Cryogenics

INDEX SECTION I INTRODUCTION 1.1

CRYOGENICS

1

1.2

HISTORICAL BACKGROUND

1

1.3

CRYOGENIC REFRIGERATION METHODS

3

SECTION II APPLICATIONS AND PROGRESS 2.1

SPACE 2.1.1 WHAT IS CRYOPUMPING?

5

2.1.2 CRYOGENICS IN SPACE VEHICLES TESTING

14

2.1.3 CRYOGENIC WIND TUNNELS

19

2.2

ROCKET AND JET PROPULSION

2.2.1 INTRODUCTION

24

2.2.2 LOX IN ROCKETRY

24

2.2.3 SLUSH HYDROGEN AS A FUTURE JET FUEL

25

2.3

CRYOSURGERY AND MEDICINE

2.3.1 CRYOSURGERY

26

2.3.2 CONDITIONS CONDUCIVE TO CRYOSURGERY

28

2.3.3 CRYOSURGICAL INSTRUMENTATION

28

2.3.4 VISUALISATION OF CRYOSURGERY

30

2.3.5 CRYOSURGICAL INSTRUMENTS

31

2.3.6 CRYOSURGERY IN THE CURE OF PARKINSON'S DISEASE

2.4

39

2.3.7 CRYOGENIC EYE SURGERY

42

2.3.8 CRYOSURGERY OF TUMORS

43

2.3.9 TREATMENT OF SKIN DISEASE

45

2.3.10 BLOOD AND TISSUE PRESERVATION

45

CRYOGENIC ELECTRONICS 2.4.1 CRYOGENICS IN ELECTRONICS

49

2.4.2 CRYOELECTRONIC DEVICES

49

2.4.3 MASERS

52

2.4.4 JOSEPHSON JUNCTIONS POWER SUPERFAST CIRCUITS

53

2.4.5 SQUIDS (SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES) 2.5

54

SUPERCONDUCTIVITY

2.5.1 PERFECT CONDUCTIVITY AT CRYOTEMPERATURES 2.5.2

58

SUPERCONDUCTIVITY IN ELECTRIC POWER APPLICATIONS

59

2.5.3 POWER TRANSMISSION CABLES

60

2.5.4 ORE SEPARATION USING HTSC MAGNETS

62

2.5.5 THE MEISSNER EFFECT AND FLUX PINNING

65

2.5.6 MAGNETIC LEVITATION VEHICLES

66

2.5.7 ZERO FRICTION SUPERCONDUCTING BEARINGS

67

2.5.8 SUPERCONDUCTING MAGNETIC ENERGY STORAGE

71

2.6

CRYOMANUFACTURING

2.6.1 CRYOGENICS IN MODERN MANUFACTURING

73

2.6.2 LN2 CRYOGEN AS A CUTTING FOR PRECISION GRINDING

2.7

73

2.6.3 CRYOGRINDING FOR SCRAP RUBBER RECYCLING

74

2.6.4 CRYOGENIC SHRINK FITTING

75

2.6.5 FABRICATION OF PRESSURE VESSELS

76

2.6.6 ADVANTAGES OF CRYOMANUFACTURING

76

CRYOPRESERVATION 2.7.1 INTRODUCTION

78

2.7.2 LIQUID NITROGEN SYSTEM FOR FOOD TRANSPORTATION

79

2.7.3 CRYOGEN FREEZING & STORAGE OF FISH & SHELLFISH

80

2.7.4 CRYOPRESERVATION OF PLANTLETS AND LIVING CELLS

81

SECTION III CONCLUSION

83

Section 1

INTRODUCTION

1.1

CRYOGENICS: In present day usage, the word cryogenics refers to " all phenomenon, processes,

techniques or apparatus occuring or using temperatures below 120K." Cryogenic engineering involves the development of techniques regarding practical utilization of low temperature phenomenon both in industrial & pure science appls. From being an interesting curiosity in the times of pioneers like Linde & Claude, Cryogenic technology has grown into a diversified field of engineering. The tremendous scope of cryogenics today can be gauged from the wide range of applications that directly utilize cryogenic principles-

1.2

HISTORICAL BACKGROUND: The chronology of cryogenic technology is summarized in the following table,

beginning with the first attempt at oxygen liquefaction & ending with the development of High Temperature Superconductors (HTSCs).

YEAR

EVENT

1877 -

Calletet and Pictet liquefy oxygen

1883 -

Wroblewski liquefied nitrogen and oxygen

1892 -

Dewar developed vacuum insulated vessel for cryofluid storage

1889 -

Dewar produced liquid Hydrogen in bulk

1902 -

Claude air liquefaction system established

1908 -

Onnes liquefies helium

1911 -

Onnes discovers super conductivity

1916 -

Commercial production of argon

1926 -

First cryogenically propelled rocket test fired by Goddard

1933 -

Magnetic cooling used to attained temperature below 1K

1937 -

Evacuated powders used at insulation for cryofluid storage systems

1939 -

First vacuum insulated railway car for cryoliquid transport buillt

1942 -

V-2 weapon system test fired

1947 -

Collins cryostat developed

1957 -

LOX - RP -1 propelled ATLAS ICBM test fired, BCS theory for super conductivity developed

1.3

1958 -

High efficiency Multi-layer Insulation (MLI) developed

1961 -

SATURN launch vehicle test fired

1966 -

Dilution refrigerator using He3 - He4 developed

1975 -

Record high superconducting transition temperature raised to 23K

1981 -

Space shuttle using LOX launched

1986 -

HTSC ceramics developed with TC = 123K

CRYOGENIC REFRIGERATION METHODS:

Absorbing or extracting heat at low temperatures and rejecting it to the ambient at higher temperatures produces refrigeration for cryogenic applications. The three general methods of producing cryorefrigeration in large scale applications area)

Liquid Vaporization Cycle Here, a refrigerant fluid with the desired low temperature boiling point is first compressed & condensed & then isenthalpically expanded to a low pressure./ the evaporation of this liquid provides the desired refrigeration. .heat rejection can be cascaded from very low temperatures to ambient levels by the use of refrigerants with different boiling points.

b)

Joule-Thompson (J-T) Expansion Cycle In this process, a refrigerant fluid is compressed & pre-cooled to below its inversion temperature, i.e. ,the temperature below which a reduction in pressure causes a temperature decrease. the cold refrigerant fluid is then isentahlpically expanded to a lower pressure to obtain the required low temperature. the low pressure fluid is partially warmed to provide cryogenic refrigeration & then further warmed to produce precooling.

c)

Engine Expansion Methods: In this process, a refrigerant fluid is compressed, pre-cooled & expanded to a lower pressure through an expander to produce work to reduce both the enthalpy & temperature.

Other refrigeration methods like use of cryocoolers provide cooling on a smaller scale by a variety of thermodynamic cycles.

The Stirling cycle follows a path of isothermal compression, heat transfer to a regenerator matrix at constant volume, isothermal expansion with heat absorption & finally heat transfer from regenerator to fluid. The Glifford-Macmahon system is a staged cryocooler that uses a displacer to reach cryogenic temperatures. Magnetic refrigeration uses the magneto calorific effect to produce cooling. When ferromagnetic materials placed in a magnetic field are removed from it, their temperature falls. Such devices are used for attaining temperatures below 1K.

2.1 SPACE

Cryopumps and cryogenic pumps are an inevitable part of space programs, from its initial developmental phase to today's advanced phase. What differentiates cryopumps from cryogenic pumps is that while the later is used to pump cryogenic fluid through various devices used in space programs, former is initially a vacuum creating device. Both these devices have been delt with in this project work. 2.1.2 What is cryopumping? “The condensation of gas on a cryogenically cooled surface to produce vacuum is called as cryopumping." The cryopumping phenomenon not only involves phase change from gas to solid at cold surface but also adsorption of gas molecules. An attractive feature of cryopumping is the extremely large cryopumping speed, which are about 10 6 liters/sec and higher for more large installations.

Principle of operation: The principle is simple. Cryogenic liquid such as liquid nitrogen or helium (Helium is used for space application) is passed through the tubes fixed to the walls of the cryopumping. As liquid helium passes through the tubes, the components of air such as nitrogen, oxygen etc are condensed at the walls which are subsequently removed and we get vacuum at that space. Liquid helium is used because it has the lowest temperature in the cryogenic temperature scale at which every other component gets liquefied. The cryopumps are largely used for space simulations. One such cryopump with big tubing in NASA lab is shown in the figure 2.1.1

Types of cryopumps: -

Basically there are three types of cryopumps i)

Liquid pool cryopump

ii)

Continuos flow cryopump

iii)

Refrigerator cooled cryopump Out of the above three types of cryopumps, continuos flow types is most commonly

used in many applications including space simulation.

Advantages of cryo pumping: Some of the advantages of cryo pumping are, 1. Clean vacuum. 2.

Final pressure of less than 10-14 torr is obtainable.

3. Pumping speed per unit area is higher than that of all other pumps. 4. Extremely high pumping speed, greater than 100000 liters/sec can be produced economically only by cryo pumps. 5. Cost of investment and operation of refrigerator cooled cryo pumps are lower than those of all other pumps at speeds greater than 10 000 liters/sec.

Advances in cryopump technology: Extensive research and development is being done on the subject till date. The latest being high efficiency, variable geometry, centrifugal cryogenic pump proposed by the scientists at Barber Nichols Inc. Arvada.

Barber Nichols cryopump: The proposed pump has a basic design that is rugged and reliable with variable speed and variable geometry features that achieve high pump efficiency over a wide range of flow rate conditions. The pump uses a seal-less design and rolling element bearings to achieve high reliability and ruggedness to withstand liquid vapor slugging. The pump can meet a wide range of variable head, off-design flow requirements and maintain design point efficiency by adjusting the pump speed. The pump also has features that allow the impeller and diffuser blades height to be adjusted. The adjustable height blades were intended to enhance the efficiency, when it is operating at constant head, off design flow rates. This pump was developed for supercritical helium service, but the design is well suited for any cryogenic application where high efficiency is required over a wide range of head flow conditions. The assembly of the newly developed cryopump is shown is fig 2.1.2. To minimize heat transfer into the cryogen, the pump is mounted with the motor end above the impeller. With this design only the impeller and impeller housing are immersed in the liquid cryo. The motor operates in warm vapor from the cryo and heat transfer from the motor into the liquid cryo is, for the most part, limited to conduction through the pump components. Design concept:To minimize this condition heat transfer, the design uses thin walled components for all parts that bridge the space between the warm motor and the cold pump. These components are made of Ti-6A1-4V which has good mechanical properties at cryo

