Mini Multi-purpose Unit

Discussion in 'Reverse Flow' started by slimtony1974, Jun 2, 2011.

  1. slimtony1974

    slimtony1974 Fire Starter

    So it begins, my Mini Multipurpose Unit. I am a full time RV’er and travel for work in MS, AL and to the in-laws in GA. So space is very limited and I have not been able to find something that fits my needs so time to build. I have been lurking for a while and have some drawings and a few Google Sketch up images, and a BUNCH of questions. I hope ya’ll can help.

    So here’s the deal. I want a RF smoker and a charcoal grill. The RF plate/pan will be removable, and replaced with a charcoal basket for grilling. (I’m going to add an inlet to the opposite side of the fire box for additional air when grilling)  Also the smoking fuel source will mainly be charcoal, but would like to be able to use propane (turkey fryer burner) and maybe electric. I have 2 stainless grates from another build, about 12x16, that are going to be the main surface area. I will have a 6x16, ½ rack up high for additional space.  The cooker chamber is going to be 13”x16.5”x20” tall (4,290ci). 2”under the plate/pan, 5” to the grate, then 13” to the top. Room enough for beer butt chickens, or a turkey, or a rib rack  and of course butts and bacon. See the pics for the lid design. I have details of the doors, latches and hinges, will post later.

    The fire box needs to be about 1,430ci, 10”x12”x12”—1,440ci, 3” under the charcoal basket/which is enough room for my propane burner, 3” tall basket, and about 6” of free space.  I will be able to move the basket up 3" when using the Propane burner. The air inlet is going to be 2.25 or 2.5” pipe, threaded npt, so I can add stuff!!. The hole between the boxes should be 12”x1” and the Chimney calcs out at 4” dia. About 6” tall.  I may use smaller pipe and taller.

    So that’s the basic idea.  I have a source to bend some steel and a friend at work with stick and bricks that has a welder I can use.

    So here’s the list of questions:

    I plan to bend the boxes from one piece of steel and weld together, What gauge is too thin?

    Should I insulate? Is it worth the effort?

    Will a smoker of this size work? Such a small volume of cooker and small box..

    Is the calculator accurate at this size?

    Will the removable pan/basket in the smoking chamber work??

    What about the tri fuel option??

    I’m going to have a couple of temp gauges, 1/2NPT?? How high/or where should they be??

    How high should the Exhaust be in the side wall, or can it go out the top??

    Can I put the fire box directly underneath the smoker box, if the smoke goes into the side of the smoke box?

    Will the door work like this?? Is it too big and will let too much heat out?

    I think the charcoal Grill idea will work, with (2) 2.25” or 2.5” inlets, plus I can add a blower. Will IT??

    I have more questions But this is a start

    I have drawings, but here are a few.

    Come on, let’s hear it. There is some talent and experience here and I want to hear every idea and criticism, thoughts and potential problems.

  2. adiochiro3

    adiochiro3 Master of the Pit OTBS Member

    Sounds complicated for a rig that has to fit in limited space!  I'll answer what I can:

    I plan to bend the boxes from one piece of steel and weld together, What gauge is too thin?  1/8" steel is generally thought to be the minimum for heat stability, but heck -- people smoke things in cardboard boxes, and you probably need this thing to be somewhat light weight for travel!

    Should I insulate? Is it worth the effort?  I would think you could insulate with an old blanket or sleeping bag, as needed in colder or windy weather; otherwise, don't worry about it.

    Will a smoker of this size work? Such a small volume of cooker and small box.  I think the size will work; people smoke great food in the little Weber Smokey Joes!  It might take some extra practice to learn how to manage the temps.

    Is the calculator accurate at this size?  Don't know.

    Will the removable pan/basket in the smoking chamber work??  I think that would work fine and is probably quite common.

    What about the tri fuel option??  Probably ambitious and unnecessary, but WTH?  My rig is set up for coal/hardwood & propane, so give it a whirl.

    I’m going to have a couple of temp gauges, 1/2NPT?? How high/or where should they be??  Temp gauges should be at grill level(s).

    How high should the Exhaust be in the side wall, or can it go out the top??  Exhaust pick up inside the unit should be an inch or 2 above grill level.  The exit point does not matter -- side, back, or top.

    Can I put the fire box directly underneath the smoker box, if the smoke goes into the side of the smoke box?  I would not, because the entire top of the fire box would act as a heat source for the cook chamber, making temp management troublesome, IMHO.  Besides, where are you gonna cook your beans & coffee????!!!!!

