Cryoref

in physics, cryogenics is the study of the production of very low temperature (below −150°C, −238°F or
123K) and the behavior of materials at those temperatures. A person who studies elements under
extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of
Celsius and Fahrenheit, cryogenicists use the absolute temperature scales. These are Kelvin (SI units) or
Rankine scale (Imperial & US units).

Definitions and distinctions
Cryogenics

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.

[edit] Etymology
The word cryogenics stems from Greek and means "the production of freezing cold"; however
the term is used today as a synonym for the low-temperature state. It is not well-defined at what
point on the temperature scale refrigeration ends and cryogenics begins, but most scientists[1]
assume it starts at or below -150°C or 123°K K (about -240°F). The National Institute of
Standards and Technology at Boulder, Colorado has chosen to consider the field of cryogenics as
that 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.

[edit] Industrial application

Cryogenic valve

Further information: Timeline of low-temperature technology

Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic
applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally
purchasable around the world. Liquid helium is also commonly used and allows for the lowest
attainable temperatures to be reached.

These liquids are held in either special containers known as Dewar flasks, which are generally
about six feet tall (1.8 m) and three feet (91.5 cm) in diameter, or giant tanks in larger
commercial operations. Dewar flasks are named after their inventor, James Dewar, the man who
first liquefied hydrogen. Museums typically display smaller vacuum flasks fitted in a protective
casing.

Cryogenic transfer pumps are the pumps used on LNG piers to transfer liquefied natural gas
from LNG carriers to LNG storage tanks, as are cryogenic valves.

[edit] Cryogenic processing

The field of cryogenics advanced during World War II when scientists found that metals frozen
to low temperatures showed more resistance to wear. Based on this theory of cryogenic
hardening, 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 called
CryoTech in 1966. Though CryoTech later merged with 300 Below to create the largest and
oldest commercial cryogenics company in the world, they originally experimented with the
possibility of increasing the life of metal tools to anywhere between 200%-400% of the original
life expectancy using cryogenic tempering instead of heat treating. This evolved in the late 1990s
into the treatment of other parts (that did more than just increase the life of a product) such as
amplifier valves (improved sound quality), baseball bats (greater sweet spot), golf clubs (greater
sweet spot), racing engines (greater performance under stress), firearms (less warping after

continuous shooting), knives, razor blades, brake rotors and even pantyhose. The theory was
based on how heat-treating metal works (the temperatures are lowered to room temperature from
a high degree causing certain strength increases in the molecular structure to occur) and
supposed that continuing the descent would allow for further strength increases. Using liquid
nitrogen, CryoTech formulated the first early version of the cryogenic processor. Unfortunately
for the newly born industry, the results were unstable, as components sometimes experienced
thermal shock when they were cooled too quickly. Some components in early tests even
shattered because of the ultra-low temperatures. In the late twentieth century, the field improved
significantly with the rise of applied research, which coupled microprocessor based industrial
controls 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 statin
drugs, must occur at low temperatures of approximately −100°C (about -148°F). Special
cryogenic chemical reactors are used to remove reaction heat and provide a low temperature
environment. The freezing of foods and biotechnology products, like vaccines, requires nitrogen
in blast freezing or immersion freezing systems. Certain soft or elastic materials become hard
and brittle at very low temperatures, which makes cryogenic milling (cryomilling) an option for
some 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 is
ambient. The only reason for this is that most heat treaters do not have cooling equipment. There
is nothing metallurgically significant about ambient temperature. The cryogenic process
continues this action from ambient temperature down to −320 °F (140 °R; 78 K; −196 °C). In
most instances the cryogenic cycle is followed by a heat tempering procedure. As all alloys do
not have the same chemical constituents, the tempering procedure varies according to the
material's chemical composition, thermal history and/or a tool's particular service application.

The entire process takes 3–4 days.

