RE: magnetic refrigeration full report
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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.
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.
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.