- #1

kandelabr

- 113

- 0

## Homework Statement

This is my problem: i have a container with humid (say

*psi*= 60%) air at a pressure of 450 bar. i need to know to what preesure can this air be expanded (adiabatically) so that it reaches temperature just above freezing.

i also need to know how much work is extracted from this expansion.

## Homework Equations

this would not be hard to calculate using ideal gas law and laws for gas mixtures, but at this pressure the compressibility factor is 1.4, that means a 40% error done with ideal gas law. this should be done using thermodynamic tables and diagrams.

## The Attempt at a Solution

as air expands adiabatically, it cools down, but condensing water adds some heat so expansion of "the dry part" is actually polytropic. if i neglect partial pressure of water vapor, i can calculate polytropic exponent

*n*, since i know how much heat the water gives out during condensation.

but i don't know how partial pressure of water vapor changes with pressure.

if dew point changes with pressure, the complication is even bigger.

if i knew enthalpies in start and end state, calculating work would be simple (w = |h

_{2}- h

_{1}|).