What makes a garn "smokeless"?

[fabguy01]

so, first off the story of how I got to where I am.
2 years ago I built myself a OWB. I built it for the most part like a central but with a vee in the bottom with fire grates. I soon got fed up with the smoke and started doing research on how to make it more efficient. so step #1 was to ad a blower and line the bottom with firebrick witch helped. step #2 was I cut a sight hole in the door with pyrex glass to see what was going on in there. central "claims" that as the fire burns the heat rises to the top and is transfered to the water by way of the top of the firebox then as the gasses cool they fall lower in the firebox, out past the last baffle and out the flue. I tried with the blower and without and my conclusion was that this claim was total B.S. . the fire would go straight to the rear past the baffle and out the flue. step#3 was to add some baffles to "force" the fire and hot gasses to the top-front of the firebox then rearward past all of the baffles. It is pretty much the same as the one on the woodheat.org site. I made my baffles out of 1/4 inch plate and put firebrick on top of the horizontal baffle to force the heat into the wave of the top of the firebox. the baffle retrofit probably increased efficiency by 40-50% . then I discovered the garn using the blower on the flue side. I wish i would have thought of it. IMHO it is hands down the best place for it for one reason no smoke in your face when reloading. so step #4 was to get the blower onto the exhaust side I was not sure on what I needed so I went cheap and ordered a dust colector from grizzly power tools . It is rated at 450 cfm but figured that i could restrict the air inlet to suit my needs. I got the new blower on, and added a "air bypass" to help cool the blower. at this point about 2 months ago I feel that my owb performs as well as a garn "although I admit I have never seen one in real life". with all the mods made at the moment the only time i get anything visible out of the stack is for about the first and last 10-15 minutes of the burn.
wow what a long lead up to a question eh?

this is where the title of the post comes into question. the blower pulls way to much air so I need to restrict my air intake to about an inch and a half to keep from an over fire. when i restrict the air the smoke disappears and there is ALOT of vacuum in the firebox. the boiler could not work better except for it being so loud. so I tried a 1725rpm motor instead of the 3450rpm and as far as noise goes it was awesome. But With the slower speed motor there seems to be nothing i can do to get rid of the smoke. and there is little to no vacuum.

The firebrick in my boiler is at least a foot away from the fire. that wouldent even remotely do what the garns reaction chamber is dooing with the fire right in it.

So, is it the fact that garn has a vacuum in their firebox,Do they have vacuum in the firebox? or is it the secondary burn chamber that eliminates the smoke?

 

[heaterman]

Basically any gasification type burn must take place in a chamber or area that is isolated from the possibility of heat transfer to the boiler fluid.
Garn does this in a secondary chamber located behind the main firebox which is insulated from the water jacket. Most downdraft gasifiers achieve this isolation in a lower chamber under the firebox where air is forced into the exhaust stream, mixing with flue gas and then burns off. The keys are the isolated high temp chamber and the introduction of the right amount of superheated fresh air into the exhaust stream.

The Garn motor is 1725rpm and moves about 350CFM through the firebox and HX tubes. The blower wheel is a very heavy, open design and weighs about 5 pounds I would guess. Sounds like your air volume was close with the 3450 motor. Maybe a larger blower wheel with the 1725 would provide the right amount of air.

 

[Gooserider]

Not a Garn expert, but I can say that with any gasser, the big thing is a combination of the secondary burn, and keeping the firebox mostly insulated from the water jacket until AFTER the combustion process is complete, and only worrying about getting the heat from the exhaust stream... I could be wrong, but far as I know, the Garn and other gassers do NOT have a significant vacuum in the firebox, since fire requires O2 to burn, putting it in a vacuum is sort of defeating the purpose. In conventional gassers, both positively and negatively pressured fireboxes are used, both burn about the same, and neither uses more than an inch or two of H2O pressure difference in the box, just enough to get the required airflow.

