G. RANQUE

Vortex Tube

Acoustic Vortex Heating / Cooling ( 100 * differential ) ... Construction, Analyses of Operation, & Patents...

Vortex Tube

Separation of a compressed gas into a hot stream and a cold stream

The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gas into hot and cold streams. It has no moving parts.

Pressurized gas is injected tangentially into a swirl chamber and accelerates to a high rate of rotation. Due to the conical nozzle at the end of the tube, only the outer shell of the compressed gas is allowed to escape at that end. The remainder of the gas is forced to return in an inner vortex of reduced diameter within the outer vortex.

There are different explanations for the effect and there is debate on which explanation is best or correct.

What is usually agreed upon is that the air in the tube experiences mostly "solid body rotation", which simply means the rotation rate (angular velocity) of the inner gas is the same as that of the outer gas. This is different from what most consider standard vortex behaviour--where inner fluid spins at a higher rate than outer fluid. The (mostly) solid body rotation is probably due to the long time which each parcel of air remains in the vortex--allowing friction between the inner parcels and outer parcels to have a notable effect.

It is also usually agreed upon that there is a slight effect of hot air wanting to "rise" toward the center, but this effect is negligible--especially if turbulence is kept to a minimum.

One simple explanation is that the outer air is under higher pressure than the inner air (because of centrifugal force). Therefore the temperature of the outer air is higher than that of the inner air.

Another explanation is that as both vortices rotate at the same angular velocity and direction, the inner vortex has lost angular momentum. The decrease of angular momentum is transferred as kinetic energy to the outer vortex, resulting in separated flows of hot and cold gas.[1]

This is somewhat analogous to a Peltier effect device, which uses electrical pressure (voltage) to move heat to one side of a dissimilar metal junction, causing the other side to grow cold.

When used to refrigerate, heat-sinking the whole vortex tube is helpful. Vortex tubes can also be cascaded. The cold (or hot) output of one can be used to pre-cool (or pre-heat) the air supply to another vortex tube. Cascaded tubes can be used, for example, to produce cryogenic temperatures.

History

The vortex tube was invented in 1933 by French physicist Georges J. Ranque. German physicist Rudolf Hilsch improved the design and published a widely read paper in 1947 on the device, which he called a Wirbelrohr (literally, whirl pipe).[2] Vortex tubes also seem to work with liquids to some extent.[3]

Efficiency

Vortex tubes have lower efficiency than traditional air conditioning equipment. They are commonly used for inexpensive spot cooling, when compressed air is available. Commercial models are designed for industrial applications to produce a temperature drop of about 45 °C (80 °F).

Proposed applications

* Dave Williams, of dissigno, has proposed using vortex tubes to make ice in third-world countries. Although the technique is inefficient, Williams expressed hope that vortex tubes could yield helpful results in areas where using electricity to create ice is not an option.

* There are industrial applications that result in unused pressurized gases. Using vortex tube energy separation may be a method to recover waste pressure energy from high and low pressure sources.[4]

References

1. ^ exair.com - Vortex tube theory -- http://www.exair.com/Cultures/en-US/Primary+Navigation/Products/Vortex+Tubes+and+Spot+Cooling/Vortex+Tubes/A+Phenomenon+of+Physics
2. ^ *Rudolf Hilsch, The Use of the Expansion of Gases in A Centrifugal Field as Cooling Process, The Review of Scientific Instruments, vol. 18(2), 108-1113, (1947). translation of an article in Zeit. Naturwis. 1 (1946) 208.
3. ^ R.T. Balmer. Pressure-driven Ranque-Hilsch temperature separation in liquids. Trans. ASME, J. Fluids Engineering, 110:161–164, June 1988.
4. ^ Sachin U. Nimbalkar, Dr.M.R. Muller. Utilizing waste pressure in industrial systems. Energy: production, distribution and conservation, ASME-ATI 2006, Milan

