MEFC104 Properties & Processes of Pure Substances
MEFC104 Properties & Processes of Pure Substances
MEFC104 Properties & Processes of Pure Substances
Module I
2
MODULE I
PROPERTIES OF PURE SUBSTANCE
INTRODUCTION
LEARNING OUTCOMES
In case you encounter difficulty, discuss this with your instructor during the
face-to-face meeting.
Lesson 1
Definition of Terms:
Pure Substance
- a working substance whose chemical composition remains the same even if
there is a change in phase
- a substance that has a fixed chemical composition throughout
Saturation Temperature
Saturation temperature is the temperature at which liquids start to boil or the
temperature at which vapors begin to condense. The saturation temperature of a given
substance depends upon its existing pressure. It is directly proportional to the
pressure, i.e. it increases as the pressure is increased and decreases as the pressure
is decreased.
Examples:
a. Water boils at 100oC at atmospheric condition (101.325 kPa)
b. Water boils at 179.91oC at a pressure of 1000 kPa
c. Steam condenses at 311.06 oC at 10 MPa
d. Steam condenses at 39oC at 0.0070 MPa
Subcooled Liquid
A subcooled liquid is one which has a temperature lower than the saturation
temperature corresponding to the existing pressure.
Example:
Liquid water at 60oC and 101.325 kPa is a subcooled liquid. This is because from
the steam tables, the saturation temperature at 101.325 kPa is 100 oC. Since the
actual temperature of liquid water of 60oC is less than 100oC, therefore, it is a
subcooled liquid.
Compressed Liquid
A compressed liquid is one which has a pressure higher than the saturation
pressure corresponding to the existing temperature.
Question:
Is liquid water at 110 kPa and 1000C a compressed liquid?
From steam tales, Psat at 1000C = 101.325 kPa.
Comparing:
The actual liquid water pressure of 110 kPa is greater than P sat at 1000C.
Therefore, it is a compressed liquid.
Saturated Liquid
A saturated liquid is a liquid at the saturations (saturation temperature or
saturation pressure), which has temperature equal to the boiling point corresponding
to the existing pressure. It is a pure liquid, i.e., it has no vapor content.
Examples:
a. Liquid water at 100oC and 101.325 kPa
b. Liquid water at 233.90oC and 3 MPa
c. Liquid water at 324.75oC and 12 MPa
Vapor
Vapor is the name given to a gaseous phase that is in contact with the liquid
phase, or that is in the vicinity of a state where some of it might be condensed,
Saturated Vapor
A saturated vapor is a vapor at the saturation conditions (saturation
temperature and saturation pressure). It is 100% vapor i.e., has no liquid or moisture
content.
Examples:
a. Steam (water vapor) at 100oC and 101.325 kPa
b. Steam at 212.42oC and 2 MPa
c. Steam at 352.37oC and 17 MPa
Superheated Vapor
A superheated vapor is a vapor having a temperature higher than the saturation
temperature corresponding to the existing pressure.
Examples:
a. Steam at 200oC and 101.325 kPa
200 C tsat P101.325 kPa 100 C
b. Steam at 300oC and 5 MPa
300 C tsat P5 MPa 263.99 C
Example:
Determine the degrees of superheat of superheated steam at 200oC and
101.325 kPa.
Example:
Determine the degrees subcooled of liquid water at 90 oC and 101.325 kPa.
From steam tables:
tsat at 101.325 kPa =100oC
o
SH = 100 oC – 90 oC = 10 oC
Wet Vapor
A wet vapor is a combination of saturated vapor and saturated liquid.
mass of vapor
x
total mass of mixture
Percent Moisture, y
The percent moisture of wet vapor is the percent by weight that is saturated
liquid.
Example:
Determine the latent heat of vaporization of water at: (a) 100 oC, (b)
o
200 C, and (c) 300oC.
From steam tables:
(a) hfg at 100oC = 2257.0 kJ/kg
(b) hfg at 200oC = 1940.7 kJ/kg
(c) hfg at 300oC = 1404.9 kJ/kg
Critical Point
The critical point represents the highest pressure and highest temperature at
which liquid and vapor can coexist in equilibrium. The state of water at critical
conditions whether it is saturated liquid or saturated vapor is unknown. Hence, the
latent heat of vaporization of water at this condition is either zero or undefined.
