JP2000018850A - Heating tube for heat exchanger - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance heat transmission efficiency by employing ceramic heating tubes for a heat exchanger exchanging heat on the inside and outside and forming a plurality of fins on the inner surface thereof and to save the space by decreasing the ratio of tube length/thickness thereby shortening the length of the ceramic tube. SOLUTION: A heating tube 1 comprises a ceramic tube provided, on the inner surface thereof, with a large number of fins 2 integrally. Outer surface of the heating tube 1 is exposed to high temperature gas and gas to be heated is passed on the inside thus exchanging heat. Consequently, heat transfer area of each fin 2 is increased and thermal efficiency can be enhanced. Alternatively, a curved face sealing part may formed continuously to one side of the heating tube 1 and fed with the gas to be heated from an inner tube 4 through the air gap between the inner tube 4 and the heating tube 1. A hollow part 3 where the fin 2 is not present is formed in the center of the inner surface at the intersection of the fins 2, i.e., the sealing part, in order to prevent the gas to be heated from striking against the sealing part directly thus relaxing thermal stress on the inner and outer surfaces.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat exchanger tube for a heat exchanger used in a high-temperature waste heat utilization system in an incineration / power generation plant or the like.

[0002]

2. Description of the Related Art In order to effectively use the thermal energy of high-temperature waste gas generated in an incineration / power generation plant and to increase the energy efficiency of the entire plant, a high-temperature gas such as the generated waste gas flows through a partition wall through a partition wall. A heat exchanger of a type for heating a gas to be heated has been used.

Specifically, a method is generally used in which a heat transfer tube is arranged as a partition in a space where a high-temperature gas is generated or passes, and a gas to be heated is sent into the tube, and the gas to be heated is heated by heat transfer. It is.

Conventionally, metals have been used for heat transfer tubes in relatively low temperature heat exchange systems.
In addition to the high temperature of 000 ° C. or higher and the high corrosiveness of the gas itself, metals are not suitable, and ceramics have begun to be used instead.

[0005]

By the way, in order to increase the heat transfer efficiency of the heat exchanger tube for a heat exchanger, it is necessary to use a heat exchanger tube having a very high ratio of tube length / wall thickness. However, even if the wall thickness is reduced, there is a limit in terms of corrosion resistance and strength. Therefore, in practice, the length of the heat transfer tube is increased in order to lengthen the gas flow path. Alternatively, a method of increasing the number of heat transfer tubes by reducing the supply speed of the fluid to be heated has been considered, but in any case, a space for the heat transfer tube is required, which hinders space saving of the equipment.

Further, when the ratio of the length / wall thickness of the heat transfer tube is very high, there is a problem that the mechanical strength and the creep resistance are low, the handling property is poor, and the heat transfer tube is deformed by use for a long time.

Further, the heat transfer tube is formed into a one-sided sealed shape, an inner tube is inserted thereinto, a gas to be heated is supplied to the heat transfer tube sealing portion through the inner tube, and the gas is passed through the gap between the inner tube and the heat transfer tube. In a heat transfer tube for a heat exchanger of a heating type, the outside of the sealed portion is at a high temperature of 1000 ° C. or more, and a low-temperature heated gas is blown from the inside, so that thermal stress is very likely to be applied. Another problem is that it is easily damaged.

[0008]

SUMMARY OF THE INVENTION The present invention provides a heat exchanger tube for a heat exchanger in which heat is exchanged inside and outside, wherein the heat exchanger tube is formed of ceramic and a plurality of fins are integrally formed on the inner surface thereof. It is assumed that.

Thus, the surface area of the inner surface of the heat transfer tube can be increased, so that the heat transfer efficiency can be increased. Because of this,
Compared with the conventional heat transfer tube, the ratio of tube length / wall thickness can be reduced, and the length of the ceramic tube can be substantially reduced, thereby contributing to space saving.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

A heat transfer tube 1 shown in FIG. 1 is a cylindrical body made of ceramics, and has a large number of fins 2 integrally formed on the inner surface thereof. If the outer surface of the heat transfer tube 1 is exposed to a high-temperature gas and the gas to be heated is passed inside, heat exchange can be performed via the heat transfer tube 1 and the gas to be heated can be heated. At this time, since the fins 2 are provided, the heat transfer area can be increased and the thermal efficiency can be increased.

