Can BTU capacity be estimated / calculated from grate area?

 
Charles Edward
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Post by Charles Edward » Thu. Dec. 08, 2022 11:12 pm

How closely does grate size / area relate to the BTU capacity of a coal burning appliance?

Is there a correlation that can be reasonably drawn?


 
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Post by McGiever » Fri. Dec. 09, 2022 10:08 pm

It absolutely does…*lsayer* had a rule of thumb for that…

 
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Post by Charles Edward » Sat. Dec. 10, 2022 10:07 pm

Any ideas on how to get him to share it with me?

McGiever wrote:
Fri. Dec. 09, 2022 10:08 pm
It absolutely does…*lsayer* had a rule of thumb for that…

 
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Post by BunkerdCaddis » Sat. Dec. 10, 2022 10:22 pm

Charles Edward wrote:
Sat. Dec. 10, 2022 10:07 pm
Any ideas on how to get him to share it with me?
It's enshrined in the annuals of the Forum, you'll want to use the search function above.

 
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Post by davidmcbeth3 » Sun. Dec. 11, 2022 3:10 am

Charles Edward wrote:
Sat. Dec. 10, 2022 10:07 pm
Any ideas on how to get him to share it with me?
Post by lsayre - BTU Output Question

lsayre seemed to indicate in the thread that grate size is not relevant ^^

 
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Post by McGiever » Sun. Dec. 11, 2022 12:26 pm

Ahhh, Let’s try that again…

“If the firebox is square or rectangular: L x W x 375 = Maximum BTU's

If the firebox is round: Radius Squared x 3.1416 x 375 = Maximum”

Need Internal Length and Width Dimensions for Your Firebox

 
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Post by Lightning » Sun. Dec. 11, 2022 12:43 pm

Seems like maximum BTU output would be contingent on how many pounds the stove can hold and how fast you could get it to burn multiplied by efficiency. But that's just my own thoughts..


 
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Post by McGiever » Sun. Dec. 11, 2022 12:48 pm

Lightning wrote:
Sun. Dec. 11, 2022 12:43 pm
Seems like maximum BTU output would be contingent on how many pounds the stove can hold and how fast you could get it to burn multiplied by efficiency. But that's just my own thoughts..
But lsayer and you went over this and he showed you by math otherwise…guess you weren’t convinced. LOL 😆

 
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Post by Lightning » Sun. Dec. 11, 2022 12:55 pm

McGiever wrote:
Sun. Dec. 11, 2022 12:48 pm
But lsayer and you went over this and he showed you by math otherwise…guess you weren’t convinced. LOL
I have the condition of "sometimers" seasoned with a little "hardheadedness" 😆

 
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Post by McGiever » Sun. Dec. 11, 2022 12:56 pm

Too Funny Lee 😆

 
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Post by Charles Edward » Sun. Dec. 11, 2022 1:48 pm

OK thanks. It took some extensive poring over the results of Advanced Search - all of which was really informative aside from my direct question - but I did eventually find Isayer's work. Albeit only with your kind assistance. <g>

CJE
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BunkerdCaddis wrote:
Sat. Dec. 10, 2022 10:22 pm
It's enshrined in the annuals of the Forum, you'll want to use the search function above.

 
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Post by Charles Edward » Sun. Dec. 11, 2022 2:00 pm

That was not my take-away.

My conclusions are that - as combustion air flow is the critical element in burning fuel - the grate size is a large part of the absolute limitations to available air flow / possible heat output.

And that conclusion seems further confirmed by the fact that control of heat output is generally accomplished by decreasing or increasing combustion air flows with air dampers. You can flow Less air with dampers throttled - but with the dampers wide open the grate area becomes the ultimate limitation to combustion air as you can't make the grate larger.

Whereas the height of the firebox is more the controlling element in length-of-burn-time - rather than having much to do with minute-to-minute heat production. You can stack the coal as deep as you like and without sufficient air flow it doesn't burn.

