# New Guy and this will be my 1st Reverse Flow Smoker Build

Discussion in 'Reverse Flow' started by imafishingfred, Oct 17, 2014.

1. ### imafishingfredSmoke Blower

Well, I'm at a standstill with my calculations. On Feldon's Calculator for propane tanks it wants to know a gallon size. My tank is 30 X 87 and on this site ( http://www.smokerplans.net/index.php?main_page=index&cPath=1_3&sort=20a&alpha_filter_id=50 ) it tells me I have a 200 gallon. The 250 is 30 X 100. I find it hard to believe that 13" X 30"Dia. would be the difference in a 200 and a 250. Any suggestions on how I should proceed with my calculations? Is there a way to find out if Feldon's used a 30 X 87 for his 200 gallon propane tank calculations? Any other way for me to find out exactly what I have?

2. ### phat ricoNewbie

If you calculate it then it makes sense.  Volume= pi*r^2*l so 1" of added length on a 30" dia tank is 706.85 cubic inches or about 3.05 gallons. That adds up to 39.65 gallons over 13 inches.  The Feldon's calculator wants a volume.  If your tank has a tag you may see the volume on there. You can also search the web for propane tank sizes.  From what I have seen they are pretty standard.

If that still doesn't make you happy you can get a 1 gallon jug of water and fill the tank one jug at a time.  Don't lose count! It would suck to start over!

So I would plug 200 Gallons into the calculator.  Are you buying the plans on the website?

3. ### imafishingfredSmoke Blower

Thanks for the reply. Guess I'll treat it as a 200 gallon and get my calculations.

No, I don't plan to buy any plans from the web site. I plan to get as much info as I can from the forum. I'd rather get 1st hand advice on what does and does not work.

5. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

That tank is 250 gallons.... 30 x 30 x 0.7854 x 82" (length adjusted for round ends) / 231 (cubic inches in one gallon) = ~ 250 gallons... Dave

6. ### imafishingfredSmoke Blower

So, thanks to DaveOmak  and Phat Rico  for making me think about this before I jumped in over my head, and after plugging the 3 possible sizes of my tank (30 X 87, 250 gal, and 200 gal) into Feldons, I've decided based on the figures that my tank is a 250 gallon and not a 200. I've been told to use 30 X 87, but I know with the ends being spherical that would not be a true volume. So now my build will be a 250. It will also be a slow build (unless I take out a new mortgage or get 2 more jobs) after finding out what steel prices are.

My 1st step once I aquire the material will be to build my firebox. My plan as of now is to build it 27 X 27 X 27.

I'll be sure to take lots of pics along the way and ask lots of questions.

7. ### imafishingfredSmoke Blower

I've been reading about a lot of the builds while gathering info for my own build and keep seeing the same thing over and over about the Feldons Calculator being off if your building a reverse flow. Seems the Recommended Firebox-to-Cooker Opening Area needs to be 1.5% larger than the Recommended Firebox size.

So these are my calculations from Feldons,

So am I correct in going from 157.46 to 236.19 and does that also include the distance at the end of the cooking chamber shelf?

8. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Yep.... and also the area under the RF plate....

9. ### imafishingfredSmoke Blower

Where do I find the area under the RF plate. Would it be the same distance (even with the top of firebox) as my half moon cut out in the end of my firebox?

10. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Set the RF plate just above the opening in the FB.... That will give you enough area/volume and not restrict the flow...

11. ### imafishingfredSmoke Blower

While I'm on a roll with getting answers to my questions, my plan is a 27 X 27 X 27 firebox per Feldons. Is that assuming the wood is on the floor of the box or should I add to the box and keep those dimensions above the grate?

12. ### lendecaturalSmoking Fanatic

That is the minimum sized firebox for your cook chamber size. Feldon's is set up to yield data required for the cook chamber to flow correctly so you want the firebox numbers to be near 100% when you run your calculations. This will give you numbers for the openings to firebox, firebox inlets, and chimney size. You can build the firebox as big as you want but you will want to at least make it taller allowing for extra height above the floor for the grate and for ash to build up and make the width match with the half moon shaped cut you have to make in the cook chamber where the firebox meets.

13. ### imafishingfredSmoke Blower

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So these are my calculations with my firebox at 100%.

I multiply 153.99 X 1.5 = 230.985. This is the opening I need from the firebox to the CC and also the  opening at the other end of the CC where my reverse flow plate ends. Now, do I use my original calculations for numbers of air inlets, chimney length, and half-moon opening size or do I trick the calculator into thinking I have this huge tank and firebox to get the numbers for the 230.985?

These are my calculations based on a 27 X 27 X 27 firebox (area above the grate) I am building from.

Not sure I understand the half-moon opening size. If I make my firebox 27” wide why wouldn’t I make the half-moon cut out 27” diameter also? The calculator calls for a 20.02” diameter.

Am I missing something here?

14. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Fred, morning.... It's nice to have someone question the calculator, such as you did....

Keep the 100% calculations for all the regular stuff.... The FB/CC opening, under the RF plate and end of the RF plate are 150% of the normal..... The reason is, the original numbers caused a restriction in air flow and that caused an over heating condition in the FB.... that caused increased fuel consumption and in some cases warping in the FB when higher temps were trying to be achieved.....

A RF smoker uses natural/convection air flow and minor restrictions will impede that air/heat/smoke flow....

Not sure I understand the half-moon opening size. If I make my firebox 27” wide why wouldn’t I make the half-moon cut out 27” diameter also? The calculator calls for a 20.02” diameter. That's one more problem with the smoker calc.

http://www.1728.org/circsect.htm

Use the calculator above for the FB/CC opening..... calculate for the GREEN area.... Click on Radius and segment height ED button..... keep changing the Segment Height ED number until it ~equals the FB/CC opening area (segment area)..... Chord AB will be the width of the RF plate....

Last edited: Nov 5, 2014
15. ### imafishingfredSmoke Blower

Now I'm really confused DaveOmak. Is the Radius I'm putting into this calculator my tank (cc) radius or 1/2 my firebox width?

16. ### lendecaturalSmoking Fanatic

The radius is for the cook chamber, the segment height (ED) that you will be changing will be how high up the cook chamber you will want to make your cut. The Segment Area is the number you are looking for to equal your 230 sq-in with that ED number you are adjusting. Your Chord will be the width of the cut on the cook chamber and you can adjust your firebox width so it matches up with the chord width.

Hope this helps.

17. ### imafishingfredSmoke Blower

Thanks. I'll play with it and let you know.

18. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Len has you covered... Think of the circle on the right.... the one with the red and green in it, as the end of the CC... the green will be the cut out of the FB and the opening to the CC.... The AB line will be the RF plate...

19. ### imafishingfredSmoke Blower

So this circle calculator is telling me what width my firebox should be. I had planned on 27" but playing with the numbers I come up with 27.80". I'm only guessing at this point because I'm not sure what the tank thickness is. I plugged in 14.25" radius guessing the tank is .375" thick.

20. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

and you can adjust your firebox width so it matches up with the chord width....

One dimension tells you what the width of the RF plate is.... You want you FB to be at least as wide as the RF plate is... and maybe an inch or 2 wider....

Once you know the size of the tank you are using for the CC, everything else is dictated.... stack, FB, air inlets, openings, RF plate size and height..... that is if you want a smoker that is proven to work well...