US 3173273 A
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March 1 5 VORTE}? TUB 3 Sheets-s 1 Filed NOV. 27, 1962 INVENTOR TON ATTORNEY March 16, 1965 c, D, ULT 3,173,273
VORTEX TUBE Filed Nov. 27, 1962 s Sheets-Sheet 2 COMPRESSED AIR March 16, 1965 c. D. FULTON 3,173,273
vomzx TUBE Filed Nov. 27, 1962 :5 Sheets-Sheet a LIQUID on GAS a: R ,1; Q o a INVENTOR 0. 0. FUL TON A'ITORNEY United States Patent 3,173,273 VQRTEX TUBE Charles D. Fulton, 440 Beechfi'ee Drive, Qincinnati 24, Ohio Filed Nov. 27, 1952, Ser. No. 240,281 9 Ciaims. (Cl. 625) The present invention relates to improvements in vortex tubes and relates more specifically to a novel vortex tube capable of emitting colder and hotter streams of gas, operating more efiiciently, being more cheaply manufactured, and being more readily applied to useful purposes.
In approximately 1931, Georges Joseph Ranque of France developed a device or apparatus commonly referred to as a vortex tube, Ranque tube, Hilsch tube, or Ranque-Hilsch tube, the basic concept of which is shown in US. Patent No. 1,952,281. During the ensuing decade little or no mention of this invention appeared in the scientific literature nor did there arise any general interest in this unique phenomenon and achievement. In 1945 Rudolf Hilsch of Germany published an account of studies which he had made on vortex tubes. Thereafter there arose a worldwide interest in the subject and many treatises were published on the perplexing phenomenon offered by the vortex tube.
The great amount of interest attaching to the vortex tube is readily apparent upon witnessing a demonstration of this device in operation. Compressed air is fed into what resembles a T fitted with pipes on either side. Cold air thereupon issues steadily from one pipe and hot air issues steadily from the other. The cold stream of air is often visible as a light blue mist and forms frost on the pipe and on other objects which it touches. Temperatures well below zero Fahrenheit are produced. At the other end of the device, the hot air stream, when the device is adjusted properly, reaches temperatures well above the boiling point of water. All of these results take place in an instrument having no moving parts and being extremely compact in size and in some cases no larger than a pencil. The simplicity of the vortex tube and its component parts enables it to be manufactured at a remarkably low cost. The absence of moving parts endows the device with extremely long life and troublefree operation. Any gas whatsoever may be utilized in the device with substantially the same results. The device may be constructed of any size according to the quantity of gas flow which is desired. The physical phenomenon displayed by the vortex tube is unique and unparalleled by any other known phenomenon. Given a source of compressed air or other gas, the vortex tube affords the simplest and most direct known means of creating heat and cold.
The development of vortex tubes capable of operating with sufiicient efiiciency and economy would make it possible to revolutionize portions of such technological fields as refrigeration, air conditioning, cryogenics, instrumentation, and controls. The primary factor impeding the widespread utilization of the vortex tube has been its low thermodynamic eificiencyi.e., the high gas pressure required to create the desired temperature changes, the small fractions of the supplied gas delivered at the lowest and highest temperatures, and altogether a large expenditure for machinery, such as compressors, and for power. The improved vortex tube which is the object of this invention is capable of alleviating these difficulties by virtue of its increased efiiciency and therefore of advancing the long-sought utilization of vortex tubes in the aforementioned technological fields.
Ranquc taught certain embodiments of his invention including what are referred to as counterfiow and unifiow types although it has since appeared that the counterfiow type is superior for the emission of separate cold ice and hot gas streams. Ranque also taught various designs of tangential nozzles for producing the vortex while Hilsch employed a single nozzle leading into a spiral ramp.
The counterfiow vortex tube comprises a long, slender tube with a diaphragm closing one end of the tube and a small hole in the center of the diaphragm, one or more tangential nozzles piercing the tube just inside the diaphragm, and a throttling valve at the far end of the slender tube. The function of the counterfiow vortex tube is to receive a flow of compressed gas through the nozzles and to discharge a stream of cold, expanded gas through the small hole in the diaphragm, and a stream of hot, expanded gas through the valve. In the event the throttling valve is not employed, a vacuum is created in the center of the tube and atmospheric air is drawn in through the small hole, and no cold gas is emitted. The same results as are yielded by the use of the valve will be yielded by the use of a fixed orifice of proper size or by a connection of the tube to a chamber containing the same pressure as ordinarily exists in the hot tube ahead of the valve.
The coldest gas is produced when only a small fraction of the gas is emitted through the small hole. This condition is obtained by opening the valve rather wide. When this is done, the hot gas is then only warm. However, on the other hand, the hottest gas is obtained by closing the valve almost entirely. Nearly all the gas is then emitted through the small hole and is only cool. The fact that only a small fraction of the gas can be extracted at the lowest temperature is a source of ineificiency in the vortex tube. Another factor is that the amount of temperature depression obtained never approaches that of a perfect expansion engine. These two ineificiencies, taken together, amount to a large loss.
In the development of this invention it has been determined that the temperatures of the three streams of gas are related by the following improved energy balance:
where T =temperature i=inlet gas c=cold gas h=hot gas f=cold fraction=mass flow of cold gas divided by mass fiow of inlet gas lT Joule-Thomson temperature drop of gas on adiabatic throttling from inlet state to outlet pressure.
The value of IT is found in thermodynamic tables. It is 4 degrees F. in air throttled from p.s.i.g. and 70 degrees F. to atmospheric pressure.
The above formula holds very accurately provided that the gas is dry and the vortex tube is insulated so as not to gain or lose heat-to the atmosphere. The quantity 1 is measured with fiow meters. By using the aforementioned formula, only two of the three temperatures need be measured. A correction for the condensation and freezing of moisture, if present, can be incorporated into the formula. The efficiency or excellence of the vortex tube in performing its function is measured by how little gas pressure it requires and how much temperature difference it produces for a given cold fraction.