General references

* G. Ranque, Expériences sur la Détente Giratoire avec Productions Simultanées d'un Echappement d'air Chaud et d'un Echappement d'air Froid, J. de Physique et Radium 4(7)(1933) 112S.
* H. C. Van Ness, Understanding Thermodynamics, New York: Dover, 1969, starting on page 53. A discussion of the vortex tube in terms of conventional thermodynamics.
* Mark P. Silverman, And Yet it Moves: Strange Systems and Subtle Questions in Physics, Cambridge, 1993, Chapter 6
* C. L. Stong, The Amateur Scientist, London: Heinemann Educational Books Ltd, 1962, Chapter IX, Section 4, The "Hilsch" Vortex Tube, p514-519.
* J. J. Van Deemter, On the Theory of the Ranque-Hilsch Cooling Effect, Applied Science Research 3, 174-196.
* Saidi, M.H. and Valipour, M.S., "Experimental Modeling of Vortex Tube Refrigerator", J. of Applied Thermal Engineering, Vol.23, pp.1971-1980, 2003.
* M. Kurosaka, Acoustic Streaming in Swirling Flow and the Ranque-Hilsch (vortex-tube) Effect, Journal of Fluid Mechanics, 1982, 124:139-172
* M. Kurosaka, J.Q. Chu, J.R. Goodman, Ranque-Hilsch Effect Revisited: Temperature Separation Traced to Orderly Spinning Waves or 'Vortex Whistle', Paper AIAA-82-0952 presented at the AIAA/ASME 3rd Joint Thermophysics Conference (June 1982)
* Gao, Chengming. Experimental Study on the Ranque-Hilsch Vortex Tube. Eindhoven : Technische Universiteit Eindhoven. ISBN 90-386-2361-5.

* Windhexe
* Helikon vortex separation process

* airtxinternational.com - AiRTX International, how vortex tubes work -- http://www.airtxinternational.com/how_vortex_tubes_work.php

* Tim Cockerill's pages on the Ranque-Hilsch Vortex Tube, including his 1995 Cambridge University thesis on the subject, and a mailing list. -- http://www.cockerill.net/rhvtmatl/

* How to Make Ice Out of Thin Air: Cool Heat Transfer, Daren Fonda, Sep. 4, 2005, Time Magazine. (Requires membership) --  http://www.time.com/time/magazine/printout/0,8816,1101299,00.html

* Oberlin college physics demo -- http://www.oberlin.edu/physics/catalog/demonstrations/thermo/vortextube.html

* itwvortec.com - Manufacturer of vortex tubes, information page -- http://www.itwvortec.com/vortex_tubes.php

* The Hilsch Vortex Tube - Online copy of the Scientific American article by C. L. Stong -- http://www.visi.com/~darus/hilsch/

* Home-brew vortex tube made from off-the-shelf parts - David Buchan's Ranque-Hilsch effect tube project using only off-the-shelf plumbing parts -- http://www.pdbuchan.com/ranque-hilsch/ranque-hilsch.html

Vortex tube uses and how do they work --
http://www.arizonavortex.com/vortex-tube

Vortex Tubes - Sub-Zero Spot Cooling from Compressed Air

Vortex Tubes are an effective, low cost solution to a wide variety of industrial spot cooling and process cooling needs. With no moving parts, a vortex tube spins compressed air to separate the air into cold and hot air streams. While French physicist Georges Ranque is credited with inventing the vortex tube in 1930, Vortec was the first company to develop and apply this phenomenon into practical and effective spot cooling solutions for industrial use.

Vortex Tube Applications:

Vortex Tubes have a very wide range of application for industrial spot cooling on machines, assembly lines and processes.

# Cool Machining Operations
# Cool plastic injection molds
# Dry ink on labels and bottles
# Dehumidify gas operations
# Cool heat seal operations
# Thermal test sensors and choke units
# Temperature cycle parts

How a Vortex Tube Works

Fluid (air) that rotates around an axis (like a tornado) is called a vortex. A Vortex Tube creates cold air and hot air by forcing compressed air through a generation chamber which spins the air centrifugally along the inner walls of the Tube at a high rate of speed (1,000,000 RPM) toward the control valve. A percentage of the hot, high-speed air is permitted to exit at the control valve. The remainder of the (now slower) air stream is forced to counterflow up through the center of the high-speed air stream, giving up heat, through the center of the generation chamber finally exiting through the opposite end as extremely cold air. Vortex tubes generate temperatures down to 100°F below inlet air temperature. A control valve located in the hot exhaust end can be used to adjust the temperature drop and rise for all Vortex Tubes.

