FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 


Translating Btus into temp.

 
Post new topic   Reply to topic    The Smoke Ring Forum Index -> General BBQ Discussion
View previous topic :: View next topic  
Author Message
SmokinDaddy
Newbie


Joined: 31 May 2006
Posts: 44

PostPosted: Thu Nov 02 06 11:32 pm    Post subject: Translating Btus into temp. Reply with quote

I've been trying to devise a formula that factors the current temperature of a smoker, species of wood, Btu of wood, weight/size of wood chunk, etc., and how to anticipate the thermal output of the wood once added to the firebox.

I know that 1 pound of Hickory will yield roughly 6000+ Btus. I also know that a stick of hickory the size of my wrist and about 8 oz will burn up in about 30 minutes (well seasoned). So this should translate into about 100 Btu's per minute.

I would take that to mean that the thermal output of my firebox from one stick of hickory should increase the pit temperature to about 100 degrees and sustain that temp over a 30 minute period.

Is my math correct? Has anyone else tried to come up with a formula like this?
_________________
Smokers
Oklahoma Joe Longhorn
Retired: Texas White-trash Modified Deluxe Water Smoker
Back to top
View user's profile Send private message
DawgPhan
BBQ Super Pro


Joined: 12 May 2005
Posts: 3444

PostPosted: Thu Nov 02 06 11:56 pm    Post subject: Re: Translating Btus into temp. Reply with quote

SmokinDaddy wrote:
I've been trying to devise a formula that factors the current temperature of a smoker, species of wood, Btu of wood, weight/size of wood chunk, etc., and how to anticipate the thermal output of the wood once added to the firebox.

I know that 1 pound of Hickory will yield roughly 6000+ Btus. I also know that a stick of hickory the size of my wrist and about 8 oz will burn up in about 30 minutes (well seasoned). So this should translate into about 100 Btu's per minute.

I would take that to mean that the thermal output of my firebox from one stick of hickory should increase the pit temperature to about 100 degrees and sustain that temp over a 30 minute period.

Is my math correct? Has anyone else tried to come up with a formula like this?



I dont know the specifics but I would say that your formula isnt going to work..

seems like BTUs refer to raising 1 pound of water 1 degree F.

I think that you would also need to consider the actual burning point of the material...you could have a 600,000 btu burner but if the thing it was burning burned at 300 degrees you are never going to get over 300 degrees while a 6000 btu burner might burn @ 500 degrees....since you are trying to determine temp seems like you need to worry more anout the temps rather than the amount of energy...

I would also imagine that a piece of wood doesnt burn at a constant rate when introduced to a fire....probably dip in temps>releasing a lot of its energy>energy trailing off...

You cant expect that a 3000 btu piece of would is going to release 100 btus per minute for 30 minutes...probably it will release 2000 btus in the first 10 minutes and then 1000 btus over then next 20...
Back to top
View user's profile Send private message
kickassbbq
BBQ Pro


Joined: 06 Jan 2006
Posts: 664
Location: mn

PostPosted: Fri Nov 03 06 12:34 am    Post subject: What? Reply with quote

SmokinDaddy Meister??????
I love it when people have time to spend on thinking about such things.
You figure that all out and let me know.
Stick Burners Rule!!!!!!!!!!!!
Smoke On!!!!!!!
_________________
Smoke On!!!!!!
www.kickassbbq.com
Back to top
View user's profile Send private message Send e-mail Visit poster's website
adolpho
BBQ Super Pro


Joined: 03 Aug 2005
Posts: 1067
Location: Austin, TX

PostPosted: Fri Nov 03 06 12:37 am    Post subject: Re: What? Reply with quote

kickassbbq wrote:
SmokinDaddy Meister??????
I love it when people have time to spend on thinking about such things.
You figure that all out and let me know.
Stick Burners Rule!!!!!!!!!!!!
Smoke On!!!!!!!


I agree.
Uhmm. 3 logs gets me to temp, and one every 30-45 minutes afterwards keeps me stable. BTUs never enters my mind. Confused
_________________
"Tag line? We don't need no stinkin' tag line!"
Back to top
View user's profile Send private message Visit poster's website
allsmokenofire
BBQ All Star


Joined: 26 Apr 2005
Posts: 5051
Location: Oklahoma

PostPosted: Fri Nov 03 06 1:11 am    Post subject: Re: Translating Btus into temp. Reply with quote

SmokinDaddy wrote:
I've been trying to devise a formula that factors the current temperature of a smoker, species of wood, Btu of wood, weight/size of wood chunk, etc., and how to anticipate the thermal output of the wood once added to the firebox.

I know that 1 pound of Hickory will yield roughly 6000+ Btus. I also know that a stick of hickory the size of my wrist and about 8 oz will burn up in about 30 minutes (well seasoned). So this should translate into about 100 Btu's per minute.

