HILSCH VORTEX TUBE

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). Ex-

ternally, except for a throttling valve at the far end of one

output tube to control air flow, the entire device manifested bi-

lateral symmetry with respect to a plane through the nozzle per-

pendicular 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 aston-

ishment 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, Wirbel-

rohr, 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 effec-

tively 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 fric-

tion between gas layers comprising the vortex would tend to es-

tablish a constant angular velocity throughout the cross-section

of the chamber. In other words, each layer of gas within the vor-

tex 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 preced-

ing 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 circum-

ferential 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 re-

quires 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, com-

pressed 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 selec-

tion 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 mecha-

nism, 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 tan-

gential introduction and swirling of gas in a stationary tube. It

is this pure tone that is purportedly responsible for the spec-

tacular 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 ge-

ometry, can be on the order of a few kilohertz - influence the

steady component of flow? The answer...was by 'acoustic stream-

ing'. 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 ab-

sence of unsteady disturbances, the air flows in a 'free' vortex

around the axis of the tube; the speed of the air is close to ze-

ro 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' vor-

tex 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 whis-

tle, and only the whistle, and see whether the radial temperature

distribution remains. To do this [Kurosaka] monitored the entire

roar with a microphone and ...decomposed it into frequencies of

which the discrete component of the lowest frequency and largest

amplitude was identified as the vortex whistle. Next, he enclosed

the Wirbelrohr inside a tunable acoustic suppressor: a cylindri-

cal 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 reso-

nant 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 temperatuare

difference between the center and periphery. In the absence of

the suppressor, an increase in pressure produced, as I had no-

ticed 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

natuare of the flow...Before the vortex whistle was suppressed,

the exhaust air swirled rapidly near and outside the tube periph-

ery in the manner expected for a forced vortex. Upon supprssion,

however, the forced vortex was also abruptly suppressed; now qui-

escent at the periphery, the air rushed out close to the center-

line."

"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 won-

dered, 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 chal-

lenge 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 ingenu-

ity and the laws of nature, there emerge sounder knowledge and

deeper understanding."

References

[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]

CONST. PROJECTS

Build Your Own Hilsch Vortex Tube

Instructables Hilsch vortex tube

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

A Universe of Atoms, an Atom in the Universe, Ch 1, " The Wirbelrohr's Roar" (google books)

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

http://cdn.instructables.com/FYU/ZA2L/F54HJ3I1/FYUZA2LF54HJ3I1.LARGE.jpg

The hilsch vortex tube, cools and heats air at the SAME time with no moving parts, and NO electricity. cool

huh? it's quite simple, and only a matter of getting the dimensions right! Not to mention the ability to

produce EXTREME temperatures! all that's needed is compressed air!

So, let's demonstrate two simple scientific principles through this tube. That's right, two principles in one!

let's start with temperature. Temperature is an AVERAGE of how fast the particles are bumping into each

other. Because it's an average, it means that air is composed of fast AND slow particles. It's hot and cold at

the same time! (see right of third picture) How will this be proved? Well, I think that if we just separated the

hot and cold, that'd do just fine for proving!

How's it going to work? By utilizing inertia! Which is defined as: "the property of an object to remain at

constant velocity unless acted upon by an outside force." Basically, once you get a boulder rolling, it's not

going to want to stop. And trust me, it'll resist you changing it's velocity! 

There are two things that make up inertia- mass, and velocity. More of either means more inertia. 

Now imagine, there's a wall that curves 90 degrees. A boulder and ping pong ball are rolling towards it.

(see fourth picture for the explanation) now, as you saw, the pingpong ball was just pushed away with

ease. Now, say the pingpong ball was the same size as the boulder (but still very much lighter) and on the

right instead of the left. The boulder would still force its way to the wall. If you had a steady stream of

boulders and giant pingpong balls, and this was a circle instead of just a curve, it wouldn't be long before

there were only boulders rolling along the wall, and all the pingpong balls were all pushed to the inside.

