unit 8 application of cryogenic systems

Cryogenics [email protected]

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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.


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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


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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.