7.5 Phase Change On P-V and T-V Diagrams
7.5 Phase Change On P-V and T-V Diagrams
7.5 Phase Change On P-V and T-V Diagrams
Diagrams
Energy, Entropy and Engines:
An Introduction to Thermodynamics
Sanjeev Chandra
What are we going to discuss?
• How do we show changes in temperature, pressure and
volume on a graph during phase change?
Volume change during phase change
Isobar
Liquid Liquid+Vapour Vapour Line of Constant Pressure
T< Tsat T= Tsat T>Tsat 𝑇 P
Temperature
Tsat L+V
Vapour Vapour L
Liquid Liquid
𝑣
Specific volume
Heat Heat Heat
Terminology
𝑣𝑓 𝑣𝑔
𝑣
Effect of increasing pressure
𝑇
V
Increasing Pressure
L+V Saturation temperature
Tsat
increases with pressure
Tsat
L
Liquid
𝑣𝑔 decreased
𝑣𝑓 increased
Higher T Higher P
Heat Volume compressed
Volume expands
𝑣𝑓 𝑣𝑔
𝑣
Critical pressure
Critical Pressure
Pc
Inflection Point
𝑇 Vapour
Pressure
Supercritical
V increasing
Fluid
L+V Liquid
𝑣𝑐
𝑣 Critical
𝑣
Volume
Constant temperature phase change
Isotherm
Line of Constant
Vapour Liquid+Vapour Liquid
P<Psat P= Psat P> Psat 𝑃 Temperature
T
Pressure
L+V
Psat
Vapour V
Vapour
Liquid Liquid
Constant Constant Constant 𝑣
Specific volume
Temperature Temperature Temperature
T T T
P-v Diagram
Supercritical
Fluid
𝑃 𝑃 Critical Point
Pc
Critical
L Pressure
Subcooled
Critical Liquid Critical
L+V Temperature Temperature
Tc Saturated Tc
V Mixture
Superheated
Vapour
Increasing Temperature
𝑣𝑐
𝑣 Critical
𝑣
Volume
Summary
• Phase change can be shown on either P-v or T-v diagrams
• A substance can be:
Subcooled Liquid: T< Tsat
Superheated Vapour: T> Tsat
Saturated Mixture: T= Tsat
• At the critical point properties of liquid and vapour become the same
and the substance becomes a supercritical fluid