2. IV B. Tech – I Semester
(19BT70309) CRYOGENICS
(Program Elective-5)
Int. Marks Ext. Marks Total Marks L T P C
40 60 100 3 - - 3
PRE-REQUISITES:
A Course on Thermal Engineering-II
COURSE DESCRIPTION:
Necessity of Low temperature, Multi-stage refrigeration, Cascade system, Applications of
low temperature, Properties of cryogenic fluids, Liquefaction of air, hydrogen and helium,
gas separation and gas purification systems, Low-temperature insulation, Storage systems
and Cryogenic fluid transfer systems
COURSE OUTCOMES:
After successful completion of this course, the students will be able to:
CO1. Demonstrate the knowledge of cryogenic systems for low temperature
applications.
CO2. Analyze the properties of cryogenic fluids for low temperature application.
CO3. Analyze the various refrigeration and liquefaction systems for low temperature
application.
CO4. Analyze the various gas separation and gas purification systems for low
temperature application.
CO5. Demonstrate the knowledge of cryogenic insulation for suitable storage and
handling systems.
DETAILED SYLLABUS
UNIT I: CRYOGENIC SYSTEMS (09 periods)
Introduction to Cryogenic Systems, Cryogenics – Definition, Historical development,
Necessity of Low temperature, Limitations of vapour compression system for the production
of low temperature, Multi stage refrigeration system - Cascade system.
Applications of Cryogenics: Applications in space, Food Processing, super conductivity,
Electrical Power, Cryobiology, Medicine-Cryosurgery, Electronics and Cutting Tool Industry.
UNIT II: PROPERTIES OF CRYOGENIC FLUIDS (09 periods)
Effects on the properties of metals - Low Temperature properties of Engineering Materials-
Mechanical properties, Thermal properties, Super conductivity and Super fluidity, Electric
and magnetic properties
T-S diagram of a cryogen; Properties of cryogenic fluids - Liquid Methane, Liquid Neon,
Liquid Nitrogen, Liquid Oxygen, Liquid Argon, Liquid Air, Liquid hydrogen and helium.
3. UNIT III: REFRIGERATION AND LIQUEFICATION (09 periods)
Manufacture of Dry ice, Joule’s Thomson effect, Liquefication of air - Linde system, Claude
system, Cascaded System, Liquefaction of neon, Hydrogen and Helium, Stirling Cycle Cryo
Coolers, Gifford McmahonCryo- refrigerator, Pulse tube refrigerator, Solvay cycle
refrigerator, Vuillimier refrigerator.
UNIT IV: GAS SEPARATION AND GAS PURIFICATION SYSTEMS (09 periods)
The thermodynamically ideal separation system properties of mixtures, Principles of gas
separation, air separation systems, Hydrogen, Argon, Helium air separation systems, Gas
purification methods.
UNIT V: LOW TEMPERATURE INSULATION (09 periods)
Types of Insulation - Reflective insulation, Evacuated powders, Rigid foams; Super
insulation; Dewar vessels; Hazards in cryogenic engineering. Cryogenic fluid transfer
systems. Transfer through un-insulated lines, vacuum insulated lines, porous insulated lines
etc.
Total Periods: 45
Topics for self-study are provided in the lesson plan.
TEXTBOOKS:
1. Randal F.Barron, Cryogenic systems, McGraw Hill, 2nd
edition, 1986
2. Klaus D.Timmerhaus and Thomas M.Flynn, Cryogenic Process Engineering, Plenum
Press, New York, 1989.
REFERENCE BOOKS:
1. Traugott H.K. Frederking and S.W.K. Yuan, Cryogenics - Low Temperature
Engineering and Applied Sciences, Yutopian Enterprises, 2005.
2. A. R. Jha, Cryogenic Technology and Applications, Butterworth-Heinemann, 2005
CO-PO-PSO Mapping Table :
Course
Outcome
Program Outcomes
Program
Specific
Outcomes
PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 PO10 PO11 PO12 PSO1 PSO2 PSO3
CO1 3 1 1 3
CO2 3 3 1 3
CO3 3 3 1 3
CO4 3 3 1 3
CO5 3 1 1 3
Average 3 2.2 1 3
Correlation
level
3 2 1 3
Correlation Levels: 3– High 2 - Medium 1– Low
4. LESSON PLAN
Name of the Subject : CRYOGENICS (19BT70309)
Class & Semester : IV B. Tech – I Semester
Name(s) of the faculty Member(s) : Dr. R. Satya Meher
S.
No.
Topic
No. of
periods
Book(s)
followed
Topics for self
study
UNIT – I: INTRODUCTION
1.
Introduction to Cryogenic Systems,
Cryogenics – Definition
01
T1
Future
refrigerants
for cascade
systems
2.
Historical development, Necessity of Low
temperature
01 T1
3.
Limitations of vapour compression system
for the production of low temperature
01 T1
4.
Multi stage refrigeration system - Cascade
system
01 T1
5.
Applications of Cryogenics: Applications in
space
01 T1
6. Food Processing, super conductivity 01 T1
7. Electrical Power, Cryobiology 01 T1
8. Medicine-Cryosurgery 01 T1
9.
Electronics and Cutting Tool Industry. 01 T1
Total periods required: 09
UNIT – II: PROPERTIES OF CRYOGENIC FLUIDS
10. Effects on the properties of metals 01 T1
Properties of
materials at
cryogenic
temperature
Space
Applications of
cryogenic fluids
11.
Low Temperature properties of
Engineering Materials
01 T1
12. Mechanical properties, Thermal properties 01 T1
13.
Super conductivity and Super fluidity,
Electric and magnetic properties
01 T1
14.
T-S diagram of a cryogen; Properties of
cryogenic fluids
01 T1
5. S.
No.
Topic
No. of
periods
Book(s)
followed
Topics for self
study
15.
Liquid Methane, Liquid Neon, Liquid
Nitrogen,
01 T1
16. Liquid Oxygen 01 T1
17. Liquid Argon, Liquid Air 01 T1
18. Liquid hydrogen and helium. 01 T1
Total periods required: 09
UNIT -III: REFRIGERATION AND LIQUEFICATION
19.
Manufacture of Dry ice, Joule’s Thomson
effect
01
T1 Mixed
Refrigerant
Cycle
Refrigeration
20. Liquefication of air - Linde system 01 T1
21. Claude system, Cascaded System 01 T1
22. Liquefaction of neon, Hydrogen 01 T1
23. Liquefaction of Helium 01 T1&R1
24. Stirling Cycle Cryo Coolers 01 T1
25. Gifford Mcmahon Cryo- refrigerator 01 T1
26. Pulse tube refrigerator 01 T1
27.
Solvay cycle refrigerator, Vuillimier
refrigerator.
01 T1
Total periods required: 09
UNIT IV:GAS SEPARATION AND GAS PURIFICATION SYSTEMS
28.
The thermodynamically ideal separation
system
1 T1
Cry coolers
29. properties of mixtures 1 T1
30. Principles of gas separation 1 T1
31. Principles of air separation systems 1 T1
32. Hydrogen separation systems 1 T1
33. Argon separation systems 1 R1 & R2
34. Helium air separation systems 1 T1
35. air separation systems 1 T1
36. Gas purification methods. 1 T1
Total periods required: 09
6. S.
No.
Topic
No. of
periods
Book(s)
followed
Topics for self
study
UNIT – V: LOW TEMPERATURE INSULATION
37. Types of Insulation -Reflective insulation 1 T2
Safety in
cryogenics
Vacuum
technology
Cryogenic
Storage &
Distribution
System
38. Evacuated powders, Rigid foams 1 T2
39. Super insulation 1 T2
40. Dewar vessels 1 T2
41. Hazards in cryogenic engineering 1 T2
42. Cryogenic fluid transfer systems 1 T2
43. Transfer through un-insulated lines 1 T2 & R1
44. vacuum insulated lines 1 T2
45.
porous insulated lines
1
T2
Total periods required: 09
Grand total periods required: 45
Text Books:
T1. Randal F.Barron, Cryogenic systems, McGraw Hill,Second edition, 1986
T2. Klaus D.Timmerhaus and Thomas M.Flynn, Cryogenic Process Engineering, Plenum
Press, New York, 1989.
Reference Books:
R1.Traugott H.K. Frederking and S.W.K. Yuan, Cryogenics - Low Temperature
Engineering and Applied Sciences, Yutopian Enterprises, 2005.
R2.A. R. Jha, Cryogenic Technology and Applications, Butterworth-Heinemann, 2005
7. UNIT – I: INTRODUCTION
Introduction to Cryogenics:
Cryogenics is basically coming from the kryo which means very cold; from greek language
this word has come and genics means to produce. So, basically cryogenic means, science
and technology associated with generation of low temperature below 123 kelvin. The funny
part about this is kryo is as cold as ice, that is what was thought about then in the Greek
mythology or Greek scientist then, but ice is the very hot temperature for cryogenic and
therefore, understand although it meant as cold as ice, it means much lower temperature
than ice temperatures. Cryogenic Engineering is very widely used in space and atomic
energy.
Historical development: Chronology in the cryogenic engineering is very important and
we will see how different events unfolded over the years worldwide. So, really speaking, the
first development happened in 1877 where as you understand, oxygen is the most
important gas for all of us; the question of life basically. So, as you can understand, the first
development was towards liquefaction of oxygen. Therefore, storage of oxygen and oxygen
gas and Cailletet and Pictet in 1877, almost 133 years before we had first liquefaction of
oxygen and oxygen liquefies at around 90 Kelvin. Both Cailletet and Pictet were professors,
Pictet was from Geneva, Switzerland while Cailletet was from Paris and both of them
actually presented a paper on liquefaction of oxygen in an conference in Paris; however,
both of them work independently. In fact, Pictet device are they are available both the
devices are available on net if you want to get more and more information or in books also.