Vortex Tubes Features & Benefits

• Vortex Tubes use only compressed air for spot cooling- no electricity or refrigerants are required

• Vortex Tubes are maintenance free - Since Vortex Tubes have no moving parts there is no maintence required

Vortex Tubes are Exceptionally reliable

Vortex Tubes are Compact and lightweight

Vortex Tube technologoy is Cycle repeatablity with ± 1 °

Vortex Tubes from Vortec drops inlet temperature by up to 100°F providing exceptional spot cooling

PDF Scans

5 -- E. Eckert & J. Hartnett : "Investigation of the Energy Distribution in a High Velocity Vortex Type Flow" ( Armour Research Symposium, May 1955 )

6 -- C. Pengelley : "Thermal Phenomena in a Vortex" ( Armour Research Symposium, May 1955 )

US Patent # 1952281
"Method & Apparatus for Obtaining from a Fluid Under Pressure Two Currents of Fluids at Different Temperatures"
G. Ranque

...    ...
...   ...   ...    ...    ...

http://pubs.acs.org/doi/abs/10.1021/ie50570a035

The Ranque-Hilsch Vortex Tube

William A. Scheller, George M. Brown

Ind. Eng. Chem., 1957, 49 (6), pp 1013–1016
DOI: 10.1021/ie50570a035
Publication Date: June 1957

Cryocoolers 12

Publisher Springer US
ISBN 978-0-306-47714-0 (Print) 978-0-306-47919-9 (Online)

Study of a Vortex Tube by Analogy with a Heat Exchanger

Y. Cao2, Y.F. Qi3, E.C. Luo3, J.F Wu3, M.Q. Gong3 and G.M. Chen2
(2)   Institute of Refrigeration and Cryogenic Engineering, Zhejiang University, Hangzhou, China, 310027
(3)   Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China, 100080

Abstract -- Based on the models of Scheper, Lewins, and Bejan, a new model has been established to study the influence of the cold mass flow fraction on the temperature separation effect in a vortex tube. The model is based on making an analogy between the vortex tube and a counterflow heat exchanger. The results show the model can accurately explain the correlation of cold mass flow fraction to the temperature separation effect.

Newsgroups: sci.physics.fusion
From: bernecky@starbase.nl.nuwc.navy.mil (W. Robert Bernecky)
Subject: Wirbelrohr or vortex tube
Sender: scott@zorch.SF-Bay.ORG (Scott Hazen Mueller)
Date: Sat, 1 Jul 1995 23:11:02 GMT

The following may be relevant to the Potapov device.

It contains excerpts from "And yet it moves...strange systems & subtle questions in physics," by Mark P. Silverman, Cambridge University Press, 1993; Chpt 6 "The Wirbelrohr's roar".

[BILL B. NOTE: also see Scientific American, November 1958 for a Hilsch-tube construction article in Stong's THE AMATEUR SCIENTIST]

"It was a Wirbelrohr, he explained; you blew into the stem, and out one end of the cross-tube flowed hot air, while cold air flowed out the other. I laughed; I was certain he was teasing me. Although I had never heard of a Wirbelrohr, I recognised a Maxwell demon when it was described."

"...he machined in his basement workshop a working model which I received from him shortly afterwards. The exterior was more or less just as he had described it: two identical long thin-walled tubes (the cross-bar of the T), were connected by cylindrical collars screwed into each end of a short section of pipe that formed the central chamber; a gas inlet nozzle (the stem of the T), shorter than the other two tubes but otherwise of identical construction, joined the midsection tangentially (Fig. 6.1). Externally, except for a throttling valve at the far end of one output tube to control air flow, the entire device manifested bilateral symmetry with respect to a plane through the nozzle perpendicular to the cross-tubes.

"Only someone with the lung capacity of Hercules could actually blow into the stem. Instead, the nozzle was meant to be attached to a source of compressed air. Taking the Wirbelrohr into my laboratory, I looked sceptically for a moment at its symmetrical shape before opening the valve by my work table that started the flow of room-temperature compressed air. Then, with frost forming on the outside surface of one tube, I yelped with pain and astonishment when, touching the other tube, I burned my fingers!"

