5Cryogenic refrigeration.pptx

CRYOGENIC REFRIGERATION SYSTEMS
(UQ APRIL 2015, NOV 2015)

(UQ APRIL 2016, APRIL 2015)
 Systems that utilize cryogenic temperatures in
operation, such as advanced electronic systems, super
conducting magnets and motors, all depend upon an
effective refrigeration system to maintain the low
temperatures required.
 The difference between the refrigeration system and
the liquefaction system is that the liquid produced is
evaporated in a refrigeration system instead of being
utilized in some other way external to the system, as
in a liquefaction system.
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IDEAL REFRIGERATION SYSTEMS
 The thermodynamically ideal isothermal-source system
 The thermodynamically ideal isobaric-source system
 The sink into which heat is rejected is usually the atmosphere.
 We have an isothermal sink in both cases.
 “Source” is to mean the source of heat for the refrigerator-that
is the space to be cooled.
 The term “Sink” refers to the region into which heat is rejected
from the refrigerator.
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THE THERMODYNAMICALLY IDEAL
ISOTHERMAL-SOURCE SYSTEM
(CARNOT REFRIGERATOR)
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 The processes involved in the Carnot Refrigerator are
as follows:-
 Process 1-2:-The working medium is compressed while
energy is rejected to the sink to maintain refrigerant
temperature constant.
 Process 2-3:-The working medium is expanded
reversibly and adiabatically from the sink temperature
Th to the source temperature Tc .
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 Process 3-4:-Energy is transferred from the source (the
region to be cooled) to the refrigeration medium, while
work is done by the medium to maintain the refrigerant at
constant temperature.
 Process 4-1:-The refrigerant is then compressed reversibly
and adiabatically (isentropic process) from the source
temperature to the sink temperature.
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 The Carnot cycle two reversible adiabaticprocesses
and two reversible isothermal processes.
 The Qa is a positive quantity (heat added to the system
is considered positive), while the net work expended is
a negative quantity (work done on the system is
considered negative; work done by the system is
considered positive)
 The figure of merit (FOM) is a number between
zero and unity.
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 FOM near unity implies a very good refrigerator.
 Small FOM implies a poor refrigerator compared with
the ideal refrigerator.
 The coefficient of performance for the Carnot system is
independent of the refrigerant. That is between the same
temperature limits, the COP would be same if helium gas
or liquid nitrogen or liquid argon were used as the
refrigerant.
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REFRIGERATORS FOR TEMPERATURES
ABOVE 2K
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JOULE-THOMSON REFRIGERATION
SYSTEMS
 Any of the liquefaction systems that do not use an
expansion engine may be classified as Joule-Thomson
Refrigerators because they depend upon the Joule-
Thomson effect to produce low temperatures.
 Instead of withdrawing the liquid formed in the
refrigerator, heat is absorbed from the low temperature
source to evaporate this liquid.
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LINDE -HAMPSON REFRIGERATOR
 The compression from point 1 to point 2 would be
isothermal in the ideal case.
 The compressed gas is passed through the heat exchanger,
cooled to low temperatures by heat exchange with the cold
outgoing gas stream, and expanded through a Joule-
Thomson valve into a evaporator.
 In the evaporator (which corresponds to the liquid receiver
in the liquefaction system), the liquid formed after the
expansion process is evaporated (at constant temperature)
by absorbing heat from the space to be refrigerated. The
vapour then returns through the heat exchanger to the
compressor.
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 Joule Thomson Refrigerator cannot be used with neon,
hydrogen or helium as the working medium, unless these
gases are first precooled below their maximum inversion
temperatures.
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EXPANSION ENGINE REFRIGERATION SYSTEMS
(CLAUDE REFRIGERATOR) OR
(COLLINS REFRIGERATOR)
 The compression from point 1 to point 2 would be
isothermal in the ideal case.
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PHILIPS REFRIGERATOR
(STIRLING CYCLE REFRIGERATOR)
(UQ APRIL 2015)
 Philips refrigerator operates on the stirling cycle.
 Philips refrigerator consists of a cylinder enclosing a
piston, a displacer and a regenerator.
 The piston compresses the gas, while the displacer moves
the gas from one chamber to another without changing the
gas volume, in the ideal case.
 The heat exchange during the constant-volume process is
carried out in the refrigerator.
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 Process 1-2:-The gas is compressed isothermally while
rejecting heat to the high temperature sink (surroundings).
 Process 2-3:-The gas is forced through the regenerator by
the motion of the displacer. The gas is cooled at constant
volume during this process. The energy removed from the
gas is not transferred to the surroundings but it is stored in
the regenerator matrix.
 Process 3-4:- The gas is expanded isothermally, while
absorbing heat from the low temperature source.
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 Process 4-1:-The cold gas is forced through the
regenerator by the motion of the displacer, the gas is heated
during this process. The energy stored during process 2-3
is transferred back to the gas. In the ideal case (no heat
inleaks), heat is transferred to the refrigerator only during
process 3-4, and heat is rejected from the refrigerator only
during process 1-2.
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IMPORTANCE OF REGENERATOR EFFECTIVENESS
FOR THE PHILIPS REFRIGERATOR
(UQ APRIL 2016)
 The success of the Philips refrigerator depends to a
large extent upon the effectiveness of the regenerator
used in the system.
 A good regenerator should be constructed of a material
with a large thermal capacity, the period of switching
should be small (i.e., the frequency of cycling the fluid
through the regenerator should be large), the heat
transfer coefficient and surface area should be large,
and the mass flow rate through the regenerator should
be small.
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 In the Philips refrigerator, a light felt (fibre) like mass of
fine wire is used as the regenerator matrix material to
thermal capacity with large heat
needed for good regenerator
attain the large
transfer area
performance.
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VUILLEUMIER REFRIGERATOR
Vuilleumier in 1918 is similar to the Stirling
Refrigerator, except the V.M. refrigerator uses a
(V.M. REFRIGERATOR)
 The Vuilleumier refrigerator, first patented by Rudolph
“thermal” compressor instead of a mechanical
compressor.
 Process 1-2:- Heat is added from a high temperature
source to the gas in the “hot” cylinder, and the displacer
moves downward to maintain the temperature of the gas
constant at Th.
 Process 4-1:-At the same time, near-ambient temperature
gas flows from the intermediate volume through the
regenerator to the hot volume.
44

