Project:RMH,horizontal feed Batchbox model 6",

[marko100]

I checked again. this applies to devices that are connected to the central heating.

Combustion plants using biomass, firewood must be in accordance with the requirements of BS EN 303-5, the following thermo-technical characteristics:
Efficiency at rated output must be greater than or equal to 90% of total dust emissions must be less than 40 mg/m3
emissions of carbon monoxide less than 500 mg/m3. Emissions combustion plants should be set in the benchmark temperature of 273 K and a pressure of 101.3 kPa and
calculating the oxygen content 13% in dry flue gas.

but it is only a matter of time when they will be enacted for other combustion plants. Maybe not so strict.

NOx is measured only on a larger boilers for biomass.

OK enough offtopic

[dvawolk]

Previous weekend me and my friend built a masonry two bell 6" system.
The plan you can see some posts back. I sticked to the sketch with three exeptions. The second bell is about 6 cm higher than on the sketch. Also the batch box is 9cm higher than on the sketch. And the first bell is one row of bricks (11cm) lower. I other words:
the floor of the bells is 12cm below the ground level of bathcbox.

First some photos linked (photobucket still does not show images or i am missing something)

So the tests has been maid. If i ignore first few firings (drying) this is how it went:

As for the bell system. It works perfectly. I measured temperatures in the far side of the bench (middle high) and these were obviously higher than exhaust (bottom level,) temperautres. Really great.

The system has inner surface area of:
3.5 sq. m for the first bell - refractory bricks
3.4 sq. m for the second bell (bench) -red clay ricks with concrete slabs on top

This was in total 6.9 sq.m. The suggested size was 7 sq.m (peter).

I measured temperatures in:
--2/3 height near the back surface of the first bell (away from the heat riser) - by the peek hole ( i don't think this temperature is correct because the robe was too near the walls of the bell so i wont be forwarding these values.
--connection between first and second bell
--exhaust hole in the second bell

All these days outside tempearture was from 0 to +5 Celsius and humidity near 100% ( snow was melting all the time)

The temperature in connection from first to second bell never reached above 140 degrees and the bench took away another 100 degrees celsius. Exhaust tempearture was not more than 40°C and severe condensation was present inside second bell and exhaust flue. Also i had a feeling that the thing lacks abit of speed /draft.
I suspect two bottle necks. First is connection between the bells and second (more likely imo) connection to exhaust flue.
I the last one the gasses have to make a 90 deg andgle turn to the exhaust flue and the CSA is changed from rectangle to round - so the turbulence should be present. Opinions?

I loaded batchbox full for two times and right at the begining of the flash over everything started to smoke - thick smoke coming out from everywhere. (here i can say that when the thing was doing OK, the second bell had underpressure, sucking air INSIDE from occasional small openings - as described by Donkey.

I thought that maybe the inner surface is taking out too much of heat and so i closed half of the bench with some rockwool. I removed close to 2 sq. me of inner surface.

Then i loaded a full batch box and observed.
The exhaust temperature came to over 50 Celsius in 30 minutes of burning. The draft was a bit better and there was no smoke-coming-from-everywhere. From the exhaust flue came MAINLY water vapours. I caught them on a piece of glass and when condensed there was no obvious dark colored deposits.
I could still smell that wet coal note in the exhaust - somewhere here i have read that this points to not totally burned carbons!?

I still think that the stove is lacking some draft. Bottlenecking?
My heat riser is 15.8 x 15.8cm =250 sq.cm (equals 15.8 DIAMETER with CSA:196 sq.cm)
My connection to the second bell is 30cm long rectangle hall with csa of 11x22cm (242 q.cm)
My connection to the exhaust flue sketch is attached. Exhaust flue is 15cm DIAMETER

Do you think that there is to little draft?
Do you think that there is too much of a surface in both bells (if the bench is not shortened)?
Do you think that there is a problem with the attachment of exhaust flue to the second bell?
Do you think there is a problem because exhaust flue is 15cm DIA and heat riser is maid for 15.8 DIA (176sq.cm CSA)?!

Some videos attached, they are from the last burn (with only a half bench attached)
20 minutes into the burn (smoke on the top of burning chamber)


35 minutes into the burn:


40 minutes into the burn:


Please, comments!

