MTO 412E Physics of Cloud and
Precipitation
Phase Changes and
Phase Diagram
1
What are the States of Matter?
Solid
Gas
Liquid
Plasma
2
Phase changes
Reactions that involve formation of inter-molecular interactions are exothermic.
Reactions that involve breaking inter-molecular interactions are endothermic.
4
5
Melting and freezing
•Melting is the process of changing state of matter from solid to liquid.
•When heat is applied to a solid the molecules in the solid phase gain kinetic
energy. When sufficient heat is applied the molecules push against each other
and break the ordered structure in which they were held. This results in the
formation of liquid, and the process is described as melting.
•The process of melting requires energy, and it is therefore endothermic.
•This process also is called fusion.
solid  liquid
∆Hfusion
liquid  solid
∆Hcrystallization
- ∆Hfusion = ∆Hcrystallization
•The reverse of fusion is crystallization. This process describes the ordered
assembly of molecules to form the solid phase. This process involves
formation of bonds, so it is exothermic.
6
Sublimation and deposition
•The process of molecules escaping directly from the solid to the gas phase is
called sublimation. Dry ice (CO2) sublimates at room temperature from
solid to vapor.
•The reverse process, the conversion of gas to solid, is called deposition.
•The molar heat of sublimation and molar heat of deposition have opposite
signs but the same magnitude.
solid  gas
∆Hsublimation
gas  solid
∆Hdeposition
- ∆Hsublimation= ∆Hdeposition
The sublimation reaction involves breaking of inter-molecular bonds.
Sublimation requires heat input and is endothermic.
The deposition reaction involves formation of inter-molecular bonds. It results
in heat output and is exothermic.
7
Phases
•
• Properties differ with different phase
–
• In order to change phase must be at the right
temperature
8
Phase Change
• Once ice starts to melt its temperature does
not change. Why?
–
•
•
• How much energy does this take?
•
– Represented by L
9
Heat of Fusion
10
Latent Heat
• For liquid-solid change:
–
• For liquid-gas change:
–
• Amount of heat:
• Substance must be at the right temperature to
undergo a phase change
–
11
Water Phase Change
• Temperature remains constant while phase is
changing
–
• Latent heat of water is very large
–
– This is why evaporating water cools you down
12
Heat and Temperature
13
Phase Change and Heat
• Adding heat will raise temperature only to a
phase change point
–
• After that additional heat will go into phase
change
–
•
• Must add all heats together
14
Phase Change and Pressure
• Water does not always boil at 100 C
•
• Examples:
–
–
• In general lower external pressure means
lower boiling point
– Easier for molecules to escape to vapor phase
15
Change in Boiling Point with
Pressure
16
Evaporation
• How can water evaporate (become gas) if it is not at
the boiling point?
–
• Evaporation only works if the air above is not
saturated with vapor
–
– Pressure is too high
•
–
17
18
The tendency for a liquid to evaporate
increases as 1. the temperature rises
2. the surface area increases
3. the intermolecular forces
decrease
19
Vapor
Pressure
20
Dynamic Equilibrium
The rate of the forward reaction = rate of the reverse
reaction
Liquid
Vapor
equilibrium vapor pressure - the pressure exerted by a
vapor above a liquid after equilibrium has been
established.
21
Vapor Pressure is Temperature Dependent
According to Kinetic Molecular Theory, a greater proportion of
molecules will have enough energy to overcome intermolecular forces
and escape into the gas phase at the higher temperature (Fig 13.5)
22
23
Diethyl ether - dipole-dipole
interactions
Ethanol and Water Hydrogen bonding
(stronger)
24
The energy required to disrupt the surface of a liquid
25
Boiling Point
“If you have a beaker of water
open to the atmosphere, the
mass of the atmosphere is
pressing down on the surface.
As heat is added, more and
more water evaporates, pushing
the molecules of the
atmosphere aside. If enough
heat is added, a temperature is
eventually reached at which the
vapor pressure of the liquid
equals the atmospheric
pressure, and the liquid boils.”
