CA1284923C - Fin of heat exchanger and method of making it - Google Patents

Abstract

SPECIFICATION

TITLE OF THE INVENTION

FIN OF HEAT EXCHANGER AND METHOD OF MAKING IT

ABSTRACT

A heat-exchanger fin is disclosed, wherein a Cu-Zn diffused alloy layer with a Zn content of not less than 1 wt % is formed on at least a portion of the surface of a Cu-based substrate for the fin. For the formation of the Cu-Zn diffused alloy layer, Zn is allowed to diffuse thermally after coating of the surface of a Cu-based substrate with Zn or Zn alloy, and processing by rolling is carried out after the thermal diffusion to finish to the desired size.

Description

1 284~23 In the drawings accompanying this application Fig. 1 is a front view showing a radiator for a car. Fig. 2 is a graph showing the distribution of average corrosition of radiator in a corrosive environment.
The present invention relates to a heat exchanger fin and a method of making it. In particular, the invention has made thinning of the fin possibIe through an improvement in corrosion resistance without decreasing heat transferability. The fin according to the invention is particularly suitable for heat exchangers used under intense conditions in corrosive environments as in automobiles etc.
For the radiating fins used for shell and tube type heat exchangers, strength and corrosion resistance are required together with heat transferability. For instance, heat exchangers for cars use a radiator for cooling the engine and a heater for air-conditioning. In all cases, a copper core fitted with fins between a plurality of tubes, through which the heat exchange medium circulates, is used and tanks are installed at both ends of the core through washer plates. In the radiator of Fig. 1, core (3) is constructed by fitting corrugated fins (2) between a plurality of tubes (1) through which the heat exchange medium circulates, washer plates (4a) and (4b) being provided at both ends of tubes (1) in core (3), and tanks (5a) and (Sb) are installed onto washer plates (4a) and (4b). Numerals (6) and (7) indicate the entrance and exit respectively for refluxing of the heat exchange medium and numerals (8) and (9) indicate injection and ejection ports for the heat exchange medium, respectively.
For the Cu-based core of the radiator, brass tubes and Ru or Cu alloy corrugated fins are generally used and the fins are fitted between the tubes by a soldering method called core burning. For the fin, Cu or Cu alloy strip having a thickness of 0.025 to 0.060 mm is used, and in order to improve the strength and the heat resistance, small amounts of Sn, Ag, Cd, P, Zr, Mg, A

lXl~4~Z3 etc. are added within a range which does not lower heat transferability. Moreover, on the radiator including the Cu core, black paint is coated for the purpose of preventing corrosion, but this treatment is confined only to the outer surfaces of the radiator and the thickness is also confined to less than 10 ~m, since thicker films are harmful to the radiation of the fin section.
In recent years, a large quantity of chlorides such as NaCl etc. is used on roads for the purpose of melting snow and ice, and corrosion of the body of a car by these chlorides is serious. The corrosion of the fin is extensive also with heat exchangers such as radiators, air conditioners, etc. for automobiles, and lowering of the radiation ability has become a disadvantage. For this reason, the use of corrosion-resistant alloys such as Cu-Ni-based alloys etc. was investigated but, because of low heat transferability, thickening became necessary to achieve a predetermined performance, which led to high prices and increase in weight. Moreover, with conventional materials, the thickening having made allowance for a margin to corrosion and the painting for the prevention from corrosion also brought about similar results making it impossible to fit for practical use.
On the other hand, decrease in weight for automobiles is desired from the point of view of energy conservation. Decrease in weight i5 desired also for the heat exchangers for automobiles. However, it has been difficult technically to æatisfy both the measures against salt damage aforementioned and the requirement of weight decrease simultaneously.
SUNNARY OF THE INVENTION
As a result of various investigations in view of this situation, a fin material for heat exchangers which has excellent corrosion resistance standing up to severe environments over a long period of time and having sufficient heat transferability, and which is not easily corroded and worn out even if thinned for decrease in weight and which exhibts radiation ability for a long tlme, has been developed as well as a method of making such fin.

