Natural Draft Air Over Fire

[Mogli]

Is there really a chance of taking advantage of the benefits of air over fire using only natural draft, didnt know if you could achieve it through throttling the AUF door near shut and leaving the AOF ducts at full open to encourage pull thru the top ducts. Just wondering if there was a way to take advantage of AOF without the use of a blower.

[RileySugarbush]

That probably won't work very well. AOFrelies on very high-speed jets of fresh air and oxygen over the fire and into the combustion zone introducing oxygen and providing complete combustion. What you're suggesting would just pull cool air over the top of fire and probably just cool your pans.

[twofer]

Have to agree with RileySugarbush. For complete secondary combustion to occur you need to introduce large amounts of air to the combustion gases coming off the fire. I don't know how you could achieve this just using natural draft.

[Mogli]

One way I was thinking it could work is ducting in the air similarly to how to conventional AOF setups where the air is preheated and introduced at the top of the fire box, only in this application the use of jets could not be used as any flow resistance would negate the natural air draw of the fire. But where my doubts, and I'm sure all of yours fall is that the fire's air pull is far from substantial enough to take advantage of this. I know its not the craziest idea as it is used on wood stoves to get gas combustion, but there its used in the top/rear of the stove for separate secondary combustion, in the case with evaporators we want all heat to be created in the fire box heat is readily pulled out and up the flue. In this case then would combusting the gases as they travel up the arch and under the flue pan. If so the vacuum created by the flow path up and under the pan could be used to draw in air if ducts were placed with the exits facing downstream. Sorry if you feel these are just rants. I am just a engineering thesis student enjoying pondering fluid flow that is not related to my thesis(Optimizing the impeller in a magnetically levitated axial left ventricle assist device, ie making a heart pump pump better while lowering or keeping blood damage at its current level)

[Mogli]

Oh ya and if these ducts worked to get combustion of wood gas as it traveled under the flue pan, would that be beneficial or potentially cause too much heat to go up the flue.

[Vermont Creation Hardwood]

In my homemade evaporator the main source of combustion air is from the lower rear of the unit, directly under the flue. The cold natural draft enters and flows in low. I have no grates, just a thick bed of ashes with the fire insulated from below by those ashes. The embers and burning wood are fed by the natural draft from the rear, creating a loop effect with the now hot combustion gasses heading back toward the rear, but above the colder incoming draft. This keeps the hot gasses right against the pans. I've found that not closing the main door all the way promotes the best burn. I assume the small amount of air entering from the front allows a little extra burn at the front of the fire where combustion might be most starved with a rear natural draft feed. The smoke stack puts out clear, hot exhaust, no smoke.

Of course this is only theory of how my evaporator is burning. It does seem to burn very well, very clean. I can't take credit for the rear draft idea, my uncle introduced me to that in a woodstove he made. The firebox is insulated on the sides by the steel I cut away from the top of the oil tank I used as a firebox. I cut out for the pans and they lay high enough to give me a good sized firebox below. The steel I cut out I is bolted to the sides, leaving a gap of 2 inches which I filled with ash. The tank has not warped a bit. All the heat is directed up to the pans and the only hot side of the firebox is the front which has no insulation.

My advice is to watch the flame and the stack. You want a clean burn without too much breeze. It seems less likely that natural draft will push too much air into the combustion chamber. Usually the problem is too little. You can always attach a stack thermometer to see if you're wasting heat out the flue.

[user587]

From my limited experience (2011 season) with fresh-built arch built to the research paper's specs, I've found that the AOF needs to be turbulent, forced through the small opening to roil around on top of the fire. That is presumably why they recommend the paddle-type blower to provide the slight fractional psi to induce the turbulent air into the fire.

When we open the valve for the AOF, there is a new roar inside the arch from the intense burning of the AOF mixed with the unburnt hot fuel from the fire below. The through-grate under-fire air seems to do little, other than help burn the fuel at the bottom of the firebox. When opening the door with both air supplies on, the fire changes very little (although the radiant heat is intense!)

So my recommendation is to have true forced air with turbulence into the AOF area. Short of that, any natural draft air is better than nothing, but without forced AOF it seems that the fire will probably be starved of available O2 where it needs it.

my $0.02...

