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500 Gallon Reverse Flow heating issues

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
I am almost finished with a 500 gallon smoker build and I took it to competition last week. I smoked a brisket for over 20hrs. The smoker did great but we had one hell of a time maintaining temperature. I put a stoker fan on the smoker and designed hardware so I can set presets with my phone/watch and I can also monitor the chamber temp. I wish it were that easy. 

I think what I am experiencing is a smoke stack that is not big enough. It is 4.5" diameter and roughly 23" long. Per calculations online it would have to be 5.5ft long with that diameter. 

Does anyone have any experience with this?

 
 

bigeee

Newbie
28
13
Joined Oct 2, 2015
Josh, please provide all the dimensions for the tank,   I.D. & length, fire box L, W, H &

the opening between the fire box and tank size.

It looks like maybe the fire box is a bit small................

I get min 1 1/2 hours of constant temp between loading more wood with my reverse flow smoker.
 

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
With the end I cut off of the tank I am at about 420 gallons (guessing approx).

The firebox is 24x30x30. above it is a warmer that is completely partitioned off. (I will double check those dimensions tonight)

The air inlet opening on the firebox isn't really a factor imho because I have a 20cfm stoker fan hooked up that adjusts temp.

Opening from firebox to cook chamber is a half moon, roughly 19" tall and 34" wide. 

Tank is roughly 100" long. The baffle separating the bottom and top of the cook chamber is 88" long.

From the baffle in the smoke chamber to the bottom of the smoke chamber is 19"

The smoke chamber is 37" in diameter. 

The firebox IS mounted a bit high, I do know that. Not 100% sure how much it will factor.

I am getting a pretty consistent temperature with my stoker fan, maybe an hr and a half, but that isn't much consolation when I'm smoking a brisket for 20hrs.

The current stack is 4.5" in diameter and 23" tall.

Here are the dimensions I punched into Feldon's bbq calc 

http://feldoncentral.com/bbqcalcula....50&fi=0,0,6,64.80,2.29&fc=14.84,20.98,172.80

Thanks,
 

bigeee

Newbie
28
13
Joined Oct 2, 2015
Josh, I plugged in the numbers you gave and here is what I found.

Fire box = 68% ............ yours is too small.   I like to be between 130% & 150%, too big is better.

4.5" pipe at 23"      It should be 77" from top of smoker to tip.  way too short.

The fire box to chamber opening 19" tall x 34" wide is roughly 460 sq inches,

It's over double what it should be at 195 sq inches.
 

johnman6996

Fire Starter
34
16
Joined May 9, 2015
The firebox being too small would really only lead to having to feed more fuel into it.  The small chimney, size of the fire inlet to the chamber, and the overall placement of the box isn't helping.

The top of the box should be as flush to the top of the opening to the chamber as you can get.  That way no heat is collecting at the  top of the box waiting to go into the chamber (or just plain not going in) and wasting fuel.  Your design has what looks to be about 10" above the top of the smoking chamber itself.  Where is the entrance to the smoking chamber?  Adding a fan to the firebox opening wouldn't really help as much as it could because of the placement. 

If you make a bigger firebox you will also have to make a longer/bigger chimney.  With your current firebox and a 10" chimney you can get away with it only being 17" long.  If you go to a 100% on the firebox (according to the amazing Feldon's Calculator) it would be over 2 feet long.  I went with a rectangular chimney because the larger 10" pipes were too expensive.  If you do go to a square/rectangular chimney add 10-20% to the length because of something to do with physics and friction (Thanks to Pete Mazz for that bit of info) .

It looks like a good smoker.  Just reposition the firebox -maybe make it bigger if you don't want to be dumping fuel into it- and get a bigger chimney... then bring me some ribs.... and beer.

Oh, show us some pics of the inside.

I'm not an expert by any stretch of the imagination.  I sell computers for a living, but BBQ as a passion.  If someone else has better reasoning go with them.  I've only built one smoker and I'm not done with it yet.
 

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
I am of the personal opinion that firebox size has zero bearing on chimney size. The chamber size and opening however, will. I agree the box is mounted too high. I got a 6" stack and am closing off a portion of the opening from the firebox to the smoke chamber. I think it will create a better flow.