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1.284923 Namely, the heat exchanger fin of the invention is characterized in that a Cu-Zn diffused alloy layer with a Zn content of not less than 1 wt % is formed on the surface of a Cu-based substrate.
Moreover, the method of making the fin of the invention is characterized in that Zn is allowed to diffuse thermally after the surface of the Cu-based substrate is coated with Zn or Zn alloy or an alloy layer with a Zn content of not less than 1 wt % is formed on the surface by carrying out rolling and tempering after thermal diffusion.
For the Cu-based substrates, thin copper alloy plates as for example Cu-Zn, Cu-Cr, Cu-Ag, Cu-Sn, Cu-Cd, Cu-Pb-Sn, Cu-In, Cu-Te, which are highly electroconductive (highly heat-transferable) and can be improved in strength through the alloy effect, for example, high electroconductive alloy plates having an electroconductivity of not less than 85% IACS, preferably 90 to 98% IACS are used besides pure Cu. On these substrates, Zn or Zn alloys as for example pure Zn or Zn-Cu, Zn-Ag, Zn-Sn, Zn-Cd, Zn-Ni, Zn-Fe, Zn-Pb, Zn-Bi-Pb, Zn-Ni-Co., Zn-As, Zn-Sb, are applied by means of electroplating, PVD, etc. by heating above the diffusion temperature of Zn to allow Zn to diffuse from the surface of the substrates.
The method by which Zn or Zn alloy is applied at high temperature and sufficient diffusion is allowed to proceed simultaneously may be useful from the viewpoint of shortening the process. The temperature is preferably higher than 350C from a practical point of view and the hot-dip and the metallization methods are advantageously used.
After the manufacturing processes described above, rolling and tempering such as annealing etc. are carried out, if necessary, to finish to a desired size and an alloy layer with a Zn content of not less than 1 wt%, preferably of not less than 10 wt%, is formed on the surface, the thickness of the alloy layer being preferably not less than 1 ~m and not more than one fourth of the thickness of the fin plate.

'~ 3 1 ~4~23 From the fact that the fin material is used usu~lly as a strip material with a thickness of 0.05 to 0.025 mm, it may be desirable to form the aforementioned diffused layer on the surface of the substrate with a thickness of about 1.0 mm and, therafter, to carry out rolling and tempering such as annealing etc. to finish to the desired size.
With the fin of the invention, such treatment as the Cu-Zn aforementioned diffused layer formed on a portion of the surface, in particular, within a range not more than 10 mm from the edge of the fin exposed to the outer circumference of the heat exchanger, is as effective as treatment of the whole surface. Besides partial covering-diffusion treatment of the fin material, covering-diffusion treatment can also be made after construction of the heat exchanger.
The fin material of the invention has made both protection against salt damage and decrease in weight possible by improving corrosion resistance under the conditions of salt damage, through the formation of an alloy layer with a Zn content of at least 1 wt~ on the surface of a Cu-based substrate and by making highly electroconductive (highly heat-transferable) through the core portion comprising an alloy with a Zn content of not more than 1 wt~.
It has been know experimentally that the addition of Zn to Cu is effective for the prevention of corrosion by salt. Pure Zn is a metal apt to be corroded under conditions of salt damage, whereas, excellent corrosion resistance is not exhibited until alloyed with Cu. Moreover, the Zn diffused layer has a distribution of concentration of Zn decreasing continuously from the surface to the interface with the core material. For this reason, the surface becomes anodic against the inner portion and the inner portion becomes cathodic over the whole period of corrosion resulting in the prevention of corrosion. Thus the general corrosion is suppressed and averaged over the whole -7r lZ~4~
surface, so that rapid deterioration in the strength of fin due to corrosion in the form of corrosion pits, observed conventionally with fins made from Cu only or Cu alloy, can be suppressed to a great extent.
When adding Zn to Cu, the electroconductivity decreases to, for example, 80 to 85~ IACS by the addition of 1 wt% of Zn, about 70~ IACS by the addition of 3 wt~, about 44~ IACS by the addition of 10 wt% and about 25~ IACS by the additon of 30 wt~. Therefore, if the desired corrosion resistance is achieved simply by the addition of Zn, the electroconductivity (heat transferability) is unsuitably lowered so, in accordance with the invention, an alloy layer with a Zn content of not less than 1 wt~, preferably of not less than 10 wt~, is formed in a thickness of not less than 1 ~m on the surface of a Cu-based substrate to improve corrosion resistance against salt damage as aforementioned and the alloy layer with a high Zn content is confined to the surface to prevent lowering of the electroconductivity.
Usually, by making the thickness of the surface layer not more than one fourth of that of the fin plate, an electroconductivity more than 70~ IACS can be displayed in most cases.
In the Zn-Cu diffused layer according to the invention, Zn or Zn alloy surface layer unreacted with the surface layer may be left behind. Although this is corroded relatively fast at the beginning, the Cu-Zn diffused layer underneath serves to prevent corrosion at the next ~tep.
As a method of increasing the heat transferability (or elctroconductivity) of the fin of the invention, Zn covering is made only on the fin portion corresponding to the outer circumference of the heat exchanger where the corrosion is most intense. Salt adheres in large amounts to the outer circumferential portion, but the adherence is confined within a distance not more than 10 mm from the edge of the fin according to many experiences in heat exchangers for cars. Fig. 2 is an .,w~, .

