Double Wall UDS

Discussion in 'UDS Builds' started by smokersask, Oct 25, 2014.

  1. smokersask

    smokersask Newbie

    Hey All,

    I finished my smoker and figured I would post a picture.  I spent a lot of time lurking on the forum getting ideas from past posts, thanks to all.  This place is a really great source for information

    The design is a double walled smoker.  I was able to source a new, sandblasted bare 55 gallon drum (we call them 45's up here because of the imperial vs U.S. barrel volumes).  I then placed the 55gal inside a new 85gal "salvage" drum (sometimes called over-pack or containment drums). I used 1/4" metal rod for spacer to float the 55gal drum inside the 85gal.  I ran 4, 3/4" pipes through the 85gal drum and into the 55gal.  I sealed up around the openings around the 85gal drum with RTV silicone (I wasn't too worried about choice of sealant as there is no communication with the inner cooking drum).

    This keeps a "dead" air space between the two barrels in an attempt to insulate for winter use.  I want to be able to run this in winter when free time is at a maximum, and these Canadian (Saskatchewan) winters can get down to -40F (not factoring in wind chill).  I ran it yesterday for about 6 hours on ribs, and it worked really good.  I would say it used about 3 lbs or lump charcoal, and seemed to settle into 230F with 2 valves closed and 2 about 1/2 open.  The outer barrel hardly gets warm with it running.

    All in all, it seems to work good with no complaints yet.  Those salvage drums are about $200 new around here (vs $50 for a new 55gal), but they seem to have nearly twice the wall thickness as the smaller drums.

    ffmedic729 likes this.
  2. SWEET!
  3. dirtsailor2003

    dirtsailor2003 Smoking Guru OTBS Member

    Nice! Looks great! Did you consider insulating between the two drums or adding thermal mass like sand?
  4. smokersask

    smokersask Newbie

    In terms of insulating, I was told that dead air space is at least as good as insulation for insulating value (though I'm not 100% sure on that - sort of the same idea as a vaccume sealed thermos). I was going to do insulation (rock wool) until I read the additives in insulation (I'm no hippie, but formaldehyde scared me off!).

    I never thought of the thermal mass perspective - good idea. The main purpose for me was insulating for the harsh winter climate.
  5. dirtsailor2003

    dirtsailor2003 Smoking Guru OTBS Member

    Well, look at it this way. Do you think that your house would be cooler if there was not insulation in the walls and just air space? The air space helps, but having the insulation or thermal mass is better. The more heat retention you have the better. You'll also burn less fuel. Your concept is very similar to rocket stoves. Which use the clay, sand as thermal mass between the inner and outer liners to help retain heat.
  6. Actually its as much the reduction in air movement (convection) as anything. The emissivity of glass is actually quite high. An interesting, cheap and easily handled insulating material in the glass family would include perlite. Just remember most glass started melting around 1500°. Perlite looks like tiny foam beads, but is in fact expanded volcanic glass. It would mold itself to any unique shape inside the machine. I would be concerned of moisture from cleaning though, and for sure the biggie is that outer skin will stop winter wind. I bet its a good machine as it is.
  7. Very neat idea.
    Keep us updated
  8. smokersask

    smokersask Newbie


    Your statements were exactly the assumptions I used too. Air spaces that are non-convective are pretty darn good insulators - and I have this thing sealed pretty tight.

    From my understanding, the biggest benefit of insulation at this point would be to help reduce the radiant heat transfer from the hot inner barrel to the cooler outer barrel. Funny about the perlite - I tried finding it around here but all the garden shops are closed up for winter. I got a bit spooked about moisture build up too when considering insulation. Biggest heat loss at this point is the fact my lid is totally uninsulated.

    I will try to do a -40F cook sometime this winter and let everyone know how it goes.
  9. One interesting point about radiant energy is the reaction that can be caused by a particularly smooth surface. The smoother the surface from which radiation tries to leave, the less high points the photons have to climb and pop off of. This idea is the exact opposite of the idea of a lightning rod. Producing, polishing, keeping (under high heat) a mirror finish in the airspace chamber would be challenging, but would produce remarkably different U factors across the inner and outer layers.

    Even polishing the exterior to a high shine would produce energy retention to some degree, but that is probably minor relative to stopping the convection losses like the design does as it is. Its easy enough to write down fuel Kg and burn time and temp for summer and winter and compare them to see if there is any need to improve the efficiency.

    Yes, I just used the word photons to talk about Q.
    Last edited: Oct 26, 2014
    ffmedic729 likes this.
  10. smokersask

    smokersask Newbie

    Mmm... photons.

    Sounds like something Homer Simpson would say.
  11. Photon Phatty. Its the best,man!
    Last edited: Oct 26, 2014
  12. gunkle

    gunkle Meat Mopper

    So how did this hold up in the cold this winter?
  13. smokersask

    smokersask Newbie

    Yes, I could get 350F+ out of it in -25C weather. It seemed to not mind wind either (not worse that it would compared to summer). I also didn't see much difference in fuel consumption in winter vs warmer weather. That double wall is very effective insulation - the outer barrel was cold to the touch on the really cold days, while running at temperature.
  14. timberjet

    timberjet Master of the Pit

    That thing looks amazing. Nice job. 
  15. paul-b

    paul-b Newbie

    Nice work man if you ever want to insulate it get some vermiculite from a garden supplies place and poor that in
    The outside will be cool to touch while retaining the heat
  16. fwismoker

    fwismoker Master of the Pit

    Dead air space is fine, my reflectix koozie on my smokers is about like smoking in summer.

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