FLEXIBLE COUNTER-FLOW HEAT EXCHANGERS FIELD OF THE INVENTION This invention relates generally to heat exchangers for the transfer of heat energy between two media at different temperatures. More particularly, it provides a novel type of flexible heat exchangers for cryogenic systems.
BACKGROUND ART Refrigeration systems are employed in a variety of applications that require cryogenic environments. US Patent 5,337,572, for example, describes a single compressor cryogenic refrigeration system; and US Patents 5,617,739 and 5,724,832 describe self-cleaning refrigeration systems suitable for operation at cryogenic temperatures. Critical to the operation of these refrigerators is a counter-flow heat exchanger that is required to pre-cool a high-pressure supply refrigerant stream with a cooler, low-pressure return refrigerant stream. The latter has been passed through a flow-restrictor, causing it to drop in pressure and consequently to cool. The low-pressure refrigerant is then used to cool a cold stage that is in contact with the object to be cooled, before returning to a compressor module by way of the heat exchanger. Different versions of the heat exchanger can be used. In most cases the heat exchanger is placed in close proximity to the cold stage. The compressor module, including a compressor, an oil separator, a condenser and, in some systems a fractionating column, is connected to the heat exchanger via a long"umbilical cord" containing separate high-pressure supply and low-pressure return lines. The umbilical cord is generally made to be flexible and entirely at ambient temperature, thereby requiring no insulation. The heat exchanger itself is contained in a vacuum-insulated (or other type of thermally- insulated) enclosure that can be quite bulky.
In many applications, such as cryosurgery, cooling of electronic devices and computer chips, the space in the vicinity of the cold stage is very limited. A great deal of effort has been made to miniaturize the heat exchanger, along with the cryogenic system. US Patents 4,386,505, 4,392,362 and 4,489,570, for example, describe microminiature refrigerators. In such a system the cold stage and the heat exchanger are fabricated on a plate, which can be placed in intimate contact or packaged with the device to be cooled. The plate is in turn connected to a compressor module via miniature tubes that are at ambient temperature. US Patent 5,901,783 describes methods for making a microminiture heat exchanger and its application in a cryosurgical probe. The miniaturized heat exchanger in this case is embedded in the distal end of the probe, which is inserted in the object to be cooled ; and the proximal end of the probe is connected to a compressor module via a flexible hose at ambient temperature.
Alternatively, the heat exchanger can be located in the compressor module. The umbilical cord in this case contains the high-pressure supply and the cooler low-pressure return lines, both of which must be insulated from ambient temperature. This is typically done by the use of a vacuum- insulated, flexible hose. The disadvantage of this approach, however, is that the heat exchanger does not immediately precede the cold stage. That is, the supply refrigerant stream, after being cooled by the heat exchanger, still needs to traverse a considerable distance, and consequently incurs unwanted thermal losses before entering the cold stage. (The cold, return refrigerant stream, too, would have to travel a long distance and suffer thermal losses before entering the heat exchanger.) This can significantly reduce the efficiency of the refrigeration system, unless a high degree of insulation is maintained between the refrigerant lines and ambient temperature.
US patent 5, 365, 749 discloses a refrigeration system for cooling a computer, where the heat exchanger directly connects the cold stage to the compressor module. The heat exchanger in this system, however, is not made to be flexible, making the entire refrigeration system rigid and less modular. A further disadvantage of such a design is that if any damage occurs in any of its components, repair is difficult.
In a paper by Little entitled,"Kleemenko Cycle Coolers: Low Cost Refrigeration at Cryogenic Temperatures" (Seventeenth International Cryogenic Engineering Conference, Eds. D. Dew- Hughes, R. G. Scurlock and J. H. P. Watson, Institute of Physics Publishers, Bristol, 1 (1998)), a cryosurgical device is described where the cold stage is located immediately adjacent to the heat exchanger, which in turn is connected to the compressor module by a long umbilical cord.
The flexible umbilical cord in this system serves only to connect the high-pressure supply and low-pressure return lines to the heat exchanger. It does not serve as a heat exchanger.
Hence, what is needed in the art is a flexible heat exchanger that replaces the umbilical cord, making the corresponding refrigeration system modular, allowing the cold stage to be more compact in size, and at the same time, maximizing the efficiency of heat exchange.
