Stack Size for custom pellet build

[JBCS]

I have a 24x42 smoker with a 4" od x 27" tall stack. I am having issue with back pressure causing fire to be pulled back into my hopper when combustion fan is at max. All the calculators out there assume you are using a traditional offset. Any input would be helpful. This a complete custom pellet system not using any pre-assembled rigs that are on the market.

[Big T]

You should probably PM Frank or Tom to get the info that you're looking for if one of them don't respond to this post in a day or so.

[JBCS]

to note: the combustion fan is 120cfm

[Frank_Cox]

Howdy!
can you post pics of the hopper assembly? actually we need to know the rpm of the auger shaft, size of the auger, estimated BTU capacity of the hopper system, what sort of burn pot you are using, how the airflow enters into the burnpot, etc.
I have built many pellet smokers both from my own homemade/ modded out hoppers and also from the SmokeDaddy system. I always shot for a 4" stack on cabinets but I always wound up installing a damper and closing it mostly off.
Honestly, I don't think your issue is with the stack, I think it is an issue with the auger system.

[The Czar]

Dang somebody ate their wheaties this morning!!


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[Frank_Cox]

Dang somebody ate their wheaties this morning!!


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hey man, I am almost caught up!!! less than 1 page of unreads!

[JBCS]

Thanks Frank! Here are some pics and requested data:
www.jbowen.com/smoker.png

3rpm auger motor
11" auger 1.5" Dia
not sure how to calculate the btu
see above link for pics.

Appreciate you!

[Frank_Cox]

so that is one heck of a nice pellet smoker!
The RPM on the auger is a little bit high I think for that size auger assuming the pitch is the same as the most common auger flights that diameter. if the auger is the same pitch as I think it is, and the diameter is 1-1/2" then the BTU's on a 2 rpm motor is 36,000 BTU's. if you can collect the pellets as they are dispensed for exactly 10 seconds or so and weigh them you can calculate the amount of btu capacity of your system. average cooking pellets are approx. 8500 btu's per pound. I have made cookers that are 48x48x48 with insulated cook chamber (rotisserie) and used a single 36,000 btu hopper. I think one that size would be close for yours.
the cause of the burn back on the auger tho, I think is caused by the end of the auger being above and too close to the burn pot.
here's the first one I butchered together...
viewtopic.php?f=108&t=1175

here's the part of the same thread where I made a ramp to get the end of the auger far enough away to prevent burn back. this cooker is still in use by the owner today...
viewtopic.php?f=108&t=1175#p15816
Hope that helps!!!

[JBCS]

Thanks Frank! As you mentioned the stack is fine and flow is great... the issue is the rate of pellets was causing too large of a fire (flame up) in the pot. I reduced the rate % and resolved the issue. It will get a small amount of smoke in the hopper on higher temps about 400f which makes since as you are going to have more flame/combustion to reach the higher temps. I also backed the auger tube out a 1/2".

Once I complete my raspberry Pi build I will have greater control over the overall output of the pellet mechanism.

Thanks again for your valued input! -Jay

[Frank_Cox]

Good deal! Would be interesting to see some of your work on the controller!

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[JBCS]

Here is the UI protoype: https://xd.adobe.com/view/70db2270-1789 ... b212fdbee/ (click on buttons)

I have a touchscreen now but will most likely port to mobile application once I get it configured and stable.