gas cooker exhaust

Discussion in 'Other Builds' started by mazatty, Jun 26, 2015.

  1. mazatty

    mazatty Newbie

    hello all, some friends and i built a trailer mounted gas fired pig roaster out of a 500 gallon propane tank. the cooking grate is 3-feet by 6-feet. it has two homemade 36-inch long 1-1/2-inch pipe burners theoretically putting out 42608 btu/hr each. i had the burners tested and they are running 96% efficient. i have a oven thermostat and millivolt gas valve for a pilot/safety and temperature control.

    the uprights for the counter weight are currently being used as exhaust. so i currently have two 2-5/8-inch, inside, square tubes for exhaust mounted at the top of the door.

    i've cooked a couple of pigs now. things went ok. the pigs came out great. my burners and pilot would go out after about 10-20 minutes or so. i was able to prop open the door about 1/2-inch or so at the bottom and get the burners to continue running. between pig roasts we added four 2-inch pipes to the bottom of the roaster. 

    one of the web pages about burners i found said you need 1-square-inch of exhaust for every 6000 btu/hr. so two burners at 42608 btu/hr would be 85216 btu/hr, divided by 6000 would be 14.2-sq-in of exhaust. now being at 10,000-ft above sea level we need more air, 30-35% according to some info i found on the internet. if we want to be able to cook at 12,000-ft we would need 35-40% more air. so on the high side 19.17-sq-in at 10,000-ft and 19.88-sq-in at 12,000-ft. so 19.88-sq-in, which would be a 5.04-inch diameter pipe.

    we also need fresh air equivalent to the exhaust. i'm estimating the holes for the burners to be about 8.5-sq-in each. we also have the four 2-inch pipes in the bottom, which would be 3.14-sq-in each. a total of 29.56-sq-in, so we should have enough fresh air.

    i've been thinking about adding a 6-inch exhaust stack with a damper that 90's up out the back side, right in the middle, below the grate. i've also thought about adding a long skinny door across the back below the grate. i would like to seal up the top of the cooker the best i can to help hold in heat. the long skinny door would be like a gas grill with the gap across the back of the lid.

    a stack seems easier. a door would be more work. i also think a stack would be better when smoking. i wasn't planning on sealing off the existing exhaust just not using them when cooking. 

    future plans include a larger heat sheild to hopefully eliminate flare ups, a smoke box for hot and cold smoke, and a thermoelectric ceiling/convection fan. first we need to get the burners to stay lit.

    i would appreciate some feedback on the exhaust. i'm leaning towards the stack, but i could be convinced to do something else. i was thinking i would make the stack 3 or 4-feet to get the smoke up over our heads. thank you.

  2. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    I'm thinking you need even heat / air flow inside the smoker for good results... Can you install 2 additional exhausts... one at each end of the Cook Chamber... I would extend them down to just above the meat level... I say that because I don't think you want the meat sitting in a stale air condition... Just a guess....

    Sounds as if you have done a good job on the research... Dave
  3. mazatty

    mazatty Newbie

    i thought about adding two stacks, one on each end. i was afraid that having the stacks above the burner holes would cause air flow just on the very ends. also, the counter weight extends over the ends of the cooker, and i don't really want to re-build the counter weight, but i can if necessary. i was hoping convection would take care of air movement in the cooker.


  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Stop the exhaust below the counter weight crossbar....
  5. Of all the gas cookers I have built the best was learning to make a good venturi. That is where the fresh air mix is critical and get the distance to the first hole right so the air has time to mix with the gas before it burns. I promise you that's your problem. Talk to Tejas Smokers in Texas They can calculate it for you and sell you a great venturi and tell you the right size pipe to use and what size holes as well as orientation. They are great . The other key is like you said getting the exhaust right. I don't want to be out of line but you are wasting your time with fresh air on the bottom it has to be mixed in the pipe. Also you may want to go to a smaller pipe like 1 1/4 instead
  6. This one has been running trouble free for almost 14 years now with the hatch closed and only two 4 x 2 fresh air vents in the back


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