value. It is a value that can be said to be lower but higher,
The heat flux ratio at 2.5 was less than 0.8, which is not high.

(4) Lp / Lp at Lr / Lw = 2.0
Distribution of Lw to heat flux ratio With Lr / Lw kept constant at 2.0, Lp / Lw was changed to 0.5, 1.0, 1.5, 2.0, 2.5 to accompany it. The change of the heat flux ratio distribution was determined. The result is shown in FIG. From this, as Lp / Lw increases, the heat flux ratio near the center between the tubes (the division position 3 of the work) decreases, and the heat flux ratio at 2.0 is below 0.9 which can be said to be a sufficiently high value. Is a value of 0.85 or more which can be said to be high, but the heat flux ratio at 2.5 was a value which was much lower than the value 0.8 which could not be said to be high.

As described above, from the results of the examination of the heat flux distribution to the workpiece between the tubes under various furnace conditions, the uniformity of the heat flux distribution increases as the pitch between the tubes decreases as conventionally known. It was found that there was a pitch range where the best uniform distribution could be obtained. And, as shown in the figure, the range is a pitch range where there are two points where the heat flux peaks between the tubes (points where the heat flux ratio = 1).

This region is more clearly shown by plotting the minimum heat flux ratio indicating the degree of variation in heat transfer to the work when Lp / Lw is changed for each Lr / Lw. For example, as shown in FIG.
Lp when 5, = 1.0, = 1.5, = 2.0
The change in the minimum heat flux ratio between /Lw=0.00 to 2.50 was determined.

FIG. 7 shows that the pitch and the minimum heat flux ratio are 2
It can be seen that there are two peaks. One is Lp / Lw
Are near 0 and another one is near 1.5. When Lp / Lw is around 0, the maximum heat flux between tubes (heat flux ratio = 1) is present at the center between the tubes, and when Lp / Lw is around 1.5, the maximum heat flux between tubes (heat flux ratio = 1) is shown. Is the case where two points exist between the tubes, and the boundary is Lp / Lw near 0.75. That is, Lp
When / Lw is 0 to 0.75, the maximum heat flux between the tubes exists at the center between the tubes, and when 0.75 to 1.75, the maximum heat flux (heat flux ratio = 1) exists at two places between the tubes. Indicates that this is an area. In these regions, the minimum heat flux ratio also shows a sufficiently high value of 0.9 or more. When Lp / Lw is 1.75 or more, the point of the maximum heat flux (heat flux ratio = 1) exists immediately below the two tubes, and above this point, the minimum heat flux ratio sharply decreases.

As a result, the range of the pitch showing a uniform heat flux distribution is Lp / Lw <1.75, and the range of the appropriate pitch considering the economical efficiency of mounting is 0.75 <Lp / Lw.
Lw. Therefore, Lp / Lw is 0.75
It is most preferable to adopt a range larger than 1.75 and smaller than 1.75.

Next, it is checked how much the temperature rise of the work after heating for a certain time varies depending on the heat flux ratio Kn, based on the rate of increase of the work temperature after heating for a certain time. The temperature rise rate te / tf is calculated by the following equation (5).

[0034]

Te / tf = 1−Exp (−KnE) where ti = 0 E = hAτ / γcv tf: furnace temperature ti: desired temperature of the work te: final temperature of the work A: heat transfer area of the work h: Heat transfer coefficient to the work at Kn = 1 c: Specific heat of the work v: Actual volume of the work τ: Heating time When this calculation result is shown in the graph, it becomes as shown in FIG. 8, where Kn> 0.85. In the soaking zone where E = 3.5 or more at which the temperature of the workpiece almost reaches saturation, the variation of the temperature rise is within 2%, which indicates that sufficient uniformity is maintained. This indicates that sufficient uniform heating is possible if Lp / Lw is in the range of 2 or less.

From the above, the ratio Lp of the pitch Lp between the radiant tubes and the distance Lw from the tube to the work, which enables uniform heating between the radiant tubes.
/ Lw <2, more preferably 0.75 <Lp / Lw <
It can be easily understood that the range is 1.75.

[0036]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the present invention will be described below in detail based on the best mode shown in the drawings.

