in physics, cryogenics is the study of the production of very low temperature (below −150°C, −238°F or123K) and the behavior of materials at those temperatures. A person who studies elements underextremely cold temperature is called a cryogenicist. Rather than the relative temperature scales ofCelsius and Fahrenheit, cryogenicists use the absolute temperature scales. These are Kelvin (SI units) orRankine scale (Imperial & US units).Definitions and distinctionsCryogenics The branches of physics and engineering that involve the study of very low temperatures, how to produce them, and how materials behave at those temperatures.Cryobiology The branch of biology involving the study of the effects of low temperatures on organisms (most often for the purpose of achieving cryopreservation).Cryosurgery The branch of surgery applying very low temperatures (down to -196 °C) to destroy malignant tissue, e.g. cancer cells.Cryonics The emerging medical technology of cryopreserving humans and animals with the intention of future revival. Researchers in the field seek to apply the results of many sciences, including cryobiology, cryogenics, rheology, emergency medicine, etc.Cryoelectronics The field of research regarding superconductivity at low temperatures.Cryotronics The practical application of cryoelectronics. EtymologyThe word cryogenics stems from Greek and means "the production of freezing cold"; howeverthe term is used today as a synonym for the low-temperature state. It is not well-defined at whatpoint on the temperature scale refrigeration ends and cryogenics begins, but most scientistsassume it starts at or below -150°C or 123°K K (about -240°F). The National Institute ofStandards and Technology at Boulder, Colorado has chosen to consider the field of cryogenics asthat involving temperatures below −180°C (-292°F or 93.15°K). This is a logical dividing line,since the normal boiling points of the so-called permanent gases (such as helium, hydrogen,
neon, nitrogen, oxygen, and normal air) lie below −180 °C while the Freon refrigerants,hydrogen sulfide, and other common refrigerants have boiling points above −180°C. Industrial applicationCryogenic valveFurther information: Timeline of low-temperature technologyLiquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenicapplications. Liquid nitrogen is the most commonly used element in cryogenics and is legallypurchasable around the world. Liquid helium is also commonly used and allows for the lowestattainable temperatures to be reached.These liquids are held in either special containers known as Dewar flasks, which are generallyabout six feet tall (1.8 m) and three feet (91.5 cm) in diameter, or giant tanks in largercommercial operations. Dewar flasks are named after their inventor, James Dewar, the man whofirst liquefied hydrogen. Museums typically display smaller vacuum flasks fitted in a protectivecasing.Cryogenic transfer pumps are the pumps used on LNG piers to transfer liquefied natural gasfrom LNG carriers to LNG storage tanks, as are cryogenic valves. Cryogenic processingThe field of cryogenics advanced during World War II when scientists found that metals frozento low temperatures showed more resistance to wear. Based on this theory of cryogenichardening, the commercial cryogenic processing industry was founded in 1966 by Ed Busch.With a background in the heat treating industry, Busch founded a company in Detroit calledCryoTech in 1966. Though CryoTech later merged with 300 Below to create the largest andoldest commercial cryogenics company in the world, they originally experimented with thepossibility of increasing the life of metal tools to anywhere between 200%-400% of the originallife expectancy using cryogenic tempering instead of heat treating. This evolved in the late 1990sinto the treatment of other parts (that did more than just increase the life of a product) such asamplifier valves (improved sound quality), baseball bats (greater sweet spot), golf clubs (greatersweet spot), racing engines (greater performance under stress), firearms (less warping after
continuous shooting), knives, razor blades, brake rotors and even pantyhose. The theory wasbased on how heat-treating metal works (the temperatures are lowered to room temperature froma high degree causing certain strength increases in the molecular structure to occur) andsupposed that continuing the descent would allow for further strength increases. Using liquidnitrogen, CryoTech formulated the first early version of the cryogenic processor. Unfortunatelyfor the newly born industry, the results were unstable, as components sometimes experiencedthermal shock when they were cooled too quickly. Some components in early tests evenshattered because of the ultra-low temperatures. In the late twentieth century, the field improvedsignificantly with the rise of applied research, which coupled microprocessor based industrialcontrols to the cryogenic processor in order to create more stable results.Cryogens, like liquid nitrogen, are further used for specialty chilling and freezing applications.Some chemical reactions, like those used to produce the active ingredients for the popular statindrugs, must occur at low temperatures of approximately −100°C (about -148°F). Specialcryogenic chemical reactors are used to remove reaction heat and provide a low temperatureenvironment. The freezing of foods and biotechnology products, like vaccines, requires nitrogenin blast freezing or immersion freezing systems. Certain soft or elastic materials become hardand brittle at very low temperatures, which makes cryogenic milling (cryomilling) an option forsome materials that cannot easily be