"...With the few parts of the Wirbelrohr laid out on my table, I understood better the significance of the German name, Wirbelrohr, or vortex tube. The heart of the device is the central chamber with a spiral cavity and offset nozzle. Compressed gas entering this chamber streams around the walls of the cavity in a high-speed vortex. But what gives rise to spatially separated air currents at different temperatures? ...the placement in one cross-tube (the cold one) of a small-aperture diaphragm effectively blocked the efflux of gas along the walls of the tube, thereby forcing this part of the air flow to exit through the other arm whose cross-section was unconstrained.

|-----------|
--------------| |------------------
| "COLD" PIPE
"HOT" PIPE

| <--- diaphragm
--------------| |------------------
|---| |---|
/ | |
CENTRAL | |
CHAMBER | |
| | | <- INLET
_____
/ \

Fig 6 - Schematic of Wirbelrohr or
/ __ \ vortex tube.
/ / \
| / | Top View
| | |
\ | | /
\ | | /
| | /
| |---
| |
| | <- INLET
| |

Room-temperature compressed air enters the inlet tube, spirals around the central chamber, and exits through the 'hot' pipe with unconstrained cross-section or through the 'cold' pipe whose aperture is restricted  by a diaphragm.

[BILLB: the 'hot' tube should be partially blocked, with either a valve, or even better, a narrow ring-slot that lets air near the inner surface escape.]

"The glimmer of a potential mechanism dawned on me. Had the in- coming air conserved angular momentum, the rotational frequency of air molecules nearest the axis of the central chamber would be higher - as would also be the corresponding rotational kinetic energy - than peripheral layers of air. However, internal friction between gas layers comprising the vortex would tend to establish a constant angular velocity throughout the cross-section of the chamber. In other words, each layer of gas within the vortex would exert a tangential force upon the next outer layer, thereby doing work upon it at the expense of its internal energy (while at the same time receiving kinetic energy from the preceding inner layer). Energy would consequently flow from the center radially outward to the walls generating a system with a low-pressure, cooled axial region and a high-pressure, heated circumferential region. Because of the diaphragm, the cooler axial air had to exit one tube (the cold side), whereas a mixture of axial and peripheral air exited the other (the hot side).

"The presence of the throttling valve on the hot side now made sense. If the low pressure of the air nearest the axis of the tube fell below atmospheric pressure, the cold air would not exit at all...By throttling the flow, pressure within the central chamber was increased sufficiently so that air could exit both tubes.

"...with some simplifying assumptions I was able to calculate the entropy change... Under what is termed adiabatic conditions - i.e. with no heat exchange with the environment - the 2nd Law requires that the entropy change of the gas, alone, be >= zero. The resulting mathematical expression, augmented by the equation of state of an ideal diatomic gas and the conservation of energy (1st Law) yields an inequality:

(x^f)[(1-fx)/(1-f)]^(1-f) >= (Pf/Pi)^(2/7)

where x= Tc/Ti
Tc is temperature of cold air
Ti is initial temperature
Pf is the final pressure
Pi is the initial pressure
f is the fraction of gas directed thru the cold side

"By setting the expression for the entropy change equal to zero, I could calculate the lowest temperature that the cold tube should be able to reach if the gas flow were an ideal reversible process. The result was astonishing. With an input pressure of 10 atmospheres and the throttling set for a fraction f= 0.3, compressed air at room temperature (20 C) could in principle be cooled to about -258 C, a mere 15 degrees above absolute zero! (The corresponding temperature of the hot side would have been 80 C.)

"...The first experimental demonstation of a vortex tube seems to have been reported in 1933 by a French engineer, Georges Ranque [1]. by German physicist Rudolph Hilsch came to the attention of American chemist R.M. Milton... In Hilsch's hands, proper selection of the air fraction f (~ .33) and an input pressure of a few atmospheres gave rise to an amazing output of 200 C at the hot end and -50 C at the cold end[2]. Hilsch, who was the one to coin the term Wirbelrohr, used the tube in place of an ammonia pre-cooling apparatus in a machine to liquify air.

"...Milton was not satisfied with the interpretation of Hilsch and Ranque that frictional loss of kinetic energy produced the radial temperature distribution...."