Of course, I barely have my feet wet - deep water sailors should jump right in to correct anything I've gotten wrong so far. Please? <g>

CJE
------
davidmcbeth3 wrote:
Sun. Dec. 11, 2022 3:10 am
Post by lsayre - BTU Output Question

lsayre seemed to indicate in the thread that grate size is not relevant ^^

 
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Post by Charles Edward » Sun. Dec. 11, 2022 2:30 pm

I didn't actually measure my present unit - I just used my thumb/little finger spread at 6" and then another closed-hand 4" to get a guesstimate of 10". So: 25 times 375 = about 30,000 BTU's.

I have a 16,000 BTU radiator to connect so, with the various inefficiencies against me, I think I can likely end up with the desired 20 degree rise across the 'boiler'.

I have largely decided against a gravity system in favor of a small wet rotor circulator. The logistics of hanging the radiator to height and so forth started to conflict with my abject laziness. <g>

I think I'm just going to put the radiator right behind the little 'boiler'.

CJE
------

McGiever wrote:
Sun. Dec. 11, 2022 12:26 pm
Ahhh, Let’s try that again…

“If the firebox is square or rectangular: L x W x 375 = Maximum BTU's

If the firebox is round: Radius Squared x 3.1416 x 375 = Maximum”

Need Internal Length and Width Dimensions for Your Firebox

 
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Post by Sunny Boy » Sun. Dec. 11, 2022 3:04 pm

Just using a grate size calculation only shows what the BTU is of that grate size, not what the stove can produce.

Without knowing how well a stove design can extract those BTUs before they get lost up the chimney you won't know what BTU the stove is capable of producing, just it's grate area.

Example. A GW #8 base heater has the same grate size and firepot depth as a GW Modern Oak 118, but the base heater will have a higher BTU rating as a "stove" than the 118 Oak because the base heater is better at extracting heat from the exhaust.

Likewise, a mica base burner with the same firepot size as a GW#8 will have a higher BTU rating because of all the mica windows better transfer of heat in addition to heating the base like just a base heater design.

Stove heat extraction design has as much to do with a stove's BTU ratings as the grate size.

So, you have to be clear about an equation, is it just comparing the BTU of a grate size, or of a stove's output ?

Paul

 
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Post by Charles Edward » Sun. Dec. 11, 2022 3:37 pm

You are right of course and I understand your point.

I meant to say that the grate size / BTU equation is for gross not net heat production. As always there are a lot of variables involved in measuring the difference between combustion efficiency and overall/total efficiency.

And also; grates themselves vary in ways other than square area. A more accurate estimate would involve only using a grates 'free area' - as no air flows through the actual grate itself - only through the spaces in the grate.

But I'm not looking to testify as an expert witness - just to get a basic idea of what I have in front of me. <g> A: do I have a 30,000 BTU unit or an 80,000 BTU unit kind of thing.

Voltaire observed that Excellence (actually "Perfection" in the French of his time) Is The Enemy of Good - and I also believe that to be true. Close enough to be adequate for accomplishing the goal is all that is required.

Sunny Boy wrote:
Sun. Dec. 11, 2022 3:04 pm
Just using a grate size calculation only shows what the BTU is of that grate size, not what the stove can produce.

Without knowing how well a stove design can extract those BTUs before they get lost up the chimney you won't know what BTU the stove is capable of producing, just it's grate area.

Example. A GW #8 base heater has the same grate size and firepot depth as a GW Modern Oak 118, but the base heater will have a higher BTU rating as a "stove" than the 118 Oak because the base heater is better at extracting heat from the exhaust.

Likewise, a mica base burner with the same firepot size as a GW#8 will have a higher BTU rating because of all the mica windows better transfer of heat in addition to heating the base like just a base heater design.

Stove heat extraction design has as much to do with a stove's BTU ratings as the grate size.

So, you have to be clear about an equation, is it just comparing the BTU of a grate size, or of a stove's output ?

Paul


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