Pictet basically came from Switzerland from University of Geneva was a physicist, even
Cailletet was a physicist and in 1877, Pictet liquefied oxygen and he what he used is
basically cascade kind of system were sulpher dioxide and CO2 or liquid CO2 was used to
liquefy oxygen. Oxygen was pressurized to very high pressure of around 320 bar and it was
liquefied at around minus 140 degree centigrade. Well, Pictet here, in this case having
precooled the gas to particular temperature, ultimately he expanded the gas using a simple
Euler Thomson expansion or sway capillary tube. Similarly, Cailletet also did the pre-cooling
of oxygen gas; however, he did it at 200 bar and he used ethylene for pre-cooling up to
minus 100 degree centigrade around and then again expanded the gas using Euler Thomson
expansion device. So, basically they followed a kind of similar technique; however, they
were precooling refrigerants very different, but the funniest part was both of them did the
independent study and presented in the same year at the same conference. Now in those
days in 1877; however, they did not know how to store liquid oxygen. They got a mist, they
8. got some fog, they got some condensate, but they did not know how to store that. So, that
time that knowledge was not there it came later on. The next development happened just
two years after that were Linde found Linde Elsmaschinen AG. This is a private company
which Linde found and Linde is a big name in cryogenics. In fact, this company which came
into existent in 1879, it became Linde AG in 1965 and most of the companies are under this
umbrella of Linde now. It is a very big name in cryogenics. It is in India also and here for
the first time, there are modern domestic refrigerator was kind of shown by Linde. The next
development happened in 1892. As I said, in those years in 1877, the facility of storing
liquid cryogen was not there. And Dewar, device is named after the scientist is called
Dewar. He developed a vacuum insulated vessel for cryogenic fluid storage. So, here for the
first time, he understood the importance of a double walled flask with vacuum in between
and he showed in principle how one can store liquid nitrogen, liquid oxygen for a long time
which otherwise is to get evaporated immediately and this is a very important development
for cryogenic engineers and the vessel in which we store liquid cryogen is also called as
nitrogen dewar or helium dewar named after this particular scientist. The next important
development happened by a very big scientist called Kamerlingh Onnes from Leiden
laboratory in Netherlands in 1895. Kamerlingh Onnes is a very big name again and he was
the physicist. He was a professor at Leiden University in Netherlands and here he found
cryogenic laboratory at this place and he invited lot of people all over the world to work at
this laboratory. Next, in 1902, Claude for the first time, he found a next company called l’Air
Liquide in France where commercial air liquefies were made available. So, first we had Linde
and then we had Claude. As two commercial companies came into existence in Europe, this
company came into existence in France and they had commercial air liquefies available
worldwide. So, it is a beginning of real commercial Cryogenic Engineering. Onnes,
Kamerlingh Onnes after founding this Leidin Laboratory, cryogenics laboratory at Leidin in
1908, he liquefied helium. See, all this development if you could see was towards lowering
and lowering of temperatures. Here we had 90 Kelvin. In 1898 Dewar for the first time
liquefied nitrogen; that means, you could 77 Kelvin here. If you come down below, in 1908,
Onnes liquefied helium; that means, he could reach the temperature of 4.2 Kelvin. This is
the most important thing that happened. Now that is a very typical connection to India in
this particular case that the helium gas which Onnes utilized for liquefaction, it had some
Indian connection he had used monocyte sand from India and when he heated this
monocyte sand, he got helium gas. So, he used this particular gas to liquefy and then he
got liquid helium in 1908. In India, we had a big conference in Indian Institute of Science,
which celebrated the centenary year of liquid helium. Having liquefied helium, the next
9. action is to utilize this liquid helium so that one could find different properties of metals and
in 1911, Kamerlingh Onnes discovered superconductivity phenomenon. As you understand,
as you go on lowering the temperature of a metal and the metal become more and more
conductive, the resistance become almost equal to 0 and it shows in principle, the
phenomena called superconductivity. So, once you get lower and lower temperature,
Kamerlingh Onnes in the year 1911 showed that mercury becomes superconducting at 4.2
Kelvin and this was a very big phenomenon which was discovered then and for this
phenomenon, he got noble prize in the year 1930. Onnes got around 60 cc of helium in this
experiment at this point.
Then next important development happened with engineering as a more in application area,
in 1926 Goddard test fired the first cryogenically propelled rocket. So, you could use the
cryogenic fluid for the propulsion of big test rockets, big rockets and here, he had used
gasoline and liquid oxygen. So, here is gasoline as a fuel and use liquid oxygen as an
oxidizer, propel the rocket. As you know the cryogenic engine what today we used uses
liquid hydrogen as a fuel and liquid oxygen as an oxidizer, but at this point in time use
gasoline as the fuel. In the year 1934, Kapitza, again a very big name in cryogenic
engineering, he designed the first expansion engine. He could get liquid helium using the
first expansion engine without pre-cooling actually and this was a major development to
have helium liquefiers in commercial applications. In the year 1952, one of the very big
institutes came into existence called National Institute of Standards and Technology or
NIST, formally known as National Bureau of Standards- NBS. It is at Colorado in USA and
here first cryogenic engineering laboratory got established were real innovation in US
started. In fact, most of the conferences what we see today as CEC that is Cryogenic
Engineering Conference or cryocooler conference, this originated from with the initiatives
taken by NIST, lot of innovations happened at this place in NIST. Going head from here, in
1966, what we got was a development of dilution refrigerator. Having achieved 4.2 Kelvin
liquid helium temperature, scientist started working towards going lower and lowers in
temperature. As I said, most of the development happened towards lowering of the
temperatures to reach first 4.2 Kelvin and then to go below 4.2 Kelvin. So, a dilution
refrigerator was a device which uses helium 4 and the other isotope of helium, that is
helium 3, in combination together will study about this also later on and one could reach
down lower in temperature below 1 Kelvin and this was a concept which is a very important
concept going below 1 Kelvin. A dilution refrigerator came into existence and lot of research
below 1 Kelvin, one could really go for the device was available then and the properties of
material could be studied below 1 Kelvin. Superconductivity research was very much high in
10. those days. What you know actually is you got a low TC material for which the low
temperature required for superconductivity and then the high TC that is the temperatures
were the high TC is obtained; that means, temperature above liquid helium temperature
and here first time one could go up to 23 Kelvin where superconductivity could be
established in material. This is a very important concept because it is very costly to reach
4.2 Kelvin in order to utilize helium at 4.2 Kelvin, but attempts were always there to use
liquid nitrogen at 77 Kelvin. So, scientist always craved to invent new and new materials
which will show superconductivity at higher and higher temperature. So, this was the first
time in 1975 that they could show superconductivity at a high temperature of around 23
Kelvin. And then in 1994, cryocooler development took a big turn where Professor
Matsubara from Newborn University, Japan developed a 4K cryocooler working on a the
pulse tube technology and called pulse tube cryocooler. Here the pulse tube cryocooler or
Gifford Mc Mahon cryocooler or Sterling cryocoolers were already there and they are as you
know close cycle cryocoolers. So, attempts were always going on. The research was always
going on in order to avoid use of liquid nitrogen or liquid helium which require continuous
replenishment because they get evaporated over a period of time. So, the research was
always going on in order to generate this lower temperature in a close cycle manner so that
different cryogens like liquid helium, liquid nitrogen or liquid oxygen etcetera will not be
required to be used and here the for the first time, he could show pulse tube cryocooler
reaching 4K temperature in a close cycle manner. Now you can imagine, I have got a device
which can produce 4K temperature in a continuous manner in a close cycle fashion whether
gas is getting compress and expanded and fluorescence temperature of 4 Kelvin
continuously and I do not have to really worry about any cryogen replenishment at all and
that was a major breakthrough and then lot of research initiated in the area of this pulse
tube cryocooler to reach lower and lower temperature and now one can reach even 1.5
Kelvin or 1.2 Kelvin using again helium 3 or helium 4 as a gas and one can really reach
lower and lower temperature below 4 Kelvin also this was a major breakthrough. As you
could see from this that lot of events happened from almost seven through to almost year
2000 and after that, where things change from oxygen liquefaction and it came down to
helium liquefaction and a temperature went below that of liquid helium.
Definition:
Cryogenics is the science and technology associated with generation of low temperature
below 123 Kelvin. What is the logic behind this. Let us see this. So, this 123 Kelvin is a
dividing line between cryogenics and refrigeration. You can see all this under the title of
11. Cryogenics. Now what was the logic of having this division at 123 Kelvin. As you can see, all
this gases were earlier called as permanent gases. It was thought that these gases could
never be liquefied. They tried to liquefy those gases at room temperature by pressurizing
these gases. One can get all these gases on the right side. They can get converted to liquid
if you pressurize this gases; however, if we talk about this gases, even if you pressurize
them at room temperature to a very high pressure, when I say high pressure it could be of
the order of 300 to 400 bar. It could be very high pressure, but in this case this gases will
not get liquefied. So, you have to use some different techniques to go below the room
temperature and then liquefy these gases. These were classified as permanent gases
thinking that these gases can never be liquefied and therefore, this dividing line came into
existence of around 120 Kelvin and on the left side of it what we call as cryogenic
engineering, on the right side of this what we call is a refrigeration
Necessity of low temperature:
Cryogenic temperatures are considerably lower than those encountered in ordinary physical
processes. At these extreme conditions, such properties of materials as strength, thermal
conductivity, ductility, and electrical resistance are altered in ways of both theoretical and
commercial importance. Because heat is created by the random motion of molecules,
materials at cryogenic temperatures are as close to a static and highly ordered state as is
possible. Cryogenics had its beginning in 1877, the year that oxygen was first cooled to the
point at which it became a liquid (−183 °C, 90 K). Since then the theoretical development
of cryogenics has been connected to the growth in capability of refrigeration systems. In
1895, when it had become possible to reach temperatures as low as 40 K, air was liquefied
and separated into its major components; in 1908 helium was liquefied (4.2 K). Three years
later the propensity of many supercooled metals to lose all resistance to electricity—the
phenomenon known as superconductivity—was discovered. By the 1920s and 1930s
temperatures close to absolute zero were reached, and by 1960 laboratories could produce
temperatures of 0.000001 K, a millionth of a degree Kelvin above absolute zero.
Temperatures below 3 K are primarily used for laboratory work, particularly research into
the properties of helium. Helium liquefies at 4.2 K, becoming what is known as helium I. At
2.19 K, however, it abruptly becomes helium II, a liquid with such low viscosity that it can
literally crawl up the side of a glass and flow through microscopic holes too small to permit
the passage of ordinary liquids, including helium I. (Helium I and helium II are, of course,
chemically identical.) This property is known as superfluidity.
12. The most important commercial application of cryogenic gas liquefaction techniques is the
storage and transportation of liquefied natural gas (LNG), a mixture largely composed of
methane, ethane, and other combustible gases. Natural gas is liquefied at 110 K, causing it
to contract to 1/600th of its volume at room temperature and making it sufficiently compact
for swift transport in specially insulated tankers.
