ES2744883T3 - Aluminum fin and tube heat exchanger - Google Patents

Abstract

A heat exchanger (10) comprising: at least one coil tube (12) having a plurality of longitudinally extending tube lengths (18); and a plurality of heat exchange fins (26) distributed in the at least one tube (12), adjacent fins of the distributed plurality of heat exchange fins having necks (28) disposed around and in contact with the at least a tube (12), wherein the at least one tube (12) comprises a tube (12) made of a first aluminum alloy having a first galvanic potential; and the plurality of heat exchange fins (26) comprise fins (26) made of a second aluminum alloy, the second aluminum alloy having a second galvanic potential, the second galvanic potential being greater than the first galvanic potential; and the at least one coil tube (12) comprises a plurality of fork tubes (14) clamped between spaced tube sheets (22, 24), each fork tube (14) having a pair of tube lengths (18) extending longitudinally and a hairpin bend (20), the plurality of hairpin tubes (14) being fluidly interconnected by a plurality of return elbows (16); characterized in that: the necks (28) are fixation necks (28) and collectively form a protective layer around the at least one tube (12); and in that the attachment necks (28) comprise a widened portion projecting outwardly from a first edge of the fin (26) and a recess provided on the opposite edge of the fin (26), the widened portion of a fin being (28) received in the recess of an adjacent fin (26).

Description

DESCRIPTION

Aluminum tube and fin heat exchanger

Field of the Invention

Generally speaking, the present invention relates to tube and fin heat exchangers and, more specifically, to a tube and fin evaporator heat exchanger completely made of a corrosion resistant aluminum alloy particularly suitable for use in relationship with transport refrigeration systems.

Background of the invention

Generally, perishable cargoes, such as fresh produce, red meat, poultry meat, seafood and other fresh and frozen foods are transported in the refrigerated box of a truck, trailer or transport container. Containers of this type are usually designed to adapt to land transport in trailers, maritime in container ships or rail in platform wagons, and even air in cargo planes. In conventional industrial practice, the truck, trailer or container that carries the perishable cargo is equipped with a refrigeration system to keep the cargo box at a temperature within a specific temperature range to maintain freshness and minimize deterioration in transit . The cooling system is mounted on a wall, usually the front wall, of the truck, trailer or container cargo box.

The cooling system includes a compressor and condensing unit isolated from the cargo box, and an evaporator unit that includes a tubular heat exchanger, commonly a tube and fin heat exchanger, operatively associated with the truck's cargo box, trailer or container The air extracted from the cargo box is passed over the evaporator heat exchanger in relation to heat exchange with refrigerant circulating through the tubes of the heat exchanger whereby the air is cooled before being supplied again to The cargo box.

Conventional evaporator heat exchangers used in transport refrigeration systems have a cylindrical tube structure and plate fins with copper tubes and aluminum fins. For example, US patents. No. 6,325,138 and 6,578,628, assigned to Carrier Corporation, describe heat exchangers of cylindrical tubes and plate fins having copper tubes and aluminum fins with improved resistance to galvanic corrosion. To enhance heat exchange, copper pipes normally have internal improvements and aluminum fins have a wavy design, to increase heat transfer on the coolant side and on the air side respectively. The price of copper pipes with the grooved interior is highly variable, since it depends on the price of the raw copper material and represents a factor that determines the cost of the heat exchanger. Also, copper is three times denser than aluminum. Therefore, in the heat exchangers of conventional copper tubes and aluminum fins, the weight of the heat exchanger is determined primarily by the weight of the copper pipes.

Aluminum pipes are commercially available, but are not currently used in transport refrigeration applications due to several factors, including not only the problem of corrosion in transport refrigeration applications and the lower tensile strength of aluminum compared to of copper and the greater wall thickness required for an identical burst resistance, but also the greater thermal expansion of aluminum with respect to copper, the sensitivity to strong welds for Al / Al joints due to the complexity in terms of geometry since the Melting temperature of Al alloys is very close to the melting temperature of strong welding alloys, and the susceptibility of aluminum to fatigue and rubbing due to high levels of vibration.

