JPH0637326B2 - Heat transfer member for heat exchanger - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat transfer member of a heat exchanger.

2. Description of the Related Art From the viewpoint of energy saving, heat exchangers are widely used for the purposes of waste heat recovery and preheating of combustion air in power plants, industrial furnaces, iron metallurgy, chemical plants, etc.
As a method of these heat exchangers, a partition wall heat transfer type (sheel
As a type), a high temperature gas on the primary side such as combustion gas is made to flow outside the tube, and the gas on the secondary side that is made to flow to the inside by heat transfer through the tube wall is heated, as a heat storage type (plate-fin type, etc.) A method is known in which high temperature gas is passed through a rotor having a honeycomb structure to store heat, and then a low temperature gas on the secondary side is caused to flow to release heat. Of these methods, the former partition wall heat transfer method is generally used in a relatively large heat exchanger, and the latter heat storage method is used in a small heat exchanger.

The heat transfer member of the heat exchanger is a material used for a heat transfer part from a high temperature gas to a low temperature gas in these heat exchangers.

Currently, heat-resistant alloys are mainly used as the material for the heat transfer member of this heat exchanger. However, heat-resistant alloys do not have sufficient heat resistance, and the temperature of the high temperature gas on the primary side that can normally be used is 800 ° C. Up to the limit, when using a gas having a temperature higher than this, cooling air or the like is introduced to cool the gas to 800 ° C. or lower for heat exchange. The temperature of the gas on the secondary side thus obtained is limited to about 500 ° C. Therefore, in order to enable the use of higher temperature gas and perform efficient heat exchange, it is required to develop a material that can withstand a temperature of 1000 ° C. or higher for a long time, and research on ceramic heat transfer members is required. It is being advanced.

Such a heat transfer member is required to have excellent heat resistance as well as excellent properties such as chemical stability, corrosion resistance, strength, thermal shock resistance, and abrasion resistance, and for a long period of time. It is desired to have durability. The ceramic member has excellent durability especially against high temperature and corrosive gas, and the partition wall heat transfer type is SiC, Si ₃ N.
No. _4, etc., in the heat storage type, cordierite, lithia ceramics such as sponge Yumen, aluminum titanate and the like are used.

Of these, SiC is excellent in high-temperature strength, heat transfer property, corrosion resistance, etc., but it is apt to cause gas leaks in the partition walls, and it is difficult to make the wall thin, and chemical stability and heat resistance against high-temperature combustion gas are high. It has drawbacks in impact and is inferior in economic efficiency.

Si ₃ N ₄ is superior to SiC in thermal shock resistance, but is inferior in high temperature strength, chemical stability and the like. Further, since both SiC and Si ₃ N ₄ are made of non-oxides, they have poor oxidation resistance in an open atmosphere or an oxidizing atmosphere at a high temperature of about 1200 ° C. or higher, and are difficult to be used for a long period of time due to oxidative decomposition. There is a drawback that.

Cordierite, lithia-based ceramics, aluminum titanate, etc. used in heat storage type heat exchangers have low thermal expansion and are excellent in oxidation resistance, but have poor heat resistance, and cordierite is approximately 1200 ° C. System ceramics is about 1000 ℃, aluminum titanate is about 1200
It can only be used up to ℃, and it is not satisfactory in corrosion resistance, high temperature stability, high temperature strength, wear resistance, etc.

As described above, the currently known ceramic heat transfer members are not sufficiently satisfactory for use at high temperatures. Therefore, the temperature is 1000 ° C. or higher, more preferably 12 ° C. or higher.
It is desired to develop a heat transfer member that can be stably used for a long period of time at a temperature of 00 ° C or higher.

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to find a heat transfer member for a heat exchanger that can withstand use at high temperatures for a long time. As a result, the heat transfer member composed of the mullite sintered body that satisfies the specific conditions can withstand repeated use for a long period of time even when a high temperature gas of 1000 ° C. or higher, further 1200 ° C. or higher is used as the heat source. It was found that there is, and the present invention was completed here.