Sensible Heat
Heat that causes change in temperature without a change in phase.
Examples:
a. Heat added in raising the temperature of steam from 100 oC at
101.325 kPa to 150oC.
b. Heat removed in lowering the temperature of water from 90 oC to
80oC.
Latent Heat
Heat that causes change in phase without a change in temperature.
Example:
Heat added in converting 1 kg of water at 100 oC and 101.325 kPa to 1
kg of steam at 100oC and 101.325 kPa.
v = vf = vfg vfg = vg – vf
hg = hf + hgf hfg = hg – hf
sg = sf + sfg sfg = sg – sf
ug = u +ufg ufg = ug – uf
x y
v v
Fig. 1-3. Specific Volume of a Wet Mixture
v = Specific volume of its saturated liquid content + specific volume of its saturated
vapor content
v yv f xvg
kg liquid m3 m3
yv f
kg mixture kg liquid kg mixture
kg vapor m3 m3
xvg
kg mixture kg vapor kg mixture
From v yv f xvg
But y 1 x
Then
v 1 x v f xvg x 1 y
v v f xv f xvg v yv f 1 y vg
or
v v f x vg v f v yv f vg yvg
v v f xv fg v vg y vg v f
v vg yv fg
Similarly,
h h f xh fg or h hg yh fg
s s f xs fg or s sg ys fg
u u f xu fg or u u g yu fg
NOTE:
At saturated conditions, p and t are dependent on one another and therefore,
are considered as one independent property. At superheated conditions, p and t are
independent from each other and therefore, are considered as two independent
properties.
Quality could not be more than 100% and per cent moisture could not be lower
than 0%.
xsf ysf
sf sfg
sg
Steam Tables
Steam tables are tables widely used by engineers and scientists in the design
and operation of equipment where steam is used as working substance in a
thermodynamic processes and cycles. It gives an idea about the phase of steam at
various temperature and pressure.
Figure 3.1 is the p–v–T surface of a substance such as water that expands on
freezing. Figure 3.2 is for a substance that contracts on freezing, and most substances
exhibit this characteristic. The coordinates of a point on the p–v–T surfaces represent
the values that pressure, specific volume, and temperature would assume when the
substance is at equilibrium.
There are regions on the p–v–T surfaces of Figs. 3.1 and 3.2 labeled solid,
liquid, and vapor. In these single-phase regions, the state is fixed by any two of the
properties: pressure, specific volume, and temperature, since all of these are
independent when there is a single-phase present. Located between the single-phase
regions are two-phase regions where two phases exist in equilibrium: liquid–vapor,
solid–liquid, and solid–vapor. Two phases can co-exist during changes in phase such as
vaporization, melting, and sublimation. Within the two-phase regions pressure and
temperature are not independent; one cannot be changed without changing the other.
In these regions the state cannot be fixed by temperature and pressure alone;
however, the state can be fixed by specific volume and either pressure or
temperature. Three phases can exist in equilibrium along the line labeled triple line.
A state at which a phase change begins or ends is called a saturation state. The dome-
shaped region composed of the two-phase liquid–vapor states is called the vapor
dome. The lines bordering the vapor dome are called saturated liquid and saturated
vapor lines. At the top of the dome, where the saturated liquid and saturated vapor
lines meet, is the critical point. The critical temperature Tc of a pure substance is
the maximum temperature at which liquid and vapor phases can coexist in equilibrium.
The pressure at the critical point is called the critical pressure, pc. The specific
volume at this state is the critical specific volume.
The triple line of the three-dimensional p–v–T surface projects onto a point on
the phase diagram. This is called the triple point. Recall that the triple point of water
is used as a reference in defining temperature scales. By agreement, the temperature
assigned to the triple point of water is 273.16 K. The measured pressure at the triple
point of water is 0.6113 kPa.