Now, one step further, now because inertia = mass x velocity, say you had lots of molecules of the same

weight going around that circle. But some were moving really fast, and others slow, because the faster

ones are moving much faster (and have more inertia), they'll push the slower ones out of the way. (yes, just

like the boulder and pingpong ball) and before you know it, there's the hot (fast) molecules around the

edge, and slow (cold) in the center! (see left side of third picture) and that's exactly what the inside of the

vortex tube looks like!

to put it very simply all the vortex tube does is get those molecules moving in one direction (rather then the

chaotic right of the third picture) so that the separation will begin!

There ya go, you've learned two scientific principles, and the basic idea of how this machine works.

If you want to know the history and a step by step process, go to step one! If not, and just want to

build, just move to the step after that.

Step 1: History and step by step explination

the history 

Georges J. Ranque, a French physicist noticed temperature differences in vortex separators. He found that

the center would get cool, and the outside quite warm. After some due thought, he wrote some theories up

and moved on. These ideas, as well as maxwell's thoughts on the subject came to Rudolf Hilsch and he

began actually researching and building a refrigerant system to try and beat the standard system for the

German military. After building a few prototypes, and getting a very good hold on the dimensions, he left

the idea alone, as, the conventional system was more efficient, and less noisy. 

Ok, enough history, now how the tube actually works in a step by step process. This is for those science

nerds who really want to know how the air separates in this cool machine. Be warned, this is complicated,

and I tried to explain it as well as I could. If you've got a good enough idea from the scientific principles

above, you may just want to go to the next step. 

-First you have the vortex chamber, this is simply where the air starts to spin. the better this is designed,

the better your tube works. for the best tube, the faster you can get the air to spin the higher the

temperature change. 

-Second the air moves down the long hot tube and the hot air separates outward, and the cold air is

pushed to the center of the vortex. (effects of inertia) 

-Third, the air makes it to the end of the pipe, and, because the ball valve is opened slightly, with a small

opening near the WALL of the hot pipe, it siphons off hot air, but, because the pressure is too great to go

out that single opening, some of the air must rebound and travel through the center of the vortex, and exit

back through the hole in the middle of the vortex chamber. 

Why wouldn't it just go through there in the first place? simply because in the vortex chamber, the air is

moving so fast, it's being smashed on the walls of the pipe and can't "pull" itself to go through it. If the ball

valve is closed, enough pressure builds, and the air just exits there, as, there's nowhere else to go.

Because the ball valve is slightly opened near the wall where the air is being smashed, it tries to exit there

first. If it can't make it out there, it is forced to go back through the center of the vortex and exit at out the

cold tube. 

-fourth as the air goes back through the center of the vortex, the faster molecules push back out towards

the edge of the tube, and the colder are forced to the inside. Because there is too much air to exit out the

hot tube, the air is forced to escape out the cold tube, and your separation is complete. hot air out one end,

cold out the other. 

Now we're done with theory, thank goodness! Back to the hands on build it part! 

Next step: the materials! 

EDIT: due to many people asking what "practical" use this might serve besides teaching a principal, once

tuned to achieve temperatures below freezing, you can use the tube to freeze all sorts of stuff! Scientists

use it for tissue sample freezing, what can you use it for? Just think carefully. Because you're producing a

concentrated blast of cold air, you can freeze things REALLY quickly! Honesly, if you've got a large air

compressor that's just sitting there, this is worth a build. In my experience, uses come after it's built, not

before.

Step 2: Supplies and such

Ok, enough with theory, let's make some hands on reality! One little bit of theory left though...

Possible hot and cold temperatures. If you build this right you get EXTREME differences, no, not a wimpy

90 degrees hot, and 60 degrees cold, we're talking -50 degrees cold and 350 degrees hot! that means you

could burn and freezer burn your hand at the same time!

But, not to disappoint, but realistically expect below freezing, and just above water boiling. which is still a

huge eye opener for friends!

