Very High Temperature/Current Help 1

[That Porter Kid]

Greeting,

I am a mechanical engineer and am trying to figure out some power transfer stuff and need some help.

I have a black-box that operates at 1100F (600C). The main lines going in/out, will be pushing 6000 Amps (peak) and a pretty low voltage comparably, 2 Volts (open circuit). I need to figure out what type of wires/cables/bars I need to allow for the electrical flow to hook up the system to my power source.

From what I could find on google, most things deal with Copper or Aluminum at high amperages, but due to the high temperatures, I have low confidence in these materials (at the moment I am looking at Nickle as a conductor). Besides, the listed information is only in low temperature applications (less than 200F (90C)).

So, I am unsure on where to start to ballpark the type of connectors I need. Any suggestions?

Thank you for your time!

 

[cranky108]

I am sure you are aware of common metal melting points, and that at those currents, a little resistance of a connection can cause heat.
Also a little connection resistance can cause a large percentage of voltage drop.

There are four metals that are commonly called good conductors, Copper, Aluminum, Silver, and Gold. So take your pick.

 

[That Porter Kid]

Hi Cranky108, thank you for the response.

Having resistances in the connection is an item of concern, so I need to make sure that the cross section for the current flow will be large enough to facilitate the the peak load. I can completely rule out Aluminum, as its melting point is 660C. The other metals listed are between 960-1090C, which are still pretty close to the steady state operating temperature. I wanted to use Moly, Nickle, or Tungsten, as its melting point and corrosion resistance is much higher, and I am hoping that the elevated temperatures will improves its electric conductivity and help mitigate its resistance (if not then introducing heat rejection might need to happen).

I feel like I am operating outside of the range for "good conductors". But this could be my inexperience speaking. Would Copper or a higher temperature alloy be a good selection for this kind of application?

 

[Compositepro]

Use gold plated copper bars.

 

[itsmoked]

As they say 'it'll never work'. You're on a very expensive fool's errand. Is there some reason you can't cool the box?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[That Porter Kid]

Hi itsmoked,
The box contains molten salt as an energy storage system, I can't get really into the details but it is for battery applications. I can cool the lines coming out, though it could be a challenge due to insulation, though not impossible. But the box itself needs to operate at the temperature, to make sure everything stays molten.

I know that we can get it to work, we just need to find the right approach to the problem!

 

[FacEngrPE]

I suggest you work out what the allowed voltage drop is in each part of the circuit, then using the material resistivity figure out the cross section area needed. If your cross section is large you will also want to understand how fast heat leaks out of your black box along your conductor.

Depending on how you balance the various factors, you might consider making your bus bar out of large rail track sections. The high temperatures will cause some oxidation, so you could consider steel used in high pressure boilers. Some of these steel alloys retain strength at higher temperatures, and have a measure of oxidation resistance. On the other hand conductivity of steel is an order of magnitude worse than copper.

Fred

 

[7anoter4]

In my opinion you may use an exposed connection in order to cool the contact with the cable up to less than 90oC.
A plate of copper of 3"x6.5"*0.75" [black lacquered] will do the job.
Let's say the transferred power will be Q= Res*I^2 [Res=1.6E-6Ω and I=6000 A]
Q=57.6 W
Rradiation=1.03
Rbe=is the convection thermal resistance from the external bottom surface=20.357
Rte=is the convection thermal resistance from the external top surface=9.1
Rve=is the convection thermal resistance from the external vertical surfaces=24.395
Rtotal=(1/Rrade+1/Rbe+1/Rte+1/Rve)^(-1)=0.855
Tcu=Rtotal*Q+Tamb=0.855*57.6+40=89.25oC

 

[bacon4life]

Resistance typically goes up with temperature, so 600C will be worse than room temperature for resistance.

Thermal expansion/contraction of electrical joints often leads to high resistance connections in components swinging between 0C and 90C. I imagine any joints operating at 600C would take some very specialized design.

The annealing temperature of copper is around 400C, so copper components would rapidly loose mechanical strength operated at 600C. I assume you would need to stay below the annealing point rather than the melting point of any other conductor.

 

[7anoter4]

I agree with you bacon4life.At first the cable lugs have to be well welded in order to avoid contact resistance. The cable from the box to plate has to be at least 1 feet length in open air-at least 2 overall diameter clearance.

 

[That Porter Kid]

Hi Team,

Thank you for all of your comments, you have given me a lot to think on!

@FacEngrPE - Thank you for your comments and suggestions, I am concerned on the oxidation and loss of mechanical strength from working at the higher temperature. We have talked internally about using steel (particularly Inconel, due to its positive traits at higher temperatures) and the resistance lose in the system because of that is something that I will still need to investigate. I'll add "check for allowable voltage drop" to my list.

@7anoter4 - Thank you for your feedback and the image! Once the cable is outside of the box, we should be able to use heat rejection methods help cool the element. Thank you for the equation for element temperature. I will be able to use this for additional calculations, and fiddle with the atmospheric conditions! The contacts onto the box will definitely be welded. *Scribble notes on comments*

@Bacon4life - Thank you for your feedback! Oh... that is unfortunate... I will have to keep this in mind, and will have to track down a curve for Temperature vs. Resistance so I can (probably) interpolate the types of resistances I will be dealing with at the higher temperatures. The annealing temp of copper was something that was worrying me, which is one of the reasons I started looking at more exotic materials for the wiring such as Moly or Tungsten. You're 100% right, dealing with a molten conductor is an additional complication I don't want to have to plan for.

 

[FacEngrPE]

To save some time this table includes some temperature coefficients.

https://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html

Two rather extreme examples are
[ul]
[li]The silicon carbide igniter used today in gas ranges that has a very substantial reduction in resistance as it's temperature increases.[/li]
[li]The tungsten filaments used in light bulbs, where high cold inrush currents can sometimes be troublesome. Resistance increases with temperature.[/li]
[/ul]

Fred