Peter van den Berg

The Testo use white filters for the sample intake, that's correct. But those won't catch all PM in the volatiles, further down I have other filters and those are catching more of the, admitted sparingly,  dust and soot.
Moreover, I use these filters at least four times in development stage. Otherwise, it would be a costly affair, the number of test runs are easily into the hundreds for each core development.

Jeremy VanGelder wrote:I believe that the Testo meters measure PM 2.5 and that there are a number of graphs from different RMH burns that show ultra-low PM 2.5.
EDIT: I might be wrong about the Testo Meters. Attached is one of Peter van den Burg's testo charts.

You are right, my Testo doesn't measure PM 2.5 nor PM 10.
But there are some things that can be said about all particulate matter: coming from a "normal" woodstove most of it is organic, so it's soot. In other words, better combustion will produce less PM. There's a second effect: in a bell construction, after combustion is done, the forward velocity of the gases are slowed down a great deal. Just because of this, the fine dust will settle at the bottom of the bell quite easily. Most of the fine dust in a well-behaving mass heater do consist of the minerals that the tree took up from the soil. This is anorganic and mostly harmless.

In my own batch box rocket heater, the bell below the core is 23 times wider than the chimney cross section area. As the result, a layer of very fine dust has been accumulating at the bottom over the years. Last year the heater seemed to be not so enthusiastic anymore, the lower ridge of the exhaust opening was reached. I managed to scoop and vacuum most of it out and after that was done it behaved as its old self again.

So, in order to substantiate the above statements, here are two test reports from two very different batch box rockets, both housed inside a bell.





The first one being a DSR2, the second a straight batchrocket with altered air inlet.

As far as I know, the EU norm is at the moment on 40 mg/m³ at 13% O².

Randy Butler wrote:
System riser is IFB, octagonal (close to 6" diameter) and 54" high.  I did not add the wedge at the base, yet.

How close to 6"? That 54" is far too high, 44" is sufficient, higher doesn't add anything.

I would suggest that you build a very simple core, bricks on edge, firebox and simple square riser. It won't produce a good roar because of the IFB though. Use some clay and fine sand to mortar the bricks together, this design is very susceptible to air leakage. You'll get a correct idea what the thing is capable of this way.

The core will produce an audible roar when all of it is hard firebrick. The soft ISB of the riser is damping the sound. I've done an open system a couple of years ago, hard firebricks all over, brick bell over it as well. It wasn't a large house and one could hear the roar everywhere, including the loo.

In your case, depth of the firebox could be 21" to 22" maximum. Riser 6" square, no need for a wedge at the base. When you built the riser as not 6" square because of the brick size, that's a mistake. In the picture the riser looks as wide as the firebox, that won't work either.
As long as the firebox is open, leave out the secondary channel altogether. Assuming everything is to specs, it should work. The secondary channel is way off specs, you should adhere to the designs that work, not just something you have lying around. Tell me what size of bricks you have there, possibly you'll need an adjusted base number to get things right.

The slow oscilating sound like a steam locomotive is pointing to a restriction somewhere in the system, showing up especially while the thing is running full bore. Did you say you had an end port at the top of the riser? Best to widen that again, this end port should be directly between the expansion chamber and the riser, in my opinion. Making an orifice at the end the riser has other, mostly negative implications.

Dave Rose wrote:Ok, next test will be 200mm height, with a little stumbler attached to the top of the final port I think.

The shape and location of the first port is as important as it's size. So, if you've got good results, don't change the shape and location, otherwise you are back to square one.

Dave Rose wrote:I’ve run a few more tests with smaller heights, but all seem to go into over fuel mode. The last one was 150mm and it seemed to oscillate between clean and over fuelling every couple of seconds, was interesting to watch. I concluded it was almost able to burn everything in the secondary chamber, but would periodically spew out the excess, resulting in puffs of smoke and or flames coming out the riser. So far then, the best burn by far has been at your suggested 200mm height, despite attempts to prove otherwise! I also tinkered with a Fibonacci, but at 100mm height - it made a nice vortex, but the burn wasn’t great, in line with the other at this height. I might test again if I find a solid configuration as I like how it looks, but it’s interesting to hear your experience.

It might be that with a greater chamber height the results are better across the board, whatever turbulencer (turbulator?) you choose. No guarantees, though.

Dave Rose wrote:Have you published any final dimensions for the Shorty? I searched your main thread and a couple of others, I did find reference to a sketchup file but couldn’t find it or the dimensions anywhere. I’d be interested to compare the dimensions of the different chambers and port sizes to my current configuration, so far I’ve been referencing the DSR3 only.

Hmmm... could you sent me the url of the page where the reference is on? I'd like to check what might be wrong.
Dimensions, here we go:
The proportions are done with the figure "B", 72.34% of the chimney's diameter, in line with the normal batchrocket.
Width of the main firebox: 2B,
height of the same: 3B,
depth of the same: 4B. This last one isn't critical, could be 25% larger.
Port size: height 2.1B width 0.5B, depth about 1B. See the following how to achieve that.
Riser depth and width: 2B, height: 5.5B.
Inside the riser box there's a liner, to reduce the sides of the square to equal of the chimney's diameter. This liner is the same left and right, and double or as much as required in order to achieve this goal, on the wall where the port is in. Height of the liner: a bit (30 mm or thereabouts) higher than the port, in such a way there's a small lintel over it. No liner at the back, at all.
Top of the riser is closed, exhaust opening in the wall facing the core's front. The opening is as wide as the riser box, height calculated so that the opening is exactly the same as the cross section area of the chimney. Placement of this opening: a piece of wall above it, equal to the opening's height.
This is about it, there are strong signals that the top gap, above the riser box, could be equal to zero if desired. This has been done with a cook stove in July. The ceramic glass cooking surface was resting directly on the riser box, effectively it was functioning as the lid. Quite spectacular to see it working like that, knowing there was nothing on the riser top but glass! We even tried it on a bare core first, with a piece of clear ceramic glass on top.

