Figuring out the Temperature of an Expanding Gas?

The cooling effect of a refrigeration system is usually attained by condensing a gas into a fluid, removing the heat at this stage, then evaporating the fluid into a gas, moving air or another fluid through some sort of heat exchanger thereby absorbing heat from the air or fluid to be cooled in the gas stage, then condensing the gas back into a fluid and removing the heat to start the cycle again...Air makes a poor refrigerant....Unless you have an indigenous dry air supply...If your air supply is extremely low in humidity you can construct an evaporative cooling system whereby you blow your dry air through a damp exchanger...The evaporation of the water has a cooling effect...

I agree with SolarEagle, you're better off to use evaporation not compression.

A typical house where I live has an AC unit that can (conceivably) make 4 tons of ice in 24 hours.

You're never gonna get that with a wind mill.

No doubt there are more effective ways to refrigerate, I'm just looking at something which can be made anywhere pretty much free. If I start involving phase changes and various chemicals then things get a lot more demanding and out of the reach of most people in the world.

I'm just looking to get a smallish insulated box down to a temp that things won't spoil, which is all that's really needed.

Air is everywhere and compressing it is easy.

As others have stated, the cooling comes from the state change from liquid to gas. If you monitor the temperature of water as it comes to a boil you will see the temp increase then it will boil and the temp will level off, you have to keep heating the water to keep it boiling but it doesn't get any hotter. This is because the heat energy is going into the phase change from liquid to gas. Refrigerators use fluids that absorb energy to boil at a lower temperature and thus remove heat from the insulated box of a fridge.

In your compressed air model, the compressed air would cool as it expands; but compressing the air will cause it to heat. You would have to compress the air, then allow the heat to dissipate then you would get some cooling effect as the air expands back to ambient air pressure. I would have to do some math to tell you how much a specific amount of air heats during compression, then how long it would take to cool in it's tank then how much it would cool as it expands again.

If you had a energy source to compress the air, you would probably find a more efficient system in using it to compress a refrigerant liquid in an ordinary air conditioner system.

Drew K

You must live in a very big house!

Nope. I just live in a very hot climate.

Perhaps I did. Maybe I should have said that a 4 ton AC unit can remove the amount of heat from air that is required to melt 4 tons of ice in 24 hours.

Maybe I should just sit on my hands.

"Maybe I should just sit on my hands."

No need to get all huffy and sensitive on me, I was just jerkin' yer chain....I know what ya meant...

I don't see that those clay pots do much without good airflow, and at a liter of water a day that's a negligible amount of cooling, ya might as well just bury it in the ground...now if you drip feed the water around a single clay pot, and had a forced air flow to evaporate the water a decent rate, and shaded the whole thing, that would be better....On the other hand if you could supply a good high heat source, you could make an absorption system and get some proper cooling....Here's a Wiki link...

http://en.wikipedia.org/wiki/Absorption_refrigerator

Here's a cool video by a guy named Adam Grosser that built a self contained absorption cooler powered by a campfire....claims to cool 3 gallons of water to 35 degrees f in a 30 degree centigrade environment for 24 hrs...at a cost of $25.

http://www.youtube.com/watch?v=HSdXqmnNCp0

"No need to get all huffy and sensitive"

Huffy, maybe; sensitive, never.

The clay pot guy says he can cool to 77°F when it's 115°F outside. That's pretty impressive considering no moving parts.

A four-ton air conditioner is toward the larger end for residential, but not at all uncommon; equivalent to 48,000 Btu/h.

OK, you increase ambient temp air at 70F to a pressure of 100 psi. Incompressing the air, it heats up to 110F +/-. You let the tank of 100 psi air cool down to ambient of 75F. Then you release the air through a valve to ambient pressure again. The air cools to about 55F. (Based on subjective experience with my air compressor and feel, not measured temperatures)

My 1/2 HP compressor takes 5 minutes to pump up a 10 gallon take. It takes about 1 minute to bleed out the air to ambient. It is not a very effective or efficient way to make things cold. Stick with the phase change systems that are currently marketed, they are much more effective.

If you are thinking of compressing air, you might also consider compressing a refrigerant. I don't know what is available these days in small externally driven compressors, which could be powered by windmill, water wheel, etc.

A concentrating type of solar collector could also power an absorption refrigeration cycle. I haven't checked it out much, but I think there is quite a bit of DIY out there.

