# Is Feldon's Calculator really flawed?

#### keny

##### Fire Starter
I've been searching all over this forum, and my eyes are getting tired...I've seen several posts stating that Feldon's Calculator is flawed. So, I just want to know before I begin my build. I have an old propane tank that is 36" I.D. and it was about 8' long, and 1/2" thick. So we cut it in half to make two smokers out of it for my brother and I. So my half is about 45" to the weld seam where the round end cap is welded on, and the end cap is about 12" tall (with my calculations, the end cap amounts to around 4 cubic feet ??) So the straight section that's 45" long amounts to 45,781 cu. inches, or around 26.5 cu. feet. With the end cap added, I think I'll be around 52,693 cu. inches or about 30.5 cu. feet. I'm not 100% sure of my calculations, but I think it's close. Any info about my cooking chamber opening size (circle segment height), firebox size etc. would be greatly appreciated! The calculator says around 24" x 24" x 32" for the firebox ??And around 32" of 6" pipe for the chimney. Also, what are everyone's thoughts about the chimney being down close to the cooking surface? How close? Should the end at the grate surface be cut at an angle? Does that make a difference? How about the height of the cooking surface from the RF plate? Any hard and fast rules on that? Like I said, I'm sure this stuff has been covered, but I'm just not finding the posts I need...I haven't really started yet, just wanted some thoughts! Thank you all for your time!

#### lendecatural

##### Smoking Fanatic
Taking your numbers for area, it equates to a cylinder of 36" diameter and a length of about 51". Your fire box size is on the money and I would recommend using the 32" dimension as the cook chamber width end. Make it as tall as you want, the Ribwizard has some clever designs on firebox floor and ash dump. The segment height will need to be 9 3/4" since Feldons gives a 147 sq-in recommended size. 147 x 1.5= ~220 and 9 3/4" will result in 223 sq-in opening and area under the RF plate. Here's your Feldons link.

http://feldoncentral.com/bbqcalcula....61&fi=6,2,0,55.30,4.61&fc=13.71,19.38,147.46

If you plug in the radius of 18" and segment height ED of 9 3/4" into the circle calculator, you get the 223 sq-in. It also shows the segment chord AB as 32" to match up to the width of your fire box. http://www.1728.org/circsect.htm

If you cut your door at 9 o'clock and 12 o'clock and install your grate at the 9 o'clock, you will have 8" above a 1/4" RF plate since you are using 9 3/4" from the bottom of the tank. That's a good distance.

Your tank is big enough for two grate levels so you may want to put the stack down 3 to 4 inches into the cook chamber, the top grate will run a little hotter than the lower grate but you can use that to your advantage once you experiment and learn the differences.

Hope this gets you off and running and feel free to ask questions, this is a great forum with lots of guys willing to lend a helping hand.

Len

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#### keny

##### Fire Starter
Excellent! Thank you so much for all that fantastic information! I'm really looking forward to getting this thing going. What I need now is some time...I will try to find some of Ribwizard's firebox ideas-that sounds great! And I assume the chimney height that the calculator shows is always the height from the TOP of the tank. Is that correct? Thanks again!

#### keny

##### Fire Starter
Excellent thread-thanks again! Soon as I get going on this, I'll start posting pics...

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Len has you covered...... Thumbs Up

#### bob1961

##### Smoking Fanatic
so is there any truth to that calc being flawed ???....

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
so is there any truth to that calc being flawed ???....

Len explained the corrections to make......

#### bob1961

##### Smoking Fanatic
so there were no issues then with it Thumbs Up ....

#### lendecatural

##### Smoking Fanatic
There are two minor issues with the calculator. First, if you put in a firebox bigger than what is required for the cook chamber, it will drive all the other values higher so you need to first size the firebox to 100% and then use the opening number and a factor of 1.5. This is what I posted earlier on another thread.

The more I've looked into this issue, it appears the problem is "one calculator does not fit all". If you notice on the bottom of the Feldon's page, there are two smoker configurations. A simple offset smoker and a reverse flow. I think the numbers are fine for a standard offset; however, for a reverse flow the smoke/heat has to turn 180 degrees and exit from the firebox end. This doubles the air friction in the smoker and using the same size opening doesn't draft as well as may be needed. That's the main reason I see the "experience factor" used around here in the reverse flow thread to recommend 1.5 times the opening and area under the RF plate. A bunch of successful smokers have been built using the 1.5 factor. Stick with what Dave recommended and plan accordingly so when you finish your build, you will have less fiddling to do to get smoking!

I'm encumbered with a couple of years of college thermo and fluid dynamics so I spent some time getting my hard head wrapped around smoke flow in these things.....