temperatures and has very low thermal conductivity (less than half of the thermal conductivity of 304 SS). The maximum shaft speed is 7900 rpm and the shaft is supported by rolling element bearings at both ends of the motor rotor. The rolling element bearings are much more robust than gas bearings that are used in other very high speed liquid helium centrifugal pumps. Experience with this bearing design has shown that they will stand up to shock loads associated with liquid-vapor slugging or magnet saturation. Since the bearings are near the motor and operate in a vapor environment, grease lubricated bearings can be used. The bearings are packed with low vapor pressure grease so outgassing is not a problem. Bearing related maintenance is very low as the bearings have an L-10 life and regrease interval of 50000 hrs and 8000 hrs respectively. The entire rotating assembly is enclosed in a hermetic pump housing. This design eliminates the need for any shaft seal, which is typically the high maintenance component in a centrifugal pump. It also insures that the pump that the pump will not leak under either vacuum or high pressure conditions. The pump uses a full emission impeller with 600 backward curved blades, a vaned diffuser section and an axial inducer. The axial inducer allows the pump t operate at its maximum flow condition with as little as one mete of NPSH. The full emission impeller and vaned diffuser section were designed for optimum efficiency at the 2.5 l/s, 39m head rise, and 7900 rpm. The pump has two features that allow it to maintain high efficiency at reduced flow rates. The pump speed can be adjusted. The pump also has adjustable height impeller and diffuser blades. By combining the adjustable speed and adjustable blade height

features, it is possible to cover a wide range of off-design flow conditions and maintain relatively a high pump efficiency. Figure2.1.3 shows that for the maximum flow condition, the pump is at its peak efficiency and is producing exactly the head rise required to match the system head drop. However, for the minimum flow condition, the pump is operating at about half of its peak efficiency and it is producing 25 times the head rise required to match the system head drop. (The excess head rise would have to be throttled across a control valve). It is apparent that if the pump is operated at a constant speed, the low off-design efficiency and the mismatch between the pump and system head flow characteristics will cause excess energy to be transferred into the cryo when it is operated at low flow rates. Testing has demonstrated that the liquid helium pump developed for the SSC cooling system is a mechanically sound, rugged, highly reliable assembly. The design point hydraulic performance of the pump is good and by operating the pump at variable speed, a wide range of off-design flow conditions can be produced at design point efficiency. To date there is relatively limited experience in the field of LHe centrifugal pumps. Barber – Nichols has built two LHe pump only for National Bureau of Standards (NBS), and one for the University Of Wisconsin (UOW). The NBS LHe pump is full emission, centrifugal pump with motor cross-coupled to the impeller. The motor runs in LHe, so all motor losses add energy to the LHe. The UOW LHe pump is a partial emission centrifugal pump with a 0.5m long hollow shaft between the motor and the impeller. The long shaft effectively limits the heat transfer, but requires the bearing at the impeller, which operates in the cryogen.

Cryopump for confined helium experiment (CHeX) The cryopump that was sent on the space shuttle for the Lambda point Experiment (LPE) was later redesigned for use in the confined Helium experiment (CHeX). The CHex uses High Resolution Thermometers (HRT) to measure heat capacity within finely spaced parallel disks. To eliminate heat flow between calorimeter and the cryogenic bath, a cryopump is used to reduce the pressure in the experimental probe. The LPE cryopump trapped activated charcoal behind the nucleopore filler. The new cryopump used activated charcoal glued to the copper plates to eliminate the nucleopore filter. Vibration tests verified that the cryopump does not generate charcoal dust contaminants. The early performance test indicated that the probe vacuum is significantly lower with the new pump, and that the pressure falls more rapidly than with the previous designs. Recent tests with CHeX experimental probe show a significant reduction in the heat leaf between the calorimeter and the cryogenic bath. The main improvements of the LPE cryopump design were to increase the thermal contact between the charcoal and the 1.7 K bath and to reduce the flow impedance from the instrument vacuum space to the charcoal supply. The LPE cryopump used loose charcoal grains trapped behind a nucleopore filter, while most cryopumps have charcoal glued to metal plates. A pump with charcoal bonded to copper plates offers several advantages. Each grain of charcoal is in good thermal contact with the substrate. The charcoal-covered plates can be arranged to offer good gas conductance to the instrument. And, since the charcoal is fixed in place, it would not be a source of contamination,

eliminating the flow impedance caused by the nucleopore filter used in the LPE cryopump. Considering these advantages, the LPE cryopump was redesigned with charcoal glued to metal plates using a high thermal conductivity epoxy. The main improvement of the LPE cryopump design were to increase the thermal contact between the charcoal and the 1.7 K bath and to reduce the flow impedance from the instrument vacuum space to the charcoal supply. The LPE cryopump used loose charcoal grains trapped behind a nucleopore filter, while most cryopumps have charcoal glued to metal plates. A pump with charcoal bonded to copper plates offers several advantages. Each grain of charcoal is in good thermal contact with the substrate. The charcoal-covered plates can be arranged to offer good gas conductance to the instrument. And since the charcoal is fixed in place, it would not be a source of contamination, eliminating the flow impedance caused by the nucleopore filter used in the LPE cryopump. Considering these advantages, the LPE cryopump was redesigned with charcoal glued to metal plates using a high thermal conductivity epoxy.

Construction of the cryopump: Figures 2.1.4 & 2.1.5 show the construction features of the CHeX cryopump. The housing holds fire copper plates coated with activated charcoal. The charcoal is embedded in a 0.8mm thick layer of aluminum filled epoxy on both the sides of each plate. The total plate surface area of 180 cm 2 holds about 9 g. of charcoal. This epoxy was also used to attach the plates in the charcoal plate supports. The plate assembly is stiffened by four 1.6 mm diameter. Copper rods glued into the holes in the plates. The screen support tube holds the plate assemblies in place inside the charcoal housing. A 100 mesh (per

inch) screen is soft soldered into a copper frame that fits lightly over the mouth of the housing. The screen retains all larger pieces of charcoal that might escape from the plates. The gold plated bottom radiation baffle reduces the heat radiated into the instrument during the bake out.

Operation of the cryopump: Several days before the launch, seven liter instrument volume was filled with 1.3Kpa (10 torr) of He gas. This gas provided enough thermal conductivity between the HRT flux tuber and the liquid helium bath at 1.7K to keep them below the superconducting transition design launch. Shortly after the launch, the mechanized valve opened to allow the probe to be evacuated through 1 m long, 6.4 mm in corrugated vacuum line. At the same time, a heater warmed the charcoal in the cryopump to above 40 k in order to expand the adsorbed. He gas After about five hours, the residual gas pressure was about 13 Pa (0.1 torr) and the evacuation valve was closed and the heater turned off. Then the charcoal cooled to 2.0 K by conduction through the weak thermal link to the liquid helium bath. The residual He gases in the probe passed through the nucleopore filter where it is trapped by the cold charcoal. For the first 30 hours after the launch, a 3W heater was cycled on and off to warm the liquid helium bath up to 2.0 K Increasing the instrument temperature reduced the time required to reach the operating pressure, about 10-7 Pa (10-9 torr).

Thus, recent tests with CHeX experimental probe show a significant reduction in heat leak between the calorimeter and cryogenic bath. The pump has also shown better performance characteristics compared to LPE pump.

Cryogenics in space vehicle testing: Introduction:The space vehicles carrying the spacecraft are sometimes required to undergo orbital and space exploration flights. Hence space simulation should provide actual ignition, restart and full-scale combustion tests under near space conditions compared to conventional environmental changes where the space crafts are tested. Testing of rockets under simulated altitude experience tests article leakage, out-gassing, large heat release, highly corrosive propellants, very high exhaust volume flows. These loads make it impossible to provide the required altitude simulation, for this type of test with any single pumping systems are combined and manipulated.

Space simulation: Cryogenic pumping offers high speed, which is essential for space simulation due to larger volume of chamber and high flow rate of gases during operation. Apart from pumping for pressure simulation, cryogenics is also used for thermal balance simulation and cooling of exhaust diffuser in rocket engine testing. Nearly perfect vacuum is required for true space simulation. Following chart gives the presently accepted values for pressure at various attitudes

Altitude (km)

Pressure (pa)

85.3

0.387

106.7 152.4 182.9 243.8 304.8 457.2 609.6 670.1 707.1

1.1x10-2 4.6 x 10-4 2.0 x 10-4 5.3 x 10-5 1.7 x 10-5 1.8 x 10-6 3.1 x 10-7 1.5 x 10-7 11.1 x 10-8

Mechanical pumps are employed to reduce the chamber pressure to 0.1 m bar range. Further reduction of chamber pressure is carried out by cryogenic pump or diffusion pump. Normally the space simulation chambers are evacuated around 1 x 10 -6 mbar (7.4 x 10-8 mtorr) to study the possibility or freezing of propellant in plumbing and also simulates the near space condition. One-kilowatt gaseous helium refrigeration system cryopump would be adequate for pumping large gas volumes under vacuum conditions. Although low

temperature refrigeration is quite expensive, cryopumping speeds are so great that the cost of large scale. Cryopumping in terms of cost per liter per second may be lower than for any other pumping system. Though at 20 k surface remove all gases, it is difficult to maintain whole surface at such a low temperature. It then requires extensive and costly refrigeration equipment. But the temperatures in the vicinity of 100 K can be produced at much lower cost using liquid nitrogen. To maintain the temperature at 20 K, “cryopumping arrays” are used. In these arrays, 20 K surface are protected from the radiant energy of the vehicle and the chamber walls by shields cooled to 100 K. Since 20 K surface may receive reflected energy from the shield surfaces, the thermal absorptivity of the shield is made as high as possible by blackening. The shields are finished with black anodic treatment from inside or sprayed with some “ space-quality” paint for high absorptivity.

The cryopump arrays are shown in figure 2.1.6

Rocket Engine Testing: The schematic of the rocket engine testing is shown in figure 2.1.7 initially is consist of an outer chamber which surrounds an inner chamber. The inner chamber is fitted in outer chamber by means of isolation bulkheads. Mechanical pump is employed for outer chamber so that the vacuum can be maintained between the two chambers, which will avoid filling of direct radiation from the atmosphere to the inner chamber.

Rocket engine is placed at the center of inner chamber. The arrangements shown in the figure are made. To avoid heat transfer from engine to port, ”heat shield” is provided in the chamber. For simulation of sun-radiation, “radiation lamp” is placed around the engine. Liquid Nitrogen is supplied around the inner chamber for simulating the “black space." Liquid Nitrogen is also supplied to the diffuser, which removes gases during firing of the engine. Simulation of space condition requires that the test object be surrounded with surfaces highly absorptive to thermal radiation and emitting minimum radiation. This is best accomplished with blackened surfaces cooled to cryogenic temperatures. These surfaces are known as cryopannels, heat sink, or thermal shrouds. The two cryogens used in simulation facility are cooled gaseous helium & liquid nitrogen. Gaseous helium is required only for the cryopump whereas liquid nitrogen does cryopumping, trapping, thermal simulation, absorbs heat from the radiating nozzles & cools the rocket exhaust gas boundary layers. Hence the efficient operation of the system depends upon the performance of the liquid nitrogen distribution system. The liquid nitrogen distribution system is shown in figure 2.1.8. The liquid nitrogen supply system for the shrouds are of two types, usually described as 1) Flash system 2) Sub-cooler system The flash system pumps liquid directly from the storage tank through the load, then expands it through the valve, either directly into the tank or into a separator from which the

remaining liquid is drained into the tank by gravity & gas from flash chamber is sent for refrigeration i.e. to convert it again into liquid nitrogen. In the subcooler system, the liquid from the tank is taken out by the pump. It cools the shroud & is discharged into the heat exchanger, where it exchanges heat with the liquid nitrogen directly coming from the LN2 supply tank. The LN2 which is converted from gaseous N2 is recirculated again. This liquid nitrogen is used to convert gaseous nitrogen into liquid. The system thus forms a closed loop. As can be gauged from the above, space simulation testing of space vehicles is much more complex as compared to simple aircraft testing and that cryogenics forms a vital & inevitable part of the space simulation program so that any future research needs to focus on methods of making the cryosystems cheaper & less complex.

