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any clever Ideas out there for preheating intake air?
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day_trippr
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PostPosted: Wed Feb 02 11 11:22 am    Post subject: Reply with quote

jigsnpigs wrote:
I believe that slightly heating/expanding the air in an indirect manner does not depreciate the oxygen level at all in fact might even create a supercharging effect.


According to Charles Law (which has only been around for a couple of centuries) you'd be wrong...

Cheers! Wink
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Get Your Rub On BBQ
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PostPosted: Wed Feb 02 11 11:39 am    Post subject: Reply with quote

jigsnpigs wrote:
Get Your Rub On BBQ, I'm glad my ideas could inspire you! JK


Been working on this for quiet a while now, just trying to get the placement of the inner workings in place without interrupting the normal use of the smoker.
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jigsnpigs
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PostPosted: Wed Feb 02 11 10:10 pm    Post subject: Reply with quote

colder air is denser, and a given volume would contain more oxygen. But that same dense, cold air also contains more nitrogen molecules and all the other components of air. So from that standpoint, the percentage of oxygen in the air doesn't change with temperature. Wink
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Michael B
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PostPosted: Wed Feb 02 11 10:44 pm    Post subject: Reply with quote

Assuming all of these "warmer air has less oxygen, so it won't work" arguments are correct, does that mean you can't BBQ in southern Florida? You can't BBQ anywhere in the summer, except maybe northern Canada? You can't BBQ in Denver, or any higher elevations?

I THINK double walling the firebox, even without using the enclosed air layer to preheat the incoming air, will improve efficiency. A portion of the heat energy normally lost through radiation to the surrounding area will stay contained in the system and routed through the cooking chamber. That has to improve efficiency.
I also believe it is the double wall insulation effect that will produce the desired effect, more than any preheating that may be accomplished. Simple preheating could be done by making a longer firebox. Build the fire near the exit from the firebox and the air is "preheated" as it approaches the fire.

Anything that reduces radiant heat loss will improve efficiency. Double walling the cooking chamber should improve it a lot! That's why the BGE/Primo cookers are so fuel stingy; more of the heat energy from the fire goes to cooking the food, while very little heats the surrounding area. I can lay my hand on the dome of my BGE while it is cooking at 350*, without getting burned.
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day_trippr
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PostPosted: Thu Feb 03 11 12:42 am    Post subject: Reply with quote

Michael B wrote:
Assuming all of these "warmer air has less oxygen, so it won't work" arguments are correct, does that mean you can't BBQ in southern Florida? You can't BBQ anywhere in the summer, except maybe northern Canada? You can't BBQ in Denver, or any higher elevations?


If there actually are "Good questions", I'm pretty sure none of those would qualify Wink Probably why nobody even suggested them.

Keeping with the original intent - increasing efficiency - I'm pretty sure preheating combustion air isn't going to do it, as the hotter that air becomes, the lower the O2 content. If you're trying to maintain the same BTU output per pound of fuel, you'll have to introduce an even larger volume of presumably pre-heated air to make up for the lower O2 content, so even more BTUs are going into heating combustion air...

Seems like a no-win situation at that point, and that's before even examining the draw needed to make that airflow increase happen.

Otoh, insulating a firebox will conserve energy, directly leading to higher efficiency, and without requiring any speculative airflow mods.

If efficiency was important, I know where I'd put my efforts...

Cheers
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evil16v
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PostPosted: Thu Feb 03 11 1:32 am    Post subject: Reply with quote

Look about 11 posts down on the third page of my build. I have conceived this idea as well. I intend to use a 55 gallon barrel to sheath my firebox when time allows. I already have the barrel. I will push through the hot area with my draft inducer fan that i already use.

as far as the hot air/oxygen argument... It does not apply. ever have an older carburetor car, with a vacuum operated door on the air breather, that drew air by a shield over the exhaust manifold? same deal... ran fine, didn't it?

The turbo-ed cars run an inter-cooler because the compressed air is hot, and causes detonation. In other words combustion is TOO GOOD and the heated charge ignites before intended ignition timing occurs (before TDC and causing Knock). The inter cooler isn't the sole cause of horse power from the cooler charge in of itself (it does, but not that much). INSTEAD you are able to provide more ignition timing and more pounds of boost -- THOSE are the intended two items that create more horsepower. Example: 84-5 Buick GNs could run more boost than the inter-cooled 86-7's. 86-7's made more power with more boost, and a more aggressive tune.

On the flip side

Now as far as drawing through something like that probably won't work well. But like I say. I am going to be using my draft inducer fan. I will also still have to use my fresh air intake (I THINK). I say this because i have to turn down my fan on my stick burner to keep from blowing the heat right out of the stack. I do have to use some air from the manual intake, which is about a 1/4 inch open on a 6" screw open circular plate intake damper. Now if the iducer air is preheated in the first place there is LESS issue with that. I am expecting to run a smaller fire and even out my temps with out the huge amount of radiant heat from the firebox end.

