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Transcript of Cryogenic Grinding
CRYOGENIC GRINDING
SEMINAR REPORT
Submitted byRAJESH.S
S7 MECHANICALREG NO; 08401042
ToThe University of Kerala
In partial fulfillment of the requirements for the award of the degreeOf
Bachelor of Technology in Mechanical Engineering
Department Of Mechanical Engineering Government Engineering College, Barton Hill, Thiruvananthapuram -35
OCTOBER 2011
DEPARTMENT OF MECHANICAL ENGINEERINGGOVERNMENT ENGINEERING COLLEGE
BARTON HILL, THIRUVANANTHAPURAM – 35
CERTIFICATE
This is to certify that the report entitled “CRYOGENIC GRINDING”, submitted by “RAJESH.S, S7 MECHANICAL, REG NO; 08401042” to the university of Kerala in partial fulfillment of the requirements for the award of the Degree of Bachelor of Technology in Mechanical Engineering (stream) is a bonafide record of the seminar presented by him.
GUIDE COORDINATOR
Mr. GOPAKUMAR Mrs. VINEETHA S DAS
HEAD OF THE DEPT.
Mr. SUNEESH
CONTENTS PAGE NO:
ABSTRACT 1
ACKNOWLEDGEMENT 2
NOMENCLATURE 3
INTRODUCTION 4
THE BEGINNINGS 5
ABOUT CRYOGENIC GRINDING 6
CRYOGENS AND IT’S APPLICATIONS 7
EXPERIMENTAL STUDY 8
CONVENTIONAL Vs CRYOGENIC GRINDING 10
CRYOGENIC GRINDING SYSTEM FOR SPICES 11
CRYOGENIC GRINDING OF PLASTICS 13
MAIN FACTORS INFLUENCING GRINDING 16
OPTIMIZED TECHNOLOGIES 19
NEW TECHNOLOGY 22
APPLICATIONS 26
ADVANTAGES 27
DISADVANTAGES 28
CONCLUSION 29
REFERENCES 30
ABSTRACT
Cryogenic grinding has been the development in the field of ultrafine
grinding of plastics, especially the ones with low softening temperature. The
word ‘Cryogenics’ originates from the Greek word ‘cryo’, which means cold.
Temperatures as low as -180ºC are attained in cryogenic systems. The
extremely low temperatures are produced by using substances called ‘cryogens
‘such as liquid nitrogen and liquid helium. Cryogens are used in various
machining operations such as cryogenic deburring, cryogenic deflashing,
cryogenic tempering and cryogenic grinding.
Cryogenic grinding, also known as freezer milling/ freezer grinding/
cryomilling is the act of cooling or chilling a material and then reducing it
to smaller particle size. Almost all materials embrittle when exposed to low
temperature. Cryogenic size reduction utilizes the cooling effect of liquid
nitrogen to embrittle materials prior to and or during the grinding process.
Materials which are elastic in nature, which have low melting points,
which have low combustion temperatures and which are sensitive to
oxygen can be ideally machined by cryogenic grinding process.
A cryogen like liquid nitrogen is applied mostly in the form of a
jet. At a low temperature of -196ºC of the cryogen, the temperature is
effectively controlled and embrittles the work piece. This enables the
grinding of the work piece.
ACKNOWLEDGEMENT
With profound respect and immense gratitude I wish to thank
Mrs. VINEETHA, Mr. GOPAKUMAR & Mr. SATHEESH Lecturers of
Mechanical Engineering, GECB for guiding me to take up this subject and also
helping me to successfully complete the seminar.
I am grateful to Prof. SUNEESH, Head of the Department of
Mechanical Engineering who has supported me with very useful suggestions
for improvement throughout the seminar.
I would like to express my deep sense of gratitude to Smt. GEETHA,
Principal GECB for providing all the facilities needed during the course of this
seminar.
Above all I thank almighty for all His blessings showered upon me and
giving me strength and presence of mind in successfully completing my
seminar.