milled at higher temperatures.Cryogenic processing is not a substitute for heat treatment, but rather an extension of the heating- quenching - tempering cycle. Normally, when an item is quenched, the final temperature isambient. The only reason for this is that most heat treaters do not have cooling equipment. Thereis nothing metallurgically significant about ambient temperature. The cryogenic processcontinues this action from ambient temperature down to −320 °F (140 °R; 78 K; −196 °C). Inmost instances the cryogenic cycle is followed by a heat tempering procedure. As all alloys donot have the same chemical constituents, the tempering procedure varies according to thematerials chemical composition, thermal history and/or a tools particular service application.The entire process takes 3–4 days. FuelsAnother use of cryogenics is cryogenic fuels. Cryogenic fuels, mainly liquid hydrogen, havebeen used as rocket fuels. Liquid oxygen is used as an oxidizer of hydrogen, but oxygen is not,strictly speaking, a fuel. For example, NASAs workhorse space shuttle uses cryogenic hydrogenfuel as its primary means of getting into orbit, as did all of the rockets built for the Soviet spaceprogram by Sergei Korolev.Russian aircraft manufacturer Tupolev developed a version of its popular design Tu-154 with acryogenic fuel system, known as the Tu-155. The plane uses a fuel referred to as liquefiednatural gas or LNG, and made its first flight in 1989. ApplicationsSome applications of cryogenics:
Magnetic resonance imaging (MRI) MRI is a method of imaging objects that uses a strong magnetic field to detect the relaxation of protons that have been perturbed by a radio-frequency pulse. This magnetic field is generated by electromagnets, and high field strengths can be achieved by using superconducting magnets. Traditionally, liquid helium is used to cool the coils because it has a boiling point of around 4 K at ambient pressure, and cheap metallic superconductors can be used for the coil wiring. So-called high-temperature superconducting compounds can be made to superconduct with the use of liquid nitrogen which boils at around 77 K. Electric power transmission in big cities It is difficult to transmit power by overhead cables in big cities, so underground cables are used. But underground cables get heated and the resistance of the wire increases leading to waste of power. Superconductors are frequently used to increase power throughput, requiring cryogenic liquids such as nitrogen or helium to cool special alloy-containing cables to increase power transmission..Cryogenic gases delivery truck at a supermarket, Ypsilanti, Michigan Frozen food Cryogenic gases are used in transportation of large masses of frozen food. When very large quantities of food must be transported to regions like war fields, earthquake hit regions, etc., they must be stored for a long time, so cryogenic food freezing is used. Cryogenic food freezing is also helpful for large scale food processing industries. Forward looking infrared (FLIR) Many infra-red cameras require their detectors to be cryogenically cooled. Blood banking Certain rare blood groups are stored at low temperatures, such as -165 degrees C. ProductionCryogenic cooling of devices and material is usually achieved via the use of liquid nitrogen,liquid helium, or a cryocompressor (which uses high pressure helium lines). Newer devices suchas pulse cryocoolers and Stirling cryocoolers have been devised. The most recent development incryogenics is the use of magnets as regenerators as well as refrigerators. These devices work onthe principle known as the magnetocaloric effect. Detectors
Cryogenic temperatures, usually well below 77 K (−196 °C) are required to operate cryogenicdetectors.INTRODUCTION :INTRODUCTION Cryogenics may be defined as the branch of physics which deals with the production ofvery low temperature and their effect on matter. It may also be defined as the science and technologyof temperatures below 120K. The word “cryo” is derived from a Greek word “kruos” which means cold.Methods to Produce Low Temperatures :Methods to Produce Low Temperatures Magnetism produces low temperatures. When a material ismagnetized it becomes warm and cold when demagnetized in controlled atmosphere thus producinglow temperatures. By compressing the gas, the gas is cooled releasing heat and later allowed to expandproducing ultra low temperatures.Cryogenics in fuels :Cryogenics in fuels Fluids are stored at 93.5k(-180degree) or below Due to air friction ,it gets ignitedCools engine on expansion. On expansion pressure increases providing high thrust Satellite payloadincreasesCryogenic Rocket Engines :Cryogenic Rocket Engines Cryogenic rocket engines are one of the important applications in the field ofcryogenics. The higher thrust levels required for a rocket engines are achieved when liquid oxygen andliquid hydrocarbons are used as fuel. But at atmospheric conditions, LOX and low molecularhydrocarbons are in gaseous state. Therefore these are stored in liquid form by cooling them downusing cryogenics. Hence the name Cryogenic Rocket Engines.VULCAIN 2 ROCKET ENGINE :VULCAIN 2 ROCKET ENGINE THRUST -1359KN INLET CONDITIONS: LH2; Pressure=182.1barTemperature=36k LO2: Pressure=153.9 Temperature=96.7k Combustion chamber pressure=117.3barThrust chamber mass=909kgCryogenic Heat Treatment :Cryogenic Heat Treatment This is a process of treating metals, plastics, ceramics at temperatures below120K to their crystal structures and properties. This increases their wear resistance, and life of metalsand plastics. They are used in the field of super conductors, cryo microbiology, and space programs.
