Note: Descriptions are shown in the official language in which they were submitted.
CA 02481477 2008-07-08
Loop-Type Thermosiphon and Stirling Refrigerator
Field of the Invention
The present invention relates to a loop-type thermosiphon and a Stirling
refrigerator using the same.
Background of the Invention
A heat sink, a heat pipe, a thermosiphon, or the like is used for cooling a
heat-
generating instrument or a thermoelectric cooling device. As for the heat
sink,
temperature distribution is caused in a base portion thereof provided with a
heat
source. Accordingly, as the distance from the heat source is increased, the
heat sink
contributes less to heat dissipation. Meanwhile, the heat pipe or the
thermosiphon
has high heat transfer capability, and is characterized by small temperature
variation
even when the heat is transferred to a portion distant from the heat source.
On the other hand, with regard to the heat pipe, vapor and liquid of a working
fluid flows in the same pipe. As such, if an amount of heat transfer is large,
a greater
number of pipes are necessary. For example, if it is assumed that the
temperature
difference is set to 5 C, a heat pipe having an outer diameter of 15.8mm and a
length
of 300mm attains an amount of heat transfer of approximately 100W. If the heat
should be ultimately emitted to an atmospheric environment, a heat pipe
including a
condensation portion having a large heat transfer area should be provided in
order to
exchange heat with air, because the heat transfer coefficient of the air is
low. A pipe-
shaped thermosiphon in which a liquid returns to an evaporation portion by
gravity
also has similar characteristics.
Meanwhile, a loop-type thermosiphon is also structured such that the liquid
condensed in a condenser returns to an evaporator by gravity. Here, however,
not
only the shape and the size of the condenser can be designed in accordance
with
cooling means of the condenser, but also the evaporator can be designed in
accordance with the shape and the size of the heat source. Therefore, two
pipes,
such as, a gas pipe and a liquid pipe connecting the condenser and the
evaporator
are enough in most cases. Here, it is natural that the condenser has to be
located
above the evaporator.
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In the loop-type thermosiphon, however, circulation flow rate is less likely
to be
stabilized and the temperature of the heat source tends to fluctuate in many
cases,
depending on the type of the contained working fluid or heat load fluctuation
in a
certain range. As is well-known, a CFC (chiorofluorocarbon) and an HCFC-based
refrigerant have been used as a working fluid or as a secondary working fluid
in
cooling equipment. The CFC-based refrigerant, however, is no longer used, and
the
use of HCFC-based refrigerants are restricted under international treaties for
protecting ozone layer. In addition, a newly developed HFC-based refrigerant,
though not destroying the ozone layer, is a potent greenhouse substance
attaining a
global warming coefficient several hundred to several thousand or more times
larger
than carbon dioxide, and subject to effluent control. Therefore, types of
refrigerants
that can be selected as a working fluid for the loop-type thermosiphon are
limited
from the viewpoint of environmental protection. Examples of environmental-
friendly
and what is called natural refrigerants include medium such as an HC-based
refrigerant, ammonia, carbon dioxide, water, and ethanol, and a mixture
thereof.
Japanese Patent Laying-Open No. 11-223404 discloses a method of cooling
the high-temperature portion of a Stirling cooler with a liquid of a secondary
refrigerant by means of a pump.
In the conventional loop-type thermosiphon, however, unstable circulation flow
rate of the working fluid has been likely, resulting in fluctuation of the
temperature of
the heat source. In particular, if the conventional loop-type thermosiphon is
operated
under a load far from the target load in accordance with design, the
temperature of
the heat source often fluctuates significantly. If the temperature of the heat
source
fluctuates significantly, not only with the performance of heat source
equipment
become unstable, but also the heat source equipment may be damaged.
Here, it is assumed that the loop-type thermosiphon is utilized for cooling
the
high-temperature portion of a Stirling cooler and the Stirling cooler is
mounted on a
refrigerator, for example. As is well-known, the heat load of the refrigerator
fluctuates
depending on the season. When the heat load of the refrigerator fluctuates,
the
amount of heat dissipation from the high-temperature portion of the Stirling
cooler
also varies. The loop-type thermosiphon often exhibits unstable operation
under
fluctuating heat load. Here, if the temperature of the high-temperature
portion of the
Stirling cooler fluctuates significantly, the influence therefrom is not
limited to
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fluctuation of a COP (Coefficient of Performance) of the Stirling cooler. If
the
temperature of the high-temperature portion is excessively high, the
regenerator of
the Stirling cooler may be destroyed.
