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UNIT-8
APPLICATION OF CRYOGENIC SYSTEMS
Space technology:
Liquid hydrogen is used together with liquid oxygen as fuel for space
vehicles. It has a high propulsive energy per unit mass, but it needs very
large volumes (in comparison with kerosene) rockets fuelled with
hydrogen and much larger than those fuelled with kerosene and have
more problems of stability during flight. The technology is similar to
that used on earth, except that weight is at a premium and once in the
space environment. Only minimum thermal insulation may be needed.
However, the absence of gravity poses serious problems. Special devices
have to be used to overcome these problems. In addition, liquid oxygen
is carried for life support and helium may be carried for pressurizing fuel
tanks.
For the space cryostats containing liquid helium, the following
precautions are used
1. The mechanical supports of the tank containing helium, are made
of low conductivity materials such as stainless steel and strong
supports withstand the high launch accelerations are added which
are eliminated when the satellite is in the absence of gravity.
2. Thermal shields connected to heat exchangers cooled by the
evaporating gas are used to drastically reduce the radiative imput.
There are two characteristics that make helium attractive for space
applications. The first is weight (about 0.185 Kg/d ) second is
superfluidity.
Space research has given a major area to the growth of cryogenics.
Cryogenics fluids namely liquid oxygen (LOX) and liquid nitrogen
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(LN2) are used as propellants in space vehicles and also as
refrigerants in space simulation respectively.
a) Space rockets:
The propellants are pumped from storage vessels of the vehicle through
an injector to the combustion chamber. The propellant pumps are driven
by small turbines which are again powered by some portion of
propellants. The fuel is usually passed through ducts in the nozzle wall
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to provide cooling before fuel is injected. The fuel and the oxidizer
combine in the combustion chamber to form products of combustion at
high pressure and temperature. These products of combustion are
exhausted from the nozzle at high velocity to provide required thrust for
the vehicle.
The cryogenic fluid has several other advantages as a rocket propellant.
1. Storing or handling propellant in the liquid state is more effective
than storing or handling in the gaseous state.
2. Controlling of engine is relatively simple.
3. The materials with the most desirable properties or liquids only at
cryogenic temperature.
Disadvantages:
1. Boil off losses during storage on board the vehicle is high.
2. Zero gravity effects in space.
Food preservation:
Food materials are perishable by nature. They required preservation
techniques to enhance the storage life. It is employed to keep the
perishable food items vegetables, meat, eggs, medicines etc for longer
period.
There are several methods of food freezing
Sharp freezing: It is slow freezing. It consists of insulated rooms
maintaining at varying temperature -15°C to -30°C. Products placed in
those rooms are cooled by free convection. It is further modified to
include banks of refrigerated shelves on which products are placed for
freezing. Belt conveyors are introduced.
So generally the foods are stored in a refrigerator for a long time of life.
One such cryogen liquid nitrogen ( ) has a tremendous potential to be
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used as refrigerant and in some time ( ) is also used as refrigerant. So
for a mass preservation most probably ( will preferred because it
does not react with any other food product. These two are ideal
refrigerants.
Mechanical freezing
The freezing of food materials is more complex than the freezing of pure
water (All food materials contain solutes such as carbohydrates, salts,
colorants of other compounds which affect their freezing behavior).
Most food products contain animal or vegetable cells forming biological
tissues.
The water content of these tissues is either inside the cells or
surrounding. During a slow freezing, there will be time for the cell to
lose water by diffusion and the water will freeze on the surface of the
crystals already formed. As the cells keep losing water, the cell shrinks
more and more until it collapses. Cryogenic food freezing differs widely
from mechanical ammonia or Freon freezing system. Thus it requires a
different procedure in determining the final exit temperature of a certain
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food product. Cryogenic freezing involves freezing the outer layers of
the food beyond its actual freezing point while the inner part of the
product remains warm.