Unlike other processes here permanent coating is completely impart through the metal surface. Thesymbol used to represent cryogenic heat treatmentIn alloy steels:- :In alloy steels:- In this process the alloy steels are treated to convert the entire austenite into amartensite matrix such it changes the molecular structure of the steel and forms an entirely new, morerefined grain structure which partly relieves the thermal stresses. The number of countable carbidesincrease from 30,000 to 80,000 per square millimeter which forms a “super hard” surface on the metal.After deep cryogenic heat treatmentSlide 9:Before CI Processing After CI Processing Comparative microphotographs (1000x) of steel samples showthe change in microstructure produced by the controlled deep cryogenic process. Uniform, morecompletely transformed microstructure and less retained austenite at right, is related to improvementsin strength, stability and resistance to wear. *Cryogenics Internationals Cryogenics International (CI)was granted a U.S. patent for its revolutionary new computerized deep cryogenic treatment systems.Cryogenics International now makes dramatic cost savings and increased productivity available to manypeople and industries around the world.Advantages of Cryogenic Processing :Advantages of Cryogenic Processing The following properties are attained to the materials treated:-Increases wear resistance Increases corrosion resistance Good dimensionality High strength Goodquality Cost reduction in the material manufactured Lower stress corrosion Cryogenic heat treatmenthelps to reduce the stored stress in the metal by creating a unified bond between the crystals.Slide 11:This process is eco friendly in nature There is no waste deposition The nitrogen which used in theprocess is liquefied from the atmosphere and later released back into it thereby creating no imbalanceto the ecosystem.Cryogenic fuels :Cryogenic fuels Cryogenics has made possible the commercial transportation of liquefied natural gas.Without cryogenics, nuclear research would lack liquid hydrogen and helium for use in particle detectorsand for the powerful electromagnets needed in large particle accelerators. Such magnets are also beingused in nuclear fusion research.
Slide 13:Cryogenic cooling is often used in space telescopes that observe objects in infrared and microwavewavelengths. More efficient and compact cryocoolers allow cryogenic temperatures to be used in anincreasing variety of military, medical, scientific, civilian, and commercial applications, including infraredsensors, superconducting electronics, and magnetic levitation trains.Slide 14:Cryogenics is used in artificial insemination to store semens and embryos. One such use in bio field isCryosurgery. Cryosurgery sometimes is referred to as cryotherapy or cryoablation. It is a surgicaltechnique in which freezing is used to destroy undesirable tissues. Liquid nitrogen, which boils at -196°C,is the most effective cryogen for clinical use. Temperatures of -25°C to -50°C can be achieved within 30seconds if a sufficient amount of liquid nitrogen is applied by spray or probe. Cryogenics in biologyOther uses of cryogenics :Other uses of cryogenics IN SPORTS:- Cryogenics are also used to treat many types of sports equipment,the most common being golf clubs. Because cryogenics increases the molecular density of treatedmaterials, it improves the distribution of energy (in this case kinetic energy) through the object. Thetreatment also increases the rigidity of the metal, which in this case might affect the shaft of the golfclub. Combined, the increases in kinetic energy distribution and rigidity of the shaft make for a longerand straighter drive.Future of Cryogenics :Future of Cryogenics Cryogenic rocket engine which will be used by NASA for its next manned moonmission. ICICLESCONCLUSION :CONCLUSION From this presentation it can be concluded as cryogenics can be applied to almosteverywhere in every field. It finds its application in military, tooling industry, agricultural industry,aerospace, medical, recycling, household, automobile industry, cryogenics is found to improve the grainstructure of everything treated be it metal or plastic or coils or engines or musical instruments or fiber.This field could be put to many other applications in various fields. Its reaches in the mentionedindustries hold a good chance of extension. Hence Cryogenics proves to be very promising for the futurein this world of materials.
