RU2043885C1 - Heat exchanger manufacturing method - Google Patents

Abstract

FIELD: power plants, particularly, water turbines and electric power generators. SUBSTANCE: method involves manufacturing of heat exchanger with finned tubes by making one part from aluminum and other part from ferrite material; cladding aluminum surface to be soldered; applying aluminum coating onto ferrite material; assembling parts and applying flux; heating assembly to temperature sufficient for at least partial fusing of cladding layer and coating for time interval sufficient for changing coating and cladding layer into ferro-aluminum intermetallic connection; flatting heat exchanger tube and making fin in the form of coil. Steel may be used as ferrite material and aluminum-silicon alloy as cladding layer and coating. EFFECT: increased efficiency and high quality of heat exchanger. 5 cl, 5 tbl

Description

 The invention relates to the field of soldering, in particular to a method for manufacturing a heat exchanger consisting of tubes and fins.

 The heat exchanger can be used in a power plant, with steam condensation as applied to drive turbines in power generation plants.

 A known method of manufacturing a heat exchanger consisting of tubes and fins, according to which parts of the heat exchanger are made of at least aluminum, a coating and a clad layer are applied to the brazed surface, the parts are assembled with a flux and heated to soldering temperature.

 However, the known method of manufacturing a heat exchanger does not provide sufficient corrosion resistance of the heat exchanger.

 The purpose of the invention is to create a heat exchanger with high thermal efficiency due to high thermal conductivity and good corrosion resistance.

 This goal is achieved by the fact that one of the parts of the heat exchanger is made of aluminum, and the other is made of a ferrite material on a brazed aluminum surface, a clad layer is applied, and on the ferrite material is an aluminum coating, parts are assembled, flux is applied and heated to a temperature sufficient for at least partial melting of the cladding layer and coating over time until the coating and cladding layer are converted to a ferroaluminium intermetallic compound.

 In one case, the heat exchanger tube is pre-flattened, and in the other, the ribs are taken in the form of a coil. Steel can be used as a ferrite material, and an aluminum-silicon alloy is used as a cladding layer and coating.

 Figure 1 shows a heat exchanger with a flattened pipe; in Fig.2 a section aa in Fig. 1; figure 3 heat exchanger with fins in the form of a coil; figure 4 assembly of the rib-tube before soldering; figure 5 assembly of the rib-tube after soldering.

 A method of manufacturing a heat exchanger is as follows.

 Tubes with ribs are interconnected. Figure 1, 2 and 3 presents the structural parts of the heat exchanger. Each branch of the heat exchanger contains a flattened pipe 1 having opposite flat sides 2 and 3.

 The fins 4 made in the form of coils are also soldered to the pipe, usually one serpentine fins 4 on each of the sides 2 and 3, i.e. the ridges 5 of each serpentine rib come into contact with the corresponding one of the sides 2 or 3 and are soldered to it, while soldered joints 6 are formed at the junction of the ribs 4 with the corresponding side 2 or 3.

 As can be seen from FIG. 3, pipes 1 with ribs 4 soldered to them are installed close to each other as a whole parallel to ridges 7, which are opposite to ridges 5 soldered to pipes 1 close to each other. Typically, the nominal gap will be on the order of 1/16 inch (1.58 mm).

 In some cases, the coil ribs 4 will overlap, i.e. if we take a view in section AA on one of the sections of each rib 4 between two adjacent ridges 5 and 7, then a series of blinds 8 that open in one direction will be located on one side of the rib, and blinds 9 that open in the opposite direction on the other side ribs. The blinds 8 and 9 can be customarily manufactured along the entire length of each serpentine fins 4. Alternatively, the fins 4 may not have blinds and be flat or corrugated.

 When the heat exchanger is filled with a liquid cooler, the pipes are made of steel. According to a most preferred embodiment, round tubes having an outer diameter of about 3 inches (7.62 cm) and a thickness of about 0.065 inches (1.65 mm) are made flattened on a hydraulic press. Prior to flattening, the pipe is usually coated with aluminum. The aluminum coating may contain about 9% silicon, while other impurities are within normal limits.

 The ribs 4 are made of aluminum with a clad layer 10. In the first stage of manufacturing the node from the ribs and pipes is the flattening of the coated aluminum pipe.

 The flattened pipe is then subjected to the usual degreasing operation, and then, but not necessarily, can be subjected to flux treatment.

Potassium fluoroaluminate can be used as the flux. It is preferred that the flux is applied by spraying onto the outside of the pipes. The application of flux is carried out with a concentration of approximately 30-40 g / m ² the surface area. Rib 4 undergoes a normal degreasing operation, and after that is performed, but not necessarily, flux treatment using the same flux and the application concentration.

