|
Post by esbjornaneer on Dec 20, 2017 2:14:45 GMT -8
Is 'true global efficiency' only to do with the combustion of the fuel or also to do with the retention of heat within the house? If the second is the case, it would make for a more things to manouver, but how about a bell that absorbs the heat until the coaling phase and then gets sealed off/separated with a valve. That way the heat would stay in the bell while the heat from the embers gets 'wasted' up the chimney or stays as coals for the the next fire, as Peter suggests.
|
|
|
Post by Dan (Upstate NY, USA) on Dec 20, 2017 2:20:42 GMT -8
Another thing to remember is that your calculation of "true global efficiency" at 71.1% is an average of efficiency based off of time, not mass or weight of fuel being burned.
90% or more of you fuel is burnt off in the first 50 minutes... you would have to weigh the fuel, kill the fire and then weight the coals after they cooled and subtract them from the original weight.
Then do some more math, not worth you effort in my opinion.
|
|
|
Post by rakettimuurari on Dec 20, 2017 2:52:55 GMT -8
How to shorten the red coal phase is easy: like the Austrians do, close the air inlet and keep the chimney damper open. The coals will die by themselves and there's a coal bed to start the next burn on. If I remember correctly, this will lower the CO production significantly. Good to know that this would be also optimal solution in relation to CO. I got interested in this as I have seen here many ways to handle the embers, which are considered "right". Of course the big difference around here is that Testo has seldom been used. Heat-wise I cannot observe remarkable difference when doing it either way... more air burns coals faster and more completely allowing quicker damper closing. With closed/ decreased inlets the cooling excess air is less but embers die slowly (but still quite soon) and the damper is longer open for safety reasons. When comparing these two ways I have found no difference in heat retention whatsoever... I should measure this too, especially by starving the majority of embers from air just after the flame dies. MHA firing instructions presented on the link's webpage interestingly kind of agree the same except the scenario of embers entirely starved from air is not actually discussed. (Scroll to section "Unwanted embers".) Peter: Would that Austrian coal bed method be also beneficial (or not detrimental at least) for burning in the sense it decreasing the ISA of firebox to be directly heated in the beginning of each burn as the floor is coal/ ash insulated? Maybe thus the firebox achieves hotter temps sooner? I remember you mentioned somewhere earlier that if system is burning well like yours, this method seems not to cause any remarkable ash accumulation as most of the excess coals are burned away during the following burn? For me it also seems that with a bell, the damper closing ASAP is not really so relevant (actually a no-brainer). I will do a temp difference comparison later between open and closed damper with my bell system to see how much difference there really is.
|
|
|
Post by pianomark on Dec 20, 2017 6:18:01 GMT -8
It seems to me that in a properly functioning bell, where gases can thermally stratify, heat will always be retained in the upper bell, regardless the inlet/damper settings. Physics prevents heat escaping up the chimney. This is one of the big advantages of bells. The heat stays trapped in the living space.
To put it another way, as soon as the gases coming from the firebox begin to cool (by dilution and/or diminished fuel), those cooler gases should pass through the bottom of the bell to the chimney, leaving the heat in the upper bell undisturbed. Of course this won't apply to a classic rocket with an oil drum and a piped bench, where all the gases are mixed and swept through the system together.
There may be other reasons for closing the damper ASAP during coaling, such as retaining heat in the firebox and chimney, which might have some effect on overall efficiency. I find unburned charcoal annoying, so I prefer to let it burn completely, rather than deal with separating it from the ashes.
|
|
|
Post by matthewwalker on Dec 20, 2017 6:27:26 GMT -8
Yasin, I often do runs from the lighting of the match to the coal phase. I do typically post graphs of second runs, but if it's 70 minutes until warmup, I agree, the Testo charts are deceptive. Here's a run of mine from match light to almost 19%. This isn't a great run, I was playing with settings as you can see by the little spikes, but it's fairly efficient from early in the burn. As Peter says, another way to get a good idea of overall performance is test from match light, then continue throughout three or four loadings. That's what got me to start to insist on insulated materials and more control over the fire. Here's an older core test on my Tiny Cook Stove, before it has much mass so eff. is not great. You can see how this is helpful for overall efficiency though. Perhaps it would be useful to include both a best effort test as Peter's protocol illustrates, and a "real world" protocol that shows lighting through two loads?