26
Normal boiling point - the temperature at which
the vapor pressure of a liquid is equal to the external
atmospheric pressure of 1 atm.
Increasing the external atmospheric pressure increases the
boiling point
Decreasing the external atmospheric pressure decreases
the boiling point
27
Location
San Francisco
Salt Lake City
Denver
Mt. Everest
Elevation (ft)
sea level
4400
5280
29,028
Boiling Point H2O (oC)
100.0
95.6
95.0
76.5
28
Phase Diagram
• Whether a substance is a solid, liquid or gas depends
on the temperature and pressure
–
• Keeping T constant while increasing P usually
produces a solid
• Water is an exception, increasing pressure on ice
produces water
–
– This causes ice skates to melt ice and freezing water to
expand and produce frost heaves
29
Phase
Diagrams
States of matter as a function of temperature and pressure
Pressure 
liquid
solid
gas
Temperature 
30
State of Matter Definitions
• Phase Diagram
– Plot of Pressure versus Temperature
• Triple Point
– A point on the phase diagram at which all three
phases exist (solid, liquid and gas)
• Critical Point
– A point on the phase diagram at which the density
of the liquid a vapor phases are the same
31
Pressure, Temperature, and State
Pressure
Liquid
Pcritical
Solid
Ptriple
Triple
Point
Critical
Point
Plasma
Gas
Vapor
Ttriple
Tcritical
Temperature
Phase Diagram Features
•
• Beyond the critical point there is no distinction
between a liquid and gas
–
• Solid and liquid phases separated by a fusion curve
•
• Solid and gas phases separated by a sublimation
curve
33
34
Phase Diagram for Water
35
Phase Diagram for Water
1. Curve AD is the equilibrium vapor pressure curve
where the liquid and gas are in equilibium.
2. Line AC is the solid-liquid equilibrium curve. Note
that the slope is negative for water - most substances
have a positive slope here. A negative slope means that
the substance becomes a liquid when the pressure is
increased.
3. Point A is the Triple Point - all three phases are in
equilibrium at once.
36
Water has a Maximum Density at 4 oC
37
Anomalous Properties of Water A consequence of hydrogen bonding
38
Velocity Distribution for Water
39
Water Vapour Saturation
• At any given air temperature, the density (or
vapour pressure) of water vapour cannot
exceed a maximum value.
• When that value is reached, the air is said to
be saturated; if any more water vapour is
added, the excess will condense out as liquid
*****
40
Saturated Vapour Pressure (SVP)
Equation
• The saturated vapour pressure es(T) increases
exponentially with temperature.
 19.65T 
e s (T)  611exp

T

273


where T is the temperature in oC
and es(T) is the vapour pressure in Pa
****
41
Example of SVP Calculation
for T = 9 oC
 19.65 x 9 
e s (9C)  611exp

282


= 611 exp [0.627] Pa
= 611 x 1.872 Pa
= 1144 Pa
*****
42
SVP/ Pa
Variation of SVP with Temperature
14000
12000
10000
8000
6000
4000
2000
0
-20 -10 0 10 20 30 40 50
Temperature/ C
43
Values of SVP
Temperature, oC
-10
0
10
20
30
40
es (T), Pa
286
611
1230
2340
4240
7380
es (T), mbar
2.86
6.11 12.30 23.40 42.40 73.80
44
How to find dewpoint temperature
vapour pressure/ Pa
14000
From any point A, decrease
T without changing vapour
pressure until SVP curve is
reached. That is Td
12000
10000
8000
6000
4000
A
2000
Td
0
-20
-10
0
10
20
30
40
50
Temperature/ C
45
No-go area
vap o u r p ressu re/ P a
14000
12000
Can have a point
anywhere below line,
not above line
10000
8000
6000
4000
2000
0
-2 0
-1 0
0
10
20
30
40
50
T e m p e ra tu re / C
46