Summary of the Invention
An object of the present invention is to provide a loop-type thermosiphon
capable of maintaining a stable temperature of a high-temperature heat source
in
spite of large fluctuation of heat load and a Stirling refrigerator equipped
with the
same.
In accordance with an aspect of the present invention, there is provided a
loop-type thermosiphon for transferring heat from a cylindrical high-
temperature heat
source having a cylindrical heat dissipation surface and extending in a
horizontal
direction using a working fluid, comprising an evaporator having a heat
absorption
portion attached to the heat dissipation surface of the high-temperature heat
source
and evaporating the working fluid by depriving the high-temperature heat
source of
heat through the heat absorption portion; a condenser located above the high-
temperature heat source and condensing the working fluid that has evaporated
in the
evaporator; and a gas pipe and a liquid pipe connecting the evaporator and the
condenser so as to form a loop; wherein an outlet of the liquid pipe is
positioned
adjacent to a top portion of the heat absorption portion that the working
fluid, that has
passed through the condenser and has been condensed is dropped, on the top
portion of the heat absorption portion.
A loop-type thermosiphon according to the present invention transfers heat
from a cylindrical high-temperature heat source using a working fluid. The
loop-type
thermosiphon includes: an annular evaporator having a heat absorption portion
attached to the high-temperature heat source and evaporating the working fluid
by
depriving the high-temperature heat source of heat through the heat absorption
portion; a condenser located above the high-temperature heat source and
condensing the working fluid that has evaporated in the evaporator; and a pipe
connecting the evaporator and the condenser so as to form a loop. The working
fluid
that has passed through the condenser and has been condensed is made to fall
on
the heat absorption portion.
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According to such an arrangement, the cooled and condensed working fluid is
preheated after falling on the heat absorption portion instead of being
directly
supplied to the liquid pool, and thereafter it is supplied from above by
gravity.
Accordingly, a flow is produced in the liquid pool and evaporation of the
working fluid
as a whole, including the working fluid in the liquid pool, is promoted.
Naturally,
evaporation of the working fluid that has been introduced and initially
exchanges heat
with the heat absorption portion is also promoted in an ensured manner,
whereby
temperature distribution in the high-temperature heat source can be uniform.
In
addition, separation of bubbles adhered to the heat absorption portion or the
like can
be promoted. Therefore, heat exchange adapted to fluctuation of the heat load
can
be performed, and the temperature of the high-temperature heat source can be
stabilized. In addition, as the high-temperature heat source has a cylindrical
shape
and the evaporator has an annular shape, an apparatus having a compact
structure
and ensuring heat exchange efficiency can readily be manufactured.
Brief Description of the Drawings
Preferred embodiments of the present invention will now be described, by way
of example, in conjunction with the following drawings, in which:
Fig. 1 illustrates a basic arrangement of a loop-type thermosiphon in a first
embodiment of the present invention;
Fig. 2 shows a variation of the loop-type thermosiphon in the first embodiment
of the present invention;
Fig. 3 shows a Stirling refrigerator in a second embodiment of the present
invention;
Fig. 4 shows stability of a temperature of a heat source when a loop-type
thermosiphon in a third embodiment of the present invention is employed;
Fig. 5 shows an arrangement of a general loop-type thermosiphon;
Fig. 6 shows an evaporator in a conventional loop-type thermosiphon; and
Fig. 7 shows fluctuation of a temperature of a heat source when the
conventional loop-type thermosiphon is used.
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Detailed Description of the Invention
Shown in Fig. 5, is a conventional loop-type thermosiphon which is structured
by connecting an evaporator 101, a condenser 103 and a gas-liquid separation
tank
106 using pipes 102, 104. A heat source 105 is cooled in evaporator 101.
Condenser 103 is provided above evaporator 101. The working fluid liquefied in
condenser 103 is separated into gas and liquid in the gas-liquid, separation
tank 106
provided between the condenser 103 and the evaporator 101. The liquid of the
working fluid transverses pipe 104 by gravity, and is introduced in the
evaporator 101
at a position in the lower portion of evaporator 101. In addition, the working
fluid that
has deprived the heat source of heat is vaporized in evaporator 101, and the
vapor of
the working fluid is introduced into condenser 103 through pipe 102 by a vapor
pressure difference between the condenser 103 and the evaporator 101. In most
cases, evaporator 101 is designed in accordance with the shape of the heat
source.