2.1.3 CRYOGENIC WIND TUNNELS: Until recently, the problem of low test Reynolds numbers had limited the usefulness of wind tunnels, especially at ultrasonic speeds. Perhaps the best solution to the problem of low Reynolds number applications comes by operating a big tunnel (2x2 m test section or larger) at relatively high pressures (upto 5 bar) & at cryogenic temperatures using nitrogen as the test gas. The first cryogenic wind tunnel was a low speed tunnel built at the at the NASA Langley research center in 1972. Since then, workers at research centers around the world have started a large number of cryogenic wind tunnel projects. The most successful tunnels have been built using skillful combination of cryogenic technology & wind tunnel theory.

Wind tunnels have played an important part in the development of aircraft right from the 1903 “Wright Flyer” to today’s state of the art supersonic jets, even with today’s supercomputer supported Computational Fluid Dynamic technology, the need for obtaining accurate experimental data from wind tunnels increases with the development of complex aircraft structures . Liquid nitrogen is injected into the air circuit & the resulting nitrogen gas result in an increase of the Reynolds number by a factor of about 7 with no increase in the dynamic pressure , while the drive power actually reduces.

Principle of operation : Fig. 2.1.9 shows the basic principle of the cryo wind tunnel. It shows the effects of reducing temperature on the gas properties, test conditions & drive power for a fan driven cryogenic wind tunnel using N2 as the test gas. For comparison purposes, a temperature of 332 K for ambient temp transonic tunnels is assumed. The left side figure shows the properties of gas that varies with the temperature. The right hand side figure shows the corresponding variations in he test conditions & drive power. It is clear from the figure that as the stagnation temperature decreases, the Reynolds number increases steeply. These curves are for the same tunnel with constant size & Mach number. Cooling the test gas to very low temperatures increases the Reynolds by more than a factor of 7. This increase is obtained with no increase in the dynamic pressure (model loads) & with a large reduction in the drive power, a clause which makes cryo wind tunnels a far more attractive choice as compared to previous methods.

NDA CRYOGENIC WIND TUNNEL: National Defence Academy (NDA) in Japan planned to replace its old transonic wind tunnel of the induction type with the new type of transonic tunnel which would be used for basic research works. The preliminary study for the new wind tunnel was started in 1981, and the cryogenic wind tunnel concept was selected for the tunnel, as the concept also can be applied to small wind tunnels used for basic research of fluid dynamics and aerodynamics. The NDA cryogenic wind tunnel was designed as a fan driven, high subsonic, twodimensional tunnel. The free stream Mach number should at least upto 0.8 to provide aerodynamic testing capability at a supercritical condition. The lowest stagnation temperature of 108K was selected to avoid oxygen rich condition at any outside part of the tunnel pressure shell. The design parameters of the tunnel are Type

closed circuit

Material of construction

SUS 304, SCS 13 stainless steel

Insulation

External

Cooling

Liquid nitrogen

Test Gas

Nitrogen

Test section size (H,W,L)

0.3 x 0.06 x 1.0 m

Mach no range

upto 0.80

Contraction ratio

14:1

Total pressure

upto 117 K Pa

Total temperature

108 K ambient

Running time

upto 30 min

Maximum Reynolds no (1/M)

90 million

Fan type

centrifugal compressor

Fan speed

upto 2250 rpm

Drive motor

75 KW

LN2 tank volume

4.9 m3

The cooling method of the working gas for the present tunnel is the same method that the NASA Langley Research Center developed. Liquid nitrogen is injected into the circuit at the middle section of the second diffuser, which is located upstream of the compressor as indicated in figure2.1.10. Originally the LN2 flow rate was controlled by a manually operated needle valve.

CONSTRUCTION OF THE TUNNEL Six nozzles for LN2 injection are placed on the circumference of the 2nd diffuser and set 600 from each other. They are connected in two groups of three nozzles, 1200 apart. The direction of spray of LN2 is perpendicular to that of the tunnel circuit flow. The test section is rectangular in cross section, & has solid sidewalls & a slotted top & bottom walls. The test section is housed in the cylindrical plenum chamber with a diameter of 711mm. This chamber is mounted on a trolley & is drawn back to access the test section. The construction of the chamber is very heavy & has a higher thermal capacity than that of the test section. This difference of thermal capacities has significantly influenced the operational efficiency of the present cryogenic tunnel. In order to avoid

excessive differences in the cooling rates between the chamber & the test section walls, a dedicated LN2 line for precooling the plenum chamber is installed . Since the NDA cryogenic wind tunnel was built in 1985,continual efforts have been made to improve the cryogenic operational system & procedure, & to develop airfoiltesting techniques at cryogenic conditions. The operational experience with further improvement of the system showed that the cryogenic operation of the present tunnel is relatively easy & safe except the cool down. But the cool down characteristics can be improved by exhausting nitrogen gas from the plenum chamber during the cooldown. It saves about 18 % of the amount of LN2 and 20% of the time for cooldown. Preliminary airfoil testing experiments also indicate that adopting suitable wall corrections, the present tunnel has a possibility to perform two dimensional airfoil tests with a model of low aspect ratio. Further experimental research is however needed to remove some of the deficiencies of the system The chart shows the specifications of some of the world's famous cryo tunnels. NASA developed its first cryogenic wind tunnel & this small & simple fan driven low speed atmospheric tunnel made it possible to build the 0.3-m transonic cryo tunnel (TCT) .The fan driven 0.3-m TCT can operate at pressures upto 6 Atm & Mach no. greater than 1.4. Success of the 0.3-m TCT led to the decision to build a very large cyro tunnel, the US National Transonic Facility (NTF). The NTF with a 2.5x2.5 m test section & operating pressures can test at flight values of Reynolds no. for many configurations. In the pat 20 years, researchers in 8 countries have built over 20 cryo tunnels a of various types. Recently a very large wind tunnel is being built in Japan at NAL. The 9 Atm 3x3 m transonic tunnel is proposed at Sawado; thus with the improvements in hardware,

software, instrumentation & operating procedure we can expect more cryo tunnel projects to fall by the way.

2.2

ROCKET AND JET PROPULSION

2.2.1 INTRODUCTION: One of the best known areas in which cryogenic fluids like LOX, LH2 are used in the field of Rocketry, with slush hydrogen being considered as the future prospect for use in jet propulsion systems. To carry the large amounts of oxygen in gaseous form would require very large volume low pressure containers or small volume but heavy thick walled cylinders, both of which would go against the stringent space and weight requirements of rockets. Hence, liquid oxygen (LOX) is used, it being 700 times denser than gaseous oxygen and having the advantage that it can be carried in low pressure, light weight insulated containers. To obtain the maximum thrust with the least possible mass of fuel, the designer searches for ways by which mass may be ejected at very high velocities, ie, by rapid burning and ejection of fuel. LOX and LH2 produce a large amount of thrust and hence are obvious choices for rocket fuels.

2.2.2 LOX IN ROCKETRY: Thrust is produced in a rocket engine by the combustion of fuel for which an oxidizer is required, oxygen being the natural choice. The rocket cannot depend upon the

atmosphere to provide the required oxygen and hence it must be carried on the rocket itself. The first long range successful rocket, the German V-2 Rocket Bomb used 5000kg of LOX as its oxidizer and 3700kg of Alcohol as the fuel and had a maximum range of 200 miles. Almost all designs from then on have used separate LOX and fuel tanks from which the fuel and oxidizer are piped separately to the combustion chamber. In the 60s NASA, developed design in which the fuel and LOX chambers formed the skin of the rocket to save weight, while others use the LOX piping to cool the combustion chamber.

2.3

CRYOSURGERY AND MEDICINE

2.3.1 INTRODUCTION Cryosurgery is the destruction of tissue by freezing. Modern Cryosurgical techniques were first introduced in the mid 1960's but they only achieved modest application in selected medical specializations. Newer advances in cryosurgical devices and Magnetic Resonance Imaging have great promise in making cryosurgery a primary therapeutic modality. The goal of cryosurgery is to destroy cells within a limited diseased area and to allow the complete recovery of surrounding cells. Cooling of tissue with a cryoprobe occurs via conductive heat transfer. When a cryoprobe is in contact with tissue, an ice front advances in a radial direction away from the probe to form a cryolesion. The growth of the cryolesion is incremental and well controlled. Frozen tissue is white in appearance

and is hard to the touch, clearly delineating it from unfrozen tissue. It is common to create a cryolesion beyond the margins of the desired treatment area to ensure exposure of unwanted cells to the coldest possible temperature. After freezing, the tissue is left in place to thaw. The tissue dies predominantly by a septic necrosis and is absorbed or sloughed by the body.

Characteristics of cryosurgery The most important characteristics of cryosurgery are -

any tissue can be destroyed from the skin to bone

-

any quality of tumor can be destroyed

-

the loss of blood during the procedure is minimal or absent

-

the possibility of using of cryoprobe avoids large incisions for deep organs

-

one of the most important characteristic of cryotherapy is its painlessness (the cold is itself an anesthetic)

-

the quantity of cryodestruction can be controlled because it is exactly proportional to quantity of freezing

-

it is safe. The edge of cryodestruction is precise without damage to adjacent tissue

-

though the rules for the applications must be rigorously followed, yet cryotherapy is simple to apply

-

it is rapid. It is possible to make a transurethral cryotherapy in 30 min

-

it is possible with thermocouple and impedance to document the intensity of cryodamage

-

it is reversible at the initial stage; this is important in any field, especially in neurosurgery.

-

it is possible to know before, during and after the treatment where the cryodestruction will finish

-

one of the most important characteristic of this treatment is that the scar is not retractive. This is especially important for the skin, the urethra, the prostate, the bronchus and so on.

Basic science studies have important implications of cryosurgical devices and procedure development. 1.

Cells in the treated region should be subjected to the highest cooling rate possible or at least to a cooling rate for cell survival. For most invasive cryosurgical procedures, probes must be introduced in the body warm. To achieve the necessary cooling rates, cryosurgical devices should be developed with high cooling capacity to achieve a rapid cool down.

2.

Minimum temperature achieved should be -200 c or lower.

3.

Thawing should be carried out at the slowest rate possible or natural rate that can be achieved in the body.

4.

Repeated freezes should be carried with a complete thaw in between freezes.

5.

Because heat transfer occurs, conductivity probe contact must be maximized to assure the highest possible cooling rate and deepest penetration of lethal temperature.

6.

Multiple probes may be placed within a single surgical sight to ensure exposure of the tissue to the greatest cooling rate possible.