A thermodynamic formulation of the problem of optimizing the design of the counterflcw vortex tube is that given the kind of gas, its pressure, temperature, and rate of flow, and a certain cold fraction to be delivered at a certain lower pressure, there exists a combination of geometric dimensions of all the parts of the vortex tube such that the cold gas will be delivered .at the lowest possible temperature. The optimum design for the combination may be found as well as the temperature. The temperature of the hot gas is inherent in the energy balance and so long as the pressure of the hot gas is destroyed by throttling through the valve, that pressure does not enter into the optimization. If, however, the hot gas is to be removed under pressure and utilized in, for example, an ejector, the optimization takes on an additional dimension, namely the pressure of the hot gas, which must be taken into account.
At the present status of this technology and art, the optimization problem may be solved only by performing a very large number of parametric experiments where one dimension and another are changed step by step and the cold gas temperature measured. At least fifteen important dimensions exist. This poses a problem of such intricacy that to achieve a thorough optimization even for one operating condition is very tedious and expensive. Should any condition be changed, a new optimization is required. For this reason, research is likely to continue on the vortex tube indefinitely.
Emphasis is more often placed on the cold gas because it is as a refrigerating means that the vortex tube has its greatest importance and number of potential uses. Heat or hot gas can ordinarily be obtained more economically by combustion or electricity. In fact if a compressor is used, it produces more heat than the vortex tube. The reason that refrigeration is usually the more valuable commodity resides in the greater rarity of reversible processes over irreversible ones, as treated in the Second Law of Thermodynamics. Nevertheless, there are instances where the hot gas of the vortex tube is usable as a byproduct and there are others where the hot and cold streams are usable alternatively. Examples of the latter are found in laboratory apparatus which sometimes requires heating and sometimes cooling, and in the air conditioning of persons or spaces which sometimes require heating and sometimes cooling. There are also instances where it is desirable to use the vortex tube entirely for its heating effect. For simplicity, the following description will be given mainly in terms of the production of cold gas, but it will be understood that because of the aforementioned energy balance, the production of cold gas is always accompanied by the production of hot gas, and the improvements that will be described are equally applicable to both purposes.
The need for an improvement in the internal efficiency of the vortex tube has been mentioned. Another significant problem is that the vortex tube requires novel means of application in order to utilize what internal efficiency it does possess. Attempts to substitute the vortex tube into refrigerative procedures which suit the use of common vapor-liquid compression machines employing such refrigerants as dichlorodifiuoromethane often lead to power consumptions of between six and ten times those of the vapor-liquid machines. This arises partly from the difference in the kinds of refrigerating duty that can be performed eificiently by an evaporating liquid and a warming gas. A solution to the latter problem is to devise means of changing the duty to suit the use of cold gas where possible.
The principles of thermodynamic efiiciency show that as the cold gas grows colder, every degree of additional cooling is worth more than the preceding one. By worth is meant the power or work required to produce the same cold gas reversibly, according to the Second Law of Thermodynamics. For small amounts of cooling, the worth of the cold gas is proportional to the square of its temperature depression. Thus cold gas at degrees F. below the environment is worth four times as much as at 5 degrees F. below the environment. For large amounts of cooling, the worth increases still faster. Therefore, if a vortex tube which gives 100 degrees F. of cooling is improved to give 110 degrees F. of cooling, the increase in etficiency is approximately percent. A similar principle holds with respect to the hot gas.
4,. Small improvements in the vortex tube can therefore mount up to a large gain in overall efiiciency.
Therefore, one of the objectives of this invention is to provide improvements in vortex tubes so that they may emit colder gas and a larger fraction of cold gas with substantially increased efficiency and reduced economic expenditure for a greater range of applications.
Another objective of this invention is to provide an improved vortex tube, and various components as well as the assembly of such components, to produce an economic vortex tube having optimum efliciency.
A further objective of this invention is to provide a novel generator for a vortex tube capable of yielding optimum performance through the utilization of a more powerful vortex and the presentation of the vortex core and other components in conjunction with the generator.
Yet another objective of this invention is to provide a vortex tube of improved freedom from leakage, reduced cost of manufacture, capability of performing with maximum efiiciency over a wide range of capacities through the use of changeable inserts, and increased ability to utilize high gas pressures efiiciently.
Still a further objective of this invention is to provide vortex tubes that will emit colder gas with larger fractions of cold gas than have been capable of being produced heretofore under given operating conditions.
Other objects and many of the attendant advantages will become more readily apparent to those skilled in this art from the following detailed description of the invention taken in conjunction with the accompanying drawings wherein like characters of reference designate corresponding parts throughout the several views, and from the method of forming the generator and positioning the components in the vortex tube assembly, and wherein:
FIG. 1 is a side elevational view of the counterfiow vortex tube for the emission of the coldest and hottest possible gas streams corresponding to given available gas pressures and flows and desired cold fractions;
FIG. 2 is an enlarged longitudinal sectional view, with portions of the line removed, presenting the structural components mounted within relationship to each other in the vortex tube;
FIG. 3 is a greatly enlarged vortex generator of the vortex tube in transverse section taken substantially along the plane of section line 3-3 of FIG. 2;
FIG. 4 is a transverse sectional view taken substantially along the plane of section line 44 of FIG. 2;
FIG. 5 presents a partial transverse section of the vortex generator illustrated in FIG. 3 and a mandrel for forming a tangential nozzle in the generator fabricated of thermoplastic material;
FIG. 6 is illustrative of the aerodynamic design of the heart of a vortex tube with surrounding portions removed;
FIG. 7 is a partial longitudinal sectional view embodying the aerodynamic design of a vortex generator with integral cold-gas orifice and diffuser, and hot-gas bushing, as incorporated in FIG. 1;
FIG. 8 is a partial longitudinal sectional View of another mechanical embodiment of the aerodynamic design incorporating a vortex generator with integral hotgas bushing, cold-gas orifice and diffuser;
FIG. 9 is a longitudinal sectional view of a vortex tube producing cold gas which is an inversion of FIG. 1 with hot gas emerging from one end and cold gas emerging from the other end;
FIG. 10 is a partial transverse sectional view of a modified or direct heat-transfer vortex tube providing for direct contact of an object to be cooled with the vortex coge, all the gas emerging from the opposite end of the tu e;
FIG. 11 is a partial longitudinal sectional view of a further modified heat-transfer vortex tube producing cold fluid with an axial central tube through which fluid to be cooled may flow counterflow through the vortex and. past the sealed diaphragm; and
FIG. 12 is a longitudinal sectional View of a further modification for a vortex tube embodying an anti-icing cold-gas orifice and duct in which the knife-edge orifice is adjacent to the counterbored diffuser having a greater diameter.
