JP5742074B2 - Latent heat recovery type hot water generating device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a latent heat recovery type hot water generating device such as a heat exchanger incorporated in a latent heat recovery type hot water supply device, and a method for manufacturing the same.

  In the latent heat recovery type hot water supply apparatus, when latent heat is recovered from the combustion gas using a heat exchanger, condensed water is generated. Therefore, it is desired that the heat exchanger for recovering latent heat is made of a material having excellent corrosion resistance, and conventionally, the material is generally made of austenitic stainless steel having excellent corrosion resistance. . However, this type of stainless steel contains nickel (Ni), which is an austenite forming element, and is expensive. Therefore, it has been proposed to use ferritic stainless steel in place of the austenitic system (see Patent Documents 1 and 2).

However, in general, ferritic stainless steel is inferior in corrosion resistance compared to austenitic stainless steel. For example, when latent heat is recovered from a combustion gas containing sulfur and nitrogen components using a heat exchanger, strongly acidic condensed water of about PH3 is generated, and the heat exchanger converts the combustion gas into a relatively high temperature combustion gas. Be exposed. Under these conditions, the pitting resistance index (PRE: Pitting)
Resistance Equivalent = Cr% + 3.3 × Mo% + 16 × N%) is desirably 30 or more. However, such a ferritic stainless steel having a pitting resistance index is expensive because of its high chromium and molybdenum content. For this reason, conventionally, it has been difficult to reduce the raw material cost of the heat exchanger.

JP 2002-106970 A JP 7-292446 A

  The present invention has been conceived under the circumstances as described above, and can reduce the manufacturing cost by reducing the raw material cost of stainless steel, and also has a latent heat recovery type having excellent corrosion resistance. An object of the present invention is to provide an apparatus for generating hot water and a manufacturing method thereof.

  In order to solve the above problems, the present invention takes the following technical means.

The latent heat recovery type hot water generating device provided by the first aspect of the present invention includes a heat transfer tube, a case for housing the heat transfer tube, and a header connected to the heat transfer tube, and the heat transfer tube is a ferrite. is a system of stainless steel, a latent heat recovery type hot water generating device configured as a heat exchanger for latent heat recovery, as the material of the heat transfer tubes, containing aluminum and titanium concentration in the surface layer portion of 25% [ The material of the ferritic stainless steel is less than the cation atom%], and the heat transfer tube is subjected to a heat treatment for brazing the side wall of the case and the header, so that the surface layer portion thereof Is characterized in that an aluminum oxide film and a titanium oxide film having a titanium concentration of 25% [cation atom%] or more are formed .
Here, the “cation atom%” is an atomic concentration of titanium (cation) in all cations (excluding oxygen as an anion) contained in the ferritic stainless steel.
The manufacturing method of the apparatus for generating latent heat recovery hot water provided by the second aspect of the present invention includes a heat transfer tube, a case for housing the heat transfer tube, and a header connected to the heat transfer tube. The heat pipe is made of ferritic stainless steel, and is a method for producing a latent heat recovery type hot water generating device configured as a heat exchanger for recovering latent heat, which contains aluminum as a material for the heat transfer pipe, and A ferritic stainless steel member having a titanium concentration of less than 25% [cation atom%] in the surface layer portion is used, and the heat transfer tube is subjected to heat treatment for brazing the side wall of the case and the header. In the surface layer portion, an aluminum oxide film and a titanium oxide film having a titanium concentration of 25% [cation atom%] or more are formed.

According to such a configuration, as will be understood from the data to be described later, even if the heat transfer tube made of ferritic stainless steel is a material having a low pitting corrosion resistance index, the titanium oxide film formed on the surface layer portion Based on its presence, it exhibits sufficiently excellent corrosion resistance even for strongly acidic condensed water of about PH3, for example. As a result, it is possible to provide a latent heat recovery type hot water generating device having excellent corrosion resistance while reducing the raw material cost of the heat transfer tube and reducing the cost.
Further, a heat treatment step for forming a titanium oxide film and a brazing process for a predetermined member are performed simultaneously. Therefore, the operation is reasonable and more preferable in reducing the manufacturing cost of the latent heat recovery type hot water generating device.

In the present invention, preferably, pitting index of the heat transfer tube, Ru der less than 30.

