Analysis of Modified Fire Grate

SteveWPBFL · May 2012

Check this out for laughs!

Analysis – Effect of Increasing BGE Grate Hole Diameter from
.625 to .750

Large Big Green Egg (mine), nominal dimensions (inches) and flow areas

Analysis – Effect of Increasing BGE Grate Hole Diameter from
.625 to .750

Large Big Green Egg, dimensions (inches) and flow areas

A1 = Max Intake 3.95 x 2.1 = 8.3 sq in

A2 = Max Chamber inlet area = 13.7 sq in, consists of:

Firebowl slot .125 x
6 = .7 sq in

Firebowl holes 6 x .625 dia. = 2.6 sq in

*Grate holes 34 x
.625 dia. = 10.4

A3 = Max Exhaust 3.85 dia. = 11.6 sq in

Assuming the BGE burns fuel rich and burns all available O2
the Egg is designed to meter O2 at the smallest of the three ports (A1, A2,
A3), which is the intake (A1). (No surprise!)

Assume a 50% intake area results in a dome temperature of
350F: A1 = 4.15 sq in

When 50% intake area A1 results in less than 350F that means
the chamber inlet A2 has been reduced to < 4.15 sq in. In this case:

The blockage is A2 – 4.15 = 13.7 – 4.15 = 9.5 sq in

The percent blockage is 9.5/13.7 = 70%

The equivalent number of chamber holes blocked is .7 x 34 =
23.8 holes

Grate holes of .75 results in A2’ = Max Modified chamber
inlet area = 18.3 sq in from:

Firebowl slot .125 x
6 = .7 sq in

Firebowl holes 6 x .625 dia. = 2.6 sq in

* Fire bowl holes 34
x .75 dia. = 15.0 sq in

When 50% intake area A1 results in less than 350F that means
chamber inlet A2’ has been reduced to < 4.15 sq in. In this case

The blockage is A2’ – 4.15 = 18.3 – 4.15 = 14.1 sq in

The percent blockage is 14.1/18.3 = 77%

The equivalent number of holes blocked is .77 x 34 = 26.2
holes

"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language:EN-US;
mso-bidi-language:AR-SA">Conclusion: Opening up the grate holes from .625 to
.750 in dia. means that 2-3 more holes would have to be blocked to provide the
same restriction in O2.

"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language:EN-US;
mso-bidi-language:AR-SA">How 'bout that!

Duganboy · May 2012

Yep, sounds about right to me :D

SteveWPBFL · May 2012

Sorry 'bout all the font commands, can't seem to get rid of them all after pasting in from Word .....

Mighty_Quinn · May 2012

:-B

cortguitarman · May 2012

Huh? I teach 5th graders. This is a bit above the scope of the math that I teach. Anybody need to know how to simplify fractions? I'm your guy.

Chubbs · May 2012

Huh?

SamFerrise · May 2012

Hey, you can slice and dice it any way you please. It works!

Doc_Eggerton · May 2012

Ugggh, fire good, fire make food good, fire make warm. Me like fire.

Doc_Eggerton · May 2012

On a more serious note. Steve I am so happy to have you around, you keep some of the other guys off my back :-)

XLBalco · May 2012

so what youre saying is

MikeP624 · May 2012

Any proof that slightly big holes just don't get clogged with slightly bigger lump?

SteveWPBFL · May 2012

What I think I meant to say was this.

1 - First. Opening up the grate fire holes does nothing to increase the max airflow through the Egg since that is set by the wide open intake sliding door position, which is the smallest max flow area in the chain. Smaller than the unplugged grate/firebox holes area. Smaller than the max exhaust area. I think we can agree on that.

2 - As far as clogging up the grate with ash: With the .625 dia. fire grate holes you have to block the equivalent of 23.8 holes to restrict the flow through the grate more than the airflow being set by the half open intake door when trying to set 350F. The number of holes clogged will change if you're trying to set a different temperature with a different intake door position.

3 - As far as clogging up the grate with ash: With the .750 dia. fire grate
holes you have to block the equivalent of 26.2 holes to restrict the
flow through the grate more than the airflow being set by the half open
intake door when trying to set 350F. Again, the number of holes clogged will change if you're trying to set a different temperature with a different intake door position.

Bottom Line - Assume there's no difference between the lump/ash ability to more readily plug the .625 dia holes than the .750 dia. holes. Intuitively that might appear to be true but it doesn't necessarily matter. It's a small difference and the trade is to drop more unburned lump through. It's a tough call, depends a lot on the lump in the bowl at the time. But, the remaining fact is that it will take less unplugged .750 dia. holes to deliver enough O2 for the fire to burn at the set position (350F in the example above) because they are bigger. Over time, you would look at how much unburned lump is falling through the grate to decide if the grate is performing better.

There. Clear as mud!