Accordingly, there is a desire to develop a heat exchanger having a fully aluminum alloy structure, that is, a heat exchanger having aluminum alloy tubes, aluminum alloy fins and aluminum alloy tube sheets , which has an acceptable corrosion resistance for use in transport refrigeration applications, with less weight and substantially the same heat transfer performance as conventional copper tube and aluminum fin heat exchangers used in refrigeration systems Of transport.

US 2005/0155750 discloses a flap neck with a shape that enhances the application and the flux of flux solder coating within the tube to the fin joint to provide an improved structural and thermal bond.

EP 1887304 describes a heat exchanger with a plurality of fins with fixing necks.

US 3443634 describes a heat exchanger comprising fins comprising fixing flanges.

Summary of the Invention

The present invention provides a heat exchanger comprising: at least one coil tube having a plurality of longitudinally extending tube lengths; and a plurality of heat exchange fins distributed in the at least one tube, adjacent fins of the distributed plurality of heat exchange fins having necks arranged around and in contact with the at least one tube, where the at least one tube it comprises a tube made of a first aluminum alloy that has a first galvanic potential; and the plurality of heat exchange fins comprises fins made of a second aluminum alloy, the second aluminum alloy having a second galvanic potential, the second galvanic potential being higher than the first galvanic potential; and the at least one coil tube comprises a plurality of fork tubes held between sheets of spaced tubes, each fork tube having a pair of longitudinally extending tube lengths and a fork curve, the plurality of fork tubes being fluidly interconnected by a plurality of return elbows, characterized in that the necks are fixing necks and collectively form an integral protective layer around the at least one tube; and because the fixing necks comprise a widened part that projects outward from a first edge of the fin and a recess provided on the opposite edge of the fin, the widened part of a fin received in the recess of an adjacent fin.

Brief description of the drawings

For a better understanding of the description, reference will be made to the following detailed description to be read in relation to the accompanying drawings, where:

Fig. 1 is a perspective view, partially exploded, illustrating a single bank of a tube and fin heat exchanger;

Fig. 2 is a perspective view of the return elbow end of the mounted heat exchanger of Fig. 1; Fig. 3 is an elevational and sectional view of the fin and tube interface of an aluminum tube and aluminum fin heat exchanger according to the description herein;

Fig. 4 is a sectional view through an aluminum tube with the grooved interior of an embodiment of the heat exchanger described herein;

Fig. 5 is a cross-sectional view of the tube with the grooved interior taken substantially along line 5-5 of Fig. 4; Y

Fig. 6 is an elevation view of an enlarged section of the tube with the grooved interior of Fig. 5.

Detailed description of the invention

First, with reference to Figs. 1 and 2 of the drawings, an exemplary embodiment of a cylindrical tube and plate heat exchanger 10 of the general type commonly used as an evaporator in transport refrigeration systems showing a bench is shown of coil tubes 12. It is to be understood that the typical heat exchanger 10 will have a plurality of coil tube banks 12 arranged in a lateral spacing relationship, for example, as shown in Fig. 2. The tube of coil 12 is formed by a plurality of fork tube sections 14 connected in fluid communication by return elbows 16. Each fork tube section 14 has a pair of generally parallel lengths of tube 18 extending longitudinally and a fork elbow with U-turn 20. Tube sheets 22 and 24, which are arranged spaced apart at opposite ends of tube lengths 18 that extend den longitudinally, they are traversed by each tube length 18 of the plurality of tube banks 12 and provide structural support for the tube banks. It is to be understood that two sections of straight tubes connected by a return bend 16 can be replaced by a single fork tube section 14. Also, one or more sheets of intermediate tubes can be incorporated in the heat exchanger 10.