OBJECTS AND ADVANTAGES Accordingly it is a principal object of the present invention to provide a flexible and modular heat exchanger that extends from the compressor module of a refrigeration system to the cold stage and allows counter-flow heat exchange along the entire connection. It is a further object of the present invention to provide methods for manufacturing such a heat exchanger and for coupling the heat exchanger to the refrigeration system.
An important advantage of the present invention is that by using the flexible heat exchanger both as a hose bridging the cold stage and the compressor module and as a heat exchanger, the refrigeration system becomes more compact, modular, and more efficient. Another advantage of the present invention is that it frees up space in the vicinity of the cold stage, thus allowing the cold stage to be more compact in size. Further advantages of the heat exchanger of the present invention are manifest in its simple design, ease of repair, and adaptability for a variety of applications.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
SUMMARY OF THE INVENTION This invention describes a flexible heat exchanger that provides both a direct connection between a cold stage and a compressor module of a refrigeration system and heat exchange along the entire connection.
In the present invention, a flexible heat exchanger plays a dual role as a hose connecting the cold stage with the compressor module, and as a heat exchanger, itself. Hence, the need for a cumbersome heat exchanger at one end or the other of the connecting link (between the compressor module and the cold stage), and additional connecting lines between the heat exchanger and the compressor module (or the cold stage) are eliminated. Moreover, the flexibility of the heat exchanger makes the corresponding refrigeration system modular and easily adaptable to different applications.
Further, counter-flow heat exchange between the supply and return refrigerant streams takes place along the length between the compressor and the cold stage, thereby maximizing the efficiency of heat transfer.
The heat exchanger of the present invention comprises a flexible outer jacket ; a flexible return tube disposed within the outer jacket; and one or more flexible supply tubes placed inside the return tube. It is configured such that a high-pressure input refrigerant, while flowing through the supply tubes, is cooled by a counter-flow of low-pressure return refrigerant surrounding the supply tubes inside the return tube.
One end of the heat exchanger is designed to be coupled to the compressor module, while the other end is coupled to the cold stage of the refrigeration system. The coupling means hereof can be made to be adaptable and removable, making the repair of the heat exchanger and the transportation of the refrigeration system much easier.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.
BRIEF DESCRIPTION OF THE FIGURES FIG. 1 shows an exemplary representation of a refrigeration system according to the present invention; FIG. 2 depicts a cross-sectional view of an exemplary embodiment of a heat exchanger according to the present invention; FIGS. 3A-3B depict side views of two exemplary embodiments of a heat exchanger according to the present invention; and FIG. 4 provides an exemplary embodiment illustrating how a plurality of high-pressure supply lines (contained within the low-pressure return line) are combined to provide a common connection at the compressor module end of the heat exchanger, or at the cold stage end of the heat exchanger.
DETAILED DESCRIPTION Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiment of the invention described below is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
FIG. 1 depicts an exemplary embodiment of a refrigeration system 100 according to the present invention. By way of example to illustrate the general layout of the refrigeration system of the present invention, a compressor module 10 and a cold stage 11 are bridged by a flexible heat exchanger 12. More specifically, a first end 13 of the heat exchanger 12 is coupled to the compressor module 10, and a second end 14 of the heat exchanger 12 is coupled to the cold stage. The length between the first end and the second end of the heat exchanger can be up to several meters, and is typically about 3 meters.
FIG. 2 depicts a cross-sectional view of a first exemplary embodiment of the heat exchanger 12 shown in FIG. 1. By way of example, the heat exchanger 12 comprises a flexible outer jacket 20, typically made of bellows or helical tubing about 2 cm (e. g., 3/4") in inner-diameter (ID) and 2.5 cm (1") in outer-diameter (OD). The outer jacket is commonly made of stainless steel, or bronze. Disposed within the outer jacket 20 is a flexible return tube 21, typically made of bellows or helical tubing of about 1 cm (e. g., 3/8") in ID and 1.6 cm (0.64") in OD. The return tube is generally made of copper, bronze, or stainless steel. The return tube 21 may be further provided with a braided outer cover to prevent it from changing length with variations in the return refrigerant pressure. Enclosed within the return tube is a plurality of flexible high-pressure supply tubes 22. The supply tubes are preferably made of fully annealed stainless steel, with 0.1 cm (e. g., 0. 04") ID and 0.16 cm (1/16") OD, to provide the necessary flexibility. The supply tubes can be made of copper, bronze, nickel, or cupro-nickel tubing.
The space between the return and supply tubes constitutes a low-pressure return line 23.
In the above embodiment, the supply tubes 22 are disposed within, and completely surrounded by the return line 23.
