Basics on Air over Fire?

[MapleMark753]

If somebody could explain the air over fire process, how its set up, how it works, maybe how they've built one and had good success with it that would be great.
If there's an active thread elsewhere here I didn't see it. From the one I saw it looks like a person who can build stuff can build one, but it needs welding. Is that right? thanks, Mark

[mapleack]

http://mapletrader.com/community/showthread.php?17239-AUF-AOF-ad-blower-questions
http://mapletrader.com/community/showthread.php?7884-Adding-over-fire-combustion-air
http://mapletrader.com/community/showthread.php?18203-AOF-arch-build-help

[maple flats]

Here is a link I posted 3 yrs ago. http://mapletrader.com/community/showthread.php?7884-Adding-over-fire-combustion-air In the first post is another link to the resource I used to design the project. I am a very poor welder at best so I cut all of the pieces to length and had a friend weld it. I used 2x2 tubing 1/16" wall for the air duct. The directions suggested 3" but I used what I had available.
I am very pleased with the performance. My boil increased and my wood consumption dropped. I give some numbers in the original post that have changed as I got used to the set up. I now fire every 9 minutes, only about 1 arm load at a time of wrist sized wood. I fill one half at a fueling and the other half 9 minutes later. I refuel by a timer. When the timer sounds I reset it for 9 minutes and then fuel as fast as is practical. In the beginning I shut off the high pressure blower to fuel, but I then discovered it was unnecessary, when I fuel, I leave the blower running which supplies both over fire and under fire air. I just open the door and add wood. I get no smoke or sparks out the front. I get a nice hard boil once
I have been running about 30-40 minutes (I need the coals to build up some). My air distribution is thru a 4" PVC pipe from the blower outside to under the arch. There I split into 3 PVC pipes 3" each. Each has a valve, one for under fire air the other for over fire air. After trying many different positions I now have the valves set with no adjustment (for about 2 seasons) The under fire air is open about 1/3 and the over fire air is open about 3/4, both on 3" ball valves. The turbulence in the firebox is truly impressive. I now run lower stack temps and I see no sparks or flame at the top of the stack like I did before the AOF was added.

[MapleMark753]

Thanks for the links, and for the information! I knew there had to be good info and experience here, just didn't find it at first. Mark

[14ledo81]

The arch I plan on building next year will be made out of a 16" steel pipe laying on its side. I will make one of the round ends a complete door. I had planned on putting the grate about 3" up from the bottom, leaving me with a firebox around 16" wide (at its widest) 13" high and I had planned on making it 26" long. I was going to cut an 8" wide slot the total lenght of the pipe and then welding some flat steel out from there to get a wider pan. My question is, "with the size arch and fire box, does it pay to try to mess around with aof?"

[maple flats]

While with AOF and AUF gives a very complete burn at only 3" under for ashes you will need to clean the ashes more often, even thou most will blow out the stack. It does sound like a sweet little unit, but you will need to come thru from the outside to run the (high pressure) HP air nozzles in, no room for a nozzle manifold and protective refractory in such a small space. Lets see some pictures as this progresses.
One other issue to consider is the cost of a HP blower, even small ones are pricey.

[14ledo81]

Dave, thanks for the reply. I was thinking about using black steel pipe (sch 40) for my manifolds for air. I work for a mechanical shop that's why all the use of pipe. I was also wondering if, because my fire box is so small, I could get by with a 1 1/2" under the grate with slots or holes drilled for auf. That way I could get the air all over under the grate and not just in the back. Is that necessary? Or should I just put the auf in the back and not worry about extending it to the front.
For the aof, again my reasoning being the smaller size of the unit, I was thinking about putting (2). 1 1/4 pipes at the edge of the 8" slot from front to back. I was thinking maybe not use nozzles and just drill holes. Or will my sch 40 pipe not last long inside the firebox?
The 16" pipe is probably 5/8 " thick and I was thinking of using 3/8" thick flat steel to build the remainder. I was hoping with the thickness of material it would hold up with out protection. What do you think?
I will certainly post pics when I build. I plan on starting next January when things slow down. I have already got confirmation from a welder that he will help.

[RileySugarbush]

Dave, thanks for the reply. I was thinking about using black steel pipe (sch 40) for my manifolds for air. I work for a mechanical shop that's why all the use of pipe. I was also wondering if, because my fire box is so small, I could get by with a 1 1/2" under the grate with slots or holes drilled for auf. That way I could get the air all over under the grate and not just in the back. Is that necessary? Or should I just put the auf in the back and not worry about extending it to the front.
For the aof, again my reasoning being the smaller size of the unit, I was thinking about putting (2). 1 1/4 pipes at the edge of the 8" slot from front to back. I was thinking maybe not use nozzles and just drill holes. Or will my sch 40 pipe not last long inside the firebox?
The 16" pipe is probably 5/8 " thick and I was thinking of using 3/8" thick flat steel to build the remainder. I was hoping with the thickness of material it would hold up with out protection. What do you think?
I will certainly post pics when I build. I plan on starting next January when things slow down. I have already got confirmation from a welder that he will help.