Dirtnaponhold · May 2012

Check this out for laughs!
Analysis – Effect of Increasing BGE Grate Hole Diameter from
.625 to .750

Large Big Green Egg (mine), nominal dimensions (inches) and flow areas

Analysis – Effect of Increasing BGE Grate Hole Diameter from
.625 to .750

Large Big Green Egg, dimensions (inches) and flow areas

A1 = Max Intake 3.95 x 2.1 = 8.3 sq in

A2 = Max Chamber inlet area = 13.7 sq in, consists of:

Firebowl slot .125 x
6 = .7 sq in

Firebowl holes 6 x .625 dia. = 2.6 sq in

*Grate holes 34 x
.625 dia. = 10.4

A3 = Max Exhaust 3.85 dia. = 11.6 sq in

Assuming the BGE burns fuel rich and burns all available O2
the Egg is designed to meter O2 at the smallest of the three ports (A1, A2,
A3), which is the intake (A1). (No surprise!)

Assume a 50% intake area results in a dome temperature of
350F: A1 = 4.15 sq in

When 50% intake area A1 results in less than 350F that means
the chamber inlet A2 has been reduced to < 4.15 sq in. In this case:

The blockage is A2 – 4.15 = 13.7 – 4.15 = 9.5 sq in

The percent blockage is 9.5/13.7 = 70%

The equivalent number of chamber holes blocked is .7 x 34 =
23.8 holes

Grate holes of .75 results in A2’ = Max Modified chamber
inlet area = 18.3 sq in from:

Firebowl slot .125 x
6 = .7 sq in

Firebowl holes 6 x .625 dia. = 2.6 sq in

* Fire bowl holes 34
x .75 dia. = 15.0 sq in

When 50% intake area A1 results in less than 350F that means
chamber inlet A2’ has been reduced to < 4.15 sq in. In this case

The blockage is A2’ – 4.15 = 18.3 – 4.15 = 14.1 sq in

The percent blockage is 14.1/18.3 = 77%

The equivalent number of holes blocked is .77 x 34 = 26.2
holes

"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language:EN-US;
mso-bidi-language:AR-SA">Conclusion: Opening up the grate holes from .625 to
.750 in dia. means that 2-3 more holes would have to be blocked to provide the
same restriction in O2.

"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language:EN-US;
mso-bidi-language:AR-SA">How 'bout that!

Huh!

stike · May 2012

The stack effect may torpedo any thought that relationships are linear. Dunno

SteveWPBFL · May 2012

Assume linearity! Otherwise, have another beer.

tjv · May 2012

can't find where you took into account the gap between the top edge of the firebox and inner egg wall. That area could be as much as 50% of your chamber intake area.

i.e., not all the air coming thru the lower slider goes into the firebox, some sneaks up the egg wall. found this out couple years ago, wondering why the medium egg, at times, was difficult to get up to searing temps......

then, there's also leakage around the felt line........always somethin'

t

burr_baby33 · May 2012

Thanks for clearing that up, it's been on mind for quite a while. Now all I need to do is decide what to cook tonight.

Billy Grill Eggster · May 2012

SteveWPBFL how long did you work at the Aircraft ? ?

Too much analysis and like was said above, beyond a 5th grade math level.

The Naked Whiz · May 2012

All I can say is, "It depends...." :))

FanOfFanboys · May 2012

Sorry 'bout all the font commands, can't seem to get rid of them all after pasting in from Word .....

. Paste in Notepad first. Then can delete the commands.

SteveWPBFL · May 2012

It always depends and is more likely to depend as you get older, too. Ignore the airflow going up the sidewall, the charcoal burning is going to pull what it wants from the available inlet flow into the firebowl, but you already knew that!

lousubcap · May 2012

Not sure I understand all I know here-but taking into account all assumptions, then you need serious air hole blockage (>70%) to impact temperature control. And since that happens to some cooks the increased air flow (bored out holes or High-Que grate) should help reduce the chances???

Egg Juju · May 2012

I... uh... yeah!

SteveWPBFL · May 2012

The real benefit is that with the bigger holes more of them have to get plugged to reduce the firebox flow area below the inlet vent setting. Cheers!

Dan4BBQ · May 2012

Did you consider the relationship of the moon to the sun at the time of the burn? If not, you may need to change your calculations by 18.23%

Let me know your thoughts on this.

Ragtop99 · May 2012

I think your analysis assumes air flow and pressure are equally distributed across the fire grate, which I don't believe is the case once a fire gets going. Also it does not account for whether the airflow can get easily to the fire. Open holes on one side of the egg will not help the fire at the bottom of the lump burn on the other side as well as an open air hole right below the fire which allows for O2 to enter from below and combusted gases and heat to exit from the top.

I'd bore the holes out if you feel like cutting some steel.

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Analysis of Modified Fire Grate

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