[edit] Fuels

Another use of cryogenics is cryogenic fuels. Cryogenic fuels, mainly liquid hydrogen, have
been used as rocket fuels. Liquid oxygen is used as an oxidizer of hydrogen, but oxygen is not,
strictly speaking, a fuel. For example, NASA's workhorse space shuttle uses cryogenic hydrogen
fuel as its primary means of getting into orbit, as did all of the rockets built for the Soviet space
program by Sergei Korolev.

Russian aircraft manufacturer Tupolev developed a version of its popular design Tu-154 with a
cryogenic fuel system, known as the Tu-155. The plane uses a fuel referred to as liquefied
natural gas or LNG, and made its first flight in 1989.

[edit] Applications
Some 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.[citation needed].

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.

[edit] Production
Cryogenic 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 such
as pulse cryocoolers and Stirling cryocoolers have been devised. The most recent development in
cryogenics is the use of magnets as regenerators as well as refrigerators. These devices work on
the principle known as the magnetocaloric effect.

[edit] Detectors

Cryogenic temperatures, usually well below 77 K (−196 °C) are required to operate cryogenic
detectors.

INTRODUCTION :
INTRODUCTION Cryogenics may be defined as the branch of physics which deals with the production of
very low temperature and their effect on matter. It may also be defined as the science and technology
of 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 is
magnetized it becomes warm and cold when demagnetized in controlled atmosphere thus producing
low temperatures. By compressing the gas, the gas is cooled releasing heat and later allowed to expand
producing ultra low temperatures.

Cryogenics in fuels :
Cryogenics in fuels Fluids are stored at 93.5k(-180degree) or below Due to air friction ,it gets ignited
Cools engine on expansion. On expansion pressure increases providing high thrust Satellite payload
increases

Cryogenic Rocket Engines :
Cryogenic Rocket Engines Cryogenic rocket engines are one of the important applications in the field of
cryogenics. The higher thrust levels required for a rocket engines are achieved when liquid oxygen and
liquid hydrocarbons are used as fuel. But at atmospheric conditions, LOX and low molecular
hydrocarbons are in gaseous state. Therefore these are stored in liquid form by cooling them down
using cryogenics. Hence the name Cryogenic Rocket Engines.

VULCAIN 2 ROCKET ENGINE :
VULCAIN 2 ROCKET ENGINE THRUST -1359KN INLET CONDITIONS: LH2; Pressure=182.1bar
Temperature=36k LO2: Pressure=153.9 Temperature=96.7k Combustion chamber pressure=117.3bar
Thrust chamber mass=909kg

Cryogenic Heat Treatment :
Cryogenic Heat Treatment This is a process of treating metals, plastics, ceramics at temperatures below
120K to their crystal structures and properties. This increases their wear resistance, and life of metals
and 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. The
symbol used to represent cryogenic heat treatment

In alloy steels:- :
In alloy steels:- In this process the alloy steels are treated to convert the entire austenite into a
martensite matrix such it changes the molecular structure of the steel and forms an entirely new, more
refined grain structure which partly relieves the thermal stresses. The number of countable carbides
increase from 30,000 to 80,000 per square millimeter which forms a “super hard” surface on the metal.
After deep cryogenic heat treatment

Slide 9:
Before CI Processing After CI Processing Comparative microphotographs (1000x) of steel samples show
the change in microstructure produced by the controlled deep cryogenic process. Uniform, more
completely transformed microstructure and less retained austenite at right, is related to improvements
in strength, stability and resistance to wear. *Cryogenics International's 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 many
people 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 Good
quality Cost reduction in the material manufactured Lower stress corrosion Cryogenic heat treatment
helps 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 the
process is liquefied from the atmosphere and later released back into it thereby creating no imbalance
to 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 detectors
and for the powerful electromagnets needed in large particle accelerators. Such magnets are also being
used in nuclear fusion research.