In Fig. 5, gas-liquid separation tank 106 is not essential.
Fig. 6 shows an evaporator for cooling the heat source of a conventional loop-
type thermosiphon having a cylindrical shape. Evaporator 101 has an annular
shape
in order to cool cylindrical heat source 105. Cylindrical heat source 105 is
fitted in a
hole of the evaporator 101, so as to be in close contact with a surface of the
hole of
the evaporator 101. The surface of the hole of the evaporator 101 is provided
with
an internal fin (not shown) for increasing the evaporation area. Liquid from
the
condenser runs through pipe 104 and flows into a liquid pool 121 through a
lower
portion of the evaporator 101, and the vapor exits from an upper portion of
the
evaporator 101 through pipe 102 and flows to the condenser.
Fig. 7 shows the temperature variation of the heat source variation in an
experimental operation of the loop-type thermosiphon employing the evaporator
and
the pipe arrangement shown in Fig. 6 and containing water as the working
fluid. If
amount of heat generation from the heat source is not larger than 75% of the
designed load, fluctuation in the temperature of the heat source results, as
shown in
Fig. 7. Improvement was not observed even when a contained amount of the
working fluid was changed.
In the following, embodiments of the present invention will be described with
reference to the figures.
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Fig. 1 is a conceptual diagram illustrating the basic arrangement of the loop-
type thermosiphon according to a first embodiment of the present invention.
The
loop-type thermosiphon shown in Fig. 1 is constituted of an evaporator 1, a
condenser 3, a gas pipe 2 extending from evaporator 1 to condenser 3, and a
liquid
pipe 4 extending from condenser 3 to evaporator 1. In the present embodiment,
since the high-temperature heat source 5 to be cooled has a cylindrical heat
dissipation surface as shown in Fig. 1, the evaporator has an annular shape
with a
circular hole having a dimension adapted to the cylindrical heat dissipation
surface of
the heat source. In addition, the surface of the hole of the evaporator 1 is
brought
into close contact with the cylindrical heat dissipation surface of heat
source 5 in
order to reduce thermal contact resistance. Condenser 3 is of a fin-tube type,
and
cools the working fluid flowing inside the pipe by flowing air around the
same.
The pipe 4 of the condenser for flowing the working fluid may be any of a
parallel flow type and a serpentine type. The condenser 3 is provided such
that an
inlet of a gas is located higher than an outlet of a condensed liquid. Gas
pipe 2
extending from evaporator 1 to condenser 3 has a larger diameter than liquid
pipe 4
extending from the condenser 3 to the evaporator 1. Therefore, gas pipe 2 has
a
flow resistance smaller than the liquid pipe 4, so as to prevent backflow of
the
working fluid and hard starting. The diameter of the liquid pipe is determined
based
on the designed heat load and thermal property of the working fluid. In order
to form
a thermosiphon, condenser 3 is located above evaporator 1.
In the present embodiment, pure water is contained as the working fluid.
Here, the contained amount is assumed to be the mass of the working fluid
which fills
1/3 to 2/3 of the total of a possible volume of liquid pool in the condenser 3
(for
example, a header pipe at an outlet of the condenser), the volume of the
liquid pipe
and the volume of the evaporator, and of which saturated vapor fills a
remaining
volume at an operating temperature. Such a contained amount allows smooth
operation of the working fluid.
As to the operation, as shown in Fig. 1, the water evaporates by depriving
high-temperature heat source of heat in evaporator 1. The vapor produced in
evaporator 1 runs through gas pipe 2 utilizing a vapor pressure difference
caused by
the temperature difference between condenser 3 and evaporator I and flows in
condenser 3, in which the vapor is deprived of heat by the air outside the
pipe and
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condensed. The liquid condensed in condenser 3 returns to evaporator 1 through
liquid pipe 4 by gravity. In this manner, the process circulation of the
working fluid,
heat absorption in the evaporator, and heat dissipation in the condenser is
repeated.
One feature of the embodiment of the present invention resides in introduction
of the liquid from the condenser through the upper portion of the evaporator 1
as
shown in Fig. 1, instead of introduction through the lower portion thereof
(see Fig. 5).
