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NON-CONVENTIONAL REFRIGERATION (MAGNETIC REFRIGERATION)
A Technical Paper
K.Anil Kumar. L.Subba rao
[email protected] [email protected] Contact no :9866686300
(IV/IV B.Tech Mechanical Engineering)
RVR & JC College of engineering,
Guntur.
ABSTRACT :
The objective of this effort is to determine the feasibility of designing,
fabricating and testing a sensor cooler, which uses solid materials as the refrigerant.
These materials demonstrate the unique property known as the magneto caloric effect,
which means that they increase and decrease in temperature when
magnetized/demagnetized. This effect has been observed for many years and was
used for cooling near absolute zero. Recently, materials are being developed which
have sufficient temperature and entropy change to make them useful for a wide range
of temperature applications. The proposed effort includes magneto caloric effect
material selection, analyses, design and integration of components into a preliminary
design. Benefits of this design are lower cost, longer life, lower weight and higher
efficiency because it only requires one moving part - the rotating disk on which the
magneto caloric material is mounted. The unit uses no gas compressor, no pumps, no
working fluid, no valves, and no ozone-destroying chlorofluorocarbons/hydro
chlorofluorocarbons (CFC's/HCFC's). Potential commercial applications include
cooling of electronics, super conducting components used in telecommunications
equipment (cell phone base stations), home and commercial refrigerators, heat
pumps, air conditioning for homes, offices and automobiles, and virtually any place
that refrigeration is needed.
INTRODUCTION:
Refrigeration:
Definition: Refrigeration is the process of reducing the temperature of any substance
below that of the surrounding temperature using some working medium called
refrigerants.
Initially refrigeration was used in the preservation of foodstuff by preventing
bacterial action and this technology was further developed and extended its use in
industrial applications. For example cool cutting oil helps in machining operations by
lowering the temperature of work piece to prevent overheating, Quenching baths for
heat treating operations, pharmaceutical field, etc are some of the industrial
applications.
Conventional Refrigeration Vs Non-conventional (Magnetic) Refrigeration :
In conventional refrigeration system we need a medium for the removal of
heat from the refrigerator to the surrounding atmosphere. This medium may be a
solid, liquid or a gas. Some of the refrigerants which were used initially are ammonia
(NH3), carbon dioxide (CO2), sulphur dioxide (SO2), etc. There are some drawbacks
in the use of these refrigerants so refrigerants like F-11, F-12, F-22, F-113, etc are
being used which are both economical as well as efficient.
Minimum temperature that can be obtained by these refrigerants is 0.71oK by
boiling liquid helium under the smallest pressure obtainable. Temperatures below this
range can be obtained only by the use of Non-Conventional refrigeration system.
Magnetic refrigeration is the method of refrigeration based on
MAGNETOCALORIC EFFECT, which is defined as the response of a solid to an
applied magnetic field, which is apparent as a change in its temperature.
Instead of ozone-depleting refrigerants and energy-consuming compressors
found in conventional vapor-cycle refrigerators, this new style of refrigerator uses
iron ammonium alum that heats up when exposed to a magnetic field, then cools
down when the magnetic field is removed.
NON-CONVENTIONAL REFRIGERATION :
TYPES INCLUDE :
1. Thermo Electric Refrigeration.
2. Acoustic Refrigeration.
3. Magnetic Refrigeration.
MAGNETIC REFRIGERATION:
PRINCIPLE:
Magnetic refrigerants heat up when they are subjected to a magnetic field
because the second law of thermodynamics states that the entropy - or disorder - of a
closed system must increase with time. This is because the electron spins in the atoms
of the material are aligned by the magnetic field, which reduces entropy. To
compensate for this, the motion of the atoms becomes more random, and the material
heats up. In a magnetic refrigerator, this heat would be carried away by water or by
air. When the magnetic field is turned off, the electron spins become random again
and the temperature of the material falls below that of its surroundings. This allows it
to absorb more unwanted heat, and the cycle begins again.
Producing very low temperature through the process of adiabatic
demagnetization can do refrigeration. The paramagnetic salt is suspended by a thread
in a tube containing a low pressure of gaseous helium to provide thermal
communication with the surrounding bath of pumped helium. In operation the liquid
helium bath is cooled by pumping to the lowest practical pressure, usually achieving a
temperature in the neighborhood of 1oK. The temperature of the paramagnetic salt
approaches that of the helium bath by conduction through the exchange gas.
Next the magnetic field is turned on, causing heating of the salt and a decrease
in entropy of the magnetic ions by virtue of their partial alignment in the direction of
the applied field. The heat produced is conducted to the surrounding bath of liquid
helium so that the temperature again approaches 1oK. If the magnetic field is
increased slowly the heat can flow out, as it is generated-the magnetization being
almost isothermal. Next the exchange gas surrounding the sample is removed by
pumping, and now, with the salt thermally isolated, the magnetic field is turned off.