Thus, as a warm, high-pressure input refrigerant stream passes through the supply tubes 22, it is cooled by a counter-flow of cold, low-pressure return refrigerant stream along the return line 23. To maximize the heat transfer between the warm and the cold refrigerant streams, it is preferable to have a plurality of small diameter supply tubes, such that the contact area between the two refrigerant streams is maximized. The use of a plurality of small diameter supply tubes also provides greater flexibility than a single, larger diameter tube.
To enhance the efficiency of the heat exchanger, a space 24 between the outer jacket 20 and the return tube 21 is thermally insulated, usually through a vacuum. This is achieved by evacuating and subsequently sealing off the outer jacket 20, or by connecting one end of the outer jacket 20 to a vacuum system. The return tube 21 may be further insulated by wrapping it in several layers of superinsulation, or aluminized mylar ribbon 25.
FIG. 3A provides a side view of a second exemplary embodiment of the heat exchanger shown in FIG. 1, along with the corresponding refrigeration system 101. The first ends 13A of the high-pressure supply tubes 22 and the first end 13B of the return tube 21 at the first end 13 (comprising 13A, 13B) of the heat exchanger 12 are connected to the high-pressure outlet and the low-pressure inlet of the compressor module 10, respectively. At the second end 14 (comprising 14A, 14B) of the heat exchanger 12, the second end 14B of the return line 21 is coupled to the cold stage 11, and the second ends 14A of the high-pressure supply tubes 22 are coupled to a flow restrictor 15 in the cold stage 11. (All references to flow restrictors in the description and the following claims encompass equivalent devices such as capillaries, throttles, or others known in the art of refrigeration.) In a typical application, the first ends 13A, 13B of the heat exchanger 12 are at ambient temperature, generally between about 273 and 330 K; and the second end 14B of the return line 21 of the heat exchanger 12 is at a cryogenic temperature, ranging from about 70 to 200 K. Vacuum insulation is commonly used to provide thermal insulation of the cold stage as well as of the heat exchanger by extending the outer jacket 20 to enclose the sections of the cold stage that are at cryogenic temperatures. A counter-flow of high-pressure supply and low-pressure return refrigerant streams, as illustrated by the arrows, takes place along all supply tubes 22 within the insulated enclosure provided by the outer jacket 20, extending from the compressor module 10 to the flow restrictor 15.
FIG. 3B provides a side view of a third exemplary embodiment of the heat exchanger shown in FIG. 1, along with the corresponding refrigeration system 102. In this case, a second flow restrictor 16 is disposed within the heat exchanger 12, coupled to the second ends 14A'of one or more auxiliary high-pressure supply tubes 22', while the supply tubes 22 continue to be coupled to the flow restrictor 15 in the cold stage 11. The purpose of the second flow restrictor 16 is to let the high-pressure refrigerant stream carried by the auxiliary supply tubes 22'to expand and consequently to cool. The cooled low-pressure refrigerant then returns to the compressor module 10 via the return line 21 and pre-cools the incoming high-pressure refrigerant stream along the way. The first ends 13A, 13A'of the supply tubes 22,22'and the first end 13B of the return tube 21 at the first end 13 (comprising 13A, 13A', 13B) of the heat exchanger 12 are connected to the high-pressure outlet and the low-pressure inlet of the compressor module 10, respectively. At the second end 14 (comprising 14A, 14B) of the heat exchanger 12, the second end 14B of the return line 21 is coupled to the cold stage 11, and the second ends 14A of the high-pressure supply tubes 22 are coupled to the flow restrictor 15. The first ends 13A, 13A', 13B of the heat exchanger 12 are typically at ambient temperature, approximately between 273 and 330 K; and the second end 14B of the return line 21 of the heat exchanger 12 is at a cryogenic temperature, ranging from 70 to 200 K.
Vacuum insulation is generally used to provide thermal insulation of the cold stage 11 as well as of the heat exchanger by extending the outer jacket 20 to enclose the sections of the cold stage that are at cryogenic temperatures. A counter-flow of high-pressure supply and low-pressure return refrigerant streams, illustrated by the arrows, takes place along all supply tubes within the insulated enclosure provided by the outer jacket 20.
The coupling between the heat exchanger 12 and the compressor module 10 and that between the heat exchanger 12 and the cold stage 11 in FIG. 1 may be achieved as follows.
