Date: Tue, 3 Nov 1998 00:07:27 -0600
Reply-To: Blue Eyes <lvlearn@MCI2000.COM>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Blue Eyes <lvlearn@MCI2000.COM>
Organization: Vexation Computer
Subject: Re: Catalytic Heaters.
Content-type: text/plain; charset=us-ascii
Let me first say that thoughts of sleeping in a van with any
combustion process that isn't directly vented to the outside sounds
scary to me. But if the combustion process produces byproducts that
are so benign that they aren't going to affect me much differently
than having the carbon dioxide from several humans in the van with me,
and metered testing proved it to be that biologically benign, I guess
I could be convinced to try it.
But that's not the issue I'd like to address. Consider the heating
effect per pound of propane from a direct vented catalytic heater vs.
a non direct vented catalytic heater. They are both vented in the
sense that eventually the carbon dioxide they produce goes outside one
way or the other. But in the case of direct vented units like the
Platinum cat, those combustion byproducts take out the additional heat
value that a condensing furnace saves compared to the previous lower
efficiency generation of furnaces. By precipitating water from
combustion gasses inside the heat exchanger, condensing furnaces gain
the latent heat of vaporization. You are familiar with the caloric
load required to boil water into steam. That same phase change heat
of vaporization is gained by precipitating gaseous water into liquid
water. These are equal but opposite examples of the same heat of
vaporization exchange.
Now, if you feel the flue gas output of a typical precipitating
furnace, you will notice that it's about as warm as dish washing
water. That's cool enough to capture much of the available phase
change heat. In our non-direct vented catalytic heater case, all
that additional heat gain goes into the van which is the heat
exchanger, whereas the direct vented type wastes it. This could be
corrected if the direct vented model had a condenser added to it, but
that heat exchanger would be large, require corrosion resistant
materials, and be expensive. So, for pure heating value per pound of
fuel, I'm very confident that the non direct vented style is more
efficient.
That said, I fear I have bad news for those who would use any non
direct vented catalytic heater for many hours in their camper unless
they have a strange and new wonderful VW van under the face of the
sun. Specifically, to avoid incremental damage, I believe it would
have to be made from stainless steel. The big killer for early
condensing furnaces was "rust out" caused by nasty corrosive
condensates in the heat exchanger. The same is true for boilers, and
that limits boiler efficiency where, by design, they never let the
heat exchanger to get cool enough to lower the combustion gasses to
their dew point.
But in the case of non direct vented heaters, YOUR VAN IS THE HEAT
EXCHANGER! And the dew point certainly will be reached in may little
hidden places where you can't see it's damage. I predict that long
term use of this design will cause water formation from the dew point
condensation effect, and that will cause incremental rusting. An ugly
characteristic of rusting is that once it starts, the the rust is
deliquescent. So it pulls water out of the air which sustains more
rusting. The analogy to cancer isn't without merit. While the flue
gas condensate from burned natural gas is warm acidic mix, I don't
know what all would be in propane catalytic burner condensate. But
I'm not going to find out with my Westy.
Anyway, that's my opinion.
John
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