The temperature of the sample decreases markedly as a consequence of the adiabatic
demagnetization, which allows the magnetic ions to regain some of their entropy at
the expense of the lattice energy of the salt.
The iron ammonium alum salt, originally in zero field (H=0,S=S1), is
magnetized isothermally at the temperature T1, by increasing the magnetic field to
H=H1.This magnetization, by orienting the magnetic ions of the salt and thus
decreasing their disorder, causes a reduction in entropy from S1 to S2. Now the salt is
isothermally isolated from its surroundings and thus when the magnetic field is
reduced to zero the process follows the horizontal isentropic line and the temperature
falls to 10K.The great decrease in temperature and the close approach zero is a
consequence of the peculiar shape of the entropy-temperature relation
WORKING
The process flow diagram for the Magnetic Refrigeration
system is show in the figure below. The mixture of water and
ethanol serves as the heat transfer fluid for the system. The fluid
first passes through the hot heat exchanger, which uses air to
transfer heat to the atmosphere. The fluid then passes through the
copper plates attached to the non-magnetized cooler Magneto
caloric beds and loses heat.
CYCLE FOR MAGNETIC REFRIGERATION IN POSITION 1:
A fan blows air past this cold fluid into the freezer to keep the
freezer temperature at approximately 0°F. The heat transfer fluid
then gets heated up to 80°F as it passes through the copper plates
adjoined by the magnetized warmer Magneto caloric Beds, where
Driving Shaft
Magnetocaloric BedCopper Plates
Electro magnets
Vessel
Pump
ToFan
To Atmosph
ere
Cold Heat Exchanger
Hot Heat Exchanger
it continues to cycle around the loop. However, the Magneto
caloric beds simultaneously move up and down, into and out of
the magnetic field.
CYCLE FOR MAGNETIC REFRIGERATION IN POSITION 2 :
Figure below, shows how the cold air from the freezer is blown into the refrigerator by
the freezer fan. The temperature of the refrigerator section is kept around 39°F.
Driving Shaft
Magnetocaloric BedCopper Plates
Electro Magnets
Vessel
Pump
To Fan
To Atmosph
ere
Cold Heat Exchanger
Hot Heat Exchanger
The typical household refrigerator has an internal volume of 21 cu.ft, where
the freezer represents approximately 30% of this volume. Freezers are designed to
maintain a temperature of 0oF. Refrigerators maintain a temperature of 39oF. The
refrigerator will be insulated with polyurethane foam, one of the most common forms
of insulation available. The refrigerator is kept cool by forcing cold air from the
freezer into the refrigerator by using a small fan. The control system for machining
the desired internal temperatures consists of two thermostats with on/off switches.
FIG : REFRIGERATOR WITH A FREEZER FAN
The freezer thermostat regulates the temperature by turning the compressor
off when the temperature gets below 0oF. A second thermostat regulates the fan that
cools the refrigerator to 39oF.
Driving Shaft
Magneto caloric Bed
Copper Plates
Electro Magnets
Vessel
Pump
To Atmosph
ere
To Fan
Freezer 0°F
Refrigerator 39°F
Refrigerator fan
Freezer fan
ROOM AIR
RESULTS OBTAINED FOR VARIOUS MAGNETIC FIELDS :
FIG : ENTROPY-TEMPERATURE DIAGRAM FOR IRON AMMONIUM ALUM
The above figure is an entropy-temperature diagram for iron ammonium alum
for various magnetic fields; on it is super imposed the refrigeration cycle ABCDA.
From A to B the working salt is magnetized isothermally and heat is absorbed by the
liquid alcohol-water bath. From B to C the salt is demagnetized isentropically,
causing a substantial decrease of temperature. From C to D the salt is demagnetized
isothermally, extracting heat from the experimental region. From D to A the salt is
remagnetized to starting condition.
ADVANTAGES :
1. Very low temperatures of the order of 001K can be obtained.
2. Required pressures are obtained without the aid of a compressor.
3. Does not produce toxic gases and chloro-fluoro carbons, thus reducing ozone
layer depletion.
4. Efficiency and compactness are increased where as power consumption is
reduced.
5. Larger temperature swings that will allow the technology to provide the
Cooling power required for specific markets, such as home refrigerators, air
conditioning, electronics cooling, and fluid chilling can be obtained.
6. The unit runs virtually silent and is vibration free.
7. The magnetic material in the regenerator bed will ever need to be replaced
when changing refrigerant to achieve a different temperature range.
8. When a better magnetic material is developed, the refrigerator will not need to
be redesigned.
APPLICATIONS: 1. It is used in large-scale refrigeration, food processing, heating& air-
conditioning, liquor distilling, grain drying, waste separation and treatment
systems.
2. Magnetic refrigerator can be utilized in actual engineering applications, such
as cooling sensitive electronics and optical devices on board spacecraft.
REFERENCES:
1. A textbook on “CRYOGENIC ENGIMEERING” by V.J. Johnson.
2. “Refrigeration and Air-conditioning” by Arora & Domkundwar.