Preheated secondary air

Re: Preheated secondary air

PostBy: steinkebunch On: Sun Jan 17, 2010 12:50 am

Thanks for the input Greg - I knew you had some success in the past, but had trouble with longevity. I robbed a cast iron tube from an old propane furnace (the part that mixes the gas with air and has fire at the end - guess it's called a burner?) I may be able to use that above the fire. I think the rest of the piping will last a long time. I may delete the piping above the fire (with holes drilled in it) and use the cast furnace burner, or I may just try the experiment like I've drawn, and if it works but degrades quickly, then I can try to retrofit the cast burner, use stainless pipe, etc.

LsFarm wrote:establishing and maintaining the correct mixture of heat, heated secondary air, and volitiles is quite difficult in a hand fed stove

I'm sure you're correct, and I suppose I should not expect it to be correct at all times. But hopefully I can figure out a system that works the majority of the time once I get a method down.

I'm tired of planning and speculating. If your gut feeling is that I will have enough draft to pull a significant amount of air through the piping, I'll just give it a shot. I can't really fit bigger piping anyway - all my brick supports, spaces, etc are 1-1/2". I've got Monday off, so I may get busy then and get something done. Between that and mudding sheetrock, my Monday is shot.

Thanks - I'll keep you all posted on success and failure, and try to snap a few photos along the way.

Steinke
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: steinkebunch On: Sun Jan 24, 2010 7:51 pm

OK - I got things built and tried it out. BTW - I had not burned coal in this stove yet this year. I had just been burning wood to clear some old timber out. I could not stand it any longer, and tried some coal. All I had was stoker coal, but I used it anyway.

Here are a few photos of the "finished" product (is anything ever really finished though?)
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All the fabricated parts in a pile
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view of 1" pipes under grates (grates removed)
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view of 3/4" pipes near baffle (baffle removed)
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Not shown are some butterfly valves I build to control air flow (one for primary air and two for secondary air.

I fired the stove up with a good wood fire and slowly added stoker coal. Wasted a lot of coal as my grate openings are too wide for small coal and 20% of it fell into the ash pan. Once I got the stove up to about 500F (skin temp) I tried the secondary air. It was very touchy, seemed like I just did not have enough heat.

I then slid my baffle plate forward, over the top of the air tubes, and it seemed to work better. I still had to keep things pretty hot to keep the volatiles burning. But it was pretty impressive. No photos of that - I'll try to get some next time.

Seems like I need to get some kaowool or firebrick to place above the tubes in order to keep enough heat in the area to burn the gasses. Or maybe I need to get rid of the 3/4" tubes and go to a hollow baffle that gives the air more time to preheat.

I built coal bed slowly to get these results. Not sure it would work to toss 2 buckets of fresh coal on an old bed. Guess banking may be important there. I could probably get by with it if I ran the pipe through the coalbed to heat it, but then there's that longevity issue with that.
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: steinkebunch On: Sun Jan 24, 2010 8:04 pm

Looks like it's important to have lots of insulation to keep heat directed at the proper area to succeed. I added the few firebrick I have left to the side of the stove, above the existing bricks - I just stacked them on top of the existing angle iron brick retainers. I only had 3 extra bricks, so I could only line the left side of the stove. That side of the stove seemed to achieve secondary combustion better than the non-bricked side.

So I'm hoping with the addition of kaowool board or more firebrick, things won't be quite so fragile, as it would be nice to not have to babysit the process too much.

But once the secondary combustion got going good, it maintained it for 10-15 minutes. But I had to open the ashpan door after while to get more heat pumping, or I'd loose the secondary combustion. I'm sure I could have just opened the primary air more too, and kept the whole thing in balance better, but I wasn't that patient.

I really don't need so much heat out of the stove, so I plan to reduce the firebox some soon using the "brick on grates" technique. Maybe then I can keep the primary air open more and with the extra insulation, get a better balance.

This may all change if I get some lump coal. It would be nice to burn stoker though if it works, as I have a supply of it for the stoker in the house. I can modify the grates (actually just lay 1/4" bars in the existing grate spaces) and likely keep the stoker coal from falling through the grates. I imagine the volatiles are released from the stoker coal quicker than from lump coal, but then air flow is way different too, so I'm not sure what will happen.