Down to business, here's what you need:

-air compressor (bigger is better!) if you've got a tiny little few gallon pancake compressor, you will NEED to

half all my measurements. These things hog air like no tomorrow! a big stand up shop compressor will be

best, but a laydown 2-5 CF compressor will work.

- 3/4", 5" PVC tubing (steel, copper and such do work... but PVC is easy to work with, and VERY easy to

cut)

- 1/2 inch thick 4"x8" (or larger) piece of acrylic/plastic. Remember, you CAN sandwich more pieces

together to make this piece. 

-1/4" fender washer ( doesn't have to be a fender washer, you just need a 1/4" hole in it) 

- glue (epoxy)

- ball valve that fits the PVC tubing, you'll probably want a non threaded type. 

-plastic/copper 1/4" or 1/8" tube. 

-T fitting (can do without, depending on design (just keep reading before you decide to buy this or not) for

the tubing above

-on/off fitting for the tubing

-connectors to attach to air line.

-4 or so bolts and nuts, must be longer then 1"

*links are to amazon

Step 3: Design the vortex chamber

look below at the schematic I made. You'll notice there are TWO types of vortex chambers, the Archimedes

screw, and the opposing jet (this is what I used, but I plan to test the Archimedes screw soon) 

So, depending on which one you want to build, you may, or may not need the T fitting for the air lines. 

the Archimedes screw does NOT need the T fitting. but needs 1/4" air lines 

the opposing jet design DOES. can use either 1/4 or 1/8 lines 

now, the archimedes screw all that's different about it, is that you need to drill ONE hole instead of two, and

you must design the one spin spiral (does not need to be perfect, just has to be smooth) 

we'll discuss the two opposing jet design. HOWEVER, please note, the archimedes screw must fit into the

3/4" pipe. Also note that the "two piece washer sandwich" below, is what both of them will be. the

indentations are JUST so that the PVC pipes have somewhere to slot into. They're glued in place then

screwed together so the washer is sandwiched between them.

Step 4: Hack and slash!

to start, let's cut our pieces up! 

first, cut the PVC pipe into two sections, one 24" long, the other, 4" long. Save the rest, you may need it if

you screw up. 

second, cut your piece of acrylic to size (4"x8") then cut it into two 4x4 squares. Set one aside for the

moment... 

(please note I used MDF... this is NOT a good choice. I'm regretting it at the moment... just ignore it. it's...

brown MDF acrylic replica for all you're concerned.) 

Step 5: Drill!

Ok, out come the Forstner attachments!** (drill bits are fine, Forstner are best... and really great for all sorts

of stuff...)

take it slow, and in steps, and cut all the way through the MIDDLE of both pieces. you're using 7/8" drill

bits, as, the PVC will fit snugly in the holes you drill. Nothing fancy here, just drill straight through.

Now, pull out the ruler** and sharpie** (for you chaps in the UK, and people in the rest of the world, a

sharpie is a permanent marker... sharpie brand is the best though. Correct me if I'm mistaken, but for some

reason it's mainly a US brand and other countries don't seem to know of them, how sad.) and draw two

tangents to the hole you just cut. opposing tangents are best... (see below) this is where we'll be drilling for

your air lines. (only one for the Archimedes screw type)

** available at Amazon.com!

Step 6: Drill! (part II)

This part's a wee bit tricky... those lines you drew, well, using those as a guideline, we need to drill about

3/4ths the way down the line, but not ALL the way. So, pick the drill bit that allows whatever type of air line

you're using to fit snugly (test on a spare piece of MDF... er... acrylic) and get to it. 

You need to be very careful how you drill this one... if you do it just right, the right side of the drill bit will just

scrape the inside of the circle you cut if you drilled all the way down. So just drill both sides.

Step 7: Drill! (part III)

Measure the outerdiameter of the washer (mine was a little bit under 1 3/8") and pull out the corresponding

spade or mortising attachment. The bit should be just slightly larger then the washer. 

