I have really enjoyed reading this thread (only 4000 posts to go!) and putting my UDS together. I went with the simple 3 x 3/4" intakes with one ball valve and just the 2" bung hole for exhaust. Flat top, some handles and a hook for the lid. Still looks pretty much like a regular old drum, kinda stealthy. I like it.
Some things have been running through my mind as I have built this thing, though. To start with, for all their similarities, I've noticed quite difference in performance drum to drum as far as holding temps low or high. Much can be attributed to air management, but fuel choice and lighting techniques also come into play.
One thing I think doesn't get the attention it deserves is thermal mass. The barrel I chose for my build was a very lightweight barrel, open top, no liner. It was so lightweight in fact that I was fearful of denting it while drilling my holes. When I burned the old paint off with a weed burner, I was struck at how quickly the thin gauge sheet metal cooled to ambient temperature, just a minute or two after removing the flame. Now, I'm no rocket scientist, but I know that for a given mass (in our case, metal), it will take a certain amount of BTU's to raise the temperature, and that the more mass you have, the longer it will take to cool. So this got me to thinking, should I have went with a heavier gauge drum? I can see benefits to both. The lightweight drum should, in theory, respond more quickly to changes in airflow, requiring less BTU's to heat the mass and cool more quickly. The flipside would be that a heavier drum, while taking longer to respond to temp changes would hold those temperatures a little more consistently and I'm guessing be a little more fuel efficient. Another factor would be the construction of the fire baskets and grates and their respective mass.
We know from another member who basically insulated his UDS with an 85 gallon drum that the efficiency increased (less fuel usage and more consistent temps) and I would attribute that to the insulating properties of using two drums and the added thermal mass.
I intend to do some experimenting on my next build, using a heavy barrel and as massive fire basket and heavy duty grates as I can manage. I will then be able to compare the performance of the two. If anyone has already thought about this or done a comparison, I would appreciate your thoughts. We already have certain rules of thumb, like intake area being approximately half or slightly more than that of the exhaust I think another good rule of thumb would be for the fire basket height to be approximately half of the diameter. Seems this promotes good steady burns. I'm going to make several in this manner sized for how much burn time I'm shooting for. And maybe we could come up with another rule of thumb as far as the gauge of the barrel is concerned.
Anyway, like I said, I will being doing my own testing, but I would love to hear others thoughts on these things.
