jrl
Junior Member
Posts: 101
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Post by jrl on Jul 28, 2014 19:25:19 GMT -8
I got the main door mostly finished: hinges welded, door mounted, gasket cemented, and handle/latch fabricated. I then proceeded to test fire the stove now that things are mostly airtight. I primed the chimney first by lighting a piece of kindling at the base of the plunger tube via the access door. I let it burn for a couple minutes before lighting the main batch box, and despite the summer temps outside, there was no smokeback this time.
As for performance, I burned two small half-loads back to back. The stove burned for 2 hours, about 1 hour on each half load before going to coals. The surface temp of the facing was about 200 degrees consistently during the burn. After the fire was out, the stove remained at 200 deg for 1 more hour, and then stayed around 175 for 2-3 hours. Below 175, I stopped paying attention. The room was up over 5 degrees from the rest of the house. Like before the brick and steel surface were still warm in the morning.
Now, if steel radiates some 315 btu / sq ft at 200 degrees and I have some 9 sq ft of steel, then theoretically this stove is producing, say, 3k btu. Does that sound accurate to anyone? For reference, my freestanding coal stove produces some 45k-75k btu, has approximately 40 sq ft of surface area, and operates between 450 and 600 deg surface temp. But, it does so by ripping through 20lb of coal in a 10 hour burn.
Is this rocket heater effective? Yes, it's a nice subtle, even heat in the room and it stays hot for hours. I wish the surface temp was 100 degrees hotter, but It seems like I'd never get get that kind of long heat output period if I burned that same load of wood in my freestanding stove. Essentially the thermal battery effect.
Is this setup efficient? I have no idea. How does one quantify how much heat is stored in and released by the firebrick over time?
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Post by satamax on Jul 28, 2014 20:52:35 GMT -8
Well, what would be intresting to see, is when you have the next cold spell in autumn; and you burn it for a few days. Will it get hotter? And will you be happy with it?
Normaly, with cold temps, you should have better draft, so better burns.
I know you're not harvesting all the heat, since you don't have 6m² or 64ft² of isa. Tho, you wanted quick heat.
5F° above ambient in the midst of the summer, i'd say is honorable. Colder temp performance will be more intresting to see.
Does your rocket roars?
Do you have any way of measuring your exhaust temps?
And last but not least, what is behind that fireplace? It would be a shame to loose heat to the outside world!
When you say 200f° tho, that gets me wonder. Why doesn't it go higher?
Realy, we need exhaust temp readings.
Another idea which popped into my head.
You could transform it as a double bell, by fitting another metal plate inside the chimney, above your first one (damper plate) with another plunger tube. A quick (ish!) and easyish way to gain more storage.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 29, 2014 11:42:00 GMT -8
Well, what would be intresting to see, is when you have the next cold spell in autumn; and you burn it for a few days. Will it get hotter? And will you be happy with it? Normaly, with cold temps, you should have better draft, so better burns. I know you're not harvesting all the heat, since you don't have 6m² or 64ft² of isa. Tho, you wanted quick heat. 5F° above ambient in the midst of the summer, i'd say is honorable. Colder temp performance will be more intresting to see. Does your rocket roars? Do you have any way of measuring your exhaust temps? And last but not least, what is behind that fireplace? It would be a shame to loose heat to the outside world! When you say 200f° tho, that gets me wonder. Why doesn't it go higher? Realy, we need exhaust temp readings. Another idea which popped into my head. You could transform it as a double bell, by fitting another metal plate inside the chimney, above your first one (damper plate) with another plunger tube. A quick (ish!) and easyish way to gain more storage. It makes a decent roaring sound. I'll take a video of it, next time I test fire it. I'm sure it will roar louder come fall when the temperatures drop. But, it does rocket. For my coal stove, I wrote some software that measures the surface temperature of the stove with a thermocouple and a USB Phidget, and stores that data in a remote database. I wrote an app for my android cell phone that tells me the current surface temp of my stove at all times. It's quite helpful when I'm at work to know if the stove is starting to fall off, so I can call and nag my wife to shake down the ash and add more coal. (Actually she hates that I have that app! ) I could move the hardware upstairs and put the thermocouple inside the bell and see what's going on. That would be an excellent bit of information to have. I could chart the results over time during and after the burn. Might also be helpful to know what the temps are exiting the plunger tube compared to the upper portion of the bell. I suspect with the rocket firing into the bell like this, the gases are all mixed up and not stratisfying much. I bet the gases exiting are probably close to the same temp as in the upper portion of the bell. BTW, Behind the fireplace is the chimney outside. The fireplace is lined with firebrick on all surfaces. The chimney surrounding the fireplace and flues is all red brick.