It's tempting to use free stuff laying around, but you will probably be far better off if you build a more conventional arch frame from angle steel and sheet metal, with insulation and bricks. Round stuff is a pain to adapt to flat stuff, even the thick steel walls of your pipe will get unreasonably hot, 16" is too narrow and not standard for a pan, etc. I built a 2x6 arch from thin, cheap angle and 16 gage sheet metal. Very easy and it works great.

[lpakiz]

14ledo81,
First, the general rule is to protect all metal from all flame, either with insulation, brick, refractory or some combination of these. Any metal exposed to flame will deteriorate fast, but your excessive thickness will help with that.
Second, even tho you have very thick walls, you should insulate to keep heat in for efficiency and comfort and safety. With the small beginning diameter you are starting with, it will be hard to do that on the inside and still have much firebox left. If you insulate on the outside, see (First) above.
Could you could have grate air using a "hollow" floor of flat plate, drilled to allow the air to come up thru?
Seems you need to conserve vertical space, so that might help. Grate air can be "lazy" air, just drifting into the firebox with no particular direction, only in the required amount. It doesn't take too much grate air to get a ferocious burn going.
Now, about AOF. The theory here is to have "jets" of air shooting and penetrating into the flames for optimum mixing. You won't get that with a hole thru a sheet or sidewall of a pipe. You need a nozzle to focus and aim the air. Given the smaller size of this unit, you might consider 1/4 inch shed 40 or even 80 pipe. Again, consider how you will protect these nozzles, as well as the supply manifold. It will be helpful to be able to regulate the amount of grate air to achieve the burn rate you want. I highly recommend a stack thermometer.
I hope I didn't discourage you too much. Remember, these are recommendations for optimim results. You can bend or break these suggestions as you like.

[14ledo81]

7840

See attached pic. My coworker built this arch this year. Pretty unique design. If you look close you can see the way he has the flames directed toward the center of the pan, and then both ways into the dual chimneys. He was able to get 13-15 gph with this on a roughly 2 x 2 pan! Notice his pre-heat pans sitting on the flat top portion of the chimney. Not many believed him, but I witnessed the boil, and he needed to put baffles/flat plates on the top of his pan to stop the pan from splashing out. It looked like multiple jacuzzi jets.

I was planning on using his basic design, but utilizing the 16'' steel pipe for the fire box. I plan on having my pan be a little smaller, maybe 22'' x 22'', but would still like to mazimize my gph. I figured if he could do it with his setup, and I add auf and aof, who knows what I could get!! The skys the limit!

My welder friend (who has the 16'' pipe) also has a 20'' pipe. Should I try to use that instead?

I could also, very easily, put the aof pipe outside and drill holes for the 1/4''? nozzles and weld/seal the openings.

Mainly, I was just wondering if with such a small operation, would I see any or much benifit with aof? I can add it for a relatively low cost, so I thought I may as well.

Thanks again for all of the advice. I have roughly 7 month design period, so I will have time to get it right.

[14ledo81]

7845

See attached pic. This is a sketch of what I was thinking to do. I thought I would make the sketch in case I was not clear with my description.

Any advice or critique is welcome.

[RileySugarbush]

Those are clever designs and should work well at that scale. I'd say go for it, you have a lot of enthusiasm and it is great to have new ideas tried out. It looks like your buddies arch has double walls which help keep the heat in a bit, really just keep your pants from catching on fire. You should do something to insulate yours.

AOF is a little finicky to get right. You need a high pressure blower and have to balance blower and nozzle size/number. In your case, I would just start with a simple AUF blower with inlet damper. No need for nozzles, just port it into the area under the grate

[AGR1093]

Can a large hot air furnace blower work for AOF? I have just a small blower with only AUF right now, but am reading these in hopes of modifying my arch this year. I have a big squirrel cage blower out of a house hot air furnace... but that isn' a high pressure blower, right?

[RileySugarbush]

Can a large hot air furnace blower work for AOF? I have just a small blower with only AUF right now, but am reading these in hopes of modifying my arch this year. I have a big squirrel cage blower out of a house hot air furnace... but that isn' a high pressure blower, right?