Psychrometer Equation
ea = es(Tw) - 66(Ta-Tw)
• used to find vapour pressure from Ta and Tw
• starting from unsaturated conditions, the air is
both cooled and humidified
• on the following graph, the purple line has the
slope of -66 Pa K-1; the pink line down from
the SVP curve is the wet bulb temperature
*****
47
How to find w et-bulb tem perature
vap o u r p ressu re/ P a
14000
12000
This line always has a
slope of -66 Pa/K
10000
8000
6000
4000
A
2000
Tw
0
-2 0
-1 0
0
10
20
30
40
50
T e m p e ra tu re / C
48
Use of Psychrometer Equation
• Ta = 20 oC; Tw = 15 oC
• ea = es(Tw) - 66 (Ta-Tw)
• but
 19.65T 
e s (T)  611exp
 T  273 

so
e s Tw   e s 15 oC 
 19.65 x 15 
611 exp 

1700
Pa
 288 
hence ea = 1700 - 66(20-15) = 1370 Pa
*****
49
Thermodynamics Review
• Equilibrium Curve
– Water Vapor vs. Liquid Water
es
Pressure
Equilibrium
Temperature
50
Thermodynamics Review
• Supercooled Liquid
Water (SLW)
Pressure
– Absence of ice
Equilibrium
es w
with
Liquid Water
SLW
0.01oC
Temperature
51
Thermodynamics Review
• What about water vapor vs. ice?
52
Thermodynamics Review
• Equilibrium Curve for Ice
Pressure
Equilibrium
with
Liquid Water
es w
Equilibrium
with Ice
es i
0.01oC
Temperature
53
hPa
SVP over water
minus
SVP over ice
SVP over
water
54
55
Thermodynamics Review
• Mixed Phase Cloud
-12oC
56
• Saturated With
Respect to Liquid
Water
Pressure
Thermodynamics Review
Equilibrium
es w
with
Liquid Water
5 mb
-12oC
Temperature
57
Thermodynamics Review
Equilibrium
es w
with
Liquid Water
Pressure
• Supersaturated
With Respect to
Ice
Equilibrium
with Ice
5 mb
4.7 mb
es i
-12oC
Temperature
58
Thermodynamics Review
• Supersaturations
of Up to 20%
Pressure
– Compare to 1% in
Warm Clouds
Equilibrium
es w
with
Liquid Water
Equilibrium
with Ice
5 mb
4.7 mb
es i
-12oC
Temperature
59
Condensation
• Phase Change from Vapor to Liquid
• Release of Latent Heat of Vaporization
• Occurs When e = eS
- also RH = 100% or
- mixing ratio = saturation mixing ratio
- Temperature = Dew Point ( T=Td)
60
e = eS ??
•
In unsaturated conditions
e < eS
1. Increase e until it equals eS
(addition of water vapor)
2. Lower eS until it equals e
(lower the temperature)
61
62
63
• Adding Water Vapor
Difficult over short periods of time
Difficult at upper levels away from the earth’s
surface
• Lowering eS (lowering temperature)
Cooling at the surface of the earth
Expansion Cooling (lowering pressure) by
Lifting
64
Example
• Pacel of air at the surface of the earth with a
temperature T = 15 C and a dew point
temperature of 3 C.
• How far will this parcel have to lift for
condensation to occur?
• Looking for e = eS
or T = Td
65
Lower T to Td by Lifting
• Tp must decrease by
DT = T - Td = 15 –3 =12 C
• Temperature of the parcel T decreases by 10
C for each kilometer of lift
• Amount of Lift Required is
L = (12C)/(10C/km) = 1.2 km
• Cloud Base should form 1.2 km above the
surface of the earth
66
Condensation
• e = eS is valid for flat surface of water
• In clean air RH of 800% is required to have
water condense and form droplets
• If ions are present RH of about 400% is
required to form droplets in clean air
• Condensation at RH = 100% requires a
surface to get process started
67
68
Droplet Growth – Size vs. Mass
Relative Size
(diameter)
1
Relative Mass
(depends on volume)
1
2
8
3
27
4
64
69
Growth Times
(Growth Times from 1.0 um to Given Size at RH=100.05 %
Labbook page 116)
Droplet Diameter
(um)
Growth Time
10
320 s
20
1800 s
100
1.20 hr
500
12.7 days
2000
201 days
70
Growth Times (cont.)
