Air under and over fire help

[huntingken111]

Building new 3 ft by 10ft arch and going to add air under and over fire. My question is this what size pipe do use or is it square tube. Plan is to use 3/8 nipples at 10 to 12 degrees with firebox being about 28 by 36 approx. after insulation and bricks.

[Ausable]

Wow! From a 2' x 4' to a 3' x 10' - When You upgrade - You don't fool around. I just use natural draft for making maple on my little 2' x 5' - but - in a previous life - I operated Electrical Generating Plant Boilers - among other things. What are You using for fuel? Wood? Air under fire would work for wood. How much air? You will be making a forced draft system with no induced draft - so You will have your arch under pressure when fired and it will have to be air tight or Your Sugar Shack will be full of smoke and nasty. The amount of air will be an on going experiment. Size of blower or fan - louvers of baffles to control air flow - size of stack (diameter and length). You want the bulk of the heat under your 3' x 10' pan setup and not igniting in and above the stack. It will be fun to play with and will work once you get it figured out. Many of the guys and gals on this Site use forced air and I am sure will chime in and give you some real pointers. ----Mike----

[Timberwolf]

huntingken,

I don't have a lot of experience with AOF; however, I can tell you this most I have seen do use square tubing as it is easier to drill and tap for nozzles versus round pipe. Exact size I will let others chime in on, but what you need to understand is that you are creating both a manifold and a plenum with this tube/pipe, i.e. it needs to be larger than you might think otherwise you will not have enough flow to supply all of you nozzles. If I were to guess you probably want to use 2" x 2" square minimum and go with the thinnest wall you can get enough threads in for your nozzles or better yet weld on bushings to thread in your nozzles. The thinner the wall generally = less $$ and will mean the inside dimensions are larger, i.e. larger opening for air.

[Bob 262626]

I used this UVM document as a guide to build my air system; the only exception was I used the blower outlet on my wet/dry vacuum, 100 MPH. My homemade arch is 2x6 so using the guidelines from UVM, I used a 2” inlet pipe, I have a 10” smoke pipe and using the blower unit at full velocity I do not have any smoke in my house, blower must be shut off when putting wood in the arch. The wet/day vac is located away from the Arch and is placed inside a plywood box for noise reduction, my wife wired an electrical plug to a switch by the arch so it is easy to shut down the vac during filling, the UVM article states that you will reduce wood consumption and reduce smoke, all statements are very true. It is well worth the effort to follow the UVM guidance.
http://www.uvm.edu/~pmrc/Combustion.pdf

[nymapleguy607]

On my 2x6 I feed the air manifolds with 3" lines, I think in your case I would use 1.5" pipe, try to inlay into the bricks and insulate over it. My air manifolds have 3 layers of 1" ceramic blanket covering them, if you dont keep the pipes insulated they will warp.

[RileySugarbush]

You want the majority of your restriction at the nozzles so that the air pressure drop is maximum right there. That is how you get the high speed jets.

To do that, make sure the cross section of the tubing is several times greater than the sum of the nozzles it feeds.

Example: If you have 10 nozzles on one side of the firebox, each with .375 inside diameter, the total nozzle area is 10 x 0.110 = 1.11 sq. inches IF you feed them through a round pipe with 2" inside diameter, the area of the feed pipe is 3.14 sq in. I don't think you should go any smaller than that to avoid too much pressure loss before the nozzles. The pipe feeding both sides should be bigger yet since the air flow is double.

[maple flats]

On my 3x8 I used 2" sq. tube for the AOF manifold. It works very well, even though the published report said to use 3" round for a 3' width arch. The length doesn't matter, just the width. I think 3/8 nozzles are too large, I'll check it out later as I'm just heading back to work. You need velocity not volume.

[huntingken111]

To answer some questions from all these great responses: My chimney will be 20 ft long 10 inch diam. since I have that. I an not sure what size nozzles to use but went off some research on this site. I will be using wood for fuel. To be honest a lot of numbers and engineering go behind what you guys said Don't completely understand though.

I am by trade tool and die maker so whatever I need to build time permitting I can make. Not sure on fan yet but probably some sort of squirell cage. I thought about making nozzles out of h13 since that will be exposed to all the heat. Same material we use on die cast molds. I will say this I am building this without a print and doing a lot of swag engineering (sophisticated wise a## guess). The arch is the easy part but auf/aof is a science that don't seem to be easy but from all the feed back worth putting in.

I am jumping to a 3 by 10 because pan price was cheap and I am sick of 13 hr boils plus the wife says I have the bug

[maple flats]

Read the UVM report mentioned above first.
A squirrel cage will not perform well at all, you need more pressure but less volume.
Don't worry about the material used for the nozzles, mine are common black pipe and after several years I see no erosion. I think that is because you have cold air blowing thru the nozzle and this keeps the nozzle well below a danger temperature. When I made mine I was going to cut the nozzles back to the surface of the refractory, but then I said "they will self prune" No such thing, several years and the nozzles look the same as when new, the stick out past the refractory anywhere from 1/2" to 1.5".
A blower that has paddles that throw the air is needed, rather than one the "flings" the air like a squirrel cage.