Referring to the drawings and more particularly to FIGS. 1, 2 and 3, there is illustrated an improved counterflow vortex tube for the emission of the coldest and hottest possible gas streams corresponding to given available gas pressures and flows and desired cold fractions. The construction illustrated is suitable for sizes ranging from the smallest possible unit to tube diameters of one inch or more. Compressed gas from a compressor is introduced through the fitting 21 and inlet tube 22 into the annular plenum chamber 23 formed between the body 24 and insert 25 having the diverging throat and generator 26. Plenum 23 distributes the gas, still under full pressure, to a plurality of circumferentially spaced-apart tangential nozzles 27 formed in the generator 26 as more fully shown in FIG. 3. The vortex generator 26 is a body of revolution except for nozzles 27.
FIG. 3 presents a section of generator 26 through the midplane of nozzles 27 with each nozzle constituting a cavity of revolution consisting of a tapering, inwardly converging inlet section 28 merging into a straight, round passageway, aperture or hole 29 intersecting the round cylindrical inner surface 30 of the vortex generator. Nozzles 27 are so positioned in vortex generator 26 that the outermost elements of straight portions 29 are approximately tangent to cylindrical suiface 30. In order to obtain optimum results, these elements should not reside outside of the tangent to surface 36} in order that there will not be created a step or indentation where portions 29 terminate and surface 39 resumes. It is permissible that the outermost elements be slightly inside of the tangent to surface 30.
One suitable means for forming vortex generator 26 is to utilize a thermoplastic material such as methyl methaorylate and introduce a heated mandrel 31 having a configuration resembling that of the nozzles 27. The terminal portion of mandrel 31, as shown in FIG. 5, may be introduced into a circular disk of the thermoplastic material which is only partially formed and does not have the cylindrical cavity 30 therein. The disk may be mounted in the spindle chuck of a lathe and mandrel 31 may be mounted on a suitable movable jig which may be the compound tool rest of a lathe. The vortex generator blank and mandrel 31 must have such relative positions and orientations that mandrel 31 may be advanced along its axis into the periphery of the generator to form a nozzle cavity having the proper tangency as described above. Mandrel 31 is heated by any suitable means sufficiently to pierce the thermoplastic material by melting and displacing the material to form the individual nozzles 27. Mandrel 31 will be advanced to a depth of penetration that has been predetermined depending upon the configuration of the nozzle desired. The mandrel is then cooled by quenching, thereby solidifying the thermoplastic material, after which the mandrel is withdrawn, leaving a polished and perfectly contoured nozzle 27. The generator may then be indexed to the next nozzle position and the operation repeated, depending upon the number of nozzles desired. Cylindrical surface 30 may then be machined.
Vortex generator 26 may be formed of any desired material, including the various plastics and metals, by the use of suitable manufacturing methods. A widely-applicable method of forming nozzles 27 is by drilling with the aid of suitable jigs and fixtures. Form or contour drills and other special tools may be employed. In the case of quantity production, multiple drills and automatic machinery may be used. The style of nozzles 27 shown in the various figures may be called enclosed inasmuch as the nozzles are formed in and surrounded by solid material. Alternatively the nozzles may be made openthat is, they may be formed as depressions in the face of the generator.
In that case FIG. 3 would no longer represent a cross section but rather a direct end View of the generator. Open nozzles may be formed by coining, machining, or molding. In that case when the generator is assembled into the vortex tube, the open nozzles will become bounded and sealed by the face of the next adjacent part or component, which will ordinarily be the mating insert 25, 58, or 60 and which should, in that case, have no shoulder projecting into the generator. It will be readily apparent to those skilled in the art that various modifications may be employed in the design and fabrication of nozzles 27 accord ing to the material used, the number of pieces to be manufactured, and the methods preferred.
The function of nozzles 27 is to accelerate the gas to the maximum possible velocity, which is sonic velocity, and to inject the gas tangentially into outer vortex chamber 32. The inlet or mouth of each nozzle should be not less than three times as wide as throat portion 29. The contour of section 28 should be smooth and polished and should merge gradually into portion 29, which should be smooth and highly polished. The design of the nozzles follows well-known best practices in the design of converging nozzles for any fluid.
The number of nozzles that should be used is not definite but best results may be achieved when a plurality of not less than three are employed, and it has been found that six and eight nozzles are ordinarily preferable. In large vortex tubes, slight additional gains may be had by using as many as ten, twelve or more nozzles in the vortex generator. The optimum number of nozzles results from the opposing influences of nozzle wall friction and error of tangential injection into chamber 32. In the event too many nozzle-s are employed, they are so small that friction within them becomes excessive, although the gas is injected with almost perfect tangency. In the event too few nozzles are employed, too much gas is introduced that is not tangent, thus engendering excessive turbulence, mixing, shock waves, and other losses in the vortex.
The cross-sectional shape of the nozzles can be made other than circular. This is not advisable in very small vortex tubes where nozzle friction is considerable and where, therefore, a round contour is optimal. However, in large vortex tubes, some gain in completeness of tangency can be had, without increasing the number of nozzles excessively, by employing a rectangular cross-section in which the larger dimension of the rectangle is parallel to the principal axis of the vortex tube. It will be apparent that the aforementioned method of molding the thermoplastic material in utilizing the mandrel 31 can be equally well employed to form rectangular nozzles or nozzles of any other desired shape or contour by modifying the cross-sectional configuration of the mandrel.