I would take that to mean that the thermal output of my firebox from one stick of hickory should increase the pit temperature to about 100 degrees and sustain that temp over a 30 minute period.

Is my math correct? Has anyone else tried to come up with a formula like this?


You're also assuming that the log will burn evenly over the 30 minute span, when actually it will take awhile to catch, be fully engulfed for a period of time, then be reduced to embers. More of a bell curve than a straight line. But I'm just the cook, not the math professor. Wink
_________________
Mike
Team Enoserv
Back to top
View user's profile Send private message
SoEzzy
Site Admin


Joined: 13 Oct 2006
Posts: 13183
Location: SLC, UT

PostPosted: Fri Nov 03 06 3:49 am    Post subject: Reply with quote

Now if 1lb hickory produces 6,000 Btu's and you use sticks about 1/2 this size, and 3 of them every 30 - 45 minutes you are using 9,000 Btu's / 30 - 45 minutes, the lower figure, 30 minutes, (when cold and windy or rainy days), then to cook is taking 300 Btu's / minute, effectively about 1 Btu / .

On warmer calm days you are using 9,000 Btu's in 45 minutes or effectively 200 Btu's / minute or 0.66 Btu / .

(x - y)=(z * t) * f / ((v / s * r) - m)

Where: -

x = pit temperature
y = external temperature
z = Btu's generated by the type of wood / standard size of stick
t = time the wood will burn for
f = vent damper setting = exhaust / inlet / 100 expressed as a %
v = volume of the cooker
s = surface area of the cooker
r = thermal loss coefficient of the cooker
m = mass of the cooker plus the mass of the meat in the cooker

I think that should work out, to give your cooker it's own equation, and if we can get comments from Alienbbq and some of the other pit makers, they may have other variables that need to be incorporated, but baring their input, you can run this for your cooker / temperature / mass of meat and I bet you will get a ballpark figure for your cooker that works out pretty well.

There will be a surface area variable to go in the pot, having thunk about it for a moment more, (I have now added the surface area and thermal conductivity of the cooker into the equation).

You can now test this for veracity against your cooker with a known amount for each of the variables.

It may show I am missing a variable or two, if anyone spots them please let me know, I will be happy to re-thunk the whole thing. Some parts of the equation will be constants for each cooker, and could therefore be put into the equation as a single figure, once it has been calculated for that cooker, (e.g. volume / surface area * thermal loss coefficient v / s * r).
_________________
Here's a change Robert.

I still work here!
Back to top
View user's profile Send private message Send e-mail
mikekilian1947
BBQ Fan


Joined: 02 Sep 2005
Posts: 368
Location: St Louis MO

PostPosted: Fri Nov 03 06 4:58 am    Post subject: Reply with quote

SoEzzy

Not sure if this will help -- didn't have much time to look at all the conversions.

http://www.export911.com/convert/conFac-B.htm

Mike
_________________
"I can smell the future." Nostrildomus
Back to top
View user's profile Send private message
SoEzzy
Site Admin


Joined: 13 Oct 2006
Posts: 13183
Location: SLC, UT

PostPosted: Fri Nov 03 06 5:16 am    Post subject: Reply with quote

mikekilian1947 wrote:
SoEzzy

Not sure if this will help -- didn't have much time to look at all the conversions.

http://www.export911.com/convert/conFac-B.htm

Mike


Thanks for the convertor Mike!

My equation is an attempt to calculate the cooker coefficient or cc for any individual pit, and although it may not look like it will do the job, I don't think it is all that far off either.

Now there are some, (myself included), who would do a few practical tests and find out that if you need 3 sticks of hickory every 30 - 45 minutes, and you test it under various weather conditions with different amounts of wood, and different vent settings, and different amounts of meat, you will eventually either run out of wood, run out of meat, or get an idea of how your pit works under those conditions.

The equation takes most of those variables into account, (I think), and you know for instance what the constants are going to be, and to a certain extent they could then be eliminated from the equation once they are a known factor.

This was more of a thinking or thought problem than the need for me to produce a fully working equation, but again just thinking about it, it does look to have produced a realistic equation, that might even be correct. I am in no way quallified other than by being a thinker, (sometimes correctly), and make no claims that the equation is the "be all and end all" of fuel / pit temperature regulation.
_________________
Here's a change Robert.

I still work here!
Back to top
View user's profile Send private message Send e-mail
allsmokenofire
BBQ All Star


Joined: 26 Apr 2005
Posts: 5051
Location: Oklahoma

PostPosted: Fri Nov 03 06 5:59 am    Post subject: Reply with quote

SoEzzy wrote:
Now if 1lb hickory produces 6,000 Btu's and you use sticks about 1/2 this size, and 3 of them every 30 - 45 minutes you are using 9,000 Btu's / 30 - 45 minutes, the lower figure, 30 minutes, (when cold and windy or rainy days), then to cook is taking 300 Btu's / minute, effectively about 1 Btu / .