Very low temperatures are also used for preserving food simply and inexpensively. Produce
is placed in a sealed tank and sprayed with liquid nitrogen. The nitrogen immediately
vaporizes, absorbing the heat content of the produce.
In cryosurgery a low-temperature scalpel or probe can be used to freeze unhealthy tissue.
The resulting dead cells are then removed through normal bodily processes. The advantage
to this method is that freezing the tissue rather than cutting it produces less bleeding. A
scalpel cooled by liquid nitrogen is used in cryosurgery; it has proved successful in removing
tonsils, hemorrhoids, warts, cataracts, and some tumours. In addition, thousands of
patients have been treated for Parkinson disease by freezing the small areas of the brain
believed to be responsible for the problem.
The application of cryogenics has also extended to space vehicles. In 1981 the U.S. space
shuttle Columbia was launched with the aid of liquid hydrogen/liquid oxygen propellants.
Of the special properties of materials cooled to extreme temperatures, superconductivity is
the most important. Its chief application has been in the construction of superconducting
electromagnets for particle accelerators. These large research facilities require such
powerful magnetic fields that conventional electromagnets could be melted by the currents
required to generate the fields. Liquid helium cools to about 4 K the cable through which the
currents flow, allowing much stronger currents to flow without generating heat by
resistance.
Limitations of vapour compression system for the production of low temperature:
Low-temperature evaporation requires low evaporation pressures while the condensing
pressure is at normal levels. It is often beneficial, and in some cases necessary, to separate
the evaporation and condensing pressure levels by more than one compressor step. This is
because when the pressure ratio over the compressor increases, the discharge temperature
out of the compressor will also increase. Simultaneously, the compressor efficiency
decreases, which increases operating costs. High discharge temperatures may cause both
13. the refrigerant and the lubrication oil to decompose. This in turn will shorten the life of the
compressor. Figure 1.1, point (a1), shows the higher discharge temperature of a single-step
refrigeration system with low evaporation temperature.
Figure 1.1 A log P/h diagram showing the resulting higher discharge temperature for a
larger compression step.
MLTI STAGE Refrigeration System:
Intermediate gas cooling is often used between the two compressor steps. By cooling the
refrigerant vapor after the first compressor, the discharge gas leaving the high-stage
compressor can be kept at an acceptable temperature level. The intermediate cooling also
increases the compressor efficiency, which reduces the compressor power consumption.
A two-stage system is a refrigeration system working with a two-stage compression and
mostly also with a two-stage expansion. A schematic system layout and the corresponding
process in a log P/h diagram are shown in Figure 1.2. Flash gas is separated from liquid
refrigerant in an intermediate receiver between the two expansion valves. The high-stage
compressor will then remove the flash gas, as shown in Figure 1.2. The removal of the gas
between the expansion stages reduces the quality of the refrigerant vapor that enters the
evaporator from the state 'j' (which would be the vapor quality if only one expansion valve
were used) to the state 'i', as shown in the log P/h diagram of Figure 1.2
14. Figure 1.2 Schematic layout of a two-stage low-temperature refrigeration system (right)
with a log P/h diagram (above).
Due to a lower quality entering vapor, each mass unit of refrigerant passing through the
evaporator will be able to absorb more heat, reducing the required refrigerant mass flow
rate for a given cooling capacity. This in turn reduces the required low-stage compressor
size. Because of the enhanced heat transfer coefficient in the evaporator, the heat transfer
area needed is also reduced.
Intercooler system
An intercooler system uses an intermediate evaporation step, similar to the economizer
system, to cool the discharge gas from the first compressor step. The two-stage intercooler
system is shown together with a corresponding log P/h diagram in Figure 1.3.
The refrigerant liquid leaving the condenser (state 'a' in Figure 1.3) is split into two
streams. The smaller part of the liquid (m2) is fed through an intermediate expansion valve
('a' to 'b'), and then allowed to evaporate on one side of the BPHE intercooler ('b' to 'c').
The main flow (m1) is sub-cooled by leading it through the other side of the BPHE
intercooler ('a' to 'd'). The sub-cooled refrigerant liquid leaving the intercooler is fed through
the main expansion valve ('d' to 'e') and then through the main evaporator ('e' to 'f'). The
15. sub-cooling decreases the inlet vapor quality, which reduces the refrigerant mass flow rate
through the evaporator and the required low-stage compressor size for a given cooling
capacity. The intermediate refrigerant stream (m2) is not completely vaporized when
leaving the intercooler (state 'c'). The remaining liquid is evaporated when it is mixed with
the hot discharge gas from the low-stage compressor.
This results in efficient gas cooling ('g' to 'h'). The discharge gas from the high stage
compressor can be kept at an acceptable temperature (state 'i'), and the compressor
efficiency is increased.
The high efficiency of the SWEP BPHE minimizes the temperature difference between the
evaporating stream (m2) and the sub-cooled stream (m1), which increases the overall
efficiency of the system
Figure 1.3 Schematic layout of an intercooler system (right) with a log P/h diagram
(above).
CASCADE REFRIGERATION SYSTEM: From a thermodynamic point of view, the cascade
system is very desirable for liquefaction because it approaches the ideal reversible system
more closely than any other discussed thus far. As one can see from the complexity of the
system shown in Figure 1.4. nothing is gained by trying to write down a general equation
16. for the liquid yield as we have done for the other systems. The fact that the irreversible
expanstions through the expansion valves occur across smaller pressure rages than in the
other systems would lead us to believe, however, that the cascade system would have
improved performance over the other systems mentioned. The lower pressure requirements
are another point in favor of this system. On the other hand, the cascade system does have
a serious practical disadvantage. Each loop of the system must be completely leakproof in
order to prevent the fluids form getting into the wrong place. This imposes a maintenance
hardship to make sure that leaks do not occur and introduces a safety hazard when leaks do
occur.
Fig. 1.4 Cascade refrigeration system
Applications:
Cryogenics in rocket propulsion, cooling of infrared sensors or space Simulation. These are
very important aspects of cryogenics used in space. Cryogenic engines are powered by
cryogenic propellants, you know different propellants are there, but if we use Cryogenic
17. propellants for example, Liquid hydrogen can be used as fuel to propel the rocket. Most of
you have heard about Cryogenic engine and Cryogenic Engine uses Liquid Hydrogen as a
fuel. It also uses Liquid Oxygen as an oxidizer. So, these two are important components,
Liquid hydrogen is liquid at 20 kelvin, liquid oxygen is liquid at 90 kelvin. These two form
the fuel for Cryogenic engine and therefore, it can work as cryogenic propellant and these
are very important.
The next is cooling of infrared detectors or telescopes or cold probes are some of the major
applications of cryogenics. we have to use infrared detector in space because when we are
taking night surveillance, while taking picture, infrared detectors. Now, in order to get a
good signal to noise ratio, that means, in order to get a good image, detector to be used at
very low temperature. In fact, lower the temperature of this detector is better the picture or
less is the noise and therefore, infrared detectors have to be kept cool at 80 kelvin or less.
Similarly in lot of electronic circuits in space for example, amplifiers or any electronics which
gets heated over period of time, if I got cold probes, I can keep it very cold and therefore,
the noise level will be minimum in this case. This form a very important applications of
cryogenics in space. Sterling cooler as example is very widely used to cool the infrared
detector in space and what is important therefore, is to design a miniature version of
sterling coolers; less weight and less volume which are very important things for space.
Space simulation chamber are the realistic environment for air craft. The cold space is
stimulated at cryogenic temperature by use of ln2 or liquid nitrogen. So, space simulation
chamber basically simulate space. Space has got very less of around 4 kelvin and very less
pressure that is vacuum. All this thing simulated have to be simulated on the ground if any
part you want to take in space it has to first gets space qualified which will what will happen
is space simulation chamber. Its a small, it is a small test which was done on the ground
which stimulates condition in a space where in, the low temperature are generated by
different cryogenics like liquid nitrogen, liquid oxin, etcetera. At a same time, different
levels of vacuum are required in space and what we do for these we use cryo pumps or
turbo molecular pumps. In this way, we can achieve lower and lower vacuums to simulate
condition like what it is in space. This is a very important applications of cryogenics in
space.
The applications of cryogenic engineering are in Magnetic Separation, Heat Treatment of
different materials or Recycling of certain materials.
18. The magnetic separation technique is used in variety of applications like enhancing the
brightness of kaolin, this is the clay improving the quality of ultra high purity quartz
etcetera. What we use is the superconducting magnet ensures proper separation. So, what
they do basically, get this kaolin clay from different or mines on the eastern part of India for
example, it get associated with lot of metals and in order to separate this materials or
metals like silicates etcetera, and pass the whole thing through some magnate and this
materials or metals will get attracted toward this magnate, after that the kaolin will be free
of this metals; however, in order to separate out this materials from the kaolin clay, what
we need is a very high magnetic fields and this high magnetic fields can be achieved only by
super conducting magnets and not by electro magnates. If we were to use electromagnetic,
then my current passing through this wires is very high and therefore, losses will be very
high. So, what we have to use for this is basically superconducting magnet. This is very
widely use techniques. Superconducting magnets are meant for magnetic separation is very
big magnates through with the kaolin passes and it comes out without these metals at the
other end. Similarly, cryogenic heat treatment gives lot of metals. As you go on cooling the
object for example, what we do in tempering and analyzing is you first go to a very high
temperature and start cooling the object and which talked at ambient temperature. If we do
not stop at ambient temperature and go below to cryogenic temperatures, there is
something called as retain which will get converted to martin sight and what we want. The
return which will get converted to martin sight and which is what we want the retain is very
brittle sometimes and therefore, we would like to have complete conversion of this ostinite
to martinsite and therefore, cryogenic treatment of very special metal is carried out and you
get a better metal at the end of the heat treatment what do get at the end is the lives of the
tools die castings, their dies forgings jigs and fixtures etcetera increase when subjected to
cryogenic heat treatment. This is very important. The life of guitar string increased by 4 to 5
times with no need for tuning. So, here you can see that the life increase is there because
of cryogenic heat treatment carried out to liquid nitrogen temperature.