Any thoughts or suggestions appreciated.
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8


Re: Preheated secondary air

PostBy: rockwood On: Sun Jan 24, 2010 8:56 pm

Looks really good!
When the secondary combustion was going good did you check to see how much smoke was coming out of the chimney? How much less smoke do you estimate there was?
I would like to see how lump coal would burn compared to stoker coal with your preheated setup. If you get that to work with stoker coal though you'll have a good setup because stoker coal is so much nicer as far as handling goes.
rockwood
 
Hot Air Coal Stoker Furnace: Stokermatic coal furnace
Hand Fed Coal Stove: Rockwood Stoveworks Circulator
Baseburners & Antiques: Malleable/Monarch Range
Coal Size/Type: Soft coal: Lump and stoker (slack coal)

Re: Preheated secondary air

PostBy: LsFarm On: Sun Jan 24, 2010 9:59 pm

Congratualtions on getting a steady secondary burn of 15 minutes.. I'm sure that it is a touchy, finicky mix of air, heat and volitiles.

Didn't Berlin's hand fed stove that he posted plans for use a hollow flat baffle for his air 'distributor' ? Sort of like a big flat shower-head 'spraying' hot air?

Can't wait for the photos, or the video !!

Greg L
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Re: Preheated secondary air

PostBy: steinkebunch On: Mon Jan 25, 2010 12:53 am

To answer a few questions above:

I did notice a smoke reduction from the chimney, though it did not completely eliminate it. I think I have so many volatiles from the stoker coal that it's hard to eliminate all the smoke. I'm anxious to try stove or lump. I agree that stoker is nice to handle, shovel, etc. The lump I buy is about 2" minus, so it's "shovelable" too. I'm really anxious to get a topload door built, just can't find the time.

And yes, Greg, I think Berlin's plans for his stove had a drawing of a hollow baffle feed by air flowing thru a pipe traveling thru the coalbed. I could retrofit the baff;e fairly easily with my setup. I'm just hoping that the addition of some kaowool or firebrick will be enough. However, I suspect that I will need every advantage available, especially if I want to achieve success even with a full fresh load of coal on an old coalbed. If I had a hollow baffle and kaowool both, I might have a chance. If I took a chance and preheated the air with a pipe directly in the coalbed, I'd further increase my chances. I just don't want to spend more than it's worth on SS pipe or something crazier like Inconel or some other exotic nickel alloy.

I found some kaowool board online for $8 for a 24" x 36" sheet, need to see what shipping costs. Rather have 1/2" stuff, but it's all so pricey. I may get some 1/4" stuff and see if I can attach it to a frame for a baffle, or just use firebrick. But I'm sure kaowool is better. I've heard stories of it being so good that it can be 1000F on one side, and 150F on the other. It reflects all heat, absorbs none.

I'll try to get a video or photos of the secondary burn yet this week, I might reduce the firebox and add some more brick before I get photos though.

Thanks for the interest,

Steinke
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: Pete69 On: Tue Jan 26, 2010 3:00 am

Do you keep the secondary air open all the time or just for when you load a fresh load?
Pete69
 
Stove/Furnace Make: Baker/Vermont Castings/Chubby
Stove/Furnace Model: fireside /VigilantII/Chubby

Re: Preheated secondary air

PostBy: steinkebunch On: Tue Jan 26, 2010 11:32 am

I have only tried this system out with one load of coal so far, but once I got the stove up to temps (500F), I left the secondary air open. I tried closing it a few times too, and somehow I still was able to maintain the secondary burn, likely because I had built the load door to allow some overfire air in also. Not sure what this means - maybe with more firebox insulation I could have achieved secondary burn without any tubing. But I definitely saw alot more secondary burn from the tubing than from the holes in the load door, and it lasted longer than the burn from the door. I suppose after thinking out loud about that, it's just that due to the mods, the secondary burn was capable of generating enough extra heat to let the door holes work too. If you did not follow that line of reasoning, I don't blame you - I think I confused myself too.

I was assuming I'd need to close the secondary air off once the volatiles were gone, but I did not have to, and still kept a fine hot coalbed the rest of the day until the coal was gone. Our bit coal is pretty forgiving compared to anthracite. The entire project was built around being able to close the secondary air with a thermostatic control, but that may not be necessary now.

