Reversible processes, non-equilibrium,
Thermodynamic Temperature Scale,
Carnot Cycle, Carnot Refrigerator, and Heat Pump
In thermodynamics, a thermodynamic system is said to be
in thermodynamic equilibrium when it is in thermal
equilibrium, mechanical equilibrium, and chemical
equilibrium.
•Two systems are in thermal equilibrium when their
temperatures are the same.
•Two systems are in mechanical equilibrium when their
pressures are the same.
•Two systems are in diffusive equilibrium when their
chemical potentials are the same..
Quasi-Equilibrium Processes:
• A process is call a quasi-equilibrium process if the
intermediate steps in the process are all close to
equilibrium.
• In this way we can characterize the intermediate states of
the process using state variables (such as temperature,
pressure, volume, entropy, etc.)
• When a process is quasi-equilibrium we can plot the
path of the process on say a pressure vs. volume work
diagram since all the variables used to characterize the
substance's intermediate states have well define
values.
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State Variable:
Examples of
State Variables:
Temperature, Pressure,Volume
Entropy, Enthalpy, Internal Energy,
Mass Density
• State Variables are Path Independent: meaning that
the change in the value of the state variable will be the
same no matter what path you take between the two
states.
• This is not true of either the work W or the heat Q.
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• If a system is carried through a cycle that returns it to its
original state, then a variable will only be a state variable if
the variable returns to its original value.
• If X is a State Variable then:
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• State Variables are only measurable when the system is
in Equilibrium.
Reversible Processes:
• A process is reversible when the successive states of the
process are Infinitesimally close to Equilibrium States. i.e.
the process is in quasi-equilibrium.
• With a reversible process it is possible to restore the
system to its original state without needing an external
agent or changing its surroundings.
• Reversible processes are an abstraction that aids the
analysis of real processes.
• A reversible process is a standard of comparison for an
actual system.
• Truly reversible thermal processes would require an
infinite amount of time for completion.
• Imagine a cylinder, with a perfectly smooth piston, which
contains gas.
• If you push with a force only just large enough to
overcome the internal pressure, the volume will start to
decrease slowly.
• If you decrease the force only slightly, the volume will start
to increase.
•This is the hallmark of a reversible process: an infinitesimal
change in the external conditions reverses the direction of
the change.
• Heat flow is only reversible if the temperature difference
between the bodies is infinitesimally small.
• Reversible processes require the absence of friction or
other hysteresis effects.
• They must also be carried out infinitesimally slowly.
• Otherwise pressure waves and finite temperature
gradients will be set up in the system, and irreversible
dissipation and heat flow will occur.
• Because reversible processes are very slow, the system
is always very nearly in equilibrium at all times.
• In that case all its state variables are well defined and
uniform, and the state of the system can be represented
on a plot of, for instance, pressure versus volume.
Intermediate
states for an
irreversible
process is
indeterminate,
therefore these
processes are
often shown by a
dotted line
joining the initial
and final states.
• In a reversible process the state of a working fluid and the
system's surroundings can be restored to the original
ones.
• This requires that the working fluid goes through a
continuous series of equilibrium states.
• There are no truly reversible processes in practice.
The real processes are all irreversible.
• However, there are some processes that can be assumed
internally reversible with good approximation, such as some
processes in cylinders with reciprocating piston.
• The working fluid is always in an equilibrium state in an
internally reversible process.
• But the surroundings undergo a state change that can
never be restored.
• A reversible process between two states may be shown by
a continuous curve on any diagram of properties. Different
points on the curve represent the intermediate states.
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The work input to a system during a reversible process is:
W= Marked area on the P-V diagram.
and the heat supplied to a system during a reversible
process is: Q= Marked area on the T-s diagram.
• This may be simply illustrated by imagining a
cylinder with a frictionless piston on the top.
• Further imagine that there is a quantity of sand
on top of the piston (exerting the pressure).
• A good approximation to a reversible process
would be realized by removing the sand one grain
at a time and carefully recording the
thermodynamic variables (temperature and
pressure in this case) after each grain of sand is
removed.
• This would be a reversible expansion and one
could individually return the grains of sand one
at a time and reproduce each intermediate state
exactly, thus reversing the transformation.