Cryogenic freezing Mechanical freezing
Investment cost Lower cost of capital
equipment and simpler
Higher cost of capital
equipment and
complex
Operating cost Higher energy cost
with or as
energy source
Generally low
Maintenance cost Low High
Freezing temperature -160°F for and -
80°F for
Typically -30°F
Food quality Rapid freezing reduces
dehydration loss
Slower freezing
Environmental
consideration
Environmentally
friendly way of
freezing food
Ammonia is a great
refrigerant but high
toxic
Plant space usage Quick freezing then
the space also large
Less space
Operational flexibility Can easily be adopted
or expanded for
different production
lines
Not suitable for
product changes.
Quick freezing techniques:
The main consideration of quick freezing is the rate at which the
temperature of the food is reduced whether it is meat, vegetables or
baked products. There is a good reason for this. The longer the freezing
process takes the more time there is for the water molecules contained in
the food to come together to form large ice crystals. These can pierce the
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cell membranes and damage the tissue with the result that the frozen
food loses its form and structure while vitamins, nutrients and flavors
are also lost.
Fig above shows the direct immersion type of freezing. As the
conductivity liquid quit high, goods can be frozen in less time, by simply
immersion of the commodity in cold bath. The process consists of
pumping the cold liquid or moving the commodity in cold bath. If the
commodity is not affected by refrigerant it can be kept in the same. The
packed or unpacked commodity can be used for freezing.
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The cold gas control mechanism of the cryogen – rapid tunnel freezer as
shown in fig . It is suitable for large quantities of food freezing and
is sprayed into the tunnel freezer. The resulting cold gas is swirled
around the product surfaces by circulating cryogenic fluid. The
temperature rapidly drops to well below zero. The unit is particularly
suitable for high quality fish, meat and baked products as well as
convenient products. Conveyor belts and product contact parts can be
easily cleaned. The products frozen with cryogenic technology show a
matrix of small ice crystals and a better texture than products frozen
using slower heat transfer processes.
Super conductive devices:
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Superconductive devices require temperature below transient
temperature of superconductors. One of the early application of
superconductivity was in gyroscope with extremely low internal friction
that were designed in internal guidance system. A superconducting rotor
supported by a magnetic field is especially suited for operation as a high
vacuum gyroscope. Atleast four limitations on ordinary gyroscope are
eliminated for superconducting gyroscope are friction, bearing
instability, requirement for continuous power supply. Dimensional
instability.
a) Super conducting bearings:
The principle of support of gyrorotors within a magnetic field applies for
superconducting bearings. A superconducting sleeve is attached to the
rotating shaft and magnetic coils are placed around the sleeve. These
coils act as bushing of the bearing and the magnetic field produced when
the current flows in the coil acts as lubricants to support the loading. If
the lines of magnetic flux are compressed (due to high load) so much
that the magnetic field under sleeve reaches critical field,
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superconductivity is destroyed. The magnetic field then penetrates the
sleeve and the shaft no longer floats on the field because of this limit,
superconducting bearings have been restricted to such applications
namely gyroscope, instrumental drives. To produce bearings capable of
supporting larger loads material with higher critical speed are required it
depends on material selection.
b) Superconducting motors:
Superconducting motors have practically zero electrical and internal
mechanical losses to be completely free from windage losses. The
motors must be operate in a vaccum, however if small windage losses
can be tolerated the motors can operate in Helium gas. One of the main
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difference between superconductivity and conventional motors is rotor
configuration. If the rotor were cylindrical lines of force acting normal
to the rotor surface would produce no torque one way to develop torque
is to shake.
c) Cryotons:
One of the basic elements in high speed computers is cryton used as a
logic memory a rectifier element. The wire wound cryton as shown in
fig. As a switching time of above 150μ sec. One even more, which is
relatively slow in addition the manufacture of a computer using 1000
of wire around crytons would be relatively expansive because of
many soldering connections. To avoid these two problems the thin
film was developed fig (a)
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CRYOGENIC APPLICATION FOR FOOD PRESERVATION:
In the food industry, liquid nitrogen is used to freeze foods quickly.
