Hi, I have an EKO40Supper that has a blower port that is about 1 3/4" x 2" (that is if memory serves me but that is hard to tell as I am often forced to look for it). The air supplied supports a variable primary combustion nozzle with 8 sq in area and two variable secondary combustion tubes that I haven't looked at in a long time. Regardless of the size of the nozzle and the combustion tubes the heat exchanger has 8 tubes with a total area near 25.5 inches and the exhaust (chimney) @ 6" has 28+ inches and an 8" is just over 50" of area. Since there is so much diversity in actual design among models and manufacturers how would one know if the model they are looking at is truly designed in a scientific/mathematical manner that would yield optimum performance? Where does one find the actual equations for mass/energy displacement for comparison? Then in an offhand sense...since water-counterflow is best for water/air handlers why is gasifier exhaust not constructed in a counter-airflow manner to elicit peak performance (that is where water is heated in a manner where the flow directs it to the coolest [exhaust] part of the heat exchanger first and the hottest part last)? With the prices of some of these things (gasifiers that is) is it for the sake of expediency, safety or the dollars and sense (pun intended) side that we are sold a decent product but are not sold the most scientically accurate design for our money?
Displacement questions for gasifier project???
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I had thought about this also. The only thing I can think of is that the natural flow of heated water would be to rise. Yes you can force it the opposite way but then if you had to gravity flow the boiler in an emergency you would have a tough time going against any installed check valves.
my guess is that it doesn't make that big a difference. Wood varies so much that the design is pretty imprecise, maybe the controls are more important to functioning? Counterflow also doesn't make much difference, the difference in water temp is usually something like 150 in and 180 out, both are so much cooler than the 2000-300 degree flue gas that it just doesn't matter.
One of the German boilers, KOB, does counter flow (with the exhaust gas coming out the bottom) so the water can still thermosipon. I believe Viessmann bought KOB. We may be seeing some of them over here soon according to rumor.
I suspect that one of the critical relationships is between at least these four values:
1) The effective pyrolizing wood surface area (which depends on primary air inlet location, primary chamber volume, and primary chamber cross-sectional area)
2) The nozzle cross-sectional area
3) Total airflow volume
4) Primary / secondary airflow ratio
There are doubtless many more that I'm blissfully ignorant of. By working backwards from the chemical constituents of wood and making some assumptions about the combustion sequence, you could calculate the total airflow required for a desired BTU output. Rule of thumb is 1.6 x stoichiometric, I think. Even this calculation is complex and still requires a lot of assumptions. It doesn't tell you the primary/secondary ratio.
I think you're pretty much on your own when it comes to calculating how much wood surface area you need to generate the desired amount of wood gas. I suspect that different primary chamber designs and materials might have a significant influence on wood gas generation.
Finally, there's a lot of variation in fuel. Different species pyrolize at different rates. Moisture content can vary. The volume/surface area ratio can also vary quite a bit. A good design has to aim for some sweet spot in the middle of this multidimensional jungle, and allow the user some ability to compensate (or include closed-loop lambda controls).
My advice - allow LOTS of adjustments. I'm assuming you don't want to build and test a series of prototypes to arrive at a final design.
1) The effective pyrolizing wood surface area (which depends on primary air inlet location, primary chamber volume, and primary chamber cross-sectional area)
2) The nozzle cross-sectional area
3) Total airflow volume
4) Primary / secondary airflow ratio
There are doubtless many more that I'm blissfully ignorant of. By working backwards from the chemical constituents of wood and making some assumptions about the combustion sequence, you could calculate the total airflow required for a desired BTU output. Rule of thumb is 1.6 x stoichiometric, I think. Even this calculation is complex and still requires a lot of assumptions. It doesn't tell you the primary/secondary ratio.
I think you're pretty much on your own when it comes to calculating how much wood surface area you need to generate the desired amount of wood gas. I suspect that different primary chamber designs and materials might have a significant influence on wood gas generation.
Finally, there's a lot of variation in fuel. Different species pyrolize at different rates. Moisture content can vary. The volume/surface area ratio can also vary quite a bit. A good design has to aim for some sweet spot in the middle of this multidimensional jungle, and allow the user some ability to compensate (or include closed-loop lambda controls).
My advice - allow LOTS of adjustments. I'm assuming you don't want to build and test a series of prototypes to arrive at a final design.
DaveBP said:
One of those would be sweet to do a thorough going over. didn't know about them.. Thanks Dave!
"My advice - allow LOTS of adjustments. I’m assuming you don’t want to build and test a series of prototypes to arrive at a final design." You are right about that nofossil! The virtue of patience and the allotments of time and cash seldom come together in quantities large enough to allow our inspirations to become part of the hardline world of practicallity. Especially whe we intend to be the immediate beneficiary of our labors. It's a bit of an amitious project that includes being able to swing from heat production full time to electrical power production (with heat production as a by product) suffiecient enough to allow going off grid. Thanks for your input.
This type of analysis is routinely done in the aerospace industry; engine combustion chambers, rocket engine flow fields, turbine engine cooling problems, avionics cooling simulations. The simulation tool is called Computational Fluid Dynamics (CFD) and if all the "boundary conditions" which means materials, surface conditions, and gas properties are all well defined so the problem can be modeled very accurately. Problem here, unlike in the aerospace field, is the time and effort to develop a $150-250K simulation will promptly go down the toilet as all the boundary and surface conditions go to crap with ash, dirt, rust, and all the other nasty stuff I see accumulate in my boiler. Sometimes you go fancy, sometimes brute force. This would be a job for brute force. Known quantity of energy in, keep tweaking while measuring all the energy out (circ and exhaust). Do over and over until you're out of money or time or both. Cheers!
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