Radiant heat off barrel vs heat from exhaust

[hockeymanvt]

I have built my first rocket stove as a proof of concept for a greenhouse mass heater. I am impressed with the temperature created at the top of the 55 gallon drum (700 degrees F) but less impressed with its ability as a space heater for my workshop. The exhaust gasses exiting the stove and going up my chimney are about 250 degrees. Does anyone have a seat of the pants estimate about how much heat (BTU) is produced by the stove compared with how much heat is leaving the stove in the exhaust. Clearly the rapid burn and large airflow is going to move heat out of the stove. In my greenhouse I will be collecting and storing the heat in a thermal mass but I would like to have some idea of the percentage of btu released by the stove vs how much will be reclaimed in the mass. Anybody want to take an educated guess?

[mechartnik]

try searching...I think Ernie Wisner(and others) have published some "potential" BTU/hr equivalents for standard 6" and 8" systems, also some BTU input numbers out there for pellet fed systems

You are not providing enough info for an "educated" guess

[peterberg]

Feb 24, 2013 16:21:07 GMT -8 hockeymanvt said:

The exhaust gasses exiting the stove and going up my chimney are about 250 degrees.

That 250 F. is not bad at all, since the combustion temperature in a well-running stove is easily 2000 F.

As an aside, there's a difference between exhaust temperature and flow volume. Larger volume and lower temp isn't any more desirable as compared to the other way around. So the heat level of the exhaust gases shouldn't be the only factor to be reckoned with. In other words, would you be happy with more air passing through at a lower temp?

I don't know anything about BTU's, sorry. What is the configuration of your workshop stove?

[hockeymanvt]

My question is related to the balance of heat production between the radiant heat generated as the flu gasses reignite within the metal barrel and the exhaust gasses the exit the stove. I realize I don't have the hard data to answer this question with precision but am looking for opinions of those of you who have experience with this technology. What I am hoping to get is a feel for is how much MORE heat can I reasonably expect to get once I run the exhaust gasses through a thermal mass. Ex. will I get 10% more heat from the stove by setting up a thermal mass or 50% or 90% ? Any observations will be welcome.

[Donkey]

Umm.. Firstly, there should be NO re-ignition inside the barrel. If you haven't burned ALL of the fuel in the J-tube, something is wrong.

As to how much heat a bench or bell will harvest, it all depends. How long is the thing (internal surface area), what kind of conditions exist for heat transfer, etc? I CAN say that the thermal mass makes a SIGNIFICANT difference. I am hesitant to throw out numbers without HARD data to back me up. Peterberg may be able to do this, I don't have the testing equipment, nor the inclination to pin it down.

Really, the major trick of ALL high mass stoves is NOT that they produce more heat but that they hold and slow the delivery of that heat. The aim of a stove is to heat the people and the heat that the people find useful is the heat that they can get INSIDE of or come in contact with.
It's quite possible (not that hard really) to run an old fashioned potbelly stove to burn very efficiently. Those old stoves can actually produce MORE BTUs than your average rocket stove, it's just that the heat produced flies off the stove, zings through and past the people and is lost VERY quickly, not to mention all of the heat that is wasted directly out the chimney. Potbellies burn the load quickly and go out, then go cold. You need a lit of wood to get anywhere with them.
Airtight stoves were invented to deliver the heat of a load of wood over a longer time, lengthening the heating time and reducing the amount of wood used. Unfortunately, airtight stoves cause other problems, smoldering wood is not in an efficient burn condition. These stoves create dangerous amounts of creosote, catalytic converters are expensive and are used up over time, etc.

Having said all that, he mass WILL harvest more heat from a rocket stove, though I suppose the same amount of heat can be harvested through successive metal bells or something like that. You may produce (into the living space) just as much heat as the thermal mass, it will be lost to the outside world MUCH quicker and will be less useful to you.