The heat exchanger 10 includes a plurality of fins 26 arranged in a parallel spacing relationship and extending transversely to the lengths of tube 18 that extend longitudinally. Each flap 26 comprises a sheet-type plate fin having a set of openings therein, each opening serving to accommodate a length of tube 18 that passes through said opening when the heat exchanger 10 is mounted. To mount the heat exchanger 10 , each fin 26 of the plurality of sheet type plate fins slides by the set of tube lengths 18, one behind the other, each opening of each fin 26 receiving a corresponding length of tube 18, until all fins 26 are mounted in the plurality of tube lengths 18. Therefore ^{, the lengths of tube} 18 ^{expand radially outwardly from within tube lengths 18} whereby the outer wall of each length of tube 18 is forced into contact with tightening with each of the neck portions 28 surrounding the openings of each of the fins 26. When the set of fins 26, commonly referred to as the fin pack, is mounted on the tube lengths 18, the fin pack extends between the spaced tube sheets 22 and 24 and above substantially the length full of tube lengths 18. The spacing between the fins within the fin pack is generally defined by the neck portions 28. The fins 26 may have thermal performance improvement features such as corrugations, waves, crevices, trellis or similar characteristics.

The neck portions 28 have a widened portion that projects outwardly from a first edge of the fin and a recess provided on the opposite edge of the fin. When the fins 26 are distributed in a face-to-face spacing relationship during the assembly process, the flap neck portions 28 fit the widened portion of a fin received in the recess of the adjacent fin. When the tube lengths 18 expand radially outward, the flap neck portions 28 are crushed thereby fixing the enlarged portions with the associated recesses. As a result, any gap or gap that may have existed between adjacent fixing fins 26 in the assembled fin packet assembly is eliminated and an integral layer is formed around the outer wall of the tube lengths 18.

In one aspect of the heat exchanger 10 described herein, the tubes 12, which include the fork tube sections 14 and the return elbows 16, the tube sheets 22 and 24 and the fins 26 are all made of alloys. of aluminum. The tubes 12 are made of a first aluminum alloy, the tube sheets 22, 24 are made of a second aluminum alloy, and the fins 26 are made of a third aluminum alloy, having the second and third aluminum alloy greater galvanic potential compared to the first aluminum alloy. Therefore, the second and third aluminum alloy act as sacrificial anodes to protect the first alloy. Therefore, aluminum alloy tube sheets 22, 24 and aluminum alloy fins 26 act as sacrificial anodes to protect aluminum alloy tubes 12.

The selection of the combination of tube alloy, that is, the first aluminum alloy, and fin alloy, that is, the third aluminum alloy, is critical to provide the aluminum heat exchanger with adequate corrosion resistance. long-term. The alloy of the fin must act as a sacrificial anode to protect the tube in environmental operating conditions. As a general rule, the difference in galvanic potential between the third aluminum alloy, that is, the fin alloy, and the first aluminum alloy, that is, the tube alloy, is greater than about 20 millivolts. Likewise, the third aluminum alloy, that is, the fin alloy, should show reduced corrosion rates in environmental operating conditions. Examples of good combinations of fin alloy and tube alloy for an evaporator heat exchanger for transport cooling applications include: a tube made of AA3003 aluminum alloy with fins made of AA1100 or AA7072 aluminum alloy, a tube made of AA30048 aluminum alloy and a fin made of AA7072 aluminum alloy, and a tube made of AA3102 aluminum alloy and a fin made of AA7072 aluminum alloy. The aluminum alloy specifications are those of the standards of the American Aluminum Association. Using such combinations, the fixing aluminum fins 26 form a cover layer that acts as a sacrificial anode to protect the underlying aluminum fork tubes 14 and that provides added corrosion resistance in the fin pack area.

Similarly, the tube sheets 22, 224 should also act as sacrificial anodes to protect the fork tubes 14 that pass through said sheets. The second aluminum alloy from which the tube sheets 22, 24 are made should have a significant galvanic potential difference, generally also greater than about 20 millivolts, relative to the first aluminum alloy from which the tubes are made. of fork 14. Examples of good combinations of tube alloy and tube sheet alloy for an evaporator heat exchanger for transport cooling applications include: tubes made of AA3003 aluminum alloy with tube sheets made of alloy AA.5052 aluminum, tubes made of AA30048 aluminum alloy with tube sheets made of AA7072 aluminum alloy, and tubes made of AA3102 aluminum alloy with tube sheets made of AA7072 aluminum alloy.