Cryogenic recycling turns the scrap into raw materials by subjecting it to cryogenic
temperatures. There are certain materials like rubber or plastic which we want to scrap and
it is not very easy to scrap these things. So, what you do you expose this material to very
low temperature we put them it is a liquid nitrogen bath, as soon as you do that thing as we
saw in the experiments this material become very hard, it becomes very rock hard and then
one can use some kind of press in order to crush the whole scrap, can be crushed like a
metal in those cases and it will ultimately get converted to powder, it is very easy to scrape
in the form of powder. So, if we got a huge raw material for example, PVC, rubbers and
19. plastics etcetera you can give a cryogenic treatment to those things, you can give a
cryogenic recycling treatment to those subjected to low temperature and crush it to powder,
various designs are available, various system are being fallowed all over the world this
forms a very important applications of cryogenics in mechanical engineering. Moving away
from mechanical engineering to medicine this is again is a very important aspect of
cryogenic engineering usage in medicine.
Cryosurgery: One can use Cryogenics in Cryosurgery. Cryosurgery is a novel technique in
which the harmful tissues are destroyed by freezing them to cryogenic treatment. This
surgery is carried out for various aspects, for example, Dermatology’s use this surgery just
to remove unwanted moles or something like that on your body, some treatment is given on
your skin by doing a small surgery at very low temperature. Why? Cryosurgery has shorter
hospital stay, less blood loss and small recovery time, this very important aspect one can go
in the morning and come back in the evening that kind of cryosurgery also possible which is
done at very low temperature. It is generally used in patients with localized prostate or
kidney cancer, skin disorders, retinal problems etcetera. This list is actually increasing with
time; however, the data’s keep coming, there are certain hard elements also that could be
treated using cryogenic techniques.
Cell Preservation or Food Preservation: Preservation industry is a huge as for as cryogenics
is concerned from the medical point of view . When you come to preservation, the
preserving food at low temperature is a well known technique this is what we do in a
domestic refrigerator. Cooling of sea food meat, milk products long time preservation is
achieved using liquid nitrogen; liquid nitrogen is very widely used for transporting sea food
or meat or milk from a port A to port B one can keep everything reserved in a original form,
frozen at cryogenic temperature and therefore, is a very important technique, very massive
work is being carried out as far as preservation of food is using cryogenic technique is
considered. Systems are developed to preserve blood cells, this is again a usage in medicine
you can preserved blood cells, plasma cells, human organ and animal organs at cryogenic
temperature. So, the preservation using liquid nitrogen bath to preserve blood cells, plasma
cells, human cells, stain cells also is latest addition to this list, this all thing is done at
cryogenic temperature.
Gas Industry: Coming next from here to the gas industry; as you know gas industry is
basically the gases the industrial gases is a very sort after for example, oxygen, nitrogen
are used. Oxygen is used in medical hospital and therefore, the gas industry is a very big
20. industry and gas industry therefore, use as cryogenic techniques for Liquefaction Separation
and Storage the very important aspect of cryogenics very important usage of cryogenics.
The transportation gases across the world is done in liquid state, one need not do the
transportation of the gases in the gaseous form because if you want to do the transport of
gases in gaseous form the gases are compressed at very high pressure which is sometimes
unsafe, which is not excepted sometimes. So, what you do, you convert those gases to
liquid by lowering the temperature by liquefying those gases using cryogenic techniques and
this is done by storing the liquid at cryogenic temperature, if we get liquid of those
particular gases we can store those liquid at cryogenic temperature and in this way we can
do transportation much easily as compared to those in case of gases. The use of inert gases
in welding industry has initiated higher demand for gas production in the recent past.
Nitrogen, oxygen all these are very widely used in the industry and therefore, this gases
require cryogenic technique to liquefy those gases and transport those gases and store
those gases and it is a very high demand of gases or cryogenic gases like nitrogen, oxygen,
neon, argon etcetera. cryogenic likes lock which is liquid oxygen LH2 liquid hydrogen are
used in rocket propulsion as we have just seen earlier; well liquid hydrogen is also being
considered for automobile, you may know that lot of research being carried out to use
hydrogen as a fuel in a automobile. In fact, a car is already ready which uses liquid
hydrogen as a fuel like what we do in using petrol or diesel. So, here all these gases, all
these liquefied gases play very important role as they are needed to be consumed in these
forms. Liquid nitrogen is used as precoolant in most of the cryogenic systems. Whenever we
want to do cooling, we can use liquid nitrogen as a very cost effective way which we will
reduce from room temperature to 77 kelvin. As you know, nitrogen is available in air. So,
nitrogen is freely available one can say, it is a very cost effective solution to reach 77 kelvin
and most of the liquefaction cycles, nitrogen is used as a precoolant in order to reduce the
temperature from room temperature 77 kelvin. The steel industry is a very important
consumer of these gases and you can find lot of liquid oxygen plants on the campus where
the steel industry is housed. So, in steel industry oxygen is used in the production of steel.
Basic oxygen furnace uses oxygen instead of air. So, we will find lot of places, liquid oxygen
becomes a very important requirement for the steel industry. Nitrogen and argon are
primarily used to provide an inert atmosphere in chemical, metallurgical and welding
industries. So, these gases being inert gases, they are stored in the form of liquid nitrogen
or liquid organ, instead of storing in the forms of very highly compressed cylinders. It create
problems for a safety considerations are taken.
21. Super conductivity: The next biggest application is super conductivity. Superconductivity
has got various uses in NMR, MRI, magnetically levitated trains, transformers and
generators. This is a very important aspect of cryogenics. In fact, super conductivity came
into existence because of cryogenics and it has got important usage in various aspects
important being The NMR or the nuclear magnetic resonance and MRI; it is a magnetic
resonance imaging which you find in hospitals. So, one of the major important usages of
super conducting magnet is a NMR which is a nuclear magnetic resonance. It is used by the
various pharmaceutical industry to study the molecular structure. If we want to device a
new drug against any particular disease, we have to do NMR in order to understand the
molecular structure in three dimensions. What e do for that is NMR and NMR has super
conducting magnets here. What you can see here is super conducting magnet and in this
super conducting magnet, what we have got is a small sample of a chemical of which we
want to study certain properties. In order that this magnet become super conducting, this
magnet could be kept dip in liquid helium and one can have liquid nitrogen outside. So, if
you have seen any NMR facility, we can always see that NMR facility will continuously
require a liquid nitrogen or liquid helium in order to keep this super conducting magnets in a
super conducting state all the time because slowly the liquid helium level starting down and
slowly liquid helium state also will start going down and therefore, we need cryogenics to be
supplied all the time. The magnetic field which is generated by this is around 10 to 25 Tesla,
higher the magnetic field better is the structure that is visible to us and therefore, the need
of super conducting magnate in this case. Similar to that, what you have got is an MRI
which is a magnetic resonance imaging and this is again used for body scanning. MRI is
meant for basically body scanning while NMR is meant for chemicals in order that this
magnet are kept in super conducting state also the time their dip in liquid helium here and
then you can find different shields outside. Sometime this liquid helium will be surrounded
by liquid nitrogen or we can use a cryocooler which can produce shields of 40 to 50 kelvin or
20 kelvin outside of this liquid helium so that the boil of is minimum in this case. This is a
very important applications of cryocoolers in MRI field. In fact, MRI field has really initiated
the research better and better efficient cryocoolers and functioning and also minimum
vibration in this cases and this is the way MRI will be done. The super conducting magnets
for both NMR and MRI machines are cooled with liquid helium and now a days with
cryocoolers also. Next application is magnetically levitated trains which run on the principle
of magnetic levitations. The train gets levitated from the guide way by using
electromagnetic forces between superconducting magnets in the vehicle and coils on the
ground. This is the very important applications of super conducting magnet which uses
22. cryogenics to go into super conducting region. This particular thing results in no contact
motion and therefore, no friction. So, imagine a train running with a speed of 600 kilometer
per hour, having no contacts with the rails and therefore, no friction. Therefore, no wear
and tear therefore, no service in requirements. So, great application of cryogenics. Similar
to this, the next applications are superconducting transformers, motors and generators.
Wherever there are windings involved, wherever there are I square R losses are there, if we
could put those winding in liquid nitrogen or liquid helium, the I square R losses are going to
be absolute minimum or 0. There are different ways in which the super conducting wires
could be achieved. Super conductivity of this wires could be achieved by various cryogenics
arrangements, cryogenic systems that we have to study; however, because of
superconducting transformers, generators and motors, cryogenic is reaching to a
commercial state now.
Cutting Tool Industry: Metal cutting is an important manufacturing operation in the
manufacturing industry. The machining processes are done on work piece to get finished
product of required quality. The manufacturing industries are continuously striving to reduce
the cutting cost and hence final product cost. The development of cutting tool material is
necessary to produce high quality of product with high tolerances of manufactured goods
with the required production rate. Cutting tool is important element in any of the
manufacturing operation such as drilling, facing, milling, turning and so on. The materials
which are used in production of cutting tool should have the different properties such as hot
hardness, wear resistance etc. to impart these properties. Conventionally the tool is surface
treated by using coating material or heat treated such as to increase the life of the tool at
last. There are mainly three costs associated namely labor cost, material cost, tooling cost
for the final product. The material cost and labor cost are varying and increasing, so tooling
cost can be reduced with improved cutting tools performance. The cutting tools performance
is improved by surface coating and cryogenic heat treatment. Cutting tool’s life plays an
important role in increasing the productivity of machines. The economy of cutting tool
depends on the tool life. Thus any improvement in cutting tool performance reduces the
cost of production, reduces tool changing time and helps to achieve required production
target.The development of new cutting tool with greater productivity and tool materials
capable of higher cutting speeds and feed rates is required. The main applications of tool
steels are for broaches, drills, milling cutters, taps etc. The tool life of cutting tools plays an
important role in increasing the productivity and tool life is an important factor for economic
consideration. Conventional heat treatment of steels produces retained austenite in steel.
The austenite is transformed into martensite causes nearly 5% volume expansion resulting
23. in distortion of cutting tools. Cutting tools life is affected by the various factors like depth of
cut, feed and cutting speed, material used for cutting tool, heat treatment carried out on
cutting tool, work piece material and nature of cutting. The characteristics of a good cutting
tool material are abrasion resistance, heat conductivity, hot hardness, impact resistance,
strength and wear resistance. Presence of various elements in tool material play important
role in deciding these characteristics of cutting tool.