The high-speed jets of gas emerging from the nozzles enter chamber 32 and create therein an intense vortex or rapidly revolving gas mass. In small vortex tubes this mass revolves at one million revolutions per minute or more. Chamber 32 is one of the features distinguishing this invention and may be refer-red to as the outer-vortex chamber, the adapting chamber, the free-vortex chamber, the supersonic chamber, or the impedance-matching chamber. The specification of chamber 32 is that it is a narrow annular cavity surrounding, larger than, and merging into the long, principal vortex cavity or tube of the vortex tube. The axial width of chamber 32 is preferably made from one and one half to two times the axial width of nozzle throats 29. It can be made slightly wider without a substantial loss in performance provided that the flat surface 33 which terminates the vortex chamber on the cold end is-located closely adjacent to the nozzle throats 29. The preferred outer diameter of chamber 32, that is, the diameter of the cylindrical cavity 30, depends upon the absolute pressure ratio applied to the vortex tube as will be hereinafter described.
Results achieved utilizing chamber 32 as shown and described herein consistently yield from ten to twenty percent greater cold-air temperature depression (the quana tity T T at pressures between 80 and 140 p.s.i.g., with the cold air discharging at atmospheric pressure, than does the best-designed vortex tube without a chamber comparable to the chamber 32.
The basic concept of chamber 32 is that it drives the main portion of the vortex as a faster rotatory speed than is possible otherwise and it, therefore, enables the cold gas to execute a more nearly reversible expansion to the outlet pressure. Converging nozzles 27 can produce no more than sonic velocity in their throats 29 and can utilize effectively no more than the well-known critical pressure ratio of the gas. That ratio is approximately equal to 0.528 for air. If the vortex tube is supplied with a greater pressure ratio than will produce sonic velocity, as it usually is, then the extra velocity, if it is achieved, cannot be achieved in the nozzles and must be generated in the outer portion of the vortex. Without chamber 32, the outer portion of the vortex cannot generate more than a fraction of the extra velocity for two reasons: (a) if the emergent nozzle jets immediately accelerate through an additional pressure fall, they execute an unrestrained expansion which creats only a fraction of the extra velocity since part of the expansion is undesirably executed in the radial direction and also because the expansion overshoots and creates irreversible shock waves; and (b) the outer portion of the vortex cannot generate the extra velocity since it is a substantially forced vortex, one of constant angular velocity behaving as a solid body, because of the tremendous turbulent viscosity existing in it. The phenomena described in (a) are well known to nozzle designers and are sometimes overcome by employing a correctly contoured supersonic, diverging nozzle portion as in rocket motors, but the use of supersonic nozzles in the vortex tube would be undesirable for reasons disclosed hereafter. With respect to reason (b), should the vortex receive only sonic velocity, then since because of its forced nature it can possess velocities that only decrease with decreasing radius, it cannot be supersonic. To be supersonic, it would have to have a free-vortex outer portion where the received sonic velocity could be augmented to supersonic before the forced vortex is encountered. Such cannot occur in the turbulent, straight, uniform tube. However, in the sequestered outer vortex chamber 32, this desirable result for achieving supersonic velocity can be obtained and does occur.
In explanation, when air is supplied to a vortex tube at 100 p.s.i.g. and the core of the vortex is at atmospheric pressure, a common operating condition, the pressure at the nozzle exits is slightly less than 46 p.s.i.g., which value is found by applying the aforementioned critical pressure ratio. Because of nozzle friction, the actual pressure is usually in the range of 40 to 42 p.s.i.g., or, on the average, 41 p.s.i.g. If the vortex revolved as completely forced, were isentropic, and had sonic peripheral velocity, it would generate a peripheral pressure, due to centrifugal force, of only approximately 17 p.s.i.g. This is computed by integrating the pressure difference in the vortex, using well-known gas equations, with L16 result that the pressure ratio across the aforementioned kind of vortex is found to be equal to where k is the well-known ratio of specific heats of the gas. For air, with k=1.4, the pressure ratio across the vortex would then be equal to 0.458. Although the vortex is not exactly isentropic, the result is approximately applicable. There would then be a mismatch of pressures of approximately 4117 or 24 p.s.i. between the nozzle exits and the main vortex periphery. This pressure fall or drop will take place by unrestrained expansion, as described hereinbefore, with a partial realization of the theoretical velocity, such that the vortex will finally be driven at a slightly supersonic speed and will generate a peripheral pressure between and 25 p.s.i.g. The remaining pressure mismatch, amounting to between 16 and 21 p.s.i., is a total loss.
Calculations made in this manner show that a perfect match of nozzles and forced vortex exists, with air, up to an inlet pressure of approximately 55 p.s.i.g. With higher inlet pressures, the lossful phenomenon described begins to appear. It is large at p.s.i.g. Still higher pressures are almost totally wasted.
Therefore, the need for a remedy begins at approximately 55 p.s.i.g. and increases thereafter. There are several reasons why supersonic nozzles are undesirable in the vortex tube. One is that experimental efforts to utilize them have failed to yield any benefit. This may be because the abrupt juncture of the supersonic jet with the vortex produces a shock wave that destroys the extra velocity. Another reason is the cost and difiiculty of forming supersonic nozzles in a vortex tube. Still another is that even if they did function, they would require reshaping for different operating conditions.
The chamber 32 which is sequestered from the main turbulent vortex and not tightly coupled to it by the turbulent viscosity resolves the aforementioned difficulty. Furthermore, the narrowness of chamber 32 causes the air to spiral promptly inward therein and generate large Coriolis forces which speed it up. This establishes a substantially free vortex in chamber 32 and augments the entering sonic velocity efficiently to supersonic. In a free vortex, angular momentum is conserved and velocity is inversely proportional to radius. The supersonic air then enters the main, forced vortex and drives it at the maximum possible speed.
The minimum diameter of chamber 32 required to accomplish this function for any given operating pressure ratio can be computed by those highly skilled in the art with the aid of the aforementioned principles. No chamber 32 is needed below 55 p.s.i.g. with air discharging to the atmosphere. At 100 p.s.i.g., the diameter of chamber 32 should be at least 20 percent greater than that of bore 35. At still higher pressures, chamber 32 should be still larger. lowever, at extremely high pressures and high supersonic velocities, the frictional losses rise rapidly and the efficiency of the vortex tube, like that of many devices, begins to fall.
