Cryogenicss is the subdivision of natural philosophies which surveies the production really low temperatures and besides the methods to obtain these temperatures. At really low temperatures the gases changes it stage to liquid. The word is originated from two Grecian words “ kryos ” which means ‘frost ‘ and “ genics ” intending ‘to green goods ‘ . The word was foremost used in the twelvemonth 1894 by Professor Kamerlingh-Onnes of the University of Leiden in Netherlands.
The first successful liquefaction of gas was made by Michael Faraday in the twelvemonth 1823. Cryogenics is now used in different Fieldss such as infinite research, medical specialty and even in car industry.
The gases cooled down to below 0K tends to alter its stage to liquids. Such liquid gases are by and large called cryogenic liquids or cryogens. Cryogens have different belongingss compared to their gaseous signifier. Cryogens should be stored in a extremely pressurized container. The cryogens are stored at a force per unit area higher than atmospheric force per unit area in order to cut down the rate of vaporisation. Successful liquefaction of He in 1908 changed the stage of low temperature natural philosophies. Liquid He is one of the most used cryogens.
Harmonizing to the jurisprudence of thermodynamics, there is a bound to the lowest temperature that can be attained. Such temperature is termed as absolute nothing. At absolute zero, molecules are in their least energy province. Cryogenic temperatures are measured utilizing the absolute graduated table besides known as Kelvin graduated table. The unit of Kelvin graduated table is kelvin and its symbol is ‘K ‘ . 0K ( -273.15A°C ) is considered as absolute nothing. The cryogenic part is considered to be about below 120K. 2.7K is the lowest temperature bing in the existence. In research lab it is possible to stop dead stuffs to every bit low as 10-6K.
The cryogenic temperatures can non be measured utilizing intoxicant or quicksilver thermometers. The intoxicant or quicksilver in them freeze at really low temperature and go worthless. Platinum Resistance Temperature Detectors ( PRTD ) is normally used to mensurate cryogenic temperatures. The advantage of PRTD is that Pt can defy really low temperature. PRTD can be used to mensurate temperature runing from 4K to 866K. PRTD has the best truth and stableness.
Semiconducting stuffs, such as doped Ge can be use as electrical opposition thermometers to mensurate cryogenic temperatures. Temperature runing from 2K to 70K can be measured utilizing this type of thermometer. The Ge used has doping values runing from 1015 to 1019 atoms/cm3.
Table 1.1 Boiling point temperature of common cryogenic liquids
Boiling Point Temperature ( K )
Layout of The Undertaking
Chapter 1 gives an thought about the field of cryogenies. It besides explains about cryogenic temperature, the graduated table used and methods to mensurate them. It besides gives an thought about cryogenic liquids. Chapter 2 is a literature reappraisal which explains the history of cryogenies. It includes the experiments accomplished by scientists to liquefy gases. Brief account of methods used by scientists to liquefy the lasting gases is included in chapter 3.
History of Cryogenicss
Michael Faraday was the first individual to liquefy a gas. He successfully liquefied gases such as Cl, S dioxide ( SO2 ) and ammonium hydroxide ( NH3 ) . He was able to bring forth temperature every bit low as 163K. Even under high force per unit area he failed to liquefy gases such as C monoxide ( CO ) and methane ( CH4 ) . Permanent gases i.e. O, N, He and H could non be liquefied by his method.
Methods Created For The Liquefaction of Gases
Raoult Pictet proposed a method to liquefy O and N. This method is known as Cascade Process. A alteration of this procedure was subsequently proposed by Heike Kamerlingh-Onnes known as three-stage Cascade Process. In 1894, Kamerlingh-Onnes constructed a works for liquefaction of air that worked on cascade procedure.
In 1895, William Hampson ( England ) and Karl Von Linde ( Germany ) independently created a new method for the liquefaction of air. This method subsequently came to be known as Hampson and Linde Method.
