[IDEA] Heat scavanger / preheating blast air

[oldgoaly]

Well I thought about starting a new thread so we can swap ideas on the subject and not get off track in other threads. The heat scavaging sorta started as a bit of a joke at the local JR College sand casting class. I would say we could bake a pie on top of the crucible furnace. I did use the exhaust to heat up metal and straighten or bend into shape like a Blacksmith would. I still cringe when I think of the mini Model T leaf spring I had turn eyelets on each end and was letting it cool, Ernie Werner comes over a grabs it while saying how good it looked..... the sound of skin sizzling ... that still sticks with me. Quite a lot of heat is wasted!
Now I have a partially built cupola, and thoughts of preheating the blast. How I was going to do this was to feed the air into a exchanger at the top of the cupola then it would feed the tuyeres in the heart of the furnace, This was to allow for smaller tuyeres but more of them for more even heating.
   
not a good pic it's just sitting outside, probably will not be finished cause the amount of work need to feed and tap a cupola is a multiple person project. I do have almost 30 years of waste oil in drums, lots of old oil burners around here. A waste matbe the wat to make a workable batch of iron for all those things that need to be cast! Also a scrap aluminum ingot maker would be good too!
What I'm thinking is a double walled top hat that will go on top of the furnace, air feeds in one side out the outer using flexible exhaust tubing so we can still get the lid open easy, still need to feed the furnace. Might need to be "C" shaped??? Please feel free to think out loud!