Sidmadra - 5-11-2018 at 14:49
I'm building a furnace that I plan to use a high voltage electric sparker to ignite, but for that I need a high temp wire or sparkplug which can
handle the heat once the flame gets going. I read that there are kiln wires designed for the high temps of kilns, but the only thing I am finding are
nichrome wire. I don't need anything for resistive heating, but something through which a current can travel. Anyone have an idea where I can get such
wire or a high temp spark plug? I'm not having luck finding any online, but I'm probably searching the wrong terms.
Basically am in need of low resistivity high temperature resistant wire.
[Edited on 5-11-2018 by Sidmadra]
Sulaiman - 5-11-2018 at 15:40
Can you make a spark gap using solid steel electrodes with connections outside of the oven ?
e.g. https://www.ebay.co.uk/itm/Ideal-Ignition-Electrode/32353320...
Twospoons - 5-11-2018 at 16:41
FeCrAl wire on Aliexpress ("Kanthal")
I know its 'resistance'wire but it should work just fine for sparks.
It works out to about 3.5 ohm per m.
weilawei - 6-11-2018 at 05:20
Steam Engine is a calculator intended for people making coils for vaporizers out of various resistance wires and diameters. That ought to help you
dial it in.
DavidJR - 6-11-2018 at 05:26
Smash a lightbulb?
kulep - 6-11-2018 at 06:28
Get a tungsten tig electrode or a tungsten rod from ebay, should handle whatever temp you achieve. You could probably use a zinc carbon battery
graphite electrode but it would certainly burn out in a high temperature oxygen rich atmosphere.
As you are dealing with high voltages you don't need a low resistance material.
WGTR - 6-11-2018 at 07:35
But why not use a hot wire igniter? Small engines use glow plugs, and gas-fired central heaters use hot surface igniters. No spark necessary if the
surface is hot enough.
Jackson - 6-11-2018 at 08:54
Doesnt Nichrome and Kanthal burn out under high temps? I remember seeing something about them ablating or just permanently losing connection under
high temps. I could be wrong though.
Ubya - 6-11-2018 at 10:46
as kulep said tungsten is a good idea, thick tungsten. lightbulbs have the filament in a vacuum or in an inert atmosphere for a reason, the thin
tungsten wire burns in air if it's glowing hot.
i have a few options in mind.
nichrome or kenthal wire eventually burns if inside a flame (furnace flame rich in oxygen or just really hot) for many hours, you did not specify what
kind of furnace you are building, gas, oil or pellets powered?
i suppose you are going to use propane, as i said a hot filament directly inside the flame cone is not a good idea, but maybe a filament or spark plug
just offset the flame cone should work. propane exits the torch nozzle diffusing around it, when contacts the hot filament ora spark it ignites
forming the flame cone out of the nozzle, missing the filament or spark plug.
a problem of this system could be that if the air gas mixture doesn't ingnite in the first few seconds, the furnace would fill with the explosive
mixture, and once lit you get the idea, like hard starts for rocket engines
[Edited on 6-11-2018 by Ubya]
WGTR - 6-11-2018 at 11:35
This is what I'm talking about, by the way.
https://www.homedepot.com/p/Emerson-Hot-Surface-Ignitor-Sili...
There are all different sizes and prices available for these. If the furnace can be configured to use something like this, then it makes starting the
flame pretty easy and reliable. They are generally made from silicon carbide or nitride.
phlogiston - 6-11-2018 at 14:02
Maybe smashing a light bulb is still not a bad idea, not for the filament, but for its support wires which are supposed to be molybdenum (mp >2600
C).
They are a bit short and thin though, a tungsten welding electrode would be more robust.
Twospoons - 6-11-2018 at 14:22
Tungsten will burn up in an oxidizing atmosphere at furnace temperatures (600C+).
Morgan - 6-11-2018 at 17:37
I remember a small toaster oven I had that jacketed the footlong heating elements in a ceramic - like a coat hanger wire in thickness and a hard white
ceramic over it. Maybe something like that philosophy could work where a suitable alloy would be shielded for the most part, except for the terminal
end.
If you had some ceramic tubing with a tiny hole diameter, perhaps you could retract or back off a metal or carbon electrode sleeved inside the ceramic
once you spark started the oven.
[Edited on 7-11-2018 by Morgan]
j_sum1 - 6-11-2018 at 18:21
Well the victor apparently is hafnium.
http://theodoregray.com/periodicTable/Elements/072/index.htm...
But something prefabricated that would work well might be better. I like WGTR's idea.
Morgan - 7-11-2018 at 16:45
Another idea occurred to me, something they do with pulse jets that typically use spark plugs. Aside from just torching the side of the jet to a red
heat, (hot surface ignition), some just make a long spark ignitor rod and stick it up the tailpipe and once the jet starts, retract the rod. So maybe
one could light the flame in the furnace and then proceed to put the top lid on or close it up, no high temperature device needed if permissible or
designed for such - or have a very small access hole.
Here's a steel tube being devoured in a furnace of sorts.
https://youtube.com/watch?v=F8vqfphZ73A
[Edited on 8-11-2018 by Morgan]
Morgan - 7-11-2018 at 17:49
Hafnium tidbits
"Hafnium is subject to severe embrittlement by relatively minute amounts of impurities, especially nitrogen, oxygen, carbon, and hydrogen. They have a
high affinity for these elements at the welding temperature. Because of this high affinity for gaseous elements, hafnium must either be welded using
arc welding processes with inert shielding gases, such as argon or helium, or be welded in a vacuum."
https://www.eaglealloys.com/hafnium-guide/
"Although it has a melting temperature of 2230 °C it is generally considered a reactive metal rather than a refractory metal. It is subject to severe
embrittlement by relatively small amounts of impurities and is rarely sintered. It is generally formed by hot-rolling between 550 and 800 °C or
extrusion above 960 °C. It can be cold rolled only 15–25% between annealing treatments."
https://www.sciencedirect.com/topics/materials-science/hafni...
"A breakthrough came with the invention of the button-type electrode. The electrode, when embedded into a water-cooled copper holder, enhanced
emissive element cooling. Moreover, thoriated tungsten was replaced by zirconium when the plasma forming gas was air. The electrode life increased
from a few minutes with tungsten to a few hours with zirconium, and air cutting of mild steel became popular."
"However, it was practically impossible to use zirconium cathode at arc currents higher than 300 amps. At the end of the 1960s, the use of hafnium in
the former Soviet Union to replace zirconium, lanthanum, and strontium further improved electrode life in air plasma cutting."
https://www.thefabricator.com/article/plasmacutting/the-life...
[Edited on 8-11-2018 by Morgan]
macckone - 11-11-2018 at 10:00
Just buy a regular spark plug for a car. They are designed for exactly this kind of usage. 1000c is a normal temperature inside an engine cylinder
at the spark plug. The cylinder walls and piston tend to be cooler but head temps at the spark plug tip are closer to adibatic.
Morgan - 11-11-2018 at 11:02
I bought a few Eclipse and Auburn ignitors that have a long reach. The one on the right is an eclipse brand with a 3/4 inch pipe thread. The little
plugs have a 1/4 32 inch thread for comparison.
http://www.pulse-jets.com/phpbb3/download/file.php?id=6345
[Edited on 11-11-2018 by Morgan]