Air manifold

[treehugger]

Is there a real problem if the air nozzles on an air over fire system are not completely covered?

[maple flats]

Please explain. On my AOF I have refractory cement covering it all, but the nozzles stick out into the firebox anywhere from 1" up to 2". I left them, thinking the extreme heat in there would prune them, but I had my pan off today, changing pans and I see no sign of any change from when I built it several years ago. On the other hand, if the manifold is the only thing between the fire and the galvanized or SS sides I think you will have problems.

[treehugger]

Please explain. On my AOF I have refractory cement covering it all, but the nozzles stick out into the firebox anywhere from 1" up to 2". I left them, thinking the extreme heat in there would prune them, but I had my pan off today, changing pans and I see no sign of any change from when I built it several years ago. On the other hand, if the manifold is the only thing between the fire and the galvanized or SS sides I think you will have problems.


I am retrofitting an aof manifold. If I completely cover everything i will really reduce the size of the firebox. I was planning on adding more brick, cement and blanket but wanted to leave the nozzles exposed. Pretty much what you explained.

Did you see improvements in boiling and wood usage?

[RileySugarbush]

The nozzles are cooled by the air flowing through them. Mine are also exposed and are made of stainless hydraulic tubing. They survive fine. I see some improvement in both evaporation rate and reduce consumption of wood, but I cannot quantify since I seem to always change more than one thing at a time!

[Haynes Forest Products]

Take any wood fired arch that does not have any supplemental air and a draft door. Stick a leaf blower into the ash door while you watch the boil and you will be jumping up and down with excitement. Now have some one go out and watch the steam stack and you will see instant response.

Now I will tell you that you will blow some ash around the place so be careful. I will also warn about sparks flying out the flue pipe but hey its a real hoot to see the sap start jumping and the stack looking like a locomotive.

[treehugger]

I already have auf with high pressure air injection, but I have always felt not enough air as the thick black smoke after refueling can be seen and smelled a mile a way. That's why im adding aof too. the ratios are going to be my next hurdle (after i rebrick everything). I saw 80% over and 20 under. 50/50 seems logical. And I have a spark arrestor on my stack.

[treehugger]

15097. This is what I got going on so far.

[RileySugarbush]

A couple of observations of your photos, take em or leave'em!:

Your grate looks like the air holes are few and small. If what I'm seeing is your grate. In general you want an even, slow flow of air up through the fuel to promote the right amount of combustion, and a well distributed air path so coals get consumed and don't build up. If your grate is pretty open, the air under it can flow easily without a lot of pressure, which is why a small squirrel change blower works there. ( or a throttled bleed off the hp blower)

Looks like a lot of AOF nozzle area, depending on your blower pressure. The goal of AOF is to have very high velocity jets so you get good mixing. High velocity only occurs if you have high pressure just upstream of the nozzles. That is why it is best not to damp or throttle the HP flow to them. The amount of air coming out of the AOF nozzles is the velocity multiplied by the sum of the area of the nozzles, so if you have good velocity with a high pressure blower and lots of big nozzles, you might end up with way too much AOF volume, cooling down the area under the flues and increasing the total velocity of the gases so the fly right by the flues and up the stack before there can be sufficient heat transfer. When I changed blower from 4 inches of water to 8, I reduced my nozzle size from .4 to .25 inches to keep the flow under control. It's a big balancing act and hard to figure.

[treehugger]

Thanks for the input. I do not have grates. The floor of the firebox is poured refractory cement with 15 high pressure air injectors. Under that cement is a n air manifold that's hooked up to a high pressure high volume blower. It works pretty good as it is but I know i can be more efficient and probably gain some gph. I ran the blower today it felt like the top was a little less vigorous than lower, but the are 4 more nozzles on the upper manifold. May I can plug a few holes on the upper? As a reference, the guy who made my evaporator may have modeled it after a force 5 but did not include aof.

[mellondome]

For pressure/volume at your aof rail, you seem to have a small manifold that it is supplied by a n equally small pipe. It appears that you have more outlet surface than you have supply.
For covering.... Maybe use 1in blanket .

[treehugger]

The manifold is 2" and supply pipe is 1 1/2". The blanket to cover manifold or plug some nozzles?

[Waynehere]

A couple of observations of your photos, take em or leave'em!:

Your grate looks like the air holes are few and small. If what I'm seeing is your grate. In general you want an even, slow flow of air up through the fuel to promote the right amount of combustion, and a well distributed air path so coals get consumed and don't build up. If your grate is pretty open, the air under it can flow easily without a lot of pressure, which is why a small squirrel change blower works there. ( or a throttled bleed off the hp blower)

Looks like a lot of AOF nozzle area, depending on your blower pressure. The goal of AOF is to have very high velocity jets so you get good mixing. High velocity only occurs if you have high pressure just upstream of the nozzles. That is why it is best not to damp or throttle the HP flow to them. The amount of air coming out of the AOF nozzles is the velocity multiplied by the sum of the area of the nozzles, so if you have good velocity with a high pressure blower and lots of big nozzles, you might end up with way too much AOF volume, cooling down the area under the flues and increasing the total velocity of the gases so the fly right by the flues and up the stack before there can be sufficient heat transfer. When I changed blower from 4 inches of water to 8, I reduced my nozzle size from .4 to .25 inches to keep the flow under control. It's a big balancing act and hard to figure.