Like I say, this isn't meant to be the most effective way of cooling, just the simplest to construct.

Think of slums and villages. That's the level of tech I'm aiming for. Even if people could safely scavenge some refrigerant, it would be significantly more complex to build the phase change apparatus than what I'm proposing.

I fed the ideal gas law into Wolfram Alpha, but there are two unknowns; temperature and molar mass. I assumed that at 4 atm there would be 4 times the moles, but this gave the same temperature as the original molar mass at 1 atm, so I guess pressure is a little more complex than that.

My volume is 2.5 litres, temperature 18 C. At 1 atm this gives .1047 mol.

How do I calculate for 4 atm?

Regarding evaporative coolers, they do work, but can go through a bit of water, which isn't always desirable, and don't work nearly as well in humid conditions.

Well one of these gizmos extracts water from the air...

"the AirDrop is capable of "pulling" out of virtually thin air in order to make irrigation a possibility no matter how dry it is out there."

http://www.ubergizmo.com/2011/11/airdrop-irrigation/

And if the humidity is high you can add a dehumidifying phase to the absorption system...

"

Water spray absorption refrigeration

Water Spray Absorption Refrigeration

Another variant, depicted to the right, uses air, water, and a salt water solution. The intake of warm, moist air is passed through a sprayed solution of salt water. The spray lowers the humidity but does not significantly change the temperature. The less humid, warm air is then passed through an evaporative cooler, consisting of a spray of fresh water, which cools and re-humidifies the air. Humidity is removed from the cooled air with another spray of salt solution, providing the outlet of cool, dry air.

The salt solution is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated from the salt solution is re-condensed, and rerouted back to the evaporative cooler."

http://en.wikipedia.org/wiki/Absorption_refrigerator

I had a look at evaporative cooling, bit of a build, but seems nice tech.

The Air Drop is quite similar to another cooling system I prototyped in Australia:

http://cr4.globalspec.com/thread/67521

And I was thinking actually that this pressure system would extract humidity. There's a fair bit of condensate in the air that I vent out of the bottles, and if that's going into a cooled insulated fridge space then it stands to reason the water would liquefy.

So this could also serve as a dehumidifier / water generator, which is nice.

Airplane cabin air comes from a 4 bar feed, and when the pressure is reduced to 1 bar there is a condensate problem, for which the industry is searching for solutions:

You might find this helpful: http://en.wikipedia.org/wiki/Joule-Thomson_effect

You say: "Energy dissipates through the bottle wall ..." so the compressed gas is losing enthalpy only by heat exchange with the environment, which is slow.

Note that the possibility exists of the compressed gas expanding and doing work (losing enthalpy), maybe turning a turbine. Tired air is cool. Maybe you can think farther than just a bare throttling process, which is the Joule-Thomson valve. It's a good problem, and your intention is good too, so I hope the assembled experts here can contribute something positive. Here is an application of the principle in the field of exhaust steam condensing: http://www.freepatentsonline.com/7987677.pdf

> the compressed gas is losing enthalpy only by heat exchange with the environment, which is slow.

Relatively, yes. But building up the pressure will likely take a little bit anyway, if it's using some kind of salvaged air pump or similar as the compressor. And the bottle can be buried or otherwise cooled. Thin plastic doesn't insulate well.

Also, as it's only for cooling a smallish, well insulated box opened a couple times a day, it's not so much speed as minimum temperature which is the main issue.

> Note that the possibility exists of the compressed gas expanding and doing work

Interesting. It wouldn't make a lot of sense in terms of getting work done; using a wind turbine to drive an air pump, to pressurise air, to drive a turbine... but if it knocked some degrees off the air temp then it would be worth looking into.

What would count as work tho? I was thinking of passing the air output through some open cell foam or similar to diffuse the noise, which is pretty loud when the thing vents.

The original idea (yesterday morning) was to have, as you say, a throttle in the form of a pinhole, and make sure the compressor has a lower maximum pressure than the bottle's, but am now leaning towards a valve which trips at a certain pressure and lets go all the air at once. It seems colder that way, tho I'm not sure why that would be. Maybe just because it's more air...

