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82073930-Magnetic-Refrigeration-Assignment Powered By Docstoc


Refrigeration is the process of removing heat from an enclosed space, or from a substance, and
moving it to a place where it is unobjectionable. The primary purpose of refrigeration is
lowering the temperature of the enclosed space or substance and then maintaining that lower
temperature. The term cooing refers generally to any natural or artificial process by which heat
is dissipated. The process of artificially producing extreme cold temperatures is referred to as
cryogenics. Cold is the absence of heat, hence in order to decrease a temperature, one
“removes heat", rather than "adding cold." In order to satisfy the Second Law of
Thermodynamics, some form of work must be performed to accomplish this. This work is
traditionally done by mechanical work but can also be done by magnetism, laser or other

1.1 Methods of refrigeration:
1.2 Non cyclic refrigeration:-

In these methods, refrigeration can be accomplished by melting ice or by subliming dry ice.
These methods are used for small-scale refrigeration such as in laboratories and workshops, or
in portable coolers. Ice owes its effectiveness as a cooling agent to its constant melting point of
0 °C (32 °F). In order to melt, ice must absorb 333.55 kJ/kg (approx. 144 Btu/lb.) of heat.
Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid
carbon dioxide, known as dry ice, is used also as a refrigerant. Having no liquid phase at normal
atmospheric pressure, it sublimes directly from the solid to vapor phase at a temperature of -
78.5 °C (-109.3 °F).Dry ice is effective for maintaining products at low temperatures during the
period of sublimation.

[B] Cyclic refrigeration:-

This consists of a refrigeration cycle, where heat is removed from a low-temperature space or
source and rejected to a high-temperature sink with the help of external work, and its inverse,
the thermodynamic power cycle. In the power cycle, heat is supplied from a high-temperature
source to the engine, part of the heat being used to produce work and the rest being rejected
to a low-temperature sink. This satisfies the second law of thermodynamics.

A refrigeration cycle

Describes the changes that take place in the refrigerant as it alternately absorbs and rejects
heat as it circulates through a refrigerator. It is also applied to HVACR work, when describing
the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split
system. Heat naturally flows from hot to cold. Work is applied to cool a living space or storage
volume by pumping heat from a lower temperature heat source into a higher temperature heat
sink. Insulation is used to reduce the work and energy required to achieve and maintain a lower
temperature in the cooled space. The operating principle of the refrigeration cycle was
described mathematically by Sadi Carnot in 1824 as a heat engine. The most common types of
refrigeration systems use the reverse-Rankine vapor-compression refrigeration cycle although
absorption heat pumps are used in a minority of applications. Cyclic refrigeration can be
classified as:

1. Vapor cycle, and

2. Gas cycle Vapor cycle refrigeration can further be classified as: 1.Vapor compression

2. Vapor absorption refrigeration

[1] Vapor-compression cycle

The vapor-compression cycle is used in most household refrigerators as well as in many large
commercial and industrial refrigeration systems.

Vapor absorption cycle

In the early years of the twentieth century, the vapor absorption cycle using water-ammonia
systems was popular and widely used but, after the development of the vapor compression
cycle, it lost much of its importance because of its low coefficient of performance(about one
fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only
where waste heat is available, where heat is derived from solar collectors, or electricity is
unavailable. The absorption cycle is similar to the compression cycle, except for the method of
raising the pressure of the refrigerant vapor. In the absorption system, the compressor is
replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which
raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor
from the high-pressure liquid.

Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much
smaller than needed by the compressor in the vapor compression cycle. In an absorption
refrigerator, a suitable combination of refrigerant and absorbent is used. The most common
combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and
lithium bromide (absorbent).
[2] Gas cycle:-

When the working fluid is a gas that is compressed and expanded but doesn't change phase,
the refrigeration cycle is called a

Gas cycle

Air is most often this working fluid. As there is no condensation and evaporation intended in a
gas cycle, components corresponding to the condenser and evaporator in a vapor compression
cycle are the hot and cold gas-to-gas heat exchangers in gas cycles. The gas cycle is less efficient
than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle
instead of the reverse Rankine cycle. As such the working fluid does not receive and reject heat
at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the
specific heat of the gas and the rise in temperature of the gas in the low temperature side.
Therefore, for the same cooling load, a gas refrigeration cycle will require a large mass flow rate
and would be bulky. Because of their lower efficiency and larger bulk,

Air cycle

Coolers are not often used nowadays in terrestrial cooling devices. The air cycle machine is very
common, however, on gas turbine-powered jet aircraft because compressed air is readily
available from the engines' compressor sections. These jet aircraft's cooling and ventilation
units also serve the purpose of pressurizing the aircraft.

