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Chapter 12A: BASIC THERMODYNAMICS
AND LAWS
Agami Reddy (July 2016)
1) Basic thermodynamic concepts
2) Changes of state
3) Sensible and latent heat
4) First law of thermo for closed systems- sensible/ latent heat
5) First law for open systems- concept of enthalpy
6) Second law of thermodynamics and entropy
HCB 3- Chap 12A: Basic Thermo and Laws
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Basic Thermodynamics
It is important to know the fundamentals of
thermodynamics since it is basic to understanding
how thermal fluid devices and systems operate.
Wide range of applications:
-
Cooling and heating systems use water and air
Coal plants and nuclear use water/steam as working fluid
Natural gas use turbines use compressed air
Gasoline and diesel engines use air
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Property, Process and Cycle
– System: objects that you are studying (defined by you!).
– Property: characteristic of system such as temperature, pressure,…
– State: condition of a system as described by its properties.
• Any property change RESULTS in state changes
– Process: a change in state (one or more properties change).
• It is related to path followed
– Extensive and intensive properties:
• A property is intensive if it does not depend on the amount of the
matter present (such as pressure, specific volume, specific enthalpy,
temperature,….). Volume is an example of an extensive property.
• Property is not process related!
– Cycle: one or more processes wherein the initial and final states of the
system are identical; i.e. all the properties have the same value.
HCB 3- Chap 12A: Basic Thermo and Laws
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Basic Thermo
-Ideal and real gases
-Pure substances and mixtures
-Determining properties
steam tables
-Process: the change in two properties
usually as a result of work done
or heat added: Work= P .dV
Process path represents the change
of pressure and volume during
compression- it is shown on
on a p-v diagram
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Changes of State (Phase)
– Substances can exist in three states: solid, liquid, and gas (vapor)
– Two factors that affect state: temperature and pressure
– Process of state change
- Temperature change or phase change
- Pressure dependent
– Molecular Theory of Liquids and Gases suggested to explain
observed phenomena
– Concept of “saturated state”
subcooled, saturated and superheated
We use saturated steam tables to determine propertiesdiscussed later
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Change of State
Molecular theory or Kinetic theory
• Temperature is a measure of
average molecule speed
• Some molecules have faster
speed and escape
• Resisting pressure plays a
role
• During boiling process,
average speed reaches a
level at which the link
between molecules break
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Approximately; cp = 1.00 kJ/kg.K (0.24 Btu/lbm.0F) for dry air, 4.19
0
kJ/kg.K (1.0 Btu/lbm.
F) for Energy
liquid water, and 1.86 kJ/kg.K (0.444
Internal
Btu/lbm.0F) for water vapor.
(b) Specific internal energy u refers to the energy stored by a
substance due to the microscopic motion and/or position of the
molecules. This form of energy consists of two parts: the internal
kinetic energy due to the velocity of the molecules, and the internal
potential energy due to the attractive forces between molecules.
Changes in the average velocity of molecules are indicated by
temperature changes of the substance. Internal energy is a property
which is accurately defined from the first law of thermodynamics as
applied to closed systems Units of specific internal energy u are in
kJ/kg or Btu/lbm.
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Laws
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First Law of Thermodynamics
Energy Balance or energy conservation law
A closed system is
one where the fluid
does not cross the
system boundaries
– Different ways to express it
– Closed System: Change in the internal energy U is the difference in
the heat Q added to the system minus work done by the system
– If no work involved: the change in total energy in a system equals the
energy added to the system minus the energy removed from the system
dU = Qin – Qout
dU: change in internal or stored energy in the system
Qin: heat added (entering ) to the system
Qin
Qout: heat removed (leaving) from the system
dU
Qout
Sign: +ve if energy added to system
–ve if energy removed from system
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Sensible Heat and Latent Heat
– Why the need to distinguish between them?
• During a process with phase change, temperature is not
the only variable that determines heat transfer rate.
– What is sensible heat?
• Energy (heat) that is added or removed during a process
where the temperature of a substance changes but there
is no change in state (phase) of the substance.
– What is latent heat change?
• Energy (heat) absorbed or released during a process
involving state (phase) change.
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Sensible Heat
Sensible heat of a body is the energy associated with its
temperature.
Example- heating water
For liquids and solids, the change in stored energy when its mass
undergoes a temperature change is given by:
Qs  m  c  T  m  c  (t2  t1 )
where Qs is the stored energy, (kJ or Btu)
 is the mass (kg or lbm)
m
c is the specific heat, (kJ/kg-oC or Btu/lbm-°F)
The same equation also applies when a fluid flow is involved.
In that case, Q is the rate of heat transfer (kW or Btu/h) and
m is the mass flow rate (kg/s or lbm/h)
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Specific Heat and Heat Capacity
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Example
The temperature of 150 lb of water in a hot water tank is to be
raised by 100 oF. The heat content of one ft3 of natural gas is
1,000 Btu/ ft3. If the efficiency of the water heater is 80%, how
many ft3 of natural gas needs to be burnt?
Amount
Of heat  m.c.T  150 x1.0 x100  15, 000 Btu
Let V be the volume of gas needed.