Vortex tubes constructed which lacked chamber 32 but comprised multiple nozzles similar to those shown in FIG. 3 yielded a rapidly appearing saturation at high pressures such that at pressures above 100 p.s.i.g. little further temperature depression was obtained. When chamber 32 was added to the system, and provided with sufiicient diameter as hereinbefore explained, the temperature depression continued to increase strongly with rising pressure, and efliciency was retained. Therefore, prior to incorporating chamber 32 into the structure, it was lossful to utilize the plurality of nozzles at high pressures and it was necessary to use Hilschs single nozzle and spiral ramp, which partially performed the supersonic function. Now with the aid of chamber 32, it is preferable to use the multiple nozzles at all times because they provide better vortex tube performance, better vortex symmetry and concentricity, better nozzle jet tangency, simpler construction, more rational nozzle design, and the manufacturing advantages encompassed in the molding process which may be utilized as well as the machining operations for forming the vortex generator. The vortex chamber 32 can also be machined and polished to a greater accuracy and degree when no spiral ramp is involved.
In the event chamber 32 is made larger than the minimum size required to match the pressures and velocities as described, the nozzles are driven into a subsonic state, but the main vortex continues to receive the maximum possible speed because the then augmented, transonic free vortex in chamber 32 executes the additional acceleration relinquished by the nozzles. The result is satisfactory unless chamber 313 is made so large that friction on its walls begins to exceed tolerable values. It is good practice to make chamber 32 somewhat oversize for the expected operating conditions.
The shape of contour 34 which merges chamber 32 into the bore 35 of insert 25 is not critical. A circular radius such as shown in FIG. 2 is satisfactory. It is not usually advisable that this radius be the largest possible radius that will produce a 90-degree corner; the radius is ordinarily made somewhat smaller so that a planar surface exists in the outer portion of chamber 32.
The purpose of bore 35 is to adapt the generator and Vortex to a hot tube 36 having a greater diameter and flow capacity than may correspond to the design of generator 26. By this means, a range of flow capacities may be covered in a single vortex-tube assembly by installing a series of alternate inserts 25 and generators 26 of various capacities. This substitution results in interchangeability of parts for producing the various desired results and increases the flexibility, adaptability, and economy of the vortex tube in its application to useful purposes.
When the maximum capacity of tube 36 is to be used, insert 25 may be cut off where contour 34 merges into surface 35 and the latter will become the inside of tube 36. The minimum diameter or throat diameter of contour 34 will be equal to the inside diameter of tube 36.
When a small capacity is to be used, insert 25 will possess the integral sleeve-like portion with bore 35 as seen in FIGS. 2, 7, 8, 9, and 11. It is unimportant whether the outside of the sleeve-like portion of insert 25 makes a snug fit inside tube 36 or not. The axial length of the sleeve-like portion of insert 25 should preferably be several times its inside diameter. Although this axial length is ordinarily not critical, it is advisable to perform experiments to determine its optimum value for any given desired operating condition or set of conditions. With the use of insert 25 and its sleeve-like portion, a range of capacities of 4 to l or more can be satisfactorily encompassed in a single main vortex-tube assembly by changing insert 25 and generator 26.
The reason that this range of capacities is achieved is that the critical portion of the vortex consists only of its first few diameters. As the distance from the generator grows, the exact diameter of the bore confining the vortex becomes of steadily decreasing importance. It is, therefore, permissible that the diameter increase Within limits. However, the presence of the long tube 36 may be essential and if it is made too short, a noticeable loss in the performance of the vortex tube may occur.
Valve 37 mounted at the terminal end of tube 36 may ordinarily be of any convenient design and may be made alternatively in the form of a fixed orifice, porous plug, capillary tube, or any means that will create a sufiicient obstruction to force the desired fraction of gas out of the opposite end of the vortex tube. It has been stated by some investigators that the internal shape of the valve or obstruction sometimes exerts a measurable influence upon the performance of the vortex tube. Other investigators have reported that, under certain conditions, it is beneficial to shorten tube 36 or to insert a vortex spoiler in the form of blades at a relatively near location in tube 36. Still others have found it beneficial to bleed hot gas from the sides of tube 36 through openings provided therein. Therefore it may be stated at the present time that the optimal length and configuration of tube 36 and valve 37 are not yet established under all conditions and will require further investigation. It is likely, however, that by making tube 36 at least 20 times as long as the diameter of bore 35 in insert 25, optimum results will always be closely approached if not fully attained. When tube 36 is long, valve 37 plays the role only of a resistance, and the only effect of importance at the extremity of tube 36 is the control of the amount of gas permitted to depart therefrom.
The inside surfaces of all parts, the insert 25, the vortex generator 26, and the long tube 36 should be smooth round and highly polished. The degree of smoothness id and polish required in the tube 36 decreases toward the valve end and becomes unimportant at that location.
A portion of the gas flows out through orifice 38. The diameter of orifice 38 is critical and must be determined by experiment for each operating condition. For the production of small cold fractions, it should be less than onehalf the inside diameter 35 of insert 25. For large cold fractions, the optimum is larger than one-half the diameter of surface 35. The optimum shape of the corner where surface 33 intersects orifice 38 is difficult to determine and depends somewhat upon the operating conditions and particularly upon the moisture content of the gas since deposition of ice occurs there. Under some conditions, a sharp edge is best. Under others, a small radius, as appears in FIGS. 2, 6, 7, 8 and 9 is best. Under still other conditions, a small snout or re-entrant mouth is best. It is advisable to investigate the effect of the shape of this corner with respect to the particular operating conditions desired.
Upon passing through orifice 38, the cold gas enters diffuser 39 which has or serves two functions. Its first function is to convert the kinetic energy of the gas flowing through orifice 38 into pressure, thus enabling the gas to flow into cold tube 4% more readily. Thus diffuser 39 lowers the pressure in the vortex core, at any given cold fraction, and enables colder gas to be produced. It also permits a smaller orifice 38 to be employed for the production of a given cold fraction with the result that colder gas is selected from the vortex core. This diffusing function is well known in fluid mechanics. The combination of vortex, orifice, and difiuser constitutes a venturi tube. It is useful and effective primarily at high cold fractions where the velocity in the orifice is high and there is much kinetic energy to be converted. In order to perform the diffusing function most effectively, divergent cone 39 should have a total included angle of between 8 and 16 degrees. However, a preferred value is approximately 14 degrees.