Thanks for the input. I think some experimenting may correct my problem at least partially without too much effort.



The firebox being too small would really only lead to having to feed more fuel into it.  The small chimney, size of the fire inlet to the chamber, and the overall placement of the box isn't helping.

The top of the box should be as flush to the top of the opening to the chamber as you can get.  That way no heat is collecting at the  top of the box waiting to go into the chamber (or just plain not going in) and wasting fuel.  Your design has what looks to be about 10" above the top of the smoking chamber itself.  Where is the entrance to the smoking chamber?  Adding a fan to the firebox opening wouldn't really help as much as it could because of the placement. 

If you make a bigger firebox you will also have to make a longer/bigger chimney.  With your current firebox and a 10" chimney you can get away with it only being 17" long.  If you go to a 100% on the firebox (according to the amazing Feldon's Calculator) it would be over 2 feet long.  I went with a rectangular chimney because the larger 10" pipes were too expensive.  If you do go to a square/rectangular chimney add 10-20% to the length because of something to do with physics and friction (Thanks to Pete Mazz for that bit of info) .

It looks like a good smoker.  Just reposition the firebox -maybe make it bigger if you don't want to be dumping fuel into it- and get a bigger chimney... then bring me some ribs.... and beer.

Oh, show us some pics of the inside.

I'm not an expert by any stretch of the imagination.  I sell computers for a living, but BBQ as a passion.  If someone else has better reasoning go with them.  I've only built one smoker and I'm not done with it yet.
 

johnman6996

Fire Starter
34
16
Joined May 9, 2015
Can you post some pics of the inside of the smoke chamber and the firebox?  I want to do a propane tank smoker at some point and love to look at other builds to give me some ideas.
 

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
My goal is to rent it out and i threw together a site which is incomplete but it has some pictures on it. www.bulldogsmokers.com

I have some still to add. I will take some and post them here in a bit, need to finish up some work first.
 

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
I think you're right about heat collecting at the top. I am working out a solution without having to cut the firebox off. 

I used an arduino with a high temp probe, LCD screen, and a transistor to kick the 20cfm fan on and off. The fan just stokes the fire so it doesn't actually disperse it through the smoker. That is what the stack and firebox opening facilitate.

Raspberri Pi would be a great way to go as well. I don't have as much experience with them but they look super easy to interface with.
 
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johnman6996

Fire Starter
34
16
Joined May 9, 2015
That looks slick.  The pi version I've seen leaves everything up to the blower.  I linked it to the spot where he shows the firebox being sealed and the HeaterMeter pi doing all the air flow:


Here is a link to the HeaterMeter itself:

https://www.raspberrypi.org/forums/viewtopic.php?f=37&t=18142 

Here is the Pitmaster pi

https://github.com/justindean/PitmasterPi

I'm not a huge coder.  I sell PCs for a living.  I'm going to let my son put it together and get it up and running for me (he is more into robotics than I am).  Just worried about the airflow in the pi design.  I like the idea of being able to go to sleep for 8 hours and being able to come back and just add more fuel.  It looks to me that the HeaterMeter or the PitMasterpi would need to be sealed off and just let the blower motor do its thing.  That would lead to one less variable with the air intake having to be adjusted.

From what I'm seeing in your design, if the blower is running and the air intake is open can't it force the heat and smoke out of the air intake?  Having it as a stoker where you do it could still force the heat and smoke out the intake.  Never seeing it in use I don't know.  Have you run into that?

Any idea where to start looking for a decommissioned tank?  I can't find them at the scrap yards and have no idea where to look.

Thanks again man!  Let us know how adjusting the chimney helps!
 

johnman6996

Fire Starter
34
16
Joined May 9, 2015
I just looked through your pics again.  How big is the space from the baffle to the rest of the chamber?  I can't see it.
 

joshbennett04

Newbie
15
11
Joined Oct 21, 2016
I dont use the baffle when i am using the fan. Call any place that refills large propane tanks. They have to inspect when they fill, at least every so often. If they find issues they will take the old tank and charge the home owner for a new tank.
 

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