Chapter 3 Properties of a Pure Substance Three familiar properties of a
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Transcript Chapter 3 Properties of a Pure Substance Three familiar properties of a
Chapter 3
Properties of a Pure Substance
Three familiar properties of a
substance in the previous
chapter —
– specific volume,
– pressure, and
– temperature.
3.1 THE PURE SUBSTANCE
has a homogeneous and invariable
chemical composition,
exist in more than one phase, and
exist with no change of phase.
Examples :
– liquid water,
– a mixture of ice and liquid water,
– a mixture of gases, such as air
A mixture of liquid air and gaseous air –
(X)
– Because the chemical composition of the liquid
phase is different from that of the vapor phase. )
Simple Compressible Substances
(system)
Those whose surface effects, magnetic
effects, and electrical effects are
insignificant when dealing with the
substances.
But changes in volume, such as those
associated with the expansion of a gas
in a cylinder, are very important.
3.2 VAPOR–LIQUID–SOLID-PHASE
EQUILIBRIUM IN A PURE SUBSTANCE
0.1MPa
20 0C,1kg
Fig.3.1
Heat, ν
99.6 0C
Heat ,ν
Saturation Temperature
– The temperature at which vaporization
takes place at a given pressure.
And this given pressure is called the
Saturation Pressure for the given
temperature.
Sub-cooled liquid
Compressed liquid
Fig. 3.2 A vapor-pressure curve
for a pure substance
Saturated liquid (state)
– A substance exists as liquid (state) at the
saturation temperature and pressure.
Subcooled liquid (Compressed liquid)
– If the temperature of the liquid is lower than the
saturation temperature for the existing pressure, it
is called either a subcooled liquid (implying that
the temperature is lower than the saturation
temperature for the given pressure) or a
compressed liquid (implying that the pressure is
greater than the saturation pressure for the given
temperature).
Quality of substance
– When a substance exists as part liquid and
part vapor at the saturation temperature,its
quality is defined as the ratio of the mass
of vapor to the total mass.
Quality has meaning only when the
substance is in a saturated state.
Saturated vapor
– A substance exists as vapor at the
saturation temperature.
The quality of dry saturated vapor is
100%.
Superheated vapor
is the vapor at a temperature greater
than the saturation temperature.
Actually, the substances we call gases
are highly superheated vapors.
oC
Supercritical
fluid
20
Fig. 3.3 Temperature–volume diagram for water showing liquid and
vapor
phases.
Table 3.1
FIGURE 3.4 T –v diagram for the two-phase liquid–vapor
region to show the quality specific volume relation.
To Derivative the Quality, x
V =Vliq +Vvap = mliq v f+mvap v g
then divide the above equation by total
mass m,
Table 3.2
FIGURE 3.5 Pressure temperature diagram
for a substance such as water.
FIGURE 3.6 Carbon dioxide phase diagram.
Fig. 3.7 Water phase
diagram.
3.3 INDEPENDENT PROPERTIES
OF A PURE SUBSTANCE
•The state of a simple compressible pure
substance is defined by two independent
properties.
• For example, if the specific volume and
temperature of superheated steam are
specified, the state of the steam is
determined.
A exception, in a saturation state, should
be noted.
Consider the saturated-liquid and saturatedvapor states of a pure substance. These two
states have the same pressure and the same
temperature, but they are definitely not the
same state. Therefore, in a saturation state,
pressure and temperature are not independent
properties.
Two independent properties such as pressure
and specific volume or pressure and quality are
required to specify a saturation state of a pure
substance.
A mixture of gases, such as air, has the
same characteristics as a pure substance
as long as only one phase is present,
concerns precisely this point.
The state of air, which is a mixture of gases
of definite composition, is determined by
specifying two properties as long as it
remains in the gaseous phase.
3.4 TABLES OF THERMODYNAMIC
PROPERTIES
oC
Pg=1.554
Pg=5.0
Pg=1.0
200
FIGURE 3.8 Listing of the steam tables.
• Example
Let us calculate the specific volume of saturated
steam at 200oC having a quality of 70%.
•
<Solution>
Using Eq. 3.1, and looking up Table B.1.3 gives
v = 0.3 (0.001 156) +0.7 (0.127 36) = 0.0895 m 3 /kg
Example. 3.1
Example 3.2
continued
Example 3.3
Example 3.4
(p.412)
3.5 THERMODYNAMIC SURFACES
3.6 THE P–V–T BEHAVIOR OF LOW- AND
MODERATE-DENSITY GASES
•At very low densities the average distances
between molecules is so large that the
intermolecular ( IM ) potential energy may
effectively be neglected.
• In such a case, the particles would be
independent
of one another, and the situation is referred to
as an
ideal gas.
•Therefore, a very low density gas behaves
according to the
ideal gas equation of state.
+
R is a different constant for each particular
gas. The value of R for a number of
substances is given in Table A.5 of
Appendix A.
Example 3.5
Example 3.6
Over what range of density will the ideal
gas equation of state hold with accuracy?
How much does an actual gas at a
given pressure and temperature
deviate from
ideal gas behavior?
As would be expected, at very low
pressure or high temperature the error is
small and the gas behavior becomes
closer to the ideal gas model.
But this error becomes severe as the
density increases (specific volume
decreases).
FIGURE 3.14 Temperature-specific volume diagram for water
that indicates the error in assuming ideal gas for saturated
vapor and for superheated vapor.
Compressibility factor, Z
A more quantitative study of the question of
the ideal-gas approximation
Z =1, for an ideal gas
The deviation of Z from unity is a measure of
the deviation of the actual relation from the
ideal-gas equation of state.
Fig.3.15 Compressibility of nitrogen
Is there a way in which we can put all of the substances on a common
basis? To do so, we “reduce” the properties with respect to the values
at the critical point.
Example 3.7
Example 3.8