reversible processes, non-equilibrium, thermodynamic temperature scale, carnot cycle, carnot...

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.



• 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:

• State Variables are only measurable when the system is in Equilibrium.

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

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.

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



Nicholas Léonard Sadi Carnot1796 - 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:



• 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 high-temperature reservoir (during the transition between points A and B) as QH.

• We will denote the heat transferred to or from the low-temperature 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 low-temperature 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 high-temperature 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 high-temperature reservoir:


• 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:


Effc = ηc

ηth = < ηc irreversible engine

=ηc reversible engine

>ηc impossible engine

⎧

⎨⎪

⎩⎪

⎫

⎬⎪

⎭⎪

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 67.2800 62.1

700 56.7

701 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 T3

Rev HEB

Rev HEC

Thermal Reservoir T1

Rev HEA

WA

WB

WC

Q1

Q2

Q3

Q2

Q1

Q3

HE(A+B) = HE (C)

Q1

Q2

= Q1Q2

Q2Q3

and f(T1, T3) = f(T1,T2 )f(T2 ,T3)

so Q1

Q3

= f(T1,T3) or QH

QL

⎛

⎝⎜⎞

⎠⎟rev

= TH

TL

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):


• 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):


• 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 one-third 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.



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



Contraction. Now the majority of the expanded gas has been shifted to the cool cylinder. It cools and contracts, drawing both pistons outward.



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.

• 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

http://www.keveney.com/bibliography.html

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 (F-32) 5/9=C (actually you can get very close by subtracting 30 and dividing by 2).

Anders Celsius

• 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.



• 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.

• 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 ice-cream 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).

• 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 Refrigeratorsif 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:

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


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

Top ten refrigerators:

1. Sub-zero

2. GE

3. Whirlpool

4. LG

5. Amana

6. Kenmore

7. Kitchenaid

8. Frigidaire

9. Maytag

10. Samsung




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.

Carnot Refrigerator and Carnot Heat Pump

Co PR = 1

QH / QL - 1, Co PHP =

1

1 - QL / QH

Co PR,Re v = 1

TH / TL - 1, Co PHP,Re v =

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.



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

• 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 S = 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.

Carnot relationships:

CoPR,rev = 1

TH /TL - 1 and CoPHP,rev =

1

1 - TL /TH

CoPR = 1

QH /QL - 1 and CoPHP =

1

1 - QL /QH

These are the highest possible CoP that a refrigerator or a heat pump can achieve for these two temperature limits.

CoPR =

< CoPR,rev irreversible refrigerator

= CoPR,rev reversible or Carnot refrigerator

> CoPR,rev impossible refrigerator

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reversible processes, non-equilibrium, thermodynamic temperature scale, carnot cycle, carnot...

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