I understand that you are trying to come up with a DIY solution that resource-poor populations can make from plans or very cheap kits. How about a compressed air reservoir under water?.. Quicker cooling and can generate lower temperatures than the water itself. However the chamber must be in a well or river... which can be hours away from the village. Burying the chamber in earth would also get some degree of cooling and, with additional heat sinking from small amounts of water in the interface between the reservoir and the ground, the temperature could be lowered more quickly. Most of the water would persist as a heat sink, but what did eveporate would aid cooling. A plastic sheet, clay, or other waterproof lining may be necessary in highly porous soils to keep heatsink from draining away.

I was actually starting to think the same thing.

Earlier this year I prototyped a ground cooling system:

http://urbangreenhouse.blogspot.com/2011_03_01_archive.html

Which basically consisted of just pumping air through a buried tank of water. Really quite surprisingly effective.

So it occurs to me that the addition of two valves would combine these two ideas quite nicely.

So what happens is:

~ Air is pumped through the compressor into the tank.

~ The air bubbles directly through the water, bringing it down to the ground temp, probably about 10-16 C / 50-60 F.

~ Pressure builds, air warms but warmth is absorbed by the water and ground around the tank.

~ At a certain pressure the second valve opens, either fully releasing all the air, or steadily.

~ Air rushes into the insulated refrigerator box, decompressing and dropping in temperature. The warmer air at the top of the fridge is forced out.

The whole thing, including the turbine, could be made from scrap materials for free, and would only require wind energy to function.

Don't know how much the cooling effect would be yet, that's what I was hoping you guys could help out on.

And it would also double as a dehumidifier/water generator.

In the lee of the wind turbine would be a place for evaporative cooling for heat rejection from the water, if you could concentrate the flow.

Seems like an adaptation of the solar chimney. A professor friend of mine from the middle east said there is something similar to this that is common there.

Drew K

> In the lee of the wind turbine would be a place for evaporative cooling for heat

> rejection from the water, if you could concentrate the flow.

Interesting idea, but I was looking at the heat being taken out of the water by the ground. The less evaporation, even tho it would be a more effective cooling mechanism, the less you have to keep the system topped up.

I was a little concerned that the ground would get saturated with heat, but the link above has the results of running the thing for 8 hours solid, with no increase at all in the water temp.

> Seems like an adaptation of the solar chimney.

Effectively yes, but using a relatively small amount of water rather than a large amount of earth. With water the air loses all its extra heat pretty much immediately, and the water has more thermal mass than soil. Also the bubbling action circulates the water and disperses the energy more evenly, so you don't get such a problem with saturation.

Ultimately the energy ends up in the soil, as per more standard ground cooling methods, but like I say, I ran this thing 8 hours solid and it never budged by more than half a degree above the ground temperature. And this was with the air going into the tank at up to 55 C, for reasons explained in the blog post.

Really?

No one knows how to calculate this?

I'm not going to have to go to physicsforums.com am I?

You were told how in post#8

There are two unknowns; the new temperature and the new volume. I need one to calculate the other.

I guess there's a crafty way to get the numbers I need from the numbers I have, I'm just not yet sure how to do it...

"Crafty way" is in AH's link too.

Find the table this is the relevant part of;

& read the table footnotes

Ok, so:

T2 = T1(P2/P1)(1 − 1/y)

where T1=20 P2=1 P1=5 y=1.4(for air)

Which gives T2 = 1.14286 degrees C.

Is that correct? Keeping in mind that I have absolutely no idea what I'm doing...

EDIT

I'm guessing I've got something horribly wrong, because

V2 = V1(P2/P1)(−1/y) where V1=2.5 P2=1 P1=5 y=1.4

gives V2 = -0.357143

Or, more correctly, using 293 for T1, as it's Kelvin

gives 16.74, which makes no sense whatsoever, so I'm clearly doing this wrong.

Doing it in the proscribed units is an advantage.

Without checking; a drop of 16 C would be in the ball park from 5-6 bar.

As many remarked, it's not 'that useful' without a change of state involved. And for air at ambient, I think 'liquid' is up around 3.5kpsi.

Not out of the question windmill compressor wise - but a little more of a challenge to make out of 'junk'.

Or why people use CO₂, ammonia and even LPG as 'alternate refrigerants'.

Is T2 the delta temperature? I thought it was the absolute, in which case the gas would be 16 Kelvin. AKA clearly not the right number.

I want to avoid refrigerants, they're tricky to source and dangerous to handle. Also most of the ease of this build is in that you're just blasting air through the whole thing, rather than having to establish a closed loop.

