# buildfor dannyp (few question) bare with me

Discussion in 'Reverse Flow' started by dannyp, Sep 8, 2014.

1. ### dannypFire Starter

I bought a used cooker and a trailer to build it on to and I have a few questions that I cant seem to find the answers to. here is what I'm starting with.

It was set up as a grill so it needs some modification to make it a reverse flow type cooker. I've had a vision in my head for years for a reverse flow with the firebox centered. I'm going to weld the 2 doors together so I can easily get rid of the gap between the doors. ill use the existing smoke stacks for the counter weights on the door.
I need to cut the bottom of the tank out because of rot. I have a couple things I need some clarification on. I don't know how many gallons this tank was initially and I am terrible with math.
The tank is 67" tip to tip inside and 52.5" to the center of the welds and 36" inside diameter, so I don't know how to enter it into the calculator to get the proper volume.
Originally I planned to make a plate steel fire box but my wife doesn't work and I have 4 kids so money savings is what I'm after. I work in the trailer business so I'm trying to use scrap I can get for next to nothing. plate steel isn't something I can get easily.

I went on a gathering spree the other day and I got 60' of 1" angle for the grill frames and slides and I got 3 pieces of 2x8 tube steel 6' long. I also got another tank to cut into a firebox from a scrap yard.
here is my idea that im hoping you guys can help me with. I've read that too big of a firebox is easier to work with than too small and since I couldn't figure out the correct volume of my CC I put in the total length of 67" into the calculator. the tank I got  for the FB is also 36" diameter. I want to cut 24" out of it so the FB would be 24"x36"..
here's where I'm needing advice. because the bottom of the CC is rotted I took the 2x8s and cut them to 52.5" and cut out the bottom from the welds and cut out 8" from the bottom which will leave me plenty of thickness to weld to.
I am hoping  to take 2 of the 2x8s and cut one side out of each and weld them together into one 4x8 to weld into the bottom and use that as my reverse flow plate. each end will be open so I am hoping this will be big enough to allow proper flow.

what im not sure of is how big the opening between the firebox to the cook chamber will need to be. I would like to use what I have on hand and run a short piece of 4x8 from the FB to the bottom of the CC 4x8. do you guys think that'll be enough.

I am going to turn the CC on the rear of the trailer and have the FB between the tires. I am planning to find a 6" pipe to use for the exhaust centered at the top of the CC. I plan to have 2 racks in the CC. this will be my build thread so any help is greatly appreciated.

Last edited: Sep 20, 2014

I did a quick calc on your tank volume and it looks like about 270 gallons or 62769 cu inches.  There still may be a tag

somewhere on the tank noting its volume.

RG

3. ### dannypFire Starter

thank you. it looks like I need a pretty big opening to the CC 15.78" round so even it I did 2 4x8s into the CC it wont be near big enough.. back to the drawing board. just so im clear on a half moon the 22ish inches its calling for is the diameter or the flat part of the half moon correct?

Danny,

I found a real cool propane tank volume calculator on-line and your tank is about 275 gallons.  I did double check this using right cylinder and elliptical sphere calcs.

I put your number into Feldons and came up with this as a start point for you:

http://feldoncentral.com/bbqcalcula....84&fi=5,6,0,66.92,2.20&fc=15.08,21.32,178.46

From Feldons you need 175 sq inch of CC to FB opening.  Feldons recommendation times 1.5 equals 263 sq-in opening needed.

If you plug in the radius of 18" and segment height ED of 11" into the circle calculator, you get the 263 sq-in. It also shows the segment chord AB as 33" to match up to the width of your fire box. http://www.1728.org/circsect.htm

Use Feldons to size your stack while keeping the firebox size at 100%.  You can make your FB larger and that is recommended.

I suggest you read this thread, it explains the tweaks needed on Feldons.  http://www.smokingmeatforums.com/t/161795/is-feldons-calculator-really-flawed

I ask that some one check my math....

Good Day!

RG

5. ### dannypFire Starter

I feel really stupid about the measuring of this tank.I ran a string around the inside of the tank and it was 6 feet ,not sure why I thought deviding it in half would give me the diameter.the really stupid part is the tanks already cut and I didn't just run a tape across the opening which is 24".I apologize and will deffinatly triple check things before posting.

Last edited: Sep 20, 2014
6. ### dannypFire Starter

I've made a little progress,and decided to stick with a standard design.I cut out the bottom to get rid of the rot.

I'm going to put this 2x8 into the hole to fill it.

I also cut off a useless clean out hole cover the previous oowner cut into it so I'll use it as the start of my CC to FB opening.

This is the tank I found to make the firebox from but I'm not sure I can bring myself to cut it apart since it would make such a nice patio cooker.

