Study And Design Of Cryogenic Machines Engineering Essay

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INTRODUCTION

Cryogenics is the branch of physics which studies the production very low temperatures and also the methods to obtain these temperatures. At very low temperatures the gases changes it phase to liquid. The word is originated from two Greek words "kryos" which means 'frost' and "genics" meaning 'to produce'. The word was first used in the year 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 year 1823. Cryogenics is now used in different fields such as space research, medicine and even in automobile industry.

Cryogenic Liquids

The gases cooled down to below 0K tends to change its phase to liquids. Such liquid gases are generally called cryogenic liquids or cryogens. Cryogens have different properties compared to their gaseous form. Cryogens should be stored in a highly pressurized container. The cryogens are stored at a pressure higher than atmospheric pressure in order to reduce the rate of vaporization. Successful liquefaction of helium in 1908 changed the phase of low temperature physics. Liquid helium is one of the most used cryogens.

Cryogenic Temperature

According to the law of thermodynamics, there is a limit to the lowest temperature that can be attained. Such temperature is termed as absolute zero. At absolute zero, molecules are in their least energy state. Cryogenic temperatures are measured using the absolute scale also known as Kelvin scale. The unit of Kelvin scale is kelvin and its symbol is 'K'. 0K (-273.15°C) is considered as absolute zero. The cryogenic region is considered to be approximately below 120K. 2.7K is the lowest temperature existing in the universe. In laboratory it is possible to freeze materials to as low as 10-6K.

The cryogenic temperatures cannot be measured using alcohol or mercury thermometers. The alcohol or mercury in them freeze at very low temperature and become worthless. Platinum Resistance Temperature Detectors (PRTD) is commonly used to measure cryogenic temperatures. The advantage of PRTD is that platinum can withstand very low temperature. PRTD can be used to measure temperature ranging from 4K to 866K. PRTD has the best accuracy and stability.

Semiconducting materials, such as doped germanium can be use as electrical resistance thermometers to measure cryogenic temperatures. Temperature ranging from 2K to 70K can be measured using this type of thermometer. The germanium used has doping values ranging from 1015 to 1019 atoms/cm3.

Table 1.1 Boiling point temperature of common cryogenic liquids

Cryogenic liquid

Boiling Point Temperature (K)

O2

90.2

N2

77.4

H2

20.3

4He

4.21

3He

3.19

CO2

194.6

NH3

293.8

SO2

263

Layout of The Project

Chapter 1 gives an idea about the field of cryogenics. It also explains about cryogenic temperature, the scale used and methods to measure them. It also gives an idea about cryogenic liquids. Chapter 2 is a literature review which explains the history of cryogenics. It includes the experiments accomplished by scientists to liquefy gases. Brief explanation of methods used by scientists to liquefy the permanent gases is included in chapter 3.

CHAPTER 2

LITERATURE REVIEW

History of Cryogenics

Michael Faraday was the first person to liquefy a gas. He successfully liquefied gases such as chlorine, sulfur dioxide (SO2) and ammonia (NH3). He was able to produce temperature as low as 163K. Even under high pressure he failed to liquefy gases such as carbon monoxide (CO) and methane (CH4). Permanent gases i.e. oxygen, nitrogen, helium and hydrogen could not be liquefied by his method.

Methods Created For The Liquefaction of Gases

Raoult Pictet proposed a method to liquefy oxygen and nitrogen. This method is known as Cascade Process. A modification of this process was later proposed by Heike Kamerlingh-Onnes known as three-stage Cascade Process. In 1894, Kamerlingh-Onnes constructed a plant for liquefaction of air that worked on cascade process.

In 1895, William Hampson (England) and Karl Von Linde (Germany) independently created a new method for the liquefaction of air. This method later came to be known as Hampson and Linde Method.

In 1902, George Claude (France) and C.W.P. Heylandt individually established a method to liquefy helium. This method is known as Claude Method.

Successful Liquefaction of Oxygen and Nitrogen

In 1877, two scientists independently liquefied oxygen using cascade process. Louis Paul Cailletet (France) and Raoult Pictet (Switzerland) were the two scientists. However, they were only able to produce a small quantity of oxygen which was insufficient for conducting experiments.

Based on their work a decade later Karol Olszewski and Zygmunt Wroblewski at the University of Krakow were able to produce large amount of oxygen, nitrogen and carbon monoxide in their liquefied form. They were able to measure the liquids boiling point temperatures and also established their properties.

Successful Liquefaction of Hydrogen

Sir James Dewar at Royal Institution, London made the first successful liquefaction of hydrogen in 1898. He used the Hampson and Linde method to produce liquid hydrogen.

The success of Sir James Dewar brought the field of cryogenics to a point where all the permanent gases except helium were liquefied and all their properties had been measured.

