Inside The Secret Underground Laboratory

[T_Bone_TL]

You could weld or bolt studs or fins to the central core to get heat from the fire to the bulk sand faster, via conduction in the metal. Safest method for the fire is to figure out how much fuel you need for the desired temperature rise, and stop loading when you've put that much in, and let it burn all the way out. Though you could also make an airtight cap for the tube.

 

[Bruce Johnson]

How do you plan to extinguish the fire after the sand is hot and rolled into the shop? Please be careful of combustion gasses in an enclosed shop. I'm sure you have good airflow so it probably won't be an issue. I know the sand won't light the cart on fire, but I'm thinking about the charging fire and the cart. Will the wood cart be OK? Using cutoffs and wood chips is a good eco-friendly source for the fire material. In my wood shop, I have a lot of mistake pieces to burn (hide the evidence). Do you know if the type of sand (course, fine, etc) will make a difference in the performance? I can't wait to see how this progresses. Very cool project. Thanks for sharing.

Hey Steve;

There's an important part missing that I haven't built yet. A shallow square cup made from steel, which slips down into the bottom of the steel furnace tube. Attached to it is a long length of 1/4" black iron pipe, which comes up out of the furnace tube and turns 90 degrees. That pipe is the air bleed. It has some small holes down at the bottom, right above the cup. At the top is a small needle valve and a quick-disconnect fitting to connect to an air line. I control a small flow of air down into the cup, to charge the fire and adjust the burn rate. The bleed tube also functions as a handle, that I can use to lift the cup up out of the furnace tube.

Yes, I'll completely extinguish the fire while it's outdoors, before rolling it into the shop. I'll disconnect the air line and pour a few ounces of water down into the top of the furnace tube. Then lift the whole air bleed and cup up out of it, carrying most of the remaining charcoal and ash. Set it into a bucket of water to quench any last embers. then leave it outside to dry until tomorrow. Any residual ash down in the furnace tube will be dampened by the water. That's the plan. No fire inside the shop. Just a box of hot sand, on wheels.

I could burn scrap wood from the shop in it. But I'm thinking that the charcoal will let me burn hotter and charge it up in a shorter time. I think a wood pellet or scrap hardwood fire would be longer and slower. But I'll try it and see. It could be that lower fire for 4 hours does a better job charging the sand than a hotter fire for 2 hours. I don't know.

 

[Bruce Johnson]

You could weld or bolt studs or fins to the central core to get heat from the fire to the bulk sand faster, via conduction in the metal. Safest method for the fire is to figure out how much fuel you need for the desired temperature rise, and stop loading when you've put that much in, and let it burn all the way out. Though you could also make an airtight cap for the tube.

Hey T_bone;

I thought about that, but I'm not sure it would add that much efficiency to the heat transfer. The square steel tube inside the square box of sand is pretty good for surface area of the steel against the sand. That's also why I made it somewhat tall and slim. The fire will be contained within a 24" long x 4" square tube, fully in contact with the sand. The heat will propagate outwards from the center of the sand. And the outer sand will help insulate and hold the heat in as it all heats up.

Yes, I'll be setting a steel plate cap on top of the furnace tube while it's burning. Not airtight, but with some smaller holes. To hold most of the heat inside the tube. I'll adjust the fire size to keep the flames entirely inside the tube.

And yes, I'm planning to work out how much fuel to put in to burn it all up in one heat-up cycle; sand temp rise vs air bleed vs burn time.

 

[ardgedee]

You could weld or bolt studs or fins to the central core to get heat from the fire to the bulk sand faster, via conduction in the metal.

I suspect anything that can conduct heat into the sand quickly can conduct it out again just as quickly.