RAJESH.S
NOMENCLATURE
D : Particle Size
Er : Relaxation Module
Wm : Specific Work of Comminution
N : Standardized Distance to Symmetric Centre
H : Standardized Temperature
EVA : Ethylene Vinyl acetate
LN2 : Liquid Nitrogen
LD-PE : Low Density Poly Ethylene
PP : Polypropylene
INTRODUCTION
Almost all materials embrittle when exposed to low temperature.
Cryogenic size reduction utilizes the cooling effect of liquid nitrogen to
embrittle materials prior to and or during the grinding process. Materials
which are elastic in nature, which have low melting points, which
have low combustion temperatures and which are sensitive to oxygen can
be ideally machined by cryogenic grinding process.
A cryogen like liquid nitrogen is applied mostly in the form of a
jet. At a low temperature of -196 ºC of the cryogen, the temperature is
effectively controlled and embrittles the work piece. This enables the grinding
of the work piece.
Cryogenic grinding usually starts with chips. This is cooled using a
chiller. The final product is a range of particle sizes which are sorted and
either used as it is or passed on and further size reduction is performed.
THE BEGINNINGS
An exact history of cryogenic grinding technology is not easily
determined. The history is unclear for reasons such as the secrecy
involved in research as this technology could prove to be very
profitable.
What is known is that the technology is derived from conventional
grinding and is used in the grinding of elastic materials.
CRYOGENIC GRINDER
ABOUT CRYOGENIC GRINDING
Cryogenic grinding , also known as freezer milling/ freezer grinding/
cryomilling is the act of cooling or chilling a material and then
reducing it to smaller particle size.
A cryogen like liquid nitrogen is applied mostly in the form of a
jet. At a low temperature of -196 ºC of the cryogen, the temperature is
effectively controlled and embrittles the work piece. This enables the
grinding of the work piece.
CRYOGENS AND IT’S APPLI CATIONS
The word ‘cryogenics’ originated from Greek word ‘cryo’, which means
cold. Temperatures as low as -180 ºC are attained in cryogenic systems. The
extremely low temperature are produced by using substances called
‘cryogens 'such as liquid nitrogen and liquid helium. Cryogens are stored in
vessels called as Dewar flask which provides good insulation.
Cryogenic fuels such as liquid hydrogen are used as rocket fuels and
propellants. Cryogens are used to achieve superconductivity in metals such as
tin and aluminium. These are also used for the preservation of bodies of animal
and humans, this process is called as cryopreservation. Cryogens are used in
food industry for food handling and processing.
Cryogenic substances are used in various machining operations
such as cryogenic tempering and cryogenic grinding. Cryogens like liquid
nitrogen are mainly used in special chilling and freezing applications.
EXPERIMENTAL STUDY
Conventional versus Cryogenic grinding
There are several processes used to produce ground rubber crumb. Two of
the most common are conventional grinding and cryogenic grinding. The
ambient process uses a high powered cracker mill and rubber is sheared and
ground into a small particle.
RUBBER GRINDER
Conventional grinding
Metal is separated using a magnetic separator. The process produces a
material with an irregular shape. In addition the process generates a significant
amount of heat. Excess heat can degrade the rubber.
Cryogenic grinding
Cryogenic grinding of rubber usually starts with chips. This is cooled
using a chiller. The final product is a range of particle sizes which are
sorted and either used as is or passed on and further size reduction
performed .The cryogenic process produces fairly smooth fracture surfaces.
No heat is generated in the process. This results in less degradation of the
rubber.
In addition, the most significant feature of the process is that almost
all fiber or steel is liberated from the rubber resulting in a high yield of usable
product and little loss of rubber.
CONVENTIONAL Vs CRYOGENIC GRINDING
Existing Grinding System Cryogenic Grinding System
The heat is developed inside the
grinding mill.