[c] Thermoelectric refrigeration:-

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of
two different types of materials. This effect is commonly used in camping and portable coolers
and for cooling electronic components and small instruments.

[D] Magnetic refrigeration:-

Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the
magneto caloric effect, an intrinsic property of magnetic solids. The refrigerant is often a
paramagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case
are those of the electron shells of the paramagnetic atoms. A strong magnetic field is applied to
the refrigerant, forcing its various magnetic dipoles to align and putting these degrees of
freedom of the refrigerant into a state of lowered entropy.

A heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal
contact with the heat sink is then broken so that the system is insulated, and the magnetic field
is switched off. This increases the heat capacity of the refrigerant, thus decreasing its
temperature below the temperature of the heat sink .Because few materials exhibit the
required properties at room temperature, applications have so far been limited to cryogenics
and research.

[E] Other methods

Other methods of refrigeration include the air cycle machine used in aircraft; the vortex tube
used for spot cooling, when compressed air is available; and thermo acoustic refrigeration using
sound waves in a pressurized gas to drive heat transfer and heat exchange.

[F] Unit of refrigeration

Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling. Commercial
refrigerators in the US are mostly rated in tons of refrigeration, but elsewhere in kW. One ton
of refrigeration capacity can freeze one short ton of water at 0 °C (32 °F) in 24 hours. Based on

Latent heat of ice (i.e., heat of fusion) = 333.55 kJ/kg ≈ 144 Btu/lb. One short ton = 2000 lb Heat
extracted = (2000)(144)/24 hr. = 288000 Btu/24 hr. = 12000 Btu/hr. =200 Btu/min1 ton
refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517kW

A much less common definition is: 1tonneof refrigeration is the rate of heat removal required
to freeze a metric ton (i.e., 1000 kg) of water at 0°Cin 24 hours. Based on the heat of fusion
being 333.55 kJ/kg, 1 ton of refrigeration = 13,898 kJ/h = 3.861 kW. As can be seen, 1 ton of
refrigeration is 10% larger than 1 ton of refrigeration. Most residential air conditioning units
range in capacity from about 1 to 5 tons of refrigeration.

2. Introduction to magnetic refrigeration:-

Magnetic refrigeration is a cooling technology based on the magneto caloric effect.
This technique can be used to attain extremely low temperatures (well below 1Kelvin), as well
as the ranges used in common refrigerators, depending on the design of the system .The
fundamental principle was suggested by Debye(1926) and Giauque(1927), and the first working
magnetic refrigerators were constructed by several groups beginning in
1933.Magneticrefrigerationwas the first method developed for cooling below about 0.3
Kelvin.Figure.1 Schematic diagram of magnetic refrigeration The system based on the magneto
caloric effect is shown in fig. 1. It has two rotating cylinders containing powdered Gadolinium-a
dense gray, rare earth metal and as upper conducting magnet. Each atom of gadolium has
seven (7) unpaired electrons in an intermediate shell, which gives the element a strong
magnetic moment. When a ferromagnetic material like gadolium is placed in a magnetic field,
the magnetic moments of its atoms become aligned, making the material more ordered. But,
the amount of entropy in the magnet must be conserved, so the atoms vibrate more rapidly,
raising the material temperature. Conversely, when gadolium is taken out of the magnetic field,
the material cools .The two cylinders containing gadolium metal can be made to rotate through
magnetic field and arrangements, should be made such that, the mixture of water and ethanol
is pumped into one of the cylinders of gadolium immediately after it moves out of the magnetic