  According to such a configuration, while using inexpensive stainless steel having a pitting corrosion index of less than 30 that is inherently unsuitable for latent heat recovery applications in which strongly acidic condensate is generated, for such applications. Since it will be able to respond appropriately, its benefits are great.

  Other features and advantages of the present invention will become more apparent from the following description of embodiments of the present invention with reference to the accompanying drawings.

An example of the heat exchanger to which this invention was applied is shown, (a) is a plane sectional view, (b) is a front sectional view. It is a figure which shows an example of the heat treatment process for forming a predetermined titanium oxide film in ferritic stainless steel. It is a figure which shows the relationship between the depth from the surface layer of the stainless steel in which the heat processing shown in FIG. 2 was performed, and the density | concentration of various atoms. It is a figure which shows the relationship between the depth from the surface layer of the stainless steel in which the improper heat processing different from the heat processing shown in FIG. 2 was given, and the density | concentration of various atoms. (A), (b) is a figure which shows a pitting corrosion potential measured value. It is a figure which shows a pitting corrosion potential measured value. (A), (b) is a figure which shows the measured value of a repassivation potential. It is a figure which shows the relationship between the titanium density | concentration of a titanium oxide film, and a pitting corrosion potential.

  Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings.

[Latent heat recovery type hot water generator]
FIG. 1 shows a heat exchanger as an example of a latent heat recovery type hot water generating device to which the present invention is applied.
The heat exchanger A of the present embodiment is incorporated in a hot water supply apparatus and the like and used for hot water heating, and the structure itself is conventionally known. Briefly, in the heat exchanger A, a plurality of spiral heat transfer tubes 2 are accommodated in the case 1, and headers 3A and 3B for water entry and hot water are attached to the ends of the heat transfer tubes 2, respectively. Has the structure. The case 1 is provided with an air supply port 11 and an exhaust port 12 for allowing combustion gas to flow in and out. As the combustion gas, for example, a combustion gas generated by a gas burner is used, and this combustion gas has been recovered by sensible heat using another heat exchanger before being supplied to the heat exchanger A. The For this reason, each heat transfer tube 2 performs latent heat recovery from the combustion gas, and strongly acidic condensed water is generated. In the figure, reference numerals b1 to b3 indicate brazed portions. More specifically, these portions b1 to b3 are a portion b1 in which each heat transfer tube 2 passes through the side wall of the case 1, a connection portion b2 between each heat transfer tube 2 and the headers 3A and 3B, a main body portion 13 and an upper lid of the case 1 14 is a joint portion b3.

  Each part of the heat exchanger A is made of stainless steel. However, the case 1 and the headers 3A and 3B are austenitic stainless steel such as JIS standard SUS304 in consideration of workability and toughness. On the other hand, the heat transfer tube 2 is ferritic stainless steel and corresponds to an example of a member to be formed with a titanium oxide film intended by the present invention. More specifically, the material of the heat transfer tube 2 corresponds to, for example, JIS standard SUS445JI, and in weight percent, C: 0.025% or less, Si: 1.00% or less, Mn: 1.00 or less, P : 0.040% or less, S: 0.030% or less, Cr: 21.00-24.00%, Mo: 0.70-1.50%, N: 0.025% or less. However, Ti: 0.1 to 0.5%, Al: 0.01 to 0.05% (representative values include Ti: 0.12%, Al: 0.018%), and the balance is Fe.

[Heat treatment and titanium oxide film formation]
A titanium oxide film having a titanium concentration of 25% [cation atom%] or more is formed on the surface layer portion of the heat transfer tube 2 described above. This titanium oxide film is formed, for example, by heat treatment under conditions as shown in FIG.
The heat treatment shown in FIG. 2 uses a vacuum furnace as the heating furnace, and is carried out by carrying the heat exchanger A shown in FIG. 1 into the heating furnace. As heating conditions, when the temperature in the heating furnace is gradually increased and reaches 1000 ° C., the heating state in the range of 1000 ° C. to 1200 ° C. is maintained for about 120 minutes, and then the heating is stopped and the heating furnace is stopped. Slowly cool inside. On the other hand, when the heating temperature is in the temperature range of 900 ° C. or higher, the inside of the heating furnace is maintained in a vacuum atmosphere of 10 ⁻² to 10 ⁻³ Pa. When the vacuum atmosphere state is released when the heating is stopped, the heating furnace is filled with an inert gas.

