HK1142949B - Heat exchanger, assembly comprising the same and method of making a heat exchanger - Google Patents

Description

Heat transfer tube, heat exchanger assembly including the same, and method of manufacturing heat exchanger

The present application is a divisional application of chinese patent application entitled "heat transfer tube made of tin brass alloy" having an application date of 4/5/2005, an application number of 200580014066.5.

Technical Field

The present invention relates generally to corrosion resistant heat transfer tubes for use in air conditioning refrigeration systems.

Background

Air conditioning refrigeration ("ACR") systems are known to be susceptible to a unique type of corrosion known as "formicary corrosion" (also referred to as "formicary corrosion"). Formicary corrosion is believed to occur only in copper-based alloys. It has a unique topography, and it appears that the interspersed pits give the impression of an ant nest shape. Typically, the pits are usually microscopic and are accompanied by discoloration of the copper surface. The discoloration may be purple, blue-gray, or black in color, which manifests itself as a catalyzed, periodic oxidation/reduction reaction that occurs between the water-soluble copper and the organic acid complex.

Formicary corrosion can cause damage to the heat exchanger coils. Most of the early observations of formicary corrosion showed that the reaction started from the inside of the tube and progressed to the outside of the tube. This is generally believed to be caused by the breakdown of certain lubricants in the presence of air. These conditions are generally limited to certain areas and can be observed once the Air Conditioning (AC) unit is loaded with refrigerant and put into service.

Recently, the heating, ventilation and air conditioning (HVAC) -related industry has been concerned with outside-to-inside corrosion. About 5 years ago, it was noted that the number of customer complaints about indoor coil leaks of ventilated AC systems, which are the predominant form of cooling systems in the us residential market, has increased dramatically. Further investigation revealed that more than half of the heat exchanger coil failed due to formicary corrosion. The major sources of this increased leakage event are believed to be attributable to the use of larger amounts of volatile organic compounds, increased emissions from new building materials, and less circulation within the room due to dense construction. In addition, reduced heat exchange tube wall thickness and increased AC system operation in hot, humid areas to control temperature and internal humidity may also be factors.

Without being bound to a particular theory, the occurrence of formicary corrosion is believed to require four elements: copper metal, oxygen, moisture, and the presence of organic acids (which may result from hydrolysis or other decomposition of oils or other organic molecules). Since it is desirable to prevent the corrosion process by removing any of these factors, copper is the preferred metal for heat exchange systems, there is a need for copper-based materials for ACR systems with enhanced resistance to formicary corrosion. Furthermore, there is a need for heat exchange tubes for ACR systems that are constructed of compositions that are resistant to formicary corrosion, such that the entire tube wall is resistant to corrosion.

Disclosure of Invention

The present invention meets the above-described needs by providing a heat transfer tube constructed of a tin brass alloy that is excellent in resistance to formicary corrosion.

Some embodiments of the invention also relate to the following items.

1. A formicary corrosion resistant heat transfer tube comprising a tin brass alloy.

2. The heat transfer tube of item 1, comprising up to 3.0% tin.

3. The heat transfer tube of item 1, comprising 0.8-1.4% tin.

4. The heat transfer tube of item 1, comprising 86% -90% copper.

5. The heat transfer tube of item 1, comprising 86% -89% copper.

6. The heat transfer tube of item 1, comprising 9.6% to 13.2% zinc.

7. The heat transfer tube of item 1, comprising up to 35% zinc.

8. The heat transfer tube of item 1, comprising no more than 0.05% lead.

9. The heat transfer tube of item 1, comprising no more than 0.05% iron.

10. The heat transfer tube of item 1, comprising no more than 90% copper, no more than 3.0% tin, and no more than 13.2% zinc.

11. The heat transfer tube of item 1, consisting essentially of 86.0% -90.0% copper, 0.8% -3.0% tin, no more than 0.05% lead, no more than 0.05% iron, no more than 0.35% phosphorus, and the balance zinc.