On warmer calm days you are using 9,000 Btu's in 45 minutes or effectively 200 Btu's / minute or 0.66 Btu / .

(x - y)=(z * t) * f / ((v / s * r) - m)

Where: -

x = pit temperature
y = external temperature
z = Btu's generated by the type of wood / standard size of stick
t = time the wood will burn for
f = vent damper setting = exhaust / inlet / 100 expressed as a %
v = volume of the cooker
s = surface area of the cooker
r = thermal loss coefficient of the cooker
m = mass of the cooker plus the mass of the meat in the cooker

I think that should work out, to give your cooker it's own equation, and if we can get comments from Alienbbq and some of the other pit makers, they may have other variables that need to be incorporated, but baring their input, you can run this for your cooker / temperature / mass of meat and I bet you will get a ballpark figure for your cooker that works out pretty well.

There will be a surface area variable to go in the pot, having thunk about it for a moment more, (I have now added the surface area and thermal conductivity of the cooker into the equation).

You can now test this for veracity against your cooker with a known amount for each of the variables.

It may show I am missing a variable or two, if anyone spots them please let me know, I will be happy to re-thunk the whole thing. Some parts of the equation will be constants for each cooker, and could therefore be put into the equation as a single figure, once it has been calculated for that cooker, (e.g. volume / surface area * thermal loss coefficient v / s * r).


...my head hurts just from reading that. Laughing
_________________
Mike
Team Enoserv
Back to top
View user's profile Send private message
SoEzzy
Site Admin


Joined: 13 Oct 2006
Posts: 13183
Location: SLC, UT

PostPosted: Fri Nov 03 06 6:29 am    Post subject: Reply with quote

Ok, so your head hurts, but did it make sense? Not that your head hurts make sense, does the equation make sense?
_________________
Here's a change Robert.

I still work here!
Back to top
View user's profile Send private message Send e-mail
allsmokenofire
BBQ All Star


Joined: 26 Apr 2005
Posts: 5051
Location: Oklahoma

PostPosted: Fri Nov 03 06 6:40 am    Post subject: Reply with quote

SoEzzy wrote:
Ok, so your head hurts, but did it make sense? Not that your head hurts make sense, does the equation make sense?


You have vastly overestimated my intellectual capabilities. Shocked

I'm not even sure a stay at a Holiday Inn Express will help me.
_________________
Mike
Team Enoserv
Back to top
View user's profile Send private message
Rubit
BBQ Pro


Joined: 04 Oct 2006
Posts: 505
Location: South Georgia

PostPosted: Fri Nov 03 06 6:59 am    Post subject: Reply with quote

This problem is solvable but once you finsh the heat transfer characteristics of your smoker it will be a constant. So many other variables could each be tested and spread sheets could give you an approximation based on the condtion of your wood.
So yes it can be done to a good guess with a fair amount of work and it can be nailed if you have the money for testing each piece of wood and had the enviromental instrumentation.

But burning wood for a good barbecue fire is an art form.
_________________
Vertical Brick pit wood burner
Converted Lazy Q Charcoal
Tank smoker Charcoal
Tank smoker wood burner
Tank Charcoal grill


Last edited by Rubit on Fri Nov 03 06 7:40 am; edited 1 time in total
Back to top
View user's profile Send private message
SmokinDaddy
Newbie


Joined: 31 May 2006
Posts: 44

PostPosted: Fri Nov 03 06 7:06 am    Post subject: Reply with quote

wow SoEzzy - great stuff! And thank you to all, I have some extra resources I'm utilizing in my effort and will post what I come up with when I get done. This is a great start!
_________________
Smokers
Oklahoma Joe Longhorn
Retired: Texas White-trash Modified Deluxe Water Smoker
Back to top
View user's profile Send private message
skybob
BBQ Super Pro


Joined: 10 Apr 2006
Posts: 1533
Location: Wichita, KS

PostPosted: Fri Nov 03 06 8:56 am    Post subject: Reply with quote

Now I realize that what my math teachers said: "Study Math, you'll never know when you need it" is true. Sad Oh, my gawd, I think I got a brain cramp just looking at this. Confused
That's what I have kids for to figure this kind of stuff out, plus of course, setting the VCR when the time change happens. Wink
Back to top
View user's profile Send private message
krimson_cardnal
Newbie


Joined: 31 Aug 2006
Posts: 41
Location: Up State New York

PostPosted: Sat Nov 04 06 5:04 am    Post subject: Reply with quote

Just keep good records of your actual experiences with burning in your cooker. By the time you are done figrin' you will have "learned" how to controll fire, and this is the secrete of great Q - the fire!!

