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View Full Version : Prototype CO2 liquifier Shell and tube issue







nzben
24-09-2007, 10:25 AM
I'm building a prototype CO2 liquefying unit only quite small scale and I think i've stuffed it up even before I've begun.

I'm making 2 shell and tube exchangers and I think i have a couple of issues, firstly I'm making out of 316 stainless (man buying stainless hurts these days) And I'm welding the tubes in the endplates - not sure if this will result in weld cracking - I know people tend to expand the pipes in heat exchangers but I wasn't sure how to do it so I figured a bead of weld should seal it up, the exchanger should be running at about -30deg C.

My main issue as I see it is the tube size I've chosen is quite wee and I'm concerned about the tubes becoming bubblepumps for the liquid refigerant that's going to be inside them. The inside diameter of the tubes is only 4.5mm altho the heat load is not that much (3kW/m^2) I'm still concerned that the bubbles generated will be big enough to drive the refrigerant up to the top of the exchanger - the center pipe I've made 1/2" so if a small amount does make it up It'll hopefully run back down but - yah I don't really know wot I'm on about - can anyone give me any ideas/thoughts???

I'm using propane as the refrigerant on the inside of the tubes and a mixture of nitrogen and CO2 will be coming down the outside of the tubes at a rate of 30kg of CO2/hr - so really small quantities in the proto. I wanted the CO2 to be on the outside of the tubes to increase my residence times to allow more to condense. Have attached a pic of the tubes on my small exchanger would love to have some helpful input as I really don't know much of what I'm doing.

Thanks

750 Valve
30-11-2007, 08:04 AM
Could you please explain your system a bit more, maybe a basic schematic. I don't understand what you are trying to achieve, are you trying to pull co2 out of the atmosphere and condense it or is this a closed loop system?

Buckiesr
09-12-2007, 07:38 PM
I am sort of like 750vlave in that I don't know exactly what your trying to accomplish with the heat exchanger. I do know that to roll in tubes there is a too that is used to do that. Just like the ones used to roll in boiler tubes, and many times the roll and weld them in. In what your doing I am quite sure you should weld them in as you indicated you did. Sure they will expand and contract so will everything else you might have used other than 316 SS. Maybe the tube sheet thickness is a secret of design to counter the stress on the weld. The thicker the tube sheet is the more the steel will be protected from flexing close to your weld. I think this would help make most of the expansions take place behind the tube sheet. Now to the thermodynamics. Why are you worried about the bubbles? they are formed in the exchange of heat. if your want to you can run it like a DX system and set the expansion rate to move the bubble through. I think you can just let the bubbles do their thing. Gravity will replace the liquid and will the natural flows due to thermodynamics laws. If the material you are cooling or heating is passed through at the proper rate the bubbles will be just as present in all of the tubes because the heat will be exchanging at all contact areas. I think your OK there. What do you accomplish by mixing the co2 and propane? Are you working on refrigerants or heat exchangers or both? I hope not both at the same time too many unknowns to work with at once. I love the experimentation aspects of all of this. When ever I work on something most people say I am crazy or worse. seems your using small amounts so unless you make a bomb you should be limiting your risks. No one ever said developing new ideas and inventions was safe and surprise free. I wonder how many times Albert Einstein or Ben Franklin had near misses? Way to go, now I am going to be labeled as a crazy person. I may have to disappear and come back under another name and location. All for being a person who thinks out side of the box.
Buckiesr