• Rate of Droplet Size Increase is Large at first
• Rate of Size Increase Quickly Decreases
because of size – mass relationship
• Typical Cloud Droplet has a diameter of about
20 um.
71
The Question of Precipitation
• Types of Precipitation
Drizzle – drops less that 0.5 mm (500 um)
Rain – 500 – 6000 um (0.5 – 6 mm)
• Average Rain Drop has a diameter of about
2000 um (2 mm)
• How many cloud droplets (20 um) are
necessary to make a rain drop (2000 um)
72
Answer
•
•
•
•
•
2000/20 = 100
NO!
Volume Proportional to (radius)3
(100)3 = 106
It takes the liquid from 1,000,000
cloud droplets to make an average rain
drop
73
Rainfall Rates
• Light
- 0.01 – 0.1 in/hr
• Moderate
- 0.1 – 0.3 in/hr
• Heavy
- more than 0.3 in/hr
74
The Question of Precipitation
• What % of all Clouds Produce Precipitation?
• What is the Minimum amount of time necessary
produce precipitation?
• How is Precipitation Produced (limitations)
75
Thermodynamically Stable
Phases
• Usually, only one phase of a given substance is
stable at any given temperature and pressure.
• At some conditions of temperature and pressure, two
or more phases may exist in equilibrium.
• A slight change in temperature or pressure will favor
one phase over others. The conversion of one phase
to another is a phase transition.
• Phase transitions occur with a decrease (spont.) or
no change (equil.) in Gibbs energy.
76
What’s Included in a Phase
Diagram
• A one-component phase diagram
represents the situation when only
that component is present.
• Thus the vapor-pressure curve plots
the pressure ONLY of the vapor (no
air)
• Water filling a container at 50 torr
WILL NOT EVAPORATE (left).
• If some of the liquid is removed
without decreasing the size of the
container (or letting any air in) vapor
will form in the space above the
liquid until the pressure of the vapor
reaches the equilibrium vapor
pressure (right).
77
The Clausius-Clapeyron
Equation
• dp/dT = DH/TDV (the
Clapeyron equation) applies
to any two phases a and b.
• However, if phase b is the
gas phase, and the gas is
assumed perfect, the
Clausius-Clapeyron
equation may be derived.
• d(ln p)/dT = DH/RT2
• ln p2 = ln p1 + DH 1 - 1
R T1 T2
• If p is known at one temp it
can be found at another
temp.
78
Critical
point
Phase Diagram
for CO2
http://chem.neopages.com/tutorials/som.shtml
79
1. Positive slope for the solid-liquid interface (normal)
2. Sublimation occurs at room temperature under 1 atm
of pressure.
3. There is a “critical point” at 73 atm and 31 oC (varies
from substance to substance).
4. Above the critical point the substance is known as a
“supercritical fluid” (not a liquid; not a gas). Increasing
the pressure normally would convert a gas to a liquid
but doesn’t happen.
5. Supercritical fluids have enhanced solvation abilities, i.e.
the fluid will dissolve greater amounts of solute than
normal.
80
Vapour pressure
of liquids and solids
• Particles escape from liquid surface – evaporation
• Particles get back to the liquid – condensation
• When rate of evaporation = rate of condensation,
dynamic equilibrium is set up
Liquid
vapour
Vapour pressure can
thus be measured. This
vapour pressure varies
with temperature.
81
Vapour pressure of different
liquids
82
Relatioship between vapour
pressure and temperature
pressure
Liquid
1 atm.
Vapour
Liquid and vapour
in equilibrium only
in a particular temp.
and pressure
It is also a boiling point
curve
100oC
Temperature
83
Vapour pressure and boiling
point
• Liquid boils when its
vapour pressure is
equal to the external
pressure.
• When external pressure
is higher, boiling point is
higher.
