Furnace Drain Holes---Procedure for use?

I've thought about this experiment for awhile and I think a drain hole is a good thing to have. The 2" dia. drain showed no hints of ever plugging up except if a mountain of frozen Al builds up in the pan underneath. That would depend on the failure mode.
I think in a catastrophic failure of a full crucible melt, you can simply poke through the melt and retract or move aside the plinth covering the drain hole.
If you spill less than two pounds of Al, the drain will be pretty much useless. Putting sand at the bottom of the furnace seems to be a real good idea.

 

I'm confused... Why is less than 2lbs of Al the point where a drain becomes useless....? I noticed in your pics that you didn't incorporate any grooves to direct spills under your plinth and toward the drain. Is that why 2lbs is your determined cut off...?

Unless I didn't have another option available I'd refrain from using sand at the bottom of the furnace. Why ruin your material by mixing it with sand...? Obviously I'd rather have ruined molten metal than a ruined furnace.

As far as the build up of material clogging the drain mentioned earlier. I haven't experienced that to date. Even the direct aluminum melting I've done hasn't left much of a skin on the drain's wall. I did of course remove that skin, but it was done so by hand, without the need for tools of any sort. It was closer to balling up tin foil than anything else.

 

When I built my furnace, we had the same discussion in that thread. Some for, some against. I ended up not putting the drain in (was thinking ribs and drain) just because I wanted to get moving on the casting. Move ahead a few months and I could kick myself right in the rear. I sure would like to stick some big chunks of aluminum in there and melt them down. My refractory is even rated for direct contact with aluminum. Oh well ... maybe next time.

 

The 2lbs was a seat of the pants estimate however, the Al recovered weighed 2#. With the sand in the bottom to seal up the cracks in my refractory, the two lbs. would not have caused any problems in removal once solid. If the melting Al slowly dribbles into the drain there is a good chance it will freeze in whatever passage way that exists beneath the plinth because that channel will be cold. If you desire to melt Al directly, you will want a drain, and I think a larger drain hole, covered with the plinth that can be easily knocked aside.

When designing a furnace I think the tuyre should be above the level in the furnace where the max. capacity of the largest crucible would fill. Or plan for metal pouring out the tuyre hole.

 

That's a interesting notion. I have a rather large steel crucible that will hold somewhere in the range of 1cuft of whatever. That would probably translate to having my tuyere at least a few of inches off the bottom of the furnace. I think mine is currently, maybe an inch off the floor. However with the drain and the grooves to direct a potential spill under the plinth, I'm not concerned at all with a spill making it's way into the tuyere. That said, the drain is a mere ~1.5", so a complete dam bursting level spill would overwhelm the drain and probably fill the burner. I don't wouldn't expect a failure of that magnitude but if the crucible walls disappeared suddenly, it would be entertaining...lol.

My point in the reply is I think you're suggestion would place the tuyere at a height where you could potentially have a cold zone at the bottom of the furnace. In my situation with my largest crucible, I most certainly would. Looks like we have another benefit to a properly designed drain.

 

Now that's the correct attitude to a disaster... don't worry just make it safe and continue!