 Process 2-3:- The displacer then moves upward and gas is
displaced from the hot volume to the intermediate volume.
 Process 3-4:- Heat is rejected from the intermediate
volume to maintain the temperature of the gas in the
volume constant at Ta.
 Process 5-6:- As the cold displacer is moved to the left,
heat is absorbed by the gas in the cold volume from the
low temperature source to maintain the gas temperature
constant at Tc.
 Process 4-5:- At the same time, gas from the intermediate
volume flows through the cold regenerator to the cold
volume.
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 Process 6-3:- The cold displacer then moves back to
the right, and gas is displaced from the cold volume
through the cold regenerator to the intermediate
volume.
 Assume that all processes are thermodynamically ideal
and that the working fluid may be treated as an ideal
gas.
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SOLVAY REFRIGERATOR
 Solvay refrigerator was invented in Germany in 1887.
 The sequence of operations for the Solvay Refrigerator is
as follows:-
 Process 1-2:-With the piston at the bottom of its stroke, the
inlet valve is opened. The high pressure gas flows into the
regenerator, in which the gas is cooled, and the system
pressure is increased from a low pressure P1 to a high
pressure P2.
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 Process 2-3:-With the inlet still open, the piston is raised to
draw a volume of gas into the cylinder. The gas has been
cooled during its flow through the regenerator.
 Process 3-4:-The inlet valve is closed, and the gas within
the cylinder is expanded ( isentropically in the ideal case)
to the initial pressure P1. As the gas expands, it does work
on the piston, and energy is removed from the gas as work.
The temperature of the gas therefore decreases.
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 Process 4-5:-The exhaust valve is opened, and the piston is
lowered to force the cold gas out of the cylinder. During
this process, the cold gas passes through a heat exchanger
to remove heat from the region to be cooled.
 Process 5-1:-The gas finally passes out through the
regenerator, in which the cold gas is warmed back to room
temperature.
 The piston is constructed of a poor heat conductor, such
as micarta, so that it may be sealed at the warm end, which
helps to avoid the problem of a low temperature moving
seal.
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 The miniature Solvay Refrigerator described by Gifford
and McMahon was capable of attaining 55K in a single
stage.
 The cylinder has a diameter of 5.6mm and length of
51mm. The working fluid was helium gas, which varied
in pressure between 345kPa and 1725kPa.
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GIFFORD McMAHON REFRIGERATOR
(G.M. REFRIGERATOR)
(A.D. LITTLE SINGLE VOLUME REFRIGERATOR)
(UQ APRIL 2016)
 G.M. Refrigerator consists of a compressor, a cylinder
closed at both ends, a displacer within the cylinder, and
a regenerator.
 This system differs from the Solvay Refrigerator in that
no work is transferred from the system during the
expansion process.
 The displacer serves the purpose of moving the gas
from one expansion space to another and would do zero
net work in the ideal case of zero pressure drop in the
regenerator.
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 Process 1-2:- With the displacer at the bottom of the
cylinder, the inlet valve is opened and the pressure within
the upper expansion space is increased from a low pressure
P1 to a higher pressure P2. The volume of the lower
expansion space is practically zero during this process
because the displacer is at its lowest position.
 Process 2-3:- With the inlet valve still open and the exhaust
valve closed, the displacer is moved to the top of the
cylinder. This action moves the gas that was originally in
the upper expansion space down through the regenerator to
the lower expansion space.
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 Because the gas is cooled as it passes through the
regenerator, it will decrease in volume so that gas will be
drawn in through the inlet valve during this process to
maintain a constant pressure within the system.
 Process 3-4:- With the displacer at the top of the cylinder,
the inlet valve is closed and the exhaust valve is opened,
thus allowing the gas within the lower expansion space to
expand to the initial pressure P1. The gas that is finally
within the lower expansion space does work to push out the
gas that leaves during this process; therefore, energy is
removed as work from the gas finally left in the lower
expansion space. This causes the gas in the lower
expansion space to drop to a low temperature. 59

