reverse flow plate details

[duck killer 1]

i recently was reading a thread on here and i couldnt find it in the short amount of time i had to search, so i was wondering if some one could answer my question. here it goes...
at the end of the reverse flow plate away from the fire box what is the required area for the opening? it seems i read that the opening should be equal to the area of the ID of the stack.
thanx for the help in advance!!

 

[meat hunter]

I believe the area should be equal to the amount of space under the heat transfer plate itself. Meaning, if the distance from the bottom of the smoker is say 5" from the plate, then the gap at the side opposite of the firebox should be 5". I may be wrong on this, but I believe this is what I read.
Someone please correct me if I am wrong on this.

 

[indy_nubie]

Pretty much the same answer as MH above I made mine basically the same area as the firebox to smoke chamber opening and when I tested steady state (ie mantaining a constant temp) I saw 1 degree delta T across the entire smoke chamber. I'm not claiming to be an expert (I've only built 1 and just finished last sat) but those where my observations.

 

[mrblack947]

Pretty sure that was my build you were reading.

I gathered the same thing from bbally that the reverse flow plate opening needed to be no more than the square inches of the exhaust pipe diameter?

Mine might be a bit more then that but I could not be more pleased with the even temperature of this rig.

 

[duck killer 1]

thats what i thought. it seems like that would be way too small of an opening if you used 4" pipe for your vent. maybe bbally can chime in and enlighten me.
thanks once again

 

[duck killer 1]

so since i am not the most brilliant person on the planet maybe somebody can help me with some simple math? i think the formula for the area of a cicle is:
area= pi X radius squared
so if i have the correct formula and i know how to use a calcultor a 4" circle would be:
3.14 X 4 = 12.56 ???


so the area of a 4" circle is 12.56".......that doesn't sound right to me.
can somebody help me with this?

 

[outlaw44]

I can't provide any help on the opening, but I can help with the math

The area of a circle is pi*r squared as you mentioned.
That would be = 3.14 * 4^2 or 3.14*16 = 50.24 square inches.

 

[duck killer 1]

wouldn't R= 2 so Rsquared would be 4? not 16?

 

[meat hunter]

so the area of a 4" circle is 12.56".......that doesn't sound right to me.
can somebody help me with this?

you are correct, 12.56

 

[outlaw44]

Ha! So much help I was! Yes, you're right. I quickly read through your post and for whatever reason my head thought you were saying it was a 4" radius. My bad home skillet.

 

[duck killer 1]

ok kool! so then in my new build i'm doing i need to have the plate stop short enough from the end so that i have 12.56" square left open, so if i used a 4" pipe for my flue then i would have a 12.5" by 1" opening at the end. the problem is my baffle plate is around 25" wide so i prolly need to make my flue a 6" so i can get a wider opening. the calculator said if i used a 4" pipe it would need to be 45" long so i think a 6" would be more practical. any opinions are welcomed! thanks in advance!
duck

 

[duck killer 1]

it's cool! if i had it figured out i wouldn't have asked LOL!

 

[outlaw44]

I have a feeling you'll excuse everything I say now, but from a heat transfer point of view it seems to make sense to have similar cross sectional openings. This way the air flow stays fairly uniform across the path of the air. If you have different size openings, you can create pressure and velocity changes. Similar to changing the diameter of a pipe with water flowing through it. However, I have no experience with building or testing to know if there's even that big of a difference. Hopefully someone more qualified will chime in...

 

[duck killer 1]

when i built my mini i didn't even think it mattered how big that opening was but i guess i must have nailed it since it works flawlessly. i guess i was lucky.

 

[dennisdocb]

Sorry but you are using the diameter not the radius...4" dia = 2" radius squared = 4 x 3.14 = 12.56 sq "

 

[outlaw44]

Have you measured it to see if you did? There's really a lot that goes into even heat distribution. The main thing is that you got it to work!

How did you calculate a 45" long 4" exhaust? Just curious, cuz I'm lost. I think I may not be picturing it correctly in my head...

 

[duck killer 1]

i used the excel calculator for a square fire box. my smoker dimensions on this particular build (i have 4 going right now

) are:
48"x 30"diameter cook chamber and a 20" tall x 24" widex 24" deep fire box. this caused my inlets for the firebox to be 2- 2" x 8.5" and my firebox to cookchamber opening to be 1- 15" x 6" opening and my stack to be 45-3/4" of 4" pipe. now if i change the stack size to 6" it changes my stack hight to 20-3/8" and my baffle plate opening to 28-1/4" square. i can try to find the link to that page if you need it.

 

[duck killer 1]

this is what it looks like,

 

[outlaw44]

Wow, sounds very interesting! If you can find it, that would be awesome...just because I'm curious now

I don't have any builds planned or anything. I'm sure it's mostly air flow driven.

 

[outlaw44]

Nice! Makes sense. It's an air volume/flow thing. That's pretty cool...well, you've made my day! And I provided you with basically nothing!