[Stoves] Water vapor calc.

[Frank Shields]

Dear Kevin,

 

Just making sure the 1.72 kj/kg / deg c value is correct for water vapor. I
think I am getting the rest. 

 

And we have all the different units! : )

 

Frank

 

Frank Shields

42 Hangar Way

Watsonville,  CA  95076

(831) 724-5244 tel

(831) 724-3188 fax

frank at bioCharlab.com

 

 

 

 

 

 

From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Kevin
Sent: Wednesday, December 21, 2011 5:41 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Water vapor calc.

 

Dear Tom and Frank

----- Original Message ----- 

From: Tom Miles <mailto:tmiles at trmiles.com>  

To: 'Discussion of biomass cooking stoves'
<mailto:stoves at lists.bioenergylists.org>  

Sent: Tuesday, December 20, 2011 8:54 PM

Subject: [Stoves] Water vapor calc.

 

Dear Stovers,

 

A few questions if you don't mind. 

 

 

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

 

# Enthalpy is simply the total energy content of the fluid with 0 C as the
reference. (Or 32 degrees F) The Joule units mask the simplicity: the
reality is that 104.73 kJ/kG = 25,015 calories/kG, which simplifies to 1
calorie per gram per degree C , over 25 degrees C.

This is the "Sensible Heat" of liquid water, also equal to 1 BTU per pound
per degree F

 

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

 

# 4.186 Joules = 1 Calorie. 75 degree rise requires 75 calories per gram.

 

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. 

 

# This is the "Heat of Vaporization of Water", or the "Latent Heat"
associated with the change in phase from liquid to vapor. 

 

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. (?)

 

# This is the "Sensible Heat of Water Vapor". 

 

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! 

 

# Yes. The Vapor at 25 C contains the "Heat of Vaporization" of the water.

 

How does humidity calculate into all this? 

 

# Humidity generally refers to the amount of water vapor in the air. When
water vapor is added to 1 pound of dry air, there comes a point where it
cannot hold any more without condensing. The "Relative Humidity", RH is then
100%. If teh air only had 75% of teh amount of water it could hold, then the
RH would be 75%.

 

Hope this helps.

 

Best wishes,

 

Kevin 

 

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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