Naked Science Forum
General Science => General Science => Topic started by: Atomic-S on 02/12/2006 04:03:30
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I got to wondering about the maximum heat obtainable from a gas burner, and it occurred to me that this may have something to do with ambient atmospheric pressure. Do the maximum BTUs/hr obtainable from a given gas burner under a given pot, vary with altitude?
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Apparently the visocisity of a gas isn't a strong function of pressure, but the density obviously is, so as you reduce the pressure (If the gas is being supplied at 1bar above atmospheric) the rate at which the gas is coming out will be reducing. Also the oxygen density will be going down so even if you had the same volume of gas coming out the flame would have to be more spread out in order to get at the oxygen. This will not affect the total power (as the same amount of gas is being burnt) but it may affect the efficiency of the transfer of heat to your pot.
If you were using butane however (and it is the same temperature as at ground level) you may have an advantage, as the pressure in the cylinder will be the same, but the pressure outside has reduced, and as Butane boils at close to 0C the pressure is going to be low so this could be significant.
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Well, the pressure of flammable gas in a source is typically much greater than atmospheric, and has to be regulated down, on the basis of which we could assume, I think, that we can obtain any flow rate of fuel gas that we would reasonably use, so that is not an issue. Let us suppose, then, that we arrange for some fixed number of moles of gas per second to pass into the burner, at various pressures of the surrounding atmosphere. Since the number of moles of fuel per second is in every case the same, if it is fully burned, the the BTUs per second generated will be the same in all cases. However it would seem to me that what will not be the same is the concentration of the heat, that is, the BTUs per second per cubic centimeter. Under lower atmospheric pressures, the flame would spread out over a larger volume and be larger, meaning that there would be problems heating a small pot as fast at high altitude, as you could at sea level.
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I got to wondering about the maximum heat obtainable from a gas burner, and it occurred to me that this may have something to do with ambient atmospheric pressure. Do the maximum BTUs/hr obtainable from a given gas burner under a given pot, vary with altitude?
I think that the pressure will make a difference. The stove will be designed to mix air and fuel at a particular rate. A lower pressure will reduce the ratio of oxygen getting into the pre-ignition fuel/air mix. I think that this will effect the heat output. Oxygen deficient (lean) burns might tend to produce incomplete combustion. For example producing CO rather than CO2.
The limits of flamability(mixed with air) for the gases normaly used are quite narrow.
Butane 1.8% to 8.4%
Gasoline 1.4 to 7.6
Propane 2.1 to 10.1
Quick disclaimer: I am no authority.
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I believe that aeroplanes alter their fuel / air mix to compensate for the reduced oxygen at altitude. Since the total amount of oxygen drops the higher you go (because the density of the air falls with altitude), you have to reduce the amount of fuel entering the engine to preserve the correct fuel / air ratio.
Obviously burning less fuel produces less energy. But, at altitude, the air is less dense so the drag (air resistance) is reduced and hence the plane flies without problem.
Chris