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CRYOGENICS-MATERIALS OF CONSTRUCTION       SOM -2         SOM-1         Testing of materials         Failure ductile brittle



Ductility is measured from the energy required to break the specimen. Some data in this respect is given in table 3.

TABLE 3: Change of ductility with low temperature

Metal Crystal lattice Energy to break (FT lb.) at 700F Energy to break  (FT lb.) at 3000 F
Al Face Centered


19 27
Cu FCC 43 50
Ni Face Centered

Cubic (FCC)

89 99
Iron BCC 78 1.5
Titanium Hexagonal Close

Packed (HCP)

14.5 6.6
Mg HCP 4 30-105


FCC structures are most suitable at cryogenic temperatures as these do not lose their ductility at low temperatures. Thus, copper and its alloys, aluminum and its alloys are most suitable. From the data, it can be noted that the ductility of FCC structure metals improves at low temperatures while that of iron falls to a very low value. However, iron can be alloyed in such a way that the ductility improves. Most metals show super-conductivity between temperatures of 4 to 15 K.  Fig. shows the phenomenon of super-conductivity.


Fig. Super conductivity( FCC Lattice materials)

(a) Copper and its alloys

Alloys of copper and Copper are easily formed and soldered. Storage and process vessels required at cryogenic temperatures are made from copper and its alloys. Addition of 3.5 % of silicon greatly improves the properties. Tensile strength improves from 200 to 400 N/mm 2.

(b) Aluminum and its alloys

It has been found that aluminum and its alloys are less costly than copper. These are light. These have excellent ductility and conductivity at low temperatures. These are easy to work with. Thus, storage vessels for liquid oxygen, Nitrogen, Hydrogen Argon and Helium and nitrogen chilling heat exchangers are made from aluminum and its alloys.

(c )Nickel steel alloys

These can be used up to -200 C with 8.5% of nickel

(d) Austenitic stainless steel

It is FCC structure. It is non magnetic. It has 3 to 19 % of Ni and 16 to 22 % Cr. It can be used for cryogenic applications.

(e) Body centered lattice materials

(f) Iron and steel become brittle. The alloying elements must be added to make these suitable at low temperatures.


These must be used under compression load applications. These are

( i) Plastics

(ii) Neoprene

(iii) Thiokol

(iv) Polyacrylate

(v) Asbestos

(vi) Teflon

(vii) Plexiglas


Such materials should have a higher values of the followings:

  1.  Yield strength
  2.  Ultimate strength
  3.  Toughness
  4.  Creep strength
  5. Weld strength


It is a compromise for various requirements namely

  1. Cost
  2. Manufacturing cost
  3. Stress level required
  4. Corrosion resistance
  5. Welding characteristics

Keeping safety first principle cost becomes the most predominant factor in the final selection.


  1. Mechanical properties of structural materials at low temperatures-NBS monograph 13, McClintock R.M. & gibbons, H.P., Washington
  1. Cryogenic materials data hand book- Durban, T.F., McClintock, R.M., Reed and R.P.

3. Scurlock, Ralph G., ed. (1993). History and Origins of Cryogenics, Oxford, Clarendon Press.

  1. Weisend, John G. II, ed. (1998). Handbook of Cryogenic Engineering, Philadelphia, Taylor and Francis
  2. A brief overview of cryogenics in China, S.M. Li, Cryogenic Laboratory,Zhejiang University, Hangzhou, 310027, China, 2000
  1. Flynn, T.M., Cryogenic Engineering, Dekker, New York, 2005



Liquefaction of Helium by Collins’s Cryostat


(i)  Polytropic expansion for pre-cooling

(ii) Throttling for liquefaction.

Liquefaction of helium(Fig.4) uses normal boiling liquid nitrogen for pre-cooling before the helium enters the first cascade heat exchanger. Since helium liquefies at a lower temperature than that of even hydrogen, more number of heat exchangers and other devices are incorporated in order to achieve significant amount of liquid helium with reasonable expenditure of energy. This liquefaction process requires about 45 KW to yield 35 to 40 liters of helium per hour.