[SeanD]

Where did you guys get your 2"+ tubing or pipe? All of the box stores and hardware stores here just have 1" and the scrap metal yards don't have any. Is using 1" steel tube and cutting the number of nozzles in half worth it or does that just defeat the whole purpose?

After doing a ton of research here, I think this project may be dead in the water.

Thanks,
Sean

[nymapleguy607]

There should be a steel supplier around you some where, they would have that size for sure. I got to know the people near me pretty well when I built my arch.

[Chicopee Sap Shack]

Local small fab shops will sell steel sometimes

[maple flats]

If you need to, order it online. All you need is light weight tube. My 2x2 manifold tube is only 1/16' thick. It is fully protected by refractory once installed. I've used online metals in the past, but check around, there may be cheaper ones. The tube can be mild steel. Then rather than drilling and tapping for the nozzles, I welded mine over the drilled holes. I cut the nozzles on a chop saw for the 10 degree angle, the angle at the nozzle end can be angled 10 degrees but mine are square, either will work. I use 1 blower for AOF, and AUF, with a 3" ball valve for each (they were free to me). Have a nozzle about every 6" along the manifold and have one each side of the fill door. Each nozzle points down 10 degrees and simply points towards the opposite side or end of the firebox. Thus, those beside the door, point rearward, those on each side point to the other side and those along the back point to the front. The biggest thing seems to be to have the right blower. It must be high pressure, not high volume. The main objective is to create turbulence in the firebox, mixing the combustible gases and thus getting complete or near complete combustion.
Once you have boiled with AOF, you will boil faster, and use less wood. I cut my fueling time from 7 minutes to 9 and I add less per fueling. I use about 35-40% less wood.

[SeanD]

Thanks for the responses. All the steel places I called do big work and won't sell me 10'-12'. One guy hung up on me when I asked him if he knew any shops in the area. That's when I posted - at the height of my frustration. There's got to be something around. I'm just hitting dead ends.

Dave, thanks for all the details. I read all the old threads on AOF and followed your work. Once I get to work, I'm going to have more questions for you.

Actually, one I did think of during all the reading was how you cut the square tubing to make the T for the manifold at the back of the arch where the two sides join from the blower. I don't have a torch and I can't think of a way to do it without one.

Sean

[maple flats]

While you do pay shipping, the online metal places I've dealt with don't care about the size of the order. Some do however have a minimum order size. Many scrap yards also resell steel, check for some of those. For that the more rural ones may be more apt to sell what they would consider a "small quantity".
Another thought, decide what you want and list it as wanted on Craig's list. Also, does your area have a guy who hauls scrap for recycling? If yes, have him help you find something.
When I built mine I got lucky, my brother's son-in-law is an engineer at a plant where they often scrap out things that weren't even used, others that were used. My brother at the time had been given about 600 lineal feet of 2" sq. tube that was 1/16 wall, in lengths of 6', 10' and 12'. Most had a base and top pad welded on. I got mine from him. Unfortunately he has used it all up since with a whole bunch of welding projects, (racks, benches, partitions, posts etc.) in his machine shop.

[Sugarmaker]

Hi,
I hope your are progressing on your AOF (air over fire) system. I am just finishing my AOF system for the second time. The first prototype system preformed well for 4-5 years.
Since I will be upgrading to new pans this next season it was a good time to review the air systems while the pans were off.
Our AOF is not conventional, as it is a "top rail" system, which means it sets on top of the arch and the pans set on top of the AOF system.
The old one was 1-1/2 dia stainless tubing about 1/16 thick. It worked but I did not like how it fit to the arch or the pan. ( could not seal it well) . Also the 1/6 thick material had many cracks from the heat.
So this time I spent the money and purchased 2 x 2 x 1/8 wall stainless tubing. Not a cheap date, about $400 for all the materials. Got this from local yard handling stee,l small orders no problem. (Gene Davis Steel) Erie PA.
Used a 4-1/2 inch grinder with cut off saw blade to cut the angles and a hole saw to cut the two large 1-1/2 holes where the old drops entered.
The nozzles are 3/8 I.D. stainless 1-1/2 long, with about a 30 degree cut to point down towards the middle of the fire box. MIG welded to the square tubes 26 places. 10 along each side and 6 across the front.
Our arch is the same size as yours 3 x 10.
MIG welded all the joints after fit up to the arch.
I like it being nice and flat and should seal the arch and pans better.

May have some pictures at some time or if requested. Will probably start another thread. Or add on to the old thread on the TRS prototype build from years back.

Good luck with your build.
Regards,
Chris