It could also be that I have such a long secondary tubing path, that very little air is actually pulled through it except at high draft. Once the volatiles are gone and the draft drops a bit, I might not be getting much air pulled through. I definitely noticed a better secondary burn once I close the ash door - which tells me that it's easier for the stove to get air primary air than secondary air. That worries me some, as I'm using stoker coal. If I use stove/lump, it will be even easier to pull air primary air, and I may have trouble getting any secondary air. Or maybe it's all about my primary air inlet opening.

I may have been better off shortening the tubing by having air inlets in the sides of the stove rather than the back, but I would not have picked up as much heat. I'm surprised how hot the 1" pipes just below the grates get, even though primary air is rushing past them.

I'm also guessing I lose some heat from the secondary air when it gets to the square tubing, as it probably conducts some of it to the stove body.

Still trying to find time to make a few simple mods to try it again, but only so many hours in a day, and the garage stove isn't making it to the top of the list.
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: rockwood On: Tue Jan 26, 2010 1:19 pm

steinkebunch wrote:If you did not follow that line of reasoning, I don't blame you - I think I confused myself too.

I follow. :)
I think it would work best if all the air for secondary burn were preheated...it would be better to close the secondary air in the doors so all the air for secondary burn is pulled through the tubes because the air though the door would be much cooler and impede the secondary burn.
Another thought is about the loading of fresh coal. You would want a very lively fire/hot firebox and you would want to load the coal as quickly but safely as possible to keep the firebox at high temperature so secondary burn can be achieved quicker. One of the worst things someone burning coal can do is leave the loading door open too long or opening loading door too often.
rockwood
 
Hot Air Coal Stoker Furnace: Stokermatic coal furnace
Hand Fed Coal Stove: Rockwood Stoveworks Circulator
Baseburners & Antiques: Malleable/Monarch Range
Coal Size/Type: Soft coal: Lump and stoker (slack coal)

Re: Preheated secondary air

PostBy: steinkebunch On: Tue Jan 26, 2010 7:13 pm

I agree with your reasoning. It's just too tempting to open the load door often to take a peek. Hopefully if this whole thing works, the volatiles will burn, my window will stay clear of soot, and I won't have to open the door to have a looksie - I can just use the window.

Hoping the firebrick are sufficient to insualte/reflect heat back for secondary burn, so I don't have to spend money on ceramic board. I'm not having any luck finding any that's a reasonable price. If anyone has any sources, please chime in. Pottery suppliers, kiln outlets, etc have some, but it's just not reasonable.

I really need to put that topload door in so I can more quickly and efficiently get coal in. Too many projects! If I could go back in time and rebuild the stove, I'd make alot of changes. I did some serious preplanning as it was, but still got lots of things wrong. Deeper coal bed, smaller firebox footprint, topload, secondary air, 8" flue/collar, ash funnels, etc. I've made many mods, just would have been quicker and simpler to get it right the first time.

Still trying to make some changes and get some pics/vids of secondary burn. Patience.
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: steinkebunch On: Wed Jan 27, 2010 4:10 pm

I think I've been successful!

Last night I placed some 5/16" diameter bars on my grates. They each lay between the 1/2" rebar I used for the grate surface, and rest on the grate axles This kept the stoker coal from falling through the grate openings, and worked well. If the bars do not hold up, I may try to find some #2 rebar, but it's a really odd size and few stores carry it.

(see this thread at Grate area vs. soot buildup for a photo of my grates without the 5/16" bars I added). The 5/16" bars just rest on the grate axles. Still shakes fine.

I also took 2 of the 3 grates out of service by reducing the firebox. Layed a piece of sheet metal cut to fit on the rear 2 grates, and stacked firebrick on them. I then removed the "connecting rod" that ties the grates together for shaking. Thus when I shake the front grate, it does not move the rear grates. Total grate area is now just 20" x 4.5"

I installed 3 firebrick above the secondary air pipes, just layed them directly on top of the pipes. Also added a few bricks to the sides of the stove, resting them on the side brick retainers. I think the additional firebrick I used to reduce the firebox size really helped reflect more heat to the right area.

Also used some firestop hi-temp caulking to seal some joints in the secondary air piping.

