Pure Substances
Thermodynamics
Professor Lee Carkner
Lecture 5
PAL # 4 First Law Pumping water uphill and then running it back to
produce energy Rate of power imparted to water = W’water = (1000)(2)(9.8)(40) =
But the pump and the turbine are only 75% efficient pump = W’water/W’pump
W’pump = 785/0.75 = Cost per year = (1046)(10)(0.03)(365) = $114500 turbine = W’turbine/W’water
W’turbine = (785)(0.75) = Revenue per year = (785)(10)(0.08)(365) = $171900 Net Yearly Profit =
PAL # 4 First Law Should you build it? No
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
Types of Phases Compressed liquid
Saturated liquid
Saturated Vapor Saturated liquid and
vapor can coexist together
Superheated vapor
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
Change of Tsat with Psat
Enthalpy
Defined as:h = u + PvH = U +PV
Very useful for flow systems such as
turbines
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
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
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
Pv Diagram
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
Regions on a Pv Diagram
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
PT Diagram
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”
PvT Diagram for Water
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
Next Time
Read: 3.6-3.8 Homework: Ch 3, P: 48, 54, 75, 86