Then, both the pipe 1 and the rib 4 with the flux applied to them are drained. For example, they can be dried in a hot air oven at a temperature of ^about 285-300 F (140-148 ^C) for about 5 min.

 At this moment, the rib 4 is assembled with the pipe 1, and usually the rib 4 is superimposed on each of the sides 2 and 3. This can be done by using appropriate clamping devices or other fittings with the guarantee that many ridges 5 of each rib 4 are adjacent to corresponding flat side 2 or 3.

 Alternatively, but not necessarily, the assembly of the ribs 4 with the pipe 10 can immediately be degreased. If this is done, then the assembled pipe and ribs are subjected to flux treatment.

 The assembly of the pipe 1 and the ribs 4 with the flux deposited is then placed in a melting furnace and its temperature rises to the soldering temperature. Of course, if vacuum brazing is performed in a smelting furnace, magnesium should usually be added to the system, while all stages of flux application can be omitted. The assembly temperature rises above the solidus of the cladding layer based on aluminum. Preferably, the assembly temperature rises just above the liquidus of the aluminum-based clad layer.

 This temperature is maintained only for the time necessary for the formation of the solder joint 6. Heating ends after the formation of the ferroaluminium intermetallic phase compound.

 Directly at the steel base 11, instead of a relatively thin ferroaluminium intermetallic compound or an intermediate phase layer 12, is a relatively iron-rich first layer 13 of a ferroaluminium intermetallic compound or an intermediate phase. Layer 13, in turn, is coated with a relatively aluminum-rich second layer 14 of a ferroaluminium intermetallic compound or an intermediate phase. The first layer 13 has more steel than the second layer 14.

 Finally, the second layer 14 will be coated with a layer 15, which is preferably intended to be made on an aluminum-based alloy.

 It can be assumed that the first layer 13 is very similar to layer 12 and can reach a thickness not greater than the thickness of layer 12, which occurs when the temperature rises during the brazing process. Layer 14 is formed by the reaction at the interface of the thickening layer 12, now becoming the layer 13, and the cladding layer 10 and / or the rest of the coating 16, as soon as the solidus temperature is reached solidus. Steel in a second ferroaluminium intermetallic compound or intermediate phase layer 14 is provided from layer 12.

 The corrosion resistance demonstrated by the resulting assembly of ribs and pipes is very unexpected. More specifically, the formation of a ferroaluminium intermetallic compound or an intermediate phase upon heating of an aluminum-coated steel is known, and it is also known that the intermetallic compound or intermediate phase is sufficiently brittle. Therefore, although the corrosion resistance of the intermetallic compound or the intermediate phase can be satisfactory for many purposes, its brittleness increases the likelihood of a brittle layer that can easily peel off of the base it protects and expose this base, leading to corrosion if a solid connection with the ribs is not ensured .

 Quite unexpectedly, by means of the invention, even in the region between the ridges 5, an aluminum layer 15 is formed on top of the layers 13 and 14 from the intermetallic compound or the intermediate phase to prevent peeling and ensure good corrosion resistance caused by aluminum as soon as the oxide is formed, and also provides good adhesion between the rib and a pipe with unpredictable strength.

 Soldering is carried out in a nitrogen-blown furnace.

First, for drying purposes, the assembled fin and pipe are passed through relatively low temperature zones. Three zones are used, while the residence time in each zone can be from 3 to 4.5 minutes. Zones are consistently rising temperature of the order of ^{200 °} F (93 ° ^C) to 250 ^o F (121 ^o C) and ^{300 °} F (148 ° ^C). Thereafter, the collected pipe and fins are subjected to preheat in five zones, the temperature is gradually increased from 650 ^° F (343 ° ^C) to 840 ^{of the} F (448 ^o C), 955 ^o F (512 ^o C), 1015 ^o F ( 546 ^o C), 1000 ^o F (537 ^o C). The residence time in each zone is in the range from 2.0 to 3.2 minutes.

Soldering takes place in five zones, the temperature in which, respectively, from beginning to end is 1000 ^o F (537 ^o C), 1120 ^o F (604 ^o C), 1130 ^o F (610 ^o C), 1130 ^o F (610 ^o C) and 1115 ^° F (601 ^° C). The time spent in the respective zones is in the following ranges: 5 2/3 8 1/2 min, 3 1/4 4 5/6 min, 2 3/4 4.2 min, 2 3/4 4.2 min, from 2 1/2 to 3.8 min.

A nitrogen stream of about 3200 cubic feet per hour (-86.4 m ³ / h) is used, so that the dew point in the furnace is minus 40 ^o F (-40 ^o C) and the oxygen content is less than 10 ppm.