2.3.3 CRYOSURGICAL INSTRUMENTATION The two current methods by which low temperature are achieved in cryosurgery are the circulation of liquid nitrogen to a probe or the expansion of high pressure gases (J - T systems) such as Nitrous Oxide or Argon. Liquid nitrogen systems circulate the coolant to vacuum insulated probe. In early liquid nitrogen devices, Liendenfrost boiling often occurred in the tip which severely hampered heat transfer and probe surface cooling. With lekdenfrost boiling, a warm gaseous film forms between the liquid nitrogen and the inner surface of the probe effectively insulating the tip. Complex staged sub cooling devices have been built that overcome this problem and allow colder probe surface temperature (-130 to -1400c). However, this significantly increases the cost and complexity of the device. Further, because the coolant is liquid nitrogen, large storage tanks are required. This limits the ability to reduce the size of the device. Also, circulation of the cold liquid from the tank to the probe causes cooling of the entire conduit. This creates the need for bulky insulation and large inflexible hoses. Finally the liquid nitrogen is consumed during use and must be continuously replenished. This creates convenience problems related to storage and replacement of liquid nitrogen and also creates the opportunity to run out of coolant during a procedure. Other currently available cryosurgical systems operate on the J-T principle. These systems utilize a single compressed gas (for example, Nitrous oxide or Argon) to cool a

probe through the J-T effect. Because cooling occurs the majority of the cooling can be focused at the tip, eliminating some insulation requirements. However to generate probe surface temperature below -1000c under biological heat loads, high press (2000+ psi) are required which creates safety problems. Even temperatures as warm as -600C require pressure of 800 psi. Similar to liquid N2 the coolant is exhausted during use and must be continually replenished. Finally the performance of these devices during the use decays as the bottle pressure decreases. Mixed gas J-T devices may circumvent the problems associated with current technologies and allow the development of a safe, low temperature, cost effective, and convenient device. By using mixture of gases, as opposed to a single gas, the non-ideal properties of the coolant are increased. This results in a greater J-T effect, such that lower temperatures and greater power can be achieved at lower pressures. Thus the advantage of J-T based systems which includes low cost, compact size, focussed cooling and flexibility can be combined into a system that achieves the necessary operating temperature for tissue destruction at safe pressures. With lower working pressures, a compressor can be incorporated into a mixed gas system reducing replenishment needs and reliability concerns.

2.3.4 VISUALIZATION OF CRYOSURGERY ULTRASOUNDS Freezing of tissue can be clearly visualized using ultrasound. The frozen tissue is a solid mass and the ice font reflects a significant fraction of the acoustic waves. This creates a bright white light at the interface between frozen and unfrozen tissue. The

remainder of the frozen tissue appears as dark region behind the hyperchoic line and is referred to as a post acoustic shadow.

MRI Like ultrasound, the growth of the ice ball can be clearly visualized using MRI. There is no effective signal from the frozen region because the T relaxation interval is sufficiently short. Subsequently the cryolesion appears black. It has been demonstrated that mathematical model for temperature distribution within a cryolesion can be utilized to obtain MRI assisted numerical solutions to the energy equations. In this manner the thermal history within the cryolesion can be clearly mapped.

2.3.5 CRYOSURGICAL INSTRUMENTS A NOVEL CLOSED LOOP CRYOSURGICAL DEVICE: A novel cryosurgical device has been developed that dramatically improves the cost, convenience, safety and effectiveness of cryosurgical devices. The device incorporates a mixed gas J-T stage. The fluid mixture is non-flammable, non toxic, non corrosive, environmentally safe, and therefore suitable for medical applications. The system is compressor driven, utilizing a novel oil free motor. Working temperature of -115 to -1250C at pressure of 300 to 350 psi are achieved. Approximately 20 W of refrigeration is delivered at -1200C.

The compressor console is portable having a 4 cu-ft volumetric footprint. The cryoprobe has a handle 3.5 cm O. D. and 18cm shaft having a 4mm O. D. and a 3.5 cm freeze zone. In bench top studies using tissue simulating media (2% gelatin, 98% saline) ice balls of 3.5-4.0 cm diameter and 5 cm length with a mass of 48gms are created. Studies using half calf liver tissue in body temperature water bath show ice balls of similar size and mass (3.5 cm diameter, 48 g). In lice goat animal studies, surface temperature of -1150C were achieved and ice balls of 3-8 cm in diameter were created in 10 min in the liver. Isotherms below -200 C were developed at 12mm radial distance from the probe.

SYSTEM Design criteria for the system were 15-20 w of refrigeration at -1200C with a maximum pressure of 350 psi. Two stages were required to achieve the desired cooling capacity. With no precooling, given the limits of the compressor technology

to

be

employed , the maximum refrigeration power yield was 9 W at -1200C. However, with approximately 20W of pre cooling at -300C, this refrigeration power increases to 28W at -1200C. The first stage, or pre cool stage, incorporated a novel oil free compressor and a novel gas mixture. The compressor had a unique electromagnetic line or drive mechanism and gas bearings and a size of approximately 0.6 cubic feet. The overall size of the integrated precooler compressor and gas mixture compressor is 2.3 cuft. The power of the system was tested using a heater mode of Nichrome wire which was wound around the freeze zone to a known resistance. A DC power supply was used

to provide current. The device was operated until the temperature reached a steady state and the power output of the heater was increased until temperature began to rise.

GAS MIXTURES Gas mixture combinations were computer modeled using an algorithm that optimizes a unitil less number termed the (H*) min. Conceptually this number provides a relative assessment of the amount of refrigeration power a gas mixture has available to extract heat from tissue for a given amount of heat exchange capacity.

CRYOPROBE: The cryoprobe was constructed by encasing heat exchangers in a metallic shell. The shell was subsequently evacuated to 10E-6 Atm to provide insulation. The metallic shell had a handle section and a shaft section. The heat exchangers of the two stages were integrated into the handle section, which had an O. D. 3.5cm, and a length of 18cm. The shell was 4mm outer diameter and 10cm in length and had a copper freeze zone or tip, at the distal end 3.5 cm in length and 4mm O. D. Braided stainless steel tubing was used to carry gas from the compressor the cryoprobe. Soldering of adjacent stainless steel tubes and mandrill winding constructed parallel tube counter flow heat exchangers. For the pre cool stage, the high-pressure gas mixed tube was soldered to low pressure gas mixture tube subsequently transitioned to a counter flow parallel tube heat exchanger from the second stage. At the end of the second stage heat exchanger, the high and low pressure tubes transitioned into a coaxial construction

that carried the gas down the shaft and to the freeze zone. A capillary tube restriction orifice was placed at the end of the high-pressure coaxial line allowing the gas mix to expand within the tip.

RESULTS: In design of a cryosurgical apparatus, the two most important criteria are probe surface temperature and cool down rate. The probe surface temperature determines the size of the ice ball created and the isothermal gradients within the ice ball. Most cells are destroyed at -200c and in some cases temperature of -400c is required. When treating tissue, it is important to achieve this necrotic temperature at the desired depth of penetration or at the margin of a tumor, making surface temperature a critical factor. The cool down rate is also relevant to cell death. A fast cool down rate results in the formation of intracellular ice-crystals, which increases the lethality of the technique. Different cells require different optimal cooling rates for destruction, but in general cooling rates above 25 0c/min are lethal. Subjecting tissue to repeated freezing increases total cell death by increasing the ice ball size and the necrotic isothermal zone. And increasing cool down rate at any given point in the frozen area. In addition, repeated freezing can lower the required necrotic temperature. This newly developed device is capable of achieving lethal temperature at radial distance of greater than 1cm from the probe surface. Cool down rates of greater than 150c/min are seen within radial distances of 6-9mm. This data indicate that the device with a single probe is capable of clinical treatment of small to moderately sized tumors. Further it would be useful in other application such as cervical disease or abnormal uterine

bleeding. A multi probe device could be developed for use in large, more complex diseases such as prostate cancer. The two stages in this system provide a number of advantages. First, the cool down rate is improved by increasing the overate the cooling capacity. Second, a smaller second stage compressor. Third, contaminants that could potentially freeze out at the J-T orifice in the tip, may first condense in the pre cooler and prevent clogging. A small Freon refrigeration loop was integrated into the system as the pre-cooler. Thermoelectric coolers meeting the pre cooling requirement were found to be impracticably large, a split stream pre cooler was considered, however the overall capacity of the current oil free compressor was too limited. An oil free compressor was chosen to improve reliability. Due to the small diameter tubing required to make the compact heat exchangers for the system, the low operating temperature, and the nature of the gas mixture, oil could easily condense in the system and cause clogging. Using an oil based compressor maximum cool time was less than five minutes and continued to decline with successive freezes. With the oil free compressor there is currently no limit on freeze time other than mechanical failure of the pump. Current run times of the system well exceed the needs to complete clinical procedures.

A CRYOGENIC CATHETER FOR TREATING HEART ARRHYTHMIA: Heart Arrhythmia is a problem for over 2 millions Americans. To date such patients have been difficult to treat with conventional drugs or surgery. Catheter therapy has proven to be more effective and less expensive method of the treatment, but the electrosurgical catheters currently used are not very effective for heart arrhythmia

treatment. They have limited destructive capability, and are difficult to keep in contact with the heart. Lesions created with electrosurgical catheters are 3-4cm diameter spheres and lines of these spheres must be connected in order to perform some of the treatment. A cryogenic catheter has the potential to deliver greater destructive power to the tip, allowing larger lesions to be formed. Linear lesions are possible and the catheter tip will adhere to the tissue during the cooling process. To treat heart arrhythmia the cryogenic catheter must reach temperature between 100 and 150K. To be inserted through Veins, it must be 3mm in diameter or smaller and must be able to make a blend with a 10-15mm radius. It must be about 1m in length and must have a surface temperature above 00c along its length. For safety reasons the maximum pressure should not exceed 3MPa. It is estimated that 10W of cooling will be required. The refrigerant should be benign, nonflammable, nontoxic and have low ozone depletion potential. The catheter also needs to be disposable for safety and sterilization reasons. These requirements have led us to choose a mixed gas J-T refrigerator as the most practical solution. The difficulty with existing J-T system is that they are too large in size to be used for catheter therapy. The cold head is too expensive to be disposable, and the units require too high pressure for catheter use. The cryogenic catheter is based upon the J-T cycle shown in fig. 1. This is a closed cycle system that does not require make-up gas at any time. The catheter has coaxial tubes for the high and low-pressure streams with a miniature heat exchanger and J-T orifice at the catheter tip. The high pressure is 2.5MPa. The largest diameter is 3mm, the length is 90cm, and all but the last 10-20mm is flexible. The gas mixture has been optimized for the required operating conditions using non-

flammable and low ozone depletion gases. Low cost techniques have been incorporated into the fabrication of the cold tip so that each catheter can be disposable.

Cryogenic catheter system The schematic of diagram of the catheter tip is shown in figure and details of both the Heat Exchangers and expansion orifice is given below. Heat Exchanger The miniature Heat Exchanger at the cold end is fabricated by diffusion bonding perforated plates of copper alternated with stainless steel spacers. Photos of the Heat Exchangers are shown in fig. By diffusion of large metal sheets containing many individual Heat Exchangers layers, large numbers of cold ends at one time, significantly reducing the cost in order to make them disposable is made. Prototype Heat Exchanger varied in length from 5 to 15mm. The O. D. of Heat Exchanger was 2.5mm. The enthalpy is more affected by changes in pressure at lower pressure, so the low pressure of Heat Exchanger must have a much lower pressure drop than the high pressure side. Expansion Orifice: The J-T expansion orifice shown in fig was fabricated of sintered Copper powder. This permitted many flow channels, limited plugging problems and provided a large area for heat transfer. Later catheters were fabricated using a single knife edge orifice and provided similar results with much less effort.

