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mdossaji (Mechanical) (OP)
8 Apr 11 13:53
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 (Structural)
8 Apr 11 15:11
I know I am just a dumb old Structural guy - but isn't hard to pump frozen water (ice)??
Helpful Member!(2)  Chance17 (Chemical)
8 Apr 11 21:35
See attached for some saturation data down to -20F
ione (Mechanical)
9 Apr 11 3:57
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
KiwiMace (Mechanical)
11 Apr 11 5:18
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 (Mechanical) (OP)
11 Apr 11 14:39
Ione, Isn't vapor pressure different from saturation pressure?
Chance17 (Chemical)
11 Apr 11 20:48
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 (Mechanical)
12 Apr 11 3:19
Antoine equation gives vapour pressure at saturation
ione (Mechanical)
12 Apr 11 7:03
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
mdossaji (Mechanical) (OP)
12 Apr 11 15:39
so saturation pressure of water is same as vapor pressure at saturation?  
Compositepro (Chemical)
12 Apr 11 16:06
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.

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