Cryogenic gases are used in transportation of large masses of frozen
food. When very large quantities of food must be transported to regions
like war zones, earthquake hit regions, etc., they must be stored for a
long time, so cryogenic food freezing is used. Cryogenic food freezing is
also helpful for large scale food processing industries.
Industrial gases (not all of them cryogenic) are used to promote seed
germination, to enrich the greenhouse environment and promote plant
and flower growth. Oxygen is added to water in aquaculture to enhance
yields. Special gas mixtures are used for pre-harvest insect control and
fruit ripening. Gases are used to stun red meat animals and poultry
during slaughter, and fish prior to freezing. This is not only kinder to the
animals, but it produces a better quality product.
Bakery Industry
In the bakery industry, nitrogen is the perfect medium for freezing
delicate products like muffins, scones and cakes. LN2 vapor is also used
to cool baked foods. Cookies that take 13 minutes to cool from 130°F to
75°F with conventional methods can be cooled in one minute with liquid
nitrogen. Space requirements are also reduced. (Baking and Snack)
Cryogenic systems cool, chill or freeze a wide variety of bakery and
related snack products, from cakes and cookies to bread dough and
bagels. They offer shortened production time—as much as 50 percent or
more. This is an advantage for products requiring several processing
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steps, including multiple layered, coated and iced products that must be
quickly set to facilitate the next processing step.
Labor and handling are reduced since bakers can immediately package
the product for a continuous, in-line operation. Yields are increased,
with product prevented from sticking to the belt, decreasing product
losses and increasing processing speeds. Cryogenic freezing also
increases production flexibility, enhances processing capability,
improves product quality, reduces maintenance time and reduces space
and capital requirements.
Both carbon dioxide and nitrogen are used in the beverage industry, and
nitrogen is used in sparging solutions to reduce the negative effects of
dissolved oxygen in brewery products, beverages, milk products, oils
and fats. Blanketing with inert atmospheres improves operational safety,
product quality and preservation of edible oils. Industrial gases are also
effective for fat and oil hydrogenation and cryogenic crystallization for
dairy, liquid, bakery and confectionary products.
MAP/CAP
Modified Atmosphere Packaging (MAP) and Controlled Atmosphere
Packaging (CAP) use a gas or a gas mixture to maximize a food
product‘s shelf life, safety, purity and freshness. Because air is replaced,
bacteria and mold growth is retarded, shrink and waste is reduced, and
taste, color, vitamins and sensory appeal are preserved without the need
for vacuum packaging or other chemical preservatives. Droplets of
liquid nitrogen are dispensed to provide quick and accurate inerting to
minimize fat rancidity during storage of nuts, milk products, peanut
butter and dried potato.
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Beverage industry packaging applications include reduction of in-can
oxygen levels and in-line systems that provide rigidity, enhancing the
stackability of cans and plastic bottles.
We all know that carbon dioxide is essential for carbonation. Also,
CO2/nitrogen gas mixtures are used in beverage dispensing.
Controlled atmosphere and pressure transfer systems establish and
maintain required conditions for stored fruits, vegetables, flowers, dairy
products and liquids. Carbon dioxide, nitrogen and special gas mixtures
provide residue-free fumigation for post-harvest disinfestation of grains,
cereals and nuts.
LN2 and combination cryogenic/mechanical in-transit refrigeration
(ITR) systems maintain the quality and safety of refrigerated foods
during transport. Using cryogenic ITR provides consistent airflow to
uniformly maintain desired temperatures and retain food product
integrity and freshness. Such systems can handle ambient, chilled and
frozen products in the same delivery vehicle or storage space.
Gas mixtures also are used for reliable and cost-effective distribution of
fresh produce, meats and seafood over long distances. Advanced
atmosphere control technology manages temperature, humidity and gas
mixture inside the transport container.
Ancillary operations which are served by the industrial gas industry
include process and wastewater treatment, condensate and food washing
water recycling systems, welding shop gases and equipment, refrigerants
and laser gases for labeling.
