Stirlingengineforum.com

I don't think there are any dedicated discussions here about the "Air-Cycle" in the forum, though I make mention of it from time to time.

The phrase, "air cycle" as of now is not searchable (common words filter) and I don't find much using Google Site-Search other than my own occasional posts mentioning the subject.

This is a rather obscure topic, but I think, important as it relates to Hot Air Engines.

A hot air engine is, literally an "air cycle system" and more and more I've come to realize that air cycle refrigeration has more in common with hot air engines than might be apparent on the surface.

Both are turning heat into work and as a consequence producing both work output and refrigeration.

The end goal of an air cycle refrigeration system is cold output, so heat and work as produces are discarded, neglected or recycled.

The end goal of a hot air engine is work output so the opposite is true. Emphasis is on maximizing work output. Heat and Cold as end products are discarded, neglected or recycled.

Finding information on air-cycle refrigeration has been a challenge, as it is not a well known system and information is scarce, I have though just recently come across a couple video tutorials on the subject worth sharing and may find more.

This video provides a fairly decent rundown, though it is of a commercial/advertisement nature, it is nevertheless educational as well.

https://youtu.be/X2zkNjDL4K0

Note that in this refrigeration system, as in most all refrigeration systems, heat is being discarded in multiple ways, only one of which involves conversion of heat into work, on the expansion side of the turbo-expander (or combined "bootstrap" compressor/expander turbines.


This NEXT video is only a teaser for an online course in refrigeration, but gives a pretty good and thorough description of the "simple air cycle system" commonly used in aircraft for cabin cooling.

The system, for the most part relies on Ram air as a primary driving force to run the system while in flight.

That is, the plane is flying through the Air anyway. The "natural" pressure differential created in flight is therefore utilized to drive much of the system.

https://youtu.be/mEticK2yunI

IMO this course looks good and is reasonable considering all that it covers, which is everything mentioned in this teaser and a lot more, so I may just sign up for the whole course myself.

Note that generally speaking the majority of these air-cycle refrigeration systems are OPEN cycle. In aircraft, even much of the cold air is wasted as the temperatures produced in an air cycle are much too cold for practical air conditioning and refrigeration which can be confusing and also results in inefficiencies. The cryogenically cold air often needs to be mixed with warm air or otherwise re-heated for use in a controlled refrigeration environment or for air conditioning, as the passengers in an airplane don't need to be cryogenically frozen, and bananas and other food hardly ever needs to be used as a hammers.

This air cycle refrigeration system has really caught my attention lately. To me it begs a few questions;

What does it mean for hot air to do work on something.

What is the importance of what that "something" is?


I think we are losing energy with piston based hot air systems. I can see pistons doing work onto air (relatively)efficiently, but I don't think the reverse holds true. It seems that a piston really only cares about max psi within a cylinder, and there is no real difference whether the pressurized air is hot or cold. Furthermore, as air is driving a piston toward BDC, most likely only a small percentage of air particles are impacting the piston as the air is pushing between cylinder head and piston. Beyond that, the piston is so heavy that the only transfer of heat energy it gets from the air is an increase in temperature. So maybe all a piston can do is expand a volume, and not use heat energy at all?

A turbine(for example), is light weight enough that the hot air may impart some direct energy to it, and all of the air is forced to travel through the blades which maximizes physical contact with "something" that is directly responsible for power production.

I've been looking into the physics of whistling lately, of course it's far more complicated than you would ever think.

But maybe whistling(or noise generation), is "something" that can be used to extract heat energy directly from the gas and subsequently lower its temperature with minimal losses. The correlation to the thermoacoustic rice engine does not elude me here.

The effect of noise generation is a direct molecule to molecule transaction, and as such is likely the most effective way to remove heat energy from air. Maybe?

The Mayekawa system is especially interesting as it's essentially ingesting its own exhaust. I'd love to know its total system efficiency.

VincentG wrote: ↑Sun Aug 13, 2023 12:39 pm This air cycle refrigeration system has really caught my attention lately. To me it begs a few questions;

What does it mean for hot air to do work on something.

What is the importance of what that "something" is?


I think we are losing energy with piston based hot air systems. I can see pistons doing work onto air (relatively)efficiently, but I don't think the reverse holds true. It seems that a piston really only cares about max psi within a cylinder, and there is no real difference whether the pressurized air is hot or cold. Furthermore, as air is driving a piston toward BDC, most likely only a small percentage of air particles are impacting the piston as the air is pushing between cylinder head and piston. Beyond that, the piston is so heavy that the only transfer of heat energy it gets from the air is an increase in temperature. So maybe all a piston can do is expand a volume, and not use heat energy at all?

