24” x 72” reverse flow

[dacolson]

If your diameter and length are 24” x 72” your stack should be about 20” with a 6” pipe.

[Dirtytires]

I assume your 6” pipe is round and the pipe is still 24”x72” with the 24”x24”x24” firebox. I calculated 692 square inch as the required volume which relates to a stack at 24.46 inches long.

[dacolson]

I assume your 6” pipe is round and the pipe is still 24”x72” with the 24”x24”x24” firebox. I calculated 692 square inch as the required volume which relates to a stack at 24.46 inches long.

I think he said it was 24x24x21 - I redid and see 21" stack w/ 6" diameter.

[Rollinsmoke]

Just a little up date. Finally finished everything! Put the last coat of paint tonight. Here’s some updated pictures. Final pics will come tomorrow.



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[dacolson]

Looks awesome. Bet you can’t wait to cook on it

[Big T]

[tinspark]

Looks real good man!!
I notice the firebox is back a ways from the axle, how is the tongue weight?
I am guessing with a 72" CC that it balances the weight forward a little more than, say a shorter 48" CC..
That thing is awesome !!

[Rollinsmoke]

Done! Had a lot of fun building it. Thank you all for the advice! Fired it up for the first time tonight. Is there usually a big temperature change from bottom rack to top with a reverse flow or is that something I’m going to have to learn how to manage?

Tinspark, the CC is 3/8”, weighs in right around 700lbs. The firebox is 1/4”. It’s fairly well balanced.


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[Rollinsmoke]

One thing I read is that the gap for the BP needs to be 75%-100% of the throat size. I’m sure that I have thay currently. Could that be the part of the problem?


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[tinspark]

Rollinsmoke,
The only thing that is glaring to me is that your side discharge stack is between the two cook grates.
I jumped over to the Smoker plans and looked at all of the RF coversheet plans and it appears that all of the RF's have the stack above the top grate on the RF plans. However there are a bunch of more experienced smokerbuilder guy's on here than me who have been building these things for years and who have seen it all, and may want to chime in. But my hunch is that your temp difference may be due to stack position.

I am guessing that the heat tests hotter at the lower racks than the top racks? (you don't really mention where the cold spots are.
Could the heat and smoke be drawn under the top racks and the top racks (especially the upper grate closest to the stack) making for cold spots ?
If this is the case I think that may be your problem. The good news is that its only metal and can be fixed, but I'd wait til a few more guys chime in and confirm my hunch..

[Rollinsmoke]

Tinspark, it’s the opposite. Top racks are 70 degrees hotter than bottom. But you may have a point.


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[Dirtytires]

Nobody wants to hear that they have to cut into a newly finished/painted pit. I would cook on it a few more times to see how it performs before you do so. Check your dimensions for firebox, throat, gap and stack to rule them out. Work on maintaining a constant temp and see if it equalizes out as it sometimes takes an hour or two to really dial the temp in and get your pit stabilized. You will always have a higher temp at the top rack but you will have to decide if yours is acceptable.

And, yes, if nothing else works I would properly locate that exhaust to the top.

[dacolson]

I've messed around a bit with the baffle plate gap spacing on mine. I just cut a couple of 2" wide x 22" pieces of steel that I could drop into the gap - they just rest in there. I found that the less gap I have, the quicker the bottom racks come up to temp and the easier it is to keep the top and bottom within 10 -15 degrees. Also, the longer the pit is running, the smaller the gap. I'm sure it has to do with the BP catching up with the air temp - I think creating a restriction at the BP gap speeds up the process. My stack is at bottom grate level - mid CC. My bottom grate is 3.5" above the BP.

[tinspark]

Tinspark, it’s the opposite. Top racks are 70 degrees hotter than bottom. But you may have a point.


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Totally agree with DT.
If you cannot dial it in after several burns, and feel that you want to make a change maybe this change would help.Not sure how much room you have, but you would need a min. of 28.27" sq. of free area at a side discharge thimble. I believe the higher the better. Maybe a rectangle would work too instead of a trapezoid. you may be able to move it a little higher with the trapezoid tho,,
Again to all smokerbuilder experts please chime in. I hate giving advice without the meeting of the minds,
Sorry for the hieroglyphic.

[tinspark]

I've messed around a bit with the baffle plate gap spacing on mine. I just cut a couple of 2" wide x 22" pieces of steel that I could drop into the gap - they just rest in there. I found that the less gap I have, the quicker the bottom racks come up to temp and the easier it is to keep the top and bottom within 10 -15 degrees. Also, the longer the pit is running, the smaller the gap. I'm sure it has to do with the BP catching up with the air temp - I think creating a restriction at the BP gap speeds up the process. My stack is at bottom grate level - mid CC. My bottom grate is 3.5" above the BP.