Lumen:

A lumen is simply the tubing used for the flexible catheter pressure lines. The inner lumen was a 1mm O. D. polyimide tube with a stainless steel reinforcing braid. The reinforcing braid provided not only strength against the 2.5MPa pressure but also helped to reduce the possibility of kinks in the tube. The outer lumen was a braided nylon derivative tube with a 0.25mm wall. For safety, the outer lumen was designed to withstand the highest pressure in the system, although a ballast volume was added to the low-pressure side to reduce the average system pressure.

Compressor: The compressor was commercial single stage oil lubricated compressor that required input powers from 300 to 500W. Some experiments operated two compressors are parallel to increase the mass flow rate. Later experiments were performed using a custom-built oil free compressor. This simplified problems with the gas mixture since higher boiling point components.

Results: Most experiments were performed with the catheter sitting on the bench top exposed to ambient air. A typical experimental result can be seen in fig. The catheter is characterized by fast cool downs and warm ups. The lowest temperature achieved was 85K with no load on cold end. Typical operating conditions were 140K with about 3W of additional heat added to the cold end. Fig shows catheter in operation.

2.3.6 CRYOSURGERY IN THE CARE OF PARKINSON'S DISEASE:

Parkinson's disease (so called "shaking palsy"), a disease of the central nervous system which causes tremors of the arms and legs, has been successfully treated by cryosurgery. A special system was developed by Linde company in co-operation with Dr. Irving S. Cooper of St. Barnabas Hospital. The probe is shown in fig is inserted through a dime sized hole in the patient's skull into the thalmus (a mast of gray matter at the base of the brain which controls the transmission of nerve impulses). The cannula or probe is made up of three capillary tube (1)

an inner tube through which liquid nitrogen flows

(2)

an intermediate tube, through which the vapor formed in boiling the liquid returns from the probe tip and

(3)

an outer tube, which serves as vacuum insulated expect for the tiny silver tip, only the material in the immediate vicinity of the tip is affected. The entire cannula has an O. D. of 0.079" (2mm) so it may be inserted easily into deep body regions. During an operation, liquid nitrogen is introduced into the probe from a storage

Dewar, and the probe tip is initially cooled to about 14 0F (-100C). The temperature is sensed by a thermocouple attached to the probe tip and the temperature is displayed on a strip chart recorder. Control of probe tip temperature is achieved by control of the flow rate of liquid nitrogen to the probe. When the probe tip has been cooled to 14 0F, the position of the thalmus in the immediate vicinity of the probe tip freezes. The frozen area will recover with no damage if it does not remain frozen for more than 30 sec. The probe is moved to several locations in the search for the defective area, and the shaking stops when the region controlling the tumors is frozen. The patient is conscious during this operation and assists the surgeon in locating the defective thalmus region. When the region controlling

the shaking is located, it is destroyed by prolonged freezing (about 3min) at temperature between -400F and -600 F. At the end of the freezing period the probe tip is rapidly warmed for easy removal. The destroyed tissue (a sphere of approximately 1/4" dia) forms a cyst after thawing and does not interfere with normal body operation. Of the more than 200 cases treated by cryosurgery during the initial development of the technique, over 90% achieved excellent alleviation of the symptoms of shaking and rigidity. In the remainder, no clear cut improvement was found but in no case were there complications or adverse affects of cryosurgery. One of the striking features of this method of brain surgery was that there was no morbidity or neurological defects after the operation. In many cases, the patient was allowed to get up and move around within 24 hr after surgery. Obviously, the control of the extent of the frozen region is quite important. The volume of brain tissue which is frozen may by accurately control by precise control of the probe tip temperature. In the steady state, the heat transfer rate through a spherical frozen region of inner radius R1 and outer radius R2 is given by

****************

WhereKs

= Thermal conductivity of solid (frozen) material

Kf

= thermal conductivity of unfrozen material

Ti

= freezing temperature

Tp

= probe tip temperature

Tw

= temperature of unfrozen tissue far away from probe tip.

2.3.7 CRYOGENIC EYE SURGERY: Two applications of cryogenic techniques have been used in eye surgery: cataract operation and correction of detached retinas. In cataract operation, lens removal is necessary; this can be accomplished easily and safely by freezing the lens on a cold probe tip. The frozen lens sticks firmly to the cold metal tip and can be lifted out of the way with the probe. Danger of rupturing the lens during extraction is minimized because it is rigid. Accidents or other circumstances some times cause the retina to become detached from the wall of the eyeball. The detachment produces a "blind spot" in the field of vision of the person. To correct the detachment retina condition, a cryogenically cooled probe tip is applied to the outside of the eyeball in the vicinity of the detachment. The reaction to the intense cold is set up within the eye tissue, and this reaction "welds" the retina back to its correct position. Burst of radiation from lasers have been used in this operation, because heat, will produce the same reaction that cold produces; however the cryogenic technique appears to be safer because there is no permanent damage to the eye tissue in cooling. The probe tip temperature and duration of application in the detached retina operation depend on the thickness of the eye wall and the vacularity of the affected region. For ex., production of frozen region or lesion near the front of the eye requires shorter

times and some what higher temperature than lesion production near the rear of the eye. Generally the temperature used in cryoprexy (cryogenic attachment of retina) range between -5 and -600F. The time during which the probe tip is applied ranges from 2 to 5 sec. The probe is applied until a white lesion appears on the eye, when the probe must be quickly warmed and removed. The white response usually lasts a few seconds and disappears. In cooperation with Dr. Charles D. Kelman, frigitronics, Inc. has developed a cryosurgery probe for eye operation which uses a thermoelectric elements to provide refrigeration, as shown in fig. The probe consists of a copper rod coated with teflon except for tip. One end of the copper rod is connected to the cold side of a thermoelectric element, and the working end of the rod is cooled by conduction for rapid warm up or the probe tip, the current to the thermoelectric element may be reversed or a small resistance heater attached to the probe rod may be turned on. The unit is limited to temperature above about -400F, but this limitation is not serious because lower temperature are not often required for eye surgery. Other probes, such as the cooper cryosurgery probe, may be used if lower temperature are needed.

2.3.8 CRYOSURGERY OF TUMORS: Cryogenic freezing has been used to produce necrosis (tissue death) in several types of tumors. Electrical or chemical destruction & irradiation have been used to kill cancerous tissue, but cryosurgery offers certain advantages - there is no bleeding to complicate the operation & little pain is noticed during freezing .In fact, cryosurgery has

been used to relive pain in terminal cases of cancer by freezing the nerves in the vicinity of the tumor. Two techniques have been used in cryosurgery of tumors: 1) the insertion of probe into the tumor & freezing from within (penetration freezing. 2) applying the probe tip to the surface of the tumor & freezing from the outside inwards (contact freezing). Penetration freezing is used primarily for large tumors, while contact freezing is best suited for small or elongated tumors. Insertion of the cryoprobe has the disadvantage of disrupting the tumor & possibly scattering the cancerous cells. This disadvantage is not thought to be serious, because the freed cells are usually imprisoned in ice & killed before they can spread into the normal regions. Both techniques have been used together for operations on large, irregularly shaped tumors. In tumor operations , temperature

of -1100F or lower are used . the time of

application of the probe is not ,as critical as for eye surgery ,except for very small tumors, the frozen tissue must be warmed up slowly for the treatment to be effective. instead of warming the tissue externally after freezing , the probe is removed & the tissue is allowed to thaw out naturally. To ensure death of cancerous tissue, two or more cycles of freezing and thawing are usually necessary. A general purpose cryosurgery probe developed by cryovac inc. is shown in the fig. the system is well suited for tumor surgery , although it may be used for eye & brain surgery as well .the probe needle is a three walled unit , similar to the cannula of the Cooper cryosurgery probe ; however , no external supply of liq. N2 is required for the cryovac inc. unit . The probe contains a miniature J-T refrigerator within the handle of the unit . High pressure gaseous Argon (2000 psi) is supplied to the probe from a standard

gas cylinder. The gas is cooled within a miniature counter flow Heat Exchanger & is expanded to ambient pressure through a capillary tube. The cold gas pressure to the probe tip absorbs heat there & returns through the Heat Exchanger to the atmosphere. Precise temperature is achieved by automatic or manual control of the gas flow rate. The temperature of the probe tip is sensed by a thermocouple at the probe tip & is indicated by a meter on the control console. a small heater within the probe tip is used when quick heating is required ,as in eye surgery or retraction of the probe from a frozen tumor.

2.3.9 IN TREATMENT OF SKIN DISEASE: Liquid N2 may be used in treatments of work and of scarring caused by acne. It is applied by cotton swab when the lesion is touched freezing occurs almost instantly. The aim being to initiate the formation of blister just sufficient to separate the surrounding tissue.

2.3.11 BLOOD AND TISSUE PRESERVATION: Much interest has been aroused by the possibility of using cryogen's for attaining "suspended animation" or for freezing whole animals although Dr. Smith and her coworkers at the national Institute for Medical Research in London have succeeded in cooling hamsters to 23F at which 50 to 60% of the water in the tissue was frozen the complete freezing and revival of a life form as complex as man or other large animal is yet an accomplishment of the future freezing and revival of simple systems such as whole blood and animal semen, has been accomplished. It has been found that freezing in itself does not cause death of cells, but the effects associated with freezing can be lethal. These

effects include mechanical damage due to ice crystal formation and chemical damage from increased salt concentration at the edge of the frozen front. These effects can be minimized by the use of additives and by control of the cooling rate. In freezing a biological system, several thermal regions are experienced as shown in the cool down curve in fig. The first region involves removal of sensible heat from the liquid may be super cooled some what (cooled below the equilibrium solidification temperature). The second region involves removal of latent heat or freezing of the liquid phase within the system. After solidification is completed, the final region involves removal of sensible heat from the solid phase until steady a state is attained. In preservation of blood or tissue it is important to cool the specimen through the dangerous temperature range quite rapidly. This danger zone or zone of high cell mortality lies between 32 and -700F. From the study of transient heat transfer, it is apparent that a volume of liquid suddenly plunged into liquid nitrogen will not experience the same temperature history throughout the volume ie the temperature will not be the some in all parts of specimen at any instant of time. Till inner portion of specimen will cool at a slower rate than the outer portions nearer the cooling liquid. For this reason blood samples are kept quite small to alleviate the problem of slow cooling (and consequent cell damage) all the central portions of the sample during freezing. Two techniques of cryogenic blood storage have been investigated: (1)

freezing red blood cells in mixture with a protective agent and

(2)

rapid freezing of whole blood to cryogenic temperature. In first case, the plasma and the red blood cells. An additive such as glycerol is

mixed with RBC and the mixture is frozen. Because of the presence of the additive cooling

rates are not as critical as for unprotected blood. Storage temperature commonly used in this technique range around -1100F. When the blood is needed the frozen mixture is thawed the glycerol is removed and the RBC are given mixed with new plasma. This method of blood storage is best suited for fairly large hospitals, because the equipment required is generally large and complex. The second method of blood storage involves immersing the whole blood in a bath of liquid nitrogen to freeze the blood in less than one min at -3200F. A protective additive such as polyvinylpyrrolidons (PVP) is often used to reduce RBC mortality. In contrast to glycerol, PVP need not be removed from the blood before transfusion. When the blood is needed, it is thawed as rapidly as it was frozen. To freeze blood as rapidly as is required for low mortality of RBC, the mass of blood sample mist be small; otherwise, the outer layer of blood exposed to the liquid N 2 wound freeze while the central portion was fluid. Two techniques have been studied to accomplish this rapid freezing (1)

use of thin flat containers

(2)

spraying small droplets of blood directly into liquid N2. A special container for rapid blood freezing suggested in a study by the Linde

company is shown in fig. The container has thin metal walls, and the thickness of the blood storage space within the container is small. The container is filled with whole blood, and the entire unit is plunged into a liquid N 2 bath. After the blood has been frozen, it may be stored indefinitely without damage to RBC. In the second method whole blood is sprayed through an atomizer directly into liquid N 2 bath.