I agree with John here. You don't want high pressure under fire, but slower volume that you can control. This is your throttle for how fast you want to burn your wood up and create heat. The more air volume under, the faster it will burn. The AOF is to mix in smaller amounts of air into the already burning fire, but make it turbulent, like it was coming from an air compressor. That is what will reduce the smoke going up the chimney because the AOF is helping with completely burning the unburnt gases/smoke. Like John says here, it is a balancing act to get the right combination of AOF and AUF for fuel effeciency. You don't want to go full throttle with the AUF if you want to save wood. However, if it is GPH you are looking for, turn your AOF way down and AUF way up and that will be the most heat you can produce. Hope you have a lot of wood though... :D

I also had a similar grate system with holes and found I just couldn't produce enough AUF volume to make enough heat. I opened it up to some heavy angle iron grates and it made a huge difference with the heat I could produce.

Any metal surface that is exposed to the corrosive heat and gases inside the arch will usually burn up/warp quickly. However, I have noticed that as long as you have cooling air running through these exposed surfaces, the metal seems to last a very long time. So if you are keeping those surfaces cooled with air, you don't need to cover them up. My 2 cents.

Good luck...

[treehugger]

I agree with John here. You don't want high pressure under fire, but slower volume that you can control. This is your throttle for how fast you want to burn your wood up and create heat. The more air volume under, the faster it will burn. The AOF is to mix in smaller amounts of air into the already burning fire, but make it turbulent, like it was coming from an air compressor. That is what will reduce the smoke going up the chimney because the AOF is helping with completely burning the unburnt gases/smoke. Like John says here, it is a balancing act to get the right combination of AOF and AUF for fuel effeciency. You don't want to go full throttle with the AUF if you want to save wood. However, if it is GPH you are looking for, turn your AOF way down and AUF way up and that will be the most heat you can produce. Hope you have a lot of wood though... :D

I also had a similar grate system with holes and found I just couldn't produce enough AUF volume to make enough heat. I opened it up to some heavy angle iron grates and it made a huge difference with the heat I could produce.

Any metal surface that is exposed to the corrosive heat and gases inside the arch will usually burn up/warp quickly. However, I have noticed that as long as you have cooling air running through these exposed surfaces, the metal seems to last a very long time. So if you are keeping those surfaces cooled with air, you don't need to cover them up. My 2 cents.

Good luck...

I think I will add a way to reduce air auf by adding a gate and will add s valve to aof. It is what it is at this point. The aof should help some and id rather have higher boil rates than higher efficiency honestly. Thanks for the info.

[mellondome]

Blanket to cover the manifold.... not nozzles. With aof, you dont need a lot of auf.

[RileySugarbush]

If the smallest area of your supply from AOF Blower to the nozzles is not at least twice as much as the sum of your nozzle area, you will not have high velocity jets and your AOF is not going to be effective. It may actually reduce your evaporation rate. You can check easily.

Put a clear tube with a u in it and some water as a manometer and connect to one of your nozzles. It will press down on one side of the water in the u and that will raise the other. The difference in height of the two water levels is by definition the pressure at your nozzles in inches of water. Now plug up all of the other nozzles and see how the pressure rises. With them all blocked, you have the static output pressure of your blower. That is the best you can get for jet velocity with that blower. If there is a big difference between the mostly open and the totally blocked pressures, then your nozzles are too big or your feed tube is too small and is restricting the flow too much.

For your reference, with 8" of water at a nozzle, the jet is exiting at about 81 mph. For 4" of water, 57 mph. AUF flow up through your grates should be wide and slow.

[treehugger]

That's some great info, thanks.

[maple flats]

When I added my AUF/AOF I did notice a big change. I went from adding wood every 7 minutes to every 9 minutes, by a timer. My boil rate went up by about 30-35%, my stack peak temps fell from around 1600-sometimes 1700 degrees F to about 800-900 and never more than about 925 F. At the top of the stack I no longer see a ball of fire, it now burns the gases under the pans where it boils the sap faster. I only see smoke for 5 to 10 minutes tops, after that, none, even when fueling. I never shut off the blower from first start until most coals have burned up at shut down. I My AUF feeds in from the back, under the original grates thru a 4" heavy ga. stove pipe with 3 rows of 5/16" holes drilled every 3" apart, 1 row top center and one each side facing the outer edge of the grates. That pipe stops at 6" from the frond and is capped. Both my AOF and my AUF are off one HP blower. If your HP blower is not big enough, use a squirrel cage blower for your AUF, but the distribution will need to be larger.

[treehugger]

When I added my AUF/AOF I did notice a big change. I went from adding wood every 7 minutes to every 9 minutes, by a timer. My boil rate went up by about 30-35%, my stack peak temps fell from around 1600-sometimes 1700 degrees F to about 800-900 and never more than about 925 F. At the top of the stack I no longer see a ball of fire, it now burns the gases under the pans where it boils the sap faster. I only see smoke for 5 to 10 minutes tops, after that, none, even when fueling. I never shut off the blower from first start until most coals have burned up at shut down. I My AUF feeds in from the back, under the original grates thru a 4" heavy ga. stove pipe with 3 rows of 5/16" holes drilled every 3" apart, 1 row top center and one each side facing the outer edge of the grates. That pipe stops at 6" from the frond and is capped. Both my AOF and my AUF are off one HP blower. If your HP blower is not big enough, use a squirrel cage blower for your AUF, but the distribution will need to be larger.

How far back does your aof go back?

[treehugger]

1516015160 15162. Update. The mani15163 is mounted, blanketed and rebricked.