  In the heat treatment, a vacuum furnace is used as a heating furnace, but a hydrogen furnace can be used instead. When using a hydrogen furnace, when the heating temperature is in the temperature range of 900 ° C. or higher, the same result as the heat treatment can be obtained by maintaining a hydrogen atmosphere with a dew point temperature of −80 to 20 ° C.

FIG. 3 shows the relationship between the depth from the stainless steel surface of the heat transfer tube 2 after the heat treatment of FIG. 2 and the concentration of main elements in the ferritic stainless steel.
According to the data shown in the figure, it can be seen that a titanium oxide film having a titanium concentration of 25% [cation atom%] or more is formed on the surface layer portion from the stainless steel surface to a depth of 1 μm. Since aluminum is easily oxidized, an aluminum oxide film is also formed on the stainless steel surface layer. When an aluminum oxide film is formed in addition to the titanium oxide film, the oxidation of Fe and Cr is more effectively prevented and protected in the deeper layer than these oxide films. In FIG. 3, only main elements such as Ti, Fe, Cr, and Al are shown, and other elements are not shown. However, this is because the other elements have a very small amount and are clearly illustrated. This is because it is difficult to do this (this also applies to the next FIG. 4).

  The data in FIG. 4 shows the state of stainless steel when the degree of vacuum in the heating furnace is insufficient and oxygen is excessive during the heat treatment. The heat treatment conditions other than the degree of vacuum are all the same as those of the heat treatment shown in FIG. In the data shown in FIG. 4, the atomic concentration of oxygen is shown. From this data, it can be seen that oxygen exists from a surface portion to a considerably deep position. In other words, in addition to titanium and aluminum, oxidation has reached Fe and Cr. In this case, the relative concentration of titanium or aluminum is reduced, resulting in a coating with low corrosion resistance mainly composed of iron oxide.

[Corrosion resistance of titanium oxide film]
FIG. 5 shows measured values of pitting corrosion potential, and FIG. 5 (a) is intended for stainless steel not subjected to the heat treatment of FIG. FIG. 5B is for the stainless steel that has been heat-treated in FIG. The pitting potential measurement is in accordance with JIS standard G0577. In this measurement, the voltage applied to the sample is gradually increased (anodic polarization). A sudden rise in the current value occurs at the moment when pitting corrosion occurs, but the potential at which the current density becomes 10 ⁻⁴ A / cm ² is the pitting corrosion potential Vc′100.
In FIG. 5A, the pitting potential Vc′100 is 0.61V, whereas in FIG. 5B, it is 0.93V. Therefore, in the stainless steel in which the titanium oxide film is formed by heat treatment, the pitting potential becomes noble by about 0.3 V and the pitting resistance is improved as compared with the stainless steel that is not.
FIG. 6 shows the pitting potential measurement values for the stainless steel subjected to the inappropriate heat treatment shown in FIG. In FIG. 6, the pitting potential Vc′100 is 0.31 V, and the pitting potential is lower than when no heat treatment is performed. Thus, when the heat treatment with respect to the ferritic stainless steel is performed under inappropriate conditions, the pitting corrosion resistance may be reduced.

FIG. 7 shows the values of the corrosion gap repassivation potential measurement, and FIG. 7 (a) is intended for stainless steel not subjected to the heat treatment of FIG. FIG. 5B is for the stainless steel that has been heat-treated in FIG. This measurement is for evaluating the corrosion resistance of the environment that receives acidic condensed water peculiar to the heat exchanger for recovering latent heat and the gap structure, and is based on JIS standard G0592. In this measurement, the gap sample is anodically polarized to cause crevice corrosion, and after the crevice corrosion is sufficiently grown with a constant current, the potential of the gap sample is gradually decreased from a high potential. The potential at which the passive film is regenerated and no current flows (the potential at which the current density is less than 0.05 mA / cm ² ) is the corrosion gap repassivation potential.
In FIG. 7 (a), the repassivation potential is −0.006V, whereas in FIG. 7 (b), it is 0.066V. Therefore, in the stainless steel in which the above-described titanium oxide film is formed by heat treatment, the repassivation potential becomes noble by about 0.07 V, and the crevice corrosion resistance is improved as compared with stainless steel that is not.