So got some actual experimental data.

Quick and nasty video of the quick and nasty setup here:

https://www.youtube.com/watch?v=yyJ1UQEJwTg

Results:

Pressure: 5 bar / 72 psi (pump it up, wet and let it cool a minute, blast it out)

Ambient temp: 19 C / 66 F

Minimum temp: 10.5 C / 51 F

Time to achieve that: didn't check, about ten minutes I guess.

==

Pressure: 7 bar / 100 psi (as above)

Ambient: 16 C / 61 F

Min temp: 5.5 C / 42 F

Time: about the same.

===

Pressure: 6-7 bar / 87 - 100 psi (continuous throttled flow)

Ambient: 16 C / 61 F

Min temp: 11 C / 52 F

T ~ same.

So, considering your average fridge is set about 4-5 C, and my setup was a long way off optimised, and that if ground temp is used to cool the air which could easily be as low as 10 C; then these results aren't actually too bad.

7 bar is quite a bit tho, so it remains to be seen if the required pressures are achievable with the available materials. I will likely build this and find out in Spain in January.

But if possible I would still very much like to get a handle on the maths of this, so I can see where I should be aiming.

Also worth mentioning is that the amount of water dropping out of the air while I was doing this was significant. After opening the tap and blasting the bottle the escaping air had the look of steam from a rapidly boiling kettle, and there was a fair bit of water left in there after only a dozen or so cycles. A nice cool metal surface would've taken out a lot more.

So this thing would probably also serve well as a dehumidifier / water generator, which in a lot of parts of the world might be at least as handy as a fridge.

5 C is ideal, but anything below 10 is a big step forward, particularly if the ambient is above 30 - as it is in large chunks of the first world.

Rather than sweating over the T2 math, have a look at what sort of mass-flow you need.

E.g. set up a junk fridge cabinet and see what air volume it takes to keep X kg of stuff (like water) at what temp against what ambient, using what Watts.

From this you can reverse engineer both storage capacity (say LPG cylinders, max ~ 250 psi ~1700 kPa) and windmill scale.

I.e you want an overview of 'full system' viability, relative scale and wot-not, before you get 'caught up' in one part, and spending money.

You might check the specs on vortex tube kits as some specs claim -50F temps running on compressed air only and up to 10,000 btus per hr at different cubic ft per minute but I'm sure you would want to blow that -50f air through a heat exchanger and then vent to atmosphere on a refrigerator to keep the storage area sealed tight and I guess since one end of the vortex also discharges up to 260f you could build a hot storage too and I guess you could reverse the vortex for so called hot gas defrost but I'm afraid shifting over to a standby electric or gasoline air compressor during low wind might not be economical compared to an electric household refrigerator but it might prove to be quite a setup if your compressed air is supplied by mother nature But matching btu's with available wind horse power and compressor cfms will be difficult

Be sure and use chest type container to prevent the cold air from falling out of area when opening the access door and eutectic cold plates might help with carry over

This is one way I thought might be a way to get refrigeration via an air compressor on a windmill and avoid the expense of building and electric generating windmill and there are many pond aerators with low pressure diaphragm air compressors running in the US as a windmill So it can be done but that is low psi and low cfm (google "pond windmill aerators")

One mfg of many vortex tube mfgs www.abbeon.com

Good luck as the world needs non-electric refrigeration before we have to revert back to cutting lake ice and mixing with salt if the chemical industry can't come up with a non-toxic or non-explosive refrigerant that is within financial reach of the common working man

Prof.Victor Papanek described this in his book Design for the real world:

Quote:

At Cal Arts one of my graduate students, Jim Hennessey, and I were more concerned
as to how Third World people could keep part of their perishables fresh for a week
or two, starting now. We developed a hand-cranked, modular produce cooler.
There is a baseboard unit that includes a tyre pump, a heat exchanger, a pump, pump
valves, and a metering valve, as well as a hand crank. This is surmounted by a 50 cm.
styrofoam cube with a lid. It forces hot pressurised air through a heat exchange which
returns the air to near-ambient temperature. The air is then metered back into the
cooler, where, as it expands, it produces a temperature drop. Other modular cubes
can be added. Certainly this is no way to keep two bottles of milk, some Coke, and a
roast of beef freezing cold. But twenty minutes of cranking will ensure that, say, a
bushel of mangoes will be kept cool enough not to rot (40° F.) for twelve hours.
More importantly, the units can be built in villages in the Third World with existing
tools and used valves. Since this problem was solved, we have begun doing re- search
into substituting a sandwich panel (made of two outer layers of used newspapers and
a core of dried native leaves) for the styrofoam. The design will be given to

UNESCO.