ImproveCryogenic refrigeration is refrigeration which uses freezing mixtures such as dry ice, soilid co2,liquid co2, or liquid nitrogen. In the liquid co2 or liquid nitrogen method, a compartment is fittedwith a temperature sensing element that can be preset. This is in turn connected to a control panelwhich activates liquid co2 or liquid nitrogen cylinder fitted with regulators to release therefrigerant. This is delivered through a spray header into the compartment until the desiredtemperature is achieved. What is cryogenicsCryogenics is the study of the production of very low temperatures (below –150 °C, –238 °F or123 K) and the behavior of materials at those temperatures. (Rather than the familiartemperature... What is the refrigerationMany people have a misconception that Refrigeration is adding cold. The absence of heat is coldand hence Refrigeration can be defined as the process of heat removal from any enclosed spaceor... What is refrigerationRefrigeration is a heat removal process in which we reduce the surrounding temperature for thepreservative of food, or in medical treatment it is the lowering of a bodys temperature fortherapeutic... What is the net refrigeration effect in the refrigeration cycleThe quantity of heat that each pound of refrigerant absorbs from the refrigerated space toproduce useful cooling. What is cryogenic refrigerantRefrigerant is the medium contained in the system. Originally salt water, it progressed throughFreon to R18.
Cryogenic Refrigeration EquipmentFreezers for the Food Processing IndustryThe Cryogenic Institute of New England, Inc. offers cryogenic refrigeration equipment for thefood processing industry. We are capable of manufacturing customized cryogenic refrigerationsystems built to our customers specifications. Typical cryogenic refrigeration systems includebatch freezers, spiral freezers, single belt tunnel freezers, linear tunnel freezers and multipasslinear tunnel freezers. Most of these cryogenic refrigeration systems have the option of usingliquified nitrogen or liquified carbon dioxide as refrigerant. In addition, all our machines aremade of stainless steel for long in-service life.At the present time, we have new batch freezers available including single batch freezers, doublebatch freezers, dual batch freezers, cryo-test batch freezers and batch freezers with a rotatingproduct trolley.Our batch freezers are available in an E-Class and S-Class trim level. The E-Class has insulatedpanels with stainless steel cladding on the outside and fully welded stainless steel claddinginside. The S-Class has fully welded stainless steel construction with injected high pressureinsulation. These machines have a small footprint, low capital cost, and are easy to maintain.The spiral freezers that we offer come in standard models, but can also be made-to-order. Up anddown cage spiral freezer systems are available. These spiral freezers are highly efficient andmade for products that need long residence time. Temperature is automatically controlled. Beltlayouts can be set at 90-180-270 degrees, however standard spiral freezers come with straightthrough belt layouts.The single belt tunnel freezers that we provide are made for continuous food procesors in thebakery, red meat, fish, poultry and vegetable industries. The standard product offerings arecapabler of cooling or freezing small items the size of a beef patty to large items such as loafs ofbread. These single belt freezers are built solely with stainless steel to ensure long service lifeand trouble-free operation. Three different standard series machines are available. The P.O.D.B.has a pneumatically operated dropping bottom. The H.O.T.L. which has a hydraulically operatedtop lifting mechanism. Lastly, the H.O.D.B. which has a hydraulically operated dropping bottom.Four standard belt widths are available including 26", 36", 48" and 60". Like all our machines,this can be modified to suit your needs.Another type of cryogenic refrigeration system that we offer is the multipass linear tunnelfreezers. In a multipass tunnel, the products move along multiple tiers of conveyer belts. Thespeed of each belt can be independently controlled. Even cold temperature distribution can beobtained through the optimum combination of recirculation and side wall fans. This allows the
belt loading density to be higher. Our standard machine has three belts. One standard series iscurrently available named the H.O.T.L.; which stands for hydraulically operated top lifting. Weoffer four standard belt widths including 26", 36", 48" and 60".Here at the Cryogenic Institute of New England, Inc., we offer an independent review of yourapplication and can specify the most appropriate cryogenic equipment solution for yourrequirements. We broker used cryogenic equipment, represent manufacturers of new cryogenicequipment, modify off-the-shelf solutions for cryogenic applications or adapt existing cryogenicequipment to specific industrial use. Our goal is to provide cost-effective cryogenic solutions thatwill fit seamlessly within your existing operations.THE CRYOGENIC REFRIGERATION PROCESSMCR REFRIGERANT COMPOSITIONThe Multi Component Refrigerant (MCR) has the following approximate composition: Nitrogen (N2) 5% Methane (CH4) (C1) 35% Ethane (C2H6) (C2) 45% Propane (C3H8) (C3) 10% Butane (C4H10) (C4) 5%The above MCR inventory is maintained by the controlled injection of required componentsobtained from the fractionation unit. (Except for the Nitrogen which is provided by the Nitrogengeneration unit). Excess inventory can be vented to flare as needed.The volume of MCR (as vapour) required to liquefy the feed gas is FOUR times the volume offeed gas processed.MCR COMPRESSION AND COOLING SYSTEM(See Figure: 16)Beginning at the MCR vapour return line from the Cryogenic towers - EC.1 (Main) and EC.2(Sub). The cold MCR vapour flow through the sub-cryogenic tower is achieved by passing theMCR from the tower bottom into a Venturi-tube placed in the main tower MCR outlet line. Thepressure drop across the Venturi gives sufficient DP across EC. 2. to provide the required MCRflow. The A bundle feed gas outlet temperature is controlled by a TCV placed in the outlet line.The combined refrigerant flow at about 20 psig now passes to the MCR compression units. Itfirst enters a suction knock-out drum to prevent any liquid from entering the 1st stage
