JPS5938320B2 - 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 by electrolytic treatment.

It is known that a high heat exchange rate can be obtained by forming a porous layer on the heat transfer surface of a 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. Many methods have been proposed for forming such a porous layer, such as sintering, plastic working, thermal spraying, plating, mesh wrapping, and etching to form a rough surface. However, all of them had drawbacks, such as being complicated and expensive to process, or lacking in functionality. In order to solve this drawback, the present inventors have developed a heat transfer tube in which a metal tube or a tube whose surface is plated with metal is used as a positive electrode, and the tube is electrolytically dissolved in an electrolyte solution to make the surface layer of the tube porous. Developed a manufacturing method (Patent Application No. 52-5, 374
No. 0), when an austenitic stainless steel pipe is used as the raw material in this method, the pipe is heated to a temperature of 400 to 8
Even when the product is heated to 50°C and then slowly cooled to make it susceptible to intergranular corrosion (sensitization treatment), it is difficult to control the homogeneity of the product.
It was also recognized that there is a problem in that the electrolytic treatment time becomes long for homogenization.

The present invention has been made to solve the above problems, and its purpose is to provide a method for manufacturing heat exchanger tubes that can homogenize the product and shorten processing time.

To summarize the present invention, the present invention is characterized in that an austenitic stainless steel tube is used as a positive electrode in an electrolyte solution, and the surface of the tube is electrolytically dissolved while stirring the electrolyte solution to make the surface layer of the tube porous. The present invention relates to a method of manufacturing a heat exchanger tube. As a result of a multifaceted study on the manufacturing method of heat transfer tubes using an electrolytic treatment method using austenitic stainless steel tubes, the present invention was developed based on the results of a multifaceted study on the manufacturing method of heat exchanger tubes using an electrolytic treatment method using austenitic stainless steel tubes as a raw material. It was found that even if the temperature was constant, the reproducibility of the product homogeneity, that is, the size of the pores formed on the tube surface and the homogeneity of the pore distribution, was poor. Under certain electrolytic conditions, in order to obtain a reproducible and homogeneous porous surface, the electrolyte solution must be stirred to a degree that causes the bubbles generated on the cathode surface to continuously peel off from the surface. We found that this is a necessary condition. The present invention is applied to the steel pipe which has been previously subjected to the above-mentioned sensitization treatment, and furthermore, before the electrolytic treatment, the steel pipe is immersed in an aqueous solution of hydrochloric acid, preferably a 20% aqueous solution of 12N hydrochloric acid, for several hours, preferably 4 to 6 hours. However, it was found that a more homogeneous porous surface could be obtained by forming an extremely minute uneven structure on the tube surface and then performing electrolytic treatment.

Even if the potential difference and current density between the two electrodes are excessively increased in order to shorten the electrolytic treatment time, a homogeneous porous structure cannot be obtained in a short time. By doing so, it became possible to greatly improve the applied potential difference and current density.

As the electrolyte solution, an inorganic acid such as hydrochloric acid or sulfuric acid or an aqueous solution of this and an electrolyte inorganic salt is used.
A mixed aqueous solution with 22% of 2NHC) is preferred.

Potential difference is 0.6-0.9V, preferably 0.8V1 Current density is 2.5-7A/Dm2, preferably 4-5A/Dm2
The electrolysis time is 1.5 to 4 hours, preferably about 2 hours, and a heat exchanger tube having a homogeneous porous layer can be obtained under these electrolysis conditions.

Next, the present invention will be described with reference to Examples, but the present invention is not limited thereto in any way.

Example In this example, an electrolytic cell shown in the attached drawings was used.

FIG. 1 is a longitudinal sectional view of the electrolytic cell, and FIG. 2 is a plan view thereof. JIS as material austenitic stainless steel pipe 1
-Using SUS3O4TP, 650 as sensitization treatment
After heating at 0.degree. C. for 1.5 hours, the mixture was slowly cooled in a furnace for about 20 hours.