Temperature below 0 ºC inside the grinding mill.
This heat, which developed during grinding, leads on the one hand to evaporation of the essential oils and on the other hand, heat- sensitive fats are melted. This is turn can lead to the grinding elements become grassy (oily) and clogged or even to machine blockages.
Minimal loss of volatile components.
Air pollution due to evaporating essential oil into the atmosphere.
No, evaporation of essential oil into the atmosphere.
Existing grinding equipments more than two times recycle into the mill for required particle size.
Approx. 2 - 3 times higher grinding capacity.
Low throughput . High throughput.
High energy consumption. Low energy consumption.
No control on particle size. Particle size under control.
CRYOGENIC SYSTEM FOR HERBS
Cryogenic grinding of spices is a method of powdering spices/herbs at sub
zero temperatures ranging from 0 to minus 196 ºC. The spices are frozen with
liquid nitrogen, as they are being ground. This process does not damage or alter
the chemical composition of the spices in any way. Normal grinding process
which does not use a cooling system can reach up to 200 ºC temp. This high
temperature can reduce volatile components and heat sensitive constituents in
spices.
The Process:
The material is feed into a feeder hopper and dropped into a conveyor.
When the material to be processed enters the pre-chilled conveyor; liquid
nitrogen is sprayed and blended directly onto the material. The material is
conveyed via a stainless steel special design auger. The auger not only
transports the grinding media, but also mixes with liquid nitrogen for greater
cooling efficiencies.
The liquid nitrogen, a cryogenic fluid with a boiling temperature of -196 ºC
absorbs heat from the material and vaporized to a gaseous state.
Liquid nitrogen is added until the temperature of the material is reduced to
a
Predetermined set point. This set point is the glass transition temperature of
the material. Finally the brittle material enters an impact (pin) mill where it is
ground to a desired particle size. Computer controls the entire process of
cryogenic grinding system.
Pin Mill
Pin mills are high-speed machines working without screen. Pin rows which
are concentrically fixed on the rotor and stator discs, crush the particles,
interchangeable pins in different sizes can be delivered. According to the size
reduction problem, the concentrically spaces between the pins can be adapted to
the material.
Cryogenic grinding of plastics with air turbine
refrigeration system
Plastics after being grounded not only maintain intrinsic
properties, but also enhance the plasticity, formability,
solubility, etc. Therefore, it enlarges a great deal of application in the
area of reuse. There is a steady increase in the demands of fine powder in
powder molding, hot melt adhesive and plastics modification.
Refrigeration principle:
The working principle of air turbine refrigerator is to
convert the pressure potential energy of compressed air
released from the compressor to the kinetic energy of high-
speed airflow through the nozzle ring.
Recooling system
The cold airflow from the apparatus for utilizing cooling
energy is sent to recooling heat exchanger to cool the airflow
coming from the aftercooler with high pressure and ambient
temperature. Therefore, the airflow temperature at turbine inlet
is effectively reduced to lower temperature airflow at turbine
outlet.
Turbo expander
The turbo expander is composed of turbine blades, compressor blades,
nozzle ring, rotating shaft, bearing, outer shell, etc. As the rotational speed is up
to 50,000 rpm/min, bearing is the key component affecting the service life. Air
bearing and oil-lubricated bearing are the most common choice in engineering
practice.
Recooling heat exchanger
High compactness and high heat transfer efficiency are the main design
targets, thus plate-fin heat exchanger is chosen for its good heat transfer effect.
For the fine refrigeration effect, countercurrent flow mode is used to enhance
the refrigeration.
Precooling and cooling chamber
The design of the precooling and refrigeration chamber is
the important means to ensure grinding effect. In this study,
the spiral rotation of spiral propeller is used to increase the
dispersion of particles, making the cold air and particles to mix
well, and achieve proper heat transfer. At the same time, the
problems of particle transportation and arch forming are solved
with optimal cooling effect.