That is, the present invention comprises i) a mullite crystal, and SiO ₂
It consists of a crystal phase containing no crystals and a glass matrix phase of 5% by volume or less, and ii) has a coefficient of thermal expansion of 5.1 × 10 5.
⁻⁶ / ° C. or less, bulk density of 3.0 g / cm ³ or more, 145
The residual expansion coefficient at room temperature after heat treatment at 0 ° C for 48 hours is 0.2.
% Or less, the present invention relates to a heat transfer member of a heat exchanger made of a mullite sintered body.

The heat transfer member of the present invention is made of a mullite sintered body that satisfies the following conditions.

a) The crystal phase in the sintered body is composed of mullite crystals,
Does not contain SiO ₂ crystals.

In the binary system of Al ₂ O ₃ and SiO ₂ , the crystal phases precipitated under normal pressure are Al ₂ O ₃ crystals, mullite crystals and S.
It is an iO ₂ crystal (mainly cristobalite crystal). Here, the mullite crystal has a chemical formula of 3Al ₂ O ₃ .2SiO.
₂ (Al ₂ O ₃ 71.8% by weight, SiO ₂ 28.2% by weight) as well as a mullite solid solution are included.

The crystal phase in the sintered body constituting the heat transfer member of the present invention,
Diffraction peaks of SiO ₂ crystal and Al ₂ O ₃ crystal should not be observed by powder X-ray diffraction of a sintered body, which consists of mullite crystal only.

When SiO ₂ crystals are precipitated in the sintered body, a liquid phase is formed during the firing process, which is effective for improving the sinterability and strength, and at high temperature, the toughness is improved by the plastic deformation of the liquid phase. However, there are the following drawbacks.

That is, due to the difference in the coefficient of thermal expansion between the SiO ₂ crystal and the mullite crystal, the strain increases in the thermal cycle, and the volatile components generated from the surrounding wall material contained in the high temperature gas and the alkali elements, sulfur, and vanadium generated from the fuel. , Ash content is SiO
₂ react with, denature the SiO ₂ crystals, and that poor durability of the heat transfer member. In addition, the presence of SiO ₂ crystals is not preferable in order to reduce the thermal expansion coefficient of the sintered body.
Therefore, in the heat transfer member of the present invention, the SiO ₂ crystal phase should not exist.

Further, when Al ₂ O ₃ crystals are present, the thermal conductivity increases, but the alkali element from the fuel or wall material reacts with the Al ₂ O ₃ crystals at high temperature to form β-Al ₂ O ₃ crystals. Then, the structure of the sintered body is deteriorated and deteriorated. Also, Al
There is also a problem that the thermal shock resistance is lowered due to an increase in strain due to a difference in thermal expansion between the ₂ O ₃ crystal and the mullite crystal, an increase in residual expansion, and the like. Therefore, in the heat transfer member of the present invention, Al
_{No 2} O ₃ crystals should also be present.

Incidentally, the mullite crystal is preferable in the case of an acicular crystal having an aspect ratio of about 2 or more because it has excellent thermal characteristics and mechanical characteristics as compared with the granular crystal.

b) The glass matrix phase in the sintered body is 5% by volume or less.

The content of the glass matrix phase is measured by the following method.

That is, first, a plate-shaped sample having a thickness of 1 mm or more is cut out from an arbitrary portion of the heat transfer member, the surface thereof is roughly finished with a diamond grindstone, and then the surface of the plate is intermediately finished with a grindstone of 800 or more. Subsequently, diamond grains or red iron oxide of 3 μm or less,
After finishing with a fine powder of chromium oxide or the like until it becomes a mirror surface, the adhered substances on the surface are removed to obtain a measurement sample. After forming a vapor-deposited film on the surface of this sample according to a conventional method, the sample surface is photographed at 3000 to 5000 times with a scanning electron microscope. This micrograph is referred to as Photo-I. Then, the vapor deposition film is removed from the sample surface, and 0 to 5 is added to the HF 1% aqueous solution.
After soaking at 24 ° C. for 24 hours, it is washed and dried, and then a micrograph is taken in the same manner as described above. This micrograph is designated as Photo-II. Photos-I and II are photographs of the same portion of the sample having an area of at least 1000 μm ² or more.