As another embodiment, as shown in FIGS. 2 and 3, one side of the heat transfer tube 1 can be formed into a shape in which a curved sealing portion 1a that is smoothly continuous with the side surface is formed. When this heat transfer tube 1 is used, as shown in FIG. 3, an inner tube 4 is inserted thereinto, and a gas to be heated is supplied to the sealing portion 1a of the heat transfer tube 1 through the inner tube 4, so that the inner tube 4 and the heat transfer tube Heating while passing through the gap 1

Here, if the number of the fins 2 is one, not only a sufficient effect of improving the heat transfer characteristics cannot be expected, but also the heat transfer tubes 1
Cannot balance the weight in the axial direction. Conversely, if the number of the fins 2 is too large, the weight of the tube will increase, and the gap inside the heat transfer tube 1 will be narrowed, and the flow velocity will be too fast.
Sufficient heat transfer characteristics cannot be expected. For this reason, the number of the fins 2 is preferably between 6 and 24.

The thickness of the fins 2 is preferably equal to or less than the thickness of the heat transfer tube 1, and the height of the fins 2 is preferably equal to or greater than the thickness of the fins 2 so as not to come into contact with other fins 2 or the inner tube 4. . This is because if the fins 2 are too thick, the thermal shock characteristics deteriorate, while if they are too thin, the strength cannot be maintained. Also,
If the height of the fins 2 is too low, it is difficult to improve the heat transfer efficiency, and if it is too high, the fins 2 come into contact with other members.

By forming the fins 2 in this manner, the mechanical strength of the heat transfer tube 1 per unit length can be improved, and the length of the heat transfer tube 1 can be reduced as described above. 1 can improve handling properties and creep resistance.

Preferably, the fins 2 become gradually thinner toward the tip end, so that the fins 2 are connected to the inner surface of the heat transfer tube 1 at the connecting portions 2.
If a curved surface having a radius of curvature of 0.3 to 3 mm is formed on the end b of the fin 2 or the fin 2, it is possible to prevent an increase in weight and a decrease in thermal shock resistance and to reduce a factor of a decrease in strength.

Further, in the heat transfer tube 1 having the structure shown in FIGS. 2 and 3, a hollow portion 3 having no fins 2 is formed at the center of the inner surface of the sealing portion 1a which is an intersection of the fins 2. This makes it difficult for the air in the hollow portion 3 to flow when used as a heat exchanger.
And a temperature boundary layer between the external high-temperature gas and the gas to be heated supplied from the inner tube 4, thereby preventing the gas to be heated from directly hitting the sealing portion 1 a. As a result, the thermal stress on the inner and outer surfaces of the heat transfer tube 1 can be reduced, and the life can be extended.

Various ceramics such as silicon carbide and silicon nitride can be used as the ceramics constituting the heat transfer tube 1, and silicon carbide ceramics are particularly preferable. This silicon carbide ceramic is 90
It contains silicon carbide (SiC) in an amount of at least% by weight and contains B, C, etc. as a sintering aid. Such a silicon carbide ceramic has excellent properties such as a thermal conductivity of 50 W / m · K or more and a thermal shock resistance ΔT of 300 ° C. or more.
It is optimal as a material for the heat transfer tube 1.

The heat transfer tube 1 of the present invention is formed integrally with the above-mentioned ceramic material by extrusion molding into the shape shown in FIGS. 1 and 2, and the obtained molded body is placed in a vacuum atmosphere at 1900 to 1900. It can be obtained by firing at 2300 ° C. In this way, the heat transfer tube 1 integrally provided with the fins 2 can be easily obtained.

[0020]

EXAMPLES Specific examples of the present invention will be described.

First, a binder is added to a ceramic raw material powder having a composition containing SiC as a main component and containing B, C, etc. to form a clay. After kneading this well, it is molded by an extruder using a mold having a desired sectional shape.

