Reverse flow theory

[daveomak]

I have a question on flow and size of exhaust stack....with my numbers input to calc it says I need 720 of cubic inch volume for stack....when I get my FR build going i'll be making my stack 4" x 9" in size and not a round stack....the cu in vol in need I take 4x9=36 cu in, then need to multiple 36x22=792 cu in vol....so my 4x9 stack needs to be at least 22" long/tall as i'm understanding it....

You got it correct Bob.....   but due to the increase in surface area of the rectangle, I would make the stack 9x5 x 30" tall...  just to be on the safe side....  that should overcome all the extra loss in flow from friction...      Dave

 

[ribwizzard]

Just felt like cooking today while doing some projects...I think the best part of this build was the cooking grates, you can see how dirty the cooking chamber is ( I havent washed it once yet) and the grates still brush off like majic with just a few pashes with the grill brush. The only time I coated them with oil was before the first cook, the rest has been what ever fat came of of the meat.

 

[ribwizzard]

I  threw some of my home made sausage ( The left is from my very first batch, to the right is my 3rd batch ) and a lttle 3.5lb picnic on about 6.30 this morning. Temps about 84 out today and taking it to 250 and holding it there is no problem right now.  Hopefully when the sun comes out and it climbs to 93 outside, I'll be able to keep the temps down.

Side note: Dont let your freinds get near your cookers with those plastic bags of garlic bread. That crap on top of the firebox and several other spots is the melted bag where they droped pieces of it all over the cooker. It will not come off!!

 

[ribwizzard]

mistake, it was 79* out , not 84*

Anyway, at 2 hours 10 minutes cooking cylcle, its still sitting right where it should be on the first batch of wood. Some of this was a couple lbs of start up charcoal that I used to start it up and burn off the cooker before cooking, but id say by 630 it was 80% on wood, so by 8;40 its all wood.

 

[ribwizzard]

but ..I'm going to classify it a two hr cooker, much longer and this fire would be dieing and it would start loosing temp.

 

[ribwizzard]

Here's what I'm using for wood, the right side is live oak ( whats in cooker ) left is red.

 

[bigsmoken]

I like that thermometer where can I get one like that?

 

[ribwizzard]

I get them off Amazon, under tru temp.  It glows in the dark, and yes , you can read it at night from 10 ft away!

 

[bigsmoken]

hmmmmmmmmm i'm gonna go check it out right now thanks

 

[ribwizzard]

They are a little pricey, but they are nice.

 

[75tranzam]

They are a little pricey, but they are nice.

I just ordered one for my build, thanks for the info!!

 

[bob1961]

I have some questions bout reverse flow air flow....still working on what size to make my smoker depending on what answers I find to some question on air flow....i know the calc to find size I need in openings and chamber sizes....what my main question is from air intake to FB to CC opening to end of RF plate to chimney size, is it better to have bigger or smaller openings from intake to exhaust ??....other words do I try to stay as close to the 1.00 the calc uses to say if opening are to small or to big ??....

 

[jarjarchef]

I have some questions bout reverse flow air flow....still working on what size to make my smoker depending on what answers I find to some question on air flow....i know the calc to find size I need in openings and chamber sizes....what my main question is from air intake to FB to CC opening to end of RF plate to chimney size, is it better to have bigger or smaller openings from intake to exhaust ??....other words do I try to stay as close to the 1.00 the calc uses to say if opening are to small or to big ??....

I have seen the build Guru's recommend going slightly larger then what the calculator says. They say it is better to have a little more air flow then needed, then not having enough. The theory is you can always choke the air intake down to get the temps down, but if you need more heat you have to work too hard to do it if air flow is the issue.

 

[ribwizzard]

I like a free flowing cooker. My taste in BBQ is for it to not have a heavy smoke flavor, but some like a heavy smoked product.

By restricting the air flow inside the smoker, and lowering the exhaust ports, I believe their attempt is to form sort of a smoke pressure cooker and force the smoke into the meat. I used to do that myself. But now my train of thought is to trap as much heat from your fuel inside the cook chamber, but as it exits from under the reverse flow plate, let it out with no back pressure.

That's what I attempted with this build and was very happy with the results, look at that slab of ribs from earlier post, has a very nice color and "smoke ring" but was not one bit bitter. Check out my pic of the exhaust, you can't even see any smoke coming from it. I'm very happy with the way this cooker performs and won't ever part with it, and I'm going to do my best on my current build to duplicate those results, but with a more traditional look to the smoker.

 

[sqwib]

I'm in the same camp as Rib.
Smoke flavor is subtle.
Usually more noticeable on leftovers. I burn a hot fire . So I may spend an hour building up a large coal bed and add 3 or 4 smallish splits at a time, this gets the wood through the combustion stage much quicker.
If I get it right... I don't have to play with the intakes most of the cook... one less thing to worry about.

Jajar x 2

 

[bob1961]

thanks guys Thumbs Up ....so from the intake go a little bit bigger from there to exhaust for good smooth fast air flow ?? ....I use royal oak lump as my main heat source with bout 3" square blocks of different woods as I feel to use....I have also used just the lump with great wood taste as well....I think my main goal is to be able to hold as low as 220 for those long smokes on large hunks of meat like large briskets and such, or whole med pig....

 

[ribwizzard]

That is the traditional concept. What I did here was Choke it down at the other end of the reverse flow plate and open up the firebox to cooking chamber opening. I wanted more heat to build up and come through the reverse plate. During the first few cooks, I thought I had a problem because I couldn't keep the heat low enough, but since have learned that this thing does not need much of a fire, and really have been cooking with what are more chunks than logs. The oversized exhaust does not really let the heat out like some would think, since you still have that restriction going on at the end of the reverse flow plate.

The math I'm using for my current build will be something like, firebox to cooking chamber=125%, Reverse flow to cooking chamber=100%, Exhaust =125%

 

[bob1961]

ok, i'll list my calc numbers in a short time Thumbs Up ....

 

[bob1961]

ok using these numbers for a 250 PP tank ??

30" D x 60" L CC

18" T x 30" W x 32" L FB

the size I want for my FB will do double duty like a flat top grill as I'm smoking, I found a large flat top grill that is 30" x 28" that will fit my FB top great....or use 2 or 3 LODGE brand cast iron griddles that are flat for eggs, pancakes and other side with raised grill for dogs and burgers and such....I know I do need the tank before I get any steel plate for FB and get it where I need/want it size wise....reading all pages here I'm thinking i'll need to go a bit bigger from intake to exhaust....

 

[bob1961]

yeah my thought on the RF plate to eliminate any hot spots on the FB end, I want to fill my RF plate with boiling water heated on top of FB as I'm heating up to do a smoke....plus the water should help with moistness for what ever I'm cooking/smoking, then drain will be at least 1" I.D. with brass 90 degree ball valve....my thinking that should keep CC pretty even temps from RF opening to exhaust....