Irreversible Processes:
• All Natural processes are Irreversible.
• The path of an irreversible process is indeterminate and
cannot be drawn on a thermodynamic diagram. (We use a
hashed line to indicate the path because the intermediate
states are in non-equilibrium.)
• The Entropy of the universe always increases during an
irreversible process.
• It is always possible to restore an irreversible process to its
original state by a reversible process, but the Entropy level
of the universe can never be restored.
• An irreversible process always requires an external agent
to restore it to its original state.
• An irreversible process is one in which the
intermediate states cannot be specified by any set
of macroscopic variables and which are not
equilibrium states.
• Since the intermediate states are unknown this
process cannot be reversed.
• This may be simply illustrated by imagining a
cylinder with a frictionless piston on the top.
• Further imagine that there is a quantity of sand
on top of the piston.
• If the sand is scooped out all at once, the piston
will rapidly slide upwards.
• Inside, the gas will rapidly expand and will
contain many random currents and pockets of
varying pressure.
• Some time will pass before these internal
currents settle and the system is at equilibrium.
• One could not drop this quantity of sand back
onto the piston and expect the currents and
pressure pockets to form exactly the same but in
reverse and clearly this process cannot be
reversed.
Examples of Irreversible Processes:
Friction
Heat Flow
Unrestrained
Expansion
Melting/Boiling
Mixing
Inelastic Deformation
Chemical Reaction Current Flow
Your house getting
dirty
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Nicholas Léonard Sadi Carnot 1796 - 1832)
born: June 1, 1796 in Paris
died: August 24, 1832 in Paris
• French engineer and physicist. Developed the physical
elements of the steam engine using a thought-experiment
(carnot cycle).
He conceived that heat is a result of the movements of
small particles and calculated (a long time before R. Mayer)
the mechanical equivalent of heat.
• In his "Réflexions sur la puissance du feu et sur les
machines propres à développer cette puissance" (Paris,
1824), he showed that the work produced by a steam
engine is proportional to the heat transferred from the
boiler to the condenser, and that in general work could
only be gained from heat by a transfer from a warmer to a
colder body (shows the importance of publishing your
work).
• (Carnot's law, was later modified by R. Clausius to the
second law of thermodynamics.)
• Carnot proposed that work was generated by the
passage of caloric from a warmer to a cooler body,
with caloric being conserved in the process.
• Clausius showed, however, that heat was, in fact,
not conserved.
• Carnot qualitatively proposed the reversible Carnot
cycle, and discovered that the efficiency of a heat
engine depended only on its input and output
temperatures.
PLAY CARNOT MOVIE
The Carnot Machine
• We consider the standard Carnot-cycle machine, which can
be thought of as having a piston moving within a cylinder,
and having the following characteristics:
• A perfect seal, so that no atoms escape from the working
fluid as the piston moves to expand or compress it.
• Perfect lubrication, so that there is no friction.
• An ideal-gas for the working fluid.
• Perfect thermal connection at any time either to one of two
reservoirs, which are at two different temperatures, with
perfect thermal insulation isolating it from all other heat
transfers.
• The piston moves back and forth repeatedly, in a cycle of
alternating "isothermal" and "adiabatic" expansions and
compressions, according to the PV diagram shown below:
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• By definition, the isothermal segments (AB and CD)
occur when there is perfect thermal contact between the
working fluid and one of the reservoirs, so that whatever
heat is needed to maintain constant temperature it (the
heat) will flow into or out of the working fluid, from or to the
reservoir.
• By definition, the adiabatic segments (BC and DA)
occur when there is perfect thermal insulation between the
working fluid and the rest of the universe, including both
reservoirs, thereby preventing the flow of any heat into or
out of the working fluid.
• The isothermal curves (but not the adiabatic curves) are
hyperbolas, according to PV = nRT.
• The enclosed area (and therefore the mechanical work
done) will depend on the two temperatures ("height") and
on the amount of heat transferred, which depends in turn
on the extent of the isothermal compression or expansion
("width"), during which heat must be transferred to
maintain the constant temperature.
• We will denote the heat transferred to or from the hightemperature reservoir (during the transition between points
A and B) as QH.
• We will denote the heat transferred to or from the lowtemperature reservoir (during the transition between points
C and D) as QL (or sometimes QC).