[RileySugarbush]

Look at how much pressure would be required to get high velocity flow through your nozzles. I have 16 nozzles of 0.40 inch diameter and the air exits them at 80 mph! That does a great job of mixing oxygen in the volume above the initial combustion zone.

[Mogli]

In my homemade evaporator the main source of combustion air is from the lower rear of the unit, directly under the flue. The cold natural draft enters and flows in low. I have no grates, just a thick bed of ashes with the fire insulated from below by those ashes. The embers and burning wood are fed by the natural draft from the rear, creating a loop effect with the now hot combustion gasses heading back toward the rear, but above the colder incoming draft. This keeps the hot gasses right against the pans. I've found that not closing the main door all the way promotes the best burn. I assume the small amount of air entering from the front allows a little extra burn at the front of the fire where combustion might be most starved with a rear natural draft feed. The smoke stack puts out clear, hot exhaust, no smoke.



Of course this is only theory of how my evaporator is burning. It does seem to burn very well, very clean. I can't take credit for the rear draft idea, my uncle introduced me to that in a woodstove he made. The firebox is insulated on the sides by the steel I cut away from the top of the oil tank I used as a firebox. I cut out for the pans and they lay high enough to give me a good sized firebox below. The steel I cut out I is bolted to the sides, leaving a gap of 2 inches which I filled with ash. The tank has not warped a bit. All the heat is directed up to the pans and the only hot side of the firebox is the front which has no insulation.

My advice is to watch the flame and the stack. You want a clean burn without too much breeze. It seems less likely that natural draft will push too much air into the combustion chamber. Usually the problem is too little. You can always attach a stack thermometer to see if you're wasting heat out the flue.


I too was exposed to this via wood stoves and also thinking of a fairly similar setup to the one you have proposed, more of a front and rear air induction then AOF induction

[Sugarmaker]

Currently using hot tub /spa blower to inject high velocity, high volume of air over fire. IT roars!

[Mogli]

I do agree that forced air would be better in terms of combustion rates, but if limited to natural draft, i still do believe that a substantial combustion gain could be had by a secondary air inlet, in fact i know as most any epa rated wood stove uses only natural draft to drive both the primary and secondary air induction, just looking to run minus plugs and to benefit anyone who is limited to no power in their shack/or wherever they run their evaporator.

[Mogli]

This is a video of the general setup I would try to take advantage of with a much more aggressive burn is mind but if it works with such a restrictive exhaust flow to actual draw in air to both sources and use it I don't see why this couldn't at least be a benefit to anyone that wants or needs to stay to natural draft.

http://youtu.be/qvc79F1pR08

[RileySugarbush]

Go for it! But don't be surprised if it doesn't improve things very much. I watched the marketing video. The combustion they are showing is a lazy fire, like 5% of what you need in an evaporator. It is another thing to have to load wood every 5 to 8 minutes.

My first attempt at AOF used a high pressure regenerative blower, but with too many nozzles and not enough flow capacity. It put quite a bit of air in over the fire, but did basically nothing for my efficiency or evaporation rate. Then I read the paper that is often referenced here, did my own calculations, bought a radial blower and a 3/4 HP high speed motor and ....WOW!

Here is the manifold explained:

http://www.youtube.com/watch?v=LhiEyMjVx5g

[wiam]

This is a video of the general setup I would try to take advantage of with a much more aggressive burn is mind but if it works with such a restrictive exhaust flow to actual draw in air to both sources and use it I don't see why this couldn't at least be a benefit to anyone that wants or needs to stay to natural draft.

http://youtu.be/qvc79F1pR08

With my aggressive air under fire I run 1200-1400 stack temps. This boils sap very fast. I do not see how you could boil very fast with that kind of a fire. Not saying it can not be done, I just don't see it.