M Kurosaka, et al [3,4], in 1982, proposed a far different mechanism, supported by experiment.

"With a loud roar air rushes turbulently thru the Wirbelrohr, just as it does thru a jet engine or a vacuum cleaner. Buried within that roar, however, is a pure tone, a "vortex whistle" as it has been called...the vortex whistle can be produced by tangential introduction and swirling of gas in a stationary tube. It is this pure tone that is purportedly responsible for the spectacular separation of temperature in a vortex tube.

"The Ranque-Hilsch effect is a steady-state phenomenon - i.e. an effect that survives averaging over time. How can a high-pitch whistle - a sound that, depending on air velocity and cavity geometry, can be on the order of a few kilohertz - influence the steady component of flow? The answer...was by 'acoustic streaming'. As a result of a small nonlinear convection term in the fluid equation of motion, an acoustic wave can act back upon the steady flow and modify its properties substantially. In the absence of unsteady disturbances, the air flows in a 'free' vortex around the axis of the tube; the speed of the air is close to zero at the center (like a hurricane), increases to a maximum at mid-radius, and drops to a small value near the walls. Acoustic streaming, however, deforms the free vortex into a 'forced' vortex where the air speed increases linearly from the center to the periphery. Acoustic streaming and the production of a forece vortex, rather than mere static centrifugation, engender the Ranque-Hilsch effect.

"The experimental test could not be more direct. Remove the whistle, and only the whistle, and see whether the radial temperature distribution remains. To do this [Kurosaka] monitored the entireroar with a microphone and ...decomposed it into frequencies of which the discrete component of the lowest frequency and largestamplitude was identified as the vortex whistle. Next, he enclosed the Wirbelrohr inside a tunable acoustic suppressor: a cylindrical section of Teflon with radially drilled holes serving as acoustic cavities distributed uniformly around the circumference. Inside each hole was a small tuning rod that could be inserted until it touched the outer shell of the Wirbelrohr to close off the cavity, or withdrawn incrementally to make the cavity resonant at the specified frequency to be suppressed.

"To simplify the experimental test, he sealed off one output of the vortex tube and monitored with thermocouples the temperature difference between the center and periphery. In the absence of the suppressor, an increase in pressure produced, as I had noticed when experimenting with my own vortex tube, a louder roar and greater temperature difference. When, however, the acoustic cavity was adjusted to suppress only the frequency of the vortex whistle (leaving unaffected the rest of the turbulent noise), the temperature difference plunged precipitously at the instant the corresponding input air pressure was reached. In one such trial, the centerline temperature jumped 33 C, from -50 C to -17 C. With further increase in pressure, the frequency of the whistle rose, and as it exceeded the narrow band of the acoustic suppressor, the temperature difference increased again.

"Additional evidence came from a striking transformation in the nature of the flow...Before the vortex whistle was suppressed, the exhaust air swirled rapidly near and outside the tube periphery in the manner expected for a forced vortex. Upon supprssion, however, the forced vortex was also abruptly suppressed; now quiescent at the periphery, the air rushed out close to the centerline."

"For all I know, the case of the mysterious Wirbelrohr is largely closed although, science being what it is, future version of that device may yet hold some suprises in store. I have sometimes wondered, for example, what would result from supplying a vortex tube, not with room-temperature air, but with a quantum fluid, like liquid helium, free of viscosity and friction.

The exorcism of the demon in the Wirbelrohr will not, I suspect, dampen one bit the ardour of those whose passion it is to challenge the 2nd Law. Despite the time and effort that has been frittered away in the past, others will undoubtedly try again. On the whole such schemes are bound to fail, but every so often, as in the case of Maxwell's own whimsical creation, this failure has its positive side: when, from the clash between human ingenuity and the laws of nature, there emerge sounder knowledge and deeper understanding."

[1] G. Ranque, "Experiences sur la Detente Giratore avec Productions Simultanees d'un Echappement d'air Chaud et d'un Echappement d'air Froid", J. de Physique et Radium 4(7)(1933) 112 S.

[2] R. Hilsch, "The Use of the Expansion of Gases in a Centrifugal Field as Cooling Process", Rev. Sci. Instrum. 18(2) (1947) 108-1113.

