Center Feed Double Reverse Flow Smoker Build

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Newbie
Good day all. Need some assistance with FB/CC throat measurements of a Center Feed Reverse Flow Smoker design that I’m busy with. I’m using 2 donated tanks for the CC and FB so I’m working with what I have. The CC dimensions are 16.14in (dia) x 41in (L) = 36.31gal and the FB is 18.9in (dia) x 23.62in (L) = 28.68gal. I know the FB tank is a bit over sized but I’m looking to make this work with what I have.

The design I’m going with is a directly below center feed FB with the FB being perpendicular(cross and centered) directly below to the CC. The CC will sit directly on top of the FB with no spaces in between, only the throat openings in both tanks. The door of the CC and FB will be facing the same direction.

My FB throat opening to the CC according to online smoker calculation websites should be in the range between 7in to 8.22in dia round hole. I’ll be using a heat deflector plate in the bottom of the CC with openings at each end of the CC to create the reverse flow. A 4in chimney stack pipe will be center on the side of the CC (opposite the door) set at main grid height.

The issue I’m having is with the FB to CC opening/throat size. Will a 7in hole be enough to let the right amount of heat and smoke through to the CC? Seeing that the design is a center feed, I don’t want to go too big with the throat size fearing that I might get too much heat in the CC.

I could not get any info on calculations that covers center feed smokers. All of the calculations seem to address offset FB designs?

I've followed the calculations on this thread but it is also based on a standard reverse flow design

[email protected]

Newbie
Exhaust Calculation..

ESV in cubic inches_____________________________ ... = Stack Length in inches (36" +/-)
0.7854 X Stack Diameter X Stack Diameter

Adjust the diameter of the stack, until the proper length is achieved... be sure to measure the actual internal diameter of the pipe used..
this is for round stacks only..
Maybe my understanding of the Exhaust Calculation is wrong? It seems like the called for length is +-36" by adjusting the diameter in the formula. I get a 2.65"ø @ 36.5" calculation? I've also read a thread on here that as a rule of thumb, the stack should not be much longer than the diameter of the CC (top of CC of course). If I go with this, the 5"ø pipe @ 18.25" will be the winner. My CC is 16.14"ø.

After looking at the drawings.. I don't see where upper vents (in the FB) would be needed as the heat from the fire will be going directly into the CC ... I would think two lower vents (1 each end) would be the better choice for this design...
As for the upper air inlet, I'll go with your advise of rather excluding this for now and including the additional lower air inlet. My FB has an existing 4"ø "flanged" hole on the body (as utilized in drawing as UAI). I'll just modify this as the FB > CC opening of 6.7"ø.

[email protected]

Newbie
View attachment 392717 View attachment 392716
Some screenshots from my drawing design. You'll note I've included the 3 types of exhaust stacks as discussed previously. Note that CC size is slightly larger than noted on drawing. Gallon size is actually 39.6258 (150L) (I have to convert everything from metric to fit pit calculator...)

[email protected]

Newbie
Some design drawings of the CFRF build. Had some questions regarding the stack diameter and length. 3 options shown on drawings. Might just go with the 5" or the 4". Still waiting 100% confirmation from forum wizards.

daveomak

Epic Pitmaster
OTBS Member
SMF Premier Member
Maybe my understanding of the Exhaust Calculation is wrong? It seems like the called for length is +-36" by adjusting the diameter in the formula. I get a 2.65"ø @ 36.5" calculation? I've also read a thread on here that as a rule of thumb, the stack should not be much longer than the diameter of the CC
The exhaust stack height is specifically intended to control draft of the smoker....

After looking at the drawings.. I don't see where upper vents (in the FB) would be needed as the heat from the fire will be going directly into the CC ... I would think two lower vents (1 each end) would be the better choice for this design...

The air inlets to the FB are designed for a specific reason..... the lower air inlet is designed to add air to the fire... The upper air inlet is designed to move air from the FB to the cook chamber without adding air to the fire.... It reduces heat build in the firebox and reduce fuel consumption...

May I suggest you follow the tutorial... It has been proven, by many members on this forum, to produces a very good smoker....

..

dcrumrine

Newbie
The exhaust stack height is specifically intended to control draft of the smoker....

After looking at the drawings.. I don't see where upper vents (in the FB) would be needed as the heat from the fire will be going directly into the CC ... I would think two lower vents (1 each end) would be the better choice for this design...

The air inlets to the FB are designed for a specific reason..... the lower air inlet is designed to add air to the fire... The upper air inlet is designed to move air from the FB to the cook chamber without adding air to the fire.... It reduces heat build in the firebox and reduce fuel consumption...

May I suggest you follow the tutorial... It has been proven, by many members on this forum, to produces a very good smoker....

..
Actually if you look at the drawing there is a difusser plate in the CC. if you put a stack at each end the smoke will come out from under the difusser plate and right out the stacks, it would never reack the food. Building the smoker in the drawing as it is drawn out is the best way to build that smoker. Nevermind Dave i read your statement wrong. Youwas talking about the vents in the firebox. The last i knew you was talking about CC and took it as the CC you was talking about but after reading it 3 times i realized you went from talking about the CC straight to the firebox.

You do not need as big of a fire with the FB under the smoker as you do with an offset. You need to build them like you are building a UDS. Use a coal basket with chunks of wood with just enough airflow to keep the fire going. It is easier to choke out a fire than to not have enough air flow. so i would put like 6 in pipe with dampers in them on each end of the firebox with a 6 in exhaust. i would only put a critter cap on the stack cause you do not choke out a fire with the stack cause that causes bad smoke. You choke out a fire with the intake so if your smoker is getting too hot choke it out. Keep in mind on a UDS it has 3 1in pipes for intakes (if you build your own) once you get to temp you close 2 off completly and leave one about halfway open. A lot of people say these run too hot but they are trying to use them as a stick burner. This design can be very effecint and great smokers but you have to get out of the stick burner mentality. Like i have said before this is basicly a gaint UDS, use a coal basket and chunks of wood. Make you basket as long as the firebox and about 12 ins tall and probably about 16 inc wide, lite one end and let it burn the length of your smoker.
Welder/Fabricator for almost 30 years also a FF for 6 years. been through fire science and other classes to understand how fire breaths and lives

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dcrumrine

Newbie
Some design drawings of the CFRF build. Had some questions regarding the stack diameter and length. 3 options shown on drawings. Might just go with the 5" or the 4". Still waiting 100% confirmation from forum wizards.

View attachment 392774

View attachment 392775
i have been working on this same design for about a year. Not building but working on figuring it out and trying to make sure it is going to work and how to make it work without it getting too hot. This is a very efficent smoker design if you know how to use it and how it works.