#### rikun

##### Fire Starter
So...Wait..

You'll want your FB to CC opening to be 1.5 times bigger than Feldon's calculator suggests when calculating using 100% sized firebox?

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
So...Wait..

You'll want your FB to CC opening to be 1.5 times bigger than Feldon's calculator suggests when calculating using 100% sized firebox?
Yes...... I think Feldon's was originally designed tor a side fire box using tuning plates... when building a reverse flow, there is too much friction loss from the RF plate and the 180 degree U-turn at the end of the RF plate causing the heat/smoke/air to slow and overheat the FB.... a 50% increase in the FB/CC opening and area under the RF plate, eliminates the overheated FB....

#### ribwizzard

##### Master of the Pit
My two cents, .....

I wouldn't call it flawed, it just needs to be used correctly. As a reference for minimum size requirements.

I think where people have had problems using it, is where they didn't compensate for material thickness, messed up on calculations, or didn't think the design through very well.

And the shape of the firebox is important as well !! , A tall firebox will yield better draft, allowing you to stay closer to minimum size, where as a short, squat firebox will need extra area in order to breath.

Long skinny cook chamber will need more room under the reverse flow plate, where as short , fat cook chambers don't need as much.

The trend to extend the chimney down into the cook chamber reduces flow as well, so allowances have to be made there also.

Best thing to do is mock up your build and sit back and look at it, ......will air flow well through the cooker? Does it allow the heat to rise naturally? Will there be enough height in the firebox for the fire to breath? With out the flames hitting the top plate? Use card board if you have to , in order to visualize the build, ....then run your measurements through the calculator and make sure you are at least as big as what it tells you is required.

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
My two cents, .....

Best thing to do is mock up your build and sit back and look at it, ......will air flow well through the cooker? Does it allow the heat to rise naturally? Will there be enough height in the firebox for the fire to breath? With out the flames hitting the top plate? Use card board if you have to , in order to visualize the build, ....then run your measurements through the calculator and make sure you are at least as big as what it tells you is required.

That, right there, is the best advice you can give someone..... Whether they take it is another story......

Seems some folks get excited about the build and skip the "ponder" part....

#### rikun

##### Fire Starter
Well, I hope the air will flow. At least for me it's a bit late for a mock-up :-) Here's a pic of how things are looking today:

My FB to CC is currently identical what Feldon's recommends for my 109% firebox. I can make it a bit larger, it wouldn't probably hurt, right?

The area under my RF plate is 1.5x of what Feldon's recommends for my 109% firebox, so that's even a bit too much if one should calculate it with 1.5x of 100% firebox.

The maximum gap I can leave at the end is a bit more than my current FB to CC opening. That I can't make any larger, or there'll be food directly above the gap, that is not recommended, right?

I'm thinking of extending my stack just a hair down, around few inches. And it's cut at 45 degree angle, I think it should allow the smoke to exit with less friction...

My FB is of similar design than Ribwizzard's, I have a huge main intake at the bottom and a smaller one just a couple of inches above the fire grate level on the door at the back. I'll have around 20" of height between my fire grate and the top of the firebox. The outer dimensions are 23" height x 25" width and 24" length.

Do you think I'll be fine with the smaller RF gap at the end with all of this? Any advantage of making the stack a bit longer than recommended?

#### ribwizzard

##### Master of the Pit
Hmmmm...I see you paid attention, pie vent slightly closer to the door!, Yep, your picturing the airflow!

#### rikun

##### Fire Starter
Yeah, after cutting the plates I though I could've put it even closer to the door. Oh well :-)

Still haven't found suitable springs for the pie vent, but that can probably wait. I'll have to get this thing up and running by the end of the week, but springs can wait for later ;-)

Going to be a busy week...

#### ribwizzard

##### Master of the Pit
Nah, that vent looks to be in the perfect spot,...

Get a cylinder heat valve spring. Spring will go under the firebox. If vent ever gets stuck from sitting too long, just reach inside the firebox and through the pie opening, tap the damper with a hammer. Don't smooth out the edges of the holes either, leave them nice and sharp!

#### rikun

##### Fire Starter
Do you think making the opening larger is needed with my FB design?

I guess I could do it, but seems like a lot of work for extra 14% increase in opening size.

And I'm a little concerned about how strong my FB is going to end up, the fit up was less than perfect. Thus I'm going to end up with some portions of the welds less thick than the walls.

I'm going to weld the firebox on both sides to the trailer frame. So it is going to see some load, but there will be 6 more joints between the smoker and trailer.

#### bob in mo

##### Newbie
Oh oh. I think I need help!

I've started my first build of a RF smoker and ran across this thread.

So far I have made the cooking chamber with the ends on it and have the firebox cylinder seam welded.

I used Feldon's calculator to come up with the design. (see below)