Endquote

http://www.mediafire.com/?r6dn05ariz7vnhe

> Rather than sweating over the T2 math

Just for the sake of seeing what's possible, tho I guess now with experimental data I've got an idea of that. But be good to know the optimal limits.

> set up a junk fridge cabinet and see what air volume it takes

I'll likely be building this thing in Spain in January. It also involves the bike parts wind turbine and the ground cooling system, which are two other bits of tech I want to flesh out. And as we're going to be at least mostly off grid, it might actually be our best option for refrigeration...

> and spending money.

Whatever happens, I won't be spending any money.

> You might check the specs on vortex tube

Yeah was just reading up on those now, and they do look interesting. I think tho slightly demanding on materials, I'm kind of aiming lower than that style of plumbing fixtures etc. Will keep it in mind tho.

> shifting over to a standby electric or gasoline air compressor during low wind might not be economical

And I guess the vortex tube wouldn't like stopping and starting all the time. Whereas an airtank shouldn't mind at all.

> Be sure and use chest type container to prevent the cold air from falling out

Good call.

> but that is low psi and low cfm

Flow rate I think shouldn't be too big a deal, but looks like I'm definitely going to be wanting something high pressure.

> bioramani

Well there you go, exact same principle. 40 F / 4 C is a good result, do they publish their design anywhere? I'll have a look.

I do not know how much power is required, but in the simplest terms you use a rotary vane compressor to compress the air that passes through a condenser (heat exchanger) that returns to the expander side of the rotary vane compressor to the cold outlet. See http://en.wikipedia.org/wiki/Air_cycle_machine.

Ok, interesting.

It seems this is largely analogous to my system, minus the turbine/fan, and extracting the humidity and adding it to the cooling airflow, which in the design is replaced by water anyway.

So the main addition would be the turbine, as Wilmot suggested.

If I did chuck a fan or similar over the outlet, it wouldn't have to be actually doing anything productive, it could just be working against friction or inertia, no? Long as it wasn't anything that added heat back into the air.

Been asking around a bit on this, and seems the difficulty in getting a solution comes down to it being a free expansion of the gas. The first law of thermodynamics has been mentioned, I might just make the thing in situ and get my numbers that way.

So next issue is, speaking of the first law of thermodynamics; work. Seems I can cool the air a reasonable amount by having it doing something on the way out. Spinning a turbine would be one way, but I don't really need that turbine to be doing anything, and would be another thing to make, so is there something super easy can be used instead?

What about just a handful of sand and small stones in the vertical tube after the release valve, so that they get kicked up against gravity? Or having the air diffused through an open cell foam?

etc?

In my experience with using CO2 to freeze down various other vapors/droplets; a small bore jet, or atomizing nozzle, is the best 'expansion' solution.

The valve can then be somewhat remote and just an on/off function, as back-pressure is provided by the jet's resistance, everything up to the jet is at system pressure, so at ambient.

If the valve is controlling expansion and remote and straight pipe is used to convey the gas, [per your diagram] then the pipe is cooled; meaning 'cold' is lost between the valve and point of application.

This may seem all a bit at odds with the 'theory' but the jet/throttle does the 'work'.

_{(then read the whole page)}

The initial idea was to have a constant pinhole outlet, rather than a sudden release at a certain pressure, but I'm tending away from this for two reasons.

One, being wind powered the pressure coming out of the compressor is going to fluctuate like mad, which with a sudden release valve is fine as it'll just build unsteadily til it hits the trip point. But with a throttle when the wind dies down the pressure will dwindle, and the temperature of the escaping air will increase, probably taking the fridge air above the level you want to keep it at. Tho, if as you say it counts as work, maybe I could have both, with a nozzle at the top of the pipe where it enters the fridge. So the valve opens, pressurising the pipe, then closes again when the system's bled out.

And two, I tried both approaches with my basic experimental rig and there was a significantly reduced cooling effect with a constant flow, as opposed to intermittent releases at the same pressure. Not sure why, tho thinking about it now I didn't adequately insulate the metal tap, so maybe that was conducting in heat, with a greater effect for the slower moving steady flow.