The second function of the diffuser 39 is to insulate the cold gas from the warm portion 41 of cold tube 40 where it joins cap 42. Vortex tubes have previously suffered from warming of the cold gas in this region before the gas could reach the distant and well insulated portions of cold tube 40. This warming has been particularly severe at small cold fractions where there is so little cold gas that the acquisition of a small amount of heat warms it by many degrees. But this is just the condition at which the coldest gas is internally produced. The result was that the coldest possible gas was never obtained. As the cold fraction was reduced, the cold gas temperature in tube 49 passed through a minimum in the range of 20 percent to 30 percent cold fraction and then rose rapidly at smaller cold fractions. This was entirely due to heat flow into the cold gas from the Warm parts which it touched on its way out.
In the various embodiments shown in FIGS. 1 through 9, this defect is almost completely overcome by the combination of diffuser 39 with thin cold tube 40 joined to counterbored cap 42 only at recessed juncture 41. The cold gas never touches warm portion 41. The portion of tube 49 where diffuser 39 ends and releases the gas is at a distance from warm portion 41 and is joined to it only by the thin section of the tube Wall which conducts very little heat. The requisite strength and insulating factors of this design for tube 40 result in improving the insulating feature for achieving minimum temperature of the cold gas measured well downstream in tube 40 and even inside fitting 43. This occurs at cold fractions of 10 percent or less and is decidedly lower than that which has been achieved heretofore. The annular air space 44 between the outside of diffuser 39 and the inside of tube 40 provides increased insulating characteristics.
In the event the compressed gas supplied to the vortex tube contains much moisture, as it often does in practice when the gas is not well dried, and if the vortex tube is operated so as to produce sub-freezing temperatures and is operated at cold fractions of approximately 50 percent or less, the moisture freezes and precipitates on the edge of orifice 38 and the inner surface of diffuser 39. It soon clogs these passges and stops the emission of cold gas. This is a very great nuisance, limitation, and source of difliculty in operating the vortex tube. It has been extremely difficult to prevent the adherence of the ice particles by the use of special materials or surface coatings. A cure can be effected, however, by employing the design shown in FIG. 12 which may be referred to as the antiicing cold-gas orifice and duct 45. What was previously referred to as diffuser 39 is bored out to the largest practicable size resulting in the cylindrical cavity 46 which leads through shallow cone 47 to the knife-edge orifice 43. The ice cannot adhere or stick to knife-edge orifice 43 and slowly accumulates in the cylindrical bore 45 which does not fill up enough to cause any appreciable obstruction until a considerable period of time has elapsed, such as a period of 15 minutes or more. At that time, valve 37 can be closed momentarily, thereby permitting the ice to melt and be driven out of the bore 46 after which normal operation can be resumed for another cycle.
The use of enlarged bore 46 spoils what was previously a diffuser but fortunately the icing problem that may be severe requires a remedy of this character only at medium and low cold fractions where the diffusing function is not greatly needed. The insulating function is most needed at those lower cold fractions and is still performed by enlarged bore or hole 46. At high cold fractions where the diffusing function is needed, ice does not ordinarily stick but is eroded and carried away by the rapidly-streaming cold gas so that diffuser 39 may be retained and employed as hereinbefore described.
Cap 42 is provided with a male screw thread which engages a female screw thread in the body 24. The inner end of the cap 42 is provided with a conical surface which engages the O ring 49. ring 49 is of the type commonly known in the mechanical art and is preferably composed of synthetic rubber. O ring 49 bears simultaneously upon the conical surface of the cap 42, the inner bore of the body 24, and the flat face of generator 26. The result is that O ring 49 performs a simultaneous radial and axial seal of all the parts and prevents the passage of gas in any direction in its neighborhood. It also simultaneously exerts a forward axial pressure upon the generator 25 so as to seat it against the insert 25 and to seat the insert 25 against the body 24 at the flat surface 56. It is ordinarily sutficient to tighten cap 42 by hand whereupon a gas pressure of up to 200 p.s.i.g. can be admitted to the inlet tube 22 without leakage. For still higher pressures or for permanence and rigidity of assembly, the cap 42 may be tightened by suitable means until it seats tightly against the lip of body 24. In order that 0 ring 49 will not be excessively compressed, the axial lengths of all parts are preferably so made that when cap 42 is tightly threaded, the compression of O ring 49 is of the maximum tolerable amount.
The use of the O ring to perform the dual seal in the manner described has considerable advantage in the assembly and is a simplification and improvement over existing designs. Previous designs have required more elaborate sealing means which include gaskets of precise thicknesses and precise adjustment of the dimensions or" all parts so that the two seals would be made simultaneously. This has resulted usually in leakage at the joint.
The body 24 and cap 42 are preferably made of brass, steel, stainless steel, monel, or any other desirable metals or may be made of plastic materials in some applications. The tubes 22, 36 and 40 are preferably made of stainless steel due to its low heat conductivity and high strength factors. The joints where the tubes 22, 36 and 4%; are joined to the body 24 and cap 42 are preferably brazed or welded for high strength. However, they may be softsoldered, glued, or fitted with screw threads when dcsir- 12 able. Insert 25 may be made of any desirable material whether of metal or plastic.
The material bounding surface 51, orifice 38 and diffuser 39 is preferably a plastic due to its very low heat conductivity, which will produce a minimum of adverse heat conduction into the cold gas as it rubs on surface 51, orifice 38 and diffuser 39. The use of a thermoplastic material will permit employment of the mandrel-type of molding in forming orifice 38 and diffuser 3 in a manner similar to that described for forming nozzles 27 in the generator, provided, however, that the molding mandrel is or" the proper contour to form the diffuser and orifice contours desired. In the event the nozzle-containing portion of generator 26 is made of metal, it is still desirable that surface 51, orifice 38 and diffuser 39 be formed in plastic for its insulating characteristics. In the latter case, the construction shown in FIG. 8 may be used.