 Process 4-5:- The low temperature gas is forced out of the
lower expansion space by moving the displacer downward
to the bottom of the cylinder. This cold gas flows through a
heat exchanger in which heat is transferred to the gas from
the low-temperature source.
 Process 5-1:- The gas flows from the heat exchanger
through the regenerator, in which the gas is warmed back to
near ambient temperature.
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REGENERATORS
 In a regenerator, the same space is occupied alternately by
the hot fluid and the cold fluid, while the energy to be
transferred is stored and released from the regenerator
packing material or matrix.
 The temperature of the gases and the temperature of the
regenerator matrix are functions both of the position within
the regenerator and of time.
 After a long period of operation, a sort of steady state
operation called periodic flow is achieved in which the
temperature distribution within the regenerator repeats
itself during each cycle of operation.
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 In both the Solvay and the Gifford McMahon
refrigerators, the regenerator is a critical component, as
in the case of the Philips refrigerator.
 For an efficient refrigerator, the regenerator
effectiveness should be 98% or better.
 Punched copper or brass screens were used as the
regenerator packing material.
 To reduce the heat conduction along the length of the
regenerator, the punched screens were separated by a
coil of stainless-steel wire.
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 For very low temperature regenerators, lead may be
used instead of copper because lead has a higher
specific heat at low temperature due to its low Debye
temperature.
 Because of the back and forth motion of the gas through
the regenerator, the impurities are deposited in the
regenerator during the intake process and are swept back
out during the exhaust process.
 Regenerators are generally less expensive, fora given
surface area, than heat exchanger.
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REFRIGERATORS FOR TEMPERATURES
BELOW 2K

MAGNETIC COOLING (UQ APRIL 2016, NOV 2015)
 To produce low temperature (about 0.6K) liquid He4 or
liquid He3 boiling under reduced pressure are used.
 About 0.4K is the limit we can attain in a practical
system because of the difficulties in maintaining such
low pressures with even moderate flow rates.
 Giauque (1927) and Debye (1926) independently
suggested a way to break this “temperature barrier”.
 They pointed out that a magnetic substance could be used
instead of a gas or liquid and that a magnetic field could
be used instead of the expansion of a fluid to attain the
low temperatures.
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 If we were to compress a gas at constant temperature, we
should increase the order (or decrease the entropy) of the
system because we move the molecules closer together
without increasing their random velocities.
 If we were to expand the gas reversibly and adiabatically,
we should not change the degree of order (because the
entropy remains constant) of the system.
 We should move the gas molecules farther apart, however,
so that the random molecular velocities (and therefore the
gas temperature) must decrease in order to maintain the
same degree of order (or entropy).
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 When we get a gas to very low temperatures, there is not
much room left for any more ordering of the system
because it is almost as ordered as it can be.
 A paramagnetic substance however has another way of
ordering itself.
 In the absence of an external magnetic field, the dipoles of
the paramagnetic material are more or less randomly
arranged, even at low temperatures.
 If we apply a magnetic field at constant temperature
(analogous to compressing a gas isothermally), we shall
tend to align the magnetic moments of the atoms of the
paramagnetic material, thereby introducing order or
decreasing the entropy of the material. 72