2.4

CRYOGENIC ELECTRONICS

2.4.1 INTRODUCTION Cryogenics and especially the phenomenon of super conductivity has shown great promise of application to electronics. Not only can totally new devices be made which can operate at this low temperature, but many ordinary electronic devices will perform much better when cooled to either liquid nitrogen or liquid helium temperatures. Several elementary super conducting circuits have been developed to produce binary memory elements, switching devices and multi vibrators for high speed computers. The small space requirements, negligible power inputs, increased speed and high reliability of these electronic units combined to promise significant advantages over other types of computer components.

2.4.2 CRYOELECTRONIC DEVICES a)

CRYOTRON Working Principle: The CRYOTRON is an electronic device like the vacuum tube or the transistor. It

operates on the principle that its electrical resistance will be considerably different in the normal state than in the super conducting state. This means that it can perform in much the same way as a switch with a low resistance on position and a high resistance of position. It uses a fact that a magnetic field will destroy superconductivity as the means for switching from one resistance value to other. The original cryoton consisted of a state piece of tantalum wire 0.010" in diameter called the gate, around which was rapped a

single layer of 0.002" niobium, wire called the control thus forming a magnet coil about 1/2" long. Tantalum become Superconducting in zero magnetic field at 4.4K when immersed in liquid helium (4.2K). Hence, only a weak magnetic field is needed to cause the tantalum to change back into its normal conductivity state. A niobium control coil which superconducts at about 8K, the magnetic field produced by a current flowing through the Niobium controlled wire will cause the tantalum gate wire to stop being superconductive, while the superconductivity of the Niobium control is not affected.

b)

THE FLIP-FLOP: One simple circuit is the Bistable Multivibrator circuit, called the Flip-Flop. The flip-

flop is able to store or remember a single piece of information. It can remember a yes or no, one or zero, on or off or any other such item of information that the designer selects. When the circuits were built, it was found that the wire Cryotron had a number of disadvantages. They were hard to make and handle, they had to be welded together an operated much more slowly than transistors. Many of these problems were overcome by thin film technology. Instead of making each Cryotron from wire and then connecting many of them together to form an electronic circuit, the circuit was made by spraying thin layers of metal and insulators through a stencil on to a flat sheet of insulating material, usually glass.

c)

MEMORY DEVICES As compared to flip-flop circuits, a much simpler method of storing information is

the persistent current memory method. Several different devices use this effect, including

the Persistor, Crowe and trapped memory flux. They store or remember information by means of a current which flows around a ring. Scientist realized that if a current can persist in a superconducting ring indefinitely, it can be used to remember information. The direction in which the current flows can be used to distinguish between a one and a zero. The presence or absence of the current may also be used. These persistent current rings may be made just as small as the stencils will permit. Together with the conductors needed to start the current flow, and to measure it later, they can be grouped in larger numbers to provide the memory capabilities needed for large computers.

2.4.3 MASERS: Masers are similar to lasers, but they shine microwaves instead of light when simulated with resonant radiation. Maser action takes place due to the phenomenon of population inversion in the molecules. Such population inversion leads to amplification of photons and thence to extremely bright microwave emissions. Currently masers are used as microwave amplifiers in wireless communication systems, electronic warfare and radar, cellular communication, base receiver filters and as very accurate clocks. When used as ultra sensitive microwave amplifiers, the thermal noise in the vicinity of the maser has to be kept to a minimum for which, it is cryocooled to liquid Helium (LHe) temperatures. Also travelling wave masers requires magnetic fields which must not very over distance of 7 inches, for which superconducting magnets are ideally suited, the magnets operating at LHe temperatures.

Another way in which the unique low temperature properties can be made use of is when superconducting metal plates are placed around the region in which the field is to be confined because the field cannot cross the superconductors.

2.4.4 JOSEPHSON JUNCTIONS POWER SUPERFAST CIRCUITS A Josephson junction is made by sandwiching a thin layer of non- superconducting material between two layers of superconducting material. Pairs of superconducting electrons “ tunneling” right through the non-superconducting barrier from one superconductor to another. Above the critical temperature, the interaction between the two electrons is repulsive. However, at the critical temp, the overall interaction becomes slightly attractive. The attraction helps the electrons into a lower energy state & makes them move without any resistance due to ionic lattice scattering. In a Josephson junction, the barrier is very thin, 30 Angstrom or less until a critical current is reach, a supercurrent can flow, but when the critical current is crossed and AC under undulating voltage develops across the junction, having a frequency of 500 GHz / milli volt. Detection of this change from one state to another is at the heart of main application for Josephson junction. Digital logic circuitry can be built using Josephson junction for use in ultra fast super conductor. The switching times of these circuits would be of the order of a few picoseconds. Hence, such super computers would be much faster, smaller and would create far less heat than present computers, thus lowering the cooling loads.

2.5 SUPERCONDUCTIVITY 2.5.1 INTRODUCTION Superconductivity is the ability of some materials, at very low temperature, characterised by a)

Perfect electrical conductivity (R = 0)

b)

Zero internal magnetic induction (B=0) in the presence of an external field.

The state of zero electrical resistance ranges between 0K and the critical temperature. Interestingly, a closed circuit consisting of superconducting elements can sustain a persistent, resistance less current without an external source of voltage. The

advent of superconductors have revolutionized many engineering and pure science applications, their zero energy loss ability having the potential of saving billions of dollars lost due to energy dissipation. Some

important

application

of

superconductors

which

look

set

to

be

commercialized in the near future are listed below i)

Superconducting Electric Transmission Cables

ii)

Superconducting Magnets for physics experiments

iii)

Superconducting Levitation application for transport and bearings

iv)

SMES (Superconducting Magnetic Energy Storage)

v)

MRI (Magnetic Resonance Imaging)

vi)

Magnetic Ore separation

vii)

Electronic Circuits and Devices

2.5.1 SUPERCONDUCTIVITY IN ELECTRICAL POWER APPLICATIONS Superconductivity brings to mind a phase of developing applications, predominantly in power generation, transmission, & distribution. The most recent demonstrations of electric power systems based on use of both HTSCs & LTSCs have been extremely impressive & have resulted in heightened expectations for the commercialization of this technology within the next decade. The fuel chemical to thermal step in the whole power production process is the least efficient and operates with a thermodynamic limit of 30 to 35%, the scope for improvement being small. Improvements are however possible in the supply side electric

generation, transmission & demand side utilization with the use of Superconductor integrated equipment.

SUPERCONDUCTING GENERATORS In generators, it would be possible to replace the iron cores with superconducting wire, which would make them lighter & lead to greater fuel efficiency.

Widespread

integration of SC generators over the next 10 to 20 years, assuming an efficiency increase of 0.8%, would lead to energy savings of about 0.1 Quads with additional savings in fuel costs & reduced emissions.

SUPER CONDUCTING MOTORS Motors made of superconducting wire would be smaller & more efficient. in industrialized countries like the US, electric motors account for 64% of all demand side consumption , with the large motors accounting for almost 50% of the above. Assuming substantial induction of large motors having efficiency improvements of 2% (including the refrigeration) over the next decade, the total reduced costs to retail customers would amount to a saving of about 0.1Quads.If in the 5000 Hp motor segment,

2.5.3 POWER TRANSMISSION CABLES

High temperature superconductors are finally ready for commercial use and applications are to be found in the electrical power supply area, for equipment such as high-tension cables current limiters and railway transformers. A HTSC compound consisting of bismath lead strontium calcium copper oxide (BPSCCO) has proven to be the most promising starting material so far. However, the process of manufacturing HTSC cables from this powdery ceramic is by no means straightforward. First of all, the ceramic material needs to be converted into a flexible strip. The strip conductor forms the core of the cable, which is manufactured in several stages; the ceramic powder is injected into silver tubes several of which are then grouped into a bundle and inserted into an additional silver tube. After that, the bundle is pressed, drawn and rolled to form a thin layer. This is followed by three stages of annealing at 820 0C leading to the synthesis of a super conductor. In this manner, the bundle is formed into a strip that increases steadily in length and whose flexibility can be attributed to the thin ceramic filaments. If these filaments were any thicker, they break when bent or pulled. Initial large scale production of the HTSC strips have taken place at Siemens Vacuum Schmelze plant in Hanau, Germany. Development activities at Corporate Technology are now geared toward achieving high current densities in conductors of the greatest possible length. At present a maximum valve of roughly 40 kilo-amp per square centimeter (KA/cm2) can be obtained. To operate competitively, however values between 50 to 100 KA/cm2 need to achieved over lengths of several hundred meters. The first stage of development of a superconducting cables consisted of designing. Constructing and testing a 10m long functional model of a conductor in Erlangen. The cable consisted of four layers of bismuth strip conductor applied to the carrier to be using a

special technique for the winding the developers came up with a special recently patented concept. A further arrangement of out bound and in bound conductors electromagnetic fields are not produced in the vicinity of the cable path. With a total cross section of 130square millimeters (35mm2 HTSC, 95mm2 silver), the cable conductor carries a current or 5000 amp (A). That is more than twice the rated current of a conventional 400-MVA cable. At a rated current of 200amp the losses in a high temperature superconducting cable amount to only 8watts per meter, even after taking into account the energy for liquid nitrogen cooling. In contrast a copper conductor with ten times the cross section (1200mm2) and carrying the same current would generate a loss of approximately 50 watts/meter. The compact nature of the superconductor is just as important for the intended applications as its low power losses. A high temperature superconducting cable of the same cross section as conventional cable can transmit three to five times the power ideal for regions where underground installation would prove too expensive or in which the terrain is too difficult. In metropolitan areas high temperature superconductors could replace the copper cables in existing cable conditions. Thus multiplying power transmission capacity further more the high current carrying capacity of HTSC allows very efficient power transmission at low voltages.

