Week 5
First Law of Thermodynamics
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First Law of Thermodynamics
⚫Conservation of energy principle:
⚫Energy can be neither created nor
destroyed;
⚫it can only change forms.
⚫Conservation of energy
the total
amount of energy in an isolated system
remains constant
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The 1st law
the application of the conservation of
energy principle to heat and thermodynamic processes;
e.g
⚫-the kinetic energy of a moving car is converted into
heat energy at the brakes and tire surfaces.
⚫-when chemical energy is released in burning and is
converted into light and heat energy.
⚫-an object can be lifted by heating a rubber band.
Heat is converted into gravitational potential energy
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NOTE:
⚫Process
changes in the properties of that system.
⚫When a very small part of a process occurs in a
system, and is accompanied by a correspondingly very
small change in the properties of the system
⚫-A large (finite) change in the quantity X
⚫-A small (infinitesimal) change in X
dX
ΔX
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⚫Q: -denotes heat, it has units of energy (it is an
energy interaction): Joules (J) or kJ (kilo-Joules).
-energy in transit due to the thermal interaction
(temperature difference between the system and
its surroundings).
⚫W: -denotes work interaction, it has units of energy
(it is also an energy interaction): Joules (J) or
kilo-Joules (kJ).
-energy in transit due to mechanical interaction
between the system and its surroundings
⚫E: -Energy contained by the system (stored energy),
property of the system.
⚫How much work can we possibly get out of heat?
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Energy balance
⚫E: represents the change in energy experienced by
the system. If the system experience a change from
state 1 to state 2, then
In many cases in thermodynamic analysis
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c
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⚫Let say u = f(T,v)
⚫Integratin
g
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c
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⚫means T2 = 300 Κ= T1
⚫means Q = 0
P2 =?
T2 = 300K
V2 = 0.05 m3 +0.03 m3
= 0.08 m3
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Example: Throttling Valves
Throttling valves are any kind of flowrestricting devices that cause a
significant pressure drop in the fluid.
The pressure drop in the fluid is often
accompanied by a large drop in
temperature, and for that reason
throttling devices are commonly used in
refrigeration and air-conditioning
applications.
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The temperature of an ideal gas does
not change during a throttling
(h = constant) process since h = h(T).
During a throttling process, the enthalpy
of a fluid remains constant. But internal
and flow energies may be converted to
each other.
Energy
balance
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5–58C Why are throttling devices commonly used in refrigeration
and air-conditioning applications?
5–59C Would you expect the temperature of air to drop as it
undergoes a steady-flow throttling process? Explain.
5–60C Would you expect the temperature of a liquid to change as it
is throttled? Explain.
5–61C During a throttling process, the temperature of a fluid drops
from 30 to 20 °C. Can this process occur adiabatically?
5–62 Refrigerant-134a is throttled from the saturated liquid state at
700 kPa to a pressure of 160 kPa. Determine the temperature drop
during this process and the final specific volume of the refrigerant.
Answers: 42.3 °C, 0.0345 m3/kg
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Example: Heat exchangers
⚫Heat exchangers are
devices where two
moving fluid streams
exchange heat without
mixing. Heat exchangers
are widely used in
various industries, and
they come in various
designs.
A heat exchanger can be as
simple as two concentric pipes.
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Mass and energy balances for the
adiabatic heat exchanger in the figure
is:
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The heat transfer associated with a heat
exchanger may be zero or nonzero depending
on how the control volume is selected.
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Mass and Energy balances for a steady-flow process
Mass balance
A water
heater in
steady
operation.
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5–76 A heat exchanger is to heat water (cp= 4.18 kJ/kg·°C) from 25 to
60 °C at a rate of 0.2 kg/s. The heating is to be accomplished by
geothermal water (cp= 4.31 kJ/kg·°C) available at 140 °C at a mass
flow rate of 0.3 kg/s. Determine the rate of heat transfer in the heat
exchanger and the exit temperature of geothermal water.
5–78 A thin-walled double-pipe counter-flow heat exchanger is used
to cool oil (cp= 2.20 kJ/kg·°C) from 150 to 40 °C at a rate of 2 kg/s by
water (cp= 4.18 kJ/kg·°C) that enters at 22 °C at a rate of 1.5 kg/s.
Determine the rate of heat transfer in the heat exchanger and the exit
temperature of water.
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