[Mogli]

I understand you do not want a lazy fire for boiling off sap, I worked in the industry for a few years and fully under you want the fire to rip and roar. Now you can get a roaring natural draft fire with just the conventional opening, I'm just wondering if a secondary upper inlet would help at all as vacuum increase with rate of burn so once you get a ripping fire going with the conventional air intake, slowly open the top to full open and throttle back the lower door to a point where burn rate is not harmed. I understand that with this type of setup the wood gas burn would not nearly reach burn rates as a forced air version but I do believe it can beneficial at least.

[Vermont Creation Hardwood]

1200 to 1400 stack temps seems a terrible waste of wood. That means a substantial portion of the heat you produce in your firebox is simply going up the stack and never warms the sap. I'd shoot for 400 or 500 degrees at the most.

[RileySugarbush]

Mogli:

You asked a question and got some answers. Looks like you don't agree with the answers, and that's ok. I guess it is time to try your idea!

Vermont Wood:

Sure it is less efficient to have high stack temps. It is a balance between efficiency and production rate. If you are in a hurry you waste some fuel, whether you are driving a car or running an evaporator.

[Billy_the_Kid]

Riley: What is the "the paper that is often referenced here" that you referred too? I really want to learn more about the AOF design. Are there other resources or threads on here that you can point me to? Thanks

[Indiana-Jones]

I like aof so much that if my shack had no electricity I would do a Gilligan's Island deal. Have Mary Anne pedal a bamboo bicycle, vine drive to a coconut air pump.

I prefer Mary Anne, some of you guys may like Ginger. I guess they could trade off.

Have a great season.

[TimJ]

I like aof so much that if my shack had no electricity I would do a Gilligan's Island deal. Have Mary Anne pedal a bamboo bicycle, vine drive to a coconut air pump.

I prefer Mary Anne, some of you guys may like Ginger. I guess they could trade off.

Have a great season.

this brought the chuckles

[RileySugarbush]

Here you go:


http://www.uvm.edu/~pmrc/Combustion.pdf

[Mogli]

I was more clarifying that i understand that the burn rates of wood stove are much different and I know in both cases forced induction is more efficient, even in engines, in fact in all combustion is more efficient if it doesnt have to pull in the air itself. Dont worry this idea will be tried I plan on making an arch over the summer and make this to first try it with just natural draft and then with a modified manifold for forced draft. Figured this community would be a little more supportive of experimentation.

[RileySugarbush]

Don't take my comments wrong! I'm all for experimenting and have some great successes and some fantastic failures.

Look back at your initial question. I think that I and several others have answered your question honestly that we don't think it will work very well in this application. I speak from my own experience, having tried lots of different things, and built and modified arches with natural and forced air flows in many different configurations. That doesn't mean it is impossible, but if I was thinking building an arch to try it out, I guess knowing what didn't work for others would help me decide which way to go.

As I said before, go for it! Let us know how it works!

[wiam]

1200 to 1400 stack temps seems a terrible waste of wood. That means a substantial portion of the heat you produce in your firebox is simply going up the stack and never warms the sap. I'd shoot for 400 or 500 degrees at the most.

For the last 2 years I have made over 400 gallons with this set up with less than 6 cord of wood. I did not boil over 4 hrs any day last year. I have other commitments and I am looking at speed not efficiency. It works for me. I would not wait for sap to simmer with 400 degree stack temp.

[Vermont Creation Hardwood]

I'm looking at the speed of the whole system. Felling, bucking, splitting and loading 6 cords into your evaporator takes an extraordinary amount of work. If you are wasting half of those 6 cords heating the outdoors at stack temps of 1200 to 1400 degrees, that's a fair amount of time you're wasting felling, bucking, splitting and loading those 3 cords for nothing.

[TimJ]

I'm looking at the speed of the whole system. Felling, bucking, splitting and loading 6 cords into your evaporator takes an extraordinary amount of work. If you are wasting half of those 6 cords heating the outdoors at stack temps of 1200 to 1400 degrees, that's a fair amount of time you're wasting felling, bucking, splitting and loading those 3 cords for nothing.

The issue for us is that we have a certain amount of sap to boil in a short time period. If I let it boil lazily with stack temps of 400 degrees I will never get to bed or get through even half my sap. If With 1400 degrees I can boil, get sleep and I have plenty of time all year to gather free pallets, hike the woods and cut the wood.