The performance of the vortex tube may be enhanced, and the axial length of body 24 reduced, if inlet tube 22 is made as small as is consistent with the minimization of pressure loss, and the end of it is partially collapsed in the axial direction so as to produce a rectangular crosssection with semicircular ends, as shown in FIGS. 1, 2, 9, 10 and 11. The rectangular cross-section at the terminal end of tube 22 will necessitate forming a shaped hole 52 in body 24 to receive this rounded rectangular cross-section. Opening 52 is located as near to the end of body 24 as is permitted by the necessary and desired thickness of the end wall in body 24.
The improved vortex tube that has been described and appears in the various figures, when supplied with dehumidified compressed air at degrees F. and p.s.i.g. and discharging at atmospheric pressure, will produce a small fraction of cold air at minus 50 degrees F. or below when tube 36 is 1 inch in diameter and the capacity is between 4 c.f.m. and 8 c.f.m. When tube 36 is /2 inch in diameter and the capacity is 16 c.f.m. to 32 c.f.m., a temperature of minus 60 degrees F. or below is produced. At 50 percent cold fraction, the same respective vortex tubes will produce minus 15 degrees F. or below and minus 22 degrees F. or below. Performance of this character has not previously been approached or produced in vortex tubes.
At still higher pressures, or with a partial vacuum induced in the vortex core, even lower temperatures than above stated have been achieved. With precooled compressed air, the temperature depression or quantity T 1" remains substantially the same, except that it decreases slowly with decreasing absolute temperatures T so that there is no limit to how cold the air may be made and it may approach the liquefaction point.
Employing a large orifice at 38 and closing valve 37 almost entirely, andinsulating tube 36 carefully, temperatures above 500 degrees F. have been obtained at the hot end of this improved vortex tube under the above-specified operating conditions.
In FIG. 6 is shown the aerodynamic heart of this improved vortex tube irrespective of the details of the mechanical embodiment or method of assembly thereof. Compressed gas enters from plenum chamber 23 into converging nozzles 27 which inject the gas tangentially into outer vortex chamber 53. Hot tube 54 surrounds the main vortex chamber 55 for receiving thehot gas, with the cold gas entering diffuser 56 through orifice 57.
In FIGS. 7 and 8 there are illustrated two mechanical arrangements of the vortex generator and diffuser wherein it is shown in FIG. 7 that the generator, orifice and diffuser 57 may be integrally formed with insert 58 being positioned adjacent to the vortex generator surface 59. In FIG. 8, the orifice and diffuser 60, 39 are integrally formed and the generator and sleeve 61 are integrally formed and positioned adjacent to surface 62 of diffuser and orifice member 60. The parting surfaces may be arranged so as to yield two, three, or even more pieces, in indefinite number of variations and combinations. The
13 inserts may also be constituted as a single integral piece depending upon the difliculty of machining or processing the member.
In FIG. 9, there is illustrated a counterflow vortex tube that is a simple inversion of that shown in FIG. 1 wherein the hot tube 63 emerges from the cap 64 and cold tube 65 emerges from the body 66 with the other internal components substantially the same as illustrated in FIGS. 1 and 2. The modification illustrated in FIG. 9 represents an alternate arrangement which may be preferable under certain circumstances or for certain purposes.
FIG. 10 illustrates a vortex tube designed to cool an object 67 by positioning it so as to contact directly the vortex core instead of being bathed in cold air at a distance. It will be noted that the orifice 68 is not provided with any extension member such as shown in FIG. 1 but terminates at the orifice. The intense turbulence and high velocity existing in the vortex core bring about the existence of an enormous heat transfer coefficient upon any object that touches the vortex core. This combined with the very low temperature existing in the vortex core inside the generator section produces a tremendous cooling effect at the location 69. The object 67 may be of any material or thing whatever, such as, a part of the human body or any material that is to be treated. In the case of freezing a portion of the human body, surgery may be performed on that portion that is frozen. It has been found that the cooling induced at this point 69 will produce an intense white spot on the skin or flesh of a human body resulting in a temperature of 50 degrees F., -70 degrees F., 100 degrees F., or even below, depending upon the operating conditions in the vortex tube.
As will be noted in FIG. 10, the valve 37 shown in FIGS. 1 and 2 has been removed and the tube 36 is left open to the atmosphere. This creates a partial vacuum in the vortex core and increases the temperature depression there, provided, however, that the object to be cooled forms the requisite seal at the point 69. It is possible to increase the vacuum and obtain still greater temperature depressions by installing a diffuser suitably where the tube 36 discharges into the atmosphere. This diffuser may consist of a diverging cone, a diverging cone with a central body of revolution contained therein to prevent backward streaming of air from the atmosphere, two parallel fiat disks, one of which contains a hole and is fitted around tube 36, or other suitable devices. A vacuum pump may also be applied to tube 36. When a diffuser is employed, the optimum length of the tube 36 is somewhat reduced and may be determined for optimum results by experiment.
The mechanical differences between the vortex tube of FIG. 10 and that of FIG. 9 consist in eliminating the ditfuser and cold tube and bringing the vortex forward to provide the closest possible access to the externally applied object. This close access is obtained by using conical conjunctive surface 70 terminating in the orifice 68 between the body 71 and the vortex generator 72 by providing cone 70 extending from the exits of the generator nozzles 73 to the cold orifice 68, by making the cold orifice extremely short and in the limit a knife edge, and in general by minimizing the distance between the outer wall of the body and the plane of nozzles 73.
FIG. 11 illustrates a vortex tube that cools a stream of fluid which passes through a small tube 74 extending axially through the vortex cavity 75. The tube 74 conveys the fluid to be cooled from the far or discharge end of the vortex in counterflow and through the most active and cold portion of the vortex. The fluid being cooled gives up most of its heat in the far portions of the vortex and thereby heats the gas in that portion of the vortex without warming the front or most active portion excessively. This counterflow effect results in a high degree of power, economy and efficiency. As the fluid advances in tube 74 toward the vortex generator 76, it encounters progressively colder regions and, because of the great heat transfer in the vortex, it finally assumes substantially the minimum vortex core temperature. It then passes out of the vortex, till within tube 74, through the seal 77 and the insulated line 78.
