Method of making a plate fin heat exchanger

Abstract: To prevent corrosion by mercury even during nonoperating of plant facilities, plate fins 1 and flat plates 2 of flow passage members constituting cooled fluid passages and refrigerant passages are formed by an aluminum alloy. An oxide film is formed on surfaces of the plate fins 1 and flat plates 2 by a reaction between the aluminum alloy and an oxidizing component in an oxidizing gas introduced into the flow passages. Alternately a hydroxide film is formed by a reaction between the aluminum alloy and an alkaline component in an alkaline aqueous solution introduced into the flow passages.

Description: BACKGROUND OF THE INVENTION (Field of the invention) The present invention relates to a method of making a plate fin heat exchanger. (Description of the Related Art) A plate fin heat exchanger is constituted by a simple structure which is formed by an aluminum alloy having an excellent mechanical strength at low temperatures and in which cooled fluid passages and refrigerant passages are arranged alternately. Therefore, the heat exchanger is much used in plant facilities such as a liquefied natural gas plant etc. requiring heat exchange especially at low temperatures. Meanwhile, mercury is often included in raw material of plant facilities and mercury is apt to remain in a plate fin heat exchanger by exchanging heat of the raw material. At this occasion the aluminum alloy forms mercury amalgam by reacting with mercury. Further, the mercury amalgam forms aluminum hydroxide and regenerates metallic mercury by causing a hydrolysis reaction induced by presence of moisture. Accordingly, when mercury and moisture are present in raw material, in the plate fin heat exchanger, flow passage members constituting cooled fluid passages or refrigerant passages in contact with the raw material are continuously corroded by which the life of the heat exchanger is shortened. Conventionally, corrosion of a plate fin heat exchanger is prevented by carrying out (1) a measure of completely preventing invasion of moisture into plant facilities, (2) a measure of holding the facilities at low temperatures to fix moisture or (3) a measure of constructing a structure capable of completely excluding remaining mercury, to eliminate at least one of mercury and moisture which are substances causing corrosion. However, according to the measures of eliminating substances causing corrosion such as mercury or moisture etc. as in the above-mentioned conventional cases, when the facilities are completely stopped in nonoperating of the plant facilities, the elimination of the substances causing corrosion is apt to be insufficient and accordingly, there is danger of corroding the plate fin heat exchanger. Document US-A-4,189,330 reveals a method for making a plate fin heat exchanger according to the preamble of claim 1. The document discloses a method where the oxidising layer is produced by immersing the surface in an alkaline solution preferably in humid air. These conditions are such that the time period for forming the oxide film is relatively long, meaning that a film of a thickness great enough to prevent mercury particles reaching the surface is not obtained. According to the present invention, there is provided a method of making a plate fin heat exchanger comprising the steps of: forming a main body of the plate fin heat exchanger in which flow passage members constituting cooled fluid passages and refrigerant passages are formed by an aluminum alloy; and forming an oxide film on surfaces of the flow passage members by reacting the aluminum alloy of the flow passage members with an oxidising gas introduced into the flow passages, characterised in that the oxidising gas has an oxygen concentration of 25 to 35 vol% and in that the main body of the plate fin heat exchanger is heated to 250 to 350°C. Preferably, the oxidising gas is sealed in the flow passages during the oxidising reaction. Preferably, the oxide film thickness is from 20 to 170µm. According to this method, in comparison with a case where flow passage members on surfaces of which a film has previously been farmed are integrated, defects of the film caused by welding etc. in assembling operation can be prevented and a uniform film can be formed on the surface of the flow passage members. Fig. 1 is a perspective view of a plate fin heat exchanger; and Fig. 2 is an explanatory view of a dip corrosion test. An explanation will be given of an embodiment according to the present invention in reference to Fig. 1 and Fig. 2. As shown in Fig. 1, a plate fin heat exchanger of the present invention is provided with a plate fin heat exchanger main body 3 (hereinafter, heat