  As understood from the above-mentioned data, when the heat treatment shown in FIG. 2 is performed on the heat transfer tube 2 and the above-described titanium oxide film is formed on the surface layer portion thereof, compared to the case where it is not so, Corrosion resistance is clearly improved, and the heat transfer tube 2 has sufficient corrosion resistance against strongly acidic condensed water. On the other hand, since the material of the heat transfer tube 2 is a ferritic stainless steel and is a relatively inexpensive stainless steel having a pitting corrosion index of less than 30, the raw material cost of the heat transfer tube 2 is suppressed, and the heat exchanger A is manufactured. This is preferable for reducing the cost.

When the heat transfer tube 2 is subjected to the heat treatment shown in FIG. 2, a brazing material is applied in advance to the brazing target portions b1 to b3 shown in FIG. As a result, during the heat treatment, the brazing material can be heated and melted, and the heat treatment step and the brazing step can be completed at once. When such a method is adopted in manufacturing the heat exchanger A, the manufacturing cost of the heat exchanger A can be further reduced.

[Relationship between titanium concentration and corrosion resistance]
The present inventors are to investigate the relationship between the titanium concentration in the titanium oxide film formed on the surface layer portion of ferritic stainless steel (hereinafter, the unit of titanium concentration is cation atom%) and the pitting corrosion potential. As a result of the experiment, data as shown in FIG. 8 was obtained.
According to the data shown in FIG. 8, when the titanium concentration in the titanium oxide film is 20%, the pitting potential is 0.3 V and the corrosion resistance is “poor”. On the other hand, when the titanium concentration is 30% or more, the pitting potential is stable at 0.9 V, and the corrosion resistance is “good”. As the titanium concentration increases from 20% to 30%, the corrosion resistance gradually improves. For this reason, the turning point between “bad” and “good” corrosion resistance is considered to be when the titanium concentration is 25%, and if the titanium concentration is higher than that, the effect intended by the present invention can be obtained.

  The present invention is not limited to the above-described embodiment, and the specific configuration of each part of the latent heat recovery-type hot water generating device according to the present invention can be varied in design in various ways. Moreover, the specific structure of each process of the manufacturing method of the apparatus for producing latent heat recovery type hot water according to the present invention can be variously changed.

In the embodiment described above, as a specific example of the ferritic stainless steel, the one corresponding to the JIS standard SUS445J1 has been described, but other materials can also be used.

  The member to be formed with the titanium oxide film is not limited to the heat transfer tube of the latent heat recovery type heat exchanger. Of course, members other than heat transfer tubes may be applied to the latent heat recovery type heat exchanger, as well as equipment other than the heat exchanger (for example, a passage of condensed water generated in the latent heat recovery type heat exchanger is formed. The member which comprises the apparatus provided with the member to perform) can also be made into application object. The “latent heat recovery type hot water generation device” as used in the present invention is a device having a function of generating hot water using heat obtained by recovering latent heat from a gas containing latent heat, and is limited to a hot water supply device. Not.

A Heat exchanger (Latent heat recovery type hot water generator)
2 Heat transfer tube (ferritic stainless steel member)

Claims (3)

A latent heat recovery type comprising a heat transfer tube, a case accommodating the heat transfer tube, and a header connected to the heat transfer tube, wherein the heat transfer tube is made of ferritic stainless steel and configured as a heat exchanger for recovering latent heat A device for generating hot water,
As a material of the heat transfer tube, a ferritic stainless steel member containing aluminum and having a titanium layer concentration of less than 25% [cation atom%] is used,
The heat transfer tube is subjected to heat treatment for brazing the side wall of the case and the header, so that the surface layer portion has an aluminum oxide film and a titanium concentration of 25% [cation atom%] or more. A latent heat recovery type hot water generating device, characterized in that a titanium oxide film is formed . Pitting index of the heat transfer tube, Ru der less than 30, latent heat recovery type hot water generating device according to claim 1. A latent heat recovery type comprising a heat transfer tube, a case accommodating the heat transfer tube, and a header connected to the heat transfer tube, wherein the heat transfer tube is made of ferritic stainless steel and configured as a heat exchanger for recovering latent heat A method for manufacturing a device for generating hot water,
As a material of the heat transfer tube, a ferritic stainless steel member containing aluminum and having a titanium layer concentration of less than 25% [cation atom%] is used,
The heat transfer tube is formed with an aluminum oxide film and a titanium oxide film having a titanium concentration of 25% (cation atom%) or more on the surface layer portion by heat treatment for brazing the side wall of the case and the header. A method of manufacturing a latent heat recovery-type hot water generating device, characterized in that:

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