12. The heat transfer tube of item 1, consisting essentially of 86.0% -89.0% copper, 0.8% -1.4% tin, no more than 0.05% lead, no more than 0.05% iron, no more than 0.35% phosphorus, and the balance zinc.

13. The heat transfer tube of item 1, wherein the tube is formed from alloy C422.

14. The heat transfer tube of item 1, wherein the tube is formed from alloy C425.

15. A heat exchanger assembly comprising the heat exchange tube of item 1, further comprising a plurality of fins and at least one tube sheet.

16. The heat exchange tube of item 1, wherein the tube is formed by welding, extrusion, or cast rolling.

17. A heat exchanger assembly for an ACR system, the heat exchanger comprising a formicary corrosion resistant heat transfer tube comprising a tin brass alloy.

18. A method of making a heat exchanger, comprising: the heat transfer tube wall is made of a tin brass alloy.

19. The method of item 18, wherein the tube wall is formed of alloy C422.

20. The method of clause 18, wherein the tube wall comprises up to 3.0% tin.

21. The method of clause 18, wherein the tube wall comprises 0.8-1.4% tin.

22. The method of clause 18, wherein the tube wall comprises up to 35% zinc.

23. The method of clause 18, wherein the tube wall consists essentially of 86.0% to 90.0% copper, 0.8% to 3.0% tin, no more than 0.05% lead, no more than 0.05% iron, no more than 0.35% phosphorus, and the balance zinc.

24. The method of clause 18, wherein the heat exchanger is installed in an ACR system.

Drawings

The present invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which:

FIG. 1 is a perspective view of a portion of a heat transfer tube;

FIG. 2 is a perspective view of a fin-type heat exchanger; and

fig. 3 is a graphical representation of corrosion resistance data for different alloys.

Detailed Description

The present invention provides a heat transfer tube that is resistant to formicary corrosion. The heat transfer tube is constructed of a tin brass alloy. The tin brass alloy provides the heat transfer tube with enhanced resistance to formicary corrosion relative to prior art compositions used to manufacture the heat transfer tube. Some examples of these prior art compositions include C220 alloy, which is a standard brass alloy, and C122 alloy, which is currently used in AC pipe. Thus, "formicary corrosion resistance" as used herein refers to the degree of enhanced formicary corrosion resistance relative to the formicary corrosion resistance of prior art compositions used to manufacture heat transfer tubes.

In one embodiment, the heat transfer tube of the present invention is constructed of an alloy comprising copper, zinc, and tin, and may or may not further comprise lead, iron, phosphorus, or other elements. In a preferred embodiment, the heat transfer tube is constructed of an alloy designated C422, the composition of which is set forth in the specific embodiments described in tables 1 and 3 below.

The following examples are intended to illustrate the invention and do not limit its scope in any way.

Example 1

This example describes one embodiment of the present invention. Routine modifications and variations to this configuration are also contemplated, as would be apparent to those skilled in the art.

Referring to fig. 1, a portion of a tube is shown generally at 20, which may be composed of a tin brass alloy and is specifically tin brass alloy No.422 in the preferred embodiment. The tube 20 may be formed by a number of different methods including, but not limited to, welding, extrusion, and cast rolling using the alloys provided herein and standard methods well known to those skilled in the art.

The tube 20 may be used, for example, in a heating or cooling system for heat transfer between a fluid of one temperature flowing inside the tube and a fluid of a different temperature flowing outside the tube, where one example of a fluid outside the tube may be air. The tube 20 includes a wall 22 having an inner surface 24 and a longitudinal axis 26. The inner surface may be smooth walled or may be internally reinforced with reinforcement 28, as will be apparent to those skilled in the art.

Turning to fig. 2, the tube 20 of the present invention may be contained within a heat exchanger assembly 21. The heat exchanger package 21 is one example of a heat exchanger package that may be used in the manner of the present invention. The present invention also applies to other types of ACR heat exchangers, as will be apparent to those skilled in the art.

