JPS5938319B2 - Method for manufacturing heat exchanger tubes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a heat exchanger tube, and more particularly to a method for manufacturing a heat exchanger tube that is easy to process and inexpensive.

So-called boiling heat transfer, in which heat is transferred on the surface of a heat transfer tube by heating and boiling the liquid in contact with it, can significantly increase the heat load on the heat transfer surface and accelerate the transfer of heat compared to when there is no phase change. Heat exchanger tubes that utilize this phenomenon, such as boiling heat exchanger tubes or heat pipes, are used in an extremely wide range of fields. Furthermore, a higher heat exchange rate can be obtained by forming a porous layer on the heat transfer surface of the heat transfer tube, for example on the outer surface of the tube in a boiling heat transfer tube or on the inner surface of the tube in a heat pipe. It is also known that Conventionally, methods for forming such a porous layer include, for example, a sintering method,
Many methods have been proposed, including plastic processing, thermal spraying, plating, mesh wrapping, and etching to form a rough surface, but all of them are complicated and expensive, or are not functionally sufficient. There were some drawbacks, such as: In view of the above-mentioned current situation, an object of the present invention is to provide a method for manufacturing a heat exchanger tube that is suitable for mass production with simple processing and can be manufactured at low cost.

To summarize the present invention, the method for manufacturing a boiling heat exchanger tube of the present invention involves electrolytically dissolving the tube in an electrolyte solution using a metal tube or a tube whose surface is plated with metal as a positive electrode, and making the surface layer of the tube porous. It is characterized by its quality.

The structure and operation of the present invention will be explained with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of an experimental apparatus used to carry out the present invention. In FIG. 1, a metal tube 1, which is a target object, is sealed at both ends with plugs 2 and immersed in an electrolyte solution 3. A negative electrode 4 is provided in the solution 3 and connected to a DC power source 5 as shown so that the metal tube 1 becomes the positive electrode. At this time, in order to make the current density uniform, for example, in the case of a boiling heat exchanger tube, it is desirable that the negative electrode 4 be a hollow cylinder and arranged concentrically with the metal tube 1 of the positive electrode. Further, in order to speed up the dissolution rate, that is, the processing time, it is preferable to provide a heater 6 in the electrolyte solution 3 and appropriately heat it to adjust the temperature. Metal tubes to which the present invention can be applied include various alloy tubes such as brass tubes and austenitic stainless steel tubes (e.g., 18-8 stainless steel), but metal tubes with plating (e.g., chrome plating) may also be used. Applicable. As the electrolyte solution, it is preferable to use an aqueous solution of hydrochloric acid, but in addition to hydrochloric acid, various acids such as sulfuric acid, nitric acid, and hydrofluoric acid, or mixtures of these acids and their salts can also be used.

When the tube material is brass, a mixed aqueous solution of hydrochloric acid and common salt is suitable as the electrolyte solution, and by passing an electric current, the zinc in the brass is selectively eluted due to the potential difference, resulting in the so-called "
``Dezincification effect'' occurs.

That is, on the surface of the brass tube immersed in the electrolyte solution 3,
First, a basic metal salt (zinc chloride) is generated, and through this, a part of the metal gradually dissolves into the electrolyte solution 3 due to the so-called dezincification action 7. Since a current is passed through the metal tube, the elution rate is significantly accelerated, and the surface of the metal tube becomes porous in a short time. When the tube material is an austenitic stainless steel tube such as 18Cr-8Ni, it is preferable to perform a sensitizing heat treatment on the surface of the steel tube as a pretreatment.

That is, it is preferable to heat the pipe to 400 to 850° C. and then slowly cool it to impart intergranular corrosion susceptibility. As you know,
Through such pretreatment, the supersaturated solid solution of carbon in the austenite ground precipitates as carbides mainly composed of chromium (containing carbides of iron and nickel), and as a result, the amount of chromium in the adjacent areas decreases. Therefore, depending on the galvanic corrosion conditions, the chromium-deficient parts are more likely to be corroded preferentially than the inside of the grains, and sensitized steel held in this potential range undergoes intergranular corrosion and becomes porous. . In this way, in the case of austenitic stainless steel, the surface of the tube can be efficiently made porous by preferentially corroding the grain boundaries using the tube as a positive electrode. At this time, there is no practical problem if the structure is returned to its normal state by performing solid solution treatment after the corrosion treatment. In the case of plating pipes made of chromium, etc., the plating surface originally has minute cracks and small pores, and since it does not dissolve electrolytically on the average and differs locally, it is natural that cracks and pores will be formed by electrolytically dissolving it. The small pores dissolve quickly and become enlarged.

Furthermore, since the stress distribution on the plating surface is generally non-uniform, corrosion is accelerated and cracks occur. Therefore, the size, porosity, and voltage of porous cavities are affected by current density, bath temperature, current application time, etc., and by adjusting these appropriately, it is possible to obtain the optimal surface structure for boiling heat transfer. Obtainable. The above electrolysis conditions are usually a bath temperature of room temperature to 80°C, a current density of about 0.1 to 100 A/Dm2, and a few minutes to
A current application time of several hours is appropriate, but these conditions naturally depend on the material of the heat exchanger tube, the type of electrolyte, etc.

The cathode used in the present invention is preferably an electrode made of platinum, titanium, or the like, but is not limited thereto.

