"steam" from chimney.

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Edwin Pawlowski wrote:

His boiler isn't condensing any moisture. The mass of water vapor expelled isn't the issue. Temperature and absolute humidity are of primary importance.
Richard Perry
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True in absolute terms but not comparative. He wants to know why one type of heater makes more visible condensing vapor than the other. Assuming they are burning about equal amounts of fuel, under equal weather conditions, why does one seem to be more visible than the other type? You did not answer that but I'm sure you will. .
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Edwin Pawlowski wrote:

Suppose that one system cools the exhaust gases just to the point of condensing moisture in the vent pipe but without actually condensing any. This mixture will be at 100% RH. For the sake of argument alone let's set the temperature of the outlet at 120F. Now suppose the other system only cools the exhaust gas down to 150F at the outlet. The RH of the exhausted gas in this system will thus be less than 100%. We are assuming that both systems have burned gas at exactly equal rates. Both will have produced the same quantity of water vapor as byproduct, and the absolute humidity of the gasses in both vent pipes will thus be the same (neglecting differences in density due to temperature differences). The dewpoint will thus also be the same for the outlet gasses of both. The difference between these systems is that the first system's gasses are already *at* dewpoint, while the second system's gasses must drop 30F before getting to dewpoint. IOW, if the gasses of these system were to both drop 5 in temp in the first few inches of travel after leaving their respective pipes, then the first will be condensing moisture and the second will still have 25 more degrees to drop before it could condense its moisture. If the gasses from the second system travel sufficiently far to drop to 120F, where it could begin producing fog, then it will instead have mixed with the ambient air causing its absolute humidity to drop, which in turn reduces the dewpoint required to produce fog. And if it mixes sufficiently with the ambient air as its temp drops, then the required dewpoint may be pushed to well below the ambient temp, thus preventing any possibility of producing fog.
The biggest fog producer of all systems will be the first system described above. If you drop the outlet temp of the vent below the saturated temp described for it, then condensation will be occurring in the unit and/or vent pipe. The loss of moisture from the gasses will cause the absolute humidity of the output to be lowered. It will still be at dewpoint as it leaves, as in system1, but it will also be closer in temp to the ambient temp and won't therefore cool as quickly. Thus for maximum fog adjust the outlet temp such that the RH is at 100% but with no condensation actually occurring within the system. Outdoor ambient conditions also play a big part in fog formation from flue gasses.
Richard Perry
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That is what I was trying to say, except in simpler terms. Your descriptions is much more accurate and finally should settle the OP's question. thank you.
--
Ed
http://pages.cthome.net/edhome/



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