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Post by satamax on Jul 29, 2014 11:59:56 GMT -8
Quick and cheap'ish test, pile up straw bales on the back of the fireplace and around the chimney! You're loosing heat on that side. And it would be nice to know what temp you have out of the plunger tube for sure.
By the way,
What is your chimney CSA?
And your fireplace/bell ISA?
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 29, 2014 12:31:21 GMT -8
Quick and cheap'ish test, pile up straw bales on the back of the fireplace and around the chimney! You're loosing heat on that side. And it would be nice to know what temp you have out of the plunger tube for sure. By the way, What is your chimney CSA? And your fireplace/bell ISA? Bell ISA is about 3.8M^2. Chimney CSA: can't remember. I thought it was 12", but maybe it's 8.5". I'll have to measure or drive by the place I bought one from and look at them in the yard. Either way it's much bigger than my system CSA.
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Post by satamax on Jul 29, 2014 14:07:04 GMT -8
JRL, i wasn't bothered about your system CSA, just the one of the chimney, to see if some more heat could be harvested in a second bell. Tho, if your chimney is outside, it would need to be insulated, to project the heat towards the insides.
If 12x12, and your system size is 6, your chimney CSA is about 5 times bigger than the system CSA, so it would theoreticaly possible to use it as a bell. Stuffing an insulated metal plate in the chimney; with another plunger tube in the oposite corner from the first one, from bell #1. You'd have to work from the top thought. 1metre long would give you another 1.2m² for a total of 5, that would be a good system size. May be, before insulating the chimney, you could add some more mass to it.
But after all this, ye won't be able to improve it much more.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 30, 2014 9:27:03 GMT -8
What temperature should I expect to see at the top of the heat riser when the stove is firing full bore? What is typical of a batch box system or rmh?
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Post by satamax on Jul 30, 2014 9:45:23 GMT -8
Top of the heat riser? Between 600 and 900 celcius i would say!
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 30, 2014 19:21:00 GMT -8
Measured tonight with the thermocouple in the top of the riser. It ran between 500 and 750 deg F while rocketing. 600-700 being the most average range. Surface temperature of the steel facing got up to 250 deg F this time. Overall, still pretty disappointing surface temp. The core on the other hand performed wonderfully. It roared like crazy on thinly split hardwood. Now I just have to get that heat out into the room.
I think a fan/tube heat exchanger may be unavoidable. Unless, I can route the immediate output from the heat riser along the steel facing, making it downdraft along the facing. Trying to brainstorm on this.
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Post by satamax on Jul 31, 2014 22:47:09 GMT -8
To me, there's something definately wrong with thoses temps. Daft question, do you have the door on yet? It might also be the result of too much chimney draft, i which case the double bell idea i was talking about might help. Nope, definately 370C° on top of the riser doesn't sound right! Different temperatures in different parts. Highest in the riser directly behind the gate, 1200 centigrade. In the firebox between 600 and 900 centigrade, riser end about 900 centigrade. In Fahrenheit, that would be 2190, 1110 and 1650 respectively. donkey32.proboards.com/post/12353/thread
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Post by satamax on Aug 1, 2014 4:47:40 GMT -8
As for performance, I burned two small half-loads back to back. The stove burned for 2 hours, about 1 hour on each half load before going to coals. Well i was talking with Peter, about your case. He said not enough draft, while i was thinking too much. What you say in the quote above leads to believe there is not enough draft. A half load in such a rocket shouldn't last more than 20 minutes. Does your rocket blows the flame of a lighter on the P channel port like this one? What is the size of the opening in your door? Have you tried to run it door semi open?
