mech technobeat
Sunday, April 20, 2014
Microstructures of Plain Carbon Steel
0.1% carbon steel
0.2% carbon steel
0.4% carbon steel
0.6% carbon steel
0.75% carbon steel
1.2% carbon steel
Microstructures of Non ferrous Alloys
Cast Brass
Die cast alloy Al-Zn
Gun Metal
Rolled Brass
Rolled Copper
Sand Cast Alloy(Al Si 12% modified)
Friday, April 11, 2014
Microstructures of Cast Iron
chilled cast iron
Feritic SG iron
Grey Cast Iron
Mealleable cast iron
pearlitic SG iron
white cast iron
Monday, March 17, 2014
Wednesday, March 12, 2014
Magnetic Refrigeration
ABSTRACT
Ten years from now, it is highly likely that
new room temperature magnetic refrigerators (RTMR) will be available to
consumers as a more efficient and more environmentally safe alternative to
conventional type refrigerators. Significantly, it has been estimated that
improving our Nation’s refrigerators and freezers by just 4.2 percent would
save an estimated power consumption equivalent to an average base-level power
plant (479 million kWh). Magnetic
refrigeration cycle is estimated 50% more efficient than conventional fluid
cycle, which would save the average power of 12 new base-level plants. The magneto caloric effect (MCE) of rear
earth alloys will be able to provide the adiabatic entropy change that is
required achieving a refrigeration cycle at near room temperatures, which are
presently achieved by the use of refrigerant fluids.Magnetic refrigeration is a cooling technology based on the magneto caloric effect. This technique
can be used to attain extremely low temperatures
(well below 1 Kelvin),
as well as the ranges used in common refrigerators,
depending on the design of the system. This technique has been used for many
years in cryogenic
systems for producing further cooling in systems already cooled to temperatures
of 4 Kelvin and
lower. In England, a company
called Cambridge
Magnetic Refrigeration produces cryogenic systems based on the magneto
caloric effect
The MCE is currently being explored to produce
better refrigeration techniques, especially for use in spacecraft.
There are still some thermal and magnetic hysteresis
problems to be solved to make it really useful in industrial and household
applications. This is a subject of current research.
Recent
discovery has succeeded using commercial grade materials and permanent
magnets on room temperatures to construct a magneto caloric refrigerator which
promises wide use.
Key words: Refrigeration,
Magnetic refrigeration, Magneto caloric effect (MCE),
1)
INTRODUCTION
Refrigeration is the process of removing heat from an enclosed
space, or from a substance, and rejecting it elsewhere for the primary purpose
of lowering the temperature of the enclosed space or substance and then
maintaining that lower temperature. The term cooling 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. Magnetic refrigeration, or adiabatic demagnetization, is a cooling
technology based on the magneto caloric effect. It is 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. The
magnetic refrigeration could be used in any possible application where cooling,
heating or power generation is used today. Since it is only at an early stage
of development, there are several technical and efficiency issues that should
be analyzed. Magneto caloric effect (MCE) is the emission or
absorption of heat in a magnetic material
in response to a changing magnetic
field.The magneto caloric
refrigeration system is composed of pumps, electric motors, secondary fluids,
heat exchangers of different types, magnets and magnetic materials. These
processes are greatly affected by irreversibilities and should be adequately
considered. Appliances using this method could have a smaller environmental impact if the method
is perfected and replaces hydro fluorocarbon (HFCs) refrigerators which have
considerable greenhouse effect. At present, however, the
superconducting magnets that are used in the process have to themselves be
cooled down to the temperature of liquid
nitrogen, or with even colder, and relatively expensive, liquid helium. Considering
these fluids have boiling points of 77.36 K and 4.22 K respectively, the
technology is clearly not cost-efficient and efficient for home appliances, but
for experimental, laboratorial, and industrial use only.The magnetic refrigeration
based on MCE is becoming a promising technology to replace the conventional
gas-compression/expansion technique.[a],[b].
2) THE MAGNETO CALORIC EFFECT
The Magneto caloric effect (MCE, from magnet and calorie) is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. This is also known as adiabatic demagnetization by low temperature physicists. In that part of the overall refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a chosen (magneto caloric) material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the material is isolated so that no energy is allowed to (e) migrate into the material during this time (i.e. an adiabatic process), the temperature drops as the domains absorb the thermal energy to perform their reorientation. The randomization of the domains occurs in a similar fashion to the randomization at the Curie temperature. Magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added. One of the most notable examples of the magneto caloric effect is in the chemical element gadolinium and some of its alloys. Gadolinium's temperature is observed to increase when it enters certain magnetic fields. When it leaves the magnetic field, the temperature returns to normal. The effect is considerably stronger for the gadolinium alloy alloyed with nickel (PrNi5) which has strong magneto caloric effect. [b], [e].
