Posted 11 March 2010 - 08:48 PM
It is successful in lighting wood chips, even melting glassware. It has also withstood a propane torch and being left on for 30 minutes.
However, there is a concern that the wires themselves connecting to the element may or may not hold up. We will see; Bear has wrapped the wires inside a metal protecting tube leading upwards to the top of the reactor.
For this first run, it will be situated between two of the air nozzles laying flat along the top of the reduction bell. The intention of this positioning is to minimize bridging, although we are also proposing to add a stainless collar around the air nozzles to increase fuel flow.
In future runs, we might consider its placement closer to the air nozzles, but it might turn out that there is little difference.
Bear, anything you want to add on the specs?
Posted 11 March 2010 - 09:24 PM
The wires are rated to 250°C. It's going to be hotter than that in there. We'll likely need to look at ways to skip the insulation that comes with the elements and attach to something more robust.
About a year ago I looked through the available resistive heating elements to find materials that might work. Nothing looked very attractive. These elements are quite interesting. Silicone nitride seems to be a new technology in resistive heating.
One of the issues with resistive elements is that many are not self regulating. But some materials/designs increase in resistance as the element temperature goes up, reducing the power (Watts, heating) of the element, preventing it from over heating. This is called PTC (positive temperature coefficient). If you over-insulate some elements, they'll overheat and fry. It seems (though I haven't looked it up) that the silicon nitride has a good PTC. That's why I tried heating the element w/ a propane torch while it was on: if it didn't have a good PTC, it would overheat.
We'll see how the material holds up in a harsh environment.
These will also be useful to keep the flare lit.
Posted 13 March 2010 - 01:44 AM
We've used alumina tubes in other settings before. Not mullite though. Insulation is a solvable problem. This round I'm using some ceramic/mica insulated wire from McMaster rated to 842°F and crimping the leads (solder would melt). Embedding the assembly in high temp mortar to insulate the crimps.
For some reason I nixed nichrome as a heating element, I think partly from the difficulty if we made something in house. Seems its performs OK in both oxidizing and reducing environments. There may be kiln elements and insulators that could attach to the interior insulation tube if we went that route. This element is looking rather attractive though. We'll see how it performs.
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