From Photo-I, bubbles, cracks, etc. on the surface of the sample are observed to be concave. The area ratio of the concave portion is calculated by obtaining the area of the portion observed as the concave shape. From Photo-II, bubbles,
In addition to the cracks, the glass matrix parts removed by the HF treatment are also observed to be concave. The difference between the area ratio of the recesses calculated from Photo-II and the area ratio of the recesses calculated from Photo-I is defined as the surface area ratio of the glass matrix phase in the sample. Since the glass matrix phase exists substantially uniformly throughout the sintered body, the area ratio of the glass matrix phase obtained by the above method can be defined as the volume% of the glass matrix phase in the sintered body. The glass matrix phase obtained by such a method is formed by the reaction of the alkali metal oxide mixed in the raw material with Al ₂ O ₃ and SiO ₂ to form a low melt.

When the glass matrix phase exceeds 5% by volume, the high temperature strength decreases due to the difference in thermal expansion between the mullite crystal and the glass matrix phase, and the glass matrix phase contains alkali elements, sulfur, and sulfur from fuel and wall materials. Components such as vanadium preferentially permeate and expand the structure, resulting in a decrease in strength due to heat cycles, resulting in poor durability, which is not preferable. The glass matrix phase is more preferably 3% or less.

c) The coefficient of thermal expansion is 5.1 × 10 ⁻⁶ / ° C. or less, and the residual expansion coefficient at room temperature after heat treatment at 1500 ° C. for 48 hours is 0.2% or less.

In the present invention, the coefficient of thermal expansion is represented by the average coefficient of linear expansion when the temperature is raised from room temperature to 800 ° C. at 5 ° C./min.

The expansion coefficient of the mullite sintered body varies depending on the components dissolved in the mullite crystals, the components of the glass matrix phase, the amount, and the like. The heat transfer member of the present invention is limited to one having a thermal expansion coefficient of 5.1 × 10 ⁻⁶ / ° C. or less, and more preferably 4.9 × 10 ⁻⁶ / ° C. or less. When the coefficient of thermal expansion exceeds 5.1 × 10 ⁻⁶ / ° C., the difference in thermal expansion between the primary side wall and the secondary side wall surface of the heat transfer member becomes large due to the thermal gradient between the primary side gas and the secondary side gas. As a result, a large stress is generated, which may lead to damage, which is unsuitable.

The residual expansion rate is measured by the following method. That is, first, a rod-shaped sample whose both end faces were ground and polished in parallel was 1450 in an electric furnace.
The sample is heated to 0 ° C., kept at 1450 ° C. for 48 hours, then gradually cooled, and the ratio of the elongation remaining after the heat treatment is calculated from the length of the sample measured at room temperature before and after the heat treatment to obtain the residual expansion coefficient.
When the residual expansion coefficient exceeds 0.2%, both ends of the heat transfer member of the partition wall heat transfer type exchanger are fixed, so mechanical stress is applied during use, causing damage, deformation, etc. Therefore, the durability against heat cycle becomes poor. In addition, since the sealing of the secondary gas introduction / exhaust portion is inferior, a gas leak may occur, which is not suitable. Further, also in the heat storage type exchanger, if the residual expansion coefficient exceeds 0.2%, the thermal shock resistance, the corrosion resistance, the wear resistance, etc. are deteriorated. Therefore, it is necessary that the residual expansion coefficient is 0.2% or less, and preferably 0.1% or less.

d) The bulk density is 3.0 g / cm ³ or more.