A common method of manufacturing the heat exchanger assembly 21 is to first assemble a plurality of fins 32 between two tube sheets 34 and then pass a plurality of hairpin tubes 36 through selected holes 38 in the fins 32 and similar holes 38 in each tube sheet 34. Next, a bell is formed at the end of the hairpin tube 36, and the tube arms are then extended to ensure a strong mechanical fit between the tube and the fin. The heat transfer area of the heat exchange tubes is increased by the area of the fins due to the robust fit between the tubes and the fins. The fin and tube heat exchanger provides improved heat transfer performance over a light pipe type heat exchanger of the same size due to the increased surface area. The heat exchange assembly is completed by providing a plurality of U-bends 42 at the ends of the hairpin tubes 36 to form one or more closed fluid flow paths through the heat exchanger tubes.

The tube-to-sheet connection may also be made by brazing, as will be apparent to those skilled in the art.

Example 2

This example demonstrates the effect of exposure to formic acid on metal strip and pipe, including the tin brass alloy of the present invention, and those commonly used in air conditioning refrigeration systems, such as the standard brass alloy C220 alloy, and the C122 alloy currently used in many AC ducts.

For formicary corrosion resistance testing, one approximately 1 "x 1" sample of each alloy (as summarized in table 1) separated with teflon was hung in the headspace of three sealed cylinders, each containing 100ml aliquots of 500ppm formic acid solution.

TABLE 1

Nominal alloy composition (%)

Alloy (I)

Cu

Pb

Fe

Sn

Zn

P

C122

rem＊(containing Ag)

0,015-0,040

C220

89.0-91.0

0.05 max

rem.＊

C422

86.0-89.0

0.05 max

0.05 max

0.8-1.4

rem，＊

0.35 max

C425

87.0-90.0

0,05 max

0.05 max

1.5-3.0

rem.＊

0.35 max

^＊rem is the rest

The other three tanks containing 100ml aliquots of acetic acid solution were also 500ppm in concentration. The seventh jar was controlled with a 100ml aliquot of deionized water (DIW). The cylinder is subjected to cyclic heating substantially as described in example 3 below. One jar containing formic acid solution and another jar containing acetic acid solution were removed from the test on days 10, 20 and 30. The DIW control was removed on day 30.

Upon removal, the metal coupons were mounted on epoxy to evaluate the depth of formicary corrosion and pitting density. The pitting density was calculated as the number of pits visible at 25x magnification on the 16mm side. Semi-quantitative data relating exposure time to depth of corrosion across the cross section of the test specimen was obtained. The depth of corrosion was determined using eye reticules (eye reticules) of a metallographic microscope.

The results in Table 2 show that the 30-day formicary corrosion sensitivity (in order of maximum pit depth) trend for the formic acid test is C422 < C425 < C122 < C220. For the acetic acid test, the trend for formicary corrosion sensitivity (in order of maximum pit depth) at 30 days was C422 < C122 < C220 < C425.

TABLE 2

Outokumpu formicary strip test at 40/20 deg.C, 500ppm formic acid or acetic acid

Table 3 provides data from similar experiments, also including tubes consisting of the compositions in table 1.

TABLE 3

Comparison of Ant nest sensitivity of alloys C220 and C422 with alloy C122

Table 3 also provides data relating to the formicary corrosion resistance properties of C422, expressed as a fold of C122 resistance. Thus, this example demonstrates that the tin brass alloy C422 is significantly more resistant to formicary corrosion than other alloys, including those commonly used in air conditioning and refrigeration systems to make heat transfer tubes.

Example 3

This example demonstrates that a heat transfer tube comprising the tin brass alloy C422 has a higher resistance to formicary corrosion relative to other compositions. The composition of the tubes used in this example is summarized in table 4.

TABLE 4

Alloy composition (%)

Alloy (I)

Cu

Pb

Fe

Sn

Zn

P

C122

Rem＊(containing Ag)

0.015-0.040

C220

89.0-91.0

0.05 max

0.05 max

Rem＊

C422

86.0-89.0

0.05 max

0.05 max

0.8-1.4

Rem＊

0.35 max

^＊Rem is the balance

Formicary corrosion resistance comparisons for C122, C220, and C422 were performed as follows.