The problem with any traditional OWB design is that you are essentially building the fire in a box of water, which prevents the fire from burning hot enough to reach secondary combustion, so all that nice combustible fuel gas goes up the stack as smoke... Your mods sound like they are working towards getting the fire away from the water jacket, which helps, but you also need to introduce the air into the right places at the right temperature. Part of what the Garn and other gassers do is preheat the air going into the fire, especially the secondary combustion chamber, so that it encourages the burn, rather than cooling the smoke and putting the fire out. Remember the old "fire triangle" - you need fuel, O2 and HEAT - Gassers do heat right, OWB's don't... They also pay a lot of attention to getting the "carburetor" right so that they are mixing the optimal ratio of air into the fuel gases - to much air and you dilute the gases, not enough air and you don't have enough O2 to get a complete burn...

Lots of different issues, all of which need to be "right" if you want the most efficient burn.

Gooserider

 

[fabguy01]

so does garn have both primary and secondary air? how about when it has run for a while is there still steam from the flue, or does it blow compleately clear? sorry for all the questions, I'm kinda amazed/obsessed :red: with the fact that you can burn wood with no smoke.

 

[tom in maine]

This link takes you to the research that helped start Garn.

http://hotandcold.tv/wood furnace.html

(Be sure to use the whole link!)

This covers the basics.

 

[heaterman]

so does garn have both primary and secondary air? how about when it has run for a while is there still steam from the flue, or does it blow compleately clear? sorry for all the questions, I'm kinda amazed/obsessed :red: with the fact that you can burn wood with no smoke.

Combustion air enters the firebox after being somewhat preheated via passage through the intake tube running through the water jacket. The primary air is directed through the bottom of the air intake collar (around the loading door) and enters the firebox directly on the wood. The secondary air enters the firebox at the top of the air intake collar, passes over the fire which does 2 things. Number one, it heats the air to very high temps allowing combustion when it mixes with smoke and particulates from the wood and, number two, helps "wash" the firebox wall keeping creosote formation very low. Both air streams enter the insulated secondary combustion chamber where the secondary, O2 rich air, mixes with the high heat and combustion by products in the primary air stream and spontaneously ignite. Sounds like a giant propane torch when it lights off.

Steam, or water vapor occurs under conditions caused by a variety of factors. Different combination's of wood moisture content, outdoor air temp and dewpoint, relative humidity of the combustion air can yield anything from a high volume of steam to little or nothing. To quote Hot Rods favorite reply, "It depends......"

 

[Jim K in PA]

so does garn have both primary and secondary air? how about when it has run for a while is there still steam from the flue, or does it blow compleately clear? sorry for all the questions, I'm kinda amazed/obsessed :red: with the fact that you can burn wood with no smoke.

Check out the videos on my web site if you have not already. 90% of the time there is no smoke and no steam. I do get steam when the dew point is high and also when the wood MC is more than ideal.

. . . O2 rich air, mixes with the high heat and combustion by products in the primary air stream and spontaneously ignite. Sounds like a giant propane torch when it lights off.

Hell yeah! I can vouch for that description. It doesn't make that sound every time, but usually when it lights off, you know it.

 

[heaterman]

BTW Fab.........nothing wrong with obsessing about doing it right.

 

[flyingcow]

This link takes you to the research that helped start Garn.

http://hotandcold.tv/wood furnace.html

(Be sure to use the whole link!)

This covers the basics.

Wouldn't link

 

[Gooserider]

This link takes you to the research that helped start Garn.

http://hotandcold.tv/wood furnace.html

(Be sure to use the whole link!)

This covers the basics.

Wouldn't link

For some reason the forum software didn't do the link parsing right, probably because of the space. If you cut and paste the ENTIRE link, from the first "h" to the last "l" it will work - at least it did for me... It was an interesting paper for sure, though it takes a fair bit of imagination to see how they got from the initial design in the paper to the actual Garn "firebox in a tank" configuration - without changing the principles involved...