The major process we will study is food freezing in its many forms,
using ammonia, carbon dioxide or liquid nitrogen. When discussing
cryogenics and food processing, we usually take a broad definition of
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the term ―cryogenics,‖ extending it to include carbon dioxide, which in
its various forms is a very useful food cooling substance. But the most
common cryogenic material used for cryogenic freezing is liquid
nitrogen.
―The objective of both refrigeration and freezing is to remove sensible
and latent heat from a food. Freezing—the conversion of the aqueous
part of a food from water into ice—preserves food by dropping it to a
temperature at which spoilage organisms are unable to grow and
chemical reactions that affect product degradation are slowed or
inhibited. Freezing basically makes water unavailable for
microorganisms and for chemical reactions.
―To freeze a food, the product must first be cooled to the transition point
of water, 32°F (0°C), by placing the food in a still-air freezer (sharp
freezing), passing cold air over the product (convection), bringing the
food into contact with cold plates (conduction), immersing it in a
cryogenic fluid (cryogenic) or using a combination of these processes.
―Cryogenic freezing is defined as freezing at -75°F (-59°C) or below.‖
(Baking & Snack, April 1998) High-velocity cryogen immediately
impacts heat transfer from product. Cryogen temperatures can reach as
low as -320°F, while overall internal freezer temperatures can reach -
150°F.
This freezing technique exploits features of nitrogen that make it an
ideal natural refrigerant for use in the food industry. In its liquid form,
nitrogen, at 196°C, is one of the coldest substances, and is completely
inert, colorless, tasteless and odorless. As a natural part of the earth‘s
atmosphere it has no adverse environmental effects, unlike other
refrigerants. The same can be said of carbon dioxide, except for its
reputed effect on the ozone layer.
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Benefits of LN2
Many benefits result from use of liquid nitrogen in freezing: the high
quality resulting from individually quick freezing (IQF); preservation of
texture when products are thawed; improved yield resulting from less
moisture loss; products frozen in LN2 do not stick to belts or get
misshapen by conforming to the shape of the belt; there is no need for
specialized maintenance personnel and production rates are very high.
INSTANT QUICK FREEZING TECHNIQUES:
That cryogenic freezing is much quicker than mechanical freezing is
evidenced by a small food processor cited in Food Engineering,
February 1997, as having cut processing time for frozen prepared foods,
primarily breakfast foods for airlines, hospitals, schools and casinos.
Their business was expanding and the old mechanical freezer took four
or five hours to freeze product. Nitrogen tunnels from Praxair, Inc. cut
processing time down 600 percent—to three to four minutes! The
customer was also pleased with quality improvements: color and flavor
retention was enhanced with the quick freezing, freezer burn was
eliminated and dehydration was reduced, increasing yield.
Types of Cryogenic Freezers
As the food industry develops, the different types of customer require
different freezing technologies and skill sets to solve their freezing
problems. Therefore, manufacturers continue to develop different types
of freezers.
Traditional tunnel freezers were the first type of freezer, produced in
response to the demands of the infant hamburger patty business. They
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are now used broadly in all segments of the industry, with prepared
foods, ethnic foods, appetizers and airline-type products.
Tunnels have a cold end and a warm end. LN2 is typically sprayed at the
exit end and cold gas drawn forward toward the entrance. Gas is
exhausted as warmly as possible. Tunnels are modular and can be
configured in length and width to meet customer needs. The longer
tunnels handle more volume. The longest are generally about 60-70 feet.
They are limited by the limits of customer facility space.
Conversion to LN2
Tunnels either use CO2 (-80°F) or LN2 (-150°F). A recent article in
Food Engineering, October 1998, cited a case in which AGA Gas
helped a customer processing value-added potato products to slash
annual freezing costs by 27 percent ($9,000 in the first month) by
changing from CO2 to LN2 in its straight tunnel freezer. The magazine
says AGA advised that although LN2 can be more expensive than CO2
per pound, it is far colder (-320°F versus -109°F) and provides up to 30
percent more cooling capacity per pound in some applications. The
customer‘s freezing costs have dropped from 4.6 to 3.2 percent of
product costs—the cost of goods sold. In seven months, the customer
saved $37,000 in freezing costs over the same period in the previous
year and produced an additional $440,000 worth of product.