Magnetic Cooling

Very Low temperatures of the order of 0.001 K have been obtained by demagnetization of a para magnetic salt such as gadolinium-sulphate (Ferric Ammonium Alum, Fe 2(So 4)(NH 4So 4 24 H o  ). The salt is cooled to a lower temperature of about 1 K by liquid helium at 0.2 atmospheric pressure. Now a strong magnetic field of 25000 Gauss intensity is applied, the molecules of the salt act as tiny magnets and arrange themselves in a regular arrangement and releases large amount of heat energy in doing so. The various steps for magnetic cooling (Fig.5) are as under:

(i) Cool the salt to 1 K by liquid helium at 0.2 atm (vacuum)

(II) Apply a strong magnet of 25000 Gauss intensity. Under this, salt molecules will arrange in a ordered manner. These molecules will behave as tiny magnets. In this process, large amount of heat will be produced. Continue cooling of the salt till 1 K is achieved.

(iii) Thermally insulate the salt.

(iv) Remove the magnet. Molecules will come in a disorder form. But these molecules need lot of energy for attaining this disorder. Since the salt is thermally insulated, the energy will be derived from the salt itself. Thus the temperature of salt will plunge to a very low temperature.

(v) Such a low temperature can be measured using Madam Curie law.

T = C/χ

Where T is the absolute temperature

C is salt constant

χ is magnetic susceptibility of the magnetic field











































Liquefaction of Nitrogen

A modified Claude cycle is used for the liquefaction of nitrogen. It has the used advantage of both the turbo-ex pander and Joule Thomson valve [Fig.3(a)]. Three heat exchangers are used in Claude cycle. Whereas, two numbers of heat ex changers are used in this liquefier. Last two heat ex changers of the Claude cycle are combined to a single heat exchanger. It reduces the cost of the liquefier.



Ordinary hydrogen is the simplest of atoms.  It consists of a proton in the center and an electron moving in the orbit around the nucleus. Both proton and electron are also spinning on their axes as well. As such there are four different types of hydrogen atoms (A,B,C and D). Firstly atoms A and C combine to form ortho-hydrogen molecule (protons spinning in same directions but electrons spinning in the opposite directions). Secondly the atom A and D combine to form para molecule of hydrogen. In atoms A and D, the protons as well as electron are spinning in the opposite directions. Thus at room temperature, Hydrogen gas consists of two molecular varieties, Ortho-hydrogen 75% and Para-hydrogen 25%.


Thus at room temperature, Hydrogen gas consists of two molecular varieties, Ortho-hydrogen 75% and Para-hydrogen 25%. This composition does not change even in freshly liquefied hydrogen. However, over the passage of time, ortho- form tends to change to para form till the equilibrium is reached at 99% para form.  It is an extremely a slow process. This conversion process is highly exothermic and releases heat (1420 kJ/kg) of hydrogen. This tremendous heat evolution causes liquid hydrogen to boil. This is called boil off loss. This loss  is reduced by employing catalysts like Cr 3 or Al 23 . This catalyst accelerates the conversion of ortho into para even before complete liquefaction of hydrogen is achieved. The liquid hydrogen is used as a fuel in a jet aircraft. It has a number of environmental and technological advantages over conventional fuels. The liquefaction of hydrogen requires a large expenditure of energy.



Principle of Hydrogen liquefaction

  • Pre-cooling by liquid nitrogen to a temperature below its inversion temperature of (-780C)
  • Throttling

Yield in this process is around 25%. (Fig.3)


Fig.3 Hydrogen Liquefaction



Principle of Liquefaction

Liquefaction of gases is achieved by employing.

  1. Joule Thomson expansion alone as in Linde’s process


Isentropic expansion as well as Joule Thomson expansion as used in Claude’s process

 Air liquefaction

It is carried out with two processes. First is by Linde’s process and second is Claude’s process respectively.

Linde’s (or hampson) air liquefaction method

Underline principle – Uses only Joule Thomson expansion for liquefaction

In Linde’s process (Fig.1(a) and (b)),the air is compressed to 200 atm and the yield is about 10 percent. Further, liquid oxygen and nitrogen are obtained by fractional distillation of liquid air.