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Dec 20, 2017 10:16:08 GMT -8
Hi everyone, Thanks for all your comments and ideas. Firstly, I started doing again some testo measurements. I haven't put the airtightness joints yet, but since the plancha is flattened I can get comparable results. The configuration is still the fully octagonal riser and I've been playing around the tipping point for some time. I stopped pushing the core as soon as the plancha temperature hit 400°C. I don't want to bend it again ! The results : - It's not possible to have a well formed double vortex. Even at 8.5% O2 with the flames raging there was an asymetrical one only. - At best, and for quite a long time the CO was around 130-200 ppm which is very nice. Overall the CO stayed under 3000 ppm - the tipping point is around 9.0-9.5% O2 so the fully octagonal shape is a lesser option in this configuration. Once again it's systematic.. everytime the O2 level goes under this 'tipping point', the CO starts to go up. Combustion analysis : Secondly, this combustion analysis is different than the previous ones because I wanted to check Peter's comment. So I didn't stop the analysis at 16.8% O2. When the fire was at 18.8% O2 there was a big pile of red coals with little blue flames. At this moment (t=76min) I closed the main air intake. The CO decreased and 5 minutes after that the testo stopped detecting a fire. 10 minutes after the closing of the air inlet, the coals were nearly all black. At this moment I closed the closing valve also called "damper". The CO stayed low but the testo started to detect a fire once again. At t=76 min (when I closed the air intake) : At t=86 min (when I closed the closing valve) : To check whether or not there could be CO coming inside the house I measured the draft with both the air inlet and the closing valve CLOSED. - 7 minutes after the closing : draft = 8 Pa; TF = 69°C - 17 minutes after the closing : draft = 7.4 Pa; TF = 66°C Conclusion :- Very nice results ! Indeed there will be a bed of coals for the next fire BUT this procedure stopped the energy waste drastically. I feared that the CO would go up with the air inlet closed but it was the opposite. - There is a strong draft even with everything closed so there will be no CO problem in the house. - I think the main air inlet can even be closed a little bit earlier. Maybe at 18.2 %O2 or something like that. The whole firing, from the start to the complete closure would last 75 min approximately. Regards,
|
|
|
Post by rakettimuurari on Dec 20, 2017 12:55:40 GMT -8
It seems to me that in a properly functioning bell, where gases can thermally stratify, heat will always be retained in the upper bell, regardless the inlet/damper settings. Physics prevents heat escaping up the chimney. This is one of the big advantages of bells. The heat stays trapped in the living space. To put it another way, as soon as the gases coming from the firebox begin to cool (by dilution and/or diminished fuel), those cooler gases should pass through the bottom of the bell to the chimney, leaving the heat in the upper bell undisturbed. Of course this won't apply to a classic rocket with an oil drum and a piped bench, where all the gases are mixed and swept through the system together. There may be other reasons for closing the damper ASAP during coaling, such as retaining heat in the firebox and chimney, which might have some effect on overall efficiency. I find unburned charcoal annoying, so I prefer to let it burn completely, rather than deal with separating it from the ashes. I agree this train of thought with relation to bell theory. In actual use the effect of damper really seems to be almost marginal when compared to systems operating with forced gas movement principle such as contraflows. On the other hand however; chimney damper open and chimney warm + not fully 100% airtight doors etc. the draft is still going on surprisingly strong. In layman terms thinking +23'C air going in the heater and +60'C air escaping to chimney the equation is obvious. I still believe the damper is really helping to minimize the cooling airflow on its part. With contraflows and such the effect of damper is of course more obvious as no stratification has occurred -> gases are warmer 'cause colder and warmer ones are just mixed in one flow so we have much more to lose to chimney. Tomorrow morning I will starve the embers!
|
|
|
Post by peterberg on Dec 20, 2017 13:08:50 GMT -8
I think the main air inlet can even be closed a little bit earlier. Maybe at 18.2 %O2 or something like that. The whole firing, from the start to the complete closure would last 75 min approximately. There's no closing valve in my heater but the door is pretty tight. Temperature in the chimney pipe is a mere 30º C within 45 minutes after closing. Actually I do wait a little longer so the coals are diminished a bit more before closing. Next run the coals will burn away for the best part and new coals are left behind so in all only ash content is slightly larger the next run. I tried it a couple of times now and I'd think the firebox could be a bit higher. The effect of the threshold is less because there is a dark coal bed at the start, elevating the fresh fuel. Years ago somebody else suggested to close the inlet of a contraflow heater early, yielding similar results. Edit: Could it be that you need the backsweep together with the full octagon? Just a thought.