Klemen

[dvawolk]

sorry for the videos they are playing twice the natural speed. (that would mean the draft is twice faster :-)

[Vortex]

Nice build. First off I'd put a better chimney stack on it - out of straight 6", make sure it's higher than the top of the bell, 12 feet high from the ground at least. That bent dented wrinkly flexible pipe will slow it down for sure.. Also if your stove is wet and very cold your not going to get efficient combustion, give it a few good firings to dry out and warm up. Well done.

[peterberg]

Klemen, I do suspect you are too impatient, that bell system can't possibly be dried out yet. A large masonry heater will take the best part of 6 weeks to dry out completely by stoking it, so this one will take 3 to 4 weeks at least. The trick to block part of the bench is a good one, this will aid to properly running in of the stove.

It is of the utmost importance not to have any fuel in the port itself and you'd better not load it to the ceiling for now, take it a little bit slower.

The connection between the bells are alright, but I would suspect the exhaust port. Namely the transition from rectangle to round. You'd better to see this as a funnel, the round flue as is should fit inside the rectangle.

When the flash-over phase is the trickiest, avoid it by loading the firebox not more than 2/3 full. It is best to keep the thing going at a lower rate for now.

The point with smelling wet charcoal is that it is detectable down to a couple of molecules. These are called aromatic hydrocarbons, for a reason. The glass didn't catch dark deposits, that's very good. Try this with a choked-down steel box wood stove and you'll see what I mean.

I would think Vortex is right, your exhaust flue is dented, not smooth inside and on the short side. The rectangle opening of the exhaust should be at least 15 cm high, to accommodate the round flue. Admittedly, the flue is a bit smaller than system size but that shouldn't be a problem at this low stack temperatures.

Again, you have to get rid of the moist inside the main stove first, after that you can connect the rest of the bench again. Whether or not the inner surface of both bells is too large I honestly don't know. The surface recommendation is an educated guess, nothing more. Only time will tell whether this is too much or not.

[marko100]

Certainly correct output in the chimney of the second bell, as suggested by Peter.
Outlet temperature at the outlet of the second bell should be above the point of condensation of water vapor (100°C).
Proper surface area is also adapted to this. Calculate the difference in the temperatures (combustion and output in the chimney) and the coefficient of heat transfer materials that you use.

[dvawolk]

First, thank you all for guide points on last reply.
UPDATES:
-i removed that flexible aluminium pipe for the stack
-I quickly rebuilt the second bell that connects to stack (it is not perfect, it has many edges and am sure that turbulence will occur. But the CSA surely is the same or above of 16cm DIAMETER circle. The stack is now 3 meters high from the ground (the bell is about 1.7 meters)
- in the last few firings i lengthened the bench so that now its surface measures 1.9 sq. meters. (first bell is 3.5 sq. meters)

The difference is obvious. I had a few firings again and that window on the first bell (with a view on heat riser) can stay clear for a whole burning.
This is a video somewhere in the middle of the burning (other half).


Temperature shown (309°C) is from sensor located 45cm from the top of the first bell, opposite to heat riser location.It is by the window in the first bell. So the generated heat touched approximately 1.3 sq. meters of bricks. The flashower flame is high almost to the top of the bell (60cm) but with my camera it is not seen that clearly...

Temperature on the exit of second bell is 45 to 55 °C and condensing. Outside temperatures are 5 to 10 °C and humidity near 100%. And we are expecting more rain and snow. Will this bricks EVER dry out?! :-)

Anyhow, now it runs much more reliably. The flashover mode is not causing problems. BUT i do not load the batchbox over 2/3 or 4/5 full. That window in the first bell stays clean to the end of the burn...

Exhaust can be smelly with wet charcoal or not smelly at all. The only exception is first 5 or so minutes at the beginning of firing with totally cold bricks - at that time the smoke is unpleasant.

PETER:
That drying period is really interesting info. The bad thing here is that my heater is outside and whenever it is not burning it sucks that moisture from the air. Rain and snow in the following days really wont help much... Am i losing a battle if i make one burn a day and the humidity around is close to condensation 24/7?

MARKO:
Do you think that in these calculations the speed of gases also plays a role? And that pretty much complicates the calculation. I have no idea of how to measure the velocity of passing gasses in the bells... Do you?


