Pure Substances

1
Pure Substances

• Thermodynamics
• Professor Lee Carkner
• Lecture 5

2
PAL 4 First Law

• Pumping water uphill and then running it back to
  produce energy
• Rate of power imparted to water
• Wwater (1000)(2)(9.8)(40)
• But the pump and the turbine are only 75
  efficient
• hpump Wwater/Wpump
• Wpump 785/0.75
• Cost per year (1046)(10)(0.03)(365) 114500
• hturbine Wturbine/Wwater
• Wturbine (785)(0.75)
• Revenue per year (785)(10)(0.08)(365) 171900
• Net Yearly Profit

3
PAL 4 First Law

• Should you build it?
• No

4
Pure Substances

• A pure substance consists of either a single type
  of atom or molecule or a homogeneous mixture
• Can exist as a mixture of phases
• Gas
• Liquid
• Solid
• Strong intermolecular forces, high density, fixed
  molecular position

5
Types of Phases

• Compressed liquid
• Saturated liquid
• Saturated Vapor
• Saturated liquid and vapor can coexist together
• Superheated vapor

6
Saturation

• A substance changes phase when it is at the
  saturation temperature for a given pressure
• Can be compiled in tables for reference
• Tsat increases with Psat
• Need more internal energy to become vapor at
  higher pressure

7
Change of Tsat with Psat
8
Enthalpy

• Defined as
• h u Pv
• H U PV
• Very useful for flow systems such as turbines

9
Specific Volume and Quality

• Mass of saturated liquid mf
• Mass of saturated vapor mg
• Since both phases often co-exist we define the
  quality, x
• The quality is the relative fraction of the
  mixture in the vapor phase

10
Quality Relations

• The total volume of a mixture is
• the volume occupied by each phase is
• We can use these relationships to write an
  expression for the combined specific volume of
  the mixture
• vavg vf xvfg
• x (vavg vf )/vfg

11
Property Diagrams

• We will plot three key variables P, T, and v as
• Each region of the diagram represents a phase or
  mixture of phases

12
Pv Diagram
13
Pv Diagram Properties

• Saturation curves define boundaries of
  liquid-vapor mixture region
• Critical point is on the critical isotherm
• Densities of both are the same
• Above the critical point, no amount of pressure
  can condense the vapor to a liquid

14
Regions on a Pv Diagram
15
PT Diagram

• Three curves can be drawn on the PT diagram
• Fusion curve
• Vaporization curve
• Sublimation curve
• The curves bound three distinct regions, one for
  each phase
• Juncture of the three curves is the triple point
  where all three coexist

16
PT Diagram
17
Other PT Features

• An isobar at standard atmospheric pressure
  intersects the normal boiling and melting points
• The critical point is on the vaporization curve
• Gas above critical T is called gas, below it is
  called vapor

18
PvT Diagram for Water
19
PVT Diagram

• P, V and T relations can be formed to create a
  three dimensional diagram
• The PT and PV diagrams are formed by projection

20
Next Time

• Read 3.6-3.8
• Homework Ch 3, P 48, 54, 75, 86