Vortex mill
Impact mechanical field is the most effective grinding means in
cryogenic grinding. In consideration of the comprehensive factors such as
material properties, grinding method, production capacity, energy
consumption a vortex mill is adopted as the grinder. The
rotating component of vortex mill consists of four stages. Each
stage is composed of blades and multiple cells, which are
formed by blades and their side conjunctive clapboard. There is
a lining with many grooves inside the outer shell. Moreover, the
clearance between blades and the lining is adjustable.
Improvement on vortex mill
Free path denotes a movement interval, within which a
particle moving in suspension with a certain concentration does
not collide with other particles. When a particle entering the
grinding area, if its distance to the blade is equal to or less than
the free path, it would collide with blades or lining first before
meeting other particles. According to the grinding mechanism,
the mutual collision between particles and blades or lining is
the most effective way of grinding in vortex mill. Therefore, the
probability of such collision is expected to be as big as possible.
So if the free path is too short, a particle entering the vortex
mill would collide with other particles first. Hence, the average
free path of particles is preferably raised to a certain extent by
appropriately increasing the blade number of each stage, which
would improve the grinding effect.
MAIN FACTORS INFLUENCING GRINDING
1. Material properties: Three kinds of plastic particles are grinded in the
same experimental equipments and condition. Mechanical property is the most
important factor in influencing the grinding effect. The output and reduction
ratio of PVA-2488 particles with the smallest elongation at break are higher.
However, for the NBR and EVA particles, there is little difference in elongation
at break.
2. Feeding particle size: The distribution of product particle size is related
to that of feeding particle size. Usually, too big particle size is very easy to
escape from the vortex field, while the opposite although increases the heat
exchange surface, but the size effect raises its anti-destructive capability,
leading to low product quality.
3. Cooling temperature: Theoretically, when the thermoplastic particles,
such as EVA and rubber are refrigerated into brittle state, the particles would
be easier to be ground by brittle fracture. However, comparing the ground
product from embrittled samples with the experimental results at room
temperature, although there is obvious difference in the particle shape, the
particle size of the former may not be finer.
4. Clearance between blades and lining: When the clearance becomes
smaller, the particle size is also smaller and the range of distribution is narrow.
5. Collision speed: As the vortex mill rotates faster, the impact speed
towards EVA particles increases, and correspondingly the fracture probability.
However, the rotating speed of rotor is restricted by the bearing load, strength of
rotor components, and machining accuracy, it is favorable to appropriately raise
the rotating speed of rotor to improve efficiency. The tangential speed of vortex
mill is selected as 105 m/s in this study.
Assumptions for calculation
1. The observed particles have the shape of a sphere.
2. The material properties are independent of location and temperature.
3. The particles have a homogeneous temperature in the calculation.
4. The flow, heat capacity, and heat transfer coefficient of particles and cold
air are invariant in the lateral spiral propeller.
5. Heat transfer among screw, cylinder wall, and cold air is considered to
reach a balance.
6. Only the heat transfer between particles and cold air is taken into
account.
Heat transfer calculation
Under the above assumptions, heat transfer between particles and cold
air could be simplified. The calculation diagram , lateral spiral propeller is
divided into n stages.
In the first stage, heat transfer of gas–solid is given by
Q = h × tm × S
CONCLUSIONS
1. Experiments show that adopting the cryogenic grinding system
refrigerated by air turbine refrigerator to process fine EVA powders is feasible.
Compared with liquid nitrogen, the energy consumed in this system is much
lower. Moreover, compared with the common refrigeration methods such as
freon, lithium bromide, and ammonia, this system possesses advantages of low
refrigeration temperature, simple equipments, and easy maintenance. And
compared with grinding at room temperature, the value of Ki in this system is
three times as large as that of the latter. Furthermore, another important
advantage is that introducing cold air into the grinding plant avoids overheating
of powder and dust explosion without any additional security equipment. In
addition to grinding EVA, this system could also be used as a kind of general
technology to grind other kinds of plastics by adjusting the related parameters.