exchanger main body 3) having a structure in which pluralities of plate fins 1 which are wavily formed and flat plates are alternately laminated and cooled fluid passages and refrigerant passages are alternately arranged among the contiguous flat plates 2 such that a cooled fluid and a refrigerant are brought into contact via the flat plates 2. An aluminum alloy such as 3003 series material or 5083 series material etc. is used in flow passage members (plate fin 1, flat plate 2) constituting the above-mentioned cooled fluid passages and refrigerant passages and an oxide film is formed on the surface of the flow passage members to prevent corrosion by mercury. This film is provided with a film thickness of 20 through 170 µm such that it is not easily eroded by the flowing cooled fluid or refrigerant and direct contact of mercury that is present in the cooled fluid or the refrigerant with the aluminum alloy that is the material of the flow passage members, is prevented. Further, there exists a naturally formed oxide film on an unprocessed surface of the aluminum alloy. However, in this case the film thickness of the oxide film is not sufficient and accordingly, it is easily eroded by the flowing cooled fluid or refrigerant, mercury invades into defect portions of the films by stress variation or vibration in operation and mercury corrosion is progressed. By contrast, according to the above-mentioned constitution the oxide film is positively formed and the film is provided with a sufficient film thickness whereby the film is not easily eroded and therefore, deficiency of the film caused by erosion by raw material or stress variation and vibration in operation can be prevented. As a result corrosion by mercury can be avoided by preventing contact of mercury with the aluminum alloy over the entire period of time in operating and nonoperating of the plant facilities. The above-mentioned film is formed by introducing an oxidizing gas into internal portions (cooled fluid passages and refrigerant passages) of the heat exchanger main body 3, hermetically sealing inlets and outlets of all the passages, mounting the heat exchanger main body 3 in a heating furnace and leaving the heat exchanger main body 3 in a heating atmosphere for several hours by which the aluminum alloy and the oxidizing component in the oxidizing gas are made react with each other. Further, when an atmospheric gas having the oxygen concentration of 25 through 35 % is used as the oxidizing gas, and the temperature of the heating atmosphere is in a range of 250 through 350°C, it is preferable that the time for leaving the heat exchanger main body (processing time) is approximately 5 hours. The reason for rendering the oxygen concentration in the range of 25 through 35 % when an atmospheric gas is used as the oxidizing gas and the reason for rendering the heating atmosphere in forming the oxide film in the range of 250 through 350°C are as follows. When either one of the oxygen concentration and the heating atmosphere is below a lower limit value (25%, 250°C), the oxygen concentration or the heating temperature is so low that a time period for forming the oxide film is prolonged, it becomes difficult to increase the film thickness and as a result it becomes difficult to form a film to a degree by which mercury particles do not reach material face of aluminum. On the other hand, when either one of the oxygen concentration and the heating atmosphere exceeds an upper limit value (35%, 350°C), while the oxide film is easy to grow, the oxygen concentration or the heating temperature is so high that crystal grains are magnified and accordingly, a film defect to a degree by which mercury particles reach material face of aluminum is formed. In the above-mentioned constitution, it has been confirmed by carrying out the following test that corrosion resistance is improved by the film formed on the heat exchanger main body 3. Firstly, two kinds of aluminum alloy plates having the plate thickness of 3mm and made of 3003 series material and 50.83 series material were prepared. Further, test pieces of 3003 series material and test pieces of 5083 series material were provided by cutting these aluminum alloy plates into a dimension of 10mm x 150mm. Further, as shown in Table 1, as film forming conditions the test pieces were left in a heating atmosphere having the oxygen concentration of 20% at 200°C and with respect to the test pieces of the respective materials, ones formed with oxide films after leaving them for 1 hour and ones formed with oxide films by leaving them for 10 hours, were provided. Thereafter, the heating atmosphere as one of the film forming conditions is changed to 300°C and 