|
|
|
Post by peterberg on Dec 20, 2017 13:23:31 GMT -8
Peter: Would that Austrian coal bed method be also beneficial (or not detrimental at least) for burning in the sense it decreasing the ISA of firebox to be directly heated in the beginning of each burn as the floor is coal/ ash insulated? Maybe thus the firebox achieves hotter temps sooner? In my heater there's a marked difference between a firebox with a layer of ash at the bottom or one that's sqeaky clean, in terms of heating up quick. I always leave some ash on top of the floor channel for this reason. I remember you mentioned somewhere earlier that if system is burning well like yours, this method seems not to cause any remarkable ash accumulation as most of the excess coals are burned away during the following burn? That's right, most of the old coals burn away and new coals are left behind. So the layer of coal and ash is growing very slowly because of more ash over time. For me it also seems that with a bell, the damper closing ASAP is not really so relevant (actually a no-brainer). I will do a temp difference comparison later between open and closed damper with my bell system to see how much difference there really is. To be honest, I used a thermometer alongside the Testo so I have a pretty good idea what to do at what chimney temp. But I got sloppy over time and sometimes I tend to forget about the heater at the end of a run because it stopped making noise and I had to look at the thing several times whether it was closing time or not. The coal starving method feels handy because shortly after the rumbling noise stops it's time to close the air inlet.
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Dec 21, 2017 9:14:46 GMT -8
Hi everyone,
I've made some more tests today with :
- a half octagonal riser - a half octagonal riser being just a little bit higher than the port (3B) and the rest of the riser being square (this was the initial configuration of the riser for the tests)
The second configuration seems to go to a lower CO level but concerning the tipping point, it's similar and it's around 9.0-9.5 % O2. Both yielded a better double vortex.
The problem is that during the first tests, the second configuration had a tipping point of 8.5% O2.. This means that my measurements considering O2 levels have now drifted ! Surely during the "hard testing" I broke some clay mortar joints so now there is more air coming in and making the O2 level higher than they should be.
As a conclusion I'd say that the best configuration until now is the "half octagonal riser being just a little bit higher than the port (3B) and the rest of the riser being square" + the bell
So I'll get along with that and now the next trials will be about the same riser configuration BUT with insulating firebricks.
Regards,
|
|
|
Post by matthewwalker on Dec 21, 2017 9:23:48 GMT -8
Yasin, I understand. I believe that once a masonry heater is settled and dry, there is air coming in and Testo results are skewed. Some more than others! (Meaning mine are pretty leaky!)
|
|
|
Post by rakettimuurari on Dec 21, 2017 14:29:26 GMT -8
... For me it also seems that with a bell, the damper closing ASAP is not really so relevant (actually a no-brainer). I will do a temp difference comparison later between open and closed damper with my bell system to see how much difference there really is. Significance of the damper in my bell system Measured the temps outside the heater during 12hrs from highest burn of one full batch (6-7kg's of cord-wood spruce). Outside snowy and 0'C. Difference to first iteration is that chimney damper was left entirely open after the burn in purpose to find out about the usefulness of the damper in a bell system. Air inlets were closed of course. Temp comparison closed vs open damper
** all doors gasketed and quite gas tight, except the ash box door below firebox. That door is nevertheless quite tight too. I am closing my damper in future too.
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Dec 21, 2017 23:20:42 GMT -8
Very nice testing rakettimuurari ! Were the temperatures of the heater equal at the initial time, right before the two firings ? regards,
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Dec 22, 2017 1:26:31 GMT -8
Hi everyone, This morning I tried the same configuration as yesterday (half octagonal riser up to 3B, then square + the bell) but with insulating firebricks inside the riser. I still do a lightning from the bottom and use small logs (4-5cm in diameter). Visually the flames were longer and more symmetrical at the beginning than all previous configurations. Two minutes after the beginning of the fire : As of now, CO-wise, the results seems equals (120 ppm at best) BUT the heater is clearly more stable -- the tipping point is around 8.6% O2 so the fire can be more intense without CO spikes. The tipping point happened 4 times (8.5, 8.6, 8.5, 8.7% O2) in the testing and then the plancha became too hot (450°C) to keep pushing the heater. Compared to the same core without insulating firebricks inside the core, the tipping point is 0.4 to 1.0 %O2 lower which is very nice ! Combustion analysis : I'm still very satisfied of this core, but maybe the most important points that could be improved are : - more mixing in every part of the burn, with a focus at the end - less coals at the end The next tests will be with a shorter threshold and/or a larger floor channel. peterberg , in your tests why did you add a threshold instead of making the horizontal or the vertical part of the floor channel larger ? The effect should be equivalent, no ? Regards,
|
|
|
Post by peterberg on Dec 22, 2017 2:04:48 GMT -8
peterberg , in your tests why did you add a threshold instead of making the horizontal or the vertical part of the floor channel larger? The effect should be equivalent, no? During testing I tried 6 different lengths and two different widths for the vertical part. Also with slits in the sides, one slit in the far corner, open top or not, closer to the port, threee different tubes for the horizontal part, etcetera. The configuration I am using now turned out to be best, the threshold was added later on (5 iterations) to damp down the first violent part of the burn and feeding the floor channel adequately at all times. So your statement above do sound far too simple, I would say.
|
|