Cryogenic cooling is often used in space telescopes that observe objects in infrared and microwave
wavelengths. More efficient and compact cryocoolers allow cryogenic temperatures to be used in an
increasing variety of military, medical, scientific, civilian, and commercial applications, including infrared
sensors, 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 is
Cryosurgery. Cryosurgery sometimes is referred to as cryotherapy or cryoablation. It is a surgical
technique 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 30
seconds if a sufficient amount of liquid nitrogen is applied by spray or probe. Cryogenics in biology

Other 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 treated
materials, it improves the distribution of energy (in this case kinetic energy) through the object. The
treatment also increases the rigidity of the metal, which in this case might affect the shaft of the golf
club. Combined, the increases in kinetic energy distribution and rigidity of the shaft make for a longer
and straighter drive.

Future of Cryogenics :
Future of Cryogenics Cryogenic rocket engine which will be used by NASA for its next manned moon
mission. ICICLES

CONCLUSION :
CONCLUSION From this presentation it can be concluded as cryogenics can be applied to almost
everywhere 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 grain
structure 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 mentioned
industries hold a good chance of extension. Hence Cryogenics proves to be very promising for the future
in this world of materials.

Improve
Cryogenic 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 fitted
with a temperature sensing element that can be preset. This is in turn connected to a control panel
which activates liquid co2 or liquid nitrogen cylinder fitted with regulators to release the
refrigerant. This is delivered through a spray header into the compartment until the desired
temperature is achieved.

What is cryogenics

Cryogenics is the study of the production of very low temperatures (below –150 °C, –238 °F or
123 K) and the behavior of materials at those temperatures. (Rather than the familiar
temperature...

What is the refrigeration

Many people have a misconception that Refrigeration is adding cold. The absence of heat is cold
and hence Refrigeration can be defined as the process of heat removal from any enclosed space
or...

What is refrigeration

Refrigeration is a heat removal process in which we reduce the surrounding temperature for the
preservative of food, or in medical treatment it is the lowering of a body's temperature for
therapeutic...

What is the net refrigeration effect in the refrigeration cycle

The quantity of heat that each pound of refrigerant absorbs from the refrigerated space to
produce useful cooling.

What is cryogenic refrigerant

Refrigerant is the medium contained in the system. Originally salt water, it progressed through
Freon to R18.

Cryogenic Refrigeration Equipment
Freezers for the Food Processing Industry
The Cryogenic Institute of New England, Inc. offers cryogenic refrigeration equipment for the
food processing industry. We are capable of manufacturing customized cryogenic refrigeration
systems built to our customers' specifications. Typical cryogenic refrigeration systems include
batch freezers, spiral freezers, single belt tunnel freezers, linear tunnel freezers and multipass
linear tunnel freezers. Most of these cryogenic refrigeration systems have the option of using
liquified nitrogen or liquified carbon dioxide as refrigerant. In addition, all our machines are
made of stainless steel for long in-service life.

At the present time, we have new batch freezers available including single batch freezers, double
batch freezers, dual batch freezers, cryo-test batch freezers and batch freezers with a rotating
product trolley.

Our batch freezers are available in an E-Class and S-Class trim level. The E-Class has insulated
panels with stainless steel cladding on the outside and fully welded stainless steel cladding
inside. The S-Class has fully welded stainless steel construction with injected high pressure
insulation. 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 and
down cage spiral freezer systems are available. These spiral freezers are highly efficient and
made for products that need long residence time. Temperature is automatically controlled. Belt
layouts can be set at 90-180-270 degrees, however standard spiral freezers come with straight
through belt layouts.

The single belt tunnel freezers that we provide are made for continuous food procesors in the
bakery, red meat, fish, poultry and vegetable industries. The standard product offerings are
capabler of cooling or freezing small items the size of a beef patty to large items such as loafs of
bread. These single belt freezers are built solely with stainless steel to ensure long service life
and 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 operated
top 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 tunnel
freezers. In a multipass tunnel, the products move along multiple tiers of conveyer belts. The
speed of each belt can be independently controlled. Even cold temperature distribution can be
obtained 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 is
currently available named the H.O.T.L.; which stands for hydraulically operated top lifting. We
offer 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 your
application and can specify the most appropriate cryogenic equipment solution for your
requirements. We broker used cryogenic equipment, represent manufacturers of new cryogenic
equipment, modify off-the-shelf solutions for cryogenic applications or adapt existing cryogenic
equipment to specific industrial use. Our goal is to provide cost-effective cryogenic solutions that
will fit seamlessly within your existing operations.