Hmmm....

The pipe would be well insulated, so as to keep the ground temp from warming the air between the tank and the fridge. PE foam wrap or similar should do a good enough job.

If that page was new to me, I doubt I would have read it, thought about it, and typed a response in 20 something minutes.

The Joule-Thomson page? I did read (most) of it when it was mentioned earlier, but didn't extract from it that work in the thermodynamic sense is involved in the process.

Not that that shouldn't probably be obvious to someone who knows what they're doing with these things, just that I don't know what I'm doing with these things.

I linked there so you would read " (the gas must not be expanded through a turbine, for example)."

When reading 'theory', do not put away your intuition. If something seems 'at odds', take the time to figure out what they are actually trying to say. They are often quite bad at it.

It can be as simple as it took 'work' to compress it, so 'work' results from un-compressing it.

Other things you might want to explore are; the insulation properties of earth, does evaporative cooling work in a closed vessel and how cold is it at night in the target geographic area. This may influence your cooling/storage approach.

> the gas must not be expanded through a turbine, for example

Because it's not work, or because it messes with the J-T effect?

Would this be the same for a sudden release?

(From the wikipedia page on Air Cycle Machines: The compressed, cooled air then travels through the expansion turbine which extracts work from the air as it expands, cooling it to below ambient temperature (down to -20°C or -30°C). It is possible for the ACM to produce air cooled to less than 0°C even when outside air temperature is high. http://thisisecs.com/blog/)

What does count as work, as in will lower the temperature?

> the insulation properties of earth

Depends strongly on the type of earth. I did prototype the ground cooling system, the soil (dry sand, in this case) didn't saturate even after 8 hours of 50-55 C air at 2 litres per second through 20 litres of water at 1 meterish depth.

> does evaporative cooling work in a closed vessel

It's not an evaporative cooler, at least not unless the input air is almost completely dry. Otherwise the cool water will actually dehumidify the air.

> how cold is it at night in the target geographic area

It will vary, but most places most of the time have a ground temp at a meter or more of about 10-16, day round, with some seasonal variation. Where I was it came up to 24.5, but that was height of record breaking summer, and would be considered unusually warm.

> This may influence your cooling/storage approach

It will, yes. However the addition of work may well end up making more difference than the pre-cooling.

Also of interest in the ACM wiki article, which is of course obvious now I think about it; passing the air through water to cool it on the way to the pressure chamber will of course mean less work for the compressor, as it'll take less energy to pressurise cooler air.

From memory there are certain temperatures that it is best to be below to best advantage J-T effect.

You can probably get some indication of the value of a 'turbine' buy measuring the drop across a common air tool.

What time the wind blows is also worth a look. If it tends to dusk and dawn, then air cooling in/at the tower might be 'less complex'.

I think it was Edison who ran an early commercial refrigeration business with air cooled by expansion. The ideal gas law gives the wrong answer because the heat transfer and energy transformations in the expanding gas depends on the shape of the expansion chamber, friction,final velocity of the gas, etc.which all can vary. The expansion valves used to be available for sale. Have you tried to buy one?

So it's all a bit too real world to calculate accurately. I'll get experimental data.

Expansion valves as in thermal expansion valves used in air conditioners? I'm not so much worried about regulating the temperature precisely, tho I do need a valve which trips open fully at a settable pressure and then restores itself....

Check this link for info on a Rovac air conditioner.

http://books.google.com/books?id=EgEAAAAAMBAJ&pg=PA84&dq=Rovac&hl=en&ei=DbvLTrPHHYLk8QP3pLgG&sa=X&oi=book_result&ct=result&resnum=1&ved=0CD8Q6AEwAA#v=onepage&q=Rovac&f=false

It uses air only, no intermediate gas, and maybe could be power by a wind turbine.

Interesting, looks to be exactly what I'm doing but slightly fancier; heat exchangers and whatnot.

Interesting about using water mist on both sides to improve effectiveness, I'm kind of doing the same on the compression side, but will have to think a bit about it's effect on the expansion as well.

They're getting about 90 C air temp at 2 bar? I'm on a lot less, but guess they'd be compressing the air much faster.

Seems that I should be able to get fairly usable cooling out of this with a bit of tweaking..

Nice to see you are still at it and getting some numbers. And thanks for the feedback.