2.5.4 ORE SEPARATION USING HTSC MAGNETS A separator for processing Kaolin will incorporate the largest high temperature superconductivity magnet in terms of combined size, magnetic field and stored energy that has ever been built. The 0.8m diameter magnet will deliver a 2.5 Tesla magnetic field and

store 800KJ of energy. Kaolin and titanium dioxide are the white pigments used in paint paper and plastics manufacturing. However as the ores contain magnetic contaminants such as Hematite (Fe2O3) and Magnetite (Fe3O4), which discolor the pigments, turning bright white into off-white. Kaolin mineral deposits also contain Ilmenite, a discoloring compound of Titanium dioxide and Iron. Most ore processing is done on site using either traditional copper or low temp superconductivity magnetic separators. Low temperature superconductivity magnetic separators shrink power consumption, but require liquid Helium, some thing that remote suppliers can’t readily deliver and with which local operators have had little experience. High temperature superconductivity or HTS can mirror the low power consumption of its low temperature counterpart but does it at a higher operating temp 20K versus 4K the boiling point of liquid Helium. Since Carnot efficiency dictates that the higher the temperature the easier it is to remove the heat, a higher operating temperature translates to a simpler lower cost refrigeration system. It is so simple that the equipment doesn’t ever need liquid system.

PROCESSING OF ORE The system processes one in batch made by pumping unprocessed clay slurry through a non magnetic 3000 series stainless steel pipe; the pipe contains an in line canister filled with a 400 serial "soft magnetic" stainless steel wool filter. Encircling the canister is an 0.8m high temperature. Superconductivity solenoidal magnet wound from silver sheathed Bismuth Strontium Calcium Copper Oxygen (Bi-2223) power-in- tube multifilament tape.

As the unprocessed slurry passes through the canister the magnetic field is cycled on. When the magnet is energized the magnetic contaminants are attracted to and trapped by the steel wool filter, while the nonmagnetic particles pass through the filter and are collected as processed product. When the magnet is cycled of, the magnetic force drops, releasing the contaminants, which are then flashed out with water. The cycle repeats with a batch every 10 to 20 minutes depending on the processing the volume and the magnetic fraction of the clay. Kaolin typically has magnetic fraction of 2 to 5%, while the titanium dioxide can measure as high as 50%. A typically filter will list for approximately 8000 hrs at continuous operation. Functionally the HTS system operates the same way also conventional copper or low temperature superconductivity system, except that it was a high temperature magnet to generate the magnetic field. The separators high-temperature superconductive coil consists of a stacked double pan cake structure in which the pan cakes are spliced together to for, a continuos conductor. The assembly mounts inside the toroidal vacuum cryostat held inside an Iron yoke. The size was not chosen arbitrarily; it matches the processing volume of the smallest commercial magnetic separator. Because processing volume is proportional to the radius squared, a smaller magnet would reduce capacity significantly.

LOOKING AHEAD

A successful HTS separator may have a potential future beyond Kaolin and titanium oxide processing. There are other mineral applications worth investigating. In addition some research work has been done in environmental remediation areas. It has been experimentally verified in the lab that it is possible to remove Uranium oxide compounds from contaminated soils and biological contaminants from waste water. 2.5.1 THE MEISSNER EFFECT AND FLUX PINING In addition to zero resistance, the other important property of super conductor is the Meissner effect, by which magnetic fields are driven out of super conductor. This property can be used to levitate a magnet. Even though Meissner discovered the expulsion of magnetic fields from super conductors in 1933, it was not until 1987 with the advent of type II superconductors or HTSCs that levitation effects could be studied in open air using LN2 induced superconductivity at 77K. When a magnet is placed near a superconductor and its magnetic field is excluded from entering the superconductor, the distortion of field lines results in a force which can lift the magnet against gravity, provided the magnetic forces are strong enough. At equilibrium, the magnet "floats" a short distance from the superconductor. The HTSC generally used in YBCO. The effect generally dies away once the HTSC block warms to above the critical temperature. Flux pinning magnetic place in TBCCO HTSCs. When magnetic flux lines become pinned in the HTSC at 77K, the base can be rotated which leads to the magnets rotating as well because the flux lines have to remain stationary.

The Meissner effect provides a force to lift a magnet above a superconductor, but whenever a magnet is in motion, there is also a drag force. The combination of these two affect the performance and design of any practical magnetic bearing surface.

2.5.2 MAGNETIC LEVITATION VEHICLES The most well advertised possible application of superconductivity is that of maglev trains, proposed to run at 500 km/hr. Present prototype technology can achieve speeds up to 550 km/hr (Japanese MLX01, Dec-97). Two different kinds of maglev are competing in today's prototype phase of development, the difference being the way by which the vehicle attaches to the guide way. i)

Electromagnetic (or attractive) suspension:- it depends on the force of attraction between the electromagnets and a steel guide. Because the force of attraction decreases with increasing distance, this system is inherently unstable. The prototype German Transrapid vehicle is proposed to carry 200 people at 500 km/hr with levitation height of 8mm, the power consumption being 43 MW.

ii)

Electro-dynamic (or repulsive) suspension:- It depends on the repulsive forces that come in to play between moving magnets and eddy currents that they induce in a conducting guide way. The repulsive force is inherently stable with distance and comparatively large levitation heights can be achieved (8-10 cms). The null flux arrangement on Japanese design tends to reduce the magnetic drag force and thus the propulsion power needed. Problems with HTSCs range from questionable mechanical strength to high AC

losses. HTSCs are well known for their brittleness & low fatigue strength. Repeated

stressing of the superconductor due to magnetic pulses leads to crack propogation and early failure. Also, AC variations in the magnetic fields of the permanent magnets leads to dissipation of energy. These losses are quite significant in HTSCs like YBCO. All in all, if maglev vehicles are to contribute significantly to future transport systems, the drawbacks listed above need to be overcome & a way has to be found to reduce the initial capital investment required for the system setup.

2.5.3 ZERO FRICTION SUPER CONDUCTING BEARINGS A frictionless bearings is every mechanical designers dream, and a bearing in which the two surfaces never make contact is close to that ideal. magnetic bearings achieve mechanical separation & when other dissipative effects are minimized ,they can be extremely good bearings, allowing rotational speeds not achieved by any other way. Though conventional magnetic bearings have been widely used & have many advantages, as compared to hydrodynamic or roller bearings, like low power dissipation & higher speeds, they require mechanical support in at least one dimension or an active feed back in the electromagnetic circuit. The use of super conducting components in magnetic bearings offers the potentials to eliminate the control system and further reduce power dissipation of the system. The obvious disadvantage is that superconductors need a cryogenic refrigerant. Conventional coil-magnet bearing system, used successfully for many years in the ultrahigh speed rotor suspension now look set to be replaced by Passive Meissner effect

bearings using type II superconductors (HTSCs) which can be stable without a feed back system and yet achieve speeds in excess of 100,000 rpm. Magnetic stiffness is an important parameter for bearing design and is defined as the restoring force due to flux pinning that acts on a rotor which drifts off-center. Efforts to increase the stiffness require clever design and new materials. Superconducting magnets generally require to be controlled in five direction, up, down, side to side, pitch and yaw. Magnetic pressures of about 105 N/m2 have been achieved for superconducting magnets which is comparable to that for conventional magnetic bearings. a)

Melt textured YBCO bearings For bearings, the key factors are load capacity, stiffness and damping. The Allied Signal co. has built a prototype device using melt texture YBCO which satisfies the above factors even at speeds nearing to 500,000 rpm. Load capacities of 5-30 psi have been achieved.

b)

Hybrid superconductor magnet bearings (HSMBs) The concept of hybrid bearings utilizing both superconductors and permanent magnets uses the magnet to magnet repulsion to support the load and a superconductor to stabilize the system. The magnetic system between two permanent magnets is quite unstable and the superconductor, with its good diamagnetic properties acts as a stabilizing agent. HSMBs can be built in various combinations like thrust, radial or thrust-radial. The trade off in HSMB design is between the cost of refrigeration and the benefit of lower frictional losses in the bearings. Generally, HSMBs require only 20 mW of dissipitive cooling as compared to 20 W needed for bearings presently used

by NASSA for space application. Because refrigeration is free of charge in outer space, HSMBs are sure to find a place in future space programs. c)

Superconducting bearings for space telescope applications The HSMBs system for a lunar telescope is based on passive magnetic levitation and the flux pinning effect of HTSCs. The unique ability of HTSCs to adopt to low temperature and vacuum environments of space or on the moon and their ability to enhance system stability passively without power consumption make them ideally suited for space application. Diamagnetism is the property that makes superconductors shield their interiors from an applied magnetic field. However, in HTSCs, any magnetic field already present would remain trapped inside the superconductor. For HTSCs, the superconducting state exists below 90K. importantly, the temperature of the lunar surface away from the sun can reach as low as 30K-60K. The use of HSMBs circumvents problems related to use of conventional lubricants in cryogenic and vacuum environments such as freeze and evaporation. It also allows use of less tight tolerance in its manufacturing and assembly as compared to conventional mechanical bearings. Clogging problems due to lunar dust are also eliminated in HSMBs. Thus a superconducting bearings is an attractive replacement for conventional bearing in the cold and dusty lunar environment for an extended period of time. For the equatorial mount of the telescope, a superconducting bearing and driving mechanism capable of tracking stars to a precision of better than 0.25 arc seconds rotates the platforms at a constant velocity 23 m RPM.

d)

Fly wheel energy storage using HTSCs A promising future application of HTSC bearings is in energy storage via fly wheels. The entire apparatus is housed in a vacuum chamber to eliminate losses due to air drag. Only the HTSC bearing need be cooled to LN2 temperature: that bearing acts on a permanent magnet attached to the fly wheel, both to suspend it and keep it stable. Energy is transferred into and out of the flywheel as followsCurrent flows through a coil adjacent to the fly wheel magnet, repelling it and causing the fly wheel to spin, proper synchronization of current and magnet velocity being maintained to accelerate the fly wheel. The current is switched off once the flywheel reaches top speed, losing almost no energy to friction or air drag because of the vacuum. Later, when the circuit is reconnected the magnet spinning on the flywheel generates electricity in the coil which can be used later to run an external load. For electric utilities wishing to store power for 12 hours, flywheels becomes competitive only when the energy loss in the bearing is very low so that the fly wheel would loose energy of the order of 1% in 10 hrs, or have a slow down rate of 10-5 to 10-4 Hz/sec. Lab models have achieved slow down rates of below 10-4 Hz/sec.

2.5.4 SUPERCONDUCTING MAGNETIC ENERGY STORAGE (SMES) SMES stores electricity for long periods of time in superconducting coils. This systems will be used by electric power companies of the future to reduce power

interruptions, which currently account for losses of over 15 billion dollars/year in the US. With the advent HTSC SMES, the whole concept of SMES changed from being a design dream to a costly but attainable working prototypes. SMES systems, designed to provide power for relatively short periods of time, are characterised by high efficiency and rapid response. A SMS is characterised by both its power rating and storage capacity. In a study conducted by the US Department Of Energy, a storage range of 2-200 MWh at power levels of 4-4MW were considered suitable for commercialization. The unit geometry could be of either toroidal of solenoidal type and either of two kinds of conductors (BSCCO or YBCO) can be used. While making an investment decision about building an SMES, future managers would have to strike a trade of between refrigeration cost (and reliability), structural strength, ground trip efficiency and so on. As of now, the toroidal configuration has look most promising due to its technical superiority and the fact that it has a low external magnetic field which is advantageous in locating the unit near a utility or customer load.