Good call Dave..We were typing at the same time with my last post and your great advice. test test test before hacking into things..

[dacolson]

I've messed around a bit with the baffle plate gap spacing on mine. I just cut a couple of 2" wide x 22" pieces of steel that I could drop into the gap - they just rest in there. I found that the less gap I have, the quicker the bottom racks come up to temp and the easier it is to keep the top and bottom within 10 -15 degrees. Also, the longer the pit is running, the smaller the gap. I'm sure it has to do with the BP catching up with the air temp - I think creating a restriction at the BP gap speeds up the process. My stack is at bottom grate level - mid CC. My bottom grate is 3.5" above the BP.

Good call Dave..We were typing at the same time with my last post and your great advice. test test test before hacking into things..

Lately I cut first, think later. trying to get better ...
Rollinsmoke, that's a sweet looking build. Hate to cut it, but it would bother me not to love how it runs even more. Keep us posted.

[Rollinsmoke]

I wondered if I was placing that stack to low. Thank you guys for all the input. I’ve only had one burn in it so far. This weekend I’m going to fire it up a few more times, mess with the gap and angles of the trailer, level vs tilted up or down, and I will go from there. Will keep you guys updated.


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[tinspark]

Check out Daves build (Dacolson), his too is between the grates with his stack, and has managed to do some killer meats in his RF
I went back and looked at that, now I need lunch!! Wish I could cook like that !!
https://smokerbuilder.com/forums/viewto ... 58#p111658

[dacolson]

Check out Daves build (Dacolson), his too is between the grates with his stack, and has managed to do some killer meats in his RF
I went back and looked at that, now I need lunch!! Wish I could cook like that !!
https://smokerbuilder.com/forums/viewto ... 58#p111658

Haha. You all put out way better product than I do. Just messing around here.

[Big T]

I agree with the guys that you should do some testing before you start chopping it up. The easiest way to describe what the BP gap does for a RF smoker is to imagine what happens when you turn a water hose on fully open and the water is just pouring out. Now think about what happens when you put your thumb over the end of the hose, the more that you pinch down the opening the faster the water moves as it comes out of the hose, up to a certain point and then it just chokes off the water. The BP gap changes the rate at which the hot air escapes from under the BP and how fast it travels through the cook chamber and out the stack. If you get everything tuned just right where the hot air stays under the BP for the right amount of time and then exits through the BP gap, moves very quickly across the CC and out the stack then you'll end up with a CC that is pretty even in temperature throughout. I don't think you'll ever achieve a the exact temp everywhere but you should be able to get it close enough that it isn't really an issue.

[tinspark]

I agree with the guys that you should do some testing before you start chopping it up. The easiest way to describe what the BP gap does for a RF smoker is to imagine what happens when you turn a water hose on fully open and the water is just pouring out. Now think about what happens when you put your thumb over the end of the hose, the more that you pinch down the opening the faster the water moves as it comes out of the hose, up to a certain point and then it just chokes off the water. The BP gap changes the rate at which the hot air escapes from under the BP and how fast it travels through the cook chamber and out the stack. If you get everything tuned just right where the hot air stays under the BP for the right amount of time and then exits through the BP gap, moves very quickly across the CC and out the stack then you'll end up with a CC that is pretty even in temperature throughout. I don't think you'll ever achieve a the exact temp everywhere but you should be able to get it close enough that it isn't really an issue.

Great analogy BigT. Very easy to understand those word pictures!

[Rollinsmoke]

Well after running it half the night last night and all day today I was getting no where. Still getting any where from 100 and 120 degrees difference between the 2. So I moved the exhaust.

Tinspark, I made sure I had 28.27” sq of free space, actually have 28.26 so I just ran with it.
I’d say it got a little better but maybe by only 20 degrees. I was getting ready to shut it down for the night, but then I seen BigT’s response.

BigT, you hit the nail on the head!! Just when I was ready to give up on the night. Thank you for the explanation. Great analogy! Made perfect sense. Closed off my BP gap and everything came together!

Can’t thank you all enough! Tomorrow will be paint, again. But that’s ok I’m glad the exhaust is correct now. Plus I think it looks better lol.
Again thank you!!


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[tinspark]

Ha ha!! glad it worked R.S.
sweet looking rig..
Shoot some pics when u get it pertied up again and U get a chance..

[Big T]

I'm glad we were able to help out!

[Dirtytires]

We all love a success story.....