Thread: Basic thermodynamics: please help

Hi everybody, hope someone can help me with this problem.

I'm a chemical engineer and I feel a bit ashamed to present you the following problem. It is about the saturated water vapor pressure of water. We all know it is dependant on ambient conditions like temperature, and described by the Antoine relation. My question is what happens with the saturated water vapor pressure (or % RH) of water when you decrease the ambient pressure?

Example: we have a closed system (say a piston of 1 m3) at 1 bar, 20C and 50% RH. Now, what happens with the % RH when you expand the volume of this piston from 1m3 to 2m3, thus decreasing the pressure to 0.5bar? The use of piston makes sure it a closed system maintained.

I tried Perry's chemical engineer's handbook with little succes. Can someone provide a formule for this one, or some other kind of relation describing the mentioned parameters? Thanks in advance

Regards, Jellis

In my 6th Edition of Perry the formulae for correcting to different pressures are Eqs. 12-6 and 12-7. Ideal gas behavior is assumed.

well, we are inside the saturation curve, therefore in your steam tables, look up specific volumes for your new saturation pressure.
you know that your new volume is 2.

so you will have 2 = vf + x(vg-vf)

solve for x

...

v is actually just used for notation in this case. so vf - vg is not v(f-g)
vf is the notation for the specific volume of the fluid at certain conditions.
vg is the notation for the specific volume of the vapour at certain conditions.