A turbine(for example), is light weight enough that the hot air may impart some direct energy to it, and all of the air is forced to travel through the blades which maximizes physical contact with "something" that is directly responsible for power production.

I've been looking into the physics of whistling lately, of course it's far more complicated than you would ever think.

But maybe whistling(or noise generation), is "something" that can be used to extract heat energy directly from the gas and subsequently lower its temperature with minimal losses. The correlation to the thermoacoustic rice engine does not elude me here.

The effect of noise generation is a direct molecule to molecule transaction, and as such is likely the most effective way to remove heat energy from air. Maybe?

The Mayekawa system is especially interesting as it's essentially ingesting its own exhaust. I'd love to know its total system efficiency.

I think in terms of kinetic energy or maybe "vibration". "Heat" is a human subjective experience and is also relative. So pistons don't use heat. But "pressure", theoretically is also kinetic energy, but I think of a different kind.

A gas particle can "hit' a piston, or it could "press on" a piston.

The first is due to movement of the particle through space relative to the piston, the second, I'm speculating, is due to the gas molecules being too close together and repelling each other for a given temperature.

A cold gas is also dense, so a lot of molecules are packed tight together almost like a solid and not very compressible.

If both heat and pressure are "vibration" then a hot gas is a few molecules vibrating rapidly and cold gas is a whole lot of molecules vibrating just a little bit because of the greater numbers and greater density can still pack a wallop to the piston.

If hot gas actually transfers heat to a cold metal piston that does not contribute to work output. The "vibration" is just absorbed into the metal and makes it a bit warmer.

Of course, I have no real idea what's "really" going on. Quantum "probability waves" deciding which way to manifest, forward and backward in time or some such thing.

I looked up Mayekawa. Seems they make compressors for ice skating rink refrigeration systems. Not sure what you mean by "ingesting its own exhaust". Do you have a reference to this? I haven't been able to find anything that would help to decipher that comment.

Not sure what you mean by "ingesting its own exhaust". Do you have a reference to this? I haven't been able to find anything that would help to decipher that comment.

Sure if you watch the video it shows the cooled air from the cold box being used to chill the air(via heat exchanger on the right) before it goes through the expander. So the system is self cooling to an extent.

I've only seen this before on new automobile ac systems that now use r1234yf, the replacement for r134a.

VincentG wrote: ↑Mon Aug 14, 2023 9:52 am

Not sure what you mean by "ingesting its own exhaust". Do you have a reference to this? I haven't been able to find anything that would help to decipher that comment.

Sure if you watch the video...

Which video?

I've found several channels of videos about Mayekawa compressors, both reciprocating and screw type, but haven't found a video showing what you describe.

Could you post a link?

It's shown in the first video above. They state that the cooled air from the box is circulated through an exchanger to reduce the temperature of air entering the expander, before it is again circulated into the cold box.

VincentG wrote: ↑Tue Aug 15, 2023 4:39 pm It's shown in the first video above. They state that the cooled air from the box is circulated through an exchanger to reduce the temperature of air entering the expander, before it is again circulated into the cold box.

Oh right, silly me. I posted that video.

But yes, the trick to reaching cryogenic temperatures with an expansion engine is to compress the gas/air and cool the compressed air before it is used to power the engine (or turbine). That pre-cooling before expansion can be accomplished in a variety of different ways.

Using the already expanded cold air to cool the incoming air to be expanded is a common method, going all the way back to Linde (that Tesla wrote about in connection with his ambient heat "cold hole" engine).

Linde however only used a simple needle valve through which he allowed the gas to expand.
Any cold air that did not liquify after expansion through the valve was used to cool the incoming compressed air just before it expanded. So even though the air only cooled a degree or two each pass, it eventually cooled enough to liquify

An expansion engine or turbine is much more effective and can cool the air rather dramatically in one pass, but the same old "self cooling" method is still used.

The best thing to do, really is to start by compressing the gas/air into pipes that are cooled by cold water from a natural stream or river or lake, or the ocean.The "free refrigeration" cools the compressed gas AS IT IS BEING COMPRESSED which causes it to cool and CONTRACT so the compression is relatively effortless.

That is why Tripler could make 10 gallons of liquid air, then boil just 3 gallons liquid air in the boiler of his steam engine to make 10 more gallons.

Of course, the 2nd law fanatics had to shut him down. That looked too much like perpetual motion!

