DRAFT

[grizbach]

OK, here is a topic I think deserves some attention.

Draft. This is a direct relation to cfm(obviously) and more importantly oxygen to the fire. As the draft increases, so does the O2 level. I have played with this variable with my original modular rocket and also my current batch load. I believe it is the reason behind my percieved slight disbelief of my “bottom intake” topic.
When one first lights a fire, there is an excess of oxygen. Put this fire into a rocket setting and I have been able to actually blow out the fire with it’s own draft. Now I have my system piped to my existing chimney that pulls draft without even having a fire. My start up sequence involves setting my damper about half open. Once the fire is underway and more hydrocarbons are being produced, the damper is opened ¾. Just before full engulfment, the damper is opened all the way to pull in the most oxygen. Once the fire reaches the coal stage I start to close down on the damper again because the extra cfm would just cool down the burn quicker.

Primary intake opening on a batch load. I believe there is no magic opening percentage, rather a range it can be in. My system has a rather large opening on my door(24sq”). Now I have a set amount of draft my system can pull, and with it, a set amount of oxygen. Necking down the door to the standard setting will increase the velocity right at the opening, but once it passes the door it expands to the csa of the wood filled box and slows.

All this being said, I have no scientific data to back it up. Just common sense and observation. I have seen a few people here confused about primary intake setting and I hope this will give them something to chew on.

[peterberg]

Feb 4, 2013 1:31:26 GMT -8 grizbach said:

Primary intake opening on a batch load. I believe there is no magic opening percentage, rather a range it can be in. My system has a rather large opening on my door(24sq”). Now I have a set amount of draft my system can pull, and with it, a set amount of oxygen. Necking down the door to the standard setting will increase the velocity right at the opening, but once it passes the door it expands to the csa of the wood filled box and slows.

All the above is true.
And here is the great BUT...

There is a certain range of air injection which will result in a good burn. It's possible to have no door at all in a Rumford fireplace. Efficiency of such a device is 25% at best. When it's well made and you are lucky.

The trick is to have the stove burning clean, using as little air as possible. This is called the amount of excess air, usually represented by the symbol Lambda, λ. Lambda is excess air plus one. The goal of many stove developers is a lambda figure between 2 and 3.
Here's a good article about gas analyzing and why it's virtually impossible to optimize a stove without.

Remember, air is composed of a lot more than oxygen alone. In fact 79% of it doesn't contribute to the combustion at all. On the contrary, it cools down the fire, often leading to incomplete combustion. Efficiency is dependent mainly on two factors: end temperature and excess air. The last one is making a far greater impact than temperature.

Now, what I've been trying to achieve is a simple construction capable of burning wood cleanly being as frugal as possible on air. Clean combustion paired to high efficiency. It's easy to reach sky-high efficiency without being bothered by the filth that do escape the chimney.

This batch box rocket is able to burn very clean and have very low excess air at the same time. The lowest Lambda figure I've seen is 1.38, 1.5 is coming by almost every burn.

Griz, you are using a primary air opening of 6 times the figure that delivered my best results. Of course, it's your stove, it appears to burn clean, you are satisfied. The only thing about it is the efficiency, which is several tens of percentage points lower than it could be.

And yes, right behind the air inlet the gases will expand and slow down. But at the spot where it counts, the port, there it moves with high velocity again. Not only because the port is smaller compared to the riser c.s.a., but also because of the fact that the fuel is blocking part of it.

Of course I've tried the same trick as you did. Using no door at all, half a door, one quarter system size and finally 15% of the riser csa. I've tried smaller openings as well, using that the stove wasn't reliable anymore.

This csa has always been calculated back to a round riser equivalent, by the way.

Long story, I had to think long and deep about it.

[grizbach]

Peter,
I know what you mean about the incoming air cooling the fire. Yesterday I looked into one of those analyzers. $2500 american! Looks like I won’t be getting one anytime soon.

With the cold temps I we have had recently(0’s F), I believe what I see come out of my chimney is mostly fog. Also some ice forming on the chimney stack. When the temps were close to freezing, the exhaust was transparent. We had 50’sF with a lot of rain a few weeks ago so my wood supply is wetter than it should be. That said, I have only ran 2 full batches. When approaching and entering full burn, I get a cloudiness in the exhaust where my flue enters the chimney. I don’t get this when my batch is ¾’s full. I assume it’s from not enough 02.

With my first fires, I had plates I would move around to get the least smoke in the firebox. Since I’ve added the bench to it, the extra restriction has changed the parameters of my air intake. I have a false door behind the main one that pivots so it rests on the wood. This helps channel the incoming air into the coals instead of the upper firebox. I believe I can get away with a larger opening this way(I like to see the fire). I have necked down the opening to 15sq” and it was marginally better.

Thank you very much for your response, as I said elsewhere, I value your input.

[tferr]

Feb 5, 2013 7:48:21 GMT -8 peterberg said:

Feb 4, 2013 1:31:26 GMT -8 grizbach said:

Primary intake opening on a batch load. I believe there is no magic opening percentage, rather a range it can be in. My system has a rather large opening on my door(24sq”). Now I have a set amount of draft my system can pull, and with it, a set amount of oxygen. Necking down the door to the standard setting will increase the velocity right at the opening, but once it passes the door it expands to the csa of the wood filled box and slows.

All the above is true.
And here is the great BUT...

There is a certain range of air injection which will result in a good burn. It's possible to have no door at all in a Rumford fireplace. Efficiency of such a device is 25% at best. When it's well made and you are lucky.

The trick is to have the stove burning clean, using as little air as possible. This is called the amount of excess air, usually represented by the symbol Lambda, λ. Lambda is excess air plus one. The goal of many stove developers is a lambda figure between 2 and 3.
Here's a good article about gas analyzing and why it's virtually impossible to optimize a stove 

Peter do you know if this article is still available the link doesnt seem to work....I guess it is 9 years old

[peterberg]

This particular pdf is published in Boiling Point magazine in 2008. Being that old, there's a fat chance it isn't available anymore.
I was able to find it again through a completely difference route and saved it in my own webspace.
Here's the new link:pberg0.home.xs4all.nl/Bestanden/bp55-pembertonpigott.pdf

[tferr]

Much appreciated Thanks