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Post by peterberg on Aug 1, 2014 8:03:58 GMT -8
Hmmm, so this results are obtained with half loads? The temperature levels I quoted were for a stove already heated up and at full bore. I'd suggest you try that, heat it up gently and load it to the top. It should roar like a hungry bear, much louder than with a half load. Oh, and load it lengthwise only, i.e. not criss cross or campfire style, avoid pieces that stick into the port, keep at least a space of 2" between door and fuel and that full load could exist entirely of 2" square pieces or something like that. When you use a door, don't switch from door closed to open with the stove at full bore. This system can't cope with that and smoke will get into the room, almost quaranteed.
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jrl
Junior Member
Posts: 101
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Post by jrl on Aug 4, 2014 19:20:59 GMT -8
Thanks for the input and suggestions on exhaust temps. I will try your suggestions and see if I can at least get up over 1000F. Perhaps fall temperatures will give me the draft needed. Right now my effort has been with extracting heat from the stove out into the room. I found a 3 ft scrap of 2"x3" rectangular steel tubing today at a local metal supplier. It should be just right to work with a standard stove blower. I hope a small blower will allow the system to draw on the thermal battery for hours after the fire is out, and give me quicker heat into the room during the burn.
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jrl
Junior Member
Posts: 101
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Post by jrl on Aug 15, 2014 9:20:54 GMT -8
I think I've reached proof of concept on my stove design. I built a blower tube from rectangular steel tubing that runs in, and over, and out of the stove in a simple U shape. I fabricated a manifold that connects a cheap stove blower fan to the blower tube. It's not the prettiest design, but it's proven effective. I will try to post some pics of the setup tonight.
In my last test firing I was able to get the stove to rocket like "an angry bear" but only on a 1/4 load after the second full load was down to coals. I think once I insulate the heat chamber and riser I'll reach that tipping point a lot faster. It was 62 outside when I was test firing, so the draft was a little better, but still not like it will be in the fall. On a side note, the access door on the right hand side has proven quite helpful in priming the stove. I've lit a small piece of kindling directly under the plunger tube entrance and let it burn for a few minutes before starting the main burn chamber. And, voila.... no smoke backs.
Anyway, after a couple loads are burned, the stove is at 250 deg F surface temperature, and the fan cranks some hot air out of the blower tube around 150-165 deg F. The fan has given me the quick heat in the room that I was looking for. It starts pumping heat out of the burn chamber almost immediately and really brings the room up fast. But, what's really amazing to me is the thermal battery effect of all that masonry in this setup. Based on my experiments, when two loads are burned to coals in about an hour, the fan continues to pump hot air out of the stove for 6+ hours after the fires out. I need to collect more data on the effectiveness of this heat pump. I'm going to hook the thermocouple up to the blower outlet and log the temp over time. I went to bed long before this thing was done pumping hot air, but overall I'm thrilled to get that kind of output over such a long period after the stove is out, and on such a small amount of wood.
I used only 1.5 14" log lengths. That is just couple splits of well seasoned oak firewood producing 7+ hours of heat to my living room. Of course, I'm really only heating a couple 10x10 rooms with this, and if it were a larger space the stove would not keep up. But, with a tradional wood stove I'd also drive everyone right out of the room with all that heat. It wouldn't be comfortable. So, in my assessment here, I think I'm heating the living space appropriately, only using a tiny bit of firewood to do so, and getting slow/steady heat output long after the fire is out that will heat the living space into the night, possibly into the early morning.
This is just my initial take on it. The fall will prove/disprove everything more conclusively.
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Post by satamax on Aug 15, 2014 11:45:42 GMT -8
That's good news. I didn't give too bad an advice for once! Well, obviously, insulate your firebox and heat riser! And also, insulate your thermal mass from the outside, so you don't loose too much heat to the outside world!
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