The mold comprises three dies, a core pin and a die head. The tip of the core pin has a hemispherical surface or a smooth curved surface similar to it to form the inner surface shape of the sealed part of the heat transfer tube molded body, so that fins are formed on the surface of the molded body. It has a shape with a deep groove. The die has a circular shape so that the outer surface of the heat transfer tube molding is formed, and the inner surface of the die head has a hemispherical shape or a smooth curved surface equivalent to it to form the outer surface of the sealing portion of the heat transfer tube molding. I have.

At the start of extrusion molding, a die head is fixed to the die, and the molded body is filled between the die, the die head and the core pin. Thereafter, the die head is removed, and extrusion molding is advanced to obtain a heat transfer tube molded body provided with fins 2 on the inner surface and sealed on one side as shown in FIG.

After the compact obtained by the above-mentioned method was sufficiently dried, it was fired under optimum conditions to obtain a sintered body.

Example 1 A single-side sealed heat transfer tube 1 having an outer diameter of 72 mm, an inner diameter of 64 mm, a length of 1100 mm and a length of 800 mm, in which a plurality of fins having a height of 12 mm were formed on the inner side, was produced by the above-described manufacturing method.

Then, the heat transfer tube 1 was inserted into a test device as shown in FIG. Here, the effective length indicates the length that the heat transfer tube 1 can contribute to heat exchange. Further, an inner tube 4 having an outer diameter of 36 mm and an inner diameter of 30 mm and open at both ends was inserted into the heat transfer tube 1 until the distance from the sealed end of the heat transfer tube 1 to the end surface of the inner tube 4 became 100 mm.

Then, the inside of the test apparatus was heated to 1200 ° C., and air heated to 200 ° C. was supplied into the inner tube 4. And in Table 1, No. 5 at the open end side in one heat transfer tube 1
The flow rate of the supplied air was adjusted to be 00 ° C., and the other heat transfer tubes 1 were fixed so that the flow rate could be evaluated at a constant flow rate.
Arrows in FIG. 4 indicate the direction of air flow.

Then, at a constant flow rate, as shown in Table 1, the temperature of the air obtained under the conditions in which the effective length of the heat transfer tube 1 and the number of fins were changed was measured.

As shown in Table 1, it was confirmed that by forming the fins 2, the air temperature could be increased and the heat exchange efficiency could be increased. For example, no. 1 and No. Comparing No. 7, by forming 12 fins, it was possible to obtain a fluid having an effective length of about 70% and a temperature equal to or higher than that of a heat transfer tube without fins.

However, in the heat transfer tube 1 having the 24 fins 2, the flow rate of the gas to be heated becomes too high due to the decrease in the gap between the heat transfer tube 1 and the inner tube 4, and the heat transfer due to the increase in the surface area of the heat transfer tube. It is considered that the improvement effect could not catch up and the obtained air temperature was lower than the heat transfer tube 1 having 12 fins.

[0031]

[Table 1]

Experimental Example 2 The above-mentioned heat transfer tube 1 was dropped freely on concrete from a height of 0.5 m to check for breakage. Table 2 shows the results. The number of test pieces is 7, and No in the table indicates N in Experimental Example 1.
It corresponds to o.

It should be noted that the number of broken pipes that could not be used completely as heat transfer tubes was investigated without including minute damage such as chipping of the end face.

As shown in Table 2, the results are as shown in FIG. No. 1 heat transfer tube and No. 1 The heat transfer tube of No. 7 shows the same heat transfer performance in Experimental Example 1, but compared to the former case where six tubes were broken,
The latter has decreased to three. It is considered that the overall length can be shortened by forming the fins, and that the fins themselves also increase the strength of the heat transfer tube, which has led to an improvement in handling.

[0035]

[Table 2]

EXPERIMENTAL EXAMPLE 3 A heat transfer tube having the same material and dimensions as above and having 12 fins with a height of 12 mm formed inside and open at both ends was manufactured.