[hockeymanvt]

Thanks Donkey, What attracts me to the rocket stove is getting the most efficient combustion out of my scrap wood. Clearly the high air volume is going to move quite a bit of heat out the exhaust which will be recovered when I run the exhaust through a thermal mass. I really LOVE seeing almost nothing coming out of the chimney. My real application for the stove is to keep the plants (Kale) warm in the greenhouse when it gets below 0 F at night. I expect I will just charge up the mass with a couple hours of intense burn and have the mass keep the plants warm through the night. Thanks for your collective wisdom. "If I knew what I was doing I wouldn't call it research"

[grizbach]

hockey, Before I ran my exhaust through my bench the room would feel slightly warmer. Now that it's going through 450lbs of bench, the surface flue temp drops from 220F to 140F. And after two thermal charges the room is quite warm and will stay that way for about 3 hrs. After 6 hrs the heat can be felt standing next to the stove. Anything over 9 hrs is a pretty cool stove.
Hope this helps

[Donkey]

My mass is easily over a ton (of cob). The stove will keep my house warm all night (on cold nights) and the bench will still be VERY nice to sit on the following morning.

[hockeymanvt]

Thanks for all the help. Next fall when the greenhouse is all set up I can report back how the growing beds/ heat sinks worked out. Now it is time to start sugaring.

[stephenson1]

In March I am planning a rebuild of an outdoor rocket oven I've been developing since last summer.
I've been browsing around this forum and every day I get so many new ideas it's hard to settle on a final plan.

My latest is to try and build a sacrificial wooden form based on Matt Walker's home-cast method. By using wood instead of Hardibacker as he did, I'm thinking I can carve the surface to match Peterburg's core design so that when it burns out I have that shape as my interior.
Can anyone advise as to whether Matt's castable recipe is likely to hold up with Peterburg's interior shape? Are detailed dimensions for Peterburg's 8" core posted somewhere?

Also, I'm thinking of using thin hard firebrick for my heat riser. I've been thinking that if I rotate each course of bricks say 1/4" I can create a spiral riser with little edges to create turbulence for a cleaner burn. I'd surround the riser with a few inches of loose vermiculite fill.

Any merit to these ideas?

[peterberg]

Feb 27, 2013 4:08:40 GMT -8 stephenson1 said:

Are detailed dimensions for Peterburg's 8" core posted somewhere?

I'm not sure, but it's simple enough if you know how to do the calculations yourself.
Take the diameter of the riser. When using a square riser, take the length of one side. Multiply this by 0.72, this is the base dimension. Firebox width, height and depth is 2xbase, 3xbase, 4xbase. The port is 2.2xbase high and 0.5xbase wide. Depth of port about 2". Total length of the riser 8 to 10xbase, measured from the floor of the firebox.
Add some sloped sides in the firebox, left and right and a sloped bottom at the back of the riser. P-channel is as wide as the port and it's csa is 5% of the riser csa, thought of as round. The primary air inlet low in front, about 25% of the riser csa. This is about it, should work.

Feb 27, 2013 4:08:40 GMT -8 stephenson1 said:

Also, I'm thinking of using thin hard firebrick for my heat riser. I've been thinking that if I rotate each course of bricks say 1/4" I can create a spiral riser with little edges to create turbulence for a cleaner burn.

Please don't rotate the bricks 1/4 each course. A riser built like that will provoke lots of friction, certainly not resulting in a cleaner burn.

Feb 27, 2013 4:08:40 GMT -8 stephenson1 said:

I'd surround the riser with a few inches of loose vermiculite fill.

Make that 4 inches if at all possible.

[matthewwalker]

I've been running an 8" batch loader in my shop this last week, made from my casting mix. It is holding up to the heat so far, but it has a lot of cracking. That big hanging top is going to be hard to get to stay there for long, although a shape like Griz's internal firebox profile would help carry the top some. I believe if you are looking for a permanent, never needs messing with solution, you'll need to go to a castable refractory. On the other hand, if you are excited about working with cob and are intending to play with it a bit over the summer, I'd say go for with the mix. I think you'll get a summer out of it at least, and learn a lot in the process.

[pinhead]

Using Peterburg's calculations as outlined in the above post, I made this spreadsheet that will automatically calculate the dimensions for a given heat riser diameter.

Attachments:

[Donkey]

Peterberg and Pinhead,
If you believe that this information is golden, ready.. Perhaps you can drop it into the Library area in some kind of coherent way?

(this is GREAT! Thanks!)

[pinhead]

Before I answer you in a definite affirmative, I have one question for Peterberg re: port height.

Does the calculated port height take into consideration the overhang of the P-Channel?

In the example of a 6-inch system, will the P-Channel overhang the 9.5" port by 0.65" or does the 0.65" overhang need to be added to the height of the port to make the "opening" the full 9.5 inches?