Spiral freezers operate much like a tunnel, CO2 at -80°F and LN2 at -
100°F. Spirals have uniform temperatures throughout and provide lots of
freezing within a limited space. There can be as much as 400-500 feet of
belt inside. Spirals are used in high volume applications because a lot of
retention time is needed to process 5-6,000 lbs./hr. of product.
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LN2 immersion freezers use a bath of LN2 to crust-freeze the surface of
a variety of products which are literally dropped into the bath. They are
usually conveyed into a tunnel or spiral freezer to finish the freezing.
Immersion is ideal for a high water-content product such as shrimp. The
outside of the product is frozen to prevent pieces from sticking together
and forming a frozen ball. It is also useful in the poultry industry with
skinless, de-boned chicken breasts, for example, which are very wet and
can conform to the mesh of the conveyor belt. Immersion prevents this
and freezing is completed in a more traditional ammonia-type freezer or
cryogenic spirals or tunnels.
CO2 immersion freezers are not possible, since CO2 is a liquid only at
high pressure; at atmospheric pressure it turns to snow, so there really
can‘t be a bath for immersion. Alternative approaches with CO2 snow
are used, however.
CO2 As a Spot Cooler
CO2 snow can be applied at many stages in food production process,
offering versatile solutions on the line.
BOC Gases, for example, produces a dual horn snow generator they say
provides ―a simple, cost-effective method to control product temperature
in processing or storage and handling operations.‖ This generator
deposits CO2 snow at a low velocity into open containers and blenders.
The horns deliver granular snow without loss from snow blowout
sometimes encountered with conventional snow horns. The generator
―goes everywhere‖ in the processing plant to chill perishables.
Another CO2 application from BOC Gases is the Dri-Pack chilling
system for chilling of perishables directly in their packing boxes.
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Adjustable and adaptable, this simple to use the system places snow
precisely.
Small Volume Immersion Freezer
Air Products introduced a new CRYO-QUICK RH LN2 immersion
freezer at the recent International Poultry Exposition in Atlanta. This is a
small immersion freezer that can be used with other freezing systems or
with an Air Products tunnel. The new freezer has a belt that is half the
width of their other RH immersion freezers, specially designed for
smaller throughput to serve the needs of smaller volume customers.
These freezers control immersion times by changing the amount of belt
that passes through the LN2 or by changing the belt speed. This freezer
is used to produce IQF poultry products, seafood, fruits, vegetables and
beef, and increases quality of products which have been diced, pre-
cooked, breaded and/or marinated. The company has a broad line of
immersion freezers.
SUPERCONDUCTIVE DEVICES:
History of Superconductivity:
Superconductivity was discovered in 1911 by the Dutch physicist, Heike
Kammerlingh Onnes when he was able to liquefy helium by cooling it to
4 Kelvin, or -452°F. This enabled him to cool other materials close to
absolute zero and investigate their electrical properties.
He noted that at these cold temperatures certain materials would lose all
resistance to the flow of electrons and become essentially perfect
conductors of electricity. He called this newly discovered state
superconductivity. An electrical conductor with no resistive heating can
carry current any distance with no losses, giving it essentially 100%
efficiency. Once direct current is introduced into a superconducting
loop, it can flow undiminished forever.
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The discovery in 1986 by Georg Bednorz and Alex Müller, working at
IBM in Zurich, Switzerland, of ceramic-based materials that could
achieve the state of superconductivity at relatively higher temperatures
opened the possibility of applying this technology to electric power
devices such as transmission cable, transformers, motors and generators.