[3] M. Kurosaka, "Acoustic Streaming in Swirling Flow and the Ranque-Hilsch (Vortex Tube) Effect", J. Fluid Mech. 124(1982)139.

[4] M. Kurosaka, J.Q. Chu, & J.R. Goodman, "Ranque-Hilsch Effect Revisited: Temperature Separation Traced to Orderly Spinning Waves or Vortex Whistle", conference of Am Inst. of Aero & Astro 1982.

[OTHERS]

C. L. Stong, The "Hilsch" Vortex Tube, The Amateur Scientist, Scientific American, 514-519.

J. J. Van Deemter, On the Theory of the Ranque-Hilsch Cooling Effect, Applied Science Research 3, 174-196.

C.L. Stong : "The Scientific American Book of Projects for the Amateur Scientist," pp. 514-520 ' Simon and Schuser, NY, 1960.
C.L. Stong : Scientific American, November 1958, p. 45.
Illustrations by Roger Hayward

# THE "HILSCH" VORTEX TUBE

With nothing more than a few pieces of plumbing and a source of compressed air, you can build a remarkably simple device for attaining moderately low temperatures. It separates high-energy molecules from those of low energy. George O. Smith, an engineer of Rumson, N. I., discusses its theory and construction

The 19th century British physicist James Clerk Maxwell made many deep contributions to physics, and among the most significant was his law of random distribution. Considering. the case of a closed box containing a gas, Maxwell started off by saying that the temperature of the gas was due to the motion of the individual gas molecules within the box. But since the box was standing still, it stood to reason that the summation of the velocity and direction of the individual gas molecules must come to zero.

In essence Maxwell's law of random distribution says that for every gas molecule headed east at 20 miles per hour, there must be another headed west at the same speed. Furthermore, if the heat of the gas indicates that the average velocity of the molecules is 20 miles per hour, the number of molecules moving slower than this speed must be equaled by the number of molecules moving faster.

After a serious analysis of the consequences of his law, Maxwell permitted himself a touch of humor. He suggested that there was a statistical probability that; at some time in the future, all the molecules in a box of gas or a glass of hot water might be moving in the same direction. This would cause the water to rise out of the glass. Next Maxwell suggested that a system of drawing both hot and cold water out of a single pipe might be devised if we could capture a small demon and train him to open and close a tiny valve. The demon would open the valve only when a fast molecule approached it, and close the valve against slow molecules. The water coming out of the valve would thus be hot. To produce a stream of cold water the demon would open the valve only for slow molecules.

Maxwell's demon would circumvent the law of thermodynamics which says in essence: "You can't get something for nothing." That is to say, one cannot separate cold water from hot without doing work. Thus when physicists heard that the Germans had developed a device which could achieve low temperatures by utilizing Maxwell's demon, they were intrigued, though obviously skeptical. One physicist investigated the matter at first hand for the U. S. Navy. He discovered that the device was most ingenious, though not quite as miraculous as had been rumored.

234

It consists of a T-shaped assembly of pipe joined by a novel fitting, as depicted in Figure 234. when compressed air is admitted to the "leg" of the T, hot air comes out of one arm of the T and cold air out of the other arm! Obviously, however, work must be done to compress the air.

The origin of the device is obscure. The principle is said to have been discovered by a Frenchman who left some early experimental models in the path of the German Army when France was occupied. These were turned over to a German physicist named Rudolf Hilsch, who was working on low temperature refrigerating devices for the German war effort. Hilsch made some improvements on the Frenchman's design, but found that it was no more efficient than conventional methods of refrigeration in achieving fairly low temperatures. Subsequently the device became known as the Hilsch tube.

235

The Hilsch tube may be constructed from a pair of modified nuts and associated parts as shown in Figure 235. The horizontal arm of the T-shaped fitting contains a specially machined piece, the outside of which fits inside the arm. The inside of the piece, however, has a cross section which is spiral with respect to the outside. In the "step" of the spiral is a small opening which is connected to the leg of the T Thus air admitted to the leg comes out of the opening and spins around the one-turn spiral. The "hot" pipe is about 14 inches long and has an inside diameter of half an inch. The far end of this pipe is fitted with a stopcock which can be used to control the pressure in the system [see Fig. 236].

