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20" diameter build questions

post #1 of 19
Thread Starter 
Well I purchased Aaron Franklin's book and it has inspired me to build my own offset, Ive cooked for years on a cheap chargriller barrel grill made from thin sheet metal and no firebox (starting fire on one side with meat on the other). I am a tool and die maker and have access to very well priced sources of steel, but unfortunately we don't have 20 inch diameter .250 wall pipe laying around and neither do a lot of steel suppliers apparently. I needed two pieces, one 40" long (cook chamber) and another 20" (firebox). I recieved quotes for the two pieces for $370, $510, and $670. Scrap yards didn't have anything and neither did well drilling companies.

So I sheared two pieces of .250 mild steel

One 62" × 40" × .250
One 62" × 20" x .250

..... took them to a large fab shop where they're rolling them into 20" diameter tubes and tacking them a few times. Should be $90 or less and the steel was next to nothing (with approval from a top dog at work of course). I will post pictures when I get them back.

I'll finish the bead on them myself for cost savings. I am planning on building a traditional offset -no reverse flow or tuning plates but I may install a baffle. However....

A) I'm planning on a 4" diameter 36 inch long stack that connects at grate height with the intention of drawing the smoke direction over the meat -appropriate size? Damper needed?

B) As for the size of the opening connecting the firebox to the cook chamber- Aaron's book says the top of the firebox should be at the center of the cook chamber? Should I cut that whole area out though (half moon shape)?

C) Interesting stuff, all of Aaron's smokers do not have firebox vents, or top access firebox doors, he instead has a simple side access door and advocates simply cracking it open a little. So...vent or no vent? Size?

Thank you for your help,

Kevin
post #2 of 19
There are some great calculators here for determining all your sizing. Hopefully Dave will chime in. He seems to be the leading authority on this stuff. @DaveOmak
post #3 of 19
Thread Starter 
I used feldons calculator after many people liked it's effectiveness. Says the firebox opening should be 50 sq inches. So I'm cutting a 5x10 rectangle equally spaced inside the football shape where the two circles overlap.

Going with 3/8 mild steel material for all end pieces and having it laser cut to fit the inner diameter of the pipe.
post #4 of 19
Quote:
Originally Posted by MrSmoklestein View Post

I used feldons calculator after many people liked it's effectiveness. Says the firebox opening should be 50 sq inches. So I'm cutting a 5x10 rectangle equally spaced inside the football shape where the two circles overlap.

Going with 3/8 mild steel material for all end pieces and having it laser cut to fit the inner diameter of the pipe.

 

 

Before you use Feldon's, there have been some improvements since that was written....   Just sayin'......  at least investigate the tutorial below....

 

http://www.smokingmeatforums.com/a/reverse-flow-smoker-how-to-calculate-build-tutorial

post #5 of 19
Thread Starter 
The link you posted confirms that 50.26 sq in is the correct firebox to cook chamber opening size (cook chamber volume x .004)

Recommended stack length is 28" with 4" pipe, but every source tells you to go longer, so I think I'll stick to 36 for now, can always dampen it.

Still don't know if I should have vents on the side fire box door, not going to install a top access door.
post #6 of 19
Quote:
Originally Posted by MrSmoklestein View Post

The link you posted confirms that 50.26 sq in is the correct firebox to cook chamber opening size (cook chamber volume x .004)

Recommended stack length is 28" with 4" pipe, but every source tells you to go longer, so I think I'll stick to 36 for now, can always dampen it.

Still don't know if I should have vents on the side fire box door, not going to install a top access door.

 

The tutorial recommends a stack length of 36" give or take...   The calculation is to figure out what inside diameter pipe fits the length...

 


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

 

 

I recommend having the FB air inlets on the end of the FB directly opposite the FB/CC openings.....

 

 

There are tools to calculate the proper size, for the football shape, if you chose that option...

 

.. ..

post #7 of 19
Thread Starter 
"The tutorial recommends a stack length of 36" give or take... The calculation is to figure out what inside diameter pipe fits the length...