Analysis Basis – for a unit mass of air liquefaction

t1       =       t2       =       t7

p1      =       p7      =       1 atm

p2      =       200 atm

Mass balance

Firstly m4     =       m6+1 =m7+1                                               (i)

Secondly  m1     =       m2     =m3   =m4

 Thirdly m6         =              m7

  Fourthly m5         =       1

Energy balance around heat exchanger.

Entering energy  = energy leaving

m2h2+m6h6 =       m3h3+m7h7                                                                  (ii)

h2, h6, h7 are known from T-S chart for air and unknowns are m2 and h3.

Use energy balance around separator

Energy entering = energy leaving

m4 h4 = m5 h5 + m6 h6                                                    (iii)

but  h4= h3                                                                     (iv)

m4 & h3 are unknown and are solved by using equations (i), ii),(iii) & (iv).

Disadvantage of Linde’s air liquefaction method

  1. It results in a lower yield of around 10%.
  2. The working pressure is very high resulting in high power consumption and requiring robust equipment.
  3. Method is costly.


This is a simple method.


Principle – uses both isentropic and throttling expansions

In Claude’s air liquefaction method (Fig.2), air is compressed to approximately 40 atm. After passing through heat exchanger I(HE1), air stream is divided into two streams. 20 percent is passed through the second heat exchanger and then throttled  to atmospheric pressure.  80 %  going to the turbine and cooled on expansion. This cooled air from the turbine cools the air in heat exchanger II and thus lowers the temperature before throttling. However, this gives a higher percentage of liquefied air.

Analysis of Claude’s air liquefaction method

It is carried out  for a unit mass of air liquefaction

t1, t2, t11, t3 are known

h1, h2, h11, h3 are known



Apply energy balance around H.E.I., II & Separator together

m2 h2 + m8 h8 = m8 h3 + (m2-1) h1 + 1.h6

In this process, yield is increased significantly as compared to Linde’s process. However some lubrication difficulties are encountered in expander.





When a gas is at a lower than its inversion temperature, cooling occurs on expansion.

Thus, must bring gases below inversion temperatures before liquefaction.

When air expands at room temperature
(a) Heating is produced
(b) Cooling is produced
(c) Neither heating nor cooling is produced
(d) None

When Hydrogen expands at room temperature
(a) Heating is produced
(b) Cooling is produced
(c) Neither heating nor cooling is produced
(d) None

When Nitrogen expands at room temperature
(a) Heating is produced
(b) Cooling is produced
(c) Neither heating nor cooling is produced
(d) None

When Helium expands at room temperature
(a) Heating is produced
(b) Cooling is produced
(c) Neither heating nor cooling is produced
(d) None

When expansion occurs at a temperature higher than the inversion temperature

(a) Cooling takes place

(b) Heating takes place

(c) Neither cooling nor heating takes place

(d) None

(Ans: b)


When expansion occurs at a temperature lower than the inversion temperature

(a) Cooling takes place

(b) Heating takes place

(c) Neither cooling nor heating takes place

(d) None

(Ans: a)


When expansion occurs at the inversion temperature

(a) Cooling takes place

(b) Heating takes place

(c) Neither cooling nor heating takes place

(d) None

(Ans: c)


The inversion temperature of air is

(a) > 0

(b) <0

(c) = 0

(d) None

(Ans: a)


The inversion temperature of hydrogen is

(a) > 0

(b) <0

(c) = 0

(d) None

(Ans: b)


The inversion temperature of Helium is

(a) > 0

(b) <0

(c) = 0

(d) None

(Ans: b)


Principle of liquefaction of a gas is

(a) Free expansion

(b) Isentropic expansion

(c) Throttling expansion

(d) None

(Ans: c)


Liquefaction of a gas is always done at a pressure

(a) > atmospheric pressure

(b) = atmospheric pressure

(c) < atmospheric pressure

(d) None

(Ans: b)


Which method of air liquefaction is more efficient

(a) Linde’s Method

(b) Claude’s Method

(c) Cascade Method

(d) None

(Ans: b)


Liquefaction brings

(a) Compactness

(b) Bulkiness

(c) compactness as well as bulkiness

(d) None

(Ans: a)




It is a science which deals with creating and maintaining temperatures lower than -1500C.