• If a Carnot machine cycles around the path clockwise, a
high-temperature isothermal expansion from A to B, an
adiabatic expansion cooling down from B to C, a lowtemperature isothermal compression from C to D, and
finally an adiabatic compression warming up from D to A, it
functions as a heat engine, removing energy from the hightemperature reservoir as heat, transforming a portion of
that energy to useful mechanical work (the enclosed area)
done on the external world, and ejecting the remainder of
the energy as waste heat to the low-temperature reservoir.
• If a Carnot machine is driven (by an external agency, such
as a motor) around the cycle counter clockwise, an adiabatic
expansion cooling down from A to D, a low-temperature
isothermal expansion from D to C, an adiabatic compression
warming up from C to B, and finally a high temperature
isothermal compression from B to A, then it functions as
either a refrigerator or a heat pump, depending on
whether removing heat from the low-temperature
reservoir or adding heat to the high-temperature
reservoir is of primary interest.
• The mechanical energy required to force the machine
around the cycle is the work done on the machine, the area
enclosed.
Efficiency
• For a heat engine, the efficiency is the ratio of useful work
performed to the heat energy consumed from the hightemperature reservoir:
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• This ratio is the interesting one because you pay for the
fuel to obtain QH, in order to get the benefit of the work
done, W.
• For a Carnot engine, this is entirely determined by the
temperatures of the hot and cold reservoirs:
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th
 < c irreversible engine 


  c reversible engine 
  impossible engine 
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Most work producing devices (i.e. heat engines)
have efficiencies less than 40% (and many much less
than that).
• The efficiency of a Carnot heat engine increases as TH is
increased or as TL is decreased.
• As TL
0 the efficiency approaches 1.
• We can speak of “energy quality”
TH (K)
Thermal Efficiency %
925
800
67.2
62.1
700
56.7
500
39.4
350
13.4
• The higher the source temperature the higher the energy
quality.
• ~100% of work can go into heat, lower quality <<100% of
heat can go into work
• What do people mean when they say they are conserving
energy? Can energy NOT be conserved?
• What is not conserved is the “quality” of energy by
converting it to a less useful form.
• An example: A high temperature source is more useful for
power generation than is a large amount of energy at the lower
temperature (like the ocean).
Thermal Reservoir T1
Q1
HE(A+B) = HE (C)
Q1
WA
Rev HE
A
Rev HE
C
Q2
Q2
Rev HE
B
WC
Q3
WB
Q3
Thermal Reservoir T3
Q1
Q1Q 2
=
and f(T1 , T3 )  f(T1 , T2 )f(T2 , T3 )
Q2
Q 2Q 3
so
 QH 
Q1
TH
 f(T1 , T3 ) or 


Q3
TL
 Q L  rev
This final equation defines a thermodynamic
temperature scale which is the Kelvin scale. This
equation only gives us the ratio of absolute
temperatures. In 1954 at an International Conference
on Weights and Measures the triple point of water was
set at 273.16 K making one Kelvin = 1/273.16. Note
1 K = 1 °C but 0 °C = 273.16 K.
• This temperature dependence is a direct consequence of
the second law of thermodynamics and the fact that all
(ideal) heat transfers occur during isothermal expansion
and contraction, with no temperature difference between
the heat reservoir and the working fluid, so that the entropy
gained by one exactly matches the entropy lost by the
other, with no net change in entropy for the system as a
whole.
• This condition is of course an ideal one, and cannot be
met in practice by any real machine.
• Thus, the Carnot efficiency is the best possible even
theoretically; all real machines will be strictly worse than
this.
• For a Carnot machine functioning as a refrigerator (focus
is on the energy removed from the cold space), the
"effectiveness" is the ratio of the energy removed from the
low-temperature reservoir to the work required to force the
machine around its cycle (the energy consumed and paid
for):
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• The effectiveness will be greater than 1 only if
the absolute temperature of the cold reservoir is
warmer than half that of the hot reservoir.
• We can see that refrigeration to extremely cold
temperatures is very difficult.
• For a Carnot machine functioning as a heat pump, the
"effectiveness" is the ratio of the energy delivered to the
high-temperature reservoir to the work required to force
the machine around its cycle (the energy consumed and
paid for):
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• This effectiveness is also known as the coefficient of
performance ("CoP").