Alternatively, the batch soldering process can be carried out in a programmable vacuum brazing furnace, for example in an Ipsen vacuum furnace. An initial low vacuum of up to 40 microns of pressure represents the first pumping. After that, to carry out the cycle at high vacuum, the conventional diffusion pump is kept open for two minutes to obtain a pressure of approximately 3x10 ^-4 Torr (a vacuum of the order of 1 Torr is equivalent to 1000 μm). During a cycle of high vacuum in the furnace temperature is increased at a rate of about 50 ^o F (10 ° ^C) per minute up until it reaches 100 ^o F (37 ^o C). When this happens, the diffusion pump closes and the cycle ends at high vacuum.

Immediately after this, the furnace is filled with nitrogen under a pressure of the order of one atmosphere. When filling is completed, a constant flow of nitrogen is maintained at approximately 9.5 cubic feet per minute (0.25 m ³ / min). During this time the temperature increases in the furnace at 100 ^o F (37 ° ^C) per minute until a temperature ^of approximately 970 F (521 ^C). For this temperature increase, approximately 8.7 minutes are required.

The temperature rises from ^about 970 F (521 ^C) to ¹¹⁰⁰ F (593 ^C) at a rate ^of about 50 F (10 ° ^C) per minute. At ¹¹⁰⁰ F (593 ^C) is commonly performed and this five minute exposure would enhance subjected to soldering temperatures assembly of fins and pipes ^of up to about 980 F (528 ^C).

Finally, the temperature is increased from ¹¹⁰⁰ F (593 ^C) to a maximum temperature ^of 1170 F (632 ^C) at a speed of ^about 25 F (3,8 ° ^C) per minute. When the temperature reached ^about 1170 F (632 ^C) requires the final exposure period of about 8 minutes to raise the temperature of the assembly of tubes and fins ^of up to about 1125 F (607 ^C). At this point, the normal nitrogen cooling cycle begins.

During this cycle of the dew point in the furnace is about ^{40 °} F (-40 ° ^C).

When using this method, the corrosion resistance of pipes made with an aluminum coating with a concentration of about 0.25 oz (7.07 g) per square foot (0.09 m ² ) is ensured. The corrosion resistance is equal to or greater than the corrosion resistance of pipes coated with a concentration of about 0.4 or even 0.6 ounces (11.3 or 17 grams) per square foot (10.09 m ² ).

 Strong cordon and corrosion resistance are obtained by coating virtually that part of the entire structure close to the ribs, which is obtained from the plating being brazed, flowing from the ribs to the outer surface of the pipe. It can also be assumed that cathodic protection for steel is provided by the use of an uncoated aluminum fin.

 In connection with the above, since only one side of the rib is connected to the pipe, cladding for soldering can be used only on this side. However, this reduces the amount of cladding for soldering that can flow from the rib to the pipe, and when ribs with blinds are used, the presence of the blinds enhances the flow of cladding for soldering to the pipe from the side of the rib opposite the pipe. That is, it can be assumed that cladding for soldering from the side of the rib opposite the pipe flows to the side of the rib at the pipe, and then to the pipe through the blinds.

 It can be expected that the use of a tube and rib assembly made according to the invention will provide a 30% increase in heating savings when used as a steam condenser compared to an all-steel construction made of galvanized ribs and pipes, or pipes equipped with wound aluminum pipes. As a result, capacitors using tube and rib assemblies according to the invention can be made with one row of pipes rather than with two or more rows to reduce the cost of their construction. In addition, the use of one row of ribs will reduce the drop in air pressure, while reducing energy costs during fan operation.

 Although the invention has been described with reference to flattened pipes and serpentine fins, it is obvious that round or other shapes of pipes and plates can also be used, or wound ribs can be used, whereby strong solder joints of steel with aluminum are achieved according to the invention.

 The assemblies of pipes and fins according to the invention can be used in many diverse environmental conditions requiring corrosion-resistant heat exchangers, and are not limited to use only in electrical installations.

Claims (5)

 1. METHOD FOR PRODUCING A HEAT EXCHANGER, consisting of tubes with fins, in which at least one of the parts is made of aluminum, a plated layer is applied to the aluminum brazed surface, parts are assembled, a flux is applied and heated to a brazing temperature, characterized in that the other part of the heat exchanger made of ferritic material coated with aluminum and heated to a temperature sufficient to at least partially melt the cladding layer and coating and for a time until the coating is transformed and cladding layer in a ferroaluminium intermetallic compound.  2. The method according to p. 1, characterized in that the tube is pre-flattened.  3. The method according to p. 1, characterized in that the fins of the heat exchanger take in the form of a coil.  4. The method according to claim 1, characterized in that steel is taken as the ferrite material.  5. The method according to p. 1, characterized in that the aluminum-silicon alloy is taken as a coating and a clad layer.

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