Designing a Liquid Nitrogen Container for Efficient Coilgun Operation

[axi0m]

I'm constructing a pulsed solenoidal accelerator (coilgun) and am looking to boost efficiency by supercooling the copper wire in the coil. I'm also getting a quote for a high-temperature superconductor film to use in place of the copper wire.

In either case, I will need submerge the coil in liquid nitrogen. I am looking for a material that is preferably non-conductive (so it won't produce eddy current losses near the coil) and can be used at liquid nitrogen temperatures. I can build the container large enough so it will not cut any significant magnetic lines of force, if I need to go with a conductive material.

Also, I need a way to seal the openings in this container to the barrel that goes through the coil and extends further out. Would a silicone rubber tubular "stopper" work for this?

Thanks

 

[Q_Goest]

Hi axi0m. Styrafoam containers are commonly used. Can you get all the wires in through the top? Silicone won't seal very well on something like that, it'll crack. If you need something that goes through the side, please explain what it is and why it has to go through.

 

[Topher925]

I would think that just about any expanded polymer would work well. I'm not really sure what would work well as a sealant though. Teflon maybe?

 

[axi0m]

Here's a quick visual I threw together (misspelling included) that shows the idea I currently have to make this work.

It involves a brass barrel with the solenoid coil wrapped around it. This is all encased in a brass tank made of a larger brass tube with donut shaped circles brazed onto its ends. This tank is sealed to the barrel by brazing as well. I would have some kind of pressure relief valve connected to the tank for the evaporating LN. I would also have, obviously, a way to pour more in. The tank would be covered with 1" of polyurethane foam (heat flow rate: 0.25-0.30 Btu/hr. x in./sq. ft. @ 75° F).

http://img191.imageshack.us/img191/9606/tempdh.png"

Anyone think this wouldn't work?

 

[Q_Goest]

Is this for an industrial application or is this a college level project? It sounds like industrial which is all the more reason to design it per standard practice. I'd suggest 304 SS pipe for the outer jacket and similar material for the inner pipe. You could use tube as well. The ends could be welded on. The tricky part will be having access to the solenoid, so you may want to flange one end. A plate flange would work fine since you don't need to rate this for pressure (ie: you don't need a 150# flange). If it's small enough, you might also consider pipe thread for the flange. Expanded Teflon makes a great gasket, though there are lots of other materials. What size is the outer jacket/pipe?

If you flange the end cap and you want to access to the solenoid, the smaller line can be sealed using a bored through Swagelok fitting. These are commonly used for tube in tube heat exchangers or allowing access to a line for thermocouples. See page 16 of their catalog here:
http://www.swagelok.com/downloads/webcatalogs/EN/ms-01-140.pdf

Swagelok and pipe thread fittings can also be used for fill, vent and relief valve ports. I'd suggest pipe thread for the relief valve. The valve can be a http://www.generant.com/crv.shtml" which is an industry standard for this type of installation. They're very inexpensive - in brass they're dirt cheap.

One inch of polyurethane foam isn't much. Typical insulation is 3 to 6 inches. Putting a vapor barrier on the outside is important if you want it to last long. One inch is ok but it will get frost on the outside for sure.

The inner pipe will also get liquid air condensing inside of it. You don't show what's on either end of the inner line but if atmosphere can get it, it will condense and you'll have an oxygen rich mixture inside which is pretty dangerous if not treated properly. Is the inner line sealed at the ends? If you'd like help with that, please provide details around what's going on inside this tube.

PS: Silicone sealant isn't going to work on anything here. It will crack very quickly and begin leaking.