These materials are called High Temperature Superconductors (HTS)
and can achieve their critical temperature (77K) using inexpensive liquid
nitrogen, rather than the more expensive liquid helium required by the
original, ‗Low‘ Temperature Superconductors (LTS) which are
commonly used in the superconducting magnets that power Magnetic
Resonance Imaging (MRI) systems. The reduced cooling needs of HTS
offer performance advantages to electric power devices that do not exist
with LTS.
Commercial applications of Superconductors:
Electric Power: generators, transformers, underground cables,
synchronous condensers, fault current limiters, industrial motors,
magnetic energy storage
Transportation: ship propulsion systems, magnetically levitated
trains, railway traction transformers, electric vehicles
Medicine: magnetic resonance imaging [MRI], particle beam
therapy
Industry: magnetic separators, large motors
Communications: HTS filters for cellular communications systems
Scientific research: accelerator magnets
Military: airborne generator, ship propulsion, directed energy
weapons
Energy Storage:
With power lines increasingly congested and prone to instability,
strategic injection of brief bursts of real power can play a crucial role in
maintaining grid reliability. Small-scale Superconducting Magnetic
Energy Storage (SMES) systems, based on low-temperature
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superconductors, have been in use for many years. These have been
applied to enhance the capacity and reliability of stability-constrained
utility grids, as well as by large industrial user sites with sensitive, high-
speed processes, to improve reliability and power quality.
Larger systems, and systems employing HTS, are a focus of
development. Flywheels, based on frictionless superconductor bearings,
can transform electric energy into kinetic energy, store the energy in a
rotating flywheel, and use the rotational kinetic energy to regenerate
electricity as needed. Using bulk HTS self-centering bearings allows
levitation and rotation in a vacuum, thergy reducing friction losses.
Conventional flywheels suffer energy losses of 3-5% per hour, whereas
HTS based flywheels operate at <0.1% loss per hour. Large and small
demonstration units are in operation and development.
Magnets:
Particle physics uses accelerators to recreate the conditions of the early
universe in an attempt to piece together the complex puzzle of how we
got to where we are today. These huge machines are used to accelerate
particles to very high energies where they are brought together in
collisions that generate particles that only existed a few moments after
the Big Bang that created the universe 15 billion years ago.
The rings of particle accelerators are made of superconducting magnets,
strung together like beads on a necklace. In the Large Hadron Collider,
two concentric rings are made up of thousands of superconducting
magnets. The high energies required could not be economically achieved
without superconducting magnets. The largest are the main dipoles that
steer the particles around the ring. These magnets contain over 1,500
tons of superconducting cable. Superconductivity also enable
construction of giant magnets for the detectors at the LHC used to
measure the properties of the particles produced in the collisions.
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Magnetic Levitation:
Magnetic-levitation is an application where superconductors perform
extremely well. Transport vehicles such as trains can be made to ―float‖
on strong superconducting magnets, virtually eliminating friction
between the train and its tracks. Not only would conventional
electromagnets waste much of the electrical energy as heat, they would
have to be physically much larger than superconducting magnets. A
landmark for the commercial use of MAGLEV technology occurred in
1990 when it gained the status of a nationally-funded project in Japan.
The Minister of Transport authorized construction of the Yamanashi
Maglev Test Line which opened on April 3, 1997. In December 2003,
the MLX01 test vehicle attained an incredible speed of 361 mph (581
kph).
Although the technology has now been proven, the wider use of
MAGLEV vehicles has been constrained by political and environmental
concerns (strong magnetic fields can create a bio-hazard). The world‘s
first MAGLEV train to be adopted into commercial service, a shuttle in
Birmingham, England, shut down in 1997 after operating for 11 years. A
Sino-German maglev is currently operating over a 30-km course at
Pudong International Airport in Shanghai, China. The U.S. plans to put
its first (non-superconducting) Maglev train into operation on a Virginia
college campus.
Magnetic Resonance Imaging:
An area where superconductors can perform a life-saving function is in
the field of biomagnetism. Doctors need a non-invasive means of
determining what‘s going on inside the human body. By impinging a
strong superconductor-derived magnetic field into the body, hydrogen
atoms that exist in the body‘s water and fat molecules are forced to
accept energy from the magnetic field. They then release this energy at a
frequency that can be detected and displayed graphically by a computer.