compressor. Any liquid which may separate out here is re-vaporised by a hot sparge gas injectioninto the vessel bottom via a perforated pipe. This is done to preserve the total inventory of theMCR vapour.On leaving the KO drum, the make-up components are added to the MCR flow via controlsystems. The MCR now passes into the 1st stage MCR compressor. This is a multi-stage,centrifugal compressor drive by a condensing steam turbine. (A combustion Gas Turbine may beused in some locations).The first stage of compression increases the MCR to 125 psi and about 225 °F. It is then cooledto about 110 °F in the inter-coolers and passed into the inter-stage drum where again, any liquiddropout is re-vaporised by hot sparge gas.A computerised flow controller (FRC) is placed in the compressor discharge line. Should theMass Flow drop below a pre-set point, the controller will begin to open the recycle valve andtake discharge gas back to the compressor suction to maintain a pre-set Minimum Flow throughthe compressor. This is necessary to prevent Surging in the compressor.(Surging in this type of machine must be avoided in order to prevent damage to the compressor,its internals and associated equipment and piping caused by high vibrations set up by the surgingaction).The 2nd stage MCR compressor takes suction from the inter-stage drum and raises the gaspressure to 450 psi and about 250 °F. The MCR is discharged through the salt water cooled after-coolers and piped to the first MCR separator D.1 on the main cryogenic tower EC. 1.Again, as in the first stage, a computerised flow element meters the mass flow and controls arecycle system for minimum flow protection against surging.Both MCR compressors may be driven by condensing steam turbines or by combustion gasturbines as required by the Company. Each machine also has over-pressure protection by safetyvalves to flare, installed in the discharge line. Suction and discharge lines are fitted with electricmotor Remote Operated Valves (ROVs) for quick operation if emergency operation is required.The following simplified diagram (Figure: 16) shows the layout of the MCR compressionsystem.
Refrigeration - CRYOGENIC EXCHANGER - BASIC OPERATIONCRYOGENIC EXCHANGER - BASIC OPERATION(See Figure: 17)The MCR from the 2nd stage compression and cooling process enters the first separator D.1.Here, the butane (C4) and some propane (C3) separate out as liquid.REFRIGERANT VAPOUR FLOWThe MCR Vapour from D.1. is piped through the B vapour coils of the main cryogenic towerEC.1. (cooled by the sprays from D.5.), and into separator D.2. where C3 and some C4 separateas liquid.
Vapour from D.2. is piped through the C vapour coils (cooled by the sprays from D.6.), and intoseparator D.3. where C2 and some C1 separate as liquid.From D.3, the vapour is cooled in the D bundles by sprays from D.7. then passes through Ebundle cooled to -262 °F.From the E bundle the MCR, now mainly liquid Methane (C1) and Nitrogen (N2) passes intoD.4. via an expansion valve where the pressure gives a final refrigerant temperature of about -275°F which feeds the E bundle sprays. The vapour from D.4. is piped directly into the maintower top.The expansion valve at D.4. maintains the upstream MCR at high pressure - some pressure lossfrom the original pressure occurs through the system, due to the liquids formed in the precedingseparators and friction due to the restriction to flow through the tube bundles and other fittings inthe system.2. REFRIGERANT LIQUID FLOWThe C4/C3 liquid from D.1. is passed through the B liquid coils and piped to Refrigerant drumD.5. via an expansion valve (Temperature Control Valve) which controls the temperature of theMCR vapour leaving the B bundle. The expansion valve causes a sudden pressure drop in D.5.and, due to the Joules-Thomson effect the expansion of the high pressure liquid causes itspartial vaporisation and therefore a large decrease in its temperature. The cold vapour from D.5.is passed directly into the main cryogenic tower. The sub-cooled liquid is sprayed over the Bbundles i.e. the refrigerant liquid and vapour coils, and the Feed Gas coils, cooling the bundles tothe operation requirement. This begins the cryogenic process.The MCR liquid (C3/C2), that separates out in D.2. passes through the C MCR liquid bundlesbefore passing into D. 6. via the expansion valve. The pressure drop further reduces this liquidtemperature. This liquid is sprayed over the C bundles thus further decreasing the temperatureof the three streams. (Upstream of the D.6. expansion valve, some refrigerant is piped to thefractionation unit for the cooling process in the recovery of Methane & Ethane. This MCR isreturned to the system downstream of the D.5. expansion valve).The MCR vapour from the C bundles passes into D. 3. where C2/C1 liquids drop out to be pipedthrough the D MCR liquid bundle, through the expansion valve into D. 7. where the pressuredrop produces a much lower liquid MCR temperature.This is sprayed over the D bundles to give their required outlet temperatures.The liquid refrigerant formed in D. 4. consists mainly of methane and some nitrogen after theexpansion valve produces a final MCR temperature of about - 275 °F.This liquid is sprayed over the E MCR vapour bundle and feed gas bundle to give an E bundleoutlet temperature of - 262 °F.