UNIT – II: PROPERTIES OF CRYOGENIC FLUIDS
Effects on the properties of metals
The properties of material change when cooled to cryogenic temperatures and then
sometimes these changes are drastic. For example, we have seen a video earlier and we will
see the experiment again. We can see from this experiment that rubber when it got
quenched in to liquid nitrogen, it turns hard and it broke like a brittle material, which we
just saw. We can also see sometimes, you know another experiment which we have not
done yet. We can see that the wires made of material like niobium, titanium it exhibits zero
resistance when subjected to liquid helium temperature. That means, this wire becomes
superconducting wire, it shows R is equal to 0, I squared R is equal to 0; that is no joule
heating These are very important changes that happen in these materials and these
examples therefore, show that the material becomes hard and brittle at low temperature. At
the same time, the electrical resistance decreases as temperature decreases for certain
materials. These two examples show that, material property do change drastically. In
addition to these two property, which we have just shown to you as an example, we got
several other properties like strength, ductility of material, thermal and electrical
conductivity, specific it capacity, thermal expansion, all this property change at low
temperature. Therefore, the knowledge of this material property changes, at low
temperature is very important for proper design of a material, which is going too subjected
to low temperature.
study various properties of material which are Mechanical properties, Thermal properties,
Electrical properties, Magnetic properties. So, there are several of these properties which we
can study, because this course is meant for mechanical engineers. I am going to talk about
24. mechanical properties and thermal properties from mechanical engineering point of view.
While, there are several electrical properties, several magnetic properties,
The mechanical properties can be well understood. Once we understand the stress-strain
curve for any material, where the stress is kept on the y axis, while the strain is kept on the
x axis. Now, when a ductile specimen is subjected to a tensile test, the stress strain
relationship is shown as follows. So, you can see that in this case, the stress is varying in
the relation with the strain and the variation is linear variation or we can say that, the stress
is directly proportional to strain in this case. Now, you could see that, when the material is
subjected to tensile, the loading, the stress is increased in a straight line up to the point
called proportional limit or PL, What is this PL? This PL is the limit, in which the elongation of
the specimen is directly proportional to the stress applied. So, we can say that during this
period, stress is directly proportional to strain. The slope of this line which constant during
this entire line is called as you know young’s modulus. So, stress upon strain is constant
during this period.
Now, if this elongating force is removed in this case, the material will come back to its
original shape and size. And that is why, we call that this particular behavior is nothing but
elastic behavior of the material. Even if you remove the elongating force, the material will
come back to its original shape and size. However, if we apply the force beyond its
particular strain and beyond its particular stress; you can see lot of different things happen.
We can see first at this point, at point C, that there is a tremendous increase in strain
immediately by the application of very small stress. If we apply more stress or if we apply
more force, we can follow the behavior in this pattern. This is what a stress strain curve of
any ductile material would be. What you can see from this is, we can see the elastic region
up to PL. We can see that the material yields during this place; this is called yielding and
the material shows plastic behavior at this point. Now, during this period if the stress or the
elongating force is removed, the material will not come to the original shape and size. You
will find some deformation in the material. Now, there are various points which are shown
here C, D, E, F, G, the F what we call as ultimate tensile stress, the point C is called as the
yield point and the stress is called as the yield stress, the value of the stress at this
particular point is called as yield stress. While the point f we called is ultimate tensile stress,
if increase the stress beyond this value, we find this kind of behavior and at this particular
point G is the breakage point. In principle, the force at point G will be actually more than
that of point F, but as you go beyond this point, the area cross section goes on decreasing.
Therefore, the force upon area or the stress starts coming. However, the engineering
25. diagram will show that the G is more than F or this is what we call as the typical stress
strain curve, for any material. And what is important to note here are, the two properties,
one is the yield stress and other one is the ultimate tensile stress. We will study the
behavior of these two properties at low temperature
This is typical diagram for a ductile material and we can see now the brittle material also
has a stress strain curve and it will also have its proportional limit. So, if I want to compute,
if I want to draw a stress strain curve for a brittle material, let us say this is the brittle
material, it will have its own proportional limit now during which time this behavior is an
elastic behavior. If I increase the stress beyond this value, this is the behavior and G is the
breakage point and the material will break. So, a stress strain curve of brittle material is as
shown over here and stress, when exceeds the PL value, it will break at this point G
exceeds. So, in summary, we have a two stress strain curve; one is showing the ductile
material and one is showing the brittle material. We will study the two properties more, that
is the yields strength and ultimate tensile strength. What happens to this property at lower
temperature?
The mechanical properties which we want to study in this particular course are the yield and
ultimate strength of which we just talked about. We talk about the fatigue strength of a
material, we talk about the impact strength of a material, we talk about the hardness and
ductility of the material and we talk about Elastic Moduli of the material. So, all these 5
properties in fact, 6 properties, they are basically the mechanical properties. In the next
lecture, may be we talk about the thermal properties and the electrical property on the
magnetic properties. Now, these are the five mechanical properties which we will discuss in
this particular lecture for different materials.
First, we will come to yield and ultimate strengths of the material. Just to redefine those
properties which we just saw, what is the yield stress? It does the stress at which the strain
of a material shows a rapid increase with an increasing stress value. When subjected to a
simple tensile test, we found that suddenly, the strain increased by giving a small stress at
a particular location and that is called as the yield point and stress is called yield stress.
Ultimate Stress is the maximum nominal stress attained by a test specimen during a simple
tensile test. So, ultimate stress is also a very important property a vary characteristic
property of any material and also yields stress also is a characteristic of characteristics of a
any material
26. Now, we will study what happens to this property at low temperature? This particular figure
gives the variation of yield strength at low temperature. You can see the y axis is giving the
yield strength, while the X axis gives the temperature variation in the reverse direction; that
means, what I am seeing here is a 300 Kelvin is a room temperature and in I am coming
down towards 0 Kelvin or towards cryogenic temperature. What you can see? I have shown
different curves here there meant for different materials. For example, this curve is meant
for 3 0 4 stainless steel, SS 3 naught four, the second curve is 9 percent nickel steel, the
third curve is carbon steel C 1020 and then we got aluminum alloyed. all these vertexes are
shown in this region in the order of decreasing the strength value. So, we can see that, in
all these cases, the strength as if come down lower in temperature, the strength has
increased most of the cases in all the cases almost, while the stainless steel shows
maximum strength as compare to the other materials. So, we can conclude from this, yield
strength of various commonly used materials increases with decreases in temperature; this
is what we see from this particular curve.
These materials are normally alloys of iron and aluminum etcetera. When we saw the curve
it was for yield strength and now, sees the similar character of a curve for ultimate
strength. So, again in this particular map, what you see is an ultimate strength plotted on a
y axis and again temperature on the X axis. And again you can see that, as a temperature is
lowered from room temperature to 0 Kelvin, the ultimate strength of the material have
increase, the way it was shown for yield strength. So, we can see that in both the cases, the
yield strength and ultimate strength increase as a temperature is lowered.
At the same time what we again can see from this, that the stainless steel shows maximum
ultimate strength in this four materials followed by 9 percent nickel, followed by carbon
steel, followed by aluminum as shown in the order in given in this region. So, again we can
conclude similar to the yield strength, the ultimate strength of the material also increases
with decrease in temperature; this is the very important thing. At the same time what it
shows is, the room temperature is actually the uncertain thing if the material is safe at
room temperature; that means it is definitely safe in the cryogenic temperature. So, for any
design we were worst design will be at room temperature, because your ultimate strength
or the yield strength is lowest at room temperature. So, the safest case, if we want to see
the failure mode, it should happen at room temperature as far as the failure based on
ultimate strength and yield strength is considered. We could also from the two figures that
stainless steel has the high strength and is mostly preferred material at cryogenic
conditions. So, many applications what you see is having stainless steel as a material. I
27. have just got a small specimen over here you can see a very thin wall tube which has got
around 0.15 millimeter thickness and this is the welded to a flange, what is constituting one
tube and if I close from this side, if I got some welded structure at this point, we have use
this tube for pulse tube cooling and we have seen that this particular tube of thickness 0.15
millimeter can stand at very high pressures and as high as 25 bar pressure of gas. It shows
that such a small thickness of a material, also can stand very high pressure and in most of
the application. Therefore, what we have use is stainless steel which is SS 3 naught four, we
use it stands very high as higher 30 to 35 bar also. One has to really calculate the hooks
stress in this case and then calculate dimensions of this, but a thickness of 0.15 to 0.2
millimeter or 0.3 millimeter can stands very high pressure. I just bought the specimen to
show you that such a small thin wall tube can stand very high pressure SS 3 naught four is
the material, which is being used for this particular purpose right. Going back from having
study this ultimate strength behavior and yield behavior at low temperature let us try to
understand why it happens like this?
Ultimate and yield strength of the material largely depend on the movement of dislocations.
We have earlier studied the movement of dislocations, and its relations with the slip planes
and its relations with the lattice structure. The movement of dislocation also depends on the
internal energy or the lattice vibrations, and at lower temperatures, the internal energy of
the atoms is very much low. As you know, the internal energy is basically a function of
temperature and if the temperature is low the internal energy of the atoms also is low. As a
result, the atoms of the material vibrate less vigorously with less thermal agitation. So, at
lower and lower temperature, the vibration the lattice vibration is very very minimum and
therefore, the material vibrate less vigorously with less thermal agitation.
When these agitations are low, when these agitations are low at low temperature, the
movement of dislocation is hampered. You can imagine that, when the dislocation
movement is ultimately it may be a long dislocation movement, a very large dislocation
movement at room temperature; however, at lower and lower temperature, the motion has
come down and this vibration if they are reduced, it does not allow the dislocation to move,
it will hampered the movement of the dislocation when the movement of the dislocation is
hampered. What will have to do? This dislocation movement now will require a very large
stress to tear the dislocations from their equilibrium position. If we want move the
dislocation now, when the thermal vibrations are very large or the lattice vibrations are very
large, we will require very large stress to tear this dislocations from their equilibrium
position, which means that the material will exhibit high yield and ultimate strength at lower
28. temperature. So, the behavior of the material at low temperature in terms of increased yield
strength and increased ultimate tensile strength at low temperature could be understood by
this particular analysis that, the vibrations are very very low at low temperatures and
dishampers the dislocation movements at low temperature. So, till now we studied the yield
strength and ultimate tensile strength and variations at low temperature in cryogenics. We
also have to study fatigue strength. What is fatigue strength? The material exhibit fatigue
failure when they are subjected to fluctuating loads.