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

I was confused by this, the Feldon's calc simply asks for your inside diameter and gives you a length needed for the stack. I do not know what they're refering to in this calculation for the tutorial however. It seems silly that they would give you the length as a variable and then determine what diameter pipe you need as the solution. If this is what's happening, one would think that everyone who owns a smoker fabrication shop should have lathes equipped to run extremely long boring bars for custom size tube making.....and I know they don't, but rather order set tube sizes.....
post #8 of 19

 I do not know what they're refering to in this calculation for the tutorial however. It seems silly that they would give you the length as a variable and then determine what diameter pipe you need as the solution.

 

 

 

From Feldon's calculator....

 

Chimney Size
Enter the diameter of your chimney pipe to find out how long it should be.
  • A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
  • Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
  • If you are building a horizontal smoker with a vertical cooking cabinet, realize that the cabinet partly acts as an exhaust, thus you may greatly shorten the chimney.

 


 

As silly as it may seem, my thoughts were to provide you with the insight, as best as I could, as to how things came about for the building of both tutorials....   

Maybe folks would learn something, as I did, and come to their own conclusions about the vagueness of building a RF smoker and all things are not set in concrete..  Numbers from previous builds were used to develop Feldon's....   I used Feldon's as a base, a very good and substantiated base, to build upon....  Members here were good enough to test my, and others, theories....  eventually we all arrived at the building blocks for a very good smoker..

Hey, if you don't like it......  you know what you can do ??.... 

I'm trying to help you build a great smoker...

 

If you find having to decide the ID of a pipe to fit it's "best use" length doesn't work for you, well......

 

On the other hand, if you have a 3" ID piece of pipe in the corner of the shop, cut it to length and weld 'er up....

post #9 of 19
Feldon's FB air inlet calculation.....    12,560  41.87 x 0.003 = 12.6 sq. in. opening....
 
Recommended
Air Inlet Area
(firebox volume * .003)
 
square in.
 
4187 x 0.003 =
 
Evidently you didn't read Feldon's correctly.....  "I used feldons calculator after many people liked it's effectiveness. Says the firebox opening should be 50 sq inches. So I'm cutting a 5x10 rectangle equally spaced inside the football shape where the two circles overlap."
 
The calculating tutorial on this forum uses 0.004 x CC volume for the FB/CC opening....12,560 x 0.004 = 50.24 sq. in.
 
 
Recommended Firebox-to-Cooker
Opening Area
(firebox volume * .008)
 
square in.
 
4187 x .008 = 33.46 sq. in.
 

 

I really screwed up this AM confusing FB and CC stuff...  I think I have it straight now...


Edited by DaveOmak - 7/24/16 at 8:09am
post #10 of 19
Thread Starter 
Didn't mean to sound insulting here if it came across that way and I do in fact appreciate your feedback.. My firebox is exactly 50% of my cook chamber volume however, not 4100ish cu in.

50% of 12560 cu in (cc volume) = 6,280 cu in. That is my firebox volume. 20" diameter x 20" long.

6,280 (my firebox volume) x .008 = 50.24 firebox to cook chamber opening.

6,280 (my firebox volume) x .003 = 18.84 firebox vent opening.

You are calculating based on the recommended fb size, and not the actual 20" diameter x 20" long firebox I'm using.
Edited by MrSmoklestein - 7/24/16 at 2:45pm
post #11 of 19

I'm just following the instructions from Feldon's....   FB = 33% of the Cook Chamber....

post #12 of 19
Quote:
Originally Posted by MrSmoklestein View Post

I used feldons calculator after many people liked it's effectiveness. Says the firebox opening should be 50 sq inches. So I'm cutting a 5x10 rectangle equally spaced inside the football shape where the two circles overlap.

Going with 3/8 mild steel material for all end pieces and having it laser cut to fit the inner diameter of the pipe.

I just use 3/16" steel for my end cap for cost and weight reasons. Works fine as all it is doing is blocking air. 