FIGS. 9A and 9B show an embodiment applied to an atmosphere furnace as an embodiment of the present invention. This atmosphere furnace mainly includes a furnace body 1, a heat storage type radiant tube burner system 2 serving as a heat source, and a work transfer means 3.

The furnace body 1 is not particularly limited in its structure, material and the like. For example, the furnace body 1 is constituted by a steel casing provided with a refractory lining. Is provided with a work transfer means 3 for transferring the work. Further, a fan 4 for stirring the atmosphere in the furnace is installed on the ceiling surface at the top of the furnace.

The regenerative radiant tube burner 2 system comprises a U-shaped radiant tube 5 arranged, for example, vertically through the inside of the furnace along the side wall of the furnace body 1;
A pair of radiant tube burners (hereinafter simply referred to as burners) 6,6 connected to the parts installed outside the furnace at both ends and burning alternately, and a pair of radiant tube burners 6,6 used for combustion to alternately burn the pair of burners 6,6 It is composed of a combustion air supply system 8 for selectively supplying air and fuel, an exhaust system 9 and a fuel supply system (not shown). The burner 6 is not particularly limited to a gun structure or the like, but includes a heat storage body (not shown) or is directly connected to the outside, and alternately exchanges exhaust gas and combustion air via the four-way valve 10. The combustion is performed in a closed space in the radiant tube 5 using high-temperature combustion air obtained by passing the air through the heat storage body. Here, the replacement cycle of the burner to be operated is a fixed time regardless of the presence or absence of combustion. For example, the burner is periodically switched in a short time of about 30 seconds, and the burner is provided so as to perform necessary combustion.

Although the radiant tube 5 is not particularly limited, ceramics such as SiC may be used in addition to commonly used metals such as heat-resistant cast steel. In the case of a ceramic tube, there is an advantage that it is not only oxidized but also lightweight and has a low heat content (because the specific gravity is small and the wall thickness can be reduced). Further, as the heat storage body, it is preferable to use, for example, a honeycomb-shaped ceramic (for example, cordierite or mullite) having a constant passage cross-sectional area and a straight passage through the passage. The honeycomb-shaped ceramic has a large heat capacity and a high durability, but has a relatively low pressure loss, so that exhaust and air supply are alternately performed without stagnation.

The number and the arrangement direction of the heat storage type radiant tube burner system 2 are selected as required, and the distance between the tube pitch Lp of the radiant tubes 5 and the distance Lw between the work W is determined. Requires a certain positional relationship. That is, the pitch Lp between the radiant tubes
Ratio Lp / to the distance Lw from the tube 5 to the workpiece W
Placing radiant tube 5 in a furnace with Lw of 2 or less, more preferably 0.75 <Lp / Lw <1.75
Is to place a radiant tube in the range. Here, the distance Lr between the radiant tube 5 and the solid wall surface 7 of the furnace body 1 is not particularly limited, and may be set to an arbitrary distance as long as the distance Lr does not extremely approach or separate. For example, according to experiments, Lr / Lw = 0.5, Lr / Lw = 1.
At 0, Lr / Lw = 1.5, and Lr / Lw = 2.0, the distribution of the heat flux ratio is not significantly affected. Therefore, assuming that the tube 5 is arranged at a constant distance Lr from the solid wall surface 7 of the furnace body 1, the tube 5
Distance / interval Lw from the position to the heated surface of the work W
(Or the position through which the heated surface of the workpiece W passes), the pitch Lp between the tubes 5 is determined from the distance Lw as described above, that is, Lp / Lw is 2 or less, more preferably 0. It is determined such that 75 <Lp / Lw <1.75. Specifically, it is preferable that the furnace wall be brought close to the area where sparing does not occur due to the heat radiated from the tube 5, and that the furnace wall be spaced away within a range where the furnace space is not unnecessarily enlarged. Lp / Lw is selected in a range where the above positional relationship can be maintained even if the installation positions vary.

