MadSci Network: Engineering

Re: how can i use a CO2 cartrige to make dry ice?

Date: Wed Feb 6 13:46:24 2008
Posted By: David and John Free, Post-doc/Fellow, MFA, MFA
Area of science: Engineering
ID: 1196121903.Eg

Hello Karim

Get a thin-wall copper tubeof say 1/8" inch diameter and wind it into s 
spiral of pitch 1/4" and diameter 3/4".
Get a Dewar vacuum flask 1" neck diameter and deep enough to contain your 
spiral helical copper tube.
Crinp or solder a very fine nozzle to the lower end of the spiral helix 
so that the CO2 exhausts VERY fast (say a hole 1/100th inch diameter: 
experiment woth different sizes).
At the top of the copper helix put a runbber or other seal so the CO2 
cartridge can be attached without leaks.
Now the cartridges I am familiar with have weak necks which you break to 
release the gas. If yours work that way you will have to work out a 
cunning way to break the neck WITHOUT losing the gas. It must pressurise 
the copper helix with the ONLY leak beive via the jet at the lower end of 
the helix!

Now due to the Joule-Thompson effect (see Wikipedia: this is how they 
make liquid air) the expanded gas is very cold. While exiting the vacuum 
flask the cold gas PRE-cools the tube and the gas within. So we have 
POSITIVE FEEDBACK - the cooling, by expansion, of precooled gas!

As liqid CO2 is not stable at atmospheric pr3essure, soon dry ice will 
coat the copper helix. It may take LOTS of cartridges!
Then when you have enough dry ice you can remove the helix and do what 
you like with the dry ice.
A LOOSE-fitting foam polystyrene lid on the Dewar flask will help. Watch 
that you do not get so much dry ice you cannot get it past the neck of 
the flask!

Good luck Karim and above all HAVE FUN!


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