    Will the door work like this?? Is it too big and will let too much heat out?  The door will be fine as long as you don't peek too often.

    I think the charcoal Grill idea will work, with (2) 2.25” or 2.5” inlets, plus I can add a blower. Will IT??  I think it will work fine if you build intake vents into the bottom area of the cook chamber to provide good oxygenation to the coals for direct grilling.  I doubt you'll need a blower in this rig.

    Please document the build for us.  It may help other RVers in their plans and schemes!!!
    Last edited: Jun 2, 2011
  3. crashdavis

    crashdavis Fire Starter

    I like the idea and I think it would work.  My concern is that with a cook chamber that small at approx 17 inches wide, that's not a lot of reverse flow.    I think regarding your question about the firebox directly underneath...that may be a better option.  You could build as all one cabinet that would then have a smaller footprint.  This smaller footprint would allow you to insulate and still use way less space than were you to insulate it as a sidefirebox.  You could have a removeable ashpan and charcoal grate for grilling. 

    Of course, if a side firebox/reverseflow is what you want, then that is what you should build.  The stack can go out the top, but with a small smoker, having it go out the side allows for more smoke/heat travel before heading up.

    Funny...two chiropractors first to answer.  :)

  4. slimtony1974

    slimtony1974 Fire Starter

    Thanks for the responses, I think the box below, would make storing easier, but like adiochiro3 said what about the heat transfer from the fire box to the smoke box. Or will that added heat be helpful?? What if i left some air space between the 2?? And i have been thinking about making the Fire box removable and sized to fit into the smoke box??? Smaller for storage??

    Keep the ideas coming. I'm going to start mocking it all up in card board before i cut or bend anything. Did i mention the steel guy is 800 miles away!!! I'm hoping my Dad's friend will do the bending. While not ideal, it will keep my dad and i close.

    Also i was thinking, most is going to be bent, may i should make the chimney Square?? I need to re calculate!!

    Last edited: Jun 12, 2011
  5. crashdavis

    crashdavis Fire Starter

    I think that a heat diffusing plate on the bottom with a small, hot fire controlled with a ball valve like a UDS would control the heat nicely into the cook chamber.  You will need to make sure you're able to seal it up nice and tight to prevent air leaks.  You could certainly make the firebox small enough to slide into the cook chamber for storage.  If you insulate the whole thing, you wouldnt need the legs.  You could shorten them up to a couple of inches and set it on a table.  With a couple of inches of insulation, I doubt the outside would get much hotter than the ambient air temp.  There is no reason the chimney couldnt be square.  It just needs to have the right volume for correct draw.  It would be a cool little build. 

  6. slimtony1974

    slimtony1974 Fire Starter

    I am going to use a ball valve on the intake, i saw a few Stump Smokers and like the idea!!!  I think i am going to insulate, I like the idea of no legs and sit on a table or a bench of a picnic table!!

    How can i seal the fire box to the smoke box and have it removable and still keep a good tight seal??

    Studs on the fire box, holes in the bottom of the smoke box, a small short "chimney" on the fire box with high temp gasket and maybe a thicken/reinforced bottom to the smoke chamber, maybe a few wing nuts....

    Thanks!! Keep the ideas coming..

  7. crashdavis

    crashdavis Fire Starter

    I think you just need one open box. There wouldnt seem to be a need to seal the firebox off from the smoke chamber.  A UDS is all in the same box, and my understanding is that they are very efficient.  You just need to be able to seal the main door to the rest of it with a gasket and some latches. 
  8. crashdavis

    crashdavis Fire Starter

    Of course, as small as this is going to be, maybe it would be a good idea to have a separate fire area with some air inlets between the two.  have a small access door for the fire on the front with gasket, and your flip top door as you had drawn previously.  Just because there wouldnt be a tremendous amount of space between your heat source and your food, like there is in a UDS.  But it still does not need to be air tight between the two.  Since you are losing the legs, you could certainly extend the total height of the box to like 30-34 inches.  You could still  have a 20 inch cook chamber and 10 inch firebox area.  I would think primarily charcoal fired.  Slip a tray over the diffuser plate, another ball valve for air inlet and some charcoal grate and there is your grill.  Just a small access door for the fire on the front with gasket, and your flip top door as you had drawn previously. 

    There was an awesome build here that I'll try to find.  Essentially a mini version of it would be perfect.

  9. crashdavis

    crashdavis Fire Starter

  10. slimtony1974

    slimtony1974 Fire Starter

    Thanks Darin, now i may have to go back to the drawing board!!! Can you sketch up what you are describing??