Tripler actually had patent priority before Linde but Tripler's patent seems to have been lost somewhere after being declared invalid.

The author of this reference considers Tripler to have been a fraud who secretly kept a store of liquid air to deceive reporters and other visitors who came to see his operation. It makes some interesting reading anyway.

http://www.douglas-self.com/MUSEUM/POWE ... uidair.htm

The number of people who came out to attack and denounce Tripler and utterly ruin him is rather astonishing.

Tripler was simply cooling his compressed air with water from a nearby river which made the air remarkably easy to compress and liquify.

It is more scientifically explained in more detail in this book:

https://onlinebooks.library.upenn.edu/w ... a100581832

Follow the links for downloading the book.

Now IMO if you can make liquid air in unlimited quantities using the cooling power of cold water from an ordinary river, it could also be done using evaporative cooling or even air cooling with a radiator. Not as fast or as efficiently as with the virtually unlimited supply of cold water from a river but I'm pretty sure it could be done.

So the big secret??

Compress the air directly into the condenser that is circulating the cold water.

Probably anybody could make their own liquid air "fuel" pretty inexpensively. At least in the winter.

Sorry that previous link does not have a download page:


https://books.google.com/books?vid=UCAL:$B28010

It should be noted that Tesla wrote about liquid air:

By the use of such machinery as I am perfecting, its cost will probably be greatly lessened

This was in the passage relating to his ambient heat engine where he also said regarding liquid air:

for motive-power purposes its cost is still by far too high. It is of interest to note, however, that in driving an engine by liquid air a certain amount of energy may be gained from the engine, or, stated otherwise, from the ambient medium which keeps the engine warm, each two hundred pounds of iron-casting of the latter contributing energy at the rate of about one effective horse-power during one hour. But this gain of the consumer is offset by an equal loss of the producer.

Much of this task on which I have labored so long remains to be done. A number of mechanical details are still to be perfected and some difficulties of a different nature to be mastered, and I cannot hope to produce a self-acting machine deriving energy from the ambient medium for a long time yet, even if all my expectations should materialize.

Tesla seems to have been unaware of Tripler's success in inexpensively producing large quantities of liquid air.

He also wrote:

at the close of 1894 I had completed these two elements of the combination, and thus produced an apparatus for compressing air, virtually to any desired pressure, incomparably simpler, smaller, and more efficient than the ordinary. I was just beginning work on the third element, which together with the first two would give a refrigerating machine of exceptional efficiency and simplicity, when a misfortune befell me in the burning of my laboratory, which crippled my labors and delayed me.

Shortly afterward Dr. Carl Linde announced the liquefaction of air by a self-cooling process, demonstrating that it was practicable to proceed with the cooling until liquefaction of the air took place. This was the only experimental proof which I was still wanting that energy was obtainable from the medium in the manner contemplated by me.

So it seems liquid air would play some role in Tesla's "Self Acting Engine" but seems to have considered liquid air itself to be of little commercial value.

Considering the simplicity of the Linde/Tripler air liquefaction method with essentially just one moving part, a piston in a cylinder or compressor piston, some liquid air would likely serve as a "cold hole" for an ambient heat engine.

Apparently, I suppose, he had no intention of expanding liquid air to drive an engine. Instead, I imagine, he would use the liquid air as a kind of cold battery for his ambient heat engine

Likely, IMO, at this point, his quest for the ultimate "cold hole" was misguided.. Heat engines don't actually seem to have much use for any extreme cold.

Here is a video that provides a pretty good look at one air-cycle type air conditioning system used on airplanes.


https://youtu.be/i84748xKNqw


It is good, I think, to compare this with Catl Linde's simple air liquefaction setup. Both follow the same basic simple cycle of 1) compression 2) pre-cooling and 3) expansion

For #1, in Linde's system a reciprocating compressor is used. In the aircraft, a turbo-compressor.

#2 Pre-cooling in Linde's air liquefaction machine takes place in the water cooled "condenser". In the aircraft, Ram air is used for cooling the compressed air. Ram air is just the natural stream of air that the plane happens to be flying through picked up by a scoop.

#3 expansion in the Linde system is through an "expansion valve", don't let the name fool you, there is nothing particularly special about this valve, it is simply a restriction that holds back the air enough so that it can be compressed while letting some air escape at high velocity. In the aircraft ACM this function is taken over by the expansion turbine which is more effective at cooling because, not only is the compressed pre-cooled air expanded but it is also made to do some work in driving the turbine.