1 ~4~23 example thereof, which shows the distribution of the corrosion of a radiator (fin: Cu-0.15 Sn alloy, 0.046 mm thickness x 30 mm width) having operated for a mileage of 1,000 km in a corrosive environment i.e. a seashore area. As evident from the diagram, the distribution is effectively biased toward 10 mm from the front and 7 mm from the rear.
Moreover, with the fin material of the invention, the Zn diffused layer can be formed on the surface through coating the surface by means of electroplating, hot dip, PVD, mechanical cladding methods, etc. followed by thermal diffusion. In particular, by means of electroplating, the coating of Zn or Zn alloy accurate as to thickness and uniformity is possible.
Moreover, in order to form the alloy layer with a predetermined thickness, the heat treatment may be carried out at a temperature of 250 to 700C or higher than this. Furthermore, by passing the Cu-based substrate through Zn vapor at higher than 500C, coating with Zn and diffusion thereof can be done simultanously.

Example 1 Zn was electroplated on heat-resistant Cu strips (electroconductivity 95.9~ IACS) having a thickness of 0.07 mm and containing 0.06 wt~ of Cd, in a bath described below to the thicknesses shown in Table 1 and, after diffusion treatment under the conditions shown in Table 1, the strips were submitted to rolling to convert to fin materials with a thickness of 0.038 mm.
The electroconductivity of these fins was measured, while the cross-section was analyzed by the use of an X-ray microanalyzer to determine the Zn contents on the surface and at depths of 1 and S ym below the surface. Moreover, the corrosion test described below was carried out to determine the average amount of corrosion by the weight method and, further, a tensile strength test was carried out on the fin before and after corrosion to determine the 1 2~4~'~3 reduction in strength. These results are shown in Table 1 in comparison with those of heat-resistant Cu strip plated only with Zn and heat-resi$tant Cu strip without any treatment.
Plating bath NaCN 50 g/~
Zn(CH)2 70 g/~
NaOH 100 g/.
Bath temperature 30 C
Current density 3 A/dm2 Corrosion test After saline was sprayed for 1 hour according to JIS Z2371, the strip wa6 kept for 23 hours in a conditioning oven regulated to 60C and 95% RH. This procedure was repeated 30 times.
As evident from Table 1, in the cases of Zn-plated fin No. 4 and untreated fin No. 5, the amount of corrosion reached 8 to 9 ~m ~one side) on average and the reduction in strength was about 85%, the state of the strips having become almost crumbly. Whereas, it can be seen that, in the cases of fins Nos. 1 and 2 formed with an alloy layer with a Zn content of not less than 1 wt% on the surface, the deterioration by corrosion was only slight. In particular, the reason why the amount of corrosion and the reduction in strength are small is due to the fact that pit corrosion causing deterioration in strength is prevented through the diffusion of Zn on the surface layer. On the other hand, in the case of fin No. 3, the Zn content in the alloy layer at a depth of 5 ~m from the surface layer being less than 1 wt%, the amount of corrosion and the reduction in strength are greater than for fins No. 1 and 2 described above, suggesting that the improvement is insufficient under severe conditions.