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Magnetic Resonance Imaging (MRI) was actually discovered in the mid
1940′s. But, the first MRI exam on a human being was not performed
until July 3, 1977. And, it took almost five hours to produce one image!
Today‘s faster computers process the data in much less time.
CRYOGENIC APPLICATIONS FOR SPACE TECHNOLOGY:
Cryogenic has two types of application in Space, namely as a Fuel and
to cool the detectors below their operational temperatures. Liquid
Hydrogen and oxygen are considered the best rocket propellants for
space application because of their high specific impulse. Liquid
Hydrogen is used as a fuel and Liquid oxygen (LOX) as an oxidizer.
They combine together in the combustion chamber and produce thrust.
Both the gases being light need large volumes and therefore used in the
form of liquids to conserve space. These liquids are stored in insulated
vessels and transported to the combustion chamber through pipes, valves
and turbo pumps. The entire assembly, called Cryogenic Engine has
been developed and successfully tested by ISRO. The next GSLV Mark
III is expected to be launched using this indigenous engine. Many of the
space missions use infrared, gamma ray, and x-ray detectors that operate
at cryogenic temperatures. The detectors are cooled to increase their
sensitivity. Astronomy missions often use cryogenic telescopes to reduce
the thermal emissions of the telescope, permitting very faint objects to
be seen. Vibration free Mini size cryo coolers are used to cool these
detectors.
NASA's workhorse space shuttle used cryogenic hydrogen/oxygen
propellant as its primary means of getting into orbit. LOX is also widely
used with RP-1 kerosene, a non-cryogenic hydrocarbon, such as in the
rockets built for the Soviet space program by Sergei Korolev.
Russian aircraft manufacturer Tupolev developed a version of its
popular design Tu-154 with a cryogenic fuel system, known as the Tu-
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155. The plane uses a fuel referred to as liquefied natural gas or LNG,
and made its first flight in 1989.
Space Cryogenics is the application of cryogenics to space missions.
These applications fall into two broad areas, supporting space science
missions and supporting the space transportation infrastructure.
Science applications: The atmosphere is opaque to much of the electro-
magnetic spectrum. In space, the absence of an atmosphere has been a
great boon to doing astronomy at these wavelengths. Being in space has
enabled Earth and atmospheric science missions to gather global data.
Many of these science missions use infrared, gamma ray, and x-ray
detectors that operate at cryogenic temperatures. The detectors are
cooled to increase their sensitivity. Astronomy missions often use
cryogenic telescopes to reduce the thermal emissions of the telescope,
permitting very faint objects to be seen. A broad range of cryogenic
technology is needed to support these missions. For instance, materials
change their properties (strength, dimensions, thermal, electrical,
magnetic, and optical properties all change). These changes need to be
considered when building an instrument for space. It is a challenge to
design a telescope that is assembled at room temperature and then
cooled to 20 kelvin (-253°C) or so and launched into space. After
surviving the high vibration environment of launch and the dimensional
changes of cooling down, the instrument must be in focus and provide
an undistorted image. All of this, while being well insulated and having
very low mass.
Then there is the matter of how to cool the instrument. Radiators
(blackened surfaces shielded from the Sun and Earth) can cool
instruments to the 100 kelvin (-173°C) range in Earth orbit. In orbits far
from the Earth (such Spitzer uses) 30k (-243°C) can be reached. For
lower temperatures, instruments have used stored solid cryogens (such
as nitrogen, neon, or hydrogen). Solid hydrogen will work for
requirements down to 6 kelvin (-267°C). For lower temperatures, liquid
helium can be used in the 1-2 kelvin range. Containing liquids while
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venting the effluent vapor has been a challenge. The disadvantage of
using a stored cryogen is that it is converted to vapor by heat dissipated
in the instrument or that comes in through the supports and insulation.
