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Post by Donkey on Jan 8, 2007 15:00:49 GMT -8
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Post by Donkey on Jan 9, 2007 12:36:20 GMT -8
I did a bit of math last night.. Taking the formula on the link provided earlier, I plugged in some numbers. The results are in the graph below which I think is somewhat interesting. First, the assumed conditions: Six inch heat riser (sized for six inch pipe) at .192 square feet. The heat riser is 2 1/2 feet tall. The exterior temperature is 68 degrees F. The discharge coefficient is at 0.65 in all cases. I fed the formula internal (stack) temperatures from between 400 deg. F to 3000 deg. F in 200 deg. intervals. Which gave me 14 seperate numbers in flow rate of the gasses through the stove in cubic feet/second. All numbers here are approximate, using no more than 3 decimal points in ALL calculations. (easier on my poor skull and my dinky calculator) (Now the graph) What I find interesting is that the flow rate curve flattens out somewhat at around 1200 deg.. This tells me that from about the point where wood begins to burn (around 400 plus) to 1200 degrees there will be a marked improvement in flow rate throughout the stove. However, from around 1200 deg. and above, the change(s) in flow rate won't be as noticable.. Fiddling with the numbers further, i've found that a way to improve flow rate (given the same dimentions/conditions) is to improve the discharge coefficient, that is the efficiency of discharge from the stove.. So, now off I go. Thumbing through physics texts to find how to effect discharge conditions...
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Post by Donkey on Jan 9, 2007 13:31:01 GMT -8
Um... Yeah.. To be specific, the formula I used was: where: Q = stack effect draft/draught flow rate, ft³/s A = area, ft² C = discharge coefficient (usually taken to be from 0.65 to 0.70) g = gravitational acceleration, 32.17 ft/s² h = height or distance, ft T i = average inside temperature, °R T o = outside air temperature, °R
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Post by Donkey on Jan 10, 2007 17:20:18 GMT -8
Well, all this is well ang good.. Some kind of comparison(s) can be made with it. Though this all assumes open-ended heat riser pipes etc. Really, I need some more concrete data on real temperatures and all that.. I should be getting a ceramicists thermostat soon from a friend. I'll post actual temps. when I have 'em.
Later.
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Post by Donkey on Jan 11, 2007 22:01:39 GMT -8
I forgot to say above that the graph objects were all scaled up 200% as a visual aide. I've got here a new graph, or actually 2 side by side. The original graph (this time to scale) run across the front. Again, six inch pipe, 2 1/2 feet tall, outside temp 68 deg. F, values run from 200 deg. (added one) to 3000 deg. F. The row in the back is for a 24 foot pipe, (imagine a stovepipe out the top of the house) all other parameters being equal. I ran the 24 foot pipe internal temps from 200 to 1200. 'Course in the real world I hope you never see a 1200 deg. F stovepipe!! Obviously, pipe height (probably also volume) has more to do with throughput than anything else. Again, this doesn't really reflect on any real world situations.. Standard stoves are usually stopped down and almost never run air through them at full.. Rocket Stoves (for heating) have barrels over them and long runs of pipe through thermal masses.. Reality will most likely show it's own picture. Still, it appears to be a somewhat informative exercise.
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Post by Donkey on Jan 19, 2007 19:09:56 GMT -8
Ok.. I just got gifted a used potters pyrometer. It's old and may be off a few degrees (says my friend). It maxes out at 2500 degrees F. Hopefully in a couple days I can show some temperature info from my test bed here..
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Post by Donkey on Jan 23, 2007 9:02:09 GMT -8
Ok.. Did a test burn with the pyrometer yesterday. Everything is still a bit damp out there from rains and whatnot. Using the test stove I made in the bucket forms post (in experiments area here). After starting the stove and things warmed up a little the stove hovered at around 1300 degrees F. I didn't measure the ambient (outside) temp so I'm not prepared to do any real math around it, but just as a guess I'd say it was around 70 deg. F.. Fairly close (and loosely) to the 1200 deg. numbers on my graph above. About one and a third cubic feet of air per sec. moved through the stove and was presumably processed fully in the burn.
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Post by DCish on Feb 21, 2014 3:59:05 GMT -8
Very nice illustration. I have a tall chimney, and it is quite reassuring to see numerically how big the effect of that is. It opens up design possibilities (e.g., the "broken riser" concept) a great deal.
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Post by pinhead on Feb 21, 2014 6:37:37 GMT -8
I would think in order to really get an accurate CFM calculation you would have to assume a perfectly insulated chimney and calculate the ft3/s at that point, then reduce the volume of that calculation according to temperature (1200°F in the riser vs 150°F in the chimney).
This still would be inaccurate due to flow restrictions between the feed and the chimney (including the restriction created by the wood, itself).
Like you said, though, still a very interesting exercise.
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Mar 2, 2014 5:33:59 GMT -8
Assuming the system is sealed without any leaks before the final flue (chimney) a flow hood could be situated there at full burn and the actual flow could be obtained there. The flow in M/s or Ft/s could be used to determine volume at that point yielding pretty valuable information I would think. Flow hoods are quite expensive. Most good medium to large sized HVAC service companies have one though and may come out for a fee and be able to take the reading. *Just a thought*
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Post by Donkey on Mar 3, 2014 13:40:58 GMT -8
Remember.. These are theoretical models, based on completely open chimneys, no benches or really anything.. You can imagine them as open chimneys with electric heaters at the bottom providing heat. Not reflective of real life but useful for visualization purposes.
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Mar 4, 2014 16:43:04 GMT -8
Remember.. These are theoretical models, based on completely open chimneys, no benches or really anything.. You can imagine them as open chimneys with electric heaters at the bottom providing heat. Not reflective of real life but useful for visualization purposes. True but individual heaters still could have actual data regarding air volume at the stack aiding in calculations of other values. An actual value in my most humble opinion might be better than an assumed one for the sake of calculating other values within the system. Like I said, just a thought.
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Post by Donkey on Mar 7, 2014 21:10:18 GMT -8
Agreed!
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