Copper Tubing

[nwpassage]

I'm fairly new at cryo engineering. What are the problems with using copper tubing and soldered joints (other than shrinkage) for carrying:
1. Dehumidified air at its dewpoint
2. Liquid nitrogen at 120 psig
3. Nitrogen gas at 120 psig, 150 K

I'd assume that the solder joints would break due to shrinkage, and since copper is an excellent heat conductor, it would be hard to keep the room temp. heat out of the lines. Any suggestions, thoughts?

Thank you!

 

[iainuts]

Copper tubing is commonly used in cryogenic systems because it remains ductile at low temperature. Similarly, a silver brazed joing works very well at these temperatures.

Regarding heat transfer, the vast majority of 'insulation' on an uninsulated line is the convective heat transfer coefficient between the pipe and air, so you don't need to worry about the high thermal conductivity of the pipe material. Although it is very high, a stainless pipe for example, will not provide any significant additional insulation. If you're concerned about insulation, then adding the proper type of cryogenic insulation is a possibility.

 

[nwpassage]

Thank you for very valuable inputs, and in a timely fashion! Are there vendors for a proper type of cryogenic insulation?

 

[iainuts]

There's a variety of suppliers, though I can't seem to locate the contacts right now. One we use quite often makes a product called X-50, which has the insulation already attached to the copper pipe. Another manufacturer, I think it's 3M, makes a closed cell glass foam which is non-flammable even in oxygen so it is often used on liquid oxygen and nitrogen lines. There are many others.

The main features of any good cryogenic foam include a good vapor/water barrier on the outside, and generally a closed cell structure. When the pipe gets cold, you'll have a thermal gradient in the insulation which will generally liquify or freeze the gas that's inside the close cells of the foam. When it does that, the pressure in the closed cells drops to near zero so that with time, air and moisture will diffuse in. The constant thawing and freezing will end up damaging the insulation. You'll find all of the insulations end up in poor condition within a few years. Still, they're generally less expensive to replace on a regular basis than VJ.

 

[pennpoint]

nwpassage
"iainuts" is on the mark on both posts. Glass Foam made by PPG is my only correction. Glass foam has great insulative abilities but it needs to have a good quality vapor barrier as stated.
Do not use anything less than brazed or silver soldered joints with your copper-bronze, copper-stainless steel joints.

Best regards
pennpoint

 

[eric2718]

http://www.industrialinsulation.com

has many cryogenically rated insulation materials.