brett
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Post by brett on Jan 6, 2015 16:29:43 GMT -8
I am wondering if anyone has any experience with RMH (identical designs) at different altitudes. How does altitude (the reduced oxygen and air pressure compared to sea level) effect RMH temperatures and burn rate/heat output?
I ask because in fluid static of gases the pressure follows and exponential function and not linear relationship. The equation arising from the ideal gas law: (P/Patm) = exp(- M g y/ R T) where P is the pressure at your altitude Patm is pressure at sea level M the molecular weight of the gas g is the gravitational constant y is the altitude or distance above sea level R is the gas constant T is the absolute temperature
For example: my altitude 5000 ft sea level pressure 14.7 psi my altitude pressure 12.2 psi (values from atmospheric pressure table)
A 16.7% difference in air pressure (and associated oxygen).
Thus, if someone built identical RMHs at sea level and 5,000 ft would the higher altitude RMH have 16.7% lower temperatures and burn rate/heat output?
Secondary thought, when I used to hang with the performance car and motorcycle people they said they often got 20 to 30% less power from their gasoline engines at 5,000 ft when they had moved from sea level. (Confirmed by dyno tests)
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Post by shilo on Jan 6, 2015 19:23:25 GMT -8
I'm planning to check that with a portable rocket stove. at our place you needs to travel only 300 kilometer in order to pass from minus 400 meter to 2000 meter. I did not did this yet
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Post by chazvan on Jan 27, 2015 19:22:26 GMT -8
I am curious about this also. I am at 6500 feet. I have built a few experimental RS units with bricks of various sorts. Ultimately want a self feeding way to heat water.
I do a little bit of flying. It is necessary to change mixture with altitude/air density. Less dense means leaner carb setting. This means less fuel burn which means less power with altitude. You can compensate with a turbo/supercharger. The corollary would seem that less fuel would get burned for a given RS set of dimensions. If you increased the size for more airflow to get the same rate of fuel burn you would get the same heat output?
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Post by Donkey on Jan 29, 2015 8:10:18 GMT -8
No one really knows.. Those of you who live at altitude need to run the tests and report back!
It might be helpful if some folks, living at different altitudes worked together on this. Build identical test stoves, measure the output and compare.
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Post by photoman290 on Jan 29, 2015 13:51:05 GMT -8
sounds like a good experiment how would one go about measuring the output?
it be meaningful one would have to set some basic conditions. the thing that i think would be the most difficult would be finding a "standard" for the wood used, something that is going to be similar all over the area people are testing in. the only way i can see that working is for someone to be in charge of the wood supply and send out samples to everyone taking part in one area. moisture content is easy, once a standard charge for each RHM is decided. the wood can be weighed before the start. how should the output be measured? a simple rocket stove with a pot of water on the top sounds the easiest and has been done by other people. not sure how we could adapt that to a rhm?just a few thoughts to be going on with.
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Post by chazvan on Jan 29, 2015 16:01:43 GMT -8
No one really knows.. Those of you who live at altitude need to run the tests and report back! It might be helpful if some folks, living at different altitudes worked together on this. Build identical test stoves, measure the output and compare. Getting matching stoves and fuel loads might be a challenge. Measure the output how? Time versus temperature heating a given amount of water?
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brett
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Post by brett on Jan 31, 2015 6:19:56 GMT -8
I think the best way might be what Shilo proposed: to take a portable rocket stove from high elevation to low elevation with the same wood, same operator and try and match the conditions exactly.
I am going to be moving from the high desert 5,200 ft to the northeast at 800 ft. Hence, my question. Yet, I won't have a portable RMH or the same wood to perform the experiment.
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Deleted
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Post by Deleted on Jan 31, 2015 6:55:09 GMT -8
One needs an higher chimney or higher stack temperatures at high altitudes to be able to draw in the same amount of air by mass at a higher speed as with a shorter chimney on sea level.
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Post by chazvan on Feb 2, 2015 19:55:02 GMT -8
One needs an higher chimney or higher stack temperatures at high altitudes to be able to draw in the same amount of air by mass at a higher speed as with a shorter chimney on sea level. I suppose you are right about that but what is the reasoning or explanation? best regards, charles
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Post by ericvw on Feb 2, 2015 20:28:51 GMT -8
chazvan, Less oxygen at higher altitudes. Or is it more than this, @karl? HTTH, Eric VW
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Deleted
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Post by Deleted on Feb 3, 2015 4:50:39 GMT -8
There are always 21%. The composition of the air at sea or in the mountains is not different, the only difference is the pressure. To compensate for the lower pressure more air volume is required. To move the same mass at lower pressure more speed is needed. For the generation of the same stack force as at sea level more height or temperature are required.
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Deleted
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Post by Deleted on Feb 3, 2015 5:19:58 GMT -8
BTW
Molar mass (g/mol): oxygen ~ 32 nitrogen ~ 28
A cyclone separator could be used to catch oxygen at any level.
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Post by chazvan on Feb 3, 2015 16:13:41 GMT -8
There are always 21%. The composition of the air at sea or in the mountains is not different, the only difference is the pressure. To compensate for the lower pressure more air volume is required. To move the same mass at lower pressure more speed is needed. For the generation of the same stack force as at sea level more height or temperature are required. Thanks Kari. I certainly agree that more air volume is needed to get the same number of O2 molecules so you are right that you need more air velocity or a larger cross section. My head is not yet convinced about the last part as it seems to me that differential pressure is what matters and the difference rather than absolute would be the same independent of altitude??? With thinner air there should be less resistance to the molecules would naturally travel faster in thinner air??? I am not disagreeing, just my fuddled old head not yet fully understanding.
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Deleted
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Post by Deleted on Feb 4, 2015 5:00:18 GMT -8
Gravity works on mass, not anything else. All work forces in a stove are results of gravity and differences in mass per volume created by heat. For complete combustion an identical mass has to be moved at any level. To create an identical force at higher altitute the hight of the chimney or the mass volume has to be extended in a way that the mass difference in the vertical gas columns, on which gravity creates the work force, is the same.
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Post by chazvan on Feb 4, 2015 10:09:19 GMT -8
Thanks again Kari. What I take that to mean is that the mass per unit volume of space or density has to have enough difference to get the gases to flow from one place to another and yes, it would have to be the same mass of gases independent of altitude for a given amount of heat energy output.
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