LN2 Submerged Electric Heater

[earthwalker84]

Hi Everyone,

I'm working on a prototype for a LN2 evaporator and am thinking of using an immersed electric heating element (cartirdge heater style: nichrome element in ceramic matrix encased in stainless steel). I am wanting to input ~100W at 110V AC.

From the research I have done it looks like LN2 should have the same electrical properties as gaseous N2.

Does anyone know if this is true?

Is LN2 a good electric insulator?

Can anyone foresee problems with electrical connections exposed to LN2?

Thanks,

 

[dcasto]

look up super conducting in liquid He

 

[earthwalker84]

Are you thinking that nickel-chromium wire would become a super conductor at LN2 temperatures?

From what I can see, super conducting occurs only in special alloys (niobium-titanium or niobium-tin) at very low temperatures (~10K), which is why liquid helium is used (liquid at ~4K) not LN2 (liquid at ~77K).

Also, I have seen cryogenic control units which utilize a nickel-chromium cartridge-type heater (

http://www.omega.com/pptst/CYC320.html)

and nichrome based cartridge heaters sold with cryogenic ratings.

What I haven't seen however is any situations where wiring or heaters are explicitly rated for submersion in LN2.

 

[EdStainless]

I would be more concerned about traces of moisture from the electrical point of view.
What will happen is that the liquid will boil at the surface of the heater, so you will not be heating a liquid, but a cold gas. The thermal conductivity will be poor, like heating air.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[earthwalker84]

Good point, are you thinking of frost buildup on electrical connections? where should I look for moisture traces? or are you thinking that moisture traces in the ceramic could damage the heater when frozen?

I think the boiling effect is what I was hoping for. My plan was to place the heater at the bottom of a volume of LN2 and allow the generated bubbles to pass up through the LN2 to ensure a cold temperature, then utilize the cold gas flow for cooling.

 

[jedward]

1) Us a lower voltage heater to mitigate the reliability and safety concerns
2) ceramic insulators should be ok if they are not porous, if porous they could fail due to moisture absorption and refreezing causing cracking Use insulators that are low thermal expansion to enhance the reliability
3) could you heat the bottom of the LN2 by conduction with the heater above the LN?
4) consider if this is a one time use or high reliability since this affects design
5) if 110 V operation, use on GFI circuit

 

[EdStainless]

Why not use one of the flat wire-embedded-in-silicone-rubber heaters? Low enough energy density and it would give you more surface area.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[jedward]

any heaters in silicone may have cracking/reliability issues since the silicone will get rigid at below about 130C

could also consider the metal foil heaters imbedded in Kapton