  11. crashdavis

    crashdavis Fire Starter

    I've not had any real success with sketch-up.  I guess essentially what I'm describing is a lid that opens like yours in the sketch, so that you have access/ability to use as a charcoal gril with an insulated box type set up in his small build. hopefully my paint picture will attach.  In this I was thinking the blue-ish part could be the firebox and door.  The upper door would allow full depth shelves and access like a grill.  I think a diffuser plate that was open in the middle with a cap on it  (like a rain cap on a roof jack) would allow the heat to move up and around and keep grease from the fire.  The plate could slope to allow a grease drain.  Drop in an ash pan and charcoal grate and you could grill. 

  12. slimtony1974

    slimtony1974 Fire Starter

    So upon further review, some more research, a few pics from BBQ Engineer and some input from Darrin, I think a simple vertical Smoker is the best idea. It will be a more compact unit and make for easy storage!! It will be about 30-32" tall,  still 13"x16", probably insulated,  adjustable fire box shelves, with water pan, a move able charcoal basket/shelf, for smoking and grilling, a larger hole on the side for my Gas turkey burner, a lower door with 2 pipes with valves (2"dia), a space then the upper door, same as my drawing. I am unsure of the size of the chimney size, I'm guessing close to the reverse flow 72ci or 2"x4" x about 10" tall.

    Where do i locate the outlet?? 1-2" above the lowest grate or towards the top?? Where is the chimney height measured from?? Outlet to top?? My total cooking Height is about 13" tall, so if i install 3" above the grate, it will be flush with the top. Is this OK, or  does it need to extent above a bit or the full 10"??

    Drawings to follow!!

    Ideas, suggestions??

    I guess i need to move this post. Can i do that??


  13. slimtony1974

    slimtony1974 Fire Starter


    Movable Charcoal basket, show on bottom, but can be moved up for smoke pan for the gas turkey fryer burner, or up further for grilling, the white rectangle box is the water pan, or heat sink, or sand box!! I think the lower unit will have shelf options every 3" off the bottom working up, and a 3" blank space between the fire box and smoke box, lots of options.

    2 valves on the door!!

    Well, what do you think??

  14. crashdavis

    crashdavis Fire Starter

    I think I would put the chimney on the top mainly because the heat is going to want to rise and then take the path of least resistance out of the cooker.  Your heat may make it to the level of the chimney and make a run for it.  I dont think you'll need much more than a few inch long chimney 3 inches in diam or so, though I have no empirical data to support the length or diam, it just feels like it would be around that size.  If you look at the Stumps style cookers, their stacks arent very long.  I like the design though.  Looks very versatile.  Gasket the doors to seal them all tight.  The insulation will make it very efficient, and being so small it may only take a coffee can of charcoal to fire it forever. 
    Last edited: Jun 7, 2011
  15. slimtony1974

    slimtony1974 Fire Starter

    Well a few more measurements, a bunch of drawings, and some cardboard, I'm starting to build a card board mock up!!! Pics to follow!!
  16. slimtony1974

    slimtony1974 Fire Starter

    No Pics of the cutouts, i used some old cardboard and it was not flat and after some tape it was not straight enough to use. I did help out to see the actual size, it is small, but will fit perfectly for storage!!!
  17. slimtony1974

    slimtony1974 Fire Starter

    Well good news and bad news!! I got all of the detail sketches made, i making the door top smaller than the box top to allow for my rear chimney. I think the inlet for the chimney is going to be 3-4" above the main cooking surface.

    Bad, news, my 60 year old dad had a Stroke on Sunday and is in the hospital. He is doing OK and has PT, OC and Speech therapy. He should get out in a few weeks, then a few more to fully recover!!! I'm glad he is doing better but the project is now on hold till he can get out on his own!!!

    Oh well time to work on some nice drawings and keep smokin' with what i've got!!!

  18. fpnmf

    fpnmf Smoking Guru OTBS Member

    Sorry to hear the news..

    Looks good tho if full recovery is attained in weeks.

    I went thru PT ST OT for 7 months after my brain injury. ( Physical,Speech,Occupational Therapy)

    Mine was a TBI  (traumatic brain injury) Stroke is an ABI (acquired brain injury).

     The therapists are sadists. All of them.

    Good luck!!

  19. crashdavis

    crashdavis Fire Starter

    Sorry to hear about your dad.  Wishing him a speedy and complete recovery. 


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