Further, the present invention is applicable not only to heat exchanger tubes having a circular cross section but also to any other shape such as a rectangular cross section or a finned tube. The metal surface electrolytically melted according to the present invention is suitable for efficient nucleate boiling (a boiling state caused by vapor bubbles that are continuously generated from a specific point, that is, a nucleus, on a heat transfer surface and cause a disturbance effect). According to the present invention, such metal tubes or metal-plated tubes can be manufactured in large quantities at low cost through simple processing.

Therefore, the present invention is extremely useful industrially. Next, the present invention will be explained with reference to examples, but the present invention is not limited thereto in any way.

Example 1 A brass (naval brass tube) pipe having a composition of about 60% copper and about 40% zinc was prepared with a mixed aqueous solution of hydrochloric acid and common salt maintained at about 60°C (1000 cc of water with about 1009 cc of salt and hydrochloric acid with a concentration of 36%). The brass piece was used as the positive electrode and the platinum was used as the negative electrode in a sample containing about 10 cc (approximately 10 cc) of electrolysis at a voltage of about 1V and a current density of about 2A/Dm2 for about 15 minutes to dissolve zinc and perform dezincification treatment.

The result was a metal tube with a partially melted surface as seen in the micrograph of FIG. 2 (400x magnification). In the photo, the bright areas indicate the areas where the pits are combined, and the blurred areas indicate areas higher or lower than that area. As is clear from this photo, the dezinced metal surface has a spongy structure consisting of minute cavities suitable for efficient nucleate boiling. Example 2 The outer surface of a steel pipe was coated with a thickness of about 0.27111 mm by a well-known method.
The chrome-plated tube from step 1 is used as the positive electrode, copper is used as the negative electrode, and 300 cc of 36% hydrochloric acid and 700 cc of water are used as the electrolyte solution.
Use a solution dissolved in C, the liquid temperature is about 70℃, the voltage is about 3,
Electricity was applied for about 15 minutes at a current density of about 3 A/Dm2 to electrolytically dissolve a part of the plating film.

As a result, a metal tube having a vertical cross section as shown in FIG. 3 was obtained. That is, it can be seen that the chromium 8 plated on the steel pipe wall 7 has a spongy structure consisting of minute cavities as shown in the figure, and has a surface structure suitable for nucleate boiling. Example 3 18Cr-8Ni austenitic stainless steel pipe with approximately 7
After heating to 00°C and slowly cooling to perform sensitization treatment, the tube was used as a positive electrode and titanium was used as a negative electrode, and a 36% electrolyte solution was used.
Using 300 cc of hydrochloric acid dissolved in 700 cc of water, the liquid temperature is about 50°C, the voltage is about 1 V, the current density is about 1 A/Dm2.
The grain boundary corrosion treatment was carried out by applying electricity for about 30 minutes.

As a result, a metal tube with a partially melted surface as seen in the micrograph of FIG. 4 (400x magnification) was obtained. In the photograph, white parts are in focus, and uneven parts with different depths of focus are shown out of focus. In this way, the metal surface has a porous structure suitable for efficient nucleate boiling. Example 4 The same treatment method as Example 3 (however, the electrolytic conditions were: temperature:
Room temperature, voltage: 0.5-0.1V1 current density: 0.5-0
．． Intergranular corrosion was carried out using 05A/Dm2, current application time: about 18 hours, electrolyte solution: same as in Example 1).

FIG. 5 shows a record of measurements of the surface irregularities of the obtained porous stainless steel tube on a straight line along the tube axis. As seen in Figure 5, the depth of the unevenness on the tube surface is approximately 10 to 30
It is a porous structure with a pitch of about 0.1 to 0.15 μm. Approximately 17,000
We observed the formation of bubbles when boiling in water and in Freon R-11 (both at an outside temperature of 22°C and the liquid level open to the atmosphere) with a heat flow of Kca1/M2hr. It was observed that nucleate boiling was generated uniformly in large quantities from many nuclei on the tube surface, and that nucleate boiling was extremely efficient. In addition, in this experiment, when the generation of bubbles was observed under the same conditions for an untreated stainless steel pipe as a control, a marked difference was observed.
As described above, according to the present invention, it is possible to easily and inexpensively manufacture a metal tube with a porous surface suitable for boiling heat transfer, or a tube whose surface is plated with metal, and the size of the cavity on the heat transfer surface is small. Since the porosity and the porosity can be adjusted as appropriate depending on the application, the present invention is particularly suitable for mass production of boiling heat exchanger tubes and heat pipes.

[Brief explanation of the drawing]

The drawings show specific examples of embodiments of the present invention, and FIG. 1 is a schematic diagram of the experimental equipment used to carry out the present invention, and FIG. 2 is a microscopic photograph of the metal surface obtained in Example 1. , FIG. 3 is a vertical cross-sectional view of the surface layer of the metal tube obtained in Example 2, FIG. 4 is a microscopic photograph of the metal surface obtained in Example 3, and FIG. 5 is the metal tube obtained in Example 4. This shows a record of the surface of the tube measured on the surface line along the tube axis. The symbols indicate the following. 1...Metal pipe, 2...Plug, 3...
... Electrolyte solution, 4 ... Cathode, 5 ...
...DC power supply, 6... Heater, 7...
Steel pipe wall, 8...Metsukichrome.

Claims (1)

[Claims] 1. A heat transfer tube characterized in that the tube is electrolytically dissolved in an electrolyte solution using a metal tube or a tube whose surface is plated with metal as a positive electrode to make the surface layer of the tube porous. Production method. 2. The method according to claim 1, wherein the austenitic stainless steel tube is heat treated at 400 to 850°C, slowly cooled, and then electrolytically melted.