I have a friend that repairs tank railroad cars and he brought me some 1/4" plate.these 2 are 38x38" so I'm gonna hold out to see if he can get me more.

Last edited: Feb 14, 2015
7. ### dannypFire Starter

I started pulling the trailer apart too. Since I sell cargo trailers I had our rep at the factory get me 3 6" i-beams to replace this trailers 3 rotted cross members.

They're also gonna get me a couple sheets of expanded steel for my grates. I'll keep this updated as I make progress

Danny, the numbers I gave you are wrong. Measure across tank at widest point, this is diameter. 1/2 of the diameter is the radius.
Need total tank length and length between welds as well.
Someone will help calc the CC volume on your new measurements. Can't do it right now.....posting from phone.

RG

9. ### dannypFire Starter

Yes I realize it's big mistake.Thanks for the help,sorry to have wasted your time. Glad I didn't do it in any stage other than planning. At the widest it is 24" across. 52.5" to the center of the welds.67" end to end inside. I think I want to do a 24"x24"x24" firebox incase I end up wanting to line it in firebrick later.

Last edited: Sep 21, 2014
10. ### oldschoolbbqSmoking GuruOTBS Member

, I comfortable and will watch...

11. ### lendecaturalSmoking Fanatic

Danny, I ran the numbers since RG was away from his computer.

You need approximately 112 square inches of opening from the firebox to the cook chamber. If you measure up 7 1/4" up from the bottom of the cook chamber and make a cut across at that height, it should be on the money. The width of the opening across the cook chamber will be right at 22" wide so your 24" firebox width will match up nicely with an inch left over on each side. Keep that 7 1/4" clearance height under the RF plate and have 112 square inches at the end of the RF plate where the smoke turns up towards the grates. You need a 4" exhaust stack 38" tall and air inlets in the firebox equaling 28 1/2 square inches. You can use three 2"x 4" close to the base of the fire and have another 2"x 2" higher and centered to help move smoke through the firebox to the cook chamber.

Looks like your buddies are getting you hooked up good with scraps, you will have that thing looking good and it won't rust out any time soon!

Hope this helps!

Last edited: Sep 21, 2014

Thanks Len !!

RG

14. ### dannypFire Starter

Thanks every one.. ill keep this updated as I go.. turns out the opening the last owner used as a cleanout is right about 7 and a quarter inches tall so it looks like itll work out perfectly.

15. ### dannypFire Starter

Like I mentioned in the first post, im really trying to keep the costs down as much as possible using things I can scrounge. Last week the neighbor to my trailer lot who runs a log truck company towed in a log truck one of his drivers rolled. ive been bugging him daily about one of the uprights that holds the logs since I need an exhaust stack and the other places ive looked ive only been able to scrounge 3" ID pipe. Finally today he said if I could cut it off before the scrappers came to pick it up I could have what I need. I got this.

my question is will the thickness of this pipe mess up the heat transfer and cause the stack to not function ideally? and with it being 1/2" wall thickness should I build gussets around it bracing it to the plenum? I don't want this thing coming off going down the road.

Last edited: Sep 24, 2014
16. ### lendecaturalSmoking Fanatic

No it will work just fine, since it is bigger in diameter, it can be shorter. You can get by with it being 30-32" tall. Just make sure you grind a bevel on the end of the pipe before you weld it up to make sure you have good penetration.

17. ### dannypFire Starter

I made a little progress today. I havent welded in about 15 years so i started practicing last night after work and took the crappy cart that was givin me with the welder that didnt fit the welder and i made it fit the welder. I also put on some hangers for my power cord and leads.

I deffinatly still needed more practice so i made a bench for my vise and bench grinder.

I felt a little better after that so i used some angle to close in the ends of the 2x8 im plugging the hole in the bottom of the cook chamber and started welding it in.

Because of the bottom being rotted out it wasnt easy. I layed down about 5 passes onto the 2x8 to build it up and seal the gaps until it quit burning through the thinner places. Its not beautiful but i got 3 sides done and just have to finish one more side then the bottom will be fixed.

Here you can see the kind of gaps i closed.

And heres the one big booger i had about 10 passes to get it to quit burning through that thin spot.

Im still waiting to see about more free plate but im not gonna quit until i have an awesome cooker.

18. ### dannypFire Starter

Bottom is done. I ground down all the welds to make sure they were connecting on both sides.

19. ### dannypFire Starter

22"x7-1/4"

I think ill use the 2 sheets of plate i have already to make the reverse flow plate then do the slides for the grates while i wait to get enough plate for the firebox.

Last edited: Sep 28, 2014
20. ### lendecaturalSmoking Fanatic

Looking good, there is nothing like burning a hole through and getting it welded up to get back in practice!