jack vegas wrote:Matthew - I like your central wand. One reason is that it would allow the fan to blow air rather than suck. My sucking fan had to work in a humid vapor saturated environment which is not great for an electrical device. There should probably be a dam in the the middle of the wand however to divert all flow through the drum. Without it, air could preferentially just travel in one end and out the other, without much flow through the perforations. Need to think about how to handle the rubbing seal.
I am pleased, even if a little surprised, that you approve of this design. I do take your point about reversing the direction of air flow. But I don't immediately see why you couldn't reverse air flow in your original design...? And yes, that was one of my concerns, that air would follow the path of least resistance through my central wand and not out of it. Yet I did not immediately see such a poetically simple solution! I have updated my diagram to show reversed airflow and a central dam in the wand:
jack vegas wrote:A bigger hair dryer may not help much since its important not to overheat the bed beyond about 140 deg-F, otherwise it kills rather than nurtures the bacteria. I picked the one I did since it was powerful enough and also it was DC. Another option might be to use an infrared "reptile heater" bulb installed inside the box. These can be had for about $25 including a a remote thermostat.
I do not believe that an IR heating bulb, as used in reptile cages, would be a suitable alternative to a hair drier to pre-heat or re-heat the air inside the box. Those bulbs are 150W. The cheap AC hair driers I am advocating are up to 2kW. There is just no comparison in terms of power. Yes, I see that a heater so powerful it kills our bacteria is only counterproductive. But consider that I have already proposed breaking the "hair drier" part of the system into two components, an electric resistance coil and a blower fan, so that they could be placed on separate circuits and thus be controlled separately. These could be purchase separately or literally ripped from various hair driers, it doesn't matter. As a functional pair, I collectively call them "the hair drier." The "muzzle temp" of a hair drier is determined by three factors: the ambient temperature of the intake air, the wattage of the resistor coil, and the volume of air flowing over that coil. If we assume the ambient air temperature is constant, then it seems to me that we can achieve any outcome we want by tweaking the other two factors. Thus in practice I would take a powerful heater from a powerful AC hair drier and then combine it with a fan providing sufficient air flow until I'd lowered the muzzle temp to 140 degrees F. This
should allow for rapid heating of the box with maximum BTUs, yet with minimal concern about inadvertently overheating our bacteria.
jack vegas wrote:Since I was mostly experimenting, I never ran the thing more than 4-5 days without shutting it down. I cleaned it out every time, though I'm guessing it could easily have run a month before clean-out would be needed. I estimated heat by taking the total dry material added over a run and dividing by the hours that were run, estimating 8,000-9,000 Btu/lb of dry material.
How interesting that you chose to clean it out and start afresh so frequently. That means there has not actually yet been a test of the interior compost heater in operation over a prolonged duration with many consecutive daily refuelings. I wonder what might happen? I wonder how noticeably the output would decrease over time, as the volume of usable, compostable fuel inside the digester decreased? Which is to say, as the proportion of that volume comprised of spent "ash" increased?
I see now the logic you used to calculate the thermal output. If the potential thermal
energy embodied in 1 lb of fuel = X BTUs, and the heater is observed to consume 1 lb/hour, then we say that it outputs X BTUs/hr. I follow the math, but I am doubtful of the results. Or I should say, I think they should be considered a very rough estimate that I worry is optimistically high.
First of all, I look
online and I see the same estimates of 8-9K BTU/lb for the potential thermal energy from
firewood (see
here). But then I read that there is good reason to reduce that estimate to just 8K at most (see
here). Then I realize that they are talking about kiln-dried
wood, whereas we should assume at least 20% moisture content for air-dried wood. So reduce again to no more than 6.7K. Then I consider that they are assuming we will burn the wood, which we will not. Neither combustion nor composting is a 100% efficient way to convert the chemical energy in the wood to BTUs. But which conversion process is more efficient, combustion or bacterial digestion? Can we even guess? I know that, when composting, a portion of the wood's chemical energy is converted into heat, but then another portion goes to support the bacteria's metabolisms, and then another portion remains embodied in the residual compost, and finally another portion remains embodied in the
CO2, methane, ammonia, and whatever other exhaust gases result from the decomposition. I don't know how big each portion is, nor how that compares to the "portioning" of energy during combustion, nor even how the starting estimate of the wood's thermal energy does or does not take some of these concepts into account. In other words, when they say "firewood has X thermal energy potential per pound," are they already factoring in estimates for the inefficiencies of the combustion process?
Plus there are likely other factors in the process, and other assumptions baked into the reference data we are using, that I wouldn't even begin to guess at. My point is simply that, while we don't know a precise figure for the average thermal output from our compost heater, I think we can safely say that it is actually considerably less than 8-9K BTUs/lb of wood fuel.
jack vegas wrote:I finally broke down and calculated heat soak through the barrel. Can't believe I never did before. Perhaps I did and promptly forgot about it. Anyway, a 55 gallon drum has about 21.6 sqft of surface area. The maximum surface temperature I can imagine is about 140 deg-F and at that point the steel can only radiate about 145 Btu/hr-sqft. So without convection, pure radiation can only dump about 3,130 Btu/hr. Convection with air (or water) is needed to draw more heat out of the system. This is both good and bad. The good news is an un-insulated drum could reach full operating temperature so insulation isn't needed. On the other hand, a fan is needed to be to push enough air over the drum to draw heat away through convection. Both to prevent over-heating and to draw more heat out of the system. It seems my insulated box served as a duct to channel that air. I may have been operating very close to the limits of my system and not have known it! So a good design would probably use the hair dryer to start the system and to protect against sudden cooling and the "box" should be configured to provide a good air cooling jacket to maximize convective heat transfer. Just about any material could be used for this air cooling jacket since it doesn't need to provide insulation. Cardboard and duct tape might be used to build a well formed cooling jacket and a relatively quiet and inexpensive box fan might be used as the convective air source!
Very interesting. This could be another argument for a potential vertical drum design, since the one you were considering wasn't insulated, and since now you calculate that insulation may not be necessary even for your horizontal drum design. In this regard, I remind about Hank's suggestion to replace the closed lid of the vertical barrel with a screen lid. Needless to say, this is a possibility only because the barrel is vertical. If, as your new numbers suggest, we might find ourselves bumping against the limited capacity of a rip-roaring compost heater - say, a vertical barrel design 80% full of compost, instead of only 40% - to shed adequate heat via conduction through the steel barrel walls, then using a screen top would be perfect. Hot exhaust gases would just naturally, continually rise up and out of the unit.
I note that I will choose for now to take on faith your assertions that we need not be concerned with odors emanating from our heater. I have
enough experience with healthy, well-balanced compost to place at least 80% confidence in your assertion ; )
Other advantages of a vertical drum design include 1) increased usable capacity, as I alluded above; 2) ease of fueling through a removable top lid; 3) no need to penetrate the steel barrel circumference at any point, since all sensor wires, power wires, and air flow channels could enter/exit through the top opening; and 4) perhaps less mechanical complexity. #4 assumes we could perfect an aeration system that precludes the need to ever mix or turn the compost. Most likely, it would be a forced-air system. The work already put into Johnson-Su bioreactors strongly suggests that we could indeed perfect such a system. Are there other "pros" that I missed? What might any "cons" be?
I have taken what I think are the best elements so far from Jack's, Hank's, and my own input and reworked the diagram for a vertical barrel compost digester. I believe this is the most promising concept we yet have going. I left in here the optional notion of insulating the barrel:
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