This one for the Nuts, not for the Volts...

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Externet
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This one for the Nuts, not for the Volts...

Post by Externet »

Hello all...
Two identical air compressors in series, both running at the same rpm. But the second in the flow reversed, as to force expansion of the air compressed by the first one.

¿Will there be pressure in the pipe between them ?

A-------C~~~~~?~~~~~E-------A

A= atmosphere
C= compressor
?= is there pressure here ?
E= expander (reversed compressor)
---- = pipe
~~~~ = radiator or finned pipe

Anyone remembers ROVAC ?

Miguel
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haklesup
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Post by haklesup »

In theory you should be able to obtain 0 pressure (reletive to the input and output of the compressors or differential to the atmosphere but not absolute pressure) but in paracticality you will not.

There certainly will be flow of air (current) in the middle tube and if the impedance of this tube could be made to be Zero, it should be possible. If the impedance is non zero ( a more realistic scenario) this tube will impart resistance to the flow of gas and a resulting pressure gradient will form.

You would also be hard pressed to find a compressor without the equivelent of ripple at its output. I suppose if you did the experiment using bottled gas and the vacuum of space, it might be obtainable.

Measuring that pressure would also be tricky in such a high current, any turbulance intruduced by a probe would create its own pressure due to the additional impedance.
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Post by Dean Huster »

Unless the two compressors have their intake cycles synchronized, there will be a build-up of pressure in the connecting line before the second compressor opens its intake valve.

Even if both compressors were prefectly synchronized, there has to be a pressure differential somewhere or there cannot be flow.


Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Chris Smith
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Post by Chris Smith »

You will always have A pressure in the pipe, because the out side pressure is 14.7 psi, and unless one compressor [the out going] is larger than the other to the point of being "perfect and massive", there will always be air in the pipe.

According to physics, you can never evacuate a chamber to absolute ZERO pressure. In theory, you can come close.

IF one is large enough to drop the pressure, there will be a decrease in pressure, but never a perfect vacuum.

According to physics, that is impossible.
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Post by rshayes »

If the valves in the expander are properly timed, you won't even have to drive the expander. The pipe between C and E will have a positive pressure in it. Furthermore, it will get hotter than the ambient temperature. The gas exhausted from the expander will be cooler than the ambient temperature.

If you close the system by connecting the output of the expander to the input of the compressor with a heat exchanger, you have the device known as a refrigerator.
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Post by Chris Smith »

You would have a [efficient] refrigerator mechanism if..........you use a gas other than air, [Very High pressure needed if Air is the medium] you have at least a POA valve or expansion valve, and a few other minor goodies like a second heat exchanger [the cold one] called the evaporator.
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Post by Externet »

Hi.
rshayes:

..."The pipe between C and E will have a positive pressure"...

OK, good to learn that.

The compressed air from C is being sucked by E in the same amount as they are identical compressors. Please enlighten me how is there pressure if the expander is sucking all the compressor is delivering?

Another part I need to understand is if there is thermal dissipation at such joining pipe and if it truly happens, its effect in the pressure. I would guess it would lower the pressure.

My uneducated view in the subject is that there is no pressure buildup because C discharge is equal to E suction, plus if there is no pressure buildup there is no temperature rise above ambient atmosphere.

If the joining pipe was cooled below ambient temperature, then it would show some vacuum.

ROVAC is ROtary Vane Air Conditioner, a clever design from 1970 stuck in my retina and am trying to search a flaw.
Just imagine "A" at both ends of the flow being an automobile interior. The shafts of both C and E being the same The expander helps driving the compressor; or in other words, recovers a good amount of driving power.

I do understand the limitations of air used as a refrigerant gas.
Thanks,
Miguel :smile:
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Post by haklesup »

Hmmmm. IF the two compressors were loudspeakers and the gas was in a closed loop, you would be somewhat close to a thermoacoustic refrigerator.
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Post by Externet »

Hi haklesup.

You may consider the interior of an automobile (or a room) as closing the loop.

Cold air from E outlet discharges into the car, and returns to C inlet warmed up. As the refrigerant gas is air, does not need confinement of a heat exchanging evaporator.

The design is spectacularly simple, am obsessed with it, and lately been trying to find what may be flawed. What cannot understand is the pressure in the joining pipe. Or the report hides some data. :sad:

Miguel
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Post by ecerfoglio »

To work as a refrigerator, the compressor's output should be hot, and at a higher pressure than it´s input.

In "ideal" conditions, you should start the compressor before you start the expander so the pressure has time to build up.

With real machines (air leaks, friction, etc) the compressor should be a little bigger than the expander (or spin faster) in order to mantain the pressure.

It´s an interestig concept for an air conditioner, but it will have to move large volumes of air and i´m afraid that it will make lots of noise - it will be hard to lower it to acceptable levels.
E. Cerfoglio
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Post by dyarker »

Air is a poor refrigerant because there is no state change (gas to liquid, then liquid to gas).

Cheers,
Dale Y
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Chris Smith
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Post by Chris Smith »

You will always have a pressure in there because you started out at 14.7 psi.

No increase or decrease in pressure,.... can be maintained with identical compressors, but you still started of with 14.7 and nothing changed but the flow.
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Post by Will »

There has been more BS on this thread than I've seen on any other ! Of course it still has entertainment value !
Hasn't anyone ever seen a multiple compressor air distribution system ? The simply connect up two or more compressors exactly as described here - except that they have a common outlet at the discharge junction.
Without the (header) outlet the two compressors would simply lift their relief valves continuously and if the were'nt so fitted then they would either blow up or trip the breakers of the electric driver motors.
There is no way the compressed air from the discharge of a compressor could get through the suction valve of another compressor. If it could then the air from a single one cylinder compressor could get back into its' own suction valve/channel The suction valves are configured to withstand the back pressure of the compression stroke, except when the piston is on the (suction) downstroke, at which time the discharge valve is always closed.
There is also no way you can compress air then release it back to its' originbal condition and produce cooling or refrigeration. It has to be cooled before depressuring - otherwise it just returns to its' original (Atmospheric) condition.
BB
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Post by Chris Smith »

Any simple length of pipe after the heat build up will allow some cooling or expelling of the build up heat due to compression to escape the closed circuit, and any increase in pressure [heat build up] at one point followed by a decrease in pressure [heat absorption] at another point will act as a heat exchanger or mini refrigerator.

So a refrigerator IS formed in this circuit.

How efficient it is only up to the length of pipe, the gas medium, how much pressure and heat transference there are, how much loss of heat you can radiate away, and how efficiently you can re-absorb it back into the medium, and not much else. Air Vortex Chillers are used every day in the industry to reach temperatures down to minus fifty.

http://www.exair.com/vortextube/vt_page ... vortextube

Using another gas such as Freon or ammonia merely make all of the above into a commercial refrigerator, one that you can sell and use practically for larger work areas.

And to prove it you need only check for hot spots and cold spots around the loop to know you are expelling heat at one point, and attracting heat at other places.

Add in Two heat exchangers, a controlled expansion valve and make it even better.

But at no point in this circuit is there a perfect vacuum, as there will always be some pressure with differentials at best.
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Post by Externet »

What the genius Will calls BS in this thread is called "Air Cycle Refrigeration", over 100 years old. He does not know a thing about it, and chooses to trash the discussion.

http://www.wisland.net/prod_airs.html

For entertainment value, he visits the wrong forum. And comes up assumming valves and pistons and other stories.

Miguel
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