AOF needs a high pressure blower in order to get high velocity jets. A furnace blower won't do it.

[Jeff E]

Agreed, you want air pressure, not air volume. The furnace blower moves a lot of air, but wont push it through nozzles.
I have 3/8" nozzles every 6" down each side of the arch. They are offset, from one side to the other, to maximize turbulance.
The nozzles are 6" below the rail, angled down about 20 deg.
It is very obvious when I forget to open the damper to the over air. I will be splitting my wood larger this year, and I think I can extend my reload time and not comprimise evaporation rate now.
Still learning....

[chicken123]

Would a shop air compressor work? it would be high pressure and I could put a regulator on it.

[maple flats]

Wrong, no squirrel cage blower is high pressure, just high volume. A high pressure blower will have paddles of some sort and they "throw" the air out the discharge. The term high pressure is almost a misnomer. This is high pressure as compared to a squirrel cage blower which scoops and flings the air. A high pressure blower really only will have maybe a 2 or 3 psi pressure, but a squirrel cage has even less. You must have the high pressure for getting the necessary turbulence for over fire air, but under fire is fine with squirrel cage air. Without the high pressure the over fire air will not mix the gases and get a total burn of the wood gases.

[RileySugarbush]

The high pressure blowers for AOF put out even less than the 2-3 psi Dave mentions. Mine puts out a pressure of 4 inches of water at about 60 CFM. That is only 0.15 psi! My nozzles in combination with that blower result in a bit under 100 CFM at about 3.5 in h20 at the nozzle entrance giving about 80 mph jets.

But that pressure is still many times higher than a squirrel cage blower and is about the minimum you need. I wish I had one that put out about 6 in h20, but as it is it works pretty good.

An air compressor is not a good fit. Terribly inefficient to compress air to 100 psi and regulate it down to to levels we need at the flow volume required.

[lpakiz]

You know, I did go see someone's Intensofire copy. His blower is a belt-driven furnace fan. It does deliver a fair amount of pressure. He is running it with a 3450 RPM motor. The motor has about a 6 inch pulley on it and the blower has about an 8 inch. I am guessing he is at about 2900 to 3000 RPM. Like I said, he does get a fair amount of pressure out of his AOF nozzles and his volume, of course, is unlimited. Not ideal, but cheap and better than nothing. Worth doing until the right blower is available$$.

[14ledo81]

Would combustion blowers (for gas water heaters or furnaces) be classified as high pressure?

[RileySugarbush]

Would combustion blowers (for gas water heaters or furnaces) be classified as high pressure?

I doubt it. The high pressure blowers we are talking about would be 3/4 hp minimum. for your smaller rig, you could get away with less power, but I still think you want a radial blade blower. It is easy to check the static pressure of any blower you have with a home made manometer.

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

[maple flats]

I'm not sure about a gas blower but I believe in the study and design reference link I posted at my first reply in this thread, they mention that the blower on an old oil burner gun is high pressure. Then you will need to determine the volume of HP air it produces. It may work on a unit as small as you are designing. Read that report, they may answer your question.

[14ledo81]

I doubt it. The high pressure blowers we are talking about would be 3/4 hp minimum. for your smaller rig, you could get away with less power, but I still think you want a radial blade blower. It is easy to check the static pressure of any blower you have with a home made manometer.

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


John, I have access to manometers. I work for a mechanical company. To check the pressure, where would I attach the hose from the manometer? What type of readings am I looking for?

[RileySugarbush]

John, I have access to manometers. I work for a mechanical company. To check the pressure, where would I attach the hose from the manometer? What type of readings am I looking for?

Not a complete measure of the blower performance, but a good first check.

A check of static pressure is to block off the entire output with the exception of the manometer. Just duct tape over output port, completely sealing it off and poke the manometer tube through the tape. Then the manometer will then be measuring the maximum pressure of the blower at zero air flow. If this static pressure is less than 2 or 3 inches of water, I'd look for a different blower.

[14ledo81]

Not a complete measure of the blower performance, but a good first check.

A check of static pressure is to block off the entire output with the exception of the manometer. Just duct tape over output port, completely sealing it off and poke the manometer tube through the tape. Then the manometer will then be measuring the maximum pressure of the blower at zero air flow. If this static pressure is less than 2 or 3 inches of water, I'd look for a different blower.

Thanks John,

Even though I am not one of the field techs (I do project managing), I still should be able to handle that simple test. I have quite a few different blowers that I can try, lets say I do find one that has more than 3 inches of water, what is my next step?