During sugaring season my time is limited. Boiling hot and fast is the goal.

This is the latest boil - we saw 1200F to 1400F
5577

"Wasting" is relative - I wouldn't consider it wasting if I can get the evaporation rate higher. There is a point on the curve where I would stop - and for each person that point is at a different spot.


We got the EGT probe so we could be consistent with our firing - not for absolute measurements. We wanted to have a good indicator for when to fire since we felt the door opening slowed the boil down and of course we don't want to wait too long before adding more wood.

Again - every one is different and it is a tradeoff.

[Mogli]

Don't take my comments wrong! I'm all for experimenting and have some great successes and some fantastic failures.

Look back at your initial question. I think that I and several others have answered your question honestly that we don't think it will work very well in this application. I speak from my own experience, having tried lots of different things, and built and modified arches with natural and forced air flows in many different configurations. That doesn't mean it is impossible, but if I was thinking building an arch to try it out, I guess knowing what didn't work for others would help me decide which way to go.

As I said before, go for it! Let us know how it works!

I guess i misread your tone common problem when you can't have direct conversation.. Glad to hear your up for experimentation. Sorry I guess I asked the wrong initial question to move the conversation to the issues I and I can tell you guys feel can hinder and make this style of layout null or not worth it. I have read the attached paper in this thread but was wondering what the real demands are for wood fires (ie air/fuel ratio, air feed rate, fuel quality(dry or wet wood, hard or soft wood), etc..). What makes normal natural draft effective at air induction is unimpeded air flow to allow the vacuum created to easily pull in air. Now I know trying to duct in air you get resistance at the inlet, every turn or change in geometry and then at the outlet. If this resistance can be kept low enough I feel enough air could get in for partial gas ignition. Now has anyone tried something like this. I feel the rate at which air can be pulled in naturally to aide the burn is the real concern of most people including myself. Seems effective enough in wood stoves and vacuum is proportional to rate of burn so with the faster rate of burn higher vacuum will be created. The real question is that higher vacuum enough to take advantage of for AOF. My aims are to make an arch anyways I can start with the NA(Naturally Aspirated) version first and either modify or ditch and make new manifolds if forced induction is necessary.

[RileySugarbush]

You can probably pull in enough air over the fire to burn all of the fuel above the fire, even with natural draft. I think the limitation you will run into is that, working with the very low differential pressure of natural draft, the air velocity will be very low. So you will end up with an unmixed or slowly mixing stream of air that travels along with the unburned gases and doesn't really mix with them until they have traveled past the pans. Combustion in the stack doesn't help any more than the combustion we often see at the top of the stack. The real trick with AOF is to balance the mount of air you introduce: Just enough under fire to have the correct reaction rate and exactly the right rate of AOF to completely burn the gases before they are past the pans. To do that efficiently, you need very quick mixing that is best accomplished with optimally oriented, high speed jets of air, right above the fuel.

Well, that's my take on it, anyway......

I try to run mine with the under fire air as low as I can and, with my set up, the over fire air full open, which is about 100CFM.

[Mogli]

ok so there is some agreement that there will be at least partial burn of the gas but prolly not worth the effort of making such a manifold for such a gamble in burn rate. Looks like once i get a tower built this summer i'll abuse my free year of solidworks and run flow simulations on a bunch of set ups to see which gets the best mixing. It would finally give me something to work on that I want to in the terms of CFD.

[RileySugarbush]

Sounds good. I'd like to see the results. Will you be a able to model the volume change due to added combustion/thermal expansion in the mix zone?

[Mogli]

Sounds good. I'd like to see the results. Will you be a able to model the volume change due to added combustion/thermal expansion in the mix zone?
Gonna start with simple simulations to see what type of air can be drawn in under natural vacuum and how to maximize draw, velocity and swirl. Gonna model both the AOF and AUF together to see what kinda swirl they create without the heat source. I should be able to add multiphysics to it and due a heat source in the center of say () btu/hr but gotta see what type of machine i can afford to put together. Multiphysics simulations are computationally exhaustive so it depends what i have capabilities to run without crashing or literally taking over a week to solve.