The Half Barrel System

[matthewwalker]

Okay, I need to qualify that "no chimney" statement a bit. I believe that these systems don't require stack effect at the end of the system to run, but in my opinion they benefit greatly from it. So, Klemen, yes, it will run adequately with no chimney if conditions are perfect, no wind, etc. But, I definitely don't recommend trying to build a solidly functional system without a chimney.

As for my comment on system size, if the system is large/long and you have a vertical chimney, it's possible to cool the gasses too much and get a "cold plug" where the gasses don't rise at the end and the system stalls. In this case no chimney is actually better, until the wind blows and screws everything up. My approach is to prime the system as I mentioned above. Once that stack effect is going it helps speed the gasses through the system so they (hopefully) reach the chimney with enough heat to rise and keep the flow going.

Ryan, there is a max length for the barrel/bell. I've built one with 11 halves (5.5 full barrels halved) and it was, I felt, a little too long. I don't know where exactly that limit is, and I suspect it has to do with a lot of variables, but for sure you can build it too long and have the situation I describe above become a problem. Exhaust gasses too cool to rise in a vertical chimney, and a strictly horizontal exhaust is in my opinion fraught with trouble due to potential pressure differences.

Klemen, I can't really say on the exhaust temps. I've messed with so many configurations that I only have a "feel" for what works and what doesn't. I only have one system that is that short, in my shop, as a test bed. Right now it's Peter's batch design scaled up to 8" riser diameter. It's driving 3 halves, and has a vertical chimney of around 12'. I would guess your estimate is a little high on exhaust temps, but somewhere close to that is probably right. Again, I can take off the vertical chimney and the system still runs well, but it all benefits from the increased draft, gas velocity, and consistent performance the chimney brings to the system. Again, I really wouldn't recommend no vertical chimney ever. It's just too delicate of a balance to keep running smoothly in all conditions in my experience.

[matthewwalker]

Oh, Ryan, I am picking up on the fact that you understand the subtle difference in the ways I've built these with regards to configuring the barrels as flue pipe vs. configuring them as a bell. A lot of what I'm discussing above mostly refers to the "flue pipe" configuration where the exhaust is flush with the top of the barrels so gasses flow easily along/inside the highest part of the arch and up and out.

With regards to the bell configuration and system size...keep in mind you can greatly vary the volume of the bell and therefore overall system size by simply inserting the exhaust deeper into the barrel bell, or raising it up. So, that's a huge variable there. When I've thrown up temporary "campfire" systems with 4 or 5 halves, I can control how hot the exhaust is at different times in the burn by raising or lowering the exhaust within the bell. In this way I can overcome an oversized systems hard starting/cold plug characteristics by raising the exhaust until things start heating up and flowing, then lower it into the bell to keep more heat in there. I'm not saying you will want a system where this is necessary, but it is a good tool to experiment with as you are determining your system size.

[dvawolk]

Oct 10, 2013 11:05:54 GMT -8 matthewwalker said:

Klemen, I can't really say on the exhaust temps. I've messed with so many configurations that I only have a "feel" for what works and what doesn't. I only have one system that is that short, in my shop, as a test bed. Right now it's Peter's batch design scaled up to 8" riser diameter. It's driving 3 halves, and has a vertical chimney of around 12'. I would guess your estimate is a little high on exhaust temps, but somewhere close to that is probably right. Again, I can take off the vertical chimney and the system still runs well, but it all benefits from the increased draft, gas velocity, and consistent performance the chimney brings to the system. Again, I really wouldn't recommend no vertical chimney ever. It's just too delicate of a balance to keep running smoothly in all conditions in my experience.

Those three halves as one single bell? are they cobbed or just bare barrels?

This is nice option in lowering the chimney down into the barrel half...It works like bypass, but better - you have all the options from minimum to maximum draft... Nice and simple

[matthewwalker]

Yes, one single bell, although in reality I run them more like flue path, with the exhaust coupled at the top of the arch. Cobbed, but not very well. Maybe 3" over the top, and certainly not finished. Just something to hook test burn chambers up to to evaluate how well they perform when connected to a horizontal run with a little mass.

[patamos]

Hi All,

Found this relevant thread after wondering aloud in earlier post elsewhere. Am thinking of a (2) half barrel bell set up in a shortish bench.
I've noticed elsewhere Peter VDB recommending roughly 43sq.ft ISA (sans floor) for a 6" J feed with a more vertically oriented rectangular shaped bell. I wonder how the shape of the barrels with the cuts facing down might vary such an equation... If anything reduce it a bit? Or am i just over-sweating the details?
I will have a flue bypass from the downdraft exit into 20+ feet of vertical, so am feeling safe in setting my exhaust from the bell right at ground level and tweaking the damper as necessary...

Apologies for a day of mega posting. Feeling a bit twitchy as i just arrived onsite away from home and am needing to adapt in response to a few layout surprises.

[matthewwalker]

I'm going to reply to this in the other post Pat.

[oxbowfarm]

I know this is an old thread, but it was the first "half barrel" bell thread I found. Is it necessary to burn the paint off of the bell half barrels? Sure would be less hassle to leave it on and cover with cob if it isn't essential. I'm scrambling to get a 6" single bell half barrel system built before everything freezes solid.

[dino]

Dec 18, 2015 10:25:49 GMT -8 oxbowfarm said:

Is it necessary to burn the paint off of the bell half barrels?

There is no need for it.

[daniel]

Hi guys,

I have a question about your experience with half barrels as bells. Is the space inside the half barrel high enough to have enough room for the hot gases to rise and stay while they cool?