Therefore, it owns a good commercial value. In other words, the cryogenic
grinding system refrigerated by air turbine refrigerator creates a new way of
recycling renewable plastics in a large scale, with high quality and low energy
consumption.
2. Material properties, feeding particle size, collision speed, etc. are the
main factors that affect the grinding effect. The selection principle of
refrigeration temperature is to ensure that the particles could maintain a
reasonable condition of low temperature. Moreover, experiments have proved
that it is not necessary to refrigerate EVA particles to the brittle condition. The
cooling energy is mostly used to counteract the grinding heat, ensuring that the
grinding process could be running for a long time at high quality and low
energy consumption.
3. Vortex mill is regarded as the research object in this study. An
engineering calculation and analysis approach is introduced from the energy
conservation law during the grinding process, aiming to guide the practical
application. These analytical results confirm the rationality of the experimental
results, which is beneficial to the improvement of equipments and evaluation of
multiple correlated parameters.
Optimized technologies for cryogenic grinding
Fine grinding or comminution gains increasing importance. This is
reflected in the extensive trend towards fine structuring of solid materials and
the rapid spreading of research disciplines like micro, nano, and surface
technologies. In addition to the target particle size, the specific comminution
work is substantially affected by the material properties. This work is 10 to
hundred times higher for the comminution of plastic than for minerals. The
theoretical background of cryogenic grinding and a new technology based on an
alternative cooling equipment are presented. With the alternative technology,
the operation costs during cold grinding will be drastically reduced. At the end,
experimental results show effects of the variation of different running
parameters.
1. Material properties
In addition to the target particle size, the specific comminution work is
substantially affected by the material properties. This work is 10 to hundred
times higher for the comminution of plastic than for minerals. Differences in the
specific work of comminution may be attributed to losses during the
deformation of the particle which does not result in a fracture. Whereas in brittle
materials, the imported comminution energy is saved in elastic deformation by
high stress fields, major part of plastic materials are being deformed
viscoplastically. This deformation behaviour is largely dependent on
temperature and time and is defined with the relaxation module. This module
describes the stress relief of a sample of constant expansion by plastic
deformation.
2. Heat transfer at a particle
The heat transfer at particles with a different diameter should be taken
into consideration before a calculation of the theoretical lowest demand on lN2
may be performed, due to the multiply stress of particles that is necessary to
achieve the desired particle size.
Relaxation of plastics at different temperatures.
3. Calculation of lN2 consumption
The heat to be discharged at cryogenic grinding is composed of heat,
which has been introduced by the product (Q feed) and the heat introduced by
the drive capacity of the mill (Q mill), provided that the grinding air flow required
for the operation of the mill is met by the condensing nitrogen alone.
Qtotal =˙Qfeed +Qmill
4. Economic aspect of cryogenic grinding
The utilization of liquid nitrogen (lN2) has undoubtedly numerous
advantages in application. However, these advantages have to be
counterbalanced against the high energy consumption, and thus investment cost
for the supply of the cold agents which considerably affects the operation cost
of a cryogenic grinding plant. For the grinding of 1 kg viscoelastic material an
amount ranging between 0.6 and 4 kg liquid nitrogen is required in optimized
mills Liang and Hao, 2000. In-house measurements at various contract grinding
companies yielded even consumptions of up to 6 kglN2/ kg solid material.
Provided a nitrogen price of approximately 0.10 /kgln2/kg solid material, the
cost of nitrogen alone would amount to more than 100 per ton material used (at
1 kgln2/kg solid material), thus representing over 40% of the cost of cryogenic
grinding.
New technology developed by Fraunhofer
UMSICHT
Measurements at production plants showed that the real consumption is
sometimes much higher than the theoretical calculated consumption. To relate
the theoretical to the practical consumption, the efficiency factor glN2 is
defined as
Ef = (theoretical consumption/ practical consumption)*100 %
At a cryogenic grinding plant in the chemical industry, this efficiency
factor varies from 25% to 75%. That shows that there is a demand of
optimization.