Gooserider

 

[heaterman]

That's the earliest study of secondary combustion that I know of. It's a crying shame that it did not becomes mandatory to use that technology right from the start of the US made wood burner industry.

 

[brad068]

Basically any gasification type burn must take place in a chamber or area that is isolated from the possibility of heat transfer to the boiler fluid.
Garn does this in a secondary chamber located behind the main firebox which is insulated from the water jacket. Most downdraft gasifiers achieve this isolation in a lower chamber under the firebox where air is forced into the exhaust stream, mixing with flue gas and then burns off. The keys are the isolated high temp chamber and the introduction of the right amount of superheated fresh air into the exhaust stream.

The Garn motor is 1725rpm and moves about 350CFM through the firebox and HX tubes. The blower wheel is a very heavy, open design and weighs about 5 pounds I would guess. Sounds like your air volume was close with the 3450 motor. Maybe a larger blower wheel with the 1725 would provide the right amount of air.

Are you sure the inducer motor is 1725? They use to be 3450, at least the ones that I saw and worked on.

 

[heaterman]

Basically any gasification type burn must take place in a chamber or area that is isolated from the possibility of heat transfer to the boiler fluid.
Garn does this in a secondary chamber located behind the main firebox which is insulated from the water jacket. Most downdraft gasifiers achieve this isolation in a lower chamber under the firebox where air is forced into the exhaust stream, mixing with flue gas and then burns off. The keys are the isolated high temp chamber and the introduction of the right amount of superheated fresh air into the exhaust stream.

The Garn motor is 1725rpm and moves about 350CFM through the firebox and HX tubes. The blower wheel is a very heavy, open design and weighs about 5 pounds I would guess. Sounds like your air volume was close with the 3450 motor. Maybe a larger blower wheel with the 1725 would provide the right amount of air.

Are you sure the inducer motor is 1725? They use to be 3450, at least the ones that I saw and worked on.

Duh!! I have two of them sitting right on the shelf in the shop. 3450. I have no idea what I was thinking. probably due to about 80 hours per week for the last 6.

Good catch Garnification.

 

[fabguy01]

looking at jims website I noticed that the garn has a flue temp guage, is that located just after the secondary combustion chamber or is it right before it goes outside? what is the "normal reading"?

 

[tom in maine]

About 25 years ago (Yikes!) I spent a bit of time talking with Martin Lunde about boilers and tanks. At that time, he told me that he had spent
a fair bit of time talking with Dick Hill while designing the Garn.

At the time, I thought the size of the Garn would limit its acceptance. (Shows what I know!!)

Good design lasts!

I talk wood boilers every day with Dick Hill. Life is good!

 

[heaterman]

looking at jims website I noticed that the garn has a flue temp guage, is that located just after the secondary combustion chamber or is it right before it goes outside? what is the "normal reading"?

At the end of the fourth HX pass right after the combustion blower leaving one remaining pass before exit. Actual exit point temps will be roughly 40* less than the indicated flue temp. Nominal mid burn temps will be 300-375 depending on the wood and load size.
Compare that with mid burn temps as high as 800-1000* that I have measured on a variety of OWB's............kinda makes a person go hmmmmmmmm......

 

[brad068]

Heaterman, this afternoon I plumbed in a bigger circ (up40-75) which in turn connected my supply and return manifolds. The piping is all 2" except for the pump flanges. I got the idea after reading about not having enough circulation during charging. The pump runs off the timer for the inducer. I'll report later.

 

[heaterman]

Heaterman, this afternoon I plumbed in a bigger circ (up40-75) which in turn connected my supply and return manifolds. The piping is all 2" except for the pump flanges. I got the idea after reading about not having enough circulation during charging. The pump runs off the timer for the inducer. I'll report later.

That'll be interesting. How are you going to measure the top to bottom temp change when the circ is running?