For the hell of it I just smoked a 10# butt in my SnP with absolutly NO meat prep. Pork and smoke was all there was. Kept a good fire going at the correct temps using my standard "lump/chunk" method and that bad boy came out to die for. Foiled at 175*, brought it up to 195* and let it rest in a 170* oven all wrapped up in it's lushious juices. Season at the table - cider vinegar "mop", salt, pepper, the drippings - ooohlala.

The BTU's took care of themselves and we did toooo - pork, uummmm! K_C

SmokinDaddy wrote:
wow SoEzzy - great stuff! And thank you to all, I have some extra resources I'm utilizing in my effort and will post what I come up with when I get done. This is a great start!
Back to top
View user's profile Send private message
Rub N Smoke
BBQ Pro


Joined: 02 Sep 2006
Posts: 713
Location: Houston, (BY GOD!!) Texas

PostPosted: Sat Nov 04 06 11:57 am    Post subject: Reply with quote

I think, I think, I think, AH hell I'll just drink a beer. My head hurts after all of that.. Twisted Evil
_________________
Cookin' Wit' A Little Spit Aint Yo' MaMas Barbecue!!
My Toys Are:
Trailered Competition Pit
4320/ Tejas Cooker
My 7 year old Daughters E-Z Bake Oven (only in Emergencies)
Check out: www.myspace.com/cookinwitalittlespit
Back to top
View user's profile Send private message Visit poster's website AIM Address
Jeff T
BBQ Super Pro


Joined: 08 Mar 2005
Posts: 4207
Location: Norfolk, Nebraska

PostPosted: Sat Nov 04 06 1:43 pm    Post subject: Reply with quote

But..... Did it taste good?
Very Happy
Back to top
View user's profile Send private message Send e-mail
Michael B
BBQ Pro


Joined: 31 Aug 2006
Posts: 789
Location: Amherst, New Hampshire

PostPosted: Sat Nov 04 06 7:57 pm    Post subject: Reply with quote

SoEzzy wrote:
It may show I am missing a variable or two, if anyone spots them please let me know, I will be happy to re-thunk the whole thing.


Moisture content of the wood and altitude.
_________________
Michael B.
I have not yet begun to procrastinate!
Back to top
View user's profile Send private message
SoEzzy
Site Admin


Joined: 13 Oct 2006
Posts: 13183
Location: SLC, UT

PostPosted: Sun Nov 05 06 2:51 am    Post subject: Reply with quote

Thanks Michael B, I was assuming the moisture content as being constant across your wood, as presumably all your wood would have been appropriately seasoned, and you would have attempted to keep it all dry when at the contest, and therefore at about the same moisture content, (though who hasn't forgotten a couple of sticks at some time). Embarassed

I think the altitude is one I will be taking a look at, air pressure / density will be different, and climatic changes such as average day / night temperatures will be affected, now that would also bring humidity and dew point into play, there could be a variable for ground temperature to account for heat reflected back from the ground, but I think that will be a low factor and can safely (as safely as any factor can be) ignored.

The altitude difference between sea level and Denver, CO would cause a difference in Oxygen content (in a cubic yard of air) of upto 28%, the oxygen would affect the burn rate, but that should be taken care of in the equation by the vent flow factors (f).

Humidity and dew point are still in play as possible factors, I will research them today or tomorrow and post back an them.

I went back after my assumption an moisture content and found the following information on moisture content.

[quote from www.sankey.ws.wetwood.html]
So, if you have a piece of seasoned wood 2 cm thick that is at the 15% MC of typical outdoors storage here, and you want to estimate how fast it will come to equilibrium in your workshop at 30% RH (7% MC) if exposed to air both sides, L = 1 cm and the diffusion equation gives t = 1x106 s, 11 days.

The equilibrium MC change required is 8%, so in 11 days you can expect 63% of 8% = 5% lower MC, that is 10% total MC. That leaves 3% to go, and you can expect 63% of that 3% to take place over the next 11 days, to 8% MC. So, 3 weeks should be enough time for 2 cm thick wood. If your wood is 4 cm thick, it will take 3 months, 6 cm thick, 7 months.

According to the literature, most seasoned temperate woods change moisture at a rate within 20% of this.[/quote]

So the variable for moisture content can be ignored if 1) all the wood has been well seasoned and 2) all the wood has been transported under the same conditions.
_________________
Here's a change Robert.

I still work here!
Back to top
View user's profile Send private message Send e-mail
Display posts from previous:   
Post new topic   Reply to topic    The Smoke Ring Forum Index -> General BBQ Discussion All times are GMT + 8 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2002 phpBB Group