84
Pressure cooker
85
Sublimation curve of water
pressure
Liquid
1 atm.
Vapour
It is also a boiling point
curve
solid
100oC
Temperature
86
Melting point curve
pressure
Melting point curve of
water has a negative slope:
higher the pressure, lower
the melting point.
Liquid
1 atm.
solid
Vapour
100oC
Temperature
87
Phase Diagram of water – the
melting point curve
• Ice-skater apply
pressure to the ice,
thereby melting the ice
(at a lower temperature)
and water serves as the
lubricating agent
88
Phase diagram of water
•
•
•
•
•
Each area denote a phase.
In each area, P and T can
be varied independently
without altering the one –
phase condition.
Line TB is a sublimation
curve
Line TC is supercooling
curve
Critical point is the highest
temperature and pressure
where gas cannot be
liquefied by applying
pressure alone.
89
Partial Pressures
• The atmospheric air can be treated as an ideal-gas mixture
whose pressure is the sum of the partial pressure of dry air
Pa and that of the water vapor Pv,
P = Pa + Pv
90
For Saturated Air, Vapor Pressure is Equal
to the Saturated Pressure of Water
91
Saturated Air
• There is a limit on the amount of vapor the air can hold at a
given temperature. Air that is holding as much moisture as it
can is called saturated air.
92
Phase diagram
Phase diagrams describe all six phase transitions. At the end of the gas-liquid
(vapor pressure curve) is the critical point. This is where the density of liquid
and the density of gas become the same. The pressure at this temperature
is the critical pressure.
93
Phase diagram of water
•AD is the vapor pressure curve; AC is the melting point curve; BA is the
ice-water vapor curve. BA describes the only phase change possible below
0°C and 1 atm, which is the equilibrium between ice and water vapor. On a
cold day, snow can disappear if the water vapor pressure is low, especially if
the sun warms up the ice. This is because sublimation is endothermic.
•The AC slope is negative (CA falls from left to right). This is because the
density of liquid water is higher than that if solid water. This property also
allows ice to float on water, which is essential for life in oceans and ponds.
94
Phase diagram of CO2
Notice that the AC line has a positive slope. This is a usual observation because
in most substances the density of the solid phase is higher than the density of
the liquid phase.
Also note that temperatures below -57°C and pressures below 5 atm the only
equilibrium possible is between solid and gas. This is why dry ice sublimates
at pressures near 1 atm.
95
Water is an unusual solvent that is essential for life
•The most unusual property of water is that ice is less dense than liquid water.
•Water has a very tight inter-molecular interactions, and it takes a great
deal of energy to heat it or cool it, or vaporize liquid water.
•This is mostly due to the high number of hydrogen bonds that can form
between water molecules.
Structure of hexagonal ice
-6 molecules of water/closed ‘ring’ of ice
-Pink dots are hydrogen bonds, gray lines covalent bonds
-Well-ordered, rigid structure
-The hydrogen bond O...H is 1.8 angstroms. A=10-10 meters.
96
Liquid water
-dynamic structure, NOT a lattice.
-on average, each O..H bond persists about 10 psec
- O...H (hydrogen) bond length is < 1.8 Angstroms.
The hydrogen bond length differences mean that liquid water is less dense
than hexagonal ice.
97
Solid  liquid water transition
•As ice melts, around 15% of the hydrogen bonds are broken and the lattice
collapses.
•Some water molecules are free to diffuse into the spaces that used to be
empty in the ice lattice. The result is that the density (g/ml) increases.
•The density of solid water at 0°C is 0.917 g/ml.
•The density of liquid water at 0°C is 0.998 g/ml. This is why only 10% of
an ice cube in 0°C water is above water.
•The density increases until at 4°C it is 1.000 g/ml.
•At temperatures above 4°C, the density decreases gradually, in accord with
most substances.
Implication for marine life:
- Water that is frozen in winter, in a deep enough lake, will float on the surface
and will insulate the rest of the water from cold air. So the water below the
ice can be kept above 0°C and the fish don’t freeze.