400°C and test pieces having the respective materials and formed with oxide films were provided by the procedure similar to the above-mentioned. Film forming conditions Weight increase by corrosion (mg) Oxide film thickness (Å) Oxigen Vol% Temperature °C Time Hr ASME SB209M ASME SB209M 3003 5083 3003 5083 20 200 1 3.8 9.1 21.6 36.3 20 200 10 2.7 7.6 25.3 56.6 20 300 1 4.5 7.5 32.6 73.6 20 300 10 2.9 5.1 45.6 162.3 20 400 1 7.1 11.9 57.0 222.0 20 400 10 3.1 8.8 137.0 556.6 10.1 15.2 - - Next, after measuring the weight of each test piece, the test piece was mounted in a dip corrosion tester (made by Suga Tester DW-UD-3) and as shown in Fig. 2, the test piece was vertically moved in an up and down movement with respect to a water tank storing mercury having a thickness of 40mm and ion-exchanged water having a thickness of 30mm by which a state (dry state) where the test piece was present in the atmosphere and a state (dip state) where the test piece was in contact with ion-exchanged water and mercury, were repeated. Further, the dry state lasted 25 minutes at 30°C and the dip state lasted 5 minutes at 30°C. Thereafter, after repeating the drying and dipping for 1400 times, the weight of each test piece was measured and an weight increase by corrosion was calculated. Further, as test pieces for comparison, two kinds of aluminum alloy plates made of 3003 series material and 5083 series material were prepared, the respective test pieces in a state (unprocessed) in which an oxide film was not formed, were mounted in the dip corrosion tester, the drying and dipping was repeated by 1400 times and under the same conditions the weight increase was calculated. As a result, as shown in Table 1, under the film forming conditions of the oxygen concentration of 20%, the heat treatment temperature of 200 through 400°C and the processing time of 1 through 10 hours, the weight increase by corrosion of the processed test pieces was more alleviated than that of the unprocessed test pieces and it was confirmed that the effect was significant especially at the processing temperature of 300°C. Next, as shown in Table 2, the oxide film was formed with respect to test pieces of two kinds of aluminum alloy plates made of 3003 series material and 5083 series material by changing the oxygen concentration while maintaining constant the heating temperature (300°C) and the processing time (5 hours). Further, a SSRT (Slow Strain Rate Test) test was carried out by using these respective test pieces and unprocessed test pieces for comparison and elongation (mm) up to rupture was measured. Film forming conditions Elongation up to rupture by SSRT test (mm) Oxide film thickness (Å) Oxigen Vol% Temperature °C Time Hr 3003 5083 3003 5083 5 300 5 8.1 2.2 35 63 20 300 5 9.1 7.2 39 68 25 300 5 9.2 7.5 41 70 35 300 5 9.5 7.3 42 70 40 300 5 9.5 3.7 42 42 - 1.4 - - As a result, as shown in Table 2, with respect to the rupture characteristic the 5083 series material shows excellent values at the oxygen concentration of 25 through 35% and the 3003 series material shows excellent values in which the higher the concentration the better the value, under the film forming conditions of the oxygen concentration of 5 through 40%, the heat treatment temperature of 300°C and the processing time of 5 hours. Therefore, it has been confirmed that the mercury corrosion resistance of the heat exchanger can be promoted for both materials of 5083 series material and 3003 series material by maintaining the oxygen concentration at the interior of the heat exchanger at 25 through 35% and by heating the heat exchanger at around 300°C for 5 hours.

Claims: A method of making a plate fin heat exchanger comprising the steps of: forming a main body (3) of the plate fin heat exchanger in which flow passage members constituting cooled fluid passages and refrigerant passages are formed by an aluminum alloy; and forming an oxide film on surfaces of the flow passage members by reacting the aluminum alloy of the flow passage members with an oxidising gas introduced into the flow passages, characterised in that the oxidising gas has an oxygen concentration of 25 to 35 vol% and in that the main body of the plate fin heat exchanger is heated to 250 to 350°C. A method according to claim 1 in which the oxidising gas is sealed in the flow passages during the oxidising reaction. A method according to claim 1 or claim 2 wherein the thickness of the oxide film formed is from 20 to 170 µm.

Inventors: MITSUHASHI KENICHIRO, MITSUHASHI, KENICHIRO, Mitsuhashi, Kenichiro, c/o Takasago Works
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO
Application year: 1996
Main classification: F28F19
IPC codes: F28F3, F25J5, C23C8, F28F19