Dave Rose wrote:Also, I wonder if you have any thoughts on the effect of the riser after the secondary combustion chamber, so essentially a third chamber? Do you think it could potentially be quite small or even not there at all, given most of the combustion should be happening in the secondary chamber? For my implementation, the only negative of having it I can think of is it will take up space in the bell, so potentially reduce time the gasses have hang around and transfer their heat. I can of course just test this, but let me know if you have any hunches on this. And apologies for all the questions, you just get a lot of these!

Lots of questions, that's true. Much more so during the colder seasons.
As I've proven that the riser could be very short, the riser in your construction might not need to be high as well. What the actual cutoff value would be, the fine line between long enough and too short, I am unable to tell you that.

Dave Rose wrote:Actually I have a question - when reloading the Shorty or DSR3, do you typically expect some smoke when refuelling?

The DSR3 need to be down to coals before refuelling. Shove all the coals to the back and reload with one end of the fresh fuel on the coals. The Shorty have been pestered with refuelling on purpose, but provided it's hot all over, it didn't emit even a whiff of smoke, of whatever colour.

Dave Rose wrote:Oh and one more thing, I could potentially try a fibonacci spiral instead of a horseshoe. So instead of splitting the gases in two before recombining again, I could send them all the same direction and into a single vortex into the riser. The benefit (I think) would be a longer path for the gas, the drawback it would also be narrower, so may have to increase the height more, which wouldn't be ideal. But if you have any hunches on horseshoe vs fibonacci, please share!

A long time ago, I tried a fibonacci spiral in order to compare it to a simple j-tube. The spiral tended to be dirtier then the j-tube. Under only one condition it was better than the j-tube, with very high winds, more like a gale.
In short: the spiral needed much more velocity in order to burn clean so I dropped that one.

<grin>Thanks for the compliment. After 40 years of playing with fire I am getting the hang of guessing what's going on.<grin off>

I've sort of seeing this coming.
Now you reduced the height of the expansion chamber the gases aren't being able to expand sufficiently anymore. The function of this chamber and both the ports before and after is (hopefully) to provide a kind of damper to the combustion rate. The first port is providing lots of gas velocity, in the chamber the gases are allowed to expand which next need to go through a restricted end port. The net result should be something that sports almost complete combustion and a limit to the combustion rate. Both my DSR3 and Shorty designs are providing this, the latter being the simplest to built.

What I would do in your place: make the first port slightly larger, say 55% to begin with and the end port slightly smaller, like 95%. Less velocity and a restriction that kicks in earlier. OR: you could opt for skipping all development work altogether and go for a 150 mm Shorty core. This one is quite a bit lower compared to what you have now and it's already optimized. Plus the design is free of charge, only subject to the Creative Commons Attribution-ShareAlike 4.0 International license. It's entirely up to you.

Sorry to say, but I'm inclined to think you aren't on the right path. The round opening between the firebox and the secondary chamber is far too small in my opinion. None of my designs has a port that's smaller than 50% of chimney cross section area, 70% is much more common. I did some calculation: your 38 mm diameter hole is 6.4% of system csa. In order to rise that to at least 50%, it should be somewhere around 106 mm diameter. As it is now, gas speed is far too slow, turbulence is lazy and the really hot afterburner flame won't occur.

The volume of the secondary chamber should be enough to accommodate rapid expansion and lots of turbulence. To my eyes, it looks far too cramped now. What you could do is rising the secondary chamber's height to 200 mm again. The horseshoe height to 100 mm or thereabouts and have the riser resting on the horseshoe. Or, with a full height horseshoe, the top half as a complete square. By doing that, the hot gases have plenty of space and need to go down in order to get into the riser.

The firebox could be defined as high as wide, say, 250 mm, being not critical.
Signals to keep in mind:
There should be an audible roar, then you'll know there's lots of turbulence.
Assuming the whole of the thing is completely dry, smoke should disappear completely within 10 minutes, preferably in 5.
When the above points are met with an open system, all air inlet should be in the top half of the fiebox, 30% of system csa in total is enough.
If and when the thing is running like it should, the afterburner flames could rise out of the riser because there's no back pressure of a bell at present.

Of course lots of different configurations are possible, although in my experience just one or two will turn out being the best performers.

All the above recommendations are based on educated guessing. Whether or not you are implementing those is entirely up to you.

For one, all and every wood species has the same combustion value by weight. Within some percentages, that is. Surprisingly enough, conifirous softwoods are slightly better than oak, for example, because of the resin in them.
And yes, big logs burn slower than small branches but that's not an advantage per se. All these (batch)rocket heaters are burning quite fast anyway, whatever the fuel is they are fed with. Which means that the temperature difference between the hot gases and the masonry mass is huge. Due to that effect, the heat absorption by the surrounding masonry is very high as well.
Also, the dryer the fuel, the less water need to be evaporated, which in turn is better for overall efficiency.

For Burra: every kilogram of reasonable (>15% moist) dry wood of whatever species, burned completely, will generate roughly 4 kWh of energy. Chimney losses and so on are already deducted, so this is what stays in the house. Provided the chimney temperature is reasonably low, of course.
Talking to EU citizens about fuel consumption is best done in the above terms, in my humble opinion.

One kilogram is equal to 2.2 lbs, 4 kWh is equal to 13,600 BTU's. So, every pound of whatever woody fuel could potentially generate 6,181.8 BTU's of energy.