- When the air warms up and melts the ice, the water at the surface become
more dense. When the temperature at the surface reaches 4°C, the water
layer is heaviest and it sink to the bottom of the lake. With it it carries oxygen
and other nutrients that are essential for life near the bottom of the lake.
- Water properties allow a natural “turnover” of the water layers and this
distributes nutrients throughout the lake.
98
TRANSITIONS BETWEEN
PHASES
See the phase diagram for water.
Lines connect all conditions of T and P where
EQUILIBRIUM exists between the phases on
either side of the line.
(At equilibrium particles move from liquid to gas as
fast as they move from gas to liquid, for example.)
Phase Diagram for Water
Animation of
solid phase.
100
Phase Diagram for Water
Animation of
equilibrium
between solid
and liquid
phases.
101
Phase Diagram for Water
Animation of
liquid phase.
102
Phase Diagram for Water
Animation of
equilibrium
between liquid
and gas
phases.
103
Phase Diagram for Water
Animation of
gas phase.
104
Phase Diagram for Water
Animation of
equilibrium
between solid
and gas
phases.
105
Phase Diagram for Water
Animation of
triple point.
At the TRIPLE POINT all three phases
are in equilibrium.
106
Phases
Diagrams—
Important Points
for Water
Normal boil point
Normal freeze point
Triple point
T(C)
100
0
0.0098
P(mmHg)
760
760
4.58
107
TRANSITIONS
BETWEEN PHASES
As P and T increase, you finally reach
the CRITICAL T and P
108
TRANSITIONS
BETWEEN PHASES
As P and T increase, you finally reach
the CRITICAL T and P
Pcritical
Note that line
goes straight up
High Pressure
.
LIQUID
Tcritical
GAS
High Temperature
109
TRANSITIONS
BETWEEN PHASES
As P and T increase, you finally reach
the CRITICAL T and P
Pcritical
Note that line
goes straight up
High Pressure
.
LIQUID
Tcritical
GAS
Above critical T
no liquid exists
no matter how
high the
pressure.
High Temperature
110
Critical T and P
COMPD
H2O
CO2
CH4
Freon-12
(CCl2F2)
Tc(oC)
374
31
-82
112
Pc(atm)
218
73
46
41
Notice that Tc and Pc depend on
intermolecular forces.
111
Solid-Liquid Equilibria
In any system, if you increase P the DENSITY will
go up.
Therefore — as P goes up, equilibrium favors
phase with the larger density (or SMALLER
volume/gram).
Density
cm3/gram 1
Liquid H2O
Solid H2O
1 g/cm3
0.917 g/cm3
1.09
112
Solid-Liquid Equilibria
In any system, if you increase P the DENSITY will
go up.
Therefore — as P goes up, equilibrium favors
phase with the larger density (or SMALLER
volume/gram).
Density
cm3/gram 1
ICE
favored at
low P
Liquid H2O
Solid H2O
1 g/cm3
0.917 g/cm3
1.09
LIQUID H 2O
favored at
high P
113
Solid-Liquid Equilibria
ICE
LIQUID H 2O
favored at
low P
P
Solid
H2O
favored at
high P
Liquid
H2O
Normal
freezing
point
760
mmHg
0 C
T
114
Solid-Liquid Equilbria
Raising the pressure at
constant T causes
P
water to melt.
The NEGATIVE SLOPE
of the S/L line is unique
to H2O. Almost
everything else has
positive slope.
Solid
H 2O
Liquid
H 2O
Normal
freezing
point
760
mmHg
0C
T
115
The behavior of water
under pressure is an
example of
LE CHATELIER’S
PRINCIPLE
At Solid/Liquid
equilibrium, raising P
squeezes the solid.
It responds by going to
phase with greater
density, i.e., the liquid
phase.
Solid-Liquid
Equilbria
P
Solid
H2O
Liquid
H2O
Normal
freezing
point
760
mmHg
0C
T
116
Solid-Vapor Equilibrium
At P < 4.58 mmHg and T < 0.0098  C
solid H2O can go directly to vapor. This
process is called SUBLIMATION
This is how a frost-free refrigerator works.
