Preheated secondary air

Re: Preheated secondary air

PostBy: steinkebunch On: Fri Jan 29, 2010 11:20 am

OK - so I guess I did upload one of the better videos. I open the load door mid-way thru the clip, and you see the secondary burn from the tubes cease. When I close the door, it starts again. If you look closely, you can see the tubing holes in the flames.
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Preheated secondary air

PostBy: wsherrick On: Sun Feb 07, 2010 11:41 pm

Another thing you could or should do is use your air tube to support a brick arch, much like a steam locomotive firebox. To completely burn the hydrocarbons released in the burning process the firebox area must be at least 1800 degrees. I don't know if that is possible in a naturally drafted stove or not. In practice a brick arch does two things, one it delays the flow of the fire from the fuel to the exhaust thus allowing more time for the hydrocarbons to mix with oxygen and ignite. Two, the brick arch heats up and maintains a much more uniform temperature in the area of the firebox thus making it more possible to reach the proper temps for gas ignition. Locomotive designers found out during the 1880's that adding a brick arch in the firebox greatly increased locomotive efficiency and lowered fuel consumption per horsepower quite a bit.
You can burn the hydrocarbons at a lower temerature, but; it will burn to carbon monoxide with gives off much less heat. At any rate heated secondary air will get you a lot closer to your goal than unheated air.
My Glenwood base heater uses heated secondary air in its combustion process. The secondary air is admitted through little holes in the fire door which lead to a heavy cast iron tube which goes around the top of the firepot. There are little holes drilled in the tube all the way around. It looks just like a big natural gas burner ring.
It works very well. After you load a fresh batch of coal into the firepot and close the door, you can see blue flames spouting off of each little hole in the ring as the heated air hits the hot gas inside the stove causing it to ignite and burn. You might want to think of an approach that is similiar to how the Glenwood is set up around the top of your firebox.
I think what you are doing is very interesting and I wish you great success with your project.
Stoker Coal Boiler: None
Hand Fed Coal Boiler: None
Hot Air Coal Stoker Furnace: None
Hot Air Coal Stoker Stove: None
Hand Fed Coal Stove: Glenwood Base Heater, Crawford Base Heater
Hand Fed Coal Furnace: None
Baseburners & Antiques: Crawford Base Heater, Glenwood, Stanley Argand
Coal Size/Type: Chestnut, Stove Size

Re: Preheated secondary air

PostBy: steinkebunch On: Wed Feb 10, 2010 12:29 pm

Thanks for the info the the baseburner - looks interesting.

I'm doing my best to get some photos for you, but it's proving difficult to get a photo that really shows anything. All snapshots with my digitial camera require too slow of shutter speed, which makes the flame blurry. I think I'm having the best success by taking a video, and then just capturing still photos from the video.

I've not forgotten about posting photos, I'm just still trying to get them right.

Also, I purchased some insulating firebrick, and replaced the hard firebrick with them that are on top of the tubes. I also added some insulating firebrick to the back wall. This has helped greatly, as the insulating brick reflect at least 5x more heat back than the hard firebrick. I can now ignite the volatiles much quicker after loading, and don't have to keep the bed quite as hot to maintain the secondary burn.

As a comparison, I layed an extra insulating firebrick on top of the stove, and although I could not even think about putting my hand on the stove top, I could hold my hand on top of the insulating firebrick all day. The firebrick is very light, porous, and fragile, but it's amazing. I'd rather have ceramic board, but I couldn't make myself pay $50 - $70 for just a few square feet.

Patience - I'll get some photos soon.

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

Re: Preheated secondary air

PostBy: fyrewerks On: Sat Feb 13, 2010 11:02 pm

The more research I do, the more I come to the conclusion that building my own boiler is the only way to go. There's just nothing on the market that makes the best use of existing [strike]technology[/strike] knowledge. (I won't say technology because many design elements that make an ideal furnace are old as the hills.) I can imagine that is why you built your own unit. I want to burn sub bit and have the option for wood too and its totally possible to build a unit that does both and does it efficiently, but there is nothing ready-made. As far as I can tell, the Biasi-Eko Coal/Wood is closest thing out there right now. It is one of the few units, boiler, stove or otherwise that incorporates heated sec. air. Cast Iron (not good for wood, or even coal tars but it prevents clinkering, corrosion and overheating) And best of all, needs no electricity to operate - why cut the umbilical to the oilman only to establish a new one to the electricity man?

But one nagging thing keeps coming back to peeve me, and you hit on the head - firebrick. Modern heaters have been married to iron and steel since the Franklin Stove of 1742. Sure firebrick takes a whole new skillset to build with, but its the only way to efficiently burn anything. Why cut wood or haul coal only to blow half the available heat up the stack because your heater is poorly designed? I know thats the motivation for your project and mine. Your design is excellent and I just thought I'd turn you on to these Kuznetsov boilers/heaters/stoves/etc. I found on Its very similar to your design, it does the same things only simpler. You can see in the pics the clever internal risers around the firebox to preheat sec. air and the sec. air openings. The Russian dude who came up with these units is hardcore: he considers a tall chimney "forced draft" and fans totally unnecessary. The fire regulates its own air supply using its own heat. The most unique thing about his units is the "dry joint" he leaves along the firebox. He claims it allows cooler gasses not directly associated with combustion to escape and be exhausted to stack, allowing only the hot gases from the fire access to the heat exchanger. At first it seems counter productive but he backs up his theory quite well. The temps in the "bell" must be insane! Its the only design I;ve seen that sets up conditions for a truly efficient burn - heated secondary air, a secondary combustion zone separate from the exchanger permitting a long residence time at high temp all trapped by a refractory dome and no electricity needed. With the right grates you could burn wood, coal or any combination without sacrificing performance. I'm not keen to build my own boiler, but I think in the end this one will keep paying me back. Hope you get some good ideas.

Re: Preheated secondary air

PostBy: steinkebunch On: Tue Feb 23, 2010 6:33 pm

Here's a video of the secondary burn. It's about the best I can do - I had to leave the lights off, so you can't really see the stove body. If you refer back to the photos on the previous page, you can imagine where the tubes are by where the shape of the flames. If you're using Windows Media Player, you can increase the contrast and brightness some to get a better view.

I'm finding out that I may need to remove the secondary burn zone from the coal bed area - it gets the bed too hot, cooks the volatiles off, generates too much heat too quickly, and shortens my burn time significantly. That may not be an easy task though... I'll need to shield the tubes from the coal bed, using ceramic board, firebrick, etc. But I may need to make the shield movable, so that I can the secondary burn started, then slide the baffle in place to keep the secondary heat from cooking the coal so fast.

I should probably just get some larger coal like lump coal, and that may temper things out enough, but I have so much stoker coal laying around....

The file is about 18 MB.


Secondary burn
(18.28 MiB) Viewed 92 times
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8