In 1902, George Claude ( France ) and C.W.P. Heylandt separately established a method to liquefy He. This method is known as Claude Method.
Successful Liquefaction of Oxygen and Nitrogen
In 1877, two scientists independently liquefied O utilizing cascade procedure. Louis Paul Cailletet ( France ) and Raoult Pictet ( Switzerland ) were the two scientists. However, they were merely able to bring forth a little measure of O which was insufficient for carry oning experiments.
Based on their work a decennary subsequently Karol Olszewski and Zygmunt Wroblewski at the University of Krakow were able to bring forth big sum of O, N and C monoxide in their liquified signifier. They were able to mensurate the liquids boiling point temperatures and besides established their belongingss.
Successful Liquefaction of Hydrogen
Sir James Dewar at Royal Institution, London made the first successful liquefaction of H in 1898. He used the Hampson and Linde method to bring forth liquid H.
The success of Sir James Dewar brought the field of cryogenies to a point where all the lasting gases except He were liquefied and all their belongingss had been measured.
Successful Liquefaction of Helium
Helium was the most hard gas to liquefy at that clip. Helium being an inert gas could non be liquefied due to the deficiency of resources. In 1908, Kamerlingh-Onnes utilizing Hampson and Linde method was able to liquefy He ( of mass 4 ) .
Other methods were besides used to liquefy He. The methods were developed by:
In 1930 by Sir Francis Simon in Germany.
In 1934 by Kapitsa in Cambridge, England.
In 1946, Samuel Collins from Massachusetts modified Kapitsa ‘s method at the Massachusetts Institute of Technology.
In 1948, Stephen G. Sydoriak, Edward R. Grilly and Edward F. Hammel at the Los Alamos Scientific Laboratory ( New Mexico ) liquefied a rare isotope of He ( of mass 3 ) which is acquired as a girl merchandise in the decay of radioactive tritium.
METHODS OF LIQUEFACTION OF GASES
Vapour compaction infrigidation system can non be used to achieve a temperature that can liquefy gases. Using this system, an evaporator temperature of 233K can be achieved. The major drawback of utilizing this system to bring forth cryogenic temperature is the hardening of the refrigerants. The mechanical setup used besides encounter troubles when runing at cryogenic temperatures. Raoult Pictet proposed a method to bring forth cryogenic temperatures known as cascade procedure. The refrigerants used in this procedure have lower boiling temperature compared to the refrigerants used in vapour compaction infrigidation system. Cascade procedure is preferred over multistage system because in multistage system the lubricating oil used in one compressor can get away into other compressors.
This procedure can be used to liquefy O and N. It uses legion compaction type iceboxs. To cut down temperature at assorted stairss, refrigerants such as ammonium hydroxide, S dioxide, C dioxide etc. are used.
The cascade procedure is now outdated. Liquefied H and He can non be made utilizing this procedure. This is due to the deficiency of suited refrigerant that can be acquired in liquid signifier to topographic point in an evaporator. The refrigerant should besides be able to keep a really low temperature that is sufficient plenty to liquefy H and He by the procedure of compaction merely.
Single Stage Cascade System
Figure 3.1: Conventional diagram of a individual phase cascade system.
The operating stairss of a individual phase cascade system are:
Measure 1: – Compaction of gas takes topographic point in the compressor. This phase is merely completed when liquid gas is acquired in the coiling tubing immersed in the H2O chilling armored combat vehicle.
Measure 2: – The heat caused by compaction procedure is passed out of the H2O chilling armored combat vehicle.
Measure 3: – The liquid so flows through the enlargement valve cut downing its force per unit area and is boiled in the evaporator. Heat absorbed from the milieus is used for vaporization.
Measure 4: – The evaporated gas is allowed to flux into the compressor, therefore finishing the rhythm.
Two Stage Cascade System
Figure 3.2: Conventional diagram of a two phase cascade system.
