Re: [Phys-l] thermo question

[Brian Whatcott <betwys1@sbcglobal.net>]

At 08:56 AM 3/1/2008, you wrote:

> Consider a fixed amount of gas (moist air) in a container of fixed  volume.
> The container is in an environment of fixed temperature.   When 
> equilibrium is
> reached, what quantity or quantities will be constant  throughout the volume
> of gas?
>
> This question comes from my desire to understand from first  principles how
> to maintain a constant relative humidity in a fixed amount  of gas.
>
> Justin (absent from the list for a while)

We can suppose  that before equilibrium is reached, the following
quantities can vary: air speed in the container, temperature of the
 moist air, relative humidity of the moist air, amount of ice present (!),
pressure of the air mix, entropy of the air mix.
 We can consider the following quantities constant: air mass, water mass,
container mass.
To maintain a constant relative humidity greater than zero  in an air
mass, it is necessary to either:
hold the temperature of the air mass constant at the same value as
 the container
or
Add water with increasing airtemperature, and subtract water at
 decreasing temperature, paying special attention at the ambient
 freezing point.


Brian Whatcott    Altus OK    Eureka!