It came to me now: between 300°C to about 150°C is around 2.2 sq meters of firebrick bricks..
And again from 150°C to 50°C there is 1.9 sq. meters of red clay bricks.

As the first bell will be dried out faster i can get some kind of a feeling of how much inner surface can i have in the second (dry) bell.
I would say that the higher the temperature, the faster it is cooled by the bricks. Means: from 300°C to 200°C less surface is needed compared to from 200°C to 100°C. Right?

WHAT COMES NEXT:
- i will hone the exhaust to chimney conversion.
- i will measure all three temperatures all at once as that might give me a better feeling of proper inner surface to expect.
- pray wholeheartedly for dry windy weather (when not firing
- reading your replies! That's always great!

Regards,
Klemen

[peterberg]

OK Klemen,
It looks like you are getting somewhere now.
That flash-over shows an irregular rhythm, probably the same as the lower pounding noise which accompany the roaring of the flames. I would think it's best to cover the stove with a tarp or something, to protect against the rain.

Maybe you could try next time this schedule: one small fire to warm the stove up a bit, shove the glowing embers to but not through the port, add some pieces of fuel until it picks up again and load it full to the top.
I'd expect that you'll get a larger flash-over and higher temps inside the bell. When this is passing without any severe smoke, your stove has past the exam.

Condensing of the vapor in the exhaust gases isn't bad in itself, let it go and see where it's heading. Several firings after each other could cause the stove to go in thermal runaway, roaring like nothing before. This is very helpful to see what the thing is capable of, this is an experiment, isn't it?

Handy in the drying out department as well...

[marko100]

Mar 12, 2013 12:27:16 GMT -8 dvawolk said:

Temperature on the exit of second bell is 45 to 55 °C and condensing. Will this bricks EVER dry out?!

Only you have to warm the bench each time at a higher temperature than the temperature of the exhaust gases. Otherwise these condensed water vapor soak in bench inside.

Mar 12, 2013 12:27:16 GMT -8 dvawolk said:

MARKO:
Do you think that in these calculations the speed of gases also plays a role? And that pretty much complicates the calculation. I have no idea of how to measure the velocity of passing gasses in the bells... Do you?

Speed ​​of gases is important. It is important residence time of flue gases. However, since you use the bell for the heat exchanger, in which according to Peter each particle touches every corner of the surface, I think you can calculate the basis of simple equations. Of course the result will not be completely accurate. The fact is that the necessary surface varies according to the desired temp. surface of the stove.

Fundamentals you will find in manual by Bojan Kraut (strojniški priroènik).

Mar 12, 2013 12:27:16 GMT -8 dvawolk said:

I would say that the higher the temperature, the faster it is cooled by the bricks. Means: from 300°C to 200°C less surface is needed compared to from 200°C to 100°C. Right?

Yes. Important is the temperature difference between the transmitter and the receiver. The greater the temperature difference smaller area need for greater temperature drop.
Thats why is important of what is desired surface temperature outside the stove.

[marko100]

About the speed of gases...
Measure the speed, temperature and amount of air entering the stove.
As the air expands when heated it increases speed. Calculate the result of changes in temperature and cross-section of the riser and input quantities. Then calculate the speed of the gas at the intersection of the first bell and temperature on individual Hight.

[dvawolk]

Hello, All.

I have been firing the heater for three days more extensively. The chimney is now almost vertikal with diameter of 19cm (plastic tube) for the first 2.2 meters and then diameter of 15cm for the top meter. this all totaling in 3.2 meters of height of a vertical stack. I fixed conection from second bell to chimney. It doesn't matter how look at it, the CSA i always more than that of a 16cm diameter circle.

I burned hardwood beech (15% to 20% humidity).

Temperatures shown below are shown like
A/B/C

where the meaning is:
A= measured after gases have passed 1.3 square meters
B= measured after gases have passed 3.5 square meters (connection between bells)
C= measured exhaust temperature to chimney flue

First day i started firing only 3/5 of a second bell/bench. So the inner surface was 3.5 (first) and 2.1 sq. meters (second bell/bench with 1.4 sq. meters removed). That is 5.6 sq. meters in total.