Now, this fluctuating loads could be, for example, I have got a small little sample over here,
you can see that this small bearing through which the piston goes and as you know the
piston is going to have an oscillating motion or a fluctuating motion up and down and this is
called a flexure bearing and this particular bearing goes up and down and therefore, this will
be subjected to fatigue failure if at all it fails it will fail back fatigue. So, at ensure that while
designing this particular flexure bearing the stress achieved by this while in motion are less
than a particular value or particular fatigue strength of this particular material. This is very
important that the bearing which is subjected to fluctuating load can stand the stresses
generated during this motion. So, these failures can happen even if the stress applied is
much lower than the ultimate value. This is very important, that the fatigue strength is
actually much less than the ultimate strength of the material. So, one has to really
compiled, one has to really understand the failure of this particular item will because of this
fatigue, the failure for example, of this bearing is going to be because of fatigue and not
because of tension or compression. The fatigue strength of a material is the stress at which
the specimen fails after a certain number of cycles. This is what a definition of fatigue
strength is, that it is a strength at which the specimen fails after a certain number of cycle.
So, fatigue strength is normally defined in terms of cycles 10 to the power 6 cycles, 10 to
the power 8 cycles etc
Now, let us see how the strength variation happens at low temperature. So, here again you
can see a curve on the Y axis what we have plotted (Figure 2.1) is fatigue strength at 10 to
the power 6 cycles, E to the power 6 cycles over here, while on the X axis what we have
plotted is temperature and this my room temperature at 300 Kelvin and this is where you
move towards the cryogenic temperature range as you have seen earlier in the yield
strength and ultimate strength variation. Again you can see that, as the temperature gets
lower down the fatigue strength increases. So, all the three cases these are the material
which can stand fatigue strength, again we have got a stainless steel, beryllium copper over
here and carbon steel. In all these three cases, what we see is at lower temperature the
29. fatigue strength shows an increase over here. So, again the worst case for this material is
at room temperature. So, we can conclude again the fatigue strength increases as the
temperature decreases. So, as a temperature goes down toward cryogenic region the
fatigue strength increases. The fatigue strength of a stainless steel is higher as shown over
here. So, again we can conclude that stainless steel is preferred material, if it is going to be
subjected to fatigue loading
Fig.2.1 Fatigue Strength
Thermal properties: When the properties of the materials at low temperature are
considered, the properties of materials change when cooled to cryogenic temperature and
we have seen a small video. We showed to you, how the properties of the material change
at low temperature. We know that, the electrical resistance of a conductor decreases as the
temperature decreases; in fact that is why the electrical conductivity increases, which lead
to super conductivity. So, as you go on reducing the temperature, the conductivity
increases in other word, electrical resistance decreases. At the same time we know that, if
the materials temperature is increased it expands. Similarly, in cryogenics if the materials
temperature is decreased, the material will shrink and therefore, the shrinkage of material
occurs, when cooled to lower temperature and this is a very important consideration and
this is what we will see in this particular lecture. At the same time, we understand that the
systems cool down faster at low temperatures, due to decrease in the specific heat. If I
want to cool down the system at room temperature, the system will take some more time
as compare to, what it would take at lower and low temperature essentially, because they
30. will be decrease in the specific heat at low temperature. This is an aspect which has to
consider while cooling different materials at low temperature. With this small examples, we
are very general examples we have talked about, we know that the study of properties of
materials at low temperatures is necessary for the proper design, because every cryogenic
design, every cryostat design, every cryogenic device has to understand, how the materials
behave at low temperature? And their behavior has to taken in to consideration while
designing that particular device. One is thermal expansion or contraction in cryogenics will
come across more contraction if we are reducing temperature while, if we are increasing
temperature will get expansion. The second properties specific heats of solids because the
specific heat is the very important function of temperature and this is with try to understand
in this particular lecture, important thing is thermal conductivity. So, we will study these
three properties and the variations of these properties, at low temperature in detail.
Thermal expansion or contraction is nothing, but reduction in the dimensions of a material
occurring, when cooled to low temperature. Simple, we know this that; if we heat the
material its dimension increases. Similarly, if the material is subjected to very very low
temperature, its shrinks and that has to be considered in the design of cryostat or a
cryogenic device. Now, I just show a small schematic or small animation here, where, we
got a material A and material B and this two materials are joint together by brazing or
welding whatever, poses of joining we use over here and this joining naturally is done at
300 Kelvin, I mean room temperature. 300 Kelvin is always treated as room temperature.
Now, this joint when subjected to low temperature will look like this, now why does it look
like this? It shows that material A has shrunk, also material B has shrunk, we can also
understand that the joint which was perfect at room temperature. Now, that joint is not
perfect here and the joint will start leaking and why does it leak? It leak because both the
material shrunk and the joint therefore, gave way to whatever cryogen or whatever gas
flowing beneath this; that means, at 300 Kelvin the joint was perfect, at 80 k the joint has
become imperfect, the joint has started leaking. Now, this is the very important thing, this
is where cryogen comes into picture, this is where thermal engineer comes into picture, this
is the contraction of A and this is the contraction of B. Now, any designer who knows this
will have to consider the shrinkage of material A and the shrinkage of material B before he
starts joining mechanism. This requires some of the calculations to be done. Some
properties of material have to be known the shrinkage is a function of temperature. So, one
should know, how much shrinkage it comes across? How much material A shrinks? Or how
much material B shrinks? Does material A shrink more than A or more than B? That you
31. have to understand and those things have to be taken in to consideration while designing a
cryostat or any machine.
Fig. 2.2 Thermal Expansion
Electrical Properties:
The electrical conductivity as you know, it is defined as the electric current per unit cross
selection area, divided by the voltage gradient in the direction of the current flow. So, we
know that V upon I is nothing, but resistance. I am talking about the reciprocal of that that
is electric current per unit cross sectional area divided by the voltage gradient in the
direction of the current flow. This is what electrical conductivity is given as the electrical
resistivity is oppose to that. So, electrical resistivity is, it is the reciprocal of electrical
conductivity. As you know that decreasing the temperature decreases the vibration energy
of the ions. As you know that as the temperature decreases the energy decreases and
therefore, the vibration energy of the ions also decrease the, this result in smaller
interference with electron motion this ions. This positive charge actually ions, which are
basically are the resistance that is offered to the motion of electron motion, electron motion
creates the correct. So, at lower and lower temperature the resistance offered by the ions is
less and less. Therefore, the electrically resistivity in this case decreases.
Fig. 2.3 Electrical Resistivity
32. Now this particular curve gives the electrical resistivity ratio with temperature for different
material like aluminum ion and copper and what you define electrical resistivity ratio as is
defined over here the ERR defined as R T upon R 273; that means, R T is at any
temperature T and R 273 is nothing, but 273 Kelvin at that 0 degree centigrade. So, you
can understand that R T by R 273 is equal to 1 for all the materials and it is this point the
variation of electrical resistivity ratio for some commonly used materials is as shown over
here. And, what you can see now? R T upon R 273 decreases for all the materials how are
the decreases are different with different temperatures with different materials. So, what
you can see for copper R T at any temperature? Let say 100 Kelvin R T upon R 273 for
copper is much smaller as compared to what it is for aluminum. What does it mean? That, R
T at 100 Kelvin are 100 is much smaller a value as compared to R 100 for aluminum which
means conductivity of copper at 100 Kelvin is much higher as compared to what it is for iron
or it is for aluminum which is what we know. This is very important to understand that how
this resistivity varies at low temperature? Or how this ratio varies at low temperature?
Also important, understand that the electrical and thermal conductivity are related by
wiedemann-Franz expression what does it do wiedemann and Franz expression basically
finds a relationship between thermal conductivity and electrical conductivity. So, k T and k e
are nothing, but thermal conductivity and electrical conductivity. So, k T upon k e into T is
equal to 1 by 3 pi k upon e to the power 2. You can see the this parameter is all most
constant because k is Boltzmann constant is the charge for electron and we can see that
this is completely constant value which will not change with temperature. If suppose the T
comes on this side, then you can understand that k T upon k e is function of temperature
only, it means that the ratio of k T upon k e is a product of constant which is this and
absolute temperature and is represented like this. So, k T upon k e is equal to AT. It means
that, as the temperature gets lower if you reducing the value of T, the k T upon k e also get
reduced. It means that k T gets reduced or k e increases. The two possibilities we know that
the thermal conductivity decreases at lower temperature, we also know that electrical
conductivity increases at low temperature because of decrease in thermal conductivity and
because of increase in k e at lower temperature. k T by k e they move in a such way that at
a particular temperature k T by k e remain constant or if the variation of temperature is
given k T by k e will move according to this expression.
33. Superfluidity and Superconductivity
The key difference between superfluidity and superconductivity is that superfluidity is the
flow of helium 4 atoms in a liquid whereas superconductivity is the flow of an electron
charge inside a solid. The terms superfluidity and superconductivity are related phenomena
of flow without resistance, but they describe these flows for different systems.
Superfluidity is a characteristic property of a fluid that has zero viscosity and able to flow
without any loss of kinetic energy. If we stir a superfluid, it tends to form vortices that
continue to rotate indefinitely. We can observe superfluidity occurring in two isotopes of
helium: helium-3 and helium-4. We can liquify these two isotopes by cooling them to
cryogenic temperature. Superfluidity is a property of various other exotic states of matter
that come under astrophysics, high energy physics, and quantum gravity. The theory
regarding superfluidity was developed by Soviet physicist Lev Landau along with Isaak
Khalatnikov. However, this phenomenon was originally discovered by Pyotr Kapitsa and
John F. Allen in liquid helium. When considering the liquid helium-4, its superfluidity occurs
at a very high temperature compared to that of helium-3. This is mainly because a helium-4
atom is a boson particle, by virtue of its integer spin while a helium-3 atom is a fermion
particle that can form bosons only through pairing with itself at a low temperature.
Moreover, the superfluidity of helium-3 was the basis for the Noble prize in physics in 1996.
Superconductivity is a quantum phenomenon where certain materials exhibit a high
conductivity at particular magnetic and temperature regimes. This phenomenon was
discovered by Onnes in 1911. However, there was no consistent microscopic theory that
could describe why superconductivity occurs at the time of discovery. However, Bardeen
and Cooper released a paper stating the mathematical foundation for conventional
superconductivity. The discovery of superconductivity happened during the study of
transport properties of mercury (Hg) at low temperature. Onnes discovered that, below the
liquifying temperature of the helium, (at about 4.2 K), the resistivity of mercury suddenly
drops to zero. But the expectation was that the resistivity would either go to zero or diverge
at a zero temperature but not vanish suddenly at a finite temperature. This vanishing
indicated a new ground state and was discovered as a property of superconductivity.
