[FoRK] Magnesium / Water cycle hydrogen production

Stephen D. Williams sdw
Wed Oct 26 17:16:59 PDT 2005

Ken Meltsner wrote:
> ...
> Since you're starting with liquid water (except in the midwest in a
> few months...), you end up losing the heat of condensation (energy
> released when vapor -> liquid) as well.  This is not a trivial amount
> of energy -- it's the reason that high efficiency gas furnaces require
> a condenser to return the maximum amount of energy from combustion. 
> Numbers escape me -- it may be about 5%.
I have one of those and I don't think that it's the condensation that's 
a key to efficiency, it's much more a matter of not throwing away a 
temperature gradient.  Condensation is probably part of that, but it 
can't be a large part.  The condensation happens and the induced exhaust 
flow is needed because of the greatly lowered exhaust gas temperature.  
Traditional gas appliances counted on a high temperature differential to 
carry the gas (include CO) up the flue.  When the unit reclaims 95% of 
the heat, the resulting temperature differential means you can't use a 
flue anymore.

If you look at the internals of a high efficiency furnace, it has 
combustion tubes that start large and then continue, winding back and 
forth into the cold air stream, until they terminate in the inducer 
plenum.  The furthest reaches of the combustion tubes hit the coldest 
air first.  That air goes on to hit warmer and warmer parts of the 
combustion tube as it travels.  The inducer makes sure the exhaust gases 
are pumped outside and the exhaust tube is slanted so that condensation 
flows back to a trap at the inducer.  It seems that most of the 
condensation happens in the exhaust tube (a standard 4" thick wall PVC 
pipe) where heat recovery is probably marginal and definitely not part 
of the airflow.
> ...
> Ken Meltsner


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