117
118
http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html
119
BST10511
Thermodynamics
PSYCHROMETRY CHART
120
Sling Psychrometer
121
PSYCHROMETRY CHART
The properties are :
•dry bulb temperature
•wet bulb temperature
•dew point temperature
•relative humidity
•total heat of the air
•vapor pressure
•the actual moisture content of the air
•specific volume
122
Psychrometric Charts
• The properties of atmospheric air at a specified total
pressure are presented in the form of easily readable charts,
called psychrometric charts. The lines of constant enthalpy
and the lines of constant wet-bulb temperature are very
nearly parallel on these charts.
123
Identification of Lines and Scales on a
Psychrometric Chart
124
Dry Bulb (db) Temperature Lines
The db lines extend vertically upward from the sole.
Usually there is a single line for each degree of db
temp.
125
Wet Bulb (w.b.) Temperature Lines
The w.b. temp scale is located along the instep of the chart.
These lines extend diagonally downward to the right from the instep.
There is an individual line for each degree of w.b. temp.
126
Relative Humidity Lines
The relative humidity (RH) lines are the only
curved lines The various % of RH is indicated
directly on the lines.
There is no coordinate scale.
127
Absolute Humidity Lines Moisture Content (w) kg/kg (dry air)
The (w) line is located on the heel of the chart and extends vertically from the
sole to the top of the chart.
128
Dew Point Temperature Lines
The dew point temperature lines and the wet bulb
temperature lines are the same. The dew point
temperature lines extend horizontally to the
right to the back of the chart.
129
Specific Volume (v) Lines (m3/kg)
The (v) scale is located along the sole and the back of the chart.
The (v) lines extend diagonally upward to
the left from the sole and heel to the instep of the chart.
130
Specific Enthalpy (h) Lines (kJ/kg)
The enthalpy scale is located along the instep of the chart.
The (h) is a measure of the total heat of the air.
The (H) is a quick means of finding the quantity of sensible and
latent heat changes
131
When all 7 of these charts are placed together the
complete psychrometric chart e.g. CIBSE Chart
132
Schematic for Psychrometric Chart
133
Dry-Bulb, Wet-Bulb, and Dew-Point
Temperatures Identical for Saturated Air
Quality is related to the horizontal differences of P-V and T-v diagrams
134
A typical winter room condition of
21oC d.b. and 50% saturation is shown below along
with the other properties. Find other properties for
summer condition at 25oC d.b. & 50% RH
135
Example
try these indoor outdoor design conditions in
class
Conditions
Dry bulb/wet Enthalpy
o
bulb C/RH% kJ/kg
Summer
indoor
Summer
outdoor
Winter
outdoor
25/50%
Moisture
content
kg/kg
Dew point
temp
o
C
Specific
volume
3
m /kg
33/28
8/40%
136
Homeworks
Reading
• Tackling the Triple Point (look for it in)
• http://www.sciam.com/search/
• Using a Psychrometric Chart to Describe Air Properties
• http://ohioline.osu.edu/aex-fact/0120.html
137
1. Identify the changes in processes indicated in the following
phase diagram
138
3. A substance described by the same
phase diagram at normal
conditions is:
a. solid
b. liquid
c. gas
d. neither
not enough information
4. When heated at normal pressure
starting at 25°C, the substance
could:
a. sublime
b. boil
c. freeze
d. only increase its temperature
without phase transition
liquefy
139
ANSWER THE FOLLOWING QUESTIONS
1. Draw the phase diagram of a typical substance and show the
zones where gas, liquid, solid, and supercritical fluid are found.
2. Show in detail (with a drawing) why groups of atoms cannot
rotate about double or triple bonds.
3. Explain why hydrogen bonding in water makes possible life on
earth as we know it.
4.
Draw the temperature-density diagram for water and ice below
about 10 ºC and use it to show why (a) ice floats; (b) the coldest
water of a pond is found at the top during winter; and (c) there
are pairs of temperatures of water with the same density.
140