236

The "cold" pipe is about four inches long and also has an inside diameter of half an inch. The end of the pipe which butts up against the spiral piece is fitted with a washer, the central hole of which is about a quarter of an inch in diameter. Washers with larger or smaller holes can also be inserted to adjust the system.

Three factors determine the performance of the Hilsch tube; the setting of the stopcock, the pressure at which air is admitted to the nozzle, and the size of the hole in the washer. For each value of air pressure and washer opening there is a setting of the stopcock which results in a maximum difference in the temperature of the hot and cold pipes [see Fig. 237].

237

When the device is properly adjusted, the hot pipe will deliver air at about 100 degrees Fahrenheit and the cold pipe air at about -70 degrees (a temperature substantially below the freezing point of mercury and approaching that of "dry ice"). When the tube is adjusted for maximum temperature on the hot side, air is delivered at about 350 degrees F. It must be mentioned, however, that few amateurs have succeeded in achieving these performance extremes. Most report minimums on the order of -10 degrees and maximums of about + 140 on the first try. Despite its impressive performance, the efficiency of the Hilsch tube leaves much to be desired. Indeed, there is still disagreement as to how it works. According to one explanation, the compressed air shoots around the spiral and forms a high-velocity vortex of air. Molecules of air at the outside of the vortex are slowed by friction with the wall of the spiral. Because these slow-moving molecules are subject to the rules of centrifugal force, they tend to fall toward the center of the vortex. The fast-moving molecules just inside the outer layer of the vortex transfer some of their energy to this layer by bombarding some of its slow-moving molecules and speeding them up. The net result of this process is the accumulation of slow-moving, low-energy molecules in the center of the whirling mass, and of high-energy, fast-moving molecules around the outside. In the thermodynamics of gases the terms "high energy" and "high velocity" mean "high temperature." So the vortex consists of a core of cold air surrounded by a rim of hot air.

The difference between the temperature of the core and that of the rim is increased by a secondary effect which takes advantage of the fact that the temperature of a given quantity of gas at a given level of thermal energy is higher when the gas is confined in a small space than in a large one; accordingly when gas is allowed to expand, its temperature drops. In the case of the Hilsch tube the action of centrifugal force compresses the hot rim of gas into a compact mass which can escape only by flowing along the inner wall of the "hot" pipe in a compressed state, because its flow into the cold tube is blocked by the rim of the washer.

The amount of the compression is determined by the adjustment of the stopcock at the end of the hot pipe. In contrast, the relatively cold inner core of the vortex, which is also considerably above atmospheric pressure, flows through the hole in the washer and drops to still lower temperature as it expands to atmospheric pressure obtaining inside the cold pipe.

Apparently the inefficiency of the Hilsch tube as a refrigerating device has barred its commercial application. Nonetheless amateurs who would like to have a means of attaining relatively low temperatures, and who do not have access to a supply of dry ice, may find the tube useful. when properly made it will deliver a blast of air 20 times colder than air which has been chilled by permitting it simply to expand through a Venturi tube from a high-pressure source. Thus the Hilsch tube could be used to quick- freeze tissues for microscopy, or to chill photomultiplier tubes. But quite apart from the tube's potential application, what could be more fun than to trap Maxwell's demon and make him explain in detail how he manages to blow hot and cold at the same time?

Incidentally, this is not a project for the person who goes in for commercially made apparatus. So far as I can discover Hilsch tubes are not to be found on the market. You must make your own. Nor is it a project for the experimenter who makes a speciality of building apparatus from detailed specifications and drawings. The dimensions shown in the accompanying figures are only approximate. Certainly they are not optimum values. But if you enjoy exploration, the device poses many questions. What would be the effect, for example, of substituting a divergent nozzle for the straight one used by Hilsch? Why not create the vortex by impeller vanes, such as those employed in the stator of turbines? Would a spiral chamber in the shape of a torus improve the efficiency? What ratio should the diameter of the pipes bear to the vortex chamber and to each other? Why not make the spiral of plastic, or even plastic wood? One can also imagine a spiral bent of a strip of brass and soldered into a conventional pipe coupling. Doubtless other and far more clever alternatives will occur to the dyed-in-the-wool tinkerer.

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