1.
Cold start and 1/3 full.

2.
2/3 load on hot coals. Fast start and roaring, reached 333/155/63.
After the flames were gone for a few minutes 228/154/62. That is interesting for me. When the coal stage is here i would expect gases to be slower and consequentially exhaust temperature would be lower. But it was not so. Is that because the "thermal battery" was getting full?

3.
2/3 load on hot coals
5 minutes after loading on coals some 3Hz pulse was heard for 5 minutes or so...?! Like some kind of steam train...
Reached 328/178/74.
10 minutes of coals only:223/179/83

4.
4/5 load on hot coals
3 minutes after loading on coals some 3Hz pulse was heard for 5 minutes, again.
After 5 minutes from start of loading: 290/172/72There was some smelly and thick fumes and after 3 minutes mainly water vapors. From here on water vapors, sometimes with slight smell of aromatic hydrocarbons.

5.
4/5 load on hot coals
after 3 minutes, beech ignites, smoke goes away and after another minute or so slightly faster 4hz pulse288/177/81.
Temperature reached:386/225/98
Begining of coals only:308/224/97
Here the exhaust was very close to boiling point of water. My guess is that it didn't pass this temperature because the heater (IMO specially second bell) is not dry).

At that time (coals only stage) i opened the bench and removed the barrier. From here on i was heating the whole bench. If i look back now maybe i should't do it at that time already.... But anyway, i did it. The total inner surface now is 6.9 sq. meters.

6.
Loaded to 2/3.
Exhaust started with water vapors. After 8 minutes the flames were quite spread already and thick smoke appeared on exhaust for 5 minutes or so 320/177/68 (whole bench heated).
After that time thick smoke was gone and temperature got up: 364/196/70
Start of coal phase: 248/190/60


7.
Loaded to 4/5 . 190/?/?
After 5 minutes: 280/181/78
After another 5 minutes: 330/201/82
After another 3 minutes: 350/207/82
10 minutes of smoke from chimney 330/?/?
and then back to 350/206/72
and minutes after 375/213/65

Second day It was very humid with +4°C. Over night the heater was cooled down to 8/9/10. The batchbox was warm to the touch under the ashes.

1.
Lit a small fire 1/4 full.
After 15 minutes mainly water vapors in exhaust. 120/44/19

2.
1/2 loaded.
From start there are water vapours mainly.
After 5 minutes smoke that lasted for 5 more minutes.
15 minutes from start flashover 291/113/34 (flashover was usually reached at temperature of 280°C or more)

3....

4.
Here i slightly opened the primary air intake to see if it can help to remove that smoky stage just after the begining of burning... It did!
Reached:371/215/60

5....

6.
4/5 full. Reached: 370/198/61


Third day Much drier than previous days with -1°C. Over night the heater was cooled down to 2/2/2. The batchbox was warm to the touch under the ashes. I think this day was one of the very few if not the only where the humidity was lower than 80%. It was nice to see water vapors disappear in a short distance :-)
The primary air intake was slightly more opened (72 sq.cm)

1.
Small load
after 12minutes coals only 129/39/15

added full load of less than 4cm thick split beech pieces
13 minutes later: 292/93/29
reached: 309/127/36

2.
Full load of beech.154/103/35
No smoking after the flames started -just vapors.
7 min from start 251/118/36
13 min from start 330/155/40 (flashover)
18 min from start 360/171/49
25 min from start 370/181/51

[dvawolk]

When i started heating the whole bench i found out that at the bottom of the bench water is coming through the (before dry) clay slip.

I have a feeling like that when i was heating 3/5 of a bench the condensing was mainly in the chimney.
But when using the whole bench a lot of condensing was present in the (especially lower part) of the second bell.

Maybe i should just leave the heater with bench open for a week or so and after that try again and hopefully see some differences in exhaust temperatures. When the bench cools down i can fell humidity inside with my hands...

Also i was thinking.. Saying that exhaust temperatures to the chimney should be above 70 or 100°C. Are these temperatures requested at the top of the burning? It is obvious to me that they can't be reached at the beginging in the cold heater even if dust dry!

Almost all the firing had in common that at the very begining of flames all is well and the exhaust does not smell badly. But when the majority of wood begins to warm but is not yet in a full burn here comes the 5 to 8 minutes of badly smelled thick smoke.
Only the last two firings were ALMOST lacking this smoky stage (primary intake was opened more).