The tubes consisting of the individual metal compositions in table 4 were hung above about 500 parts per million (ppm) of formic acid per million Deionized (DI) water in 100ml aliquots of the bottom of a pressurized test vessel. Because the copper tubes are suspended from the test liquid, these tubes are only in contact with corrosive vapors and condensates. The tubes are exposed to steam and condensate using a temperature cycle that optimizes formicary corrosion. Typically, temperature cycling involves holding the test containers in an oven at a temperature of 40 ℃ during the night and over the weekend. The test containers were evaluated by checking the pressure at 9 hours each working day, with the oven turned off and the oven door opened to allow the test containers to cool to room temperature (20 ℃). When a significant drop in pressure is detected, the leak is located by immersion in water and/or detection of air bubbles with droplets of liquid soap. The section of the tube containing the source of the leak was cut from the copper tubing, filled with vacuum impregnated resin, the leak site polished and a photomicrograph taken. It was thus determined that tube leakage was caused by formicary pit corrosion through the tube wall.

Table 5 gives the results of the corrosion resistance tests (measured by time to tube failure) for alloys C122, C220 and C422. When both C422 tubes were not destroyed after about 26 weeks of exposure, they were removed and examined. Formicary corrosion was present in both tubes, but did not develop to half the wall thickness of 12.5 mm. The tubes made in C422 were tested for exposure to the solution to ensure that adequate levels of formic acid were maintained by the end of the experiment. The formic acid level was reduced to 342ppm and 368ppm, respectively, indicating that sufficient caustic could be maintained throughout the test.

Thus, the results summarized in table 5 demonstrate the higher formicary corrosion resistance characteristics of the C422 alloy relative to the composition of heat transfer tubes typically comprising current air conditioning refrigeration systems. In addition to the data summarized in table 5, it is noted that when the C422 sample was removed without leakage after about 26 weeks, the pit depth observed in these experiments was less than half the wall thickness. This is in clear contrast to the maximum pit depths of C122 and C220, which are multiples of the observed C422 size. These data highlight the relative benefits of tube walls constructed substantially entirely of the formicary corrosion resistant alloys described herein, as opposed to surface coatings.

TABLE 5 Ant nest sensitivity comparison

Figure 3 graphically summarizes the results from a similar experiment where a pressure drop to 0 indicates that a failure has occurred and that the tube has leaked. Some of the interruptions in the data of FIG. 3 are caused by conditions unrelated to pipe performance. Nevertheless, the results shown here show that after 150 days of exposure to a corrosive environment, the tin brass alloy C422 can hold the pressure without leakage, while the compositions typically used for ACR tubing break down more rapidly.

Thus, this example demonstrates that tin brass alloy No.422 has a higher resistance to formicary corrosion than tubes composed primarily of copper or brass.

While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Claims (6)

1. A formicary corrosion resistant heat transfer tube constructed of a tin brass alloy, wherein the tin brass alloy consists of 86.0% to 89.0% copper, 0.8% to 1.4% tin, no more than 0.05% lead, no more than 0.05% iron, no more than 0.35% phosphorus, and the balance zinc.

2. The heat transfer tube of claim 1, wherein the tube is formed by welding, extrusion, or cast rolling.

3. A heat exchanger assembly comprising the heat transfer tube of claim 1, further comprising a plurality of fins and at least one tube sheet.

4. A heat exchanger assembly for an air conditioning refrigeration system, said heat exchanger comprising the formicary corrosion resistant heat transfer tube of claim 1.

5. A method of making a heat exchanger, comprising: the heat transfer tube wall is made of a tin brass alloy, wherein the heat transfer tube wall is resistant to formicary corrosion, and the tin brass alloy consists of 86.0% -89.0% of copper, 0.8% -1.4% of tin, no more than 0.05% of lead, no more than 0.05% of iron, no more than 0.35% of phosphorus, and the balance of zinc.

6. The method of claim 5, wherein the heat exchanger is installed in an air conditioning refrigeration system.