• For heat pumps, the effectiveness is always greater than 1.
• Electrically powered heat pumps can make economic sense
only if the effectiveness of the heat pump times the efficiency
of the electrical generation and transmission process exceeds
1.
• Otherwise, only part of the fuel burned to produce the
electricity would have to be burned to provide the heat
needed. (Modern natural gas furnaces can easily transfer
more than 95% of the combustion heat to the heated space.)
• As the temperature of the cold reservoir (the outside
temperature) declines, the CoP of the heat pump
decreases toward 1.
• Because large electrical generators produce about onethird as much electrical energy as the heat value of the fuel
they consume, as soon as the CoP is less than about 3, it
would be cheaper to burn the original fuel directly for the
heat, rather than generate electricity to operate a heat
pump.
• This limits the geographical regions where heat pumps
make economic sense. (How is this changing today?)
• The Stirling engine is a heat engine of the external
combustion piston engine type whose heat-exchange process
allows for near-ideal efficiency in conversion of heat into
mechanical movement by following the Carnot cycle as
closely as is practically possible with given materials.
• Its invention is credited to the Scottish clergyman Rev.
Robert Stirling in 1816 who made significant improvements
to earlier designs and took out the first patent.
• He was later assisted in its development by his engineer
brother James Stirling.
• The inventors sought to create a safer alternative to the
steam engines of the time, whose boilers often exploded due
to the high pressure of the steam and the inadequate
materials.
• Stirling engines will convert any temperature difference
directly into movement.
• The Stirling engine works by the repeated heating and
cooling of a sealed amount of working gas, usually air or
other gases such as hydrogen or helium.
• This is accomplished by moving the gas between hot
and cold heat exchangers, the hot heat exchanger being a
chamber in thermal contact with an external heat source,
e.g. a fuel burner, and the cold heat exchanger being a
chamber in thermal contact with an external heat sink,
e.g. air fins.
• When the gas is heated, because it is in a sealed
chamber, the pressure rises and this then acts on the
power piston to produce a power stroke. When the gas is
cooled the pressure drops and this means that less work
needs to be done by the piston to recompress the gas
on the return stroke, giving a net gain in power available
on the shaft.
• The working gas flows cyclically between the hot and cold
heat exchangers.
• The working gas is sealed within the piston cylinders, so
there is no exhaust gas (other than that incidental to heat
production if combustion is used as the heat source).
• No valves are required, unlike other types of piston
engines.
• Some Stirling engines use a separate displacer piston to
move the working gas back and forth between cold and hot
reservoirs.
• Others rely on interconnecting the power pistons of
multiple cylinders to move the working gas, with the
cylinders held at different temperatures.
• In true Stirling engines a regenerator, typically a mass of
wire, is located between the reservoirs.
• As the gas cycles between the hot and cold sides, its heat
is transferred to and from the regenerator.
• In some designs, the displacer piston is itself the
regenerator. This regenerator contributes to the efficiency of
the Stirling cycle.
Expansion. At this point, most of
the gas in the system has just
been driven into the hot
cylinder. The gas heats and
expands driving both pistons
inward.
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Transfer. At this point, the gas
has expanded (about 3 times in
this example). Most of the gas
(about 2/3rds) is still located in
the hot cylinder. Flywheel
momentum carries the
crankshaft the next 90 degrees,
transferring the bulk of the gas
to the cool cylinder
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Contraction. Now the majority
of the expanded gas has been
shifted to the cool cylinder. It
cools and contracts, drawing
both pistons outward.
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Transfer. The now contracted
gas is still located in the cool
cylinder. Flywheel momentum
carries the crank another 90
degrees, transferring the gas
to back to the hot cylinder to
complete the cycle.
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• This engine also features a regenerator, illustrated by the
chamber containing the green hatch lines.
• The regenerator is constructed of material that readily
conducts heat and has a high surface area (a mesh of
closely spaced thin metal plates for example).
• When hot gas is transferred to the cool cylinder, it is first
driven through the regenerator, where a portion of the heat
is deposited.