When the bulk density is less than 3.0 g / cm ³ , the mechanical strength is lowered, and the alkaline components contained in the combustion gas are easily adsorbed on the heat transfer member, and the alkaline components react with the members to cause the members to react. Markedly deteriorates, resulting in poor durability.
Therefore, the bulk density of the heat transfer member needs to be 3.0 g / cm ³ or more, and preferably 3.05 g / cm ³
That is all.

The heat transfer member of the present invention can be produced, for example, by the method described below.

That is, first, a liquid raw material is prepared by mixing an aluminum sol, an aluminum compound such as aluminum chloride, a sulfate or a nitrate with a silicon compound such as silica sol or ethyl silicate so as to have a predetermined Al / Si ratio. It is economically preferable to increase the concentration of the liquid raw material, but in order to easily disperse both components evenly and form mullite crystals, in the case of a sol solution, 30% or less, and in the case of a salt solution, Is appropriately about 2 mol% or less.

In order to reduce the thermal expansion coefficient of the sintered body, it is appropriate that the Al / Si ratio in the liquid raw material be such that the amount of Al ₂ O ₃ is below the solid solution limit in mullite.
Al ₂ O ₃ is approximately 69 to 74% by weight, and SiO ₂ is approximately 26 to
The ratio may be 31% by weight. In addition, Al ₂ O ₃
The total amount of SiO ₂ and SiO ₂ is preferably 98% by weight or more in the sintered body. In addition, in order to reduce the coefficient of thermal expansion, it is effective to allow a small amount of MgO, B ₂ O ₃ or the like to be present in the raw material.
A small amount may be added.

After the liquid raw material is prepared, the liquid raw material is thoroughly mixed so that the Al content and the Si content are uniformly dispersed, and a powder in which an aluminum compound and a silicon compound are uniformly mixed is formed from the liquid raw material. As a method for obtaining a powder sample from a liquid raw material, a method of uniformly coprecipitating an aluminum compound and a silicon compound and then drying, a method of evaporating water from the liquid raw material to obtain a powder sample, and a method of spraying the liquid raw material A thermal decomposition method or the like can be exemplified.

The powder sample thus obtained is roasted at 900 to 1350 ° C., preferably 980 to 1280 ° C. in order to reduce the dimensional change in the firing step after molding. When a large amount of unreacted SiO ₂ , Al ₂ O ₃ , or an amorphous phase is present in the powder sample after roasting, aggregation or separation of the powder sample easily occurs in the subsequent steps, which is preferable. Absent. Further, in order to reduce the residual expansion of the sintered body, it is preferable to make the powder sample mullite in the roasting step. For this reason, it is appropriate that the roasting conditions are such that the mullite of the powder sample proceeds, and specifically, the X-ray diffraction peak of mullite is generated in the powder sample after roasting. It is preferable to roast in (1) and roast to such an extent that a diffraction peak of SiO ₂ or Al ₂ O ₃ does not occur.

Then, the obtained powder sample is pulverized and dispersed. By pulverization, the average particle size of the powder is about 2 μm or less, and the specific surface area is 1 m ^2.
/ G or more is preferable. Average particle size is 2 μm
If it exceeds the range, defects are likely to occur inside the molded body during the molding and firing of the powder, and if the specific surface area falls below 1 m ² / g, the sinterability tends to be poor, such being undesirable. In addition, by pulverizing the powder in the pulverizing step, a sintered body having a high density can be obtained. For this purpose, the average particle size of the powder is 1μ
More preferably, it is m or less. The pulverization and dispersion of the powder may be carried out according to a conventional method, for example, a ball mill, a vibration mill,
An attrition mill, a centrifugal mill or the like may be used.