Two compressors, a high temperature compressor and a low temperature compressor are used in this procedure. A heat money changer serves as a capacitor for low temperature cascade system and as an evaporator for high temperature cascade system. The evaporator of low temperature cascade system merely produces utile refrigerating consequence. The two phases require two different refrigerants. R-12 ( Dichlorodifluoromethane ) or R-22 ( Monochlorodifluoromethane ) which has high boiling temperature is used as a refrigerant in high temperature cascade system. In a low temperature cascade system, a higher Coefficient of Performance ( COP ) can be obtained by low boiling temperature refrigerants which have high force per unit area that guarantees smaller compressor supplanting. R-13 ( Monochlorotrifluoromethane ) is a refrigerating with low boiling temperature.
Three Stage Cascade System
Figure 3.3: Conventional diagram of a three phase cascade system.
A three phase cascade system is proposed by Kamerlingh-Onnes. Methane is used as a refrigerant in the first phase. Ethylene and ammonium hydroxide are used as refrigerant in the 2nd phase and 3rd phase severally. The intent of methane in the first evaporator is to chill the high force per unit area ethene of the 2nd phase. Thus ethene is liquefied in the spirals at a temperature of 183K. The temperature of the 2nd phase evaporator is maintained at 113K. This evaporator is used to liquefy the tight air in the 3rd phase.
Hampson and Linde Methods
The operation of this method is based on the rule developed by James Joule and Lord Kelvin in 1852. They discovered that when a compressed gas is permitted to get away continuously through a half closed valve, a alteration in temperature takes topographic point. They besides found that at room temperature, the gases were colder than the gases come ining the half closed valve. When get awaying the half closed valve, a little warming occurred merely for H. When the initial temperature of the gas come ining the half closed valve is low, chilling occurs. The temperature at which the consequence is reversed from heating to chilling at the half closed valve is known as inversion temperature.
The lone difference between Hampson ‘s system and Linde ‘s system is the industry of the heat money changer. In Linde ‘s system, the heat money changer consists of two Fe tubings holding a length of 100m. Their diameters are 10cm and 4cm severally and injure into a spiral one inside the other. To cut down thermic loss wool was wrapped around the tubing. Through the interior tubing high force per unit area air is passed and through the infinite between the tubings low force per unit area air is passed.
Simple Linde System
Figure 3.4: Conventional diagram of a simple Linde system.
The setup used for a simple Linde system consist of a compressor, a heat money changer and a brassy chamber or vapour-liquid centrifuge. In a compressor, air is compressed isothermally to 200atm. The tight air ( O or N ) is cooled to 166K in a heat money changer. When the compressed air flows through the enlargement valve, its temperature reduces to 83K followed by a stage alteration. This is based on the rule discovered by Joule and Kelvin. In the brassy chamber liquid air is separated from its gaseous signifier. The gas is brought back to the compressor whereas the liquid air is transferred to a container.
Double Pressure Linde System
Figure 3.5: Conventional diagram of a double force per unit area Linde system.
Simple Linde system can merely be used to liquefy little measure of air and its efficiency is low. To increase the efficiency of the system uses two compressors ( high force per unit area and low force per unit area compressors ) , two flash Chamberss ( high force per unit area and low force per unit area flash Chamberss ) and two enlargement valves are used. Atmospheric air flows through low force per unit area compressor into the high force per unit area compressor. The tight air flows to the heat money changer. From the heat exchanger the gas passes through enlargement valve 1 to the high force per unit area brassy chamber. The vapour nowadays in the brassy chamber is sent back to the end product of the low force per unit area compressor. The liquid from high force per unit area brassy chamber is passed through enlargement valve 2 to the low force per unit area brassy chamber. The vapour nowadays flows back to the heat money changer. The vapor so flows to the low force per unit area compressor.
The efficiency of both simple Linde system and double force per unit area Linde system can be increased by utilizing a vapour compaction evaporator. Using the evaporator, high force per unit area compressed air is pre-cooled. This gives the best public presentation of the system.