I hope i get more smoke from this in the winter. I burn so hot to keep temp in the smoker, that it burns TOO clean. I have a more difficult time getting smoke flavor on a really cold day (not bad but very noticeable). Much better cooks on the few warm days we have had since the build.

Part of the problem is that not all of the air that the fan puts in the fire box goes to combustion, and dilutes the heat/smoke on a really cold day. Therefor a hot air charge would not cool the air charge,allowing for a smaller fire and a less fan duration used by the PID to maintain the same temps in the cooker.
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Last edited by evil16v on Thu Feb 03 11 2:13 am; edited 1 time in total
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Rockpyle
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PostPosted: Thu Feb 03 11 2:12 am    Post subject: Reply with quote

day_trippr wrote:



Keeping with the original intent - increasing efficiency - I'm pretty sure preheating combustion air isn't going to do it, as the hotter that air becomes, the lower the O2 content. If you're trying to maintain the same BTU output per pound of fuel, you'll have to introduce an even larger volume of presumably pre-heated air to make up for the lower O2 content, so even more BTUs are going into heating combustion air...

Seems like a no-win situation at that point, and that's before even examining the draw needed to make that airflow increase happen.

Otoh, insulating a firebox will conserve energy, directly leading to higher efficiency, and without requiring any speculative airflow mods.

If efficiency was important, I know where I'd put my efforts...

Cheers


Looking at Charles' Law (thanks, Wikipedia!), my only question is that it talks about volume increase, rather than O2 decrease. Is this what you mean by decreasing the amount of oxygen (for a given volume, there is less oxygen in warmer air)?

If so, I think that the temperature change we are talking about is relatively small (in absolute temperature), and I would cretainly be interested in some data about the tradeoff between (combustion effects due to lower oxygen per unit volume) and (efficiency increase due to lower delta T of the incoming air requiring fewer BTUs to heat the cooking chamber)

In general, though, I aree that insulating a firebox would lead to greater improvements.

Rock
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jigsnpigs
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PostPosted: Thu Feb 10 11 10:48 am    Post subject: Reply with quote

What if instead of insulating a firebox it was double-walled and you drafted your air through this space reclaiming radiant heat normally lost or just trapped by insulation,

that being said...its insulating a firebox accomplishing the same goal?

Just remember this is a bbq conversation not a forum for jerks..

Thanks!
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Teleking
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PostPosted: Thu Feb 10 11 11:13 am    Post subject: Reply with quote

jigsnpigs wrote:
What if instead of insulating a firebox it was double-walled and you drafted your air through this space reclaiming radiant heat normally lost or just trapped by insulation,


Insulation is more efficient as stated above. Think of this concept with heating your house in say.... Maine in Feb. Take a house that has walls and attic with no insulation set up to recirculate the heat loss (recovery) back to the heating system as opposed to air tight walls with R19 and an attic with R36 insulation.

Which is more efficient with heat retention without even considering O2 concentrations in warm and cold air. Which house will cost more to heat for the winter???

And since when does stating the laws of physics that has been proven for centuries make someone a JERK???

Cheers
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day_trippr
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PostPosted: Thu Feb 10 11 1:10 pm    Post subject: Reply with quote

Rockpyle wrote:
Looking at Charles' Law (thanks, Wikipedia!), my only question is that it talks about volume increase, rather than O2 decrease. Is this what you mean by decreasing the amount of oxygen (for a given volume, there is less oxygen in warmer air)?


Of course. Change "volume increase" to "density decrease". And it's effect is significant enough that internal combustion engines have to account for it over a temperature range that covers a mere 150°F or so, or they'll run like crap, if at all...

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PostPosted: Fri Feb 11 11 6:05 am    Post subject: Reply with quote

I dont know if superheating the air will work with the small amount of draw produced by a normal cooker or not, but I'd give it a try. In my opinion if you reduced the size of your firebox so you had a small and very hot fire all the time, insulated everything, and built a big enough smoke stack to make the draw needed it should be very efficient. The food might suffer in flavor and quality with the extra air flow and the lack of smoke though.

Preheating the air seems to work great for high efficiency ,non catalyatic, EPA approved wood stoves.

http://www.epa.gov/Compliance/monitoring/programs/caa/whlqanda.html#types Click on What types of EPA-certified wood stoves are available for sale?

http://www.napoleonfireplaces.com/Brochures/WOODStoves.pdf Scroll to page 9

http://www.quadrafire.com/en/Ideas%20and%20Advice/Technology%20Info/QuadraFire%204-Point%20Combustion%20System.aspx

http://www.regency-fire.com/Buyer-s-Guide/FAQ-s/Wood-FAQ-s.aspx

http://www.morsona.com/index.php/videolinks?___store=default

http://www.lopistoves.com/TravisDocs/98800200.PDF Scroll to page 21.

All of them and several more that I dident list all use pre heated air in a secondary burn zone.
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