Edwin Pawlowski wrote:
"RP" wrote in message

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


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. .

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 120ºF.
Now suppose the other system only cools the exhaust gas down to 150ºF 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
30ºF before getting to dewpoint. IOW, if the gasses of these system
were to both drop 5º 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 120ºF, 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