Water saturation pressure 2

[mdossaji]

I have a flow calculation for SCFM which requires saturation pressure (Psat)of water at ambient temperature. See attached calcuations. However the saturation pressure (Psat)curve stops at 32 degF. Is there a different curve used for saturation pressure of water at below freezing temperature or shall I ignore the Psat value for 32F and below?

 

[MiketheEngineer]

I know I am just a dumb old Structural guy - but isn't hard to pump frozen water (ice)??

 

[Chance17]

See attached for some saturation data down to -20F

 

[ione]

Vapour pressures can be calculated using Antoine equation.

You can find a calculator at the link below (results given by the calculator are in agreement with those reported in the document posted by Chance17

http://www.ruf.rice.edu/~aiche/calc/calculator.html

 

[KiwiMace]

There are imperical formulae for the sat pressure over ice and water in ASHRAE fundamentals handbook (2009 chapter 1, Psychrometrics, Hyland and Wexler). These are much more accurate and computer friendly than interpolating out of a table.

Practically every hvac engineer will have this book and the formulae have been in there for years. The cd in the back of the book has both metric and imperial versions of the formulae.

 

[mdossaji]

Ione, Isn't vapor pressure different from saturation pressure?

 

[Chance17]

Question about vapor pressure vs saturation pressure?
The water molecules can have a vapor pressure less than or equal to saturation.
Saturation = chemical equilibrium and is documented by tables or equations that fit data.
One needs an analysis to determine actual vapor pressure.

 

[ione]

Antoine equation gives vapour pressure at saturation

 

[ione]

As I've written in my first post, the calculator suggested works with Antoine equation (quite simple) and gives results in agreement with those reported in the table posted by Chance17.

There are anyway many other correlations available, basically all working with logarithmic correlations between saturation pressure and actual temperature. You can find a bunch of these correlations at the link below

http://cires.colorado.edu/~voemel/vp.html

 

[mdossaji]

so saturation pressure of water is same as vapor pressure at saturation?

 

[Compositepro]

I would say so. Vapor pressure is the defined term used by scientists to describe the pressure of a vapor in equilibrium with its liquid at a given temperature. This also happens to be the "saturated" condition. "Partial pressure" is used to describe non-equilibrium states.