3) MAGNETO CALORIC CYCLE
Analogy between magnetic refrigeration and
vapor cycle or conventional refrigeration: H
= externally applied magnetic field; Q
= heat quantity; P = pressure;
ΔTad = adiabatic temperature variation.
The cycle is performed as a refrigeration cycle, analogous to the Carnot
cycle, and can be described at a starting point whereby the chosen working
substance is introduced into a magnetic
field (i.e. the magnetic flux density is increased).(Fig.1).The working
material is the refrigerant, and starts in thermal equilibrium with the
refrigerated environment. [b], [c].
a.
Steps Involved In MCE Cycle
·
Adiabatic
magnetization: The substance is
placed in an insulated environment. The increasing external magnetic field (+H) causes the magnetic
dipoles of the atoms to align, thereby decreasing the material's magnetic entropy and heat
capacity. Since overall energy is not lost yet and therefore total entropy is not
reduced according to thermodynamic laws, the net result is that the item heats
up (T + ΔTad).
·
Isomagnetic
enthalpic transfer: This added
heat can then be removed by a fluid like water or helium for example
(-Q). The magnetic field is
held constant to prevent the dipoles from reabsorbing the heat. Once
sufficiently cooled, the magneto caloric material and the coolant are separated
(H=0).
·
Adiabatic
demagnetization: The substance
is returned to another adiabatic condition so the total entropy remains
constant. However, this time the magnetic field is decreased, the thermal
energy causes the domains to overcome the field, and thus the sample cools
(i.e. an adiabatic temperature change). Energy transfers from thermal entropy
to magnetic entropy i.e. disorder of the magnetic dipoles.
·
Isomagnetic
entropic transfer: The magnetic
field is held constant to prevent the material from heating back up. The
material is placed in thermal contact with the environment being refrigerated.
Because the working material is cooler than the refrigerated environment, heat
energy migrates into the working material (+Q).Once the refrigerant and refrigerated environment is in
thermal equilibrium, the cycle continues.
b.
Entropy(S)
–Temperature (T) Diagram
Magneto caloric effect
(MCE) is the emission or absorption of heat in a magnetic material in response to a changing magnetic field. When a material is magnetized,
the magnetic entropy ΔSm, is changed due to a change in the magnetic order of the material. Under
adiabatic conditions, ΔSm must be compensated by the entropy associated with
the lattice, resulting in a change in temperature of the material, ΔTad. The
relation between ΔTad, ΔSm and magnetic
properties of the material is illustrated in Fig.2.
4) WORKING MATERIALS
The magneto caloric effect
is an intrinsic property of a magnetic solid. This thermal response of a solid
to the application or removal of magnetic fields is maximized when the solid is
near its magnetic ordering temperature. Gadolinium and its alloys are the best
material available today for magnetic refrigeration near room temperature since
they undergo second-order phase transitions which have no magnetic or thermal
hysteresis involved. Also, crystalline electric fields and pressure can have a
substantial influence on magnetic entropy and adiabatic temperature changes.
Currently, alloys
of gadolinium producing 3 to 4 K per tesla of change in a magnetic field can be
used for magnetic refrigeration or power generation purposes. Eventually
paramagnetic salts become either diamagnetic
or ferromagnetic,
limiting the lowest temperature which can be reached using this method. (Fig.3)
The originally suggested refrigerant was 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. [e].
5) MAGNETIC FIELD DEVICE
DESIGN
Magnetic
field and magnetic refrigerant can be assembled statically or dynamically. Statically
the field is pulsed and power losses are high; dynamically uses either
reciprocating or rotary movement, where extra power is required for the
mechanics of the dynamic system. Conventional refrigerators compress a volatile
gas and then permit it to rapidly expand, pulling heat from the surroundings.
In contrast, the magnetic device exploits magnetically induced heating and
cooling of a powder of the element gadolinium. The powder is stuffed in pockets
inside the ring that carries it through the field of the permanent magnet. [d],
[f].