Eventually, the cryogen is consumed, ending the mission. Recently there
have been many advances in building closed cycle refrigerators for
space applications. These coolers have extended mission durations and
extended the range of temperatures available to 0.05 kelvin. These
coolers are required to be long lived, 5-10 years, have a very low system
mass (including the mass of solar cells and electronics to power the
coolers and radiators to reject heat) and, often, have very low vibration.
Another area of space science, which makes use of cryogenics, is sample
preservation. This includes the preservation of biological samples from
experiments on the Shuttle and the Station and the preservation of
material gathered from comets, asteroids, and other planets. These
applications have used phase change materials (solid to liquid transition)
or liquid nitrogen absorbed in fine pore as coolants. Closed cycle coolers
are now being developed for these applications.
Space transportation: Liquid hydrogen and liquid oxygen are used in the
main engines of the Shuttle because they offer a very high specific
impulse (thrust per unit mass of propellant consumed). These propellants
are cryogenic with normal boiling points of 20 kelvin (-253°C) and 90
kelvin (-183°C) respectively. For the Shuttle, these propellants are
stored in the poorly insulated external tank. There is interest in
extending the storage time of cryogenic propellants from a few hours to
many years and in being able to resupply rockets with these propellants
from depots in space. While, in principle, this can be achieved, the
techniques discussed above, the size of transportation systems, many
tons of propellants, require new engineering approaches.
Cryogenics [email protected]
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ASTRONOMY IN SPACE
In thinking about the reasons to perform astronomy in space, we first
consider the effect of the earth‘s atmosphere. On a scale of decreasing
energy, gamma rays, cosmic rays, X-rays and the ultraviolet are so
energetic that they interact strongly with the elements in the atmosphere,
producing lower energy particles that are all we can detect. If we wish
to see the primary radiation, we must go to space. In the optical region,
the atmosphere is largely transparent, but there is enough interference
that the Hubble telescope can see much that is hidden from even the
large telescopes on the high peaks of Hawaii and Chile.
As we go into the infrared, the atmosphere becomes increasingly opaque
because of interaction with the molecule in the atmosphere. Below 20
micrometers, we are essentially blind until we reach the submillimeter
range. Even here, unscrambling the astronomical data from the
background requires detection in at least two bands, and preferably three
or four.
What are the kinds of instruments that are used in space? Gamma rays
are typically captured in cooled solids, and the cooling has largely been
in the range of the solid cryogens, i.e. methane, xenon, ammonia, etc.
Next we have X-rays, which are also captured in cooled solids, but the
detector signal-to-noise ratios benefits greatly from lower
temperatures. In fact, the X-ray the detectors aboard the ASTRO-E2
mission to be launched in 2005 are at 65 milliKelvin, which will be the
lowest yet in space.
Ultra-violet and optical detectors are not crucially dependent on low
temperatures, but benefit from some cooling. If you have friend who is
an ardent amateur astronomer (and either well-to-do or willing to
sacrifice creature comforts), he will have charge coupled detectors
cooled to dry ice temperatures. As we move into the infrared, cooling
again becomes crucial. Below 20 microns and into the millimeter range,
bolometers are the detector of choice. Their signal-to-noise ratio
improves as T-2.5
, so halving the temperature improves the S/N by a
Cryogenics [email protected]
27 | P a g e
factor of 5.6. Below 1 mm (300 GHz, for radio types) various
semiconductor detectors, coupled to semiconductor amplifiers such as
HEMT‘s (High Electron Mobility Transistors) have the lowest signal-to-
noise ratio. Typically, their performance does not improve below 20 K,
so they can be cooled by heat exchange if a lower temperature is also
required..
QUESTIONS:
1. Write short notes on the application of cryogenic system on: (VTU
June 2010, Dec 2010, Jan 2014, Jan 2015)
a) Space technology.
b) Super conducting devices (bearings, cryotrons).
c) Instant quick freezing techniques.
d) Food preservation.
e) Chemical rockets.