Liquefaction of air at -1910C

Liquefaction of hydrogen at -2530C

Magnetic cooling 0.001 K


1.Industrial applications

i. Shrink fitting of metals

ii. Liquid oxygen is used in welding

iii. Liquid oxygen is used in the manufacture of steel

iv. Liquid oxygen in artificial breathing in hospitals & aircrafts

v.  For the preservation of blood, dead bodies and life saving medicines

vi. For freezing the food for preservation by spray of liquid nitrogen

vii. Quick healing of wounds

viii Cooling the body parts by anesthesia

ix.  Aircraft  tires are filled with nitrogen (not compressed air) . In military aircraft, nitrogen gas is used to inert the space over the fuel in the tanks.

x. Blood of rarer group is stored at -165°C or below in liquid nitrogen

xi.  Foam using nitrogen can be more effective in fire fighting

Xii. The mass production of frozen food (chicken, Prawns, Beef burgers etc.) in seven minutes is possible using liquid nitrogen. It is  sprayed on the items. It is economical. Thus, freshness, taste and flavor is ‘locked’ into the product by the quick freezing process

Xiii. Storage of large volumes of gases in small space in the liquefied form

xiiii.  For the manufacture of cryogenic magnets

xiv. Super conductive transformers and Super conduction motors are possible. It is because of superconductivity at low temperatures

xv. Used in separation of gases i.e. air, Coke oven gas, Helium 3 from Helium 4

xvi. Economic transport of ice cream.

xvii.  Superconductivity makes computers compact

xviii.  Liquid hydrogen is used as a fuel in rockets.

xix. Bolometer is used at low temperature to measure very small quantity of radiant heat.

2. Agriculture Applications

Preservation of bull insemination for better creed.

3.  Research applications

(i).  To understand thermodynamic concepts more deeply.

(ii). Cryo-pumps produces very high vacuum i.e. 10-12 mm of mercury.

(iii). Space research

(iv). In Missile launching

(v) Super conductivity

(vi) Super fluidity




 Throttling Process

This is an expansion process at constant enthalpy.

That it is found refrigeration and air-conditioning where the LIQUID is throttled.

Further, process is also found in the liquefaction of gases.

Throttling is an irreversible process.

No work is obtained at the decrease of pressure. Thus, it is not an useful process. Rather, it is a compulsion in refrigeration as well as in the liquefaction of gases. For this expansion, a turbine cannot be used because of phase change in throttling.






T changes
u= Internal energy changes.

Throttling is also defined with the help of Joule –Thomson Coefficient ‘μ’.

μ =(бT/бP) h = C

That μ can be +

Thus  μ can be ‘0’

Also μ can be –

In case of a gas, μ is linked to the maximum inversion temperature of a gas.

Values of maximum inversion temperatures for few gases are given in the table below.


  N.B.P. C Freezing Point C Critical Temperature C Maximum inversion Temp. C
Air -191 -212.3 -140.2 330
2 -183 -218.8 -118.8 620
2 -196 -210 -147.0 347.8
2 -252.8 -259.2 -239.9 -77.8
He -268.9 -269.7 -267.9 -250.0
CO 2 -78.3 —— 31.1 1230


Case 1 for throttling

Gas at a temperature below maximum inversion temperature before expansion

On expansion (throttling), temperature of the gas will decrease.  Cooling occurs on expansion.


In an expansion, there is decrease of pressure. Here it causes  decrease in temperature making μ as positive.

It is employed in refrigeration and air conditioning.

Case 2 for throttling

Gas at a temperature at the maximum inversion temperature before expansion

On expansion (throttling),  no change of temperature will occur. No heating and no cooling.

μ =0

Such a case is normally not found in actual practice.

Case 3 for throttling

Gas at a temperature above the maximum inversion temperature before expansion

On expansion (throttling), temperature will rise. Heating on expansion

μ= — (negative)

It is found in case hydrogen or helium when expanded.

Since the maximum inversion temperature of hydrogen is –77.8C.

Whereas maximum inversion temperatures of Helium is –250C.


It is process at constant internal energy.

Free expansion process is unrestricted process.

When it happens in a thermally insulated tank, it becomes an adiabatic process.

Thus it is an irreversible adiabatic process, non-flow process, and a sudden process.

