Intake and exhaust sizes on electric smokers

Discussion in 'Electric Smokers' started by clearprop, Oct 11, 2015.

  1. clearprop

    clearprop Fire Starter

    I'm trying to figure out the correct size for intake and exhaust sizes for a 17x21x38" smoker. There will be a box under the unit for a AMNPS (mailbox size or smaller) I was planning on using 3 separate 3/4 holes on that on its front side near the bottom. Then using a 2" pipe about 4inches long straight out the top of it into the bottom of the smoker. Using a 1500 watt Brinkmann element, then a 2" pipe out the back of the smoker near the top. Will 2" pipe work or should I go 3"?
    Last edited: Oct 11, 2015
  2. dward51

    dward51 Master of the Pit OTBS Member

    I would start with 2" as you can always make the opening bigger to go 3" later.  Also make sure to get a mailbox that has metal that a magnet will stick to.  That way, you can use a flat magnet strip to act as a damper to close down part of the 3 intake holes on the mailbox to fine tune the burn and air flow (if it sucks too much air to fast, your pellets will burn way faster than expected.  It's like blowing on them constantly).
  3. clearprop

    clearprop Fire Starter

    The 3/4" holes in the AMNPS compartment will have some sort of adjustment o them.

    There is going to be 3-4" of rock wool insulation  and an outer shell. 2" intake and exhaust stack will be welded to on part and red rtv sealed to the other. It is being made to take back apart and in theory I can cut out the 2" and replace with 3"... but I don't want to have to.... 

    A  3" hole on an electric smoker SEEMS like it would allow too much heat to escape. (or won't it matter?)
  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Last edited: Oct 12, 2015
  5. dward51

    dward51 Master of the Pit OTBS Member

    Look at the MES.  It has a large top vent hole, but then has a rotating damper plate that restricts most of that opening (and can be totally closed).  With convection heating, you will get the stack effect and it will naturally draw upwards no matter what size the opening.  The opening is mainly for airflow for the smoke source as meat will cook in a closed heated environment (just like in a home oven). I think 2" is plenty for the size box you are describing, but this is an opinion and not based on any mathematical formula.

    Edit - looks like Dave and I posted simultaneously. 
    Last edited: Oct 12, 2015
  6. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    I reread the first post.... 2"... will be plenty big enough.. Just be sure to have the exhaust pipe extend to the center of the smoke chamber so you will have even flow to all areas... It should still extend out the side wall to prevent condensate drips on the food....

  7. clearprop

    clearprop Fire Starter

    the 3" idea Is gone... but now 2 vs 1.5"

    I agree the 2" will be plenty (I already have some in stainless)

    But will 1&1/2" be best? I can get that in stainless for cheap too....

    ALSO.... I was planning on going straight out the back of this unit (see pic option "A"). It sounds like you are suggesting to go out the center of the top with a 90 elbow and then straight out the back (see pic option "B")?

  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Elbows in the exhaust will cause friction loss in exhaust gasses.... think about a straight exhaust... or a stub exhaust...

  9. Great questions and answers. The Dave's are giving you first class answers. The dimensions of your proposed box are not that different than a MES  30 or 40. If you look at the exhaust damper as Dave said it is 2" but with it's construction it is a bit small. Other smokers from other manufactures are even smaller.This  is the amount of air it will exhaust in the full open position. The hole size is 3 inch but not nearly not capable of moving that volume of air.

    If you exhaust right out the top center it would be the most efficient as far as heat distribution . As far as exhaust dampers go the choice is yours. Dave provided you with a lot of choices. I went with a very conventional stack with a damper installed. The cost was great at 15.00 or there abouts.

    Unless you have a free source for the rock wool you might think about 2" fiberglass board. That would reduce the overall size and still have great insulation values. Rock wool is used alot in high heat situations you do not have that at 300 degrees.  Just a thought Do your home work and you will make good choices   Jted
  10. clearprop

    clearprop Fire Starter

    I already have rock wool and the inner and outer shells are already made. I'm just trying to figure out this exhaust before I punch the holes and weld it up.

    Good to know about the elbows. Wouldn't a vertical stack straight out the top cause too much heat loss too fast for electric as opposed to a horizontal stack out the back?
    Last edited: Oct 13, 2015
  11. dward51

    dward51 Master of the Pit OTBS Member

    That's why you want some method of controlling the air flow in/out of the chamber.  It would be better to have a tallish stack with a damper than a short stack with no-control.  You can always dial the damper down to slow the flow.  And if you find your flow is not enough, just make the stack taller.  That enhances the chimney effect.

    Also I like Dave's illustration of a pipe out the back, with the pickup inside the smoker at the center.  That looks to me to be the best option for what you described you were looking for.  Just run that into a T connection so any condensation will fall into the dead end of the T and then add your stack & damper above.  In essence it's the same end result as your "B" illustration, but without the elbow inside the smoker.
  12. clearprop

    clearprop Fire Starter

    I can run the exhaust any which way. I'm just trying to figure out the "best" way. It's beginning to seem it doesn't make any difference out the back or out the top. Is this true?
  13. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Out the top "may" cause condensation to drip on your food.... that's the drawback there....
  14. clearprop

    clearprop Fire Starter

    Would a "drip tray" hanging down an inch from the stack opening work to catch and eliminate the condensation problem?

    I have read about the condensation dripping on food in other posts, but I haven't seen anyone post a solution.
  15. bearcarver

    bearcarver Smoking Guru OTBS Member

    My Son used to have a Cook Shack Smoker, and it had one small hole in the top for a Vent.

    It was right in the middle of the top, and it used to drip disgusting stuff on the meat, so he hung a small tin can under it to catch the drippings.

  16. dward51

    dward51 Master of the Pit OTBS Member

    I like DaveOmak's rear exit version with the end of the pickup centered in the smoke chamber.  You can put a T connection at the back of the smoker and any condensation as it climbs the chimney will fall down through that T to a container you can hang there.  Easy to check and empty and it keeps it 100% off the food.
  17. clearprop

    clearprop Fire Starter

    Alright, I'll be running it from the center of the top straight out the back.
    Last edited: Oct 16, 2015
  18. clearprop

    clearprop Fire Starter

    Changed the plans. I  would like to use 3" exhaust (I think?) so that I have enough airflow for cold smoking and I can just use a 2" insert If i need to reduce the pipe for heat retention,

    I scratched the Brinkmann 120v 1500w element and I am now using a 240v 2600w oven element instead so getting up to temp or even cooking a pizza at 425 shouldn't be an issue.

    I noticed how another member used a "HEAT / SMOKE DIFFUSER" below a 4" straight up exhaust (he's pointing to it in the still shot). I haven't seen this used in the fridge builds before. Granted he used a pellet burning insert for heat, but is this something I SHOULD consider for my electric fridge build? 

    Last edited: Mar 21, 2017
  19. clearprop

    clearprop Fire Starter

    Ended up doing a combination of everything. 3"OD 14ga aluminized exhaust pipe. It has a long swooping curve, the bottom 1" has no bend/elbow. It has a slight lip near the opening, and angles back slightly so any built up creosote condensation should not drip onto the food. I driled my rear hole the the outer fridge casing about 3/16" too high because I originally thought the exhaust tube would be level....and the hole saw took out about 3/32" more than I wanted it too when I cut out the cresent from the inner piece. Nothing some steel and a welder can't fix.

    Last edited: Mar 28, 2017

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