According to the atmosphere furnace configured as described above, the burner burns through the regenerator at a high temperature, for example, 80 ° C.
Tube 5, using secondary air preheated to 0 ° C or higher
5 alternately in a short time at both ends. Then, the combustion gas generated by the combustion of one burner passes through the heat storage body of the burner on the other end while heating the radiant tube 5 and is then led to the exhaust system 9, and after being subjected to a predetermined exhaust treatment, to the atmosphere. Is discharged. For this reason, the heat of the exhaust gas is recovered by the heat storage body. Then, the heat recovered in the heat storage body is used for preheating the combustion air when burning the burner, and is returned to the tube again. Thus, a tube surface temperature distribution as shown in FIG. 10B is formed, and the tube is uniformly heated in the longitudinal direction. The heating in the direction between the tubes is performed as shown in FIG.
As shown in FIG. 6, when Lp / Lw <2, a sufficiently high minimum heat flux ratio 0.9 is obtained. Of course, Lp / L
For w <1.75, a sufficiently high minimum heat flux ratio of 0.9
5 is obtained. Then, as shown in FIG.
At 85, the temperature of the workpiece rises to almost saturation E =
In the soaking zone of 1.35 or more, the variation of the temperature rise is 2%
Within the range, indicating that sufficient uniformity is maintained. From this, when Lp / Lw is in the range of 2 or less, sufficient uniform heating is possible, and in particular, 0.75 <Lp / L
When w <1.75, extremely uniform heating can be achieved.

[0043]

As is apparent from the above description, according to the heating furnace using the radiant tube of the present invention as a heat source according to the first aspect, the radiant tube is heated by the alternate length of the heat storage type radiant tube burner. This is achieved by a uniform temperature distribution in the vertical direction and a uniform heating between the tubes. That is, heat is radiated in all directions from the radiant tube, and heat is applied to the work directly from each tube and indirectly by reflecting on the furnace wall, but when the distance between the above-mentioned tube pitch and the work is taken. The heat radiated directly from the tube and the heat reflected on the furnace wall are substantially equal, and uniform heating is realized also in the tube pitch direction. Therefore, it is possible to reduce the temperature difference in the heating of the work not only in the longitudinal direction of the tube but also in the tube pitch direction orthogonal thereto.

In particular, in the case of the atmosphere furnace according to the present invention, the minimum heat flux ratio becomes a sufficiently high value, and the difference becomes small between the tubes, so that uniform heating becomes possible.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a heat radiation state between two radiant tubes.

FIG. 2 is an explanatory diagram showing a state of heat radiation from one radiant tube.

FIG. 3 is a distribution diagram of Lp / Lw to heat flux ratio when Lr / Lw = 0.5.

FIG. 4 is a distribution diagram of Lp / Lw to heat flux ratio when Lr / Lw = 1.0.

FIG. 5 is a distribution diagram of Lp / Lw to heat flux ratio when Lr / Lw = 1.5.

FIG. 6 is a distribution diagram of Lp / Lw to heat flux ratio when Lr / Lw = 2.0.

FIG. 7 shows the minimum heat flux ratio Lr / L with respect to the ratio Lp / Lw between the pitch of the radiant tube and the distance between the workpieces.
It is a graph which shows the change for every w.

FIG. 8 is a graph showing a change in a temperature rise rate of a work with respect to a heat flux ratio.

9A and 9B are diagrams illustrating an embodiment applied to an atmosphere furnace as one embodiment of the present invention. FIG. 9A is a schematic diagram of an atmosphere furnace,
(B) is an explanatory view showing a tube arrangement of the atmosphere furnace.

10A and 10B are diagrams comparing the temperature distributions of a conventional atmospheric furnace and an atmospheric furnace to which the present invention is applied, wherein FIG. 10A is a conventional atmospheric furnace and FIG. 10B is a diagram of an atmospheric furnace to which the present invention is applied; Are respectively shown.

[Explanation of symbols]

 1 Furnace body 2 Radiant tube burner system 5 Radiant tube 6 Burner Lp Pitch between radiant tubes Lw Distance from tube to work

Claims (2)

[Claims] 1. A radiant equipped with a heat storage burner system for alternately discharging exhaust gas and supplying combustion air through both ends of a radiant tube through a heat storage body, and equipped with a heat storage burner system for alternately burning the pair of burners. In a heating furnace using a tube as a heating source, the pitch Lp between the radiant tubes and the distance Lw from the tube to the work are set.
Wherein the ratio Lp / Lw is 2 or less, and wherein the radiant tube is disposed in the furnace. 2. The ratio Lp / Lw is 0.75 <Lp / Lw.
The heating furnace using the radiant tube as a heat source according to claim 1, wherein the heating furnace is in a range of <1.75.