• When the cool gas is transferred back, this heat is
reclaimed; thus the regenerator "pre heats" and "pre cools"
the working gas, dramatically improving efficiency.3
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Absolute zero
• the zero point of the ideal gas temperature scale,
denoted by 0 degrees on the Kelvin and Rankine
temperature scales, which is equivalent to -273.16°C and
-459.67°F.
• For most gases there is a linear relationship between
temperature and pressure (see gas laws), i.e., gases
contract indefinitely as the temperature is decreased.
• Theoretically, at absolute zero the volume of an ideal gas
would be zero and all molecular motion would cease.
• In actuality, all gases condense to solids or liquids well
above this point.
• Although absolute zero cannot be reached, temperatures
within a few billionths of a degree above absolute zero have
been achieved in the laboratory.
• At such low temperatures, gases assume nontraditional
states, the Bose-Einstein and fermionic condensates.
Daniel Gabriel Fahrenheit
• Daniel Gabriel Fahrenheit (1686-1736) was the
German physicist who invented the alcohol
thermometer in 1709, and the mercury
thermometer in 1714.
In 1724, he introduced the temperature scale
that bears his name - Fahrenheit Scale.
•
Fahrenheit temperature scale (fâr'unhī")
• temperature scale in which the temperature difference
between two reference temperatures, the melting and
boiling points of water, is divided into 180 equal intervals
called degrees.
• The freezing point is taken as 32 °F and the boiling point
as 212 °F.
• William John Macquorn Rankine used it as the basis of his
absolute temperature scale, now called the Rankine
temperature scale, in 1859.
• Although the Fahrenheit scale was formerly used widely in
English-speaking countries, many of these countries began
changing to the more convenient Celsius temperature scale in
the late 1960s and early 1970s;
• a notable exception is the United States, where the
Fahrenheit scale is still in common use together with other
English units of measurement.
• Temperatures on the Fahrenheit scale can be converted
to equivalent temperatures on the Celsius scale by first
subtracting 32° from the Fahrenheit temperature, then
multiplying the result by 5/9, according to the formula (F32) 5/9=C (actually you can get very close by subtracting
30 and dividing by 2).
Anders Celsius
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• The Celsius temperature scale is also
referred to as the "centigrade" scale.
Centigrade means "consisting of or divided
into 100 degrees".
• The Celsius scale, invented by Swedish
Astronomer Anders Celsius (1701-1744),
has 100 degrees between the freezing
point (0 C) and boiling point (100 C) of
pure water at sea level air pressure.
• The term "Celsius" was adopted in 1948
by an international conference on weights
and measures.
• Celsius was not only an inventor and astronomer,
but also a physicist.
• However, the thing that made him famous is his
temperature scale, which he based on the boiling
and melting points of water.
• This scale, an inverted form of Celsius' original
design, was adopted as the standard and is used in
almost all scientific work.
• Anders Celsius died in 1744, at the age of 42.
• He had started many other research projects, but
finished few of them.
• Among his papers was a draft of a science fiction
novel, situated partly on the star Sirius.
Refrigerators
• In the kitchen of nearly every home in America there is a
refrigerator.
• Every 15 minutes or so you hear the motor turn on, and
it magically keeps things cold. Without refrigeration, we'd
be throwing out our leftovers instead of saving them for
another meal.
• The refrigerator is one of those
miracles of modern living that totally
changes life.
• Prior to refrigeration, the only way to
preserve meat was to salt it, and iced
beverages in the summer were a real
luxury.
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• The fundamental reason for having a
refrigerator is to keep food cold. Cold
temperatures help food stay fresh
longer.
• The basic idea behind refrigeration is
to slow down the activity of bacteria
(which all food contains) so that it
takes longer for the bacteria to spoil
the food.
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• For example, bacteria will spoil milk in two or three hours
if the milk is left out on the kitchen counter at room
temperature.
• However, by reducing the temperature of the milk, it will
stay fresh for a week or two -- the cold temperature inside
the refrigerator decreases the activity of the bacteria that
much.
• By freezing the milk you can stop the bacteria altogether,
and the milk can last for months.
• Ice houses were buildings used to store ice
throughout the year, prior to the invention of the
refrigerator.
• The most common designs involved underground
chambers, usually man-made, which were built close to
natural sources of winter ice such as freshwater lakes.