But the progress is obvious. In these three days the smoke NEVER come from the cracks of heater - that probably goes to solved bottleneck at the begining of a chimney.

Just a thought. The temperatures here are around 5°C. If the temperature around the stove will be, say 20°C (like inside of appartment) that would mean that exhaust would be close to 15°C higher, right? And because chimney will lead gasses out to the winter cold the difference will be higher= higher draft?
And probably i should expect a BETTER draft with 20 diameter chamotte flue chimney that is 12 meters high?
Based on that, can i safely conclude that if the heater is marginally accepted like it is now (outside and partly dried) it should perform much better inside even if partly dried...?


Peter:
-the heater is under the roof. the only humidity it gets is from air and water vapors :-)
-again, regarding drying period: 4 weeks to dry with firings included? Probably that 4 weeks period is especially true for the last (coolest) part of a heater - bench in my case?
I can imagine now you had LOTS of drying when writing rocket/bell projects...

Marko100:

Fundamentals you will find in manual by Bojan Kraut (strojniški priročnik)

Funny thing is that i just came over this book two days ago and i already decided to get it next week - will be useful in many other projects :-)

Thats why is important of what is desired surface temperature outside the stove.

...By "desired" you mean the temperature of media outside the stove? Like air in my case?


Oh, and another good news. I know a guy that uses testo or similar to testo gadget.. I might get measurements (when i am certain that the thing is dried and as all-in-all it should be!)

I look forward to replies...

[grizbach]

Klemen, I only know how my batch unit performs.
I have found with it attached to a bell, I need more primary air also. I believe this is from weak draft from cool exhaust and also the extra restriction of the bell itself.

It will perform much better when it is inside and you are starting out with room temperature and you have a more powerful chimney.

For comparison sake, my exhaust typically runs from 55-80C and it doesn’t condense in the pipe going to the chimney. I do get some drips running down the chimney though.

[peterberg]

Klemen,
That's a lot of data to wade through. All in all, I'm inclined to think your chimney is still too short and uninsulated. The stove isn't able to pull enough air in at the critical stage about 5 minutes after lighting. Almost all my diagrams do show a bump in the CO level around that moment. When that bump lead to a high peak of more than 4000 ppm CO, there will be smoke.

There are several ways to alleviate this. A better chimney is one way, a higher end temp another. In other words: the size of the second bell is depending on the quality of the chimney. As a rule of thumb the end temperature of the stove shouldn't stay below 60 C. (140 F.) during the complete burn.

About the topic of bell inner surface: the surface of a masonry bell compared to a steel bell was guessed as twice as much. That's to say, using a single bell. Utilizing a double bell is different, this struck me last night. I had to dig through the Rocket/bell project of some years ago to read my own findings about that subject.

The overall conclusion is like this: volume-wise, the single bell is less efficient than its double brother. To achieve the same efficiency the double variant could do with 25 to 30 % less volume. That's a quite important fact when considering a bell design.

Klemen, your stove could be exactly the right size "as is" when coupled to a real chimney. The one you describe of 12 meter (13.1 yard) long and 20 cm (8") diameter will surely create a more powerful draft. This will aid the combustion process inside this stove, because it's depending on a relative high gas velocity. At the same time the size of the bells will be better suited, the chimney stack should be warmed up a lot quicker.

[dvawolk]

Mar 17, 2013 2:17:00 GMT -8 peterberg said:

The overall conclusion is like this: volume-wise, the single bell is less efficient than its double brother. To achieve the same efficiency the double variant could do with 25 to 30 % less volume. That's a quite important fact when considering a bell design.

I was calculating bell sizing only on the inner surface and not on the volume.. I understand that two bells with the same summed inner surface (compared to one large bell) have much less of a volume.
But is there any difference if i have one bell with 7 sq. meters or two bells with summed inner surface of 7 sq. meters?
Also, one bell system can have the same inner surface and the same volume compared to two-bell system IF it is not plain block.

As far as i understood bell dimensioning:
the volume does not play a role as far as the CSA is 4 times or more larger compared to system CSA. Or am i missing something?

Grizbach:
It is nice to hear that it might/will perform significantly better at room temperatures/humidity + better chimney draft...

Now i should think about getting another few meters of chimney flue to test it...

Klemen