THE CRYOGENIC REFRIGERATION PROCESS
MCR REFRIGERANT COMPOSITION

The 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 components
obtained from the fractionation unit. (Except for the Nitrogen which is provided by the Nitrogen
generation 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 of
feed 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 the
MCR from the tower bottom into a Venturi-tube placed in the main tower MCR outlet line. The
pressure drop across the Venturi gives sufficient DP across EC. 2. to provide the required MCR
flow. 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. It
first 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 injection
into the vessel bottom via a perforated pipe. This is done to preserve the total inventory of the
MCR vapour.

On leaving the KO drum, the make-up components are added to the MCR flow via control
systems. 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 be
used in some locations).

The first stage of compression increases the MCR to 125 psi and about 225 °F. It is then cooled
to about 110 °F in the inter-coolers and passed into the inter-stage drum where again, any liquid
dropout is re-vaporised by hot sparge gas.

A computerised flow controller (FRC) is placed in the compressor discharge line. Should the
Mass Flow drop below a pre-set point, the controller will begin to open the recycle valve and
take discharge gas back to the compressor suction to maintain a pre-set Minimum Flow through
the 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 surging
action).

The 2nd stage MCR compressor takes suction from the inter-stage drum and raises the gas
pressure 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 a
recycle system for minimum flow protection against surging.

Both MCR compressors may be driven by condensing steam turbines or by combustion gas
turbines as required by the Company. Each machine also has over-pressure protection by safety
valves to flare, installed in the discharge line. Suction and discharge lines are fitted with electric
motor 'Remote Operated Valves' (ROV's) for quick operation if emergency operation is required.

The following simplified diagram (Figure: 16) shows the layout of the MCR compression
system.

Refrigeration - CRYOGENIC EXCHANGER - BASIC OPERATION

CRYOGENIC 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 FLOW

The MCR Vapour from D.1. is piped through the 'B' vapour coils of the main cryogenic tower
EC.1. (cooled by the sprays from D.5.), and into separator D.2. where C3 and some C4 separate
as liquid.

Vapour from D.2. is piped through the 'C' vapour coils (cooled by the sprays from D.6.), and into
separator 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 'E'
bundle cooled to -262 °F.

From the 'E' bundle the MCR, now mainly liquid Methane (C1) and Nitrogen (N2) passes into
D.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 main
tower top.

The expansion valve at D.4. maintains the upstream MCR at high pressure - some pressure loss
from the original pressure occurs through the system, due to the liquids formed in the preceding
separators and friction due to the restriction to flow through the tube bundles and other fittings in
the system.

2. REFRIGERANT LIQUID FLOW

The C4/C3 liquid from D.1. is passed through the 'B' liquid coils and piped to Refrigerant drum
D.5. via an expansion valve (Temperature Control Valve) which controls the temperature of the
MCR 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 its
partial 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 'B'
bundles i.e. the refrigerant liquid and vapour coils, and the Feed Gas coils, cooling the bundles to
the operation requirement. This begins the cryogenic process.

The MCR liquid (C3/C2), that separates out in D.2. passes through the 'C' MCR liquid bundles
before passing into D. 6. via the expansion valve. The pressure drop further reduces this liquid
temperature. This liquid is sprayed over the 'C' bundles thus further decreasing the temperature
of the three streams. (Upstream of the D.6. expansion valve, some refrigerant is piped to the
fractionation unit for the cooling process in the recovery of Methane & Ethane. This MCR is
returned 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 piped
through the 'D' MCR liquid bundle, through the expansion valve into 'D. 7.' where the pressure
drop 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 the
expansion 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' bundle
outlet 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 the
rundown 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. Provision
is 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" Cryocooler

Paul Bailey

The Need for Cooling

In the late 1970s there was a desire to learn more about the Earth's atmosphere, and a satellite
instrument called ISAMS (Improved Stratospheric And Mesospheric Sounder) was designed to
measure the vertical profiles of temperature in the atmosphere and also a number of the
atmospheric constituents.

