air liquefier attempt, questions

So I've wanted for a long time to attempt to build an air liquefier using a fridge compressor.
I've done plenty of reading, trying to understand the best I can how these devices work, and what is in the
reach of my technical capabilities.
The setup I will follow is actually similar to what i believe Carl Von Linde built about 100 years ago, managing for
the first time to liquefy air.

Anyway, I bought a new 180W fridge compressor for $80 .
2800 rpm, 4.56 cubic cm displacement, air flow of 790 litres/hour, or 220 ml/second.


I also bought 30 meters of 6.35mm OD 0.8mm thick copper tubing and 3 meters 15mm OD 1mm thick Cu tubing.
A 0-40 bar manometer, will get also a 0-200 bar one.

I plan to use the 3 m 15mm tubing to make the first heat exchanger , right after the compressor, this will be cooled by running cold tap water, which should cool the compressed air to about 8-10C. I will do this by placing the copper tubing in a larger diameter hose , and running cold water continuously through it.
Now from what I've read in industrial settings, they cool the first heat exchanger by underground piping, which cools it to about 14C, so I reckon that mine will be a bit better, I may have to use more than 3m of tubing to bring the air all the way to the temperature of the water, though

The 30 m tube will be the place where the magic happens of course.
I still have to buy 30 meters of hose, 12mm ID at least, to slide it over this copper tube to make the regenerative cooler. And also a lot of pipe insulation to go over the hose, I want to put it 1 inch thick at least.

The expansion valve I will use is an all brass screw valve, that came from a household butane tank.

While I was thinking of my setup, i soon came to realize that by far my biggest problem will be atmospheric moisture , and to a lesser extent atmospheric CO2 which accounts for about 0.04% of air.
I will have to build the air desiccators myself, and plan on placing them at the intake of the compressor, even though these devices are usually placed on the pressurised side, I can't trust a homemade tube packed with desiccator running at 40 bar or more.

Do you have some ideas on building these desiccators? What desiccant should I use? CaCl2, or NaOH? I can get NaOH very cheap, and this also has the advantage of removing CO2 from the air, on the other hand from what i know CaCl2 is more hygroscopic than NaOH and will dry the air better. I also thought of building 2 desiccator tubes one with CaCl2 and one with NaOH, placing them one after the other at the intake.

Do you have any other ideas on how to get super dry air? to ensure that no water will condense, or freeze, at the expansion valve, which will be by far the coldest point of the system?

I played a bit with the fridge compressor , and the manometer, I took it to 30 bar, it had no problem whatever. I'm under the impression that it can do at least 50-60 bar.
What I don't like about it is that it 'spits' oil , but from what I've read all fridge compressors do this. I do have a couple of ideas on how to make sure this oil doesn't go into the whole system, though.

I also drew a Paint picture of my setup:
http://s13.postimage.org/e2stnlorr/untitled.png

Thanks if you've read all this and I'd really like to hear your thoughts!!

[Edited on 12-4-2012 by condennnsa]

[Edited on 12-4-2012 by condennnsa]

[Edited on 12-4-2012 by condennnsa]

Quote: Originally posted by condennnsa

Do you have some ideas on building these desiccators?

I hope you'll succeed, because this is quite a challenge. I'm not sure if your compressor can supply enough pressure for this to work with air, but I'd give it a try. However, have lots of quality insulation like styrofoam beads. I'd wrap the cooling elements with glass wool and put it in a box and stuff it with beads.
(if you don't have the beads, acquire beaded styrofoam and few kids; kids love destroying styrofoam, I know I loved it )

Consider using tubes of scrubbers and dessicants instead of large tanks. It's always better to let the air pass through a long tube will less material, than passing it through a small tank holding lots of it.

You'll also need few one-way valves, some exposed to normal temperatures, and some that can withstand extremely low temperatures. I recommend aquarium valves with a dab of inert oil for room temperatures, and home blown borosilicate glass valves can be the solution for liquid air, if insulated metallic ones aren't available.

Use the largest thermos flask you can find for the expansion vessel. I think the pressure inside is almost atmospheric, so bursting shouldn't happen. Avoid the stupid designer ones, with metallic casing. Use plastic ones, and stuff styrofoam beads inside. I did that to one of my flasks and it works like a charm, compared to a flask without the beads.

Remember to put safety valves, too. You don't want things to blow up.

Quote: Originally posted by Bikemaster

Remember that you need to be under the critical temperature of the air (Nitrogen/Oxygen) for the systeme to start producing liquide air.