1. Optimized process design
In addition to increasing the efficient factor by optimizing the operation of
conventional plants, Fraunhofer UMSICHT has developed an innovative
technology which allows to avoid the use of lN2 partially or completely,
depending on the material. This technique is based on the employment of a
refrigeration plant. This plant has a cooling power of 40 kWth which is equal to
360 kglN2/h. The heat is transported by a cooling medium, which is still liquid
at temperatures below _1008C. This allows to cool the grinding material and the
grinding air separately in special heat exchangers down to a temperature of
about _608C. For many materials, this temperature is sufficient to become
brittle. Should this temperature be not sufficient, the material may be exposed
to further cooling in a cryoscrew with lN2 In order to perform the optimization
described, a pilot plant has been erected at the technical laboratory of
Fraunhofer UMSICHT. Planning did in particular focus on a high flexibility of
the plant. This made it possible to test both according to the traditional method
and according to the new method, as well as by combining both methods.
Injection options of nitrogen at different places of the grinding air circulation
allow cooling of the mill, independent of the cooling of the material. A number
of applicable grinding aggregates with different grinding principles may be
applied for different milling functions. The plant has a throughput performance
of 300 kg/h (depending on the material), so that the results achieved may be
readily transferred to large scale plants. To allow detailed estimates, extensive
measuring technology was included.
2. Comparison between conventional and alternative cooling technique
The supply of cold at a temperature of -196 8C is not necessary for many
plastic materials. In many cases, temperature levels noticeably above -80 8C are
sufficient to make plastics brittle. Characteristic for a process of cold generation
is the performance coefficient ‘e’ Nesselmann, 1957:
e= Q0/ W
Where ‘Q0’ represents the cold quantity, and W the required mechanical
capacity.
Process design of the cryogenic grinding facility at Fhl
UMSICHT
Cold amount of thermal energy (Qth) may be generated at a temperature
level of -80 *C utilizing one-sixth of energy input compared to a temperature
level of -196 *C. The connection between the performance coefficient of the
Carnot process eC and the parameter of the real process e is formed by the
efficiency factor ‘g’.
This plant configuration would yield a break-even point of 2676 h/a,
indicating the threshold for a cost-efficient application of the alternative
technology. For plants with lower operating hours, the conventional technology
is more efficient cost. However, the increased investment input for the
acquisition of a cold generation plant comprising the necessary equipment has
to be considered on the other hand.
Efficiency factor ‘g’ of different process
The Fig. shows the factor of four different processes (cold steam, Philips,
Thomson–Joule and adiabatic isothermal).
The connection between the performance coefficient of the Carnot process
eC and the parameter of the real process e is formed by the efficiency factor ‘g'.
This efficiency factor depends on the used cold generation process. With respect
to the temperature range between -80T C and 0T C, the cold steam process is the
efficient process
In comparison to the use of lN2, the reduced energy input of the
alternative cold technology is reflected in lower operation cost. However, the
increased investment input for the acquisition of a cold generation plant
comprising the necessary equipment has to be considered on the other hand. As
a consequence, a plant based on the alternative technology will be cost efficient
only after a certain operation time.
APPLICATIONS OF CRYOGENIC GRINDING
Cryogrinding of steel:- The large amount of heat generated during
grinding at high speed raises the temperatures at cutting zones excessively.
Cryogens such as liquid nitrogen will help in reducing the effect of heat on tool
and work piece, thereby increasing the life of the tool.
Thermoplastics: - Nylon, PVC, Polyethylene, Polypropylene are usually
machined using cryogenic grinding to form powders.
Thermo sets: - Synthetic and natural vulcanized rubber and materials
such as bakelite can be economically machined with cryogenic grinding and
recycled.