The signal-to-noise ratio of this instrument would be enhanced if the sensor was kept at a
temperature of about 80 K. On the face of it this seems easy - space is a very cold place - but in
fact, satellites are NOT cold. Because of the size and mass of radiators, satellites are typically at
about 300 K, so a sensor at 80 K would need to be cooled in some way. The thermodynamic
cycle most suitable for this was the Stirling cycle, a gas cycle which, in theory, approaches the
ideal '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 the
compressor 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 its
surroundings, and when it is compressed, the gas has been moved to the warm end, where heat is
rejected. 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 POSSIBLE

The last requirement was the problem. A simple single cylinder reciprocating machine has at
least five bearings, and all of these need to be lubricated. With the 'cold end' of the cryocooler at
80 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 rubber
diaphragms have all been used, but all of them would need periodic maintenance to survive ten
years.

Early Development

The solution to this was provided by Dr Gordon Davey, who adapted a 'Pressure Modulator'
developed by Oxford's Atmospheric Physics Department. The key features of the new cryocooler
were:

Clearance Seals are not seals - they are leaks! If the radial clearance between piston and
cylinder is made small enough, the resulting leakage can be tolerated. The clearance needed is
10-20 µm, and this requires both piston and cylinder to be very cylindrical, with good
concentricity maintained between them.

Spiral Disc Springs are used to maintain the alignment of the piston within the cylinder (see
Figure 1). They are photoetched from thin sheet - an inexpensive process which can easily
generate the curved shapes required. The spring arms defined by the slots act as cantilevers
'built-in' at both ends. Axially the springs are compliant, allowing the piston to move up and
down in the cylinder, but radially they are stiff, so that the piston remains aligned concentrically
in the cylinder. A 10 year design life at 50 Hz is equivalent to 1.6 x 1010 cycles, so the material
used for the springs - usually austenitic stainless steel or beryllium copper - must have a "fatigue
limit", and the spring is designed so that the peak stress is safely below this limit.

Figure 1: Spiral disc spring

Linear Motion. A 'loudspeaker' type moving coil, permanent magnet motor is used to drive the
compressors. With the motor, piston and springs all aligned on a common axis, there are
negligible sideways forces during operation. A typical assembly suspended on spiral springs
would 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 on
two stacks of springs, with a moving piston in the cylinder at the top. The motor is positioned
between 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, which
stores heat as gas passes between the hot and cold ends of the displacer, is housed within the
displacer.

Machines to this basic design were made by the Rutherford-Appleton Laboratory and by
Oxford's 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 cryocooler

Second Generation Cryocoolers

The first generation machines were expensive to make and difficult to assemble, and there was a
requirement for smaller, lower cost machines that would be suitable for non-space use. To meet
these requirements, an 'Integral' cryocooler was developed, which had the following
characteristics:

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 assemble

These units were developed in partnership with the Hymatic Engineering Company (now
Honeywell Hymatic) for 'tactical' and commercial markets, and there was also a transfer of

technology to TRW (now part of Northrop Grumman Space Technology (NGST)). These
licensing agreements are made through Isis Innovation, the University of Oxford's technology
transfer company.

The Third Generation

TRW had a requirement for a compressor to drive a cold head, but with tight restrictions on the
diameter 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 a
back-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 Grumman's HEC cryocooler

To 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 integration

These 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 been
developed from 26 cm3 (the High Capacity Cryocooler - HCC) to a 0.6 cm3 "Mini" unit. These
compressors 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 the
correct pressure-volume phase relationship at the cold head.

Heat Engines - the TASHE

In a collaboration with NGST, Hymatic and NASA's Los Alamos laboratory, one of the
compressors was used in a Thermo Acoustic Stirling Heat Engine. The TASHE was a prototype
for a prime mover to supply power for deep space or planetary missions, which would use
plutonium 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 is
possible to get power out of the system.