SMES Refrigeration system For an SMES system, though the coil resistance in the superconducting state is negligible, the refrigerator requires electric power which must be accounted for during efficiency evaluation. The heat loads that must be removed by the cooling system include conduction through the support system, radiation from warmer to colder surfaces, AC losses in the conductor and connections from the cold to warm power leads.

All in all, the SMES system has shown great promise in the design stage and it is hope that proper commercialization of this vital technology would lead to significant savings in the potentially important electric utility supply system.

2.6

CRYOMANUFACTURING

2.6.1 INTRODUCTION Cryogenic technology finds ever growing applications in the manufacturing industry, ranging from steel making, grinding to fabrication of complex structures used in space technology. Cryogenic technology improves the product quality after machining, minimizes or completely eliminates faults in assembly during manufacturing. The ultimate stress, Young's module and the yield stress of most engineering materials increase with cooling to Cryogenic temperature. It has been established that the chip tool interface temperature and cutting forces reduce while the chip ratio increases when cutting is done using a single point tool at Cryogenic temperature. Materials like plastic and rubber brittle at low temperature and thus become easy to grind and crush using Cryogenic technology. Some of the widely used Cryogenic manufacturing processes are discussed below.

2.6.2 LN2 CRYOGEN AS A CUTTING FLUID FOR PRECISION GRINDING Grinding of steel requires high specific energy at high speed. This results in very high temperature which reduce the dimensional accuracy and wheel life due to plastic deformation and rapid wear of cutting points respectively. This induces large tensile residual stresses, microcracks and structural changes in the ground components.

Cryogenic cooling by agents like LN2 reduces the above problem to a great extent. Cryogenic manufacturing has resulted in relatively lower cutting forces, longer tool life and better cutting surfaces. The setup for cryogrinding is shown in the figure. The LN 2 comes in from its reservoir under air pressure and impinges on to the grinding zone from suitable distance and angle. The flow of LN2 is controlled using a number of valves. The cooling to cryo temperatures leads to lower grinding interface temperatures which prevents plastic flow of material & hence smoother surfaces that are free of surface damage. as can be seen from the microphotos, cryogrinding leads to a definite improvement over conventional grinding.

2.6.3 CRYOGRINDING OF RUBBER Scrap or waste rubber can be ground and recycled so that the requirement of fresh rubber is minimized. use of this recycled rubber reduces the mould wear & lessens the total energy requirements. As compared to conventional grinding, grinding at cryo temperatures has definite advantages because rubber becomes extremely brittle at the low temperatures.

Design and construction of the grinding machine The fundamental principle of the process is that the scrap rubber is cooled to below its embrittlement temperature & then pulverized in a hammer mill. The schematic of the process is as shown in the figure. Pre-sized rubber is fed into the hopper & conveyed to

the hammer mill by means of a screw conveyor. LN2 is sprayed directly onto the rubber when it passes from the hopper to the hammer mill. The 3 main components of the machine are the screw conveyor, the hammer mill & the LN2 injection system. The carbon black filled rubber takes about 10 minutes to attain about -100 degree C when submerged in the LN2 spray. Large scale systems marketed by an American company has a production rate of about 1 to 3 tons/hr. The systems are based on a capital investment of about $40,000 & an LN2 consumption rate of 2.64kg/kg of product for a reduction rate of 150kg/hr.

2.6.4 CRYOGENIC SHRINK FITTING For the assembly of components such as shafts in the wheels and rotors, of liners in cylinders & of bushes & bearings in housings, it is necessary to bring dimensional changes in both or one of the mating parts, either by heating the female (hub) or by cooling the male part (shaft).the latter method is called the "expansion fit method" & has inherent advantages over the conventional "contraction fit"

technique. To achieve a

reasonable reduction in the diameter, the shaft must be cooled to a temperature of about 730 K, which is achievable using LN2. The advantages of the cryo fitting technique are as listed below1.

The technique is quick & requires minimal special equipment.

2.

Mechanical damage & metallurgical changes in the materials are absent.

3.

Holding ability of cryogenically shrink fitted joint is considerably greater than that for other methods.

Cryogenic shrink fitting is being widely used for making interference fits. When the two members are assembled & the temperature returns to the normal, a radial pressure sets up at the interface causing the two parts to be held firmly. Main applications of cryogenic Shrink fitting are in assembly of rims over locomotive engine wheels, assembly of kiln support rollers.

2.6.5 FABRICATION OF PRESSURE VESSELS Here, the die for manufacture of pressure vessels is cooled to LN2 temperatures. The vessel to be formed is kept in the die. The pressurized Nitrogen gas is admitted in to the vessel until the container stretches by about 15% and then the vessel is removed from the die and allowed to warm upto room temperature.

2.6.6 ADVANTAGES OF CRYOMANUFACTURING The following advantages of cryomanufacturing over conventional methods are most evident: 1.

Cryogrinding provides better surface integrity & finish when compared to conventional grinding by reducing the surface tensile residual stresses to about 20% of their original values, at the same time improving tool life to a great extent.

2.

The brittle nature of rubber at cryo temperatures facilitates easy crushing & grinding with reduced energy requirements.

3.

Cryo shrink fitting provides greater holding forces, while at the same time preventing any mechanical or metallurgical damage to the specimen.

4.

Fabrication of large pressure vessels having complex shapes is facilitated by use of nitrogen induced stretching in the moulds.

2.7

CRYOPRESERVATION

2.7.1 INTRODUCTION The preservation of perishable commodities particularly food stuffs, fruits and flowers, etc is more important now and then ever before in history. Today’s large urban population requires tremendous quantities of food, naturally these foodstuffs must be kept in preserved condition until they are finally consumed. Also, many products particularly fruits, flowers and vegetables are produced during certain seasons of the year, they must be stored and preserved if they are to be made available through the year.

Conventionally, drying, prickling and salting methods were used and still are used where no other means are available. However these methods are inadequate for certain products. The disadvantages associated with conventional one are: 1) The characteristics of fresh food and fruits such as appearance, vitamin content, taste are changed. 2) These methods lead to quality deterioration e.g. discoloration, loss of vitamins, disintegration of product because of physiochemical changes. 3) Quality of perfumes gets deteriorated in case of flowers.

Refrigeration is one of the important method for preservation of food. Food preservation at low temperature retains the quality of food. Cryopreservation is recent development in refrigeration technology. During the last decade considerable progress has been made in agriculture biotechnology, and complete plants have been regenerated from cell, and organ cultures of a number of crop species cryopreserved in liquid Nitrogen for various lengths of time.

2.7.2

LIQUID NITROGEN SYSTEM FOR FOOD TRANSPORTATION. For food preservation during transportation through trucks, the liquid Nitrogen

system is shown in fig.(2.7.1). The liquid Nitrogen containers are located behind the driver’s seat in the cabin. The temperature of the space inside the truck was registered by the vapor pressure thermometer with its sensing element located inside the space and dials located in the driver’s cabin. The liquid delivery system comprised of adapters fitted

over the containers. These adapters serve as convenient means for providing an air -tight lid over the containers and liquid Nitrogen out of them. Compressed air available in the truck for operating its air brake system is to be used for providing the higher pressure required for pumping the liquid Nitrogen from the containers. The pressure of air acting over the surface of the liquid Nitrogen in the container is to be regulated by operating a solenoid valve and relief valve. An operator travels in the driver’s cabin for operating the solenoid valve and admitting compressed air as and when desired. When the truck temperature rises by 2 C, the operator closes the air relief valve and opens the solenoid valve causing the compressed air to flow into the container and pump liquid Nitrogen into the truck. Due to the spray of the liquid Nitrogen, the truck temperature decreases and whenever it reaches –17 C, he closes the solenoid valve and opens the pressure relief valve causing the flow of liquid Nitrogen to stop. It has been found that the spray is to be maintained for the duration of 10 minutes at interval of 30 minutes. This system has a number of advantages: 1)

Liquid Nitrogen system is quite economical.

2)

The high quality of frozen product can be maintained using this system.

3)

With this system it is possible to extend the area of marketing to over the long distance.

4)

Rate of spoilage is as low as 10% compared to 40% with refrigerated system.

2.7.3

CRYOGEN FREEZING & STORAGE OF FISH &SHELLFISH For the last two or three decades, the cryogenic freezing is replacing the traditional

freezing system in western countries. Cryogenic freezing using cryogen (such as liquid Nitrogen) bears the advantages of quick freezing. The process of freezing may be divided into three phases: 1) Pre-cooling: -The product is brought down to its freezing point. 2) Freezing: -Maintains a constant temperature and phase change occurs due to formation of ice crystals. 3) Tempering - Temperature of center falls gradually towards the temperature of freezing medium. The initial investment is low for liquid Nitrogen freezing.

2.7.4

CRYOPRESERVATION OF PLANTLETS & LIVING CELLS It is possible to store plantlets and microbes in cryogen to maintain genetic stability

and viability. Liquid Nitrogen provides a storage temperature of –196 C and storage equipments and containers are commercially available. Cells undergo severe temperature changes during freezing. For successful cryopreservation requires certain procedure:1) Low cooling rate down to –30 C. 2) Fast cooling rate below –30 C. 3) Fast warming rate during thawing. 4) Minimum concentration of electrolytes in the freezing suspension. 5) Addition of cryoprotectants to the freezing suspension to protect living

cells against freeze –thaw injury. Cryopreservation is used in genetic engineering for preserving cell culture of different plants to be cloned. Cryopreserving large plantlets though difficult, attempts with young seedlings of pea and carrot have shown encouraging results. Cryopreservation offers following advantages:1) They are easy to culture and regenerate into plants. 2) They yield pathogen-free stocks for propagation. 3) Being genetically more stable they ensure stability of clones. 4) Ideal material for the international exchange of germplasm.

2.7.5 CONCLUSION Liquid Nitrogen is most preferred cryogenic liquid for food storage because it is safe, nontoxic, chemically inactive and commercially available. Cryopreservation retains the quality and natural freshness of food, fruits and flowers. During cryopreservation of plantlets and living cells care must be taken to avoid accidents. Proper ventilation must be assured. There is a possibility of splashes and explosion of glass containers on thawing. Otherwise liquid Nitrogen is best cryogen for cryopreservation.

SQUIDS (Superconducting Quantum Interface Devices) A squid consists of a loop with two JJ interrupting the loop .it is extremely sensitive to the total amount of magnetic field that penetrates the area of the loop - the voltage measured across the device is a strong indicator of the field around the loop. SQUIDs are made use of in instruments designed for detecting very low magnetic fields. Currently, fields less than 10billonth of that of the earth can be detected using such instrument. SQUIDs can be used for high resolution magnetometers, motion detectors, sensitive voltmeters, picoammeters and other detectors all used for wide variety of applications like neuromagnetology, gravity measurement, high speed scanning and non destructive evaluation (NDE) of stressed machine components.