The LNG leaving the E bundle at - 262 °F and reduced pressure, passes through a TCV which,due to its throttling action, will decrease the pressure further before the LNG finally enters therundown line to the LNG storage tanks.The tanks pressure is maintained at 0.5 psi (just above atmospheric pressure).From storage the LNG is pumped into special cryogenic tankers for shipment abroad. Provisionis made to send off-spec LNG to burn-pit during start-up and shut-down operations.
Cryogenic Compressors for Ever! The Development of the "Oxford" CryocoolerPaul BaileyThe Need for CoolingIn the late 1970s there was a desire to learn more about the Earths atmosphere, and a satelliteinstrument called ISAMS (Improved Stratospheric And Mesospheric Sounder) was designed tomeasure the vertical profiles of temperature in the atmosphere and also a number of theatmospheric constituents.The signal-to-noise ratio of this instrument would be enhanced if the sensor was kept at atemperature of about 80 K. On the face of it this seems easy - space is a very cold place - but infact, satellites are NOT cold. Because of the size and mass of radiators, satellites are typically atabout 300 K, so a sensor at 80 K would need to be cooled in some way. The thermodynamiccycle most suitable for this was the Stirling cycle, a gas cycle which, in theory, approaches theideal Carnot efficiency.Originally developed for producing power, the Stirling cycle can also be used for refrigeration.The main components are a compressor, which produces pressure pulsations, and a cold head,which contains a displacer piston and heat exchangers. The displacer is synchronised with thecompressor piston and is usually operated at a phase angle of about 90° to the compressor piston.Hence when the gas is expanding, much of the gas is at the cold end, and takes in heat from itssurroundings, and when it is compressed, the gas has been moved to the warm end, where heat isrejected. By this means heat is pumped from a low temperature to a high temperature.The specification for this cryocooler was: 1 Watt of cooling at 80 K, rejecting heat at 300 K 10 year life 230 K to 340 K survival temperature Survival of launch vibration (non-operating) Low exported vibration High efficiency NO MAINTENANCE POSSIBLEThe last requirement was the problem. A simple single cylinder reciprocating machine has atleast five bearings, and all of these need to be lubricated. With the cold end of the cryocooler at80 K, any oil would solidify and block the heat exchangers. Therefore the unit must be oil free.There are a variety of oil free compressors - oil separators, ceramic pistons and metal or rubberdiaphragms have all been used, but all of them would need periodic maintenance to survive tenyears.