In this prototype TASHE, an HEC compressor was modified by increasing the size of the
pistons, and these were mated to a thermo-acoustic engine: instead of converting electricity into
pressure pulsations, the compressors acted in reverse - absorbing acoustic power and generating
AC electricity. The complete system achieved a thermal efficiency of 18% with an electrical
power output of 40 W.

Valved Compressors

All 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 used
to create a "DC" flow through a system. One application for this is in conventional vapour
compression 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-Red
Instrument (MIRI) on the James Webb Space Telescope - the successor to the Hubble. The MIRI
instrument requires 65 mW of cooling at 6 K, and the system being developed consists of a three
stage pulse tube cooler powered by an HCC compressor, which is used to pre-cool the bottom
stage, 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 the
complete J-T system by NGST.

Computer Cooling

Oxford 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 in
CPU chips. Because they are very small, the heat flux required to cool them is higher than can be
obtained 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 extended
heat transfer surface can be formed into the surface of the chip, which would become the
'evaporator' 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 find
its way to the fine extended surface, which it would block.

The Oxford compressors provide a solution to this problem - the clearance seal/spring system
requires no lubrication and produces no debris that would foul the extended surface. Oxford has
just started a three-year project, in collaboration with Newcastle University and London South
Bank University, to develop this technology.

The moving coil compressors used to date are too expensive to be used in applications such as
these, so a new low-cost moving magnet linear motor has been designed. Instead of magnetic
yokes machined from pure iron, and delicate moving coils, the new motor uses silicon-iron
laminations and conventional windings similar to those in a standard rotary motor.

Oil Free Compressors

There 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, but
failure 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 that
the leak rate is less than 5 x 10-7 mbar l/s, which is equivalent to a pressure drop of about 1% in
25 years; pressure drops of 5% or so would be tolerable before the efficiency would fall
significantly.

Life test data compiled for "Oxford" type machines in 1998 showed a total of 49 machine-years
with no failures. More recent data from cryocoolers flown on satellites shows one failure in 38
machine-years (this was a displacer failure - the compressor was OK).

The future prospects of the "Oxford" compressor are very promising. The third generation
compressors are very compact and robust, and have a high performance. They have been used
for valved compressors and as a part of a heat engine. The new moving magnet concept will lead
to a lower cost machine, and this will encourage its much wider use.

Mechanical & Cryogenic Refrigeration

Recent reports within the food processing industry indicate that the average cost of food freezing
is up to 12 cents more per pound with the use of Cryogenic systems than with a similar
Mechanical system. This dramatic discrepancy is the result of costly, non-renewable Liquid
Nitrogen (LN) consumption.

According to conservative estimates, LN costs approximately 3 cents per pound. Depending
upon the product, cryogenic freezing can consume between 0.5 and 1.5 pounds of gas for every
pound of product. Therefore, a production system that runs 40 hours per week at 5,000 pounds
per hour will incur an operating cost of $27,000 per

month. A similar mechanical, air-blast system
will 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 initial
capital investment. In spite of the rapid pay-back period for mechanical systems, some sources
maintain that cryogenic systems are feasible for start-up operations, but should be replaced with
mechanical solutions as soon as the investment can be made.

According to Process Engineering & Fabrication president, Bob Amacker, “When testing the
market for your product, it may be savvy to keep capital expenditures low. However, once you
have gained a clearer picture of the demand for your product and increased confidence in your
ability 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 system
could incur variable costs in excess of $2 million over a 10-year period, the initial price of a
similar spiral system is negligible.

Food processors still face important considerations in the mechanical/cryogenic
debate. Cryogenic tunnels do not have a clear advantage in factory footprint over conventional
spiral freezers, but cryogenic spiral freezers are clearly the most compact option. Issues of
factory footprint, crust freezing, and freeze-rate favor cryogenic spirals, whereas spirals with
mechanical freezing reduce operation costs significantly, and are the best choice for the long-
term.

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