[14ledo81]

Well iin case anyone else was wondering, John was right about the combustion blower for a water heater not being high pressure. Not only did it not make 2-3 inches of water, it did not even budge the needle. Oh well, the blower was free. I will try some more different ones. We have quite a few sitting around our shop.

[Waynehere]

Maybe not for AOF, but I have a one of those draft inducers as a backup for my AUF.

[nymapleguy607]

Check e-bay for blowers. I found mine without the motor for under $250. The blowers from a dust collector would also work.

[14ledo81]

I tried another combustion blower last night. This one was from commercial make-up air unit (last was from a residential water heater). As soon as I plugged it in I could tell a difference. I was all ready to see a nice high reading on the manometer. Boy was I disappointed. It was able to move the needle off of the peg, but just barely. Maybe .5 to 1 inch of water.

On a side note. We have quite a few old motors around our shop. Many of these have a higher HP rating. Does anybody think it possible to modify or make a new blower using one of these motors?

[lpakiz]

More HP won't do a thing. You need more RPMs. Hard to do when the impeller is mounted to the armature. Now if the impeller is on a separate shaft, belt driven, well now you have some options.

[14ledo81]

More HP won't do a thing. You need more RPMs. Hard to do when the impeller is mounted to the armature. Now if the impeller is on a separate shaft, belt driven, well now you have some options.

Interesting. Do you think it possible to extend the shaft that the impeller sits on, put pulleys on both the impeller shaft and motor shaft, and hook together with a belt? Meaning do you think it is something I could try to make myself? Would it make much difference even if the blower is still a squirrel cage style?

[lpakiz]

Well, how handy are you?
You would need to mount the impeller on a separate shaft, along with 2pillow block bearings and a V belt pulley. This will have to be positioned and spaced so that the impeller is correctly located in the housing.
The motor wont need anything but a V pulley. The motor will have to be mounted so that you can tension the V belt against the impeller shaft. Sound simple? I had considered doing this for mine but it turned out that I found a 3 phase, VFD converter that allowed me to over-speed the original 3 phase motor a little.

I saw a setup like this with a furnace blower and it worked pretty good. His motor was a 3450 to start with.....

[RileySugarbush]

Nope. Blowers are designed for specific purposes, optimizing quietness, volume of air and pressure output as well as power/efficiency. You need a blower impeller that id specifically designed for high-pressure output. These are typically radial vane impellers, they're not very quiet and they take a lot of power. Spinning the wrong impeller at a higher speed won't get you what you want.

[14ledo81]

Nope. Blowers are designed for specific purposes, optimizing quietness, volume of air and pressure output as well as power/efficiency. You need a blower impeller that id specifically designed for high-pressure output. These are typically radial vane impellers, they're not very quiet and they take a lot of power. Spinning the wrong impeller at a higher speed won't get you what you want.

John, someone had mentioned before about leaf blowers. I checked today and smaller ones can be had relatively cheap. Do you think they would work? I apologize if you are getting sick of all my tight-a#% questions. The more I can save on my arch, the more I can put into a sap shack.

[RileySugarbush]

John, someone had mentioned before about leaf blowers. I checked today and smaller ones can be had relatively cheap. Do you think they would work? I apologize if you are getting sick of all my tight-a#% questions. The more I can save on my arch, the more I can put into a sap shack.

Years ago I used a little electric leaf blower for under fire air. Noisy but it worked fine. I have no idea if they develop sufficient pressure. I have my doubts. I still have it, maybe I'll check tomorrow!

[maple flats]

I suspect a leaf blower might give the needed pressure, but doubt it will last very long. A leaf blower is designed to run a few minutes at a time, the HP blower will need to run for several hours. The last thing you want is to go to the effort to build the AOF and then have the leaf blower fail after 2 hrs of constant use.

[nymapleguy607]

I used a leaf blower when I first tried AOF. It worked okay and it sold me on the concept, the blower didnt sound to good after running it for a while. The new blower was my next purchase

[maple flats]

The technology of building a high pressure blower is very simple. Getting the thing balanced is another issue.

[14ledo81]

Should I be looking for a blower motor with a higher HP rating or higher RPM?

[RileySugarbush]

It's not as simple as that. It is true that high pressure blowers tend to turn at high rpm, and that they require higher horsepower motors because they are putting out more power in air. But high power motors or high speed fans don't necessarily put out high pressure air.

The cheapest high pressure blowers I have seen were discussed in a thread here or on sugarbush.info talking about blowers for inflated play structures. I have not seen flow curves but I think they claim 5" h2o and continuous operation.

That said, from your design sketch I think that you will have less benefit from AOF than a more traditional arch, since your pans are only exposed to the flue gases over a very short length. I would concentrate on rapid combustion with a medium sized AUF blower and good fire grates, an insulated arch and keep it simple like that at first.