Successful Liquefaction of Helium

Helium was the most difficult gas to liquefy at that time. Helium being an inert gas could not be liquefied due to the lack of resources. In 1908, Kamerlingh-Onnes using Hampson and Linde method was able to liquefy helium (of mass 4).

Other methods were also used to liquefy helium. 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 helium (of mass 3) which is acquired as a daughter product in the decay of radioactive tritium.

CHAPTER 3

METHODS OF LIQUEFACTION OF GASES

Cascade Process

Vapour compression refrigeration system cannot be used to attain a temperature that can liquefy gases. Using this system, an evaporator temperature of 233K can be achieved. The major drawback of using this system to produce cryogenic temperature is the solidification of the refrigerants. The mechanical apparatus used also encounter difficulties when operating at cryogenic temperatures. Raoult Pictet proposed a method to produce cryogenic temperatures known as cascade process. The refrigerants used in this process have lower boiling temperature compared to the refrigerants used in vapour compression refrigeration system. Cascade process is preferred over multistage system because in multistage system the lubricating oil used in one compressor can escape into other compressors.

This process can be used to liquefy oxygen and nitrogen. It uses numerous compression type refrigerators. To reduce temperature at various steps, refrigerants such as ammonia, sulfur dioxide, carbon dioxide etc. are used.

The cascade process is now outdated. Liquefied hydrogen and helium cannot be made using this process. This is due to the lack of suitable refrigerant that can be acquired in liquid form to place in an evaporator. The refrigerant should also be able to maintain a very low temperature that is sufficient enough to liquefy hydrogen and helium by the process of compression only.

Single Stage Cascade System

Figure 3.1: Schematic diagram of a single stage cascade system.

The operating steps of a single stage cascade system are:

Step 1:- Compression of gas takes place in the compressor. This stage is only completed when liquid gas is acquired in the spiral tube immersed in the water cooling tank.

Step 2:- The heat caused by compression process is passed out of the water cooling tank.

Step 3:- The liquid then flows through the expansion valve reducing its pressure and is boiled in the evaporator. Heat absorbed from the surroundings is used for evaporation.

Step 4:- The evaporated gas is allowed to flow into the compressor, thus completing the cycle.

Two Stage Cascade System

Figure 3.2: Schematic diagram of a two stage cascade system.

Two compressors, a high temperature compressor and a low temperature compressor are used in this process. A heat exchanger serves as a condenser for low temperature cascade system and as an evaporator for high temperature cascade system. The evaporator of low temperature cascade system only produces useful refrigerating effect. The two stages 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 pressure that guarantees smaller compressor displacement. R-13 (Monochlorotrifluoromethane) is a refrigerant with low boiling temperature.

Three Stage Cascade System

Figure 3.3: Schematic diagram of a three stage cascade system.

A three stage cascade system is proposed by Kamerlingh-Onnes. Methane is used as a refrigerant in the first stage. Ethylene and ammonia are used as refrigerant in the second stage and third stage respectively. The purpose of methane in the first evaporator is to cool the high pressure ethylene of the second stage. Thus ethylene is liquefied in the coils at a temperature of 183K. The temperature of the second stage evaporator is maintained at 113K. This evaporator is used to liquefy the compressed air in the third stage.

Hampson and Linde Methods

The operation of this method is based on the principle developed by James Joule and Lord Kelvin in 1852. They discovered that when a compressed gas is permitted to escape continuously through a half closed valve, a change in temperature takes place. They also found that at room temperature, the gases were colder than the gases entering the half closed valve. When escaping the half closed valve, a slight heating occurred only for hydrogen. When the initial temperature of the gas entering the half closed valve is low, cooling occurs. The temperature at which the result is reversed from heating to cooling at the half closed valve is known as inversion temperature.

The only difference between Hampson's system and Linde's system is the manufacture of the heat exchanger. In Linde's system, the heat exchanger consists of two iron tubes having a length of 100m. Their diameters are 10cm and 4cm respectively and wound into a helix one inside the other. To reduce thermal loss wool was wrapped around the tubes. Through the inner tube high pressure air is passed and through the space between the tubes low pressure air is passed.

Simple Linde System

Figure 3.4: Schematic diagram of a simple Linde system.

The apparatus used for a simple Linde system consist of a compressor, a heat exchanger and a flash chamber or vapour-liquid separator. In a compressor, air is compressed isothermally to 200atm. The compressed air (oxygen or nitrogen) is cooled to 166K in a heat exchanger. When the compressed air flows through the expansion valve, its temperature reduces to 83K followed by a phase change. This is based on the principle discovered by Joule and Kelvin. In the flash chamber liquid air is separated from its gaseous form. The gas is brought back to the compressor whereas the liquid air is transferred to a container.

