FYI - MicroDamper + 22"WSM + HeaterMeter + AirBurner = Fail

[J Thompson]

Greetings All,

TLDR; The MicroDamper fan can't flow enough air through the bends on the Air Burner so use the blower provided in the HeaterMeter Kit.

Got fancy and figured, why not use the MicroDamper. Foolish move. This configuration had literally zero effect on the grill temp. Had I known, I would have saved three cooks trying to figure out why I couldn't establish / maintain temp in the grill. Here are some pics of my setup for comparison purposes. Ultimately, I swapped the blower in for the fan and duct taped it directly to the copper tubing. Kept 300 deg @ ~ 40 deg outside temp all night.

Measure

Drill and Insert the First Tube - I drilled straight through the bottom leg retainer plate as well

Add Copper Fittings (All the small copper pieces are 4")

More Fittings

DONE

Test

Fail lols

Hopefully these pics and findings help someone avoid wasting time trying to figure out why it's not working. Feel free to ping me for any questions regarding sizes etc.

 

[RalphTrimble]

Yep, fans aren't able to push against restriction, they're basically useless if there is any bottleneck in the air passage. A blower, however, can create static pressure which will overcome restrictions and force air to flow... Lesson learned I guess...

 

[J Thompson]

Indeed. Should have trusted my intuition but there's a strong urge to over-engineer which I did by not using what came w/ the kit

 

[Bryan Mayland]

That's a really great looking airburner setup though, I love seeing those. What size is the copper tubing, is it 1" diameter? At some point I am going to have to build one of these just for testing on the workbench at the very least.

 

[RalphTrimble]

I'm guessing 3/4" copper, that is what I made the original air burner out of.

 

[J Thompson]

That's a really great looking airburner setup though, I love seeing those. What size is the copper tubing, is it 1" diameter? At some point I am going to have to build one of these just for testing on the workbench at the very least.

Ralph is correct, it's 3/4" Copper and thanks! I didn't have enough holes drilled to sustain higher than about 255 deg during my cook last night so I'm going with the pipe only to compare on the next one. Trial and error I wonder if I could use diameter of the holes to know exactly how many I need to have the same amount of flow with the pipe only. The bends would likely need to be compensated for. More to come.

 

[Dave Smith]

Ralph is correct, it's 3/4" Copper and thanks! I didn't have enough holes drilled to sustain higher than about 255 deg during my cook last night so I'm going with the pipe only to compare on the next one. Trial and error I wonder if I could use diameter of the holes to know exactly how many I need to have the same amount of flow with the pipe only. The bends would likely need to be compensated for. More to come.

try a bigger fan or different type fan/damper; I have no problem with a Rotodamper on my burner setup, can get temps over 375 easily. Also, try shortening the pipe the fan attaches to, might be too long.

 

[MartinB]

I wonder if I could use diameter of the holes to know exactly how many I need to have the same amount of flow with the pipe only. The bends would likely need to be compensated for. More to come.

Yes.
Easy for a chemical or mechanical engr to calculate

Most dP will likely be the orifices btw.

 

[RalphTrimble]

Area=pi*r*r = 3.14*10*10 = 314 sq mm
(where r=10mm because the inside diameter of the 3/4" copper pipe is 20mm. I generally work with metric because a 10 based system is much easier to work with than US fractions.)

So your cross sectional area of the 3/4" pipe is 314 sq mm.

Lets say you want 10 holes, so you want cross sectional area of each hole to be about 31.4mm, so.

31.4mm = 3.14*r*r

r*r =31.4/3.14 = 10

r = ~3.2mm

So, in the example, 10 holes with diameter of 6.4mm would be roughly the same cross sectional area as the 3/4" pipe.

The math on that is:

Area (per hole)= pi*r*r = 3.14*3.2*3.2 = 32.15 sq mm

So, 10 of those 6.4mm diameter holes would be 321.5 sq mm, roughly the cross sectional area of the 3/4" pipe.

Just replace your desired constraints, be it number of holes, or size of holes, in the equations above to find out what size or how many hole you need.

Is that what you were looking for?

Math is great, but may not actually make for the best performing air burner. Reason being, the concept is not only to distribute the air flow evenly under the fire, it is to create air jets that blow on the fire to stoke it much like a fireplace bellows. To accomplish this you may want to restrict the air flow down a bit (making the total cross sectional area of your holes less than that of the 3/4" pipe) so the air jets out of those holes a bit more rather than flowing in a more relaxed fashion.

For this reason I suggest you just experiment with the size and number of holes, start with fewer smaller holes, see how the air jets out of them. Move up to more holes or larger holes until you get the right balance of jet vs total flow for your pit. There is no need to solder any of the copper pipe, so if you go too large you can always replace that section of pipe and move backward. That is how I tuned my original air burner and it works really great....

Good luck!

 

[MartinB]

Burning 1 lb/ hr charcoal requires about 1.2 scfm air.

@10 - 6mm holes, for example, you could pass 2.4 cfm @0.69"wc, neglecting the copper pipe.

1 m of 3/4 copper tube, .745 id would pass 7 cfm air at .69", ie the holes dp is much greater than piping.

The blower curve. At 2cfm pres is only 0.6., so it would be less than 2 cfm neglecting piping.

It is not that difficult to calculate whats needed. For an engineer. We have anecdotal charcoal/hr at different temps. At 225 the wsm 18 uses about 1 lb/hr . Much more at 325, likely in ballpark of double.


My only point is you're really restricting the airflow, and you might not realize how much, or that you don't have anywhere close to that 6.7 CFM. Likely near minimum of 1.5

Fine to do things by trial and error, even necessary, but also good to understand what your doing too.

 

[RalphTrimble]

Hitting the point re: restricting air flow...

The HM is great but one byproduct of pushing air into your pit with a blower is you cause cooling in addition to stoking the fire. If you flow too much air, particularly in cold climates, you might end up fighting the cooling effect, burning more fuel and drying out food as a result.

The air burner concept was to limit the air flow but focus the air so the fire is stoked quicker with less air pushed into the pit. Thus the small holes that jet out air directly at the fire, less air, but more focused. Hope that makes sense...