It is clear from the fig. that as
gadolinium enters a magnetic field and becomes magnetized; the material's atoms
align, causing it to get hot. A fluid (red) carries that heat away. As the
gadolinium exits the field, the atoms absorb heat from the recirculated fluid
(blue) that chills a space. (Fig.4)
6)
ADVANTAGES
OF MAGNETIC REFRIGERATION
a. Magnetic refrigeration is environment friendly because there is no production of CFC, hazardous chemicals like NH3 (Ammonia) and greenhouse gases.
b. The efficiency of magnetic refrigeration is 60% to 70% as compared to Carnot cycle.
c. The magnetic refrigeration consumes less power.
d. Magnetic refrigeration is totally maintenance free & mechanically simple in construction.
e. The C.O.P. (coefficient of performance) is very good as compared with conventional refrigeration.
7)
DISADVANTAGES
OF MAGNETIC REFRIGERATION
The only disadvantage of magnetic refrigeration is
that initial investment is more as compared with conventional refrigeration.
8)
APPLICATIONS
OF MAGNETIC REFRIGERATION
a. Magnetic refrigeration is currently being used
to produce better refrigeration techniques, especially for use in spacecraft.
b. Magnetic refrigeration is used as magnetic refrigerator in house hold applications.
c. Magnetic refrigeration is used as to produce very low temperature as 1 Kelvin.
d. As the through magnetic refrigeration we can produce 20 Kelvin temperature which is liquefied point of Hydrogen gas so we can get liquid hydrogen gas from air as a fuel. Magnetic refrigeration is used in food preservation applications.
e. Magnetic refrigeration is used to produce small as well as large capacity of crayocoolers. This crayocoolers have a lot many applications in Cryogenics.
9) CASE STUDY- LIQUIFIED HYDROGEN
In 1997, the first near room temperature proof
of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gschneidner, Jr. by the Iowa State University at Ames
Laboratory. [e].This event attracted interest worldwide that started
developing new kinds of room temperature materials and magnetic refrigerator design.Liquid
hydrogen could prove to be a perfect fuel, but first scientists and engineers
must jump a few technological hurdles. One of the biggest hurdles, an efficient
method of liquefying hydrogen has been eliminated by recent developments at
Ames Laboratory. Scientists have developed a highly efficient magneto caloric
material that makes magnetic refrigeration technology efficient enough to cheaply
produce liquid hydrogen which is used in magnetic refrigerators.Gschneidner‘s latest
discovery is a new class of alloys with significantly more cooling power than
the best existing materials. The new material gadolinium has two to three times
the magneto caloric effect of a typical ferromagnetic iron and a popular choice
for low-temperature ranges. Additional work has revealed that Gd5Si2Ge2 is one
of a family of compounds that exhibits a giant magneto caloric effect and whose
ordering temperature can be tuned from 30 Kelvin (-405.4 F) to near room
temperature (290 K or 62.6 F) by adjusting the ratio of silicon to germanium.
(Fig.5)
Another
factor helping to heat up the development of magnetic refrigeration technology
is the recent ban on CFCs and other environmentally harmful substances.
Magnetic refrigeration doesn't use CFCs and, in the case of the Astronautics
model, water is used as the heat transfer fluid. Only antifreeze is added to
allow the Astronautics unit to reach temperatures below 273 K, the freezing
point of water. [g], [e].
Large-scale applications include
supermarket-sized refrigerators and freezers, air conditioning for large
buildings, industrial chemical processing, and waste treatment.
10) CONCLUSION
a.
Magnetic refrigeration has greater efficiency and would
have beneficial effects on national power consumption. However, continued
research in material sciences will be required to find a low cost material
solution to magnetic refrigerant.
b.
In addition, because
permanent magnets account for a significant portion of the cost of prototypical
systems, the development of higher performance and lower cost permanent magnet materials
in the magnet industry will benefit the economics of magnetic refrigeration.
11)
REFERENCES
a.
R. S.
Khurmi, ‘Refrigeration & Air condition’, Eurasia Publishing house,
Ramnagar,New Delhi 110055.
b.
Stoecker and Jones, ‘Refrigeration and Air
Conditioning’, Tata-McGraw Hill Publishers
c.
Mathur, M.L., Mehta, F.S., ‘Thermal Engineering Vol
II’
d.
John Dieckmann, Member ASHRAE; Kurt Roth,
Ph.D., Associate Member ASHRAE;and James Brodrick, Ph.D., Member ASHRAE
e.
A b Karl Gschneidner, Jr. and Kerry Gibson (December 7, 2001); ‘Magnetic
Refrigerator Successfully Tested’; Ames Laboratory News Release. Ames Laboratory. Retrieved
on 2006-12-17.
g.
http://www.external.ameslab.gov/news/Inquiry/fall97/bigchill.html
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