Given the high thermal gradient in the refrigeration environment and high vibration loads in transport applications, and the lower tensile strength of aluminum with respect to copper, interference between tubes and sheet of tubes can be a problem if not Sufficient space is provided to avoid long-term damage of the tubes due to rubbing. The space between the tubes and the tube sheet must be maintained from the point of view of thermal stress but should be minimized by vibrations. The space, Ts, measured in a radial direction as indicated in Fig. 5, between the openings in the tube sheet 22, 24 through which a cylindrical tube 14 having an outside diameter D passes, can be selected so that the dimensionless ratio Ts / D has a value that range between 0.0013 and 0.0363.

The return elbows 16 and the fork curves 20 of the fork tubes 14 are disposed outside the area of the fin pack and, therefore, these portions of the coil tube 12 are not protected by the sacrificial layer provided by the fixing fins 26. Protective coating may be applied to the return elbows 16 and the fork curves 20 of the fork tubes 14 to provide corrosion resistance to the fork curves 20 and the return elbows 16 equivalent to The design life of the fin heat exchanger fin area. For example, protective coatings can be applied to the ends of the tube sheet to coat the fork curves 20 and the return elbows 16. Acceptable protective coatings include, for example, non-restrictive, base varnish transparent solvent acrylic, urethane-based varnish, conversion coatings, acrylic electrophoretic coatings, electrophoretic coatings of epoxy resin, electrophoretic coatings corrosion inhibitor, polyester powder, powder coatings and 2-component pre-treated epoxy resins, which They can be applied according to the level of protection desired.

If desired, a protective coating may be applied to the entire heat exchanger 10 to provide additional corrosion resistance under severe environmental exposure conditions that may occur in some transport refrigeration applications. Examples of protective coatings that can be applied to the entire heat exchanger 10 include, but are not limited to, trivalent chromium conversion coatings, zirconium conversion coatings, zinc phosphate coatings and iron phosphate coatings. Also, any of the previously mentioned coatings for application to the fork curves 20 and the return elbows 16 can be applied to the entire heat exchanger 10 as a pretreatment followed by the application of an additional layer of the mentioned coatings.

The heat exchanger coils of aluminum cylindrical tubes and aluminum plate fins can replace the heat exchanger coils of a similar structure with cylindrical copper tubes and aluminum fins achieving an equivalent thermal efficiency in a wide variety of environmental conditions and operating parameters. Aluminum pipes need to have a thicker wall due to the lower tensile strength of aluminum alloys compared to copper alloys. The aluminum tubes used in heat exchanger applications may have the grooved interior or, conversely, may be provided with improvements in the form of internal fins such as those illustrated in the exemplary embodiment of the tube 14 shown in Fig. 4- 6.

The table below shows several dimensionless groups representing internal tube geometry with enhanced thermal performance and improved structural rigidity (see associated Fig. 4-6 for individual parameter definitions).

Table I

Likewise, the perimeter ratio of the fin to the cross section of the fin for the internal improvements of the fin may have a value ranging from 400 to 650 (expressed in inches'1). With reference to Fig. 6, the fin perimeter / fin cross-sectional ratio is defined as (ak 2 * (((ak) / 2) A2 hA2) A0.5) / (h * (a + k) / two).

The terminology used in this document is descriptive and not restrictive. Structural details and specific functionalities described herein are not to be construed as restrictive, but simply as a basis for teaching those skilled in the art to employ the present invention. Those skilled in the art will also recognize equivalents that may replace elements described with reference to exemplary embodiments described herein without departing from the scope of the present invention as defined in the claims.

Although this invention has been shown and described in particular with reference to exemplary embodiments as illustrated in the drawings, those skilled in the art will recognize that various modifications can be made without departing from the scope of the invention as defined in the claims. Therefore, it is intended that the present description is not limited to the particular embodiment or embodiments described, but that the description includes all embodiments within the scope of the appended claims.

