Patent Application
20090159248
Tube-in-tube heat exchangers are used in a variety of applications for transferring heat from one fluid to another. Particular configurations of tube-in-tube heat exchangers are described in U.S. Pat. Nos. 5,004,047 and 6,012,514.
Traditionally, tube-in-tube heat exchangers used for swimming pool heat pumps had CuNi inner tubes enclosed in a copper or steel jacket. Later, titanium alloys were used to form the inner tubes in order to provide improved chlorine resistance, and the outer shells were made of polyvinyl chloride.
Currently, swimming pool heat pumps are available that have heat exchangers formed from an inner tube of twisted titanium in a plastic polyvinyl chloride jacket. The heat exchangers are suitable for use with R-22 refrigerant. The twisted titanium tube is connected to a copper tube transition with a lock ring connector.
It would be useful to further improve the efficiency of heat exchangers used in corrosive environments, including but not limited to swimming pool heat exchangers.
One embodiment is a heat exchanger tube comprising a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion, the transition portion including a reinforcing sleeve.
Another embodiment is a heat exchanger comprising an outer tube and an inner tube defining an annular opening therebetween, the inner tube including a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion including a reinforcing sleeve formed thereon, the inner tube being configured to withstand a hydrostatic test pressure of at least 2600 psig for at least 2 minutes without mechanical failure.
Yet another embodiment is a heat exchanger comprising an outer tube and an inner tube defining an annular opening therebetween, the inner tube including a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion including a reinforcing sleeve formed thereon, the inner tube being configured to pass a fatigue test of 250,000 cycles between 118 psig and 418 psig at a rate of 0.5 cycles/second without mechanical failure.
Another embodiment is a method of making a heat exchanger tube comprising forming a inner tube comprising a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion, and forming a reinforcing sleeve over the transition portion.
Yet another embodiment is a heat exchanger comprising an inner tube comprising a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion including reinforcing sleeve, and an outer tube or shell surrounding the inner tube.
A further embodiment is a method of extending the useful life of a heat exchanger tube comprising a first portion connected to a twisted portion, the method comprising forming a reinforcing sleeve over the connection between the first portion and the twisted portion.
Yet another embodiment is a method of forming a heat exchanger tube capable of use at test pressures up to at least 2600 psig, comprising forming a heat exchanger tube comprising a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion, and forming a reinforcing sleeve over the transition portion.
Another embodiment is a method of forming a heat exchanger tube capable of use at operating pressures up to at least 500 psig, comprising forming a heat exchanger tube comprising a first portion, a twisted portion, and a transition portion between the first portion and the twisted portion, and forming a reinforcing sleeve over the transition portion.
A further embodiment is a method of protecting a twisted heat exchanger tube from fluid impingement on its outer surface, comprising forming a reinforcing sleeve over the upstream part of the twisted portion of the tube.
A new and improved heat exchanger tube has been developed that enables environmentally favorable and efficient refrigerants to be used in corrosive environments, including in the presence of chlorinated swimming pool water. The new heat exchanger tube can be used at pressures up to 500 psig or more, or 600 psig or more, and is particularly useful when the pressure difference on opposite sides of the tube wall is 300 psig or more. The tube can be used with R-410A refrigerant, which has a zero Ozone Depleting Potential, as well as other refrigerants requiring high pressures. The new tube can be fabricated in a variety of sizes and shapes. Typically, the twisted tube is coiled and is enclosed a hard coil-shaped tube made of plastic or another suitable material. The coils can have various diameters, and various numbers of turns per linear meter.
As used herein, a “twisted portion” of a tube is a portion that has improved heat transfer resulting from increased surface area per unit length of the tube by twisting the tube. A twisted portion of tube material has one or more visible flutes on one or both of its inner and outer surfaces. As used herein, a “smooth portion” and a “straight portion” of a tube are non-twisted portions of a tube. As used herein, “mechanical failure” refers to a fracture or rupture of the tube.
Referring first to
The twisted portion 14 of the tube 10 includes a spiraling flute 17 and a corresponding valley 19, both of which spiral along the length of the twisted portion 14 of the tube 10. Tube configurations having multiple flutes and valleys also can be used.
The first portion 12 and twisted portion 14 usually comprise titanium and often are made from a titanium alloy. The sleeve 16 typically is formed from titanium or a titanium alloy. The sleeve prevents mechanical failure of the twisted portion 14 of the tube material at and near the connection with the first portion 12 of the tube material when the heat exchanger is operated at a 500 or 520 psig (or higher) working pressure. The sleeve also prevents mechanical failure of the twisted portion 14 of the tube material at and near the connection with the first portion 12 of the tube material when the tube 10 is tested at a test pressure of, e.g. 2500-3000 psig for 2-6 minutes.
For a swimming pool or spa heat exchanger, the sleeve typically has an outer diameter in the range of about 0.75 inch to 1.25 inch, and a length of about 5-7 inches. It is noted that tubing with larger and smaller dimensions can be used, and that sleeves of longer and shorter lengths can be used. The flutes typically have a peak-to-valley height of about 0.145″ to 0.150″ and a length of approximately 0.375″ per rotation. The twisted portion of the tube often has an average wall thickness in the range of 0.018″ to 0.025″. The thinness of the tube wall provides for effective heat transfer between the fluids on opposite sides of the tube wall. The ratio of the length to the outer diameter of the sleeve typically, but not necessarily, is in the range of 6-8, and often is in the range of 6.5 to 7.0.