The  process is useless because work is not recovered on expansion because there is no restriction or no resistance to free expansion.




u= Constant, thus internal energy remains constant. But internal energy is a function of temperature, therefore temperature remains constant during free expansion.

Thus T1 = T2

But it cannot be called as an iso-thermal process since work is not recovered.

Examples are

(i)                  Leakage from a system into the atmosphere

(ii)                Leakage into a system working under vacuum conditions.



Cryogenics  involves the study of temperatures below -150C,. It is of interest to know how to produce such temperatures. In addition , it is of interest is know which materials are suitable at these temperatures. There are many areas of interest where we need cryogenic temperatures such as

(i)   Storage of large volumes of gases in small space in the liquefied form

(ii)  Preservation of insemination

(iii) Very high vacuum

(iv) Fundamental research in understanding more deeply about entropy and sub-atomic structure of matter as the motion of protons, electrons reduces significantly at cryogenic temperatures


In order to liquefy a gas, only one or two types of gas expansions are employed. These are

(i)          Throttling expansion

It is a must for liquefaction of each gas and to overcome the practical difficulties associated with the reversible expansion Throttling expansion of a gas eliminates the lubrication difficulties, simplifies the equipment necessary but does so with a marked lowering of the efficiency ( Complete loss from high pressure to low pressure as no work is recovered). It is also called as Joule Thomson Expansion.

Joule Thomson coefficient= μ =(∂T/∂p)h=C

Joule Thomson Coefficient is a function of both temperature and pressure. Therefore this coefficient does not have an unique value. It can be positive, zero and negative.

 Joule Thomson Effect

It is related to expansion at constant enthalpy i.e. throttling process. In an expansion there is always a fall of pressure.

Three possibilities are possible in Joule Thomson Effect.

(a)  Heating (rise of temp) occurs after expansion if temperature before expansion is higher than INVERSION TEMPERATURE.

(b) No heating or no cooling if the temperature before expansion is INVERSION TEMPERATURE itself.

(c) Cooling occurs after expansion if the temperature before expansion is lower than the Inversion temperature. Mathematically Joule Thomson Coefficient , µ = Change of temp/fall of pressure= (t2 – t1)/ (p1 – p2) = — value shows heating after expansion µ= 0 for no heating or no cooling µ= +  for cooling effect          

(ii) Isotropic expansion (theoretical)/real expansion

It is found in Claude’s process of air liquefaction. By this process alone, liquefaction is not feasible because of lubrication and erosion problems. This expansion is helpful in decreasing the temperature before throttling and ultimately increases the percentage yields of liquefied gas. Under this process, the temperature after expansion is given by the relation given below:

For a theoretical process        (T2/T1)= (p2/p1)–1)/γ  ,    γ=1.4

For a real process      (T2/T1)= (p2/p1)(n–1)/n                n=1 to 1.4

INVERSION TEMPERATURE: Maximum temperature at which Joule Thomson coefficient μ =(∂T/∂p)h=C  is zero. It represents neither heating nor cooling on expansion which is possible only for an ideal gas. There is only one (maximum) inversion temperature for an ideal gas. The Inversion temperature for real gases is not unique as it is a function of both temperature and pressure but there is one maximum inversion temperature for each gas.

(i)        If the temperature of the gas is above the inversion temperature before expansion, then heating occurs upon expansion.

(ii)     If temperature is below the inversion temperature before expansion, cooling results on expansion.

(iii)   Consequently whenever a gas has to undergo cooling upon expansion, it must be at a temperature below its inversion temperature.

(iv)   For such a case, fall of temperature with fall of pressure makes the Joule Thomson Coefficient positive.

(v)     Final liquefaction is practically possible only with throttling process.

The maximum inversion temperature and other properties of various gases is given below:



N.B.P. 0C

Freezing Point

Critical temperature 0C

Maximum inversion temperature 0C

























Carbon dioxide




 It is important to note that the inversion temperatures of hydrogen and helium are -77.80C and -2500C. Therefore these are to be brought below the inversion temperatures before throttling to achieve liquefaction.

(ii)   Advantages of liquefaction

  1. Liquefaction of gases produces cryogenic temperatures required for fundamental research.
  2. Gases in the Liquefied form can  store large volume in a small storage.