Example 2 Employing plating baths described below in place of the Zn plating in Example 1, Zn-5 wt % Ni alloy and Zn-10 wt % Cd alloy were electroplated to the thicknesses shown in Table 2 and, after ~' ' - ' .

~ 2~4~X3 ~o~ . o~
~_ ~ o ~U ~ U
_ ~ ~ , , ~ ~ ~

l o ~ U'~ ~
~ ~ ~ 8~- 5~

E~ ~ E~ ,~ ~.X X X X ~
8 ~ o i~ o c~ ~f a~ x x x . .
~,Q ~0~
.,~ ~ O O 0 ~0 0 o o o o ' z; ~o~ O

b b 1284~3Z3 diffusion treatment under the conditions shown in Table 2, the strips were submitted to processing by rolling to convert to fin materials with a thickness of 0.038 mm. Using these fins, tests similar to those of Example 1 were carried out and the results were compared with tho~e obtained using the fin materials plated simply with Zn-5 wS% Ni alloy and Zn-10 wt% Cd alloy.
Plating bath of Zn-5 wt% Ni alloy ZnS04 75 g/~
NiS04 60 g/
CH3COONa ~0 g/,~
H3B03 15 g/l_ pH 3 Bath temperature 45 C
Current density7.5 A/dm2 Plating bath of Zn-10 wt X Cd alloy Zn(CN)2 76 g/~
CdO 4 gt~
NaCN 45 g/R
NaOH 80 g/~Q
Bath temperature 35 C
Current density2 A/dm2 ~. . . .

~2~4~Z3 As i5 evident from Table 2, in the cases of fins Nos. 6 and 7 of the invention formed with an alloy layer having a Zn content of not le~s than 1 wt~ on the surface by carrying out diffusion treatment after plating with Zn-5 wt% Ni alloy and Zn-10 wt% Cd alloy, the deterioration by corrosion was only slight. On the contrary, in the case of fin No. 8, the Zn content at S ym portion being less than 1 wt% even though that on the surface being greater than 1 wt%, the improvement in the corrosion resistance is inferior to that of Nos. 6 and 7, showing insufficiency under the severe conditions of use.

Example 3 Using a heat-resistant Cu strip (electroconductivity 98% IACS) having a thickness of 0.06 mm and containing 0.09 wt % of Ag, diffusion treatment of Zn combined with intermediate annealing was carried out by exposing the strip for 15 seconds into a Zn bath fused at 590C in an atmosphere of H2. This was submitted to rolling to a thickness of 0.035 mm to convert to the fin material. Tests similar to those of Example 1 were carried out.
The results are shown in Table 3 compared with those for an untreated fin as above.

Table 3 Electro- ~n cor~LLation (wt~ A~t of Reduction rate Fin conduct vity- Surface --~ corrosion in str~th E~entthe 89.0 18 13 1.2 3.6 21 Fin withalt 97.0 O _ _ 8.8 90 treatm~nt ~ .

~ 2~49Z3 It is obvious from Table 3 that the corrosion resistance of the fin of the invention is remarkably improved compared with that of the untreated fin.

Example 4 In the example above, after hot-dipping for 4 seconds into the Zn bath, the strip was wiped and cooled. ~olling wa~ carried out similarly to finish. Results of similar tests are shown in Table 4. As evident from the table, the corrosion resistance is improved drastically.

Table 4 Electro- Zn coxYntration (wt Z) h~nt of R~i~tion rate l l cox~t~tyl sbrea e ~ ~ (~ in str~h ~ entknn. 1 34 18 0.9 2.4 18 FiD WQ~YUt 97.0 1 0 _ _ 8.8 90 Example 5 A radiator, as shown in Fig. 1, fitted with corrugated fins consisting of Cu-0.15 Sn-O.OlP alloy and having a thickness of 0.040 mm and a width of 32 mm, was assembled in conventional manner. Besides, this radiator was provided with two rows of tubes to the width of the fin.
Under the plating conditions of Exa~ple 1, one side each of the radiator was dipped partially while Zn was plated to a thickness of 0.9 ~m at distances of 3 and 9 mm from the edge of the fin. These were heated for 3 hours at 280C.