Adhesives and waxes :- Sticky materials such as adhesives and
waxes are difficult to machine using the conventional grinding methods. By
using cryogenic grinding, they can be embrittled easily and machined into fine
particles.
Explosives :- Explosive materials explode when their temperature
increases to ignition temperature in the presence of oxygen. By using
cryogens the ignition temperature can be reduced effectively and then be
machined.
Spices :- Spices like pepper ,cinnamon can be powdered by
cryogenic grinding which helps in the preservation of the taste and aroma.
ADVANTAGES OF CRYOGENIC GRINDING
Higher material removal rate can be achieved.
Tool wear and tear is minimized to a great extent.
Grinding forces are reduced.
Cryogens act as coolant and hence the effects of overheating of the tool
and work piece are reduced.
Materials which are soft and elastic in nature such as rubber can be easily
machined with this process.
Smaller particle size can be achieved.
Better surface finish and dimensional accuracy can be achieved.
Temperature below 0 0C inside the grinding mill.
Minimal loss of volatile components.
Low energy consumption.
Approx. 2 - 3 times higher grinding Capacity.
No Fire Risk.
Low capacity motors are required to grind the material.
No, evaporation of essential oil into the Atmosphere.
Particle size under control.
High throughput.
DISADVANTAGES OF CRYOGENIC GRINDING
1. Grinding system is very complex. It consists of large number of equipments
like dewar flask, grinder, nozzle, compressor, silica gel container.
2. Very costly. It need high capital cost. (For cryogenic grinding, compression
system, moisture adsorption system and grinding system are necessary.
3. Formation of ice around delivery nozzle. Silica gel absorbs moisture and
this moisture cause the formation of ice around delivery nozzle. It cause
blockage inside of the valve.
4. Large area requirement. Cryogenic grinding system includes compression
system, moisture adsorption system and grinding system.
5. Not economic. Chemicals such as silica gel & liquid nitrogen produce
unwanted products, which cause pollution.
6. Material specific. Cryogenic grinding can be used only for Materials which
are elastic in nature, which have low melting points, which have low
combustion temperatures and which are sensitive to oxygen .
CONCLUSION
Cryogenic grinding , also known as freezer milling/ freezer grinding/
cryomilling is the act of cooling or chilling a material and then reducing
it to smaller particle size. Cryogenic size reduction utilizes the cooling
effect of liquid nitrogen to embrittle materials.
The main advantages are higher material removal rate, tool wear and tear
is minimized to a great extent, and grinding forces are reduced. Cryogens act as
coolant and hence the effects of overheating of the tool and work piece are
reduced. Materials which are soft and elastic in nature such as rubber can be
easily machined with this process.
The application of cryogen in moist atmosphere may cause formation
of ice around the delivery nozzle and the piping system carrying the cryogen.
Cryogenic grinding improves product quality by controlling thermal
effects. Oxidation and surface burning are eliminated. Surface damage is
eliminated. Finer particle size is achieved. Material removal rate is high. The
process is economical in the long run.
REFERENCES
Michael, Jurgen & Damian,(2004) “Optimized technologies for
cryogenic grinding.” Int. J. Mineral Process.
Journal of “Process Mechanical Engineering”-2011. Fraunhofer
Institute for Environmental, Savety and Energy Technology UMSICHT,
Oberhausen, Germany
E-book of Spectra Cryogenic Systems Private Limited(2005 ) Cryogenic
equipment manufacturers and consultants “Cryogenic Grinding System for
Spices & Herbs” Kota 324005, Rajasthan, India
APPENDICES
Crackermill 11
Cryogenics 8
Cryopreservation 10
Dewar flask 10
Ethylene vinyl acetate 15
Freezer milling 9
Gas solid separator 11
Low density polyethylene 23
Magnetic separator 11
Pin mill 15
Polypropylene 23
Spices 29
Theoretical consumption 25
Vortex mill 18