Volume on liquefaction

Volume of gas

Reduction ratio






























It is obtained by the magnetization followed by de-magnetization of a para-magnetic salt. It is based on the fact that there are some materials which raise their temperatures when adiabatically magnetized, and decrease their temperatures when adiabatically demagnetized. Each atom of the para-magnetic salt may be considered to be a tiny magnet. Initially the salt is not magnetized, then all its atoms (the magnets) are randomly oriented such that the net magnetic force is zero. Magnetic cooling is carried out in four steps.

Step 1    The paramagnetic salt is surrounded by boiling helium, at a low pressure ( 0.2 atm), which chills the salt to 1 K.

Step 2    A strong magnetic field (25000 Gauss intensity) is applied to the salt. Molecules become magnets and aligns themselves in a regular fashion. In this process, heat is released and the liquid helium removes the heat thus released. In this condition, helium  still maintains the salt at 1 K.

Step 3    The helium bath is removed and the salt is thermally insulated.

Step 4    Finally the magnetic field is removed. The molecules de-arrange themselves and requires large amount of energy for this disorder.  This energy is not available from anywhere since the salt is thermally insulated. The salt gives out this energy from itself and thus plunges to extremely low temperatures reaching almost absolute zero.

Very Low temperatures of the order of 0.001 K have been obtained by demagnetization of a paramagnetic salt such as gadolinium-sulphate (Ferric Ammonium Alum, Fe(So4) (NH4)So4 24 H2O).

 REFRIGERATION-INTERVIEW SHORT QUESTION ANSWERS-5       Q. Ans Air Refrigeration-2         Off and On controls-Refrigeration         Q. Ans Air Refrigeration-3          Air Refrigeration Theory



1. Define refrigeration

Refrigeration is a process of creating and maintaining lower temperatures as compared to the surrounding temperature. For example (i)   To cool a room in summer with a window air conditioner (ii) To produce lower temp in a fridge.

2. What is air conditioning?

Air conditioning  controls simultaneously temperature, humidity, purity and velocity of air

i.e. (i) Temperature between 22 to 27C

(ii) Relative humidity 35 to 60 %

(iii) Dust particles of size > 100  microns are not permitted

(iv) Velocity 8 to 10 m/s

Examples of air conditioning are:

Window air conditioner for a room

Central air conditioning plant for a big auditorium  

3. Describe Cryogenics

Cryogenics deals with creating and maintaining temperatures lower than -150C.

(i) Liquefaction of air at -191C

(ii) Liquefaction of hydrogen at -253C

(iii) Magnetic cooling 0.001 K


(i) Fridge (ii) Air conditioner (iii) Deep Freezer (iv) Water cooler (v) Display cabinets used in confectionery shops (vi) Incubators (vii) Cold storage (viii) Ice plants (ix) Distilleries (x) Milk plants (xi) Dairy industry (xii) Food processing, preservation and distribution (xiii) Chemical and process industries (xiv) Cold treatment of metals (xv) Medical (xvi) Construction, (xvii) Ice skating  (xviii) Comfort air-conditioning

5.  Comfort air conditioning Applications

(i) Window air conditioner in homes, offices and shops (ii) Air conditioning of cinema halls, (iii) Air conditioning of libraries, (iv) Air conditioning of computer centers, (v) Air conditioning of hotels, (vi) Air conditioning of hospitals, (vii) Air conditioning of cars, buses, trains and aircrafts (viii) Air conditioning of air-ports. (ix) Air conditioning of saloons (x) Air conditioning of malls (xi) Air conditioning of restaurants

6. Applications of Industrial  air conditioning

(i) Air conditioning in textile industry (ii) Air conditioning in printing industry (iii) Air conditioning in photography industry (iv) Air conditioning in power plants industry (v) Air conditioning in textile industry (vi) Air conditioning in textile industry (vii) Air conditioning in textile industry

7.   Cryogenics Applications  

(i)   Achieving superconductivity in metals (ii)  Better understanding of molecules movements and thermodynamic concepts (iii)  Preservation of bull semen for better creed (iv)  Liquefaction of air, oxygen, nitrogen, hydrogen and helium, (v)   Obtaining very high vacuum (10-7 mm of Hg) with Cryo-pumps.