Then, by using the powder thus prepared, according to a conventional method in the production of ceramics, after molding into a predetermined shape by a method such as cast molding, extrusion molding, and press molding, the present invention A member for heat transfer is obtained.
The firing temperature is usually about 1550 to 1750 ° C, preferably about 1600 to 1650 ° C under normal pressure. Generally, as the firing temperature increases, the glass matrix phase tends to increase and the density tends to increase. On the other hand, when the firing temperature is lowered, on the contrary, the glass matrix phase tends to decrease and the density tends to decrease. Further, in order to reduce the residual expansion, it is preferable to perform firing at a rather low temperature for a long time without raising the firing temperature too high.
Further, in order to increase the aspect ratio, it is preferable that the firing temperature is not too high. Therefore, a specific firing temperature may be appropriately determined so that a sintered body satisfying the above conditions a) to d) can be obtained according to the raw material composition, the molding conditions, and the like. Also, the firing time may be appropriately determined so as to satisfy the above conditions a) to d) depending on the firing temperature, the raw material composition, the molding conditions, and the like.

In the heat transfer member of the present invention, in order to reduce the existing amount of the glass matrix phase and to reduce the thermal expansion coefficient, N
The content of alkaline substances such as a and K is 0.2 in terms of oxide.
It is preferably not more than 0.1% by weight, more preferably not more than 0.1% by weight. Therefore, it is preferable to use a raw material having a low content of impurities or to carry out dealkalization treatment in the step of preparing the powder sample. Further, in order to reduce the coefficient of thermal expansion, it is preferable to prevent CaO from being mixed in the raw material.

In the present invention, by preparing a powder sample from the liquid raw material by the above-mentioned method, a powder having excellent sinterability in which Al and Si are evenly mixed to a minute portion can be obtained. Therefore, like the heat transfer member of the present invention, it is possible to obtain a mullite sintered body having high strength even when the glass matrix phase and the amount of impurities are small and having excellent thermal shock resistance and the like.

EFFECTS OF THE INVENTION The heat transfer member of the present invention is made of a mullite sintered body having excellent heat resistance, thermal shock resistance, thermal stability, etc., and high strength. Have characteristics.

1. Even at a high temperature of 1200 ° C. or higher, it has excellent mechanical strength, corrosion resistance, etc. Therefore, when it is used as a heat transfer member, it can withstand use even if the wall thickness is thin. Therefore, the heat efficiency and the weight of the heat exchanger can be improved.

2. It has excellent thermal shock resistance, and can set a high temperature difference when introducing the primary gas and the secondary gas.

3. It has good thermal stability against heat cycles and excellent durability.

4. It has high durability against corrosive components and dust contained in volatile components from combustion gas and wall materials.

As described above, the heat transfer member of the present invention has excellent properties and can be stably used for a long period of time as a heat transfer member of a heat exchanger using a primary side gas at a temperature of 1000 ° C. or more and 1200 ° C. In particular, it is useful as a heat transfer member of a partition wall heat transfer type heat exchanger. Further, the heat transfer member of the present invention can be used as a transportation ceramic roller, a furnace tube, a sagger, a setter, a burner nozzle, etc., which are required to have similar properties.

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples below.

Examples 1 and 2 and Comparative Examples 1 to 4 Solutions in which 0.5 mol% aluminum chloride solution and 20% silica sol solution were blended so as to have a ratio of Al ₂ O ₃ and SiO ₂ shown in Table 1. Was prepared. Then, this solution was thoroughly mixed so as to be homogeneous, then neutralized and coprecipitated with aqueous ammonia, and the precipitate was dried and roasted at 1250 ° C. to obtain mullite powder. However, in Example 2, the content of MgO should be 0.5% so that MgO is contained in the powder.
(NO _{3) 2} solution was added to the liquid sample to prepare a powder. Further, in Comparative Example 3, alumina crystals remained in the powder after roasting.

Next, the above powder was wet-milled for 24 hours by a ball mill and dispersed to obtain a raw material powder. After adding 2% PVA to this raw material powder, a molding pressure of 2 tons /
It was molded into a tube shape having an outer diameter of 20 mm, an inner diameter of 16 mm and a length of 600 mm in cm ² , and was fired at each temperature shown in Table 2 for about 3 hours to obtain a mullite sintered body.

