- Apr 22, 2015
- 3
- 10
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.
matt