Here's the deal, drill down with the spade bit until the flat part starts to catch (please make sure the thing is

centered first!) once this happens, stop, and pull the spade out. You'll have left a nice circular indention. (if

you used the mortising attachment just drill down enough so that the washer can sit flush, or just above

being flush.) 

we need an indention so the washer will fit flush in there and not interfere with the two halves coming

together in the "washer sandwich". so, move on and I'll show you what to do.

Step 8: Dremel!

Pull out the Dremel* and a flat headed grinding bit* and get to work. Too hard? nope, just be careful,

wear eyeglasses* and go at it. use that groove you made with the spade bit as a depth guideline.

With a steady hand, this won't take long at all!

*available at amazon.com!

Step 9: Pipe'n drill!

http://cdn.instructables.com/FMY/1ER9/F54HJ3I3/FMY1ER9F54HJ3I3.LARGE.jpg

Now, push the pipe in through the hole and make it flush with the indentation you just made. 

Now, it's time to pull out the 1/10" drill bit and drill in that hole again! this will be the actual passage for the

air to go into the hot pipe. be VERY VERY careful! how much you damage the inside of the pipe will

determine how well your vortex tube works. like before the drill bit should almost scrape the inside of the

pipe when it cuts through. Depending on how the previous holes were drilled, you'll have to move your drill

accordingly. Drill both sides and then glue the washer into the groove. 

CAREFUL!!! 

Center the washer FIRST!!! make SURE the hole is centered with the inside of the pipe.

Step 10: Line up and bolt it down!

http://cdn.instructables.com/FXF/6JDW/F54HJ3I9/FXF6JDWF54HJ3I9.LARGE.jpg

http://cdn.instructables.com/FG4/J7IP/F54HJ3I8/FG4J7IPF54HJ3I8.LARGE.jpg

Now, if you're careful, you can pull out the hot pipe and re align the holes later... otherwise, you'll have to

use the corded drill instead of a drill press (if you were using a drill press). 

center up the hole on the second 4x4 acrylic square you set aside and drill four or so holes, and using the

bolt and nuts, attach the two pieces. Re-attach the hot pipe (look down the holes and line them up, hard, I

know, but you'll figure out a way, such as lighting up the tube with a flashlight or something to make the

hole visible) and attach the cold pipe. 

Almost there! 

Now, just attach the ball valve on the end of the hot pipe (if it's snug, you probably could go without gluing

it) 

oh, and rig up the air lines! I'd go into detail... but every compressor and air line and fitting is different, so

you'll have to figure it out... but it's pretty simple, once you're done, slide the lines into place (if very snug,

you don't have to use glue) If you're using copper lines period, glue it. 

You're finished! it should look like the one below (if the dimensions look different, good, this was one of my

many pipes while trying to make the instructable... and certainly not the last! each and every one gets

better! )

Step 11: Running the beast!

Ok, attach it to the air line (nearly close the ball valve) and slowly open and close the valve until the hot side gets VERY hot. Honestly, this is trial and error here (make sure you're running the tank around 100-150 PSI) you'll get a feel for it though, However, you should probably never get the valve more then 1/4 the way open. 

Notice that the more open you get it (to an extent) will make the hot side cooler, but the cold side MUCH colder. when you find that perfect place, and if the tube is right, if you're not careful you'll get some instant frostbite! yikes! Turn it the other way and optimize for heat (more closed) and you may get blistering air out the hot end! However you work it, the pipe should ALWAYS be hot when in use, if it doesn't get hot to the touch, you probably have the valve too open. 

Webmaster's notes:

It has recently been suggested to me that credit for this article should be given to C. L. Stong who wrote most of the "Amateur Scientist" columns for "Scientific American" magazine. I had recieved it as a copy of a copy from a friend of a friend etc... One day a bunch of years ago when this thing called the "World Wide Web" got popular, I decided to make this website. It has become surprisingly popular.

I have never actually built one of these, myself. If anyone actually DOES build one based on this information, I'd certainly love to hear about it.

I do know they are commercially available from several manufacturers. I'm sure your favorite search engine can help you find them.

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.

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.

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].

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].

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.