The line representing the two-phase solid–liquid region on the phase diagram
slopes to the left for substances that expand on freezing and to the right for those
that contract. Al-though a single solid phase region is shown on the phase diagrams of
Figs. 3.1 and 3.2, solids can exist in different solid phases. For example, seven
different crystalline forms have been identified for water as a solid (ice).
p-V DIAGRAM
Projecting the p–v–T surface onto the pressure–specific volume plane results in
a p–v diagram, as shown by Figs. 3.1cand 3.2c. The figures are labeled with terms that
have already been introduced. When solving problems, a sketch of the p–v diagram is
frequently convenient. To facilitate the use of such a sketch, note the appearance of
constant-temperature lines (isotherms). By inspection of Figs. 3.1c and 3.2c, it can be
seen that for any specified temperature less than the critical temperature, pressure
remains constant as the two-phase liquid–vapor region is traversed, but in the single-
phase liquid and vapor regions the pressure decreases at fixed temperature as specific
volume increases. For temperatures greater than or equal to the critical temperature,
pressure decreases continuously at fixed temperature as specific volume in-creases.
There is no passage across the two-phase liquid–vapor region. The critical isotherm
passes through a point of inflection at the critical point and the slope is zero there.
T–v DIAGRAM
Projecting the liquid, two-phase liquid–vapor, and vapor regions of the p–v–T
surface on to the temperature–specific volume plane results in a T–v diagram as in Fig.
3.3. Since consistent patterns are revealed in the p–v–T behavior of all pure
substances, Fig. 3.3 showing a T–v diagram for water can be regarded as
representative.
PHASE CHANGES
Liquid State
For a two-phase liquid–vapor mixture, the ratio of the mass of vapor present
to the total mass of the mixture is its quality, x.
mvapor
x
mliquid mvapor
The value of the quality ranges from zero to unity: at saturated liquid states,
x=0, and at saturated vapor states, x=1.0. Although defined as a ratio, the quality is
frequently given as a percentage. Similar parameters can be defined for two-phase
solid–vapor and two-phase solid–liquid mixtures.
Vapor States
Let us return to a consideration of Figs. 3.3 and 3.4. When the system is at the
saturated vapor state (state g on Fig. 3.3), further heating at fixed pressure results in
increases in both temperature and specific volume. The condition of the system would
now be as shown in Fig. 3.4c. The state labeled s on Fig. 3.3 is representative of the
states that would be attained by further heating while keeping the pressure constant.
A state such as s is often referred to as a superheated vapor state because the system
would be at a temperature greater than the saturation temperature corresponding to
the given pressure.
Consider next the same thought experiment at the other constant pressures
labeled on Fig. 3.3, 10 MPa, 22.09 MPa, and 30 MPa. The first of these pressures is less
than the critical pressure of water, the second is the critical pressure, and the third
is greater than the critical pressure. As before, let the system initially contain a liquid
at 20C. First, let us study the system if it were heated slowly at 10 MPa. At this
pressure, vapor would format a higher temperature than in the previous example,
because the saturation pressure is higher (refer to Fig. 3.3). In addition, there would
be somewhat less of an increase in specific volume from saturated liquid to vapor, as
evidenced by the narrowing of the vapor dome. Apart from this, the general behavior
would be the same as before. Next, consider the behavior of the system where it
heated at the critical pressure, or higher. As seen by following the critical isobar on
Fig. 3.3, there would be no change in phase from liquid to vapor. At all states there
would be only one phase. Vaporization (and the inverse process of condensation) can
occur only when the pressure is less than the critical pressure. Thus, at states where
pressure is greater than the critical pressure, the terms liquid and vapor tend to lose
their significance. Still, for ease of reference to such states, we use the term liquid
when the temperature is less than the critical temperature and vapor when the
temperature is greater than the critical temperature.
As the system is heated further, the ice continues to melt until eventually the last bit
melts, and the system contains only saturated liquid. During the melting process the
temperature and pressure remain constant. For most substances, the specific volume
increases during melting, but for water the specific volume of the liquid is less than
the specific volume of the solid. Further heating at fixed pressure results in an increase
in temperature as the system is brought to point con Fig. 3.5. Next, consider the case
where the system is initially at state a of Fig. 3.5, where the pressure is less than the
triple point pressure. In this case, if the system is heated at constant pressure it passes
through the two-phase solid–vapor region into the vapor region along the line a–b–c
shown on Fig. 3.5. The case of vaporization discussed previously is shown on Fig. 3.5
by the line a”–b”–c”.
LEARNING ACTIVITY