• During the winter, ice and snow would be taken into the
ice house and packed with insulation, often straw.
• It remains frozen for many months, often until the
following winter, and could be used as a source of ice
during summer months.
• This could be used simply to cool drinks, or allow icecream and sorbet desserts to be created.
• Ice houses allowed a trade in ice that was a major part of
the early economy of the New England region of the United
States, which saw many fortunes made by people who
shipped ice in straw-packed ships to countries and colonies
throughout the Caribbean Sea.
• Many countries can claim to be the home of the inventor of
the refrigerator, as the technology was developed over a
period of time all over the world, using different types of
technology and for different purposes.
• Claimants to the name of inventor include Oliver Evans
(USA), Jacob Perkins (USA and England), John Gorrie (USA),
Alexander Catlin Twining (USA), James Harrison (Australia)
and Carl von Linde (Germany).
• One of the first uses of "home" refrigeration was at
Biltmore Estate in Asheville, North Carolina, USA, installed
around 1895, while in commercial refrigeration the Vestey
Brothers opened one of the first refrigerated cold stores in
London the same year.
•The gas absorption refrigerator, which cools by the use
of a source of heat, was invented in Sweden by Baltzar
von Platen in 1922.
• It was later manufactured by Electrolux and Servel.
• Today it is used in homes that are not connected to the
electrical grid, and in recreational vehicles.
• The first refrigerator redesigned for home use was the
Domelre, which was manufactured in Chicago in
1913. Frigidaire brand's roots date back to the invention
of the first self-container refrigerator for household use
by Alfred Mellowes in 1915.
• The "Guardian Frigerator" as Mr. Mellowes called it,
was purchased by General Motors Corporation in 1918
and the name was changed to Frigidaire.
Refrigerator
• The basic idea behind a refrigerator is very simple: It
uses the evaporation of a liquid to absorb heat.
• You probably know that when you put water on your
skin it makes you feel cool.
• As the water evaporates, it absorbs heat, creating that
cool feeling.
• Rubbing alcohol feels even cooler because it
evaporates at a lower temperature.
• The liquid, or refrigerant, used in a refrigerator
evaporates at an extremely low temperature, so it can
create freezing temperatures inside the refrigerator.
• If you place your refrigerator's refrigerant on your skin
(definitely NOT a good idea), it will freeze your skin as it
evaporates.
There are five basic parts to any refrigerator
• Compressor
• Heat-exchanging pipes - serpentine or coiled set of pipes
outside the unit
• Expansion valve
• Heat-exchanging pipes - serpentine or coiled set of pipes
inside the unit
• Refrigerant - liquid that evaporates inside the refrigerator
to create the cold temperatures
•Many industrial installations use pure ammonia as the
refrigerant. Pure ammonia evaporates at -27 degrees
Fahrenheit (-32 degrees Celsius).
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• The compressor compresses the refrigerant gas.
• This raises the refrigerant's pressure and temperature
(orange), so the heat-exchanging coils outside the
refrigerator allow the refrigerant to dissipate the heat of
pressurization.
• As it cools, the refrigerant condenses into liquid form
(purple) and flows through the expansion valve.
• When it flows through the expansion valve, the liquid
refrigerant is allowed to move from a high-pressure zone
to a low-pressure zone, so it expands and evaporates
(light blue).
• In evaporating, it absorbs heat, making it cold.
• Imagine some creature that is able to live happily in an
oven at 400 degrees Fahrenheit.
• This creature thinks 400 °F is just great -- the perfect
temperature (just like humans think that 70 °F is just great).
• If the creature is hanging out in an oven at 400 °F, and
there is a cup of water in the oven boiling away at 212 °F,
how is the creature going to feel about that water?
• It is going to think that the boiling water is REALLY cold.
After all, the boiling water is 188 degrees colder than the 400
°F that this creature thinks is comfortable. That's a big
temperature difference!
• This is exactly what is happening when we humans deal with
liquid nitrogen.
• We feel comfortable at 70 °F. Liquid nitrogen boils at -320 °F.
• So if you had a pot of liquid nitrogen sitting on the kitchen
table, its temperature would be -320 °F, and it would be boiling
away -- to you, of course, it would feel incredibly cold.)