The vortex tube illustrated in FIG. 11 operates in much the same manner as that illustrated in FIG. 10 in that there is no valve at the end of the tube and a diffuser or vacuum pump may be installed there. The description in respect to FIG. 10 is equally applicable to that of FIG. 11. Driving the vortex tube of FIG. 11 with compressed air at p.s.i.g. and 70 degrees F., a temperature of -75 degrees F. or below can be produced, and with the aid of a diffuser or vacuum pump, a temperature of l00 degrees F. or below may be achieved. This type of vortex tube may be especially useful in liquefying gases and in other applications requiring extremely great cooling. The gas supplied to the nozzles of the vortex tube in this embodiment may be any gas, and the fluid flowing in the tube 74 may be any fluid. The greatest resistance to heat transfer occurs between the inner wall of the tube 74 and the fluid being cooled therein. For
this reason, it may be advantageous in some instances to roughen the inside of tube 74 or to form fins upon it, or to pack the tube 74 with copper wool or other suitable materials.
The vortex tubes illustrated in FIGS. 10 and 11 are not as subject to clogging with ice in the event there is present humidity in the compressed gas used. Ice particles exist in the vortex but they are swept away and not deposited.
The insulation 78 in FIG. 11 should be applied to the tube 74 where it passes beyond the seal 77. The material composing the terminal wall of the vortex chamber at or near the seal 77 should be of very low heat conductivity and a plastic material has been found to be suitable.
It is desirable in each of the various embodiments to carefully insulate the exterior of the cold tube 40, 65 with a suitable material such as urethane or other foamed plastics and the insulation should extend into and fill the annular cavity between cold tube 40 and cap 42.
It will be readily appreciated that the particular contours illustrated may be modified without departing from the purpose and spirit of this invention, such as the configuration of the orifices and diffusers, and to make or fabricate some of the components integrally or to articulate them without departing from the purpose and spirit of this invention. Many modifications and variations are contemplated within the scope of the appended claims.
What is claimed is:
1. A vortex tube apparatus for cooling a fluid flowing continuously therethrough comprising a housing body having an opening for admitting a gas, a vortex generator supported in said body and receiving said gas, said generator having a plurality of nozzles and a circular vortex cavity into which said nozzles discharge said gas tangentially, a seal at one end of said vortex cavity, a discharge line communicating with the other end of said vortex cavity, and a smaller line for receiving said fluid to be cooled passing coaxially through said discharge line, vortex cavity and seal.
2. A vortex generator having an annular body with a circular vortex cavity, a plurality of nozzle openings circumferentially spaced from each other and extending from the perimeter of said body tangentially into said vortex cavity, and a cold gas receiving member extending across one end of said vortex cavity and having a tubular extension with a circular bore concentric with and of smaller diameter than the said vortex cavity, at least a portion of said bore adjacent said generator being tapered toward said generator with a total wall angularity of substantially 8 to 16.
3. A vortex generator having an annular body with a circular vortex cavity, a plurality of nozzle openings circumferentially spaced from each other and extending from the perimeter of said body tangentially into said vortex cavity, and a hot gas receiving member extending 15 across one end of said vortex cavity and having a circular bore concentric with and of'smaller diameter than the said vortex cavity.
4. In a vortex tube device, the combination of a vortex generator having an annular body with a plurality of nozzle openings therein, and a central vortex cavity, the inner ends of said nozzle openings being tangentially directed into said cavity, a hollow hot gas member extending across one end of said vortex cavity and having a central opening with an internal diameter substantially less than the internal diameter of said vortex cavity, a cold gas member extending across the other end of said vortex cavity and having a central opening with an efiective internal diameter substantially less than the diameter of the opening in said hot gas member, said members and said generator being in assembly such that a substantial annular outer portion of said vortex cavity extends beyond the openings in said last mentioned members whereby the speed of rotation of a gas vortex formed in said vortex cavity is increased, and housing means holding the said members and the said generator in assembly and shaped to provide a plenum chamber surrounding the annular body of said generator for supplying gas under pressure thereto.
5. The structure claimed in claim 4 wherein said housing means comprises an internally threaded cap element with a lateral gas inlet and an externally threaded nipple element with a tapered nose portion, one of said hot and cold gas members having a disc-like portion on its end extending beyond the annular body of said generator, the said members and said generator being assembled in said housing in such order that the said disc-like portion approaches the said nipple, and an O-ring compressiblebetween a surface o f said disc-like portion, an inner surface of said cap element and the tapered nose of said nipple.
6. The structure claimed in claim 5 wherein hot and cold gas members are made of a substance of low heat conductivity, wherein a metallic tube is attached to one of said housing elements and wherein said cold gas mem ber comprises an axial extension, the said central opening therein tapering toward said generator, said axial extension lying within said tube but spaced from the walls thereof.
7. The structure claimed in claim 6 wherein a metallic tube is attached to the other of said housing elements and wherein said hot gas member has a tubular extension lying within said last mentioned metal tube.
8. The structure claimed in claim 5 wherein said generator body and said'hot gas member are integral and wherein said cold gas member and said disc-like element are integral. i i
9. The structure claimed in claim 5 wherein said generator body, said disc-like element, and said cold gas element are integral.
References Cited by the Examiner UNITED STATES PATENTS 2,187,217 1/40 Winslow 285-332.3 2,581,168 1/52 Bramley 62-5 2,661,965 12/53 Parmesan 285332.3 2,802,530 8/57 Kaufman 18-48 2,839,900 6/58 Green 625 2,893,214 7/59 Hendal 625 2,955,432 10/60 Hardebol 625 2,979,776 4/61 Morin l856 3,057,166 10/62 Thompson 625 3,074,243 1/63 Tilden 62-5 FOREIGN PATENTS 556,867 5/57 Belgium.
WILLIAM J. WYE, Primary Examiner.