Also I cut it to 505mm diameter which is halfway between the interal and external ID's so it leaves a nice 1/8" gap to lay a weld then doesn't take much to grind a nice radius with the flap wheel..

post #13 of 19
Thread Starter 

"I just use 3/16" steel for my end cap for cost and weight reasons. Works fine as all it is doing is blocking air. 

Also I cut it to 505mm diameter which is halfway between the interal and external ID's so it leaves a nice 1/8" gap to lay a weld then doesn't take much to grind a nice radius with the flap wheel.."

 

Appreciate it, I was wondering if I should fit the piece internally or externally. You just answered the question. My OD is 20" my ID is 19.5". So I'll cut the end caps at 19.75".

post #14 of 19
Thread Starter 

"Hey, if you don't like it......  you know what you can do ??.... "

 

"Maybe folks would learn something, as I did, and come to their own conclusions about the vagueness of building a RF smoker and all things are not set in concrete."

 

 

Wow, first thing here. I'm not building a reverse flow as I stated in my initial post...."I am planning on building a traditional offset -no reverse flow or tuning plates"

 

Second thing: Your calculations, as much time and effort as you have put into them do have faults here. So don't get upset when someone gives you feedback. Your calculations given would call for a stack a little more than 3" ID at 36" long. Call Horizon, Yoder, or Lang and they will all tell you that's too narrow of a stack. Hence why I simply pointed out that it was pretty silly to change the pipe diameter based on a solid length of 36". In no way meant to be insulting or degrade the amount of time and effort you have put in here even though you took it as such.  Guys have pretty good chunks of change tied up in these builds, it would be wrong not to have feedback.

 

 

I understand, this is all subjective and that it's all a base so in reality it may not be worth feuding over. 

 

Feldon's calc does in fact have bugs as well. One is that it is contradictory. You take it at face value when it says "firebox should be 1/3 of cook chamber volume". However, the same calculator goes on to say " slightly oversized firebox preferred" . The numbers you use are based strictly on the 1/3 rule and not my specific scenario. So basically, you're not following the instructions on Feldon's because it's unclear what the actual instructions are. I would assume however that the desired numbers given from the equations (firebox opening, vent size, stack size) are being calculated based on specific scenarios and not general rules of thumb. 

 

In fact, I have seen more well built smokers that use a firebox that is 50% of the CC volume. Horizon is one of them. 

 

In real life, I would quell this problem with a case of beer and some burnt ends. It's kind of hard to do that online....:beercheer:

post #15 of 19

Hey, if you don't like it......  you know what you can do ??.... 

I'm trying to help you build a great smoker...

 

You can build the smoker any way you choose...  You can take anyone's advice...  You can ignore my advice...   

 

Have a great day.....

post #16 of 19
Thread Starter 
Quote:
Originally Posted by DaveOmak View Post

 
Hey, if you don't like it......  you know what you can do ??.... 
I'm trying to help you build a great smoker...

You can build the smoker any way you choose...  You can take anyone's advice...  You can ignore my advice...   

Have a great day...
...you always have to be careful who is giving you advice on the internet
Edited by MrSmoklestein - 7/26/16 at 7:45pm
post #17 of 19
Quote:
Originally Posted by MrSmoklestein View Post

...you always have to be careful who is giving you advice on the internet. I (foolishly) trusted metalworking advice and nonsensical mathematical advice to a fisherman instead of someone who builds these things for a living.

I tried to be civil and logical here and explained my reasoning why you are giving wrong information to people, to which you reply: "Hey, if you don't like it....you know what you can do??..."

Yeah, I know what I can do. Stop pandering to Yankee hobbyist metalworkers who ignore good (correct) information when it's right in front of them because they're too old and stubborn to want to change. You aren't helping anyone by giving incorrect information -that is incorrect based on the calculator that you created yourself. 

Good day buddy. 

Enough with the insults, take the man's advice or don't!
post #18 of 19
Funny you come here looking for advice then you don't want it.
post #19 of 19
I'm planning on making mine out of 3/8" or 1/2" steel.
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