Vx heat content x Efficiency of hot water boiler =15,000
15,000
or V=
 18.75 ft 3 of gas
1000x0.8
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Latent Heat
– How to calculate latent heat at a given temperature
• Inter-molecular force changes – Enthalpy changes
although temperature does not change
• Use latent heat equation:
Q  m  h fg
where Q is the heat transfer rate, W or Btu/h
m is the mass flow rate, kg/s or lbm/h
hfg is the latent heat of vaporization, kW or Btu/lbm
This is given as the difference between:
hf is the enthalpy of saturated liquid, kJ/kg or Btu/lbm
hg is the enthalpy of saturated vapor, kJ/kg or Btu/lbm
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Enthalpy (h)
– A property of a body that measures its heat content
– Enthalpy includes: (i) Internal energy U and
(ii) pV or energy due to flow work
– Enthalpy is a combined property which is widely used in
thermal analysis
– When T, p or V changes, H changes
– Enthalpy H = U + V.p
in kJ or Btu (V is total volume)
specific enthalpy h = u + p.v in kJ or Btu/lbm (v is specific vol.)
Instead of sensible or latent heat equations,
enthalpy equation is widely used since one
does not have to worry about state of fluid:
V
T P
Q  m  (h final  hinitial )
P- pressure
V- volume
T- temperature
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First Law of Thermodynamics contd.
– For a OPEN system:
– Extension of Bernouilli equation
On a total-mass (extensive) basis, the open-system first law is
v in2
v 2out
min ( gzin 
 hin )  Q  mout ( gzout 
 hout )  W
2
2
where
min, mout = inlet and outlet mass flow amounts, kg ( lbm)
zin, zout = inlet and outlet system port elevations, m (ft)
vin,vout = inlet and outlet fluid average velocities, m/s (ft/s)
hin, hout = inlet and outlet specific enthalpies, kJ/kg (Btu/lbm)
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First Law of Thermodynamics contd.
For a OPEN system where KE, PE and other energy sources are
negligible
- when W=0:
- when Q=0:
Q = m (hout – hin)
W = m (hin– hout)
boiler
turbine
where
hin: enthalpy of fluid entering the system
hout: enthalpy of fluid leaving the system
Sign convention for Q:
+ve when added to the system
–ve when removed from the system
Sign convention for W is opposite
Recall hfg which is the latent heat of vaporization = (hg - hf )
where hf = enthalpy of saturated liquid, kJ/kg or Btu/lbm
hg = enthalpy of saturated vapor, kJ/kg or Btu/lbm
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Example
Example: Cooling of air supplied to room
Consider a room which contains 300 kg (660 lbm) of air. If this air is to be
refreshed once each hour, and cooled from 29.4o C (85o F) to 12. 8o C (55o F),
what is the cooling capacity of the cooling coil?
• Specific heat of air cp =1.00 kJ/(kg.oC)
• Assumptions: Steady flow process, neglect changes in kinetic and potential
energies, no work is involved, density of air remains constant, duct is adiabatic
SOLUTION
Since there is no change of phase, we can use the sensible heat eqn:
Q(
300 kg/h
)  1.00 kJ/(kg. o C)  (29.4-12.8 o C)=1.38 kW (4.72 kBtu/h)
3600 s/h
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Applications of First Law of Thermodynamics
Efficiencies of
components
of a system
can be multiplied
to yield total
System efficiency
From Randolph
and Masters, 2008
HCB 3- Chap 12A: Basic Thermo and Laws
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From Randolph
and Masters, 2008
HCB 3- Chap 12A: Basic Thermo and Laws
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Source: EIA, Annual Energy Review 2004, DOE/EIA-0384(2004)
(Washington, D.C., August 2005), Diagram 5.
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All forms of energy are not equal!
Thermal energy (or heat) is a more “disordered” form of energy-
From Cengal and Boles
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Laws of Thermodynamics
In simplest terms, the Laws of Thermodynamics dictate the specifics for the
movement of heat and work.
Basically, the First Law of Thermodynamics is a statement of the conservation
of energy –
the Second Law is a statement about the quality of energy or direction of
that conservation –
and the Third Law is a statement about reaching Absolute Zero (0 K).
However, since their conception, these laws have become some of the most
important laws of all science - and are often associated with concepts far
beyond what is directly stated in the wording.
-Heat is the lowest form of energy
-Work (from which electricity is produced) is a higher form
-One unit of thermal energy at a high temperature is more VALUABLE
than the same amount of energy at a lower temperature
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Entropy
General observational statements:
a) Any system which is free of external influences becomes more
disordered with time.
b) The irrevocable loss of some energy to the environment can be
associated with an increase of disorder in that system.
The 19th century physicist, Clausius, proposed the use of a variable to
quantify disorder- entropy
Clausius worked out a general equation devoted to the measurement of
entropy change over a period of time:
entropy S = dQ / T
(the change in entropy is equal to the amount of heat added to the system
[which is an irreversible process] divided by the temperature in
Kelvin).
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Entropy contd…
• Concept of entropy: a measure of the irreversibility of the process (due
to friction, heat transfer across a temperature difference,…)
a property (just like temperature, pressure, enthalpy)
Entropy acts as a function of the state of a system :
- high amount of entropy translates into higher chaos within the system,
while
- low entropy is reflective of a highly ordered state.
Proper understanding important for energy resources sustainability
•
do not use fuel sources which can give you high temperatures for low
temperature applications- for example, using solar energy for hot
water heating is a better sustainability practice than using gas or
electricity
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Outcomes
• Understanding of basic thermodynamic definitions such as system,
process, cycle,…
• Understanding of different states and the kinetic theory of gases
• Understanding of basic thermodynamic properties such as internal
energy, enthalpy and entropy
• Understanding the difference between sensible and latent heat and
be able to solve simple problems
• Understanding of how the first law is a representation of the
conservation of energy concept
• Be able to apply the first law of thermodynamics to closed and
open system analysis
• Familiarity with the applications and insights provided by the
second law
• Familiarity with the concept of entropy
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Laws
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