Then, 50% from both ends in a reducing atmosphere firing furnace.
mm, and heat-treated at 1400 ° C. × 100 hours. The warpage of each of the two samples was measured, and the average was determined.

As shown in Table 3, it was confirmed that the formation of the fins improved the creep resistance.

[0039]

[Table 3]

Experimental Example 4 Using the same heat transfer tube as in Experimental Example 1, the tube was inserted into a test device as shown in FIG. 4, and an inner tube having an outer diameter of 36 mm and an inner diameter of 30 mm was inserted from the sealed end of the heat transfer tube to the inner tube. The distance to the end face is 50mm
Inserted up to the position.

Thereafter, the inside of the test apparatus was heated to 1400 ° C.
A heat cycle test was conducted for 60 minutes in which room temperature air was intermittently supplied from the inner tube for 60 minutes, and the number of heat transfer tubes in which damage such as cracks occurred among the five test samples was investigated.

As shown in Table 4, the heat transfer tubes with fins were not formed. The number of breaks is smaller than the heat transfer tube of No. 1. The central part of the sealing end, which is the intersection of the fins, is hollowed out, creating a temperature boundary layer around the hollowed out part and the intersection of the inner surface of the sealing part, reducing the thermal stress on the inner and outer surfaces and improving the life I found out.

[0043]

[Table 4]

EXPERIMENTAL EXAMPLE 5 A plurality of ceramics shown in Table 5 were used to obtain a height 12 inside.
A one-side sealed heat transfer tube similar to that of Experimental Example 1 in which twelve mm fins were formed was produced. Then, the heat transfer tube was inserted into a test device as shown in FIG. 4, and an inner tube was further inserted inside the heat transfer tube as in Experimental Example 1.

Then, the inside of the test apparatus was heated to 1200 ° C., and air heated to 200 ° C. was supplied into the inner tube.
In Table 1, No. The flow rate of the supplied air was adjusted to 500 ° C. at the open end side of the heat transfer tube in the silicon carbide heat transfer tube of No. 1 and the other heat transfer tubes were fixed so that the flow rate could be evaluated at a constant flow rate. And the temperature of the air obtained by the heat transfer tube of each material was measured.

Table 5 shows the results. At the same time, the measurement results of the thermal conductivity and thermal shock resistance of each material are also shown. Among the materials evaluated this time, silicon carbide having the highest thermal conductivity exhibited the highest air temperature. In addition, alumina and zirconia could not be measured because the tube was damaged. In this experiment, it was found that silicon carbide was the most excellent heat transfer tube material.

[0047]

[Table 5]

[0048]

As described above, by forming the fins inside the heat transfer tube, the heat transfer characteristics can be improved and the space saving can be promoted. Furthermore, since the creep resistance and the handling properties can be improved, the service life can be extended.

In the heat transfer tube with one side sealed, the sealing end is curved and the center of the inner surface of the sealing portion, which is the intersection of the fins, is hollowed out, so that the sealing portion is damaged by thermal shock. Can be suppressed.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a heat exchanger tube for a heat exchanger of the present invention.

FIG. 2 is a sectional view of another embodiment of the present invention.

FIG. 3 is a diagram showing a use state of the heat transfer tube of FIG. 2;

FIG. 4 is a schematic diagram of a test apparatus used in an experimental example.

[Explanation of symbols]

 1: heat transfer tube 2: fin 3: hollow part 4: inner tube

Claims (3)

[Claims] 1. A heat exchanger tube for a heat exchanger, wherein a heat exchanger tube for performing heat exchange inside and outside is formed of ceramics, and a plurality of fins are integrally formed on an inner surface thereof. 2. A heat-transfer tube according to claim 1, wherein one side of the heat transfer tube is formed with a curved sealing portion smoothly continuing to the side surface, and a hollow portion having no fin is provided at a central portion of the inner surface of the sealing portion. The heat exchanger tube for a heat exchanger according to claim 1, wherein 3. The heat exchanger tube for a heat exchanger according to claim 1, wherein the heat exchanger tube is made of a ceramic containing silicon carbide as a main component.