The mixture cools, as it flows through the porous bed of demagnetized gadolium and then
through a heat exchanger. Next the mixture passes through the cylinder of gadolium, which is
inside the magnetic field. The stream of mixture heats up and flows through another exchanger
providing ample refrigeration power by continuously heating one exchanger and cooling the
other. Interval, after the two cylinders of gadolium compound switch takes place and flow of
mixture is reversed. The team has developed a working system that uses two beds containing
spherical powder of Gadolium with water being used as the heat transfer fluid. The magnetic
field for such system is 5 Telsa. Magnetic refrigeration is based on a fundamental
thermodynamic property of magnetic materials: the so-called magneto caloric effect, which
causes a temperature change if the material is subject to an applied magnetic field under
adiabatic conditions. The magneto caloric effect was discovered in 1881in iron by the German
physicist Emil Warburg .Usually the temperature increases when the field is applied (and
decreases when the field is removed) and the process is reversible The magneto caloric effect
can qualitatively be understood as an interaction between the entropy (which is a measure of
the disorder) associated with the spins (magnetic moments of the atoms) of the crystal lattice
and the entropy associated with the heat motion of the atoms in the lattice: an external
magnetic field tends to order the spins, thus decreasing the magnetic entropy, if the material is
isolated from its surroundings(i.e. its entropy is constant), the decrease must be compensated
by an increase of the lattice entropy, i.e. an increase in heat motion and therefore in
temperature. The magneto caloric effect is most pronounced in the vicinity of a magnetic phase
transition of the material, e.g. from a non-ordered (paramagnetic) to a ferromagnetic state. A
magneto caloric material can be used as the active element in a refrigeration apparatus. The
apparatus can for instance be operated in a four step cycle:-

(1)The magneto caloric material is magnetized by a magnet and the temperature increases.

(2)The material cools by giving off heat to the surroundings through a heat exchanger.

(3)The magnetic field is removed and the temperature of the material drops further.

(4)The material takes up heat from the cold side heat exchanger (“the inside of the
refrigerator”) thus cooling it.
Such a magnetic refrigerator has a number of advantages compared to conventional
refrigerators, e.g. environmentally hazardous refrigeration gases such as HFC
(hydroflurocarbon) or ammonia are avoided, and higher efficiency are possible. It has been
recognizes since 1920’s that magneto caloric materials can be used for cooling purposes. In the
laboratory magnetic refrigeration using paramagnetic salts is a standard technique for
obtaining ultra-low temperatures .However, for the purpose of using magnetic refrigeration
near room temperature several problems arise magneto caloric materials are only active in a
certain material -specific temperature range significantly limiting the temperature range where
the refrigerator should function. The temperature change induced by the active material is only
of the order of a few degrees, which is too small for practical purposes. To overcome this
obstacle, the simple cycle illustrated above must be modified by the use of a regenerator. By
having the magneto caloric material act as a regenerator on the cooling fluid, the device can
span a wider temperature range. This concept, called an active magnetic regenerator (AMR),
was introduced by J.A. Barclay and W.A. Steyert in the early 1980’s.Wor in particular by Ames
Laboratory(university of lowa) and astronautics Corporation in the USA have elaborated on this
idea, resulting in a number of prototype magnetic refrigerators using metals such as pure
gadolium and gadolium alloys such as the active materials. Combining these materials with high
applied magnetic fields (applied by superconducting magnet coils), temperature spans I excess
of 40 degrees may be reached. Magnetic refrigeration is a promising technology for energy
efficient and environmentally friendly cooling. The technology uses magnetic materials as the
active components and non-volatile fluids, e.g. water, for heat transfer. When magnetic
materials are subjected to an external magnetic field, their temperature rises. This so-called
magneto caloric effect is thermodynamically reversible, which means that the efficiency of a
cooling cycle based on this may be as much as 60%greater than for conventional compressor
based refrigeration systems. Magnetic refrigeration is seen as an environmentally friendly
alternative to conventional vapor-cycle refrigeration. And as it eliminates the need for the most
inefficient part of today’s refrigerators, the compressors, it should save costs. New materials
described in this issue may bring practical magneto caloric cooling a step closer large magnetic
entropy changes has been found to occur in MnFeP 0.45As0.55 at room temperature, making it an
attractive candidate for commercial application in magnetic refrigeration.

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