me 63-lecture 2-ay 20112012
TRANSCRIPT
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Lecture 2
Chapter 3
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Pure Substance
a substance that has a homogeneous and invariable chemicalcomposition
may exist in more than one phase (i.e., mixture of phases) butchemical composition is similar in all phases
Example: A mixture of ice and liquid water
A mixture of liquid water and vapor (steam)
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Phase and State Changes of a
Substance A pure substance can exist in different phases (solid,
liquid, or gas)
It can exist in equilibrium as a mixture of differentphases
When a pure substance exists as a mixture of two ormore phases, it is in phase equilibrium when there isno tendency for the mass of each phase to change
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Phase and State Changes of a
Substance
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T-v diagram for the heating process of water at
constant pressure.
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Definitions:
Saturation Temperature
- the temperature at which vaporization takes placefor a given pressure
Saturation Pressure
- the pressure at which vaporization takes place for agiven temperature
For a pure substance, there is a definite relation
between the saturation temperature and saturationpressure which can be shown by the vapor-pressurecurve.
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A substance can exist in any of the
following phases
Compressed Liquid
Subcooled Liquid
Saturated Liquid
Saturated Vapor
Saturated Mixture
Superheated Vapor
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Compressed Liquid
a liquid whose pressure is higher than the saturation pressure
for the existing liquid temperature (sat. temp.)Subcooled Liquid
another term for compressed liquid; a liquid whosetemperature is lower than the saturation temperature for theexisting liquid pressure (the saturation pressure)
Saturated Liquidsubstance that exists as liquid only at saturation temperatureAND saturation pressure
Saturated Vapor
substance that exists as vapor only at saturation temperature
AND saturation pressure
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Saturated Mixture
substance that exists as a mixture of liquid and vapor for a
given temperature (saturation temperature) AND pressure(saturation pressure)
The liquid component in the mixture has the same propertiesas saturated liquid for the given temperature or pressure
The vapor component in the mixture has the same propertiesas saturated vapor for the given temperature or pressure
For a liquid-vapor mixture, a property that relates the mass ofvapor to the mass of the mixture is defined:
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Superheated Vapor
substance in vapor phase whose temperature is higherthan the saturation temperature for the existing
pressure
OR
substance in vapor phase whose pressure is lower than
the saturation pressure for the existing temperature
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Quality - the ratio of the mass of vapor component to thetotal mass of the liquid-vapor mixture
x = mg / m = mass of vapor / total mass of liquid-vapormixture
where
mf = mass of liquid component
mg = mass of vapor component
m = mf + mg
Note that
x = 0 for saturated liquid
x = 1 for saturated vapor
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Property Relationships
*note
liq gV V V
liq f g gmv m v m v
fg g f v v v
(1 ) f g f fgv x v x v v x v
specific volume
=specific volume of a saturated liquid
=specific volume of a saturated gas
fg
f
g
v average
v
v
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Critical Point
The state wherein the saturated liquid and saturated
vapor states are identical
It is specified by the critical temperature Tc , critical
pressure Pc , or critical specific volumeAt pressures above the critical pressure, the liquid
and vapor phases do not exist in equilibrium
At supercritical pressures, the substance is simply
called a fluid
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T-v Diagram
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P-V Diagram
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Processes that involve phase change
Sublimation direct phase change from solid to vapor along the sublimation line, the solid and vapor phases (solid-
vapor mixture) exist in equilibriumFusion direct phase change from solid to liquid along the fusion line, the solid and liquid phases (solid-liquid
mixture) exist in equilibriumVaporization direct phase change from liquid to vapor along the vaporization line, the liquid and vapor phases
(liquid-vapor mixture) exist in equilibrium
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Triple Point
the state in which three phases are present in
equilibrium
for a pure substance, there may be many
triple points but only one triple point where
the solid, liquid, and vapor phases exist in
equilibrium
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Tables of Thermodynamic Properties
The thermodynamic tables lists other properties of asubstance (e.g., v, u, h, s) as a function of two independentproperties, commonly T and P
1. Saturated Liquid
Saturated Vapor Region Tabulates properties of saturated liquid-vapor mixtures for a
given temperature ( which is also equal to Tsat ) or a givenpressure ( which is also equal to Psat )
Quality - An independent property of the liquid-vapor mixture
which can be used to specify its state, defined as
vapor vapor g
mixture vapor liquid g f
m m mx
m m m m m
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Property Relationships
liq vapV V V
liq f vap gmv m v m v
fg g f v v v
(1 ) f g f fgv x v x v v x v
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3. Compressed or Subcooled Liquid Region
Tabulates properties of compressed or subcooledliquid
Tests
1. For a given T and P, see if P > Psat) T or T 1 actual density is less than predicted from
ideal gas equation due to greater intermolecular
repulsive force caused by much reduced
intermolecular distances
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GENERALIZED COMPRESSIBILITY CHART
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Pr = P / Pc = reduced pressure
Tr = T / Tc = reduced temperature
Pc = critical pressure
Tc = critical temperature
See Table A.2 for Pc and Tc ofvarious substances
This plot of the generalizedcompressibility factor Z in termsof the reduced pressure Pr andreduced temperature Tr is
applicable to various simplesubstances.
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The generalized compressibility chart indicates that
ideal gas behavior can be assumed when P 2Tc up to P 4 to 5 times Pc
For other conditions, especially in the superheated
vapor region, it is more accurate to use thethermodynamic tables
May be used to estimate P-v-T data of substances ifno tables are available