 If the magnetic field is removed reversibly and
adiabatically (corresponding to a reversible adiabatic
expansion of a gas), the entropy remains constant but the
alignment of the dipole moments is not as great as before.
 To preserve the degree of order (or maintain the entropy
constant), the temperature of the paramagnetic material
must decrease.
 This process is called adiabatic demagnetization, and it
is the process that allows us to enter the temperature
region below 0.6K.
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 A paramagnetic salt pellet is suspended in a chamber
by silk or nylon threads. This chamber is initially filled
with gaseous helium, and the chamber is then
immersed in a liquid helium bath.
 The liquid helium is boiling under reduced pressure, so
its temperature and the temperature of the
paramagnetic salt are about 1K.
 The helium bath is surrounded by a liquid hydrogen or
liquid nitrogen shield to reduce the heat transfer from
ambient to the helium bath.
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 This entire assembly is placed between the poles of a
powerful electromagnet, which is shaped so that the field
of the magnet is concentrated around the salt pellet.
 This magnet field is turned on and maintained for
about an hour to allow the heat of magnetization
(similar to the heat of compression for a gas) to be
conducted to the helium bath by the gaseous helium in the
small chamber, thereby maintaining the salt at its original
temperature.
 When thermal equilibrium is attained, the gaseous helium
(which is called an exchange gas) is pumped away to
thermally isolate the paramagnetic salt.
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 The magnetic field is then removed, and the temperature
of the salt drops to a very low value. Temperatures as low
as 0.0014K have been attained by this method.
 This process of adiabatic demagnetization will work only
for very low temperature because of the magnitude of the
lattice thermal effects at temperatures much above 2K or
3K .
 The lattice entropy must be much smaller than the entropy
associated with the magnetic dipoles of the paramagnetic
material if a significant temperature change is to be
achieved.
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APPLICATION OF
ADIABATIC DEMAGNETIZATION PROCESS
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MAGNETIC REFRIGERATION SYSTEMS
(MAGNETIC REFRIGERATOR)
(UQ APRIL 2016, APRIL 2015, NOV 2015)
 The working medium is a paramagnetic material (iron
ammonium alum).
 The sequence of operations for the Magnetic Refrigerator
is as follows:-
 Process 1-2:- The magnetic field is applied to the
working salt while the upper thermal valve is open and
the lower thermal valve is closed. When the upper
thermal valve is open, heat may be transferred from the
working salt to the liquid helium bath, thereby
thermal valve between the working salt and
maintaining the salt temperature fairly constant. The
the
reservoir salt is closed so that heat will not flow back
into the low temperature reservoir during this process.79

 Process 2-3:- Both thermal valves are closed, and the
magnetic field around the working salt is reduced
adiabatically to some intermediate value. During this
process, the temperature of the working salt decreases.
 Process 3-4:- The thermal valve between the working
salt and the reservoir salt is opened, and the field
around the working salt is reduced to zero while heat is
absorbed isothermally by the working salt from the
reservoir salt.
 Process 4-1:- Both thermal valves are closed, and the
magnetic field around the working salt is adiabatically
increased to its original value.
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 The mass of the working salt is 15g and the helium bath
is maintained at a temperature of 1.11K. The sequence
of processes is carried out so that one cycle requires
about 2 minutes to complete.
 The working salt used in the magnetic refrigerator was
iron ammonium alum salt, and the reservoir salt (which
was used as a “thermal flywheel” to smooth out
temperature fluctuations in the space to be cooled) was
chromium potassium alum.
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 These salts have low thermal conductivities, therefore
heat transfer to and form the salts poses quite a problem.
Copper fins and 3mm lengths of fine copper wire
(0.05mm to 0.08mm diameter) were embedded in the
salt pellets to improve the heat transfer situation.
 About 1g of copper wire and 1g of silicone vacuum
grease were mixed with the 15g of paramagnetic salt,
and the pellet was formed by pressing under a pressure
of 20MPa.
 The vacuum grease acted as a binder and improved the
mechanical stability of the salt pellet.
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 One of the advantages of the magnetic refrigerator is
that it can operate effectively in zero gravity. Because of
this characteristics, magnetic refrigerators have been used
to cool infrared bolometers in space systems by NASA.
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THERMAL VALVES
 One of the critical components of the magnetic
refrigeration system is the thermal valves.
 In magnetic refrigerator, thin lead strips were used as
the thermal valves.
 The thermal valve is in the “open” position (heat flow
can take place) when it is driven normal by the valve
magnet (when valve magnetic field is applied).
 The valve is in the “closed” position (heat flow is
restricted but there is some leakage) when the valve
magnetic field is removed.
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