Next, this was immersed in a 20% aqueous solution of 12NHCe for 4 to 6 hours. After this immersion, the outer surface of the tube becomes white macroscopically.
Microscopically, it became an extremely fine uneven structure. After the treatment, the upper and lower ends of the steel pipe 1 are closed with rubber plugs 2, 2', and are fixed concentrically with the copper cylinder 4 that will serve as a cathode with rubber support rings 3, 32, which are attached to rubber seats 5, 55. It was immersed in a glass container 7 filled with an electrolyte solution 6.

Note that holes and notches are provided in the support rings 3, 32 and the rubber seat 52, respectively, so that the electrolyte solution 6 can be circulated. The electrolytic treatment is carried out by connecting the stainless steel tube 1 to the positive electrode and the steel cylinder 4 to the negative electrode using a constant potential electrolyzer 8, and providing a stirring blade 9 driven by a motor 10 in the container 7 to inject the solution 6 in the direction of the arrow 11. This was carried out by applying electricity while stirring and circulating.

As the solution 6 was stirred and circulated, air bubbles generated on the outer surface of the cylinder 4 were continuously peeled off from that surface. Electrolytic treatment is performed at room temperature (20
~27℃), the composition of the electrolyte solution, the potential difference between the two electrodes,
The pore size and homogeneity of the porous surface of the resulting boiling heat exchanger tubes were observed under a microscope by varying the current density and electrolysis time. Excellent results (homogeneity of pore size and distribution) were obtained.

Further, the electrolytic conditions that could achieve homogeneity of the porous surface of the boiling heat exchanger tube and shorten the electrolytic treatment time were as follows.

(a) Composition of electrolyte solution; NaC22O% and 12NHC
Mixed aqueous solution with e2% (b) Potential difference between the two electrodes; 0.6 to 0
．． 9V (0.8V is best) (c) Current density; 2.5-7
A/Dm2 (4 to 5 A/Dm2 is best) (d) Electrolysis conditions; 1.5 to 4 hours (about 2 hours under the best conditions of B and c) In the above example, an example of manufacturing a boiling heat exchanger tube was shown. A heat pipe with a porous inner surface can be obtained by fixing a copper electrode rod concentrically inside a steel pipe and electrolyzing it under similar conditions.

As is clear from the above description, according to the present invention, by stirring the electrolyte solution during electrolytic treatment, it is possible to obtain a boiling heat exchanger tube or heat pipe in which the size and distribution of pores in the porous layer are uniform in a short time. Can be done.

[Brief explanation of the drawing]

FIG. 1 shows a longitudinal sectional view of an electrolytic cell used in an example of the present invention, and FIG. 2 shows a plan view thereof. 1... Stainless steel pipe, 2, 22...
Rubber stopper, 3, 32...Support ring, 4...
Copper cylinder (negative electrode), 5,5'...Rubber seat,
6... Electrolyte solution, 7... Glass container, 8... Constant potential electrolyzer, 9... Stirring blade, 10... Motor 11...
Circulation direction of electrolyte solution.

Claims (1)

[Claims] 1. A transmission characterized in that an austenitic stainless steel tube is used as a positive electrode in an electrolyte solution, and the surface of the tube is electrolytically dissolved while stirring the electrolyte solution to make the surface layer of the tube porous. Method of manufacturing heat tubes. 2. The method according to claim 1, wherein the austenitic stainless steel pipe is immersed in an aqueous hydrochloric acid solution for several hours before electrolytic treatment. 3 Using a mixed aqueous solution of NaCl and HCl as the electrolyte solution, the potential difference between the two electrodes is 0.6 to 0.9 V, and the current density is 2.
The method according to claim 1 or 2, wherein the electrolytic treatment is carried out under conditions of 5 to 7 A/dm^2 and an electrolysis time of 1.5 to 4 hours.