CO2 and cascading

[Gray Mole]

I was thinking about the fact that I haven't used CO2 at all, and don't know how it really works or the best way to set it up in a cascade.

There's no liquid phase, so is that why the 290 is used? Carry oil and flushing out ice?

I know how phase changing works and all that, but CO2 is a bit of a mystery to me.

If someone could post a link or explain some detail about using CO2 in a cascade and how it's actually doing the job, I'd be grateful :)

Cheers

Gray

[Cr@sh_D1n3r]

It's used as any other gas we consider as cascade gas. The main problem with co2 is it's freezing point really near it's boiling point atm. What do you mean by no liquid phase? There is a liquid phase to co2 if there wasen't how could it condense :P Btw yes r290 is used for oil carrying and flush ice. In small evaps as ours it's not a good idea to run a deep vacuum when operating the system because dry ice will form in the evap and blocking the system. As long as you don't go below -85ish, everything should be fine.

[Gray Mole]

Friend of mine's a Chemist and I asked him about CO2, and he said there was no 'appreciable liquid phase'. He's not into refrigeration so couldn't say about using it in phase change though.

That's more why I ask, as when it's dry ice, it's straight to gas and no inbetween.

I was just wondering if the 290 was for more than just carying oil and such, maybe it blends to make a more usable liquid phase or something.

No need for sarcasm, I'm just trying to find out more about CO2 and how it actually works in a phase changer.

Anyone know what the range of liquid phase is then? Can CO2 work on it's own?

Cheers

Gray

[Cr@sh_D1n3r]

This is called sublimation. You can get the link here http://www.airliquide.com/en/business/products/gases/gasdata/index.asp?Formula=&GasID=26&UNNumber=&EquivGasID=53&VolLiquideBox=&MasseLiquideBox=&VolGasBox=&MasseGasBox=&btnMSDS=0&MSDSLanguageBox=0&RD20=29&RD9=8&RD6=64&RD4=2&RD3=22&RD8=27&RD2=20&RD18=41&RD7=18&RD13=71&RD16=35&RD12=31&RD19=34&RD24=62&RD25=77&RD26=78&RD28=81&RD29=82

to air liquide almost all useful info is found there. Remember that the system is under pressure. If you take a co2 tank and put it downward, you will get carbonic snow. But if you so it under pressure ( to test this just put co2 in a system with a sight glass), you will get liquid co2. Just think about it, The bottle isn't cold so how could it be dry ice inside the tank? :)

[jinu117]

is it me or operating high side pressure would be around 210psi with CO2 Cascade granted HX temp outlet on 2nd stage is about -45c? (actually what is typical HX outlet temp?) Some calculation yields that -38c outlet temp, I think it is about 270psi getting into terriotories where lot of condenser's max operating pressures are...
Just curious :P

[SlackeR]

As i understand the co2-thing, it is liquid at room temp and high pressure. In a cascade we get i liquid at lower temp and lower pressure. But when the pressure is released it will cross it's triple point, the point where all three stages (gas, liquid, solid) can coexist. All gases have this, but it being low is a special thing to CO2. this means that some of the liquid will evaporate and some will turn into ice.
This is where the other refrigerant, 290, 404, 507, 22 or any other comes in. The snow will be in the liquid refrigerant, making a nice soup. So the CO2 sublimates just like when we use DICE in free air, but eventually at a lower temp due to vacuum..
So the other refrigerant is there for oil return, but manily for pushing dice.

[Gray Mole]

Cheers, that kinda clears things up a little.

I don't know if I'll use it, as I've got R23 here, and still loking for Ethylene, but I've always wondered about how CO2 works in these systems.

Maybe someday I'll throw together a CO2 cascade just to try it out and see what it'll do :toast:


Gray

[kayl]

if you got r23 wouldnt bother with co2
heres my co2 cascade
http://www.xtremesystems.org/forums/showthread.php?t=45620&page=8&pp=25&highlight=kayl+co2+cascade


basically with co2 after -55c it sublimes.

basically ya need enough r290 or r22 (or anything) in there to clear the end of the captube (in side the evap) so it doesn’t block up.
what generally happens is the compressors suckes vacuum and needle on the low side gauge flickers in vacuum little as solid pass through.
if you lisen to the face of the evap you can hear the solids going thud thud thud as they pass.

I did experience complete blocks inside the evap.
http://www.xtremesystems.org/forums/showthread.php?t=45620&page=7&pp=25&highlight=kayl+co2+cascade
top of thread bit about me evap blocking.

Basically the low side and high slowly rise and rise in pressure.
The evap temps slowly fall (at load benching) to around -55c -50c that BANG the solid passes and the compressor hits power band and quickly brings the evap back down to -7x’c.
Basically there is very little co2 in there.
Ya put as much c02 in there to hold load, then when it start blocking too much remove some 744 and add more r290/r22.