The high-pressure supply tubes 22 and/or 22'are first combined at each end with suitable fittings, to provide a common feed line from the high-pressure outlet of the compressor module 10 and a common connection to the flow restrictor 15 in the cold stage 11. The return tube 21 of the heat exchanger 12 then connects with an outlet of the cold stage 11 at one end and with the low-pressure inlet of the compressor module 10 at the other by means of brazing, welding, or removable connectors (e. g., SwageLock Quick Connects: SS-QC4-D-PM, SS-QC-B1-400, B-QC4-B1-400, B-QC-D- 400, and B-DC4-D-2HC).
FIG. 4 provides an exemplary embodiment of a coupling means for combining the high-pressure supply lines (contained within the low-pressure return line) to form a common connection at the compressor module end of the heat exchanger, or at the cold stage end of the heat exchanger.
A plurality of high-pressure supply lines 30 are first brazed to a flange 31, which is in turn brazed into a fitting 32. An outgoing line 33 is then brazed to the fitting 32, connecting the high-pressure supply lines to the compressor module, or to the flow restrictor in the cold stage. The flange 31 is typically made of brass. The fitting 32 is generally made of copper, or brass. The outgoing line 33 can be made of copper, or stainless steel tubing.
A suitable refrigerant in the refrigeration system described above for operating at a cold stage temperature of 120 K includes the following mixture: 8% propane, 8% n-butane, 12% argon, 7% nitrogen, 18.5% R14,14.5% R134a, 17.5% R23, and 14.5% R123, where the percentages are given in mole percents. Another mixture suitable for operation at a cold stage temperature of 165 K consists of 35.7% R404a, 51% R14, and 13. 3% Argon. Those skilled in the art will know how to implement other refrigerant mixtures of different composition for a given application.
An important feature that distinguishes the heat exchanger of the present invention from the prior art types is that it is made to be flexible and/or conformable. The latter implies that the heat exchanger returns substantially to its original shape when the stress is removed; while in the case of the former, the heat exchanger retains its bent shape.
Both entail the use of a heat exchanger that can be bent to conform to a particular shape, and/or is capable of responding to the stress of bending or twisting by deforming.
As an illustrative example of the flexibility of the heat exchanger of the present invention, the outer jacket 20 of the heat exchanger 12 can be made of unbraided, helical or bellows corrugated hose that is commercially available from Pacific Flex, Inc., Pacific Coast Cryogenics, 1600 Shasta Avenue,. San Jose, CA 95128. Such a hose can yield considerable curvature along its length when being bent, and the degree of curvature varies with its size. For a hose of 3/4"ID and 1"OD, for instance, the minimum radius of curvature (i. e., the most severe deformation) it can withstand under a static bend is about 2". In addition, the return tube 21 can be a flexible helical or bellows tubing, made of copper, bronze, or stainless steel. It can also be a conformable tubing, comprising a material selected from the same group. The high-pressure supply tubes 22,221 are generally made of a material selected from the group consisting of stainless steel, copper, bronze, nickel, and cupro-nickel. The choice of the material and sizes of the supply tubes is such that they can be made flexible, and/or conformable. Those skilled in the art will know how to implement other types of tubing to make the heat exchanger flexible and to meet the need of a particular application.
It should be noted that the specific dimensions of the outer jacket, return and supply tubes described above provide only one exemplary embodiment of the heat exchanger of the present invention, suitable to work with a cold stage operating in a temperature range between 70 K and 200 K. It is known in the art of cryogenics that the physical dimensions and other characteristics of a heat exchanger depend upon the specific type of refrigeration system and in particular, the range of cryogenic temperatures the refrigeration system is intended to provide. Accordingly, a skilled artisan will know how to calculate the physical dimensions of the supply and return lines to achieve the desired heat exchange for a particular refrigerant to be used in the heat exchanger of a given application.
The advantages of the heat exchanger according to the present invention are apparent: being flexible, it makes a refrigeration system highly modular and adaptable to various applications; it provides an effective heat exchange along the way between the compressor module and the cold stage; it frees up the space in the vicinity of the cold stage; and it can be easily repaired should damage occur. Accordingly, it is ideally suited for a variety of applications, such as cryosurgery, cooling of electronic devices, and cooling of computer chips, etc.
Moreover, those skilled in the art will recognize that the application of the heat exchanger of the present invention is not limited to cryogenic systems. It can also be employed in other applications involving the transfer of heat energy between two fluids.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the principle and the scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims and their legal equivalents.