11 ' 12~4~3 Using the articles of the invention thus obtained and the conventional untreated article, exposure to saline (JIS Z2371) was conducted for 10 minutes and further dampening exposure under 60C
x 90% RH was made for 23 hours, was repeated 60 times. Also, in order to simulate the practical operation of a car, the aforementioned test was conducted in a wind channel and the saline was sprayed onto the radiator at a speed corresponding to the operation of a car at 60 km/hr. From the results shown in Table 5, the deterioration of the articles of the invention can be seen to be lessened significantly.

Table 5 Electro- I Zn cox~n~at~n (wt Z) R~h~tion rate Fin cox~ctivity in stn~th (~ ) rface 1 pm Dbpth 5 ~m Depth (%) Ari~le of the 80 39 21 0.8 45 invention 3 mm inventkn 9 m~ 82 36 16 0.9 36 A~i~le wi~xut 88 _ _ _ 75 trea~xnt As described, the fin of the invention has excellent corrosion re~istance and heat transferability, never loses its function as a fin for a long period of time even under a severe environment and makes thinning and decrease in weight possible. Particularly, when used in a heat exchanger for a car, it renders not only decrease in weight but also improvement in the life possible.
Therefore, it exerts remarkable effects industrially.

Claims (12)

1. A fin for a heat exchanger comprising a Cu-Zn diffused layer having a Zn content of not less than 1 wt% formed on at least a portion of the surface of a Cu-based fin substrate, wherein the concentration of Zn decreases continuously from the outside surface of the Cu-Zn diffused layer to the interface between that layer and the Cu-based fin substrate.

2. The fin of a heat exchanger according to claim 1, wherein the Cu-Zn diffused alloy layer with a Zn content of not less than 1 wt % has a thickness of not less than 1 µm and not more than one fourth of the thickness of fin plate in the diffused layer.

3. The fin of a heat exchanger according to claim 1, wherein the Zn diffused layer is formed on the surface not more distant than 10 mm from the edge of the fin exposed to the outer circumference of the heat exchanger.

4. The fin of a heat exchanger according to claim 1, wherein the fin material is in the shape of a heat exchanger of a car.

5. A heat-exchanger for a car comprising, a plurality of brass tubes running substantially parallel to one another and having two ends, heat-exchanging fluid disposed within the brass tubes, a plurality of corrugated fins being disposed between the brass tubes and being bonded thermally to the brass tubes to promote heat-exchange of the heat-exchange fluid, an upper tank disposed at one end of the brass tubes and in liquid communication with the brass tubes, a lower tank disposed at the other end of the brass tubes and also in liquid communication with the brass tubes, wherein the fins comprise fin substrate made from copper or a copper alloy, and diffuse layers formed on at least one surface of the fin substrate, the diffuse layers containing both the copper or copper alloy of the fin substrate and zinc or zinc alloy, said diffused layer having a Zn content of not less than 1 wt %, wherein the concentration of Zn decreases continuously from the outside surface of the Cu-Zn diffused layer to the interface between that layer and the Cu-based fin substrate.

6. The heat-exchanger for a car of claim 5, wherein the diffusion layers have a uniform thickness.

7. The heat-exchanger for a car of claim 5, wherein the diffusion layers are formed from zinc or zinc alloy which has been applied to the fin substrate by electroplating.

8. The heat-exchanger for a car of claim 5, wherein the diffusion layers are formed from zinc or zinc alloy which has been applied to the fin substrate by hot dipping.

9. The heat-exchanger for a car of claim 5, wherein the diffusion layers are formed from zinc or zinc alloy which has been applied to the fin substrate by an evaporation process.

10. The heat-exchanger for a car of claim 5, wherein the diffusion layers have been formed by the application of heat to the surface of the base fins.

11. The heat-exchanger for a car of claim 5, wherein the diffusion layers have been formed by allowing all portions of the zinc or zinc alloy to diffuse into the fin substrate.

12. The heat-exchanger for a car of claim 5, further comprising a first seat plate covering an end portion of the upper tank, a second seat plate connected to the brass tubes, an exhaust port being positioned within the lower tank, and an outflow port being positioned within the lower tank and being connected to the second seat plate.