Thanks guys for the support. I've been really ill for 3 days now I don't what it is, but my back hurts like hell. It's ridiculously painful to perform basic taks such as walking . I suspect a herniated disc, but i'm not sure because i had episodes of back pain before which went away...

Regarding watson's and twospoons' recommendations , was also thinking to use cold to get dry air, but i was hoping i could get away without, just with desiccants. I searched the web the best I could for conclusive data about the dewpoint that each desiccant can achieve, but couldn't find any.
For example the wikipedia air dryer page states that:
"A regenerative desiccant dryer typically delivers a dew point of between -40°F(-40°C) and -100°F (-73°C)"

according to that page a desiccant dryer will dry the air the best. Of course if this data is for P2O5 as a desiccant, which is one of the most powerful, then it's irrelevant for me. Basically I will choose CaCl2 or NaOH or both , these I can get cheaply and in pellet form to allow easy air circulation. I also read on some other page that anhydrous CaSO4 is more powerful than CaCl2, so I consider that as well.

Wizzard, yes a fridge compressor can get quite hot after tens of minutes running, but I don't see a problem, as the watercooled first heat exchanger should cool the air pretty quickly to the waters temperature. Keep in mind that since this air is high pressure, it will move very slowly through the pipe, for example at 30 bar the flow will be 220/30 ml/sec, or 7.3 ml/s . The 3 meter tube with 13mm ID has a volume of around 400ml, so the air will spend about 1 minute in the cold tube.

Endimion, I'm on a budget here . We have one refrigeration technology supplier in out city, and the safety valves they sell are ridiculously expensive, a 28 bar one is around 35 euro, as pressure goes up, price goes up exponentially.
I actually thought about using the compressors stalling pressure as a safety feature. 2 days ago I got a 0-400 bar manometer, I'll try to connect it directly to the compressor and start it to determine the pressure at which the motor is overloaded and stalls. I'm a bit worried though, what if the piping bursts... these fridge compressors, even if they look harmless, are mean little beasts

For the expansion tank I'll take your advice, i'll look into supermarkets for some thermos, as I don't own one, I also thought of just using expanded PU foam. Yes the pressure is just a tad over atmospheric , depending on the ID of the regenerative cooling tube and its length.

oh yeah, what also pisses me off is that copper fittings of small diameter <15mm are also expensive as hell with a mean price of 1.5 euro each. while a 15mm 90 degree curve is about 0.2 euro, a 12 to 8mm reduction is 1 euro, and weighs about 1/10 of the 15 mm one.

For the time being I set a goal in my mind and that is a temperature of -50C. If I will see that temp displayed on my multimeter screen, I'll jump with joy.


[Edited on 15-4-2012 by condennnsa]

[Edited on 15-4-2012 by condennnsa]

Dewpoints of air provided by different desiccants from "Handbook of Preparative Inorganic Chemistry" available in our library and provided by by S.C. Wack.

thanks bromic, for spoonfeeding me, i guess i should've been more persistent . I found another chart in "Purification of laboratory chemicals" it doesn't give dew point data, just puts them in order , it goes like this:

P2O5 >> BaO > Mg(ClO4)2, CaO, MgO, KOH (fused), conc H2SO4, CaSO4, Al2O3 > KOH (pellets),
silica gel, Mg(ClO4)2.3H2O > NaOH (fused), 95% H2SO4, CaBr2, CaCl2 (fused) > NaOH (pellets),
Ba(ClO4)2, ZnCl2, ZnBr2 > CaCl2 (technical) > CuSO4 > Na2SO4, K2CO3.

I tested the compressor and it is only able to go to 40 bar, it doesn't stall however, it seems that the compression ratio is about 1:40 so that the air eventually ends up being compressed and never leaving the cylinder .
here is a video I shot
http://www.youtube.com/watch?v=hygPGnjluL0

I will have to buy another compressor to pressurise the intake to a moderate 5-10 bar pressure that way I should be able to reach the 80-100 bar which I'm after. I know a guy who owns fridge repair shop that carries lots of them cheaply.

a few pics:






This is the 30 meter future to be regenerative cooler, the copper tube was a real PITA to push all the way through the PE tube.

The regenerative cooler is the part that's troubling me the most on how to build, I'm still undecided on whether to put piping insulation over the black hose, or to build a big box out of thick styrofoam boards, piping insulation is really expensive.





This is to collect oil from the compressor and prevent it entering the system, i can empty it when full and pour it back at the intake. It seems to work well.


[Edited on 30-4-2012 by condennnsa]

Quote: Originally posted by XeonTheMGPony

http://homemadeliquidnitrogen.com/ not my project but a very nicely built unit at the garage level!