Superfluidity is a characteristic property of a fluid having zero viscosity and able to flow
without any loss of kinetic energy. Superconductivity is a quantum phenomenon where
certain materials exhibit a high conductivity at particular magnetic and temperature
34. regimes. The key difference between superfluidity and superconductivity is that superfluidity
is the flow of helium 4 atoms in a liquid whereas superconductivity is the flow of electron
charge inside a solid. Superfluidity is a characteristic property of a fluid having zero viscosity
and able to flow without any loss of kinetic energy. Superconductivity is a quantum
phenomenon where certain materials exhibit a high conductivity at particular magnetic and
temperature regimes. The key difference between superfluidity and superconductivity is that
superfluidity is the flow of helium 4 atoms in a liquid whereas superconductivity is the flow
of electron charge inside a solid.
Cryogenic Fluids
Liquid Methane: Liquid methane has a boiling point of 111.7 Kelvin. It can be used as rocket
fuel and it is also being used as in the form of compressed natural gas or CNG. You know
CNG is basically nothing but most of them is methane. One of the other usages of methane
is in a mix refrigerant cryocooler or in a cascade system, one can use methane as one of the
refrigerants in a cascade. So, you can have different temperatures or you can have different
circuits where in methane could be one of the refrigerants which can give you temperature
between 110 to 120 Kelvin and then you can use different refrigerants with respect to
temperatures associated with them. So, these are liquid methane which is normally not very
much used as such in cryogenic activities, which normally is now below 80 Kelvin as such,
but it definitely forms one of the important constituents in cryogens
Liquid Neon
Neon is a clear, colorless liquid with a boiling point at 27.1 Kelvin. As you know, it is inert
gas. It is a very costly gas, it is again a rare gas. Neon is commonly used in neon
advertising you know this. Liquid neon is commercially used as cryogenic refrigerant,
sometimes neon is also used in refrigerator as a pure gas, but again the cost considerations
are plenty. It is a compact, inert and comparative less expensive as compared to helium. If
you compare the cost of neon as to helium, it is relatively less expensive
Liquid Nitrogen
s liquid nitrogen which is very widely used. It boils at 77.36 and freezes at 63.2 Kelvin. It
resembles water in appearance and density of 807 kg per meter cube. This is very
important. If I were to compare the density of nitrogen with water, water is 100 k g per
meter cube approximately, while you can see it is around 807 kg per meter cube which is
very comparable with water and if you see liquid nitrogen, it will be difficult for you to
35. differentiate between liquid nitrogen and liquid and water, but you can differentiate it
because of the fumes coming from liquid nitrogen because liquid nitrogen will be in a state
of boil of the fumes will always be there. The vaporization will always be happening while it
will not be true with water and therefore, this is the only difference possibly one could come
across, unless you touch, unless you put your finger in liquid nitrogen, but if you physically
see, liquid nitrogen it will resemble like water. Now, nitrogen has got two stable isotopes,
N14 and N15, this atomic mass. Normally what we is normally N14, nitrogen 14 and the
ratio of N14 to N15 is 10,000 to 38. You will have around 10000 N14 in comparison to that
what will find is only 38 nitrogen 15 isotopes. The heat of vaporization is 199.3 kilo joules.
Again, this is a latent heat we are talking about. See, if I were to get cooling effect at 77
Kelvin, what I will get from 1kg of liquid nitrogen is 199.3 kilo joules, while if I compare the
same with water, it is an order of magnitude more for water which is 2 2 5 7, 2257 kilo
joule per kg for water and it is produced by distillation of liquid here. How do I get liquid
nitrogen? So, biggest source of nitrogen is as you know air. Nitrogen is primarily used to
provide an inert atmosphere in chemical and metallurgical industries. It is a non reactive
kind of a gas and therefore, it is widely used because its available in plenty and cheap and
therefore, it is primarily used to provide an inert atmosphere in various chemical and
metallurgical industries. It is also used too as a liquid to provide refrigeration. So, lot of
activities related to food preservation or blood preservation or medicine preservation, one
would use liquid nitrogen because its cost effectiveness and availability and non reactivity.
It is safe to use liquid nitrogen in those places which gives you 77 Kelvin temperature and
also gives you cooling effect. So, liquid nitrogen is widely used because of its availability and
the cost. For food preservation, blood and for cells preservation. So, medicine as well as
food industry liquid nitrogen got tremendous usage in these industries and importantly, for
high temperature superconductivity, here one would love to use liquid nitrogen, one would
hate to use liquid helium because liquid helium is very costly. So, unless subjected unless
required, I would like always to use liquid nitrogen to get high tc or high temperature
superconductivity. As I said earlier, the research is going on in order to increase the
temperature of various materials so that they can become superconducting at higher and
higher temperatures. At moment, I have got certain materials with requisite property and
they showed they show superconductivity at liquid helium temperatures only. While if I
were to use some materials at very high temperature of around liquid nitrogen temperature,
then I have to sacrifice some important properties. That is a big problem right now so;
however, I would always prefer to use liquid nitrogen as a temperature to then
superconductivity. So, research is always going on in order that I should get some materials
36. with required properties to show superconductivity at liquid nitrogen temperatures. Lot of
work is being going on in this area
Liquid Oxygen
Liquid Oxygen normally looks a blue in color due to long chains of O4. Different oxygen gets
chained together and because of which get blue thing in the appearance. It has the boiling
point of 90.18 Kelvin and freezing point of 54.4 Kelvin. These again will be clear if one has a
look at the T s diagram. All these properties are absolutely visible if one has a look at T s
diagram of this cryogens. It has got density of 1141 kg per meter cube. Again, if you
compare with water, the density of liquid oxygen is more than that of water. O2 is slightly
magnetic and it exists in three stable isotope; O16, O17 and O18 in the ratios of 10000 to 4
to 20. This is an information, but what is most important it is magnetic. Oxygen is magnetic
and this property is utilized to separate something or to remove the magnetic materials
from some area. So, this is a very typical characteristic of liquid oxygen or oxygen gas.
Because of the unique properties of oxygen, there is no substitute for oxygen in any of its
usage. It is widely used in industries and for medical purpose. As I showed, the hole of
cryogenic engineering, the first event was liquefaction of oxygen; that means, to reach 90
Kelvin and why did it happen? It happened because of its usage in industry as well as in
medicine. One requires oxygen for living and therefore, all the attempts were on, in order to
store oxygen in plenty and that can be done only in the liquid form. So, the research
towards production of liquid oxygen was always on and this is what initiated in fact,
cryogenic engineering and just mentioned today, Cailletet and Pictet liquefied oxygen in
1877 from where we have got a existence of LOX. It is largely used in iron and steel
manufacturing industry. In fact, wherever you have got a steel making plant, liquid oxygen
plant would be there. If the plant is if the steel manufacturing is in bear quantity, they can
always afford a liquid oxygen plant on the campus instead of bringing liquid oxygen from
another site. So, this is very important property of a steel manufacturing industry. As you
know, it is one of the oxidizer propellant for spacecraft rocket applications. So, liquid oxygen
is a very important oxidizer in the rocket propulsion. As you know in cryogenic engine
again, liquid hydrogen is a fuel and liquid oxygen then oxidizer
Liquid Argon: Liquid argon is also colorless, inert and non toxic gas. Again as you know,
these are all inert gases and therefore, they are rare gases and therefore, their cost are bit
high as compared to other cryogens. Boils at 87.3 Kelvin and freezes at 83.8 Kelvin. As I
mentioned earlier, one should see the only difference of 5 Kelvin between the boiling point
and the freezing point. It has a density of 1394 as compared to water of 1000. So, we can
37. see it is a very dense liquid. It exists in three stable isotopes; 35, 38 and 40. The property
of inertness of argon is used to purge moulds in casting industry. Argon is very widely used
in casting industry and also it is very widely used in steel industry. It is used in argon
oxygen decarburization process in stainless steel industry and one of the important usages
of argon is in welding. So, it offers inert atmosphere for welding stainless steel, aluminium
and titanium etcetera; this is what makes argon is a very important gas. The steel industry
or the welding business runs on argon, as you know, argon welding is very popular for
stainless steel. Argon has tremendous usages in industry both in manufacturing or steel
industry or in casting industry.
Liquid Air:
As you know liquid air is a mixture of various components various gases, 78 percent
nitrogen, 21 percent oxygen, 1 percent argons and others means CO2, helium, moisture
etcetera, but therefore, normally we can call it 79 percent nitrogen and 21 percent oxygen if
you forget about these others. It has a boiling point of 78.9 Kelvin and a density of 874 Kg
per meter cube. Liquid air was earlier used as precoolant for low temperature application
and nowadays mostly liquid nitrogen is used as a precoolant rather than liquid air, but
previously liquid air was more prevalent to be used as precoolant. Liquid air is primarily
used in production of pure nitrogen, oxygen and rare gases. Now this is the very important
thing. Air liquefaction is a very big area, this a very big cryogenic industry and lot of air
liquefies are still because ultimately all these gases nitrogen, oxygen, helium, argon; all
these gases are coming from air and how do I get those gases? I get these gases only from
air. So, what I have to do first is to liquefy air and separate out this gases of nitrogen,
oxygen, argon, helium, carbon dioxide etcetera by carrying fractional distillation of air. But
for that, what you need to have is a air liquefaction. So, what you need to have is air
liquefier and this is a very big industry and therefore, liquid air is a very primary used
cryogen I should say for other gases or producing other pure gases like nitrogen, oxygen
and all other rare gases which basically you can find only in here. So, this is a one of the
very important cryogen and also liquid nitrogen which are primarily used everywhere in
cryogenic engineering.