Applications to Liquefy Hydrogen And Helium
The rule of the Hampson and Linde systems were used to run the liquefiers proposed by James Dewar for the liquefaction of H and Kamerlingh-Onnes for the liquefaction of He. Before these gases entered into the concluding heat money changer system, it was of import to chill the high force per unit area gas due to its low inversion temperatures. It was efficient to pre-cool the entrance H gas maintained at a force per unit area of 150atm utilizing a liquid air that boils under a decreased force per unit area. For He maintained at 30atm the pre-cooling is done by liquid H boiling under a decreased force per unit area.
Figure 3.6: Conventional diagram of a Claude system for air liquefaction.
Claude method was developed by Georges Claude and C.W.P Heylandt. A simple Linde system can be modified to a Claude system by adding an expander and a 2nd heat money changer. External work for the expander is done by utilizing a reciprocating engine. If enlargement is carried out without taking any heat from the milieus, the external work done by air is at the rate of its internal energy. Using a compressor, an isothermal compaction of air to a force per unit area of 40atm takes topographic point. High force per unit area air is cooled partly through first heat money changer.
The air end product from the first heat money changer is divided into two waies. About 80 % of air is cooled by the expander. The staying 20 % of air base on ballss through the 2nd heat money changer. When air passes through the enlargement valve a part of air is liquefied. The liquid air is removed from brassy chamber. The air from the expander is assorted with the staying air from the centrifuge, which cools the incoming high force per unit area air in the 2nd heat money changer. This air passes through first heat money changer back to the compressor.
The Claude system is more efficient than Linde system due to the presence of the expander which consequences in much lower temperature than the Linde system. This system can be farther expanded by utilizing a turbine alternatively of utilizing a reciprocating engine to make external work for the expander. These besides avoid jobs of lubrication.
Methods of Liquefaction of Helium
Method Used by Samuel Collins
Figure 3.7: Conventional diagram for liquefaction of He.
The Claude method was used by Samuel Collins and besides by Kapitsa for the liquefaction of He. To pre-cool the entrance He gas, liquid N was used by Kapitsa. Collins besides used two expanders to make the pre-cooling. Lubrication was non required for the reciprocating engines used in the expanders because they operated at 29K and 10K severally.
In this system, He is compressed to a force per unit area of 12atm by a four-stage compressor. Liquid N is used to pre-cool a part of He. By grouping four heat money changers and three expanders, the He is cooled down to about 16K. Helium can be farther cooled by go throughing it through an enlargement valve. At about 4.21K liquid He is obtained. By utilizing a brassy chamber liquid He is removed. The staying He gas flows back to the compressor by go throughing through four heat money changers.
In Simon method, a extremely compressed gas undergoes adiabatic enlargement. In the procedure of enlargement, temperature of the gas reduces when work is done against the external forces. Under favourable conditions, the condensation of gas consequences in the formation of its liquid. Helium undergoes compaction in a Cu pot which has strong walls. Solid H topographic point in a container above the Cu pot is used to chill He gas down to 11K.
The phases used for bring forthing liquid He are:
The heat produced by compaction is absorbed by H
The tight He is enclosed in a metal shell and is secluded thermally from its milieus. At room temperature, He is so permitted to spread out through a constricted tubing into a gas-holder.
Due to the procedure of enlargement He is liquefied. The Cu pot is filled with liquid He to a volume of approximately 70 % .
CONCLUSIONS AND FUTURE WORK
Study of cryogenies and cryogenic temperatures in item.
Understanding assorted belongingss of cryogenic liquids
Understanding the history of cryogenies.
Detailed survey of the methods used for the liquefaction of gases.
Future Work includes the followers:
Study of cryogenic liquids in item.
Study of low temperature sensors.
Study of safety steps to be taken when managing cryogenic stuffs.
Study of applications of cryogenies.