Table 2 shows the volume ratio of the mullite crystals and the alumina crystals in the crystal phase of the sintered body, the volume ratio of the glass matrix phase in the sintered body, the coefficient of thermal expansion, the residual expansion coefficient and the bulk density.

Test 1 In the flue of a kiln in which the tubes of Examples 1 and 2 and Comparative Examples 1 to 4 were fueled with LPG, the temperature of the combustion gas was 135.
It was installed in each part at 0 ° C., 1300 ° C. and 1100 ° C. so as to be perpendicular to the gas flow. After each tube has reached its respective gas temperature, 50 from one tube into the tube.
C. Air was introduced, the temperature was maintained for 5 hours, and then the mixture was allowed to cool. This operation required 48 hours including heating and cooling. When this operation was repeated, the number of repetitions was calculated until the tube was damaged, cracked, or the like to cause a gas leak and the tube could not be reused. Also, the bulk density 2.
A tube made of 6 g / cm ³ of SiC and a bulk density of 3.
The same test was performed on a tube made of 1 g / cm ³ of Si ₃ N ₄ . The results are shown in Table 3.

Test 2 Vanadium was added to 500 ppm of vanadium as test pieces obtained by cutting the tubes of Examples 1 and 2 and Comparative Examples 1 to 4 into 5 mm long pieces.
, Heated with combustion gas of kerosene containing 50ppm of sodium, 50 at each temperature of 1100 ℃ and 1450 ℃.
The time was kept and a corrosion resistance test was conducted. Table 4 shows the results of measuring the bending strength of the circular ring sample before and after the corrosion resistance test at room temperature.

Test 3 Outer diameter 2 mm in the same manner as in Examples 1-2 and Comparative Examples 1-4
A spherical heat transfer member was produced. This spherical heat transfer member was filled in a pipe having an inner diameter of 80 mm to a height of 10 cm, and both ends thereof were covered with a wire mesh of a heat resistant alloy. Then, a high temperature gas was blown from one end of the pipe with a burner and heated for 3 minutes to bring the temperature of the central portion of the pipe to 1100 ° C.
Then, the operation of blowing air at 30 ° C. into the pipe in the same direction as the high-temperature gas was blown for 3 minutes was repeated 100 times, and then the heat transfer member was taken out and examined for the occurrence of cracks.

As a result, in the heat transfer member of Example 1, it was recognized that cracks were contained in the sphere in the portion 4 to 6 mm from the pipe inlet, and in the heat transfer member of Example 2, no crack was generated. . In addition, when air at 30 ° C. was introduced into the pipe from the direction opposite to the direction in which the high temperature gas was introduced, Example 1
No cracking was observed in the heat transfer members of Nos. 2 and 2. On the other hand, in the heat transfer members of Comparative Example 1 and Comparative Example 4,
When air was introduced from the same direction as the high temperature gas, a large number of cracks were found in the sphere in the portion of about 4 to 5 cm from the inlet of the pipe, and for heat transfer in Comparative Examples 2 and 3. As for the members, many balls with cracks were found in the sphere in the part up to about 2 cm from the entrance of the pipe. Also, in Comparative Examples 1 to 4, cracking was observed in the balls even when the air was introduced into the pipe from the opposite direction.

From the above test results, the heat transfer member of the present invention is 1000 ° C.
It is clear that it can withstand repeated use even when the above high-temperature gas is used as a heat source, and that there is little reduction in strength due to combustion gas containing corrosive components such as vanadium and sodium.

Claims (1)

[Claims] 1. A mullite crystal, a crystal phase containing no SiO ₂ crystals and a glass matrix phase of 5% by volume or less, and ii) a coefficient of thermal expansion of 5.1 × 10 ⁻⁶ / ° C. or less and a bulk density. Is 3.0 g / cm ³ or more, and the residual expansion coefficient at room temperature after heat treatment at 1450 ° C. for 48 hours is 0.2% or less, which is a mullite sintered body.