• The coils inside the refrigerator allow the refrigerant to
absorb heat, making the inside of the refrigerator cold.
• The cycle then repeats.
• Modern refrigerators use a regenerating cycle to
reuse the same refrigerant over and over again.
• An air conditioner is basically a refrigerator without the
insulated box.
• It uses the evaporation of a refrigerant, like Freon, to
provide cooling.
• The mechanics of the Freon evaporation cycle are the
same in a refrigerator as in an air conditioner.
• Modern refrigerators use a regenerating cycle to reuse
the same refrigerant over and over again.
• You can get an idea of how this works by again imagining
our oven creature and his cup of water.
• He could create a regenerating cycle by taking the
following four steps:
• The air temperature in the oven is 400 degrees °F.
• The water in the cup boils away, remaining at 212 °F but
producing a lot of 400 °F steam.
• Let's say the creature collects this steam in a big bag.
• Once all the water boils away, he pressurizes the steam
into a steel container.
• In the process of pressurizing it, its temperature rises to
800 °F and it remains steam.
• So now the steel container is "hot" to the creature
because it contains 800 °F steam.
• The steel container dissipates its excess heat to the air
in the oven, and it eventually falls back to 400 °F.
• In the process, the high-pressure steam in the container
condenses into pressurized water (just like the butane in a
lighter).
• At this point, the creature releases the water from the
steel pressurized container into a pot, and it immediately
begins to boil, its temperature dropping to 212 °F.
• By repeating these four steps, the creature now has a
way of reusing the same water over and over again to
provide refrigeration.
• Gas and Propane Refrigerators if you own an RV or
use a refrigerator where electricity is not available,
chances are you have a gas- or propane-powered
refrigerator.
•These refrigerators are interesting because they have
no moving parts and use gas or propane as their primary
source of energy.
• Also, they use heat, in the form of burning propane, to
produce the cold inside the refrigerator.
•Generator - generates ammonia gas
•Separator - separates ammonia gas from water
•Condenser - where hot ammonia gas is cooled and
condensed to create liquid ammonia
•Evaporator - where liquid ammonia evaporates to create
cold temperatures inside the refrigerator
•Absorber - absorbs the ammonia gas in water
Maytag MFI2568AES Stainless Steel
Bottom Freezer French Door
RefrigeratorType:
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Bottom Freezer, French Door, Total Volume:
25 cu. ft. 4 shelves, With Ice Maker, Door
Opens to Left and Right $2306 - $2550
LG LRFC25750 Refrigerator French
DoorType: Refrigerator, French Door, Total
Volume: 22.4 cu. ft. 7 shelves, With Ice
Maker, Door Opens to Left and Right
$1195 - $1665
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Viking DFBB363 Bottom Freezer
Refrigerator
Type: Bottom Freezer, Total Volume:
20.3 cu. ft. 3 shelves, With Ice
Maker, Door Opens to Left and
Right
$169 - $169
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Top ten refrigerators:
1.
Sub-zero
2.
GE
3.
Whirlpool
4.
LG
5.
Amana
6.
Kenmore
7.
Kitchenaid
8.
Frigidaire
9.
Maytag
10. Samsung
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Air Conditioner
1. The compressor compresses cool Freon gas, causing it to
become hot, high-pressure Freon gas (red in the diagram
above).
2. This hot gas runs through a set of coils so it can dissipate
its heat, and it condenses into a liquid.
3. The Freon liquid runs through an expansion valve, and in
the process it evaporates to become cold, low-pressure
Freon gas (light blue in the diagram above).
4. This cold gas runs through a set of coils that allow the
gas to absorb heat and cool down the air inside the
building.
Mixed in with the Freon is a small amount of a lightweight
oil. This oil lubricates the compressor.
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Carnot Refrigerator and Carnot Heat Pump
1
1
Co PR =
, Co PHP =
QH / QL - 1
1 - QL / QH
Co PR,Re v
1
1
=
, Co PHP,Re v =
TH / TL - 1
1 - TL / TH
These are the highest CoPs that a refrigerator or a heat
pump operating between these two temperature limits can
have.
Heat Pump Cooling Mode
• The compressor (1) pumps the refrigerant to the reversing valve (2).