[14ledo81]

It's not as simple as that. It is true that high pressure blowers tend to turn at high rpm, and that they require higher horsepower motors because they are putting out more power in air. But high power motors or high speed fans don't necessarily put out high pressure air.

The cheapest high pressure blowers I have seen were discussed in a thread here or on sugarbush.info talking about blowers for inflated play structures. I have not seen flow curves but I think they claim 5" h2o and continuous operation.

That said, from your design sketch I think that you will have less benefit from AOF than a more traditional arch, since your pans are only exposed to the flue gases over a very short length. I would concentrate on rapid combustion with a medium sized AUF blower and good fire grates, an insulated arch and keep it simple like that at first.

John,
Thanks for the input. I agree with you that I will not gain as much with aof as the larger
operations. I would think the gains would be percentage based, as such, with my smaller operation the gains may very well be minimal. The thing is, I don't think it will add much to my costs to add it. 10' of pipe, a few fittings, and some 1/4" nipples. The vast majority of the cost would be a blower unless I can make something work. I think I will build it in, but maybe try it with the best blower I can find. If that does not work, maybe save for a better blower or scrap the aof. If I decide to scrap it, I would not be out much.

On a different note, I thought I read that a blower from a fuel oil burner/gun would work. Is this true?

[14ledo81]

I tried another blower today. It was a large squirrel cage blower from a furnace. I blocked off the whole outlet and cut one small hole for the manomerer hose. Once again, the needle barely moved, not even a half inch of water column.

[maple flats]

Again I emphasize, a squirrel cage will not work. You must have a blower that throws the air out rather than fling it. A squirrel cage is just flinging the air. The blower must have something that is basically paddles throwing the air.

[14ledo81]

Dave, I had read that before, but I just had to see it for myself. I am a believer now. I was actually amazed because the blower (when it was not blocked off) seemed to move a lot off air, but still did not budge the manometer. Like you and others have said, that was because the squirrel cage type is not the right kind. Now I have seen it, and believe it. apparently I need to learn the hard way.

[Indiana-Jones]

Hi, I thought that I would chime in on this blower project.

I tried the squirrel cage blower, no pressure. I read a bunch here about paddle blowers and RPM's. Learned a bit. Searched online and came up with the idea to try the draft induction blowers off of a high efficiency gas furnace.

The blowers have a 2 inch round output that is easy to work with, spin at @ 3000 RPM and are made to be quite. I put two blowers on my oil tank 2X5 and I'm very pleased. Could something be built to be better? For sure, but for what I got in to this, less than $50.00, it is wonderful.

I traded a bottle of syrup for dumpster diving privileges at a heating and cooling contractor and got blowers to spare. A 10 ft stick of 2" rigid conduit, 1 elbow cut in half and some flexible exhaust from Auto Zone. Add a hand full of 3/8 pipe nipples and you are in business.

This is what the units look like. http://www.amazon.com/Fasco-A170-Replacement-Specific-Purpose/sim/B000BUPQ7G/2/ref=pd_sxp_grid_mlt_0_1

Check out my photo bucket, There are some pictures.

[maple flats]

Looking at the link, some of the blowers MIGHT be high pressure, but I doubt it. The ones where the impeller can be seen are all squirrel cage. High pressure with have that housing shape but the impeller will be paddles rather than the shape you can see in the exhaust port on those. Those are squirrel cage design, lots of air but no pressure. Without that pressure you don't generate the turbulance needed to mix and burn the gasses headed towards the stack.

[14ledo81]

I have to agree with Dave on this one. I work for a mechanical company, and I have tried a few combustion blowers. The best one produced 1/2 inch off water column.

[AGR1093]

http://www.newegg.com/Product/Product.aspx?Item=9SIA1JB0M87936&nm_mc=KNC-GoogleMKP&cm_mmc=KNC-GoogleMKP-_-pla-_-Activity+Centers+%26+Play+Areas-_-9SIA1JB0M87936

This bounce house blolwer claims 9 inches only $188 blower and motor. They are even cheaper on ebay.

[maple flats]

That may be the right type. I question the durability, but it seems like it should be ok for a few seasons. The thing to look for is if it is rated "continuous" use. Another is whether new bearings can be installed and in fact, does it have bearings or bushings. There is a reason why a HP blower is so costly. For example, mine is likely well over 50 yrs old, since the friend who salvaged it out of a factory he worked at had worked there 41 years and it was old looking when he started there. He was their machinist and never had to work on it. I do have a different blower motor because it was 3 phase which I don't have.