Early DevelopmentThe solution to this was provided by Dr Gordon Davey, who adapted a Pressure Modulatordeveloped by Oxfords Atmospheric Physics Department. The key features of the new cryocoolerwere:Clearance Seals are not seals - they are leaks! If the radial clearance between piston andcylinder is made small enough, the resulting leakage can be tolerated. The clearance needed is10-20 µm, and this requires both piston and cylinder to be very cylindrical, with goodconcentricity maintained between them.Spiral Disc Springs are used to maintain the alignment of the piston within the cylinder (seeFigure 1). They are photoetched from thin sheet - an inexpensive process which can easilygenerate the curved shapes required. The spring arms defined by the slots act as cantileversbuilt-in at both ends. Axially the springs are compliant, allowing the piston to move up anddown in the cylinder, but radially they are stiff, so that the piston remains aligned concentricallyin the cylinder. A 10 year design life at 50 Hz is equivalent to 1.6 x 1010 cycles, so the materialused for the springs - usually austenitic stainless steel or beryllium copper - must have a "fatiguelimit", and the spring is designed so that the peak stress is safely below this limit. Figure 1: Spiral disc springLinear Motion. A loudspeaker type moving coil, permanent magnet motor is used to drive thecompressors. With the motor, piston and springs all aligned on a common axis, there arenegligible sideways forces during operation. A typical assembly suspended on spiral springswould have a radial movement of ± 3 µm for a stroke of ± 5 mm.A diagram of this machine is shown in Figure 2. The compressor has a moving shaft mounted ontwo stacks of springs, with a moving piston in the cylinder at the top. The motor is positionedbetween the two spring stacks. The cold head is of similar design, with the displacer housed in a
cold finger, the tip of which is connected to the sensor being cooled. The regenerator, whichstores heat as gas passes between the hot and cold ends of the displacer, is housed within thedisplacer.Machines to this basic design were made by the Rutherford-Appleton Laboratory and byOxfords Atmospheric Physics Department and flown on the ISAMS and ATSR experiments,and the design was also developed by British Aerospace (and then Matra Marconi and Astrium),Lucas, Ball and Hughes/Raytheon and several other companies. Figure 2: Schematic of an early split Stirling cryocoolerSecond Generation CryocoolersThe first generation machines were expensive to make and difficult to assemble, and there was arequirement for smaller, lower cost machines that would be suitable for non-space use. To meetthese requirements, an Integral cryocooler was developed, which had the followingcharacteristics: A single unit, with the displacer integral with the compressor Moving cylinder and fixed piston The long thin shaft of the early machines was replaced by a short fat tube acting as the moving cylinder Only one motor - the displacer is driven pneumatically by the pressure pulsation More robust and easier to assembleThese units were developed in partnership with the Hymatic Engineering Company (nowHoneywell Hymatic) for tactical and commercial markets, and there was also a transfer of
technology to TRW (now part of Northrop Grumman Space Technology (NGST)). Theselicensing agreements are made through Isis Innovation, the University of Oxfords technologytransfer company.The Third GenerationTRW had a requirement for a compressor to drive a cold head, but with tight restrictions on thediameter and length of the compressor. To achieve this, a new moving coil motor was designed,and improvements were made to the spiral disc springs. The original design developed into aback-to-back configuration with two identical compressors acting on a common cylinder space,and this was used as the basis for the High Efficiency Cryocooler (HEC), shown in Figure 3. Figure 3: Northrop Grummans HEC cryocoolerTo develop this new machine, a three-way collaboration was established between: Oxford University - original design & consultancy Hymatic - detailed design and production TRW - cold head and system design and integrationThese machines were made using a carefully controlled production process, with extensive in-process and acceptance testing. Over 20 of these flight compressors have now been produced -this may not seem a large number, but by the standards of space hardware it is huge.
From the original machine, which was a 6 cm3 balanced compressor, a range of units have beendeveloped from 26 cm3 (the High Capacity Cryocooler - HCC) to a 0.6 cm3 "Mini" unit. Thesecompressors are typically mated to a pulse tube cold head, which uses simple plumbing(typically an orifice and an inertance tank), rather than a mechanical displacer, to give thecorrect pressure-volume phase relationship at the cold head.Heat Engines - the TASHEIn a collaboration with NGST, Hymatic and NASAs Los Alamos laboratory, one of thecompressors was used in a Thermo Acoustic Stirling Heat Engine. The TASHE was a prototypefor a prime mover to supply power for deep space or planetary missions, which would useplutonium as a heat source.In a thermo-acoustic engine, a high temperature gradient can excite acoustic oscillations in a gas.If this source of sound is connected to a piston which resonates at the same frequency, it ispossible to get power out of the system.In this prototype TASHE, an HEC compressor was modified by increasing the size of thepistons, and these were mated to a thermo-acoustic engine: instead of converting electricity intopressure pulsations, the compressors acted in reverse - absorbing acoustic power and generatingAC electricity. The complete system achieved a thermal efficiency of 18% with an electricalpower output of 40 W.Valved CompressorsAll of the machines mentioned so far have an oscillating flow - the compressors are "AC"devices used to produce a pressure oscillation. By adding valves, these compressors can be usedto create a "DC" flow through a system. One application for this is in conventional vapourcompression refrigeration, or to produce a flow for a Joule-Thomson (J-T) cooler.An example of the latter is a system being developed by NGST to cool the Mid-Infra-RedInstrument (MIRI) on the James Webb Space Telescope - the successor to the Hubble. The MIRIinstrument requires 65 mW of cooling at 6 K, and the system being developed consists of a threestage pulse tube cooler powered by an HCC compressor, which is used to pre-cool the bottomstage, which is a J-T system with a valved HEC compressor.The prototype valved compressor was built by Oxford and Hymatic, and is being tested on thecomplete J-T system by NGST.Computer CoolingOxford has just started a new project which will further develop the valved linear compressor.Computers have reached the stage where performance is being limited by the heat generated inCPU chips. Because they are very small, the heat flux required to cool them is higher than can beobtained with a forced convection heat sink clamped to the top surface.