Got a wood fire going (just a small amount of wood), and added some bit coal. Got about an inch of glowing coals covering the grate. I then filled the firebox with about 6" of coal, leaving one side open with coals still glowing. Opened the ash door and let it roar for a few minutes. When the stove skin temps above load door raised to 200F or 250F, I closed the ash door and opened the secondary air. Got the secondary burn from the piping above the bed (at least on the glowing coal side), and let that cook about 10-15 minutes. I then filled the remaining bed with coal, and shut the load door, and the secondary burn remained. It continued to burn for several hours. I peeked in several times, played with the air settings, etc. It did not seem to be fragile balance. Stove temps hovered between 350F and 400F. Door glass was pretty clean next morning, though stove temps were down some as, the coal bed just is not deep enough.

This morning I went thru a similar process, but of course instead of starting with wood, I opened the ash door, let things heat up, shook the ash down, and then added coal. Same story - got my secondary burn back, let it cook awhile, then filled the remainder of the firebox, and the secondary burn was still going 2 hours later. When I came home for lunch, I had a nice glowing coal bed, though the secondary burn was gone since all the volatiles were all cooked out.

I checked the chimney outside about 1/2 hour after loading, and I could not see any smoke, only some 'heat'. If I squinted my eyes, I might be able to imagine some smoke was there.

Few items still need taken care of are a deeper coal bed (some bars or firebrick stacked up in front of the lower half of the load door), a top load door, and a more sophisticated control for opening and closing the secondary air inlet. But I am so excited that I'm finally getting this to work.

I took some photos and video, but I was not too impressed with them. I will try to get some better ones tonight. The photos do not do it justice. I did not use a flash, and it was OK, but it doesn't look much like it looks with my eyeballs.

I imagine the results would be different with stove coal (1" plus). I may try it, but the stoker coal is nice as you get fewer rocks, which clog up the grates eventually. The smaller stoker coal doesn't seem to contain as many rocks and junk. I'll bet with stove coal, I'd maintain the secondary burn longer, and have more even temps.

Steinke
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: LsFarm On: Wed Jan 27, 2010 8:42 pm

It sounds like you have it down pat ! To be able to just shake, load and go, and get the secondary burn without fail is a success. And the clean glass, flue and chimney are added bonus'.

Greg L
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Re: Preheated secondary air

PostBy: rockwood On: Wed Jan 27, 2010 9:29 pm

steinkebunch wrote:and a more sophisticated control for opening and closing the secondary air inlet

What ideas do you have for that...maybe a bimetal thermostat that would close sec. air damper after volatiles are gone to keep more heat in the stove?

Looking forward to photos, video. :)
rockwood
 
Hot Air Coal Stoker Furnace: Stokermatic coal furnace
Hand Fed Coal Stove: Rockwood Stoveworks Circulator
Baseburners & Antiques: Malleable/Monarch Range
Coal Size/Type: Soft coal: Lump and stoker (slack coal)

Re: Preheated secondary air

PostBy: steinkebunch On: Thu Jan 28, 2010 2:08 am

OK - here is a string of videos, explanation contained within. Entire video sequence took about 30 minutes from beginning to end.

And to answer a question above - I was thinking of inserting a bimetal fan/limit controller from a furnace to control a butterfly flapper/valve on the secondary intakes. Still pondering all that though. Baby steps.

And I don't really have it down pat yet - I closed the primary air too much after I shot the video series, and 30 minutes later I lost the secondary burn. I had to poke a small hole in the black coalbed to get things going, rev the fire up with the ash door open for a few minutes, and all was well again. I opened the primary air up some, and everything was good. I suppose it will take a few weeks to get good at this - can't expect to get it figured out in 2 or 3 tries. The stove really likes to be at about 350F or more to maintain the balance.
-----------

OK - the files are too big to upload in the time limit. I'll go to work early and use the T1 line to get it done quicker. Patience until tomorrow..
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: steinkebunch On: Fri Jan 29, 2010 11:15 am

OK - most of the video sequence is just too large, and you probably don't want to see the process from start to finish anyway. So here's just one video of the final product.

Sound you hear is just the distrubution blower.

I have some more impressive videos, but they are too large. If I get time, I'll clip one or two of the best ones and just show the best parts. On those other vids, you can see little holes in the flames where air is coming out of the tubes.
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steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8