• The reversing valve directs the flow to the outside coil (condenser)
where the fan (3) cools and condenses the refrigerant to liquid.
• The air flowing across the coil removes heat (4) from the refrigerant
• The liquid refrigerant by passes the first metering device and flows to the
second metering device (6) at the inside coil (evaporator) where it is
metered.
• Here it picks up heat energy from the air blowing (3) across the inside
coil (evaporator) and the air comes out cooler (7). This is the air that
blows into the home.
•The refrigerant vapor (8) then travels back to the reversing valve (9) to be
directed to the compressor to start the cycle all over again (1).
Heat Pump Heating Mode
• The diagram above shows the heat pump in heat mode.
• The difference in the two diagrams is the reversing valve
(2) directs the compressed refrigerant to the inside coil
first.
• This makes the inside coil the condenser and releases
the heat energy (3-4).
• This heated air is ducted to the home.
• The outside coil is used to collect the heat energy (3-7).
• This now becomes the evaporator.
• Both heating and A/C modes do exactly the same thing.
• They PUMP HEAT from one location to another.
• In these examples the heat in the air is moved out of or
into the home.
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Clausius, Rudolf
(1822-1888))
German physicist who reconciled the
results of Joule with the theories of
Sadi Carnot by abandoning the idea
that heat (not energy) was conserved.
He stated formally the equivalence
of heat and work (First Law of
Thermodynamics) and developed the
concept of entropy (which he named
in 1865) to explain the directionality
of physical processes.
• He discovered the fact that entropy can never
decrease in a physical process and can only remain
constant in a reversible process, a result which
became known as the Second Law of Thermodynamics.
• With Maxwell, he developed the kinetic theory of
gases.
• In "Über die Art der Bewegung welche wir Wärme
nennen" ("On the Kind of Motion which we Call
Heat" (1857), he provided a full account of the
kinetic theory of molecular motions.
• This was the first systematic treatment of the
kinetic theory.
• It used probabilistic arguments, introduced the
concept of mean free path, and correlated temperature
and velocity.
• Clausius resolved the paradox of Buys-Ballot by
explaining the motion of particles in terms of a
“random walk” resulting from many collisions.
• Clausius also extended Clapeyron’s equation.
Clausius Theorem and Inequality
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• The equality above represents the Clausius Theorem
and applies only the the ideal or Carnot cycle.
• Since the integral represents the net change in entropy
in one complete cycle, it attributes a zero entropy change
to the most efficient engine cycle.
• Real engines will have lower efficiencies.
• The Clausius Inequality applies to any real engine cycle
and implies a negative change in entropy on the cycle.
• That is, the entropy given to the environment during the
cycle is larger than the entropy transferred to the engine by
heat from the hot reservoir.
• In the simplified heat engine where the heat QH is all added
at temperature TH, then an amount of entropy DS = QH/TH is
added to the system and must be removed to the
environment to complete the cycle.
• In general, the engine temperature will be less than TH
for at least part of the time when heat is being added, and
any temperature difference implies an irreversible
process.
• Excess entropy is created in any irreversible process,
and therefore more heat must be dumped to the cold
reservoir to get rid of this entropy.
• This leaves less energy to do work.
Entropy Changes with Temperature
• The higher the temperature the greater the entropy of the system (unless
some other change such as compression compensates for the change in
temperature). This is as true for ideal gases as it is for liquids and solids.
• If the temperature is raised at a constant volume
∆S = Cv ln(Tf/Ti) ...where Tf = Tfinal and Ti = T initial
• If the temperature is raise at a constant pressure
∆S = Cp ln(Tf/Ti) ...where Tf = Tfinal and Ti = T initial
• The temperature change at constant pressure produces more entropy,
because to keep the pressure constant there is expansion as well as a
temperature increase.
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Carnot relationships:
1
1
CoPR =
and CoPHP =
QH /QL - 1
1 - QL /QH
CoPR,rev
1
1
=
and CoPHP,rev =
TH /TL - 1
1 - TL /TH
These are the highest possible CoP that a
refrigerator or a heat pump can achieve for these
two temperature limits.
 < CoPR,rev irreversible refrigerator



CoPR = = CoPR,rev reversible or Carnot refrigerator 
 > CoP

impossible
refrigerator
R,rev