Suitable heat fluxes can be obtained by evaporative cooling, especially if a very fine extendedheat transfer surface can be formed into the surface of the chip, which would become theevaporator of a conventional vapour compression refrigerator. The problem with an off-the-shelf system is the presence of oil, which circulates with the refrigerant, and would quickly findits way to the fine extended surface, which it would block.The Oxford compressors provide a solution to this problem - the clearance seal/spring systemrequires no lubrication and produces no debris that would foul the extended surface. Oxford hasjust started a three-year project, in collaboration with Newcastle University and London SouthBank University, to develop this technology.The moving coil compressors used to date are too expensive to be used in applications such asthese, so a new low-cost moving magnet linear motor has been designed. Instead of magneticyokes machined from pure iron, and delicate moving coils, the new motor uses silicon-ironlaminations and conventional windings similar to those in a standard rotary motor.Oil Free CompressorsThere are several other benefits of oil free compressors. The absence of oil makes these compressors suitable for use with high purity and medical gases, where oil cannot be tolerated. Oil acts as a catalyst for the breakdown of refrigerants at high temperatures in vapour compression systems. Oil free compression could widen the choice of refrigerants and increase the possible temperature range. It is inefficient to control conventional refrigerators by on/off cycling. The output of a linear compressor can be easily modulated by reducing the stroke, which can be done by dropping the supply voltage.Do They Live Forever?No.But they should last a long time; there are very few failure mechanisms in these simple machines- the springs are unlikely to fail, there is no wear and no oil or debris to cause blockages.There is the potential for electrical failure within the machine, particularly soldered joints, butfailure of external controls and drive electronics is much more likely.The one definite failure mode is gas leakage. Space cryocoolers are typically tested to ensure thatthe leak rate is less than 5 x 10-7 mbar l/s, which is equivalent to a pressure drop of about 1% in25 years; pressure drops of 5% or so would be tolerable before the efficiency would fallsignificantly.
Life test data compiled for "Oxford" type machines in 1998 showed a total of 49 machine-yearswith no failures. More recent data from cryocoolers flown on satellites shows one failure in 38machine-years (this was a displacer failure - the compressor was OK).The future prospects of the "Oxford" compressor are very promising. The third generationcompressors are very compact and robust, and have a high performance. They have been usedfor valved compressors and as a part of a heat engine. The new moving magnet concept will leadto a lower cost machine, and this will encourage its much wider use.Mechanical & Cryogenic RefrigerationRecent reports within the food processing industry indicate that the average cost of food freezingis up to 12 cents more per pound with the use of Cryogenic systems than with a similarMechanical system. This dramatic discrepancy is the result of costly, non-renewable LiquidNitrogen (LN) consumption.According to conservative estimates, LN costs approximately 3 cents per pound. Dependingupon the product, cryogenic freezing can consume between 0.5 and 1.5 pounds of gas for everypound of product. Therefore, a production system that runs 40 hours per week at 5,000 poundsper hour will incur an operating cost of $27,000 permonth. A similar mechanical, air-blast systemwill use 35 kilowatt-hours of electricity, resulting in a monthly energy bill around $630(assuming 10 cent per kilowatt-hour rates).Still, many food processors continue to turn to cryogenic solutions for the relatively low initialcapital investment. In spite of the rapid pay-back period for mechanical systems, some sourcesmaintain that cryogenic systems are feasible for start-up operations, but should be replaced withmechanical solutions as soon as the investment can be made.According to Process Engineering & Fabrication president, Bob Amacker, “When testing themarket for your product, it may be savvy to keep capital expenditures low. However, once youhave gained a clearer picture of the demand for your product and increased confidence in yourability to fill that demand, it is best to switch to the lowest long-term cost option.” With a pay-back period under 2 years, mechanical freezing is that option. Given that a cryogenic systemcould incur variable costs in excess of $2 million over a 10-year period, the initial price of asimilar spiral system is negligible.
Food processors still face important considerations in the mechanical/cryogenicdebate. Cryogenic tunnels do not have a clear advantage in factory footprint over conventionalspiral freezers, but cryogenic spiral freezers are clearly the most compact option. Issues offactory footprint, crust freezing, and freeze-rate favor cryogenic spirals, whereas spirals withmechanical freezing reduce operation costs significantly, and are the best choice for the long-term.