Dual Pressure Linde System

Figure 3.5: Schematic diagram of a dual pressure Linde system.

Simple Linde system can only be used to liquefy small quantity of air and its efficiency is low. To increase the efficiency of the system uses two compressors (high pressure and low pressure compressors), two flash chambers (high pressure and low pressure flash chambers) and two expansion valves are used. Atmospheric air flows through low pressure compressor into the high pressure compressor. The compressed air flows to the heat exchanger. From the heat exchanger the gas passes through expansion valve 1 to the high pressure flash chamber. The vapour present in the flash chamber is sent back to the output of the low pressure compressor. The liquid from high pressure flash chamber is passed through expansion valve 2 to the low pressure flash chamber. The vapour present flows back to the heat exchanger. The vapour then flows to the low pressure compressor.

The efficiency of both simple Linde system and dual pressure Linde system can be increased by using a vapour compression evaporator. Using the evaporator, high pressure compressed air is pre-cooled. This gives the best performance of the system.

Applications to Liquefy Hydrogen And Helium

The principle of the Hampson and Linde systems were used to operate the liquefiers proposed by James Dewar for the liquefaction of hydrogen and Kamerlingh-Onnes for the liquefaction of helium. Before these gases entered into the final heat exchanger system, it was important to cool the high pressure gas due to its low inversion temperatures. It was efficient to pre-cool the incoming hydrogen gas maintained at a pressure of 150atm using a liquid air that boils under a reduced pressure. For helium maintained at 30atm the pre-cooling is done by liquid hydrogen boiling under a reduced pressure.

Claude Method

Figure 3.6: Schematic 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 second heat exchanger. External work for the expander is done by using a reciprocating engine. If expansion is carried out without taking any heat from the surroundings, the external work done by air is at the rate of its internal energy. Using a compressor, an isothermal compression of air to a pressure of 40atm takes place. High pressure air is cooled partially through first heat exchanger.

The air output from the first heat exchanger is divided into two paths. About 80% of air is cooled by the expander. The remaining 20% of air passes through the second heat exchanger. When air passes through the expansion valve a portion of air is liquefied. The liquid air is removed from flash chamber. The air from the expander is mixed with the remaining air from the separator, which cools the incoming high pressure air in the second heat exchanger. This air passes through first heat exchanger back to the compressor.

The Claude system is more efficient than Linde system due to the presence of the expander which results in much lower temperature than the Linde system. This system can be further expanded by using a turbine instead of using a reciprocating engine to do external work for the expander. These also avoid problems of lubrication.

Methods of Liquefaction of Helium

Method Used by Samuel Collins

Figure 3.7: Schematic diagram for liquefaction of helium.

The Claude method was used by Samuel Collins and also by Kapitsa for the liquefaction of helium. To pre-cool the incoming helium gas, liquid nitrogen was used by Kapitsa. Collins also used two expanders to do the pre-cooling. Lubrication was not required for the reciprocating engines used in the expanders because they operated at 29K and 10K respectively.

In this system, helium is compressed to a pressure of 12atm by a four-stage compressor. Liquid nitrogen is used to pre-cool a portion of helium. By grouping four heat exchangers and three expanders, the helium is cooled down to about 16K. Helium can be further cooled by passing it through an expansion valve. At approximately 4.21K liquid helium is obtained. By using a flash chamber liquid helium is removed. The remaining helium gas flows back to the compressor by passing through four heat exchangers.

Simon Method

In Simon method, a highly compressed gas undergoes adiabatic expansion. In the process of expansion, temperature of the gas reduces when work is done against the external forces. Under favorable conditions, the condensation of gas results in the formation of its liquid. Helium undergoes compression in a copper pot which has strong walls. Solid hydrogen place in a container above the copper pot is used to cool helium gas down to 11K.

The stages used for producing liquid helium are:

The heat produced by compression is absorbed by hydrogen

The compressed helium is enclosed in a metal casing and is secluded thermally from its surroundings. At room temperature, helium is then permitted to expand through a constricted tube into a gas-holder.

Due to the process of expansion helium is liquefied. The copper pot is filled with liquid helium to a volume of about 70%.

CHAPTER 4

CONCLUSIONS AND FUTURE WORK

Conclusions

Study of cryogenics and cryogenic temperatures in detail.

Understanding various properties of cryogenic liquids

Understanding the history of cryogenics.

Detailed study of the methods used for the liquefaction of gases.

Future Work

Future Work includes the following:

Study of cryogenic liquids in detail.

Study of low temperature detectors.

Study of safety measures to be taken when handling cryogenic materials.

Study of applications of cryogenics.

CHAPTER 5

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