Hydrogen: hydrogen, which has got its boiling point at 20 Kelvin. Hydrogen exists in
diatomic form as hydrogen, everybody knows about this; and these are the normal
properties of hydrogen gas. What are its properties? It has got a normal boiling point of
20.27 Kelvin at one bar, one atmosphere. It has a normal freezing point of 13.95 Kelvin. It
has a critical pressure of 1.315 M p a, or around 13 bar pressure. It has a critical pressure
38. of 33 Kelvin, that is, 33.19 Kelvin to be exact. Now, it has got a density of 70.79 kg per
meter cube, and the latent heat is 443 Kilo Joule per kg, when it gets converted from gas to
liquid or, from liquid to gas. These are very important properties; and this means that I can
use liquid hydrogen to give me a cooling effect at around 20 Kelvin approximately, or
above. I cannot come below 20 Kelvin. If I have to come, and use hydrogen at temperature
lower than 20 Kelvin, then I have to remove the pressure over it; that means, I have to go
into vacuum to touch down lower and lower temperature, and at 13.95 Kelvin, liquid
hydrogen will get converted into solid hydrogen. This is the temperature entropy diagram of
hydrogen. I always told you that, all the mechanical engineers or all the cryogenic engineers
first should refer to T - s diagram to understand different property variations with
temperature. So, on the y axis what we have is the temperature, and what x axis what you
have got is an entropy. You may not be able to see properly, it is possible; but, just have a
look at different lines on this diagram. For example, you can see the dome over here; it
means that, inside this dome what you have is a 2 phase mixture, that is, liquid plus vapor.
You can see all these lines which are coming from the top to the bottom; it means, they are
all isobaric lines, that is, the pressure remaining constant. You can see all these curved
lines; they are all isenthalpic line, or the enthalpy remaining constant. Why I am stretching
this point is, when you go in the next lectures, when you go for liquefier and refrigeration,
we will deal with these diagrams in every problem. We have to understand the property
variation, enthalpy variation, and entropy variation at every point, and for that, we will
always have to refer to the T - s diagram of different gases, or different cryogens. So here,
you can see a line which is at one bar, and this is the temperature corresponding to the
change of phase from gas to liquid, or nothing but the boiling point of hydrogen, which is
one atmosphere pressure and 20.27 Kelvin as the temperature. Now this is the critical
point, and the properties of critical point are – 13.15 atmospheres and 33.19 Kelvin
temperature. So, this is the critical temperature, and this is the critical pressure. In addition
to that, I have just given data to understand the density of vapor at one atmosphere in the
saturated condition, which is 1.33 kg per meter cube, and that of liquid is 70.79 kg per
meter cube hydrogen. Now, some general information about hydrogen - it has 3 isotopes
hydrogen, deuterium, and tritium. The relative percentage of existence of these 3 isotopes
is 6,400 and 1 for hydrogen and deuterium respectively. The atomic mass in this case is, all
these things have got 1 proton in the nucleus, and the number of neutrons will vary
depending upon what isotope we are talking about, for example, for hydrogen we have got
0 neutron, for deuterium we have got 1 neutron, and for tritium what we have is 2
neutrons. Now, this tritium is a very rare substance, a very rare gas in comparison to what
39. you see for hydrogen and deuterium. In addition to that, the tritium gas is a radioactive
gas, and is unstable with a half life of 12.5 years. So, whenever we have to deal with
tritium, one has to be very, very careful. One has to worry about all its radioactivity and all
the measures have to be taken, to deal with this tritium. In fact, this is not normally used in
commercial operations.
Now, the important fact about hydrogen is that, it exists in 2 molecular forms, that is, Ortho
and Para. So, we have got an Ortho form, and we have got a Para form. This is very
important; and we will talk about this Ortho and Para forms of hydrogen with a little bit
detail in the coming slides. from this definition of spin, we understand what Ortho hydrogen
is, and what Para hydrogen is. So, what is the spin? The spin is defined as the rotation of a
body about its own axis. When it rotates, it spins about its own axis. So, this is what I am
talking about hydrogen molecule, and you have got 2 atoms of hydrogen together, which
means you have got 2 protons. So, hydrogen molecule has 2 protons and 2 electrons. These
protons will be spinning all the time. The distinction between the 2 forms of hydrogen is
what we talked about as Ortho hydrogen, and Para hydrogen, is basically because of the
direction of the spin of these protons. So, as I have just said these 2 protons will have some
spin, and what is the direction of that spin? With it we will decide if it is a Ortho hydrogen,
or if it is a Para hydrogen. As soon as we talk about Ortho and Para, lot of properties
associated with the energy of hydrogen will get decided; and we will talk about that in the
next slide. The 2 protons possess a spin, which gives the angular momentum. You have got
hydrogen molecule, and this proton immediately gets aligned to the magnetic field because
of the spin. Similarly, I am talking about the same thing here. The 2 protons possess a spin,
and this gives the angular momentum, which will have a direction. So here, you can see 2
protons, and you can see 2 clockwise directions over here, which indicate the spin of these
protons. If the nuclear spins are in the same direction for both the protons, it is what we call
Ortho hydrogen; that means, if both of them are in the same direction, clockwise in this
case, it is nothing but Ortho hydrogen One has a spin in this direction, and one has a spin in
the opposite direction. So, one is clockwise, and other one is anti-clockwise. This is what we
call as Para hydrogen. So essentially, if the nuclear spins are in the opposite direction for
both the protons, it is Para hydrogen, and if both the spins are in the same direction, we call
it Ortho hydrogen. So, it is just the difference of spin of the protons, which makes it Ortho
or Para. Now what happens? Why are we studying all these? As soon as you start lowering
the temperature of the hydrogen gas, this Ortho will start getting converted to Para; that
means, the molecules, which had the spin in the same direction, will now have the spins in
the opposite direction. So, with the decrease in the temperature, the Ortho hydrogen is
40. converted to Para hydrogen. What is happening as you lower the temperature? The gas –
hydrogen, is slowly starting to get converted to liquid, and here, when we start lowering the
temperature, or when we start liquefying the gas, now basically if we want to reach down to
20 Kelvin from room temperature, what will happen? Ortho hydrogen will get converted to
Para hydrogen. How does that happen? What are the percentages of Ortho and Para? At
300 Kelvin, we got 75% Ortho, and 25% Para. At 20 Kelvin, after the whole conversion had
taken place, and equilibrium hydrogen has formed, what we have got is, almost all the
Ortho has got converted to Para. So, Para hydrogen at 20 Kelvin, which is nothing but the
boiling point of hydrogen, the 75 to 25 ratio has got converted to almost 100%. Only
0.179% Ortho has remained there, while Para is 99.821% here. So, almost 100% Ortho has
got converted to Para. Now this is the clear distinction of what exactly happens when you go
on lowering the temperature of the gas, from 300 Kelvin to 20 Kelvin. Here, Ortho gets
converted to ParaNow, this Para form is a low energy form, and therefore, heat is liberated
during conversion. This is a very important thing that, Ortho has got a higher energy form,
while Para is a low energy form. So during this conversion, the heat is liberated because
Ortho has higher energy, while Para has lower energy. During this transformation from
Ortho to Para, lot of heat gets liberated, which means that this is basically an exothermic
reaction. So, conversion of Ortho to Para, as one goes on lowering the temperature, is
basically going to result in a release of energy; and to release of energy means, increase of
temperatures, or there is a lot heat energy involved over there. So, conversion of Ortho to
Para form of hydrogen is an exothermic reaction, and this conversion is a very slow process.
This is the most important point again about this conversion. Just to summarize, as you go
on lowering the temperature, 75:25 Ortho will get converted to Para, which is 0.179%, and
99.8% respectively. This process of Ortho to Para conversion is an exothermic reaction, and
also this conversion is a very, very slow process; it does not happen fast, it happens very,
very slowly. It is very important for liquefaction that this Ortho to Para conversion takes
place faster; and it is also very important that this conversion is complete during
liquefaction. What will happen, we will see later; but if we want to make this process of
conversion very fast, then what Iwehave to do is, we have to add a catalyst to this reaction.
So, in order to make this conversion faster, catalysts are added. There are different kinds of
catalysts; there are 3 or 4 types. They have to add it in the correct quantity, in order to
convert the Ortho hydrogen to Para hydrogen as fast as possible, or to make the reaction.
The heat of conversion is getting evolved, that is, it being an exothermic reaction, lot of
heat is being liberated, and it evaporates; because of the effect of this heat release,
whatever liquid has got formed gets evaporated immediately. If we get the liquid, 70% of
41. that liquid will get evaporated; because of this conversion from Ortho to Para. This is a very
important constraint in liquefaction, and storage of hydrogen. So the conversion is a very,
very slow process; and if the conversion does not take place during liquefaction, we will
store it in the form of liquid. In this liquid form, this Ortho will slowly get converted to Para.
During this conversion from Ortho to Para a lot of heat will get released and therefore, a lot
of liquid will get evaporated. It means that, we should ensure that all the conversions
happens during liquefaction only; that means, all the conversions from Ortho hydrogen, to
Para hydrogen happens during liquefaction only. If that does not happen, then whatever
liquid you get at the end of liquefaction, the Ortho will get converted to Para in the liquid
form and therefore, it will cause the evaporation of most of the liquid that is stored. This is
very, very typical of hydrogen, and this has to be taken care off. So, what do we do? We
add catalyst during liquefaction. The catalyst makes this reaction faster, and this will ensure
that the whole conversion of Ortho to Para takes place during liquefaction only, and
whatever liquid has got formed will not get evaporated.
Helium: it is a inert gas, it is a non-reactive gas, and it has got a lowest possible boiling
point at 4.2 Kelvin at one atmosphere; it means that, this gas will remain in a gaseous
condition till 4.2 Kelvin. All other gases will get liquefied, because, their boiling points are
above 4.2 Kelvin temperature, and therefore, if I want to achieve temperatures very close
to 4.2 Kelvin, or below, or in the range between, let us say, 50 Kelvin to 4.2 Kelvin, we
have got no other option, but the only safe, inert gas is helium. Helium has got a
tremendous importance in cryogenic engineering.
The evidence of helium was first noted by Janssen, during solar eclipse in 1868. It was
discovered as a new line in the solar spectrum. So, the discovery of the gas only happened
around 1868. In the year 1895, Ramsay discovered helium in a Uranium mineral called as
Clevite. So, this was the discovery on the earth for the first time, in 1895. So, you can
understand that it is a gas, which is just 115 years old on the earth. In the year 1908,
Kamerlingh Onnes at Leiden university liquefied helium using helium gas, which he obtained
by heating Monazite sand procured from India. So, this is the Indian connection to the first
helium liquefaction that happened in 1908, by Kamerlingh Onnes at Leiden University, in
Netherlands. The year 2008 we had the centenary year of helium liquefaction. So, all the
cryogenics and physicists associated with lower temperature research, celebrated the year
2008 as the centenary year of helium liquefaction.
Helium is an inert gas, and exists in monatomic state. These are the properties of normal
helium. What are they? It has got a boiling point of 4.25 Kelvin. Normal freezing point does
