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Fluidized sand pot, question


dan pfanenstiel
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Hey guys, I’ve been mulling a design in my head about a high temperature sand pot and thought I’d ask.

 

If one were to build this, pretty much a 24” vertical heat treating oven with a pot (muffle tube) running down the middle, PID and SSR control, is there any reason why the coils can’t be very close to the pot? Not touching the pot but running in grooves in the soft fire brick with say a half inch of space to the pot? Everything I see has some sort of chamber size, which is correct on a gas burner design, but electric?

 

Thanks

Dan

Dan Pfanenstiel

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The potential issue I see is the sand itself.  Most "sand" is a gooey liquid at HT temperatures.  What are you planning on using?  

 

Other than that, you need a little empty space to keep the coils from arcing to the tube.  

 

BTW, good to see you back around!

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There has been a few sands tried, Alan. Most common is aluminum oxide in 120 grit (sand blast medium). Think of a salt pot, but with sand and a way to introduce air from the bottom. Takes very little air. Eliminates most of the danger of salts, but introduces decarb potential if not using inert gas. There’s a few YouTube vids on the subject. I played with home built salt pots and there was just enough anxiety over it to give it up.

 

I keep with the forum regularly, just not a lot to say. Finished work has been sparse, trying to correct that :-)

 

Dan

Dan Pfanenstiel

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Gotcha!  Yeah, AlO would be great, and I was wondering how you were going to fluidize it.  I think, again, the only danger with electric here is that the elements are live wires.  Enough of an air gap to prevent any danger of arcs and it should be fine.

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18 hours ago, Jaro Petrina said:

I dont think you are going to melt AL2O3 at home, its about 2100 Celsius melting point. Anything that melts that is more dangerous than any salt bath. 

hes not melting it using it to even out hot spots the air introduced in the bot um is to keep the sand flowing so you can easily put the knife in unless im totally wrong

Brandon Sawisch bladesmith

 

eagles may soar but weasels don't get sucked in to jet engines

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9 minutes ago, dragoncutlery said:

hes not melting it using it to even out hot spots the air introduced in the bot um is to keep the sand flowing so you can easily put the knife in unless im totally wrong

 

 

That would make sense, but it seems to be complicated to me, because the electric owen with a hard brick would give him just about that.

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I have done salt pots in the past, propane fired, and it does work well. In an effort to move away from that, the sand pots made sense. I cobbled together a small sand pot, with gas burner, and had trouble with it. Like Gilbert, getting it tuned in was a problem. In an ‘ah ha!’ moment, thought why not an electric version. Since I had pieces and parts from past electric oven builds, I started thinking about it. I know Paragon offered an electric version sand pot, but big and cumbersome and pricey. 
 

The sand does offer up even heating and temperature control. If the electric version can get stable temps and proper sand fluidization, the only downside would be scaling on the blade unless an inert gas (argon) is used for the air supply, or protective coating on the blade. I think of it as an improvement over an electric heat treating oven, sort of like putting a tube in the forge to do heat treating.

 

More thinking and tinkering...

 

Dan

Dan Pfanenstiel

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Heating and cooling sand is a bit of a big issue in foundries, and I have done a lot of work on it.  With that being said, I would not think a fluidized sand bed would be a good choice for heat treating.  Thermal conductivity of sand isn't great (some much more than others; chromite being much more than silica, for example).  So you would have to be really stirring it to ensure you actually are getting an even temperature.  And stirring it with cold air only makes the problems worse.  So now you have to start looking at heating your air to HT temps before it hits the sand.  And then you have to start worrying about contaminants.  A little bit of the wrong thing in your sand can dramatically reduce the melt/sinter temperature for the system.  I am working on an industry meeting presentation I will be giving (pre-recorded and broadcast) that touches on this, as it was a massive problem for our foundry (looks like Mg was our biggest problem contaminant).  All this is very hard to really put into practical usage without experimentation.  For example, zircon sand is used as a chilling material in silica sand molds because it draws out heat faster than the silica and does not pose a problem with sintering or melting like chromite would (5% chromite contamination would be very problematic).  When you look at the thermal conductivity of zircon and silica, you would think that the zircon would be insulating.  It actually does chill more than silica because it is considerably denser than silica.  

 

This is all conjecture based on my experiences, which don't actually include trying to heat and maintain elevated temperatures in multiple sands.  I have dealt with thermal reclaimers, which is burning the binders off of foundry sand at 1800+ F.  But the goal there is to simply heat the sand in a fluidized bed (the fluidizing air is the combustion byproduct and recycled air / recuperators to maintain the heat) and keep it moving through the system at the right speed.  Then it runs through a fluidized bed to cool it back down for re-use.  Just wanted to add this little bit of info in case it helps.  If you do figure out a process that woks well that would be awesome, because there are significant drawbacks to molten salt!  Good luck!  

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Interesting, Jerrod. I was thinking about bringing the air supply in from the bottom of the pot. Your comment about the air cooling factor made me remember that some of the first iterations of home built sand pots ran the air line down from the top, inside the pot. Pre-heated air.

 

Just for clarity, the air coming up from the bottom is to agitate the sand. In my test pot, it took very little air. Like needle valve almost closed. Just enough to get the sand gurgling, but not blow the sand out the top, and allowing a blade to move down into the sand mass easily rather than being shoved into it. The mass of the sand, at temperature, provides the thermal transfer to the blade.

 

This is one of those things that could be good, or not work at all. I wish there was a full set of plans.

 

Dan

 

Dan Pfanenstiel

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It would be great if you have a thermocouple probe to stick into the sand to see what kind of variances you have (with and without a test blade in there).  I would worry about things not being as even as desired.  I hope you get something that works.  Salt pots are on my "someday" list, and I would really rather not have molten salt, but I really want the control.  

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It also occurred to me that I should point out that a fluidized sand bed can make a pretty good quenchant.  Use it at room temp for when you aren't sure if you want to rink an oil quench but aren't sure if air is fast enough.  Obviously this is only of use for a select few alloys.  

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Yes on the thermocouple. I would have a PID control with thermocouple reading directly in the sand. I was hoping to have somewhere in the 24” long range for the pot, dunno if that long a probe (nichrome sheath, etc.) is doable. Don’t know why not. The electric coils are in hopes of getting even temps down the pot, rather than one or two burners. There’s a slim hope that having the coils spiral just around the outside of the pot with a half inch air gap, that target temps of around 1500 deg was possible with 110v and 1500 watts of power. I may be dreaming.

 

I had not heard of sand quenching, interesting. 
 

Dan

Dan Pfanenstiel

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