Pressure-Volume Diagram for Heating a Vapor-Liquid Mixture at a Constant Volume

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This Demonstration shows the phase behavior of 1 mol of water on a log pressure versus a log volume (-) phase diagram.


Use the sliders to change the specific volume and add heat at constant volume. The temperature, vapor quality and pressure are shown on the left. The cylinder fitted with a piston on the right represents the volumes of liquid (blue) and vapor (green) present. The critical point (orange), which is the highest temperature and pressure where two phases coexist, is at 647 K and 22.1 MPa for water. Depending on whether the volume selected is smaller or larger than the critical volume, the vapor phase or liquid phase volume decreases, respectively, as heat is added.


Contributed by: Adam J. Johnston and Rachael L. Baumann (September 2017)
Additional contributions by: John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



Mass and energy balances are used to calculate the pressure and quality when two phases are present:

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