Claims (12)

1. A heat exchanger (10) comprising: at least one coil tube (12) having a plurality of lengths of tube (18) extending longitudinally; Y a plurality of heat exchange fins (26) distributed in the at least one tube (12), adjacent fins of the distributed plurality of heat exchange fins having necks (28) arranged around and in contact with the at least one tube (12), wherein the at least one tube (12) comprises a tube (12) made of a first aluminum alloy having a first galvanic potential; and the plurality of heat exchange fins (26) comprises fins (26) made of a second aluminum alloy, the second aluminum alloy having a second galvanic potential, the second galvanic potential being greater than the first galvanic potential; Y the at least one coil tube (12) comprises a plurality of fork tubes (14) held between spaced tube sheets (22, 24), each fork tube (14) having a pair of tube lengths (18) that they extend longitudinally and a fork curve (20), the plurality of fork tubes (14) being fluidly interconnected by a plurality of return elbows (16); characterized in that: the necks (28) are fixing necks (28) and collectively form a protective layer around the at least one tube (12); and because the fixing necks (28) comprise a widened part that projects outwardly from a first edge of ^{the fin (26) and a recess provided on the opposite edge of the fin (} 26 ^{), the widened part of a fin} (28 ) received in the recess of an adjacent fin (26). 2. The heat exchanger (10) as described in claim 1 wherein the protective layer around the at least one tube (12) comprises a layer that acts as a sacrificial anode to protect the at least one tube (12). 3. The heat exchanger (10) as described in claim 1 or 2 wherein each tube (12) comprises a sheet of tubes (22, 24) made of a third aluminum alloy, the third aluminum alloy having a third Galvanic potential, the third galvanic potential being greater than the first galvanic potential. 4. The heat exchanger (10) as described in claim 3 wherein the first aluminum alloy comprises the AA3003 aluminum alloy and the third aluminum alloy comprises the AA5052 aluminum alloy. 5. The heat exchanger (10) as described in claim 3 wherein the first aluminum alloy comprises an aluminum alloy selected from the group consisting of AA30048 and AA3102 and the third aluminum alloy comprises the AA7072 aluminum alloy. 6. The heat exchanger (10) as described in claim 3, 4 or 5 wherein the second aluminum alloy and the third aluminum alloy comprise the same aluminum alloy. 7. The heat exchanger (10) as described in any of the preceding claims wherein the protective coating comprises a coating to enhance the corrosion resistance selected from the group that includes acrylic solvent-based varnish to the transparent solvent, varnish with urethane base, conversion coatings, acrylic electrophoretic coatings, epoxy resin electrophoretic coatings, corrosion inhibiting electrophoretic coatings, polyester powder, powder coatings and pre-treated 2-component epoxy resins. 8. The heat exchanger (10) as described in any of the preceding claims wherein the heat exchanger (10) is coated with a protective coating to further enhance corrosion resistance. 9. The heat exchanger (10) as described in claim 8 wherein the protective coating comprises a coating to enhance the corrosion resistance selected from the group including trivalent chromium conversion coatings, zirconium conversion coatings , zinc phosphate coatings and iron phosphate coatings. 10. The heat exchanger (10) as described in claim 1 for use as an evaporator coil in relation to a transport cooling system, Acting the tube sheets (22, 24) as sacrificial anodes to protect the fork tubes (14). 11. The heat exchanger (10) as described in any of the preceding claims wherein each tube sheet (22, 24) has a plurality of tube holes therein, each tube hole receiving a tube length of a respective tube of the plurality of fork tubes (14), each tube length having an outer diameter, D, and each hole having a space, Ts, with respect to the tube length, where the ratio Ts / D has a value that oscillates between 0.0013 and 0.0363. 12. The heat exchanger (10) as described in any of the preceding claims wherein the fork curves and return elbows are coated with a protective coating to enhance corrosion resistance.