In
The outer tube 102 can be made of a material that is resistant to corrosion by the fluid that flows in the annular opening 104. When the fluid is chlorinated water, a hard plastic material such as polyvinyl chloride can be used. The reinforcing sleeve provides for use of the tube in a tube-in-tube heat exchanger in which the pressure difference between the inside and the outside of the tube is at least 300 psi. or at least 450 psi, or at least 550 psi. When a refrigerant such as R-410A is used, the pressure on the inside of the inner tube typically reaches a maximum of 600 psig and the pressure on the outside of the inner tube typically is in the range of 25 to 100.
Various tests were conducted during the process of forming the new heat exchanger tubing. Test procedures and results are shown in Examples 1-3. The examples are included to illustrate features of the invention but are not intended to be limiting.
Seven different sets of tubes having a straight portion and a twisted portion (Samples 1-6 and 9) were tested using fatigue test UL 1995 to determine their suitability for use in a heat exchanger operating with R-410A refrigerant. Two sets of tube material that only contained smooth tubing also were tested (Samples 7-8). The twisted tubes were formed from a titanium alloy and had the following general dimensions: ⅞ inch outer diameter, 0.020 wall thickness along the straight portion, and 0.020 average wall thickness along the twisted portion. The fluted portion had an outer diameter of about 0.810-0.850, a height of about 0.145 from flute to valley as measured on the outer wall, and approximately 33 turns per linear foot. According to the literature, the titanium alloy used in making the tube material has an ultimate tensile strength of approximately 65 ksi, a yield strength of approximately 50 ksi and a nominal composition typical of type 2 titanium. The smooth tubes had the same composition as the twisted tube and the same dimensions as the smooth portion of the twisted tubes.
In the fatigue test (as per UL 1995), the tubes were cycled between 118 psig and 418 psig for 250,000 cycles. Hydraulic oil was used as the test fluid. The outer surface of the tube was maintained at atmospheric pressure. To pass the test, no failure can occur. The hydraulic system was controlled by a servo valve to pressurize the test articles. The system included a hydraulic pump, interconnecting piping, on/off valves, servo valve, filters, accumulator and PLC electronic controls. UL 1995 requires that the high and low pressure in any cycle be maintained for at least 0.10 seconds. Tests were run at around 0.5 cycles/sec or 2 sec/cycle.
As is shown on Table 1 below, none of the twisted tube Samples 1-6 and 9 passed the fatigue test. Both of the smooth tube Samples 7-8 passed the test, showing that the non-twisted material is acceptable for use at the pressures required for R-410A refrigerant.
Five different types of tubes having a straight portion and a twisted portion (Samples 10-14) were tested using a hydrostatic strength test to determine their suitability for use in a heat exchanger operating with R-410A refrigerant. The twisted tubes were formed from a titanium alloy and had the following general dimensions: ⅞ inch outer diameter, 0.020″ wall thickness along the straight portion, and 0.020″ average wall thickness along the twisted portion. The fluted portion had an outer diameter of about 0.810-0.850″, a height of about 0.134″ from flute to valley as measured on the outer wall, and approximately 33 turns per lineal foot. According to the literature, the titanium alloy used in making the tube material has an ultimate tensile strength of approximately 65 ksi, a yield strength of approximately 50 ksi and a nominal composition typical of type 2 titanium.
To pass the hydrostatic strength test, a tube is required to withstand 5 times the desired working pressure for a minimum of 2 minutes. Thus, for a desired working pressure of 520 psi the test is run at 2600 psi, and for a desired working pressure or 600 psi the test is run at 3000 psi. Pressure outside the tube was atmospheric. Water was used as the fluid in the high pressure test.
As is shown on Table 1 below, none of Samples 10-14 passed the hydrostatic strength test at 2600 psig, thus showing the difficulty in using twisted tube material in high pressure environments.
Reinforcing sleeves made of type 2 titanium alloy having an inner diameter of about ⅞ inch and a length of 6 inches were placed (swaged) around the transition point from smooth tubing to twisted tubing of several samples of the same type of tube material of the same dimensions as was used in Example 2. The tubes with the reinforcing sleeves were subjected to the hydrostatic strength test described above in Example 2 using water as the high pressure test fluid. The inside of the twisted tube was brought to a pressure of 2,600 psig, and this pressure was maintained for 2 minutes. Subsequently, the pressure was increased to 2,700 psig for 2 additional minutes, and the pressure was then increased to 3,000 psig for 2 more minutes. The outer surface of the tube was at atmospheric pressure. The twisted tube was then examined for failures.
For Sample set 15 (TR 2007-024) which included the reinforcing sleeve, no failures were detected at 2600 psig but failure occurred at 3000 psig. For tube sample TR 2007-039, which included both a reinforcing sleeve and a tight fit reducer bushing positioned around the plain end, no failure occurred even at 3000 psig.
The same type of tube samples as were used in Example 3, Sample 16, were subjected to a fatigue test in accordance with UL1995, described above in Example 1, using hydraulic oil as the test fluid. The set of samples passed two full rounds of testing without any failures. The results are shown in Table 1 as Sample 17 (Example TR-2007 041).
An important advantage of the embodiments disclosed herein is that the twisted tubes can be used in conjunction with refrigerants that replace HCFCs. Nonlimiting examples of refrigerants that can be used with the embodiments described herein are provided below on Table 2.
It will be appreciated that features disclosed above and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Furthermore, currently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
This application claims priority from U.S. Provisional Patent Application No. 61/008,807 filed Dec. 21, 2007.
| Number | Date | Country | |
|---|---|---|---|
| 61008807 | Dec 2007 | US |
