[Stoves] Water vapor calc.

Crispin Pemberton-Pigott crispinpigott at gmail.com
Tue Dec 20 21:56:58 CST 2011


Dear Frank

 

Humidity is just a way of expressing the moisture content of the air.
Instead of expressing it in % (of what it could hold without condensing at
that temperature and pressure) one could express it as the mass of water per
cubic metre of air at that temperature and pressure. For example you could
say 14 g of water vapour per m3. 

 

Cooling that air causes the moisture to release its latent heat, warming the
air (opposing the cooling) causing it to cool more slowly. If the humidity
is really low, air will drop in temperature very rapidly at night.

 

You could calculate (in absolute terms) the quantum of energy in any
particular cubic metre of air if you know the temperature, pressure and mass
of water vapour in it.

 

Regards

Crispin

 

+++++++

 

Dear Stovers,

 

A few questions if you don't mind. 

 

 

Enthalpy of water is 104.73 kj/kg at 25 deg. C

We need to add 4.186 kj/kg/deg. C (4.186 X 75) 314 to equal 419 to reach 100
deg. C

We need to add (2675 - 419) =2256 kj/kg to get to a vapor. And have enthalpy
of 2675 kj/kg at 100 deg. C. 

 

But then the water vapor goes up to 400 deg. C in a stack. 

 

So we add 1.72 kj/kg / deg. C or 300 X 1.72 = 516. So  516 + 2675 = 3191
kj/kg in the water vapor at 400 deg C. (?)

 

When the vapor cools off it releases a lot of energy. Its interesting to me
that water vapor at 25 deg. C has so much more energy than water in a
liquid! 

 

How does humidity calculate into all this? 

 

Thanks

Frank

 

 

 

 

 

 

 

Frank Shields

42 Hangar Way

Watsonville,  CA  95076

(831) 724-5244 tel

(831) 724-3188 fax

frank at bioCharlab.com

 

 

 

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