In the arrangement of the conventional loop-type thermosiphon shown in Figs. 4
and
5, a cold liquid is supplied to the lower portion of the evaporator 1.
Accordingly, the
temperature gradient in the liquid pooled in the evaporator 1 does not
considerably
affect the flow, without promoting evaporation. If the evaporator 1 operates
under a
condition far from the designed heat load, particularly under such a condition
as small
heat load, bubbles adhered to a heat transfer surface takes longer time to
form.
Then, the liquid is further pooled in the evaporator I and the bubbles are
less likely to
escape. As described above, in the conventional thermosiphon, significant
temperature fluctuation results in the heat source due to variation of
circulation flow
rate of the working fluid or suspension of circulation (see Fig. 7).
In the loop-type thermosiphon according to the embodiment shown in Fig. 1,
the liquid from the condenser 3 is introduced through the upper portion of the
evaporator 1, so that the supercooled liquid initially falls on the heat
absorption
portion at a high temperature or on a internal fin (not shown), on which the
liquid is
preheated. The internal fin is attached to the heat absorption portion and
formed
inwardly, so that evaporation of the liquid pooled in the evaporator 1 is
promoted. In
addition, when a colder liquid is introduced from above the liquid level in
the
evaporator 1, the liquid tends to move downward by gravity due to a difference
in
density. Then, the liquid in the evaporator 1 is stirred and evaporation is
promoted,
whereby the bubbles present on the heat transfer surface tend to be separated
and
destroyed. In this manner, the loop-type thermosiphon according to the present
embodiment can achieve a stable temperature of the heat source even under a
condition far from the designed heat load.
Though the gas-liquid separation tank is not provided in the loop-type
thermosiphon shown in Fig. 1, a gas-liquid separation tank 6 may be provided
between the condenser and the evaporator as shown in Fig. 2. It is noted,
however,
that the inner volume of the gas-liquid separation tank should be regarded as
a
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portion of the liquid pipe in determining the contained amount. Provision of
the gas-
liquid separation tank may be effective for attaining a stable operation of
the loop-
type thermosiphon.
Addition of ethanol to the water serving as the working fluid by not larger
than
60% can lower a tolerable temperature of an environment during operation or
transportation.
Figure 3 is a conceptual diagram of a Stirling refrigerator according to a
second embodiment of the present invention, provided with the loop-type
thermosiphon. The Stirling refrigerator in Fig. 3 is constituted of a Stirling
cooler
provided in a refrigerator main body 19, the loop-type thermosiphon attached
in order
to cool a high-temperature portion of the Stirling cooler, a low-temperature
side heat
exchange system transferring the cold of a low-temperature portion of the
Stirling
cooler to the inside of the refrigerator, the refrigerator main body, and the
like.
Though the low-temperature side heat exchange system is implemented by the
loop-
type thermosiphon, it is the loop-type thermosiphon not of interest in the
present
embodiment.
A Stirling cooler 11 having cylindrical high-temperature and low-temperature
portions is arranged on a back surface of the refrigerator 19. Evaporator 1 of
the
loop-type thermosiphon cooling a high-temperature portion 13 of the Stirling
cooler is
attached to and brought into close contact with the high-temperature portion
of the
Stirling cooler. In addition, the condenser 3 is placed on the refrigerator
main body
19 and evaporator 1 and condenser 3 are connected to each other by a pipe as
shown in Fig. 1, so that the loop-type thermosiphon in the present embodiment
is
mounted on the Stirling refrigerator. Liquid pipe 4 is inserted in evaporator
1 through
its upper portion. As a working fluid, pure water or a mixture of pure water
and
ethanol is contained.
The low-temperature side heat exchange system supplies the cold of a low-
temperature portion 12 of the Stirling cooler to the inside of the
refrigerator with a
refrigerator cooling apparatus 15 utilizing a secondary refrigerant.
Refrigerator
cooling apparatus 15 is provided by a cold-air duct in the refrigerator.
When Stirling cooler 11 operates, the temperature of high-temperature portion
13 of the Stirling cooler is raised. Then, the working fluid is heated and
evaporates in
evaporator 1 and flows in condenser 3 through gas pipe 2. At the same time,
outside
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air is introduced by the rotation of a fan 7, so that the gas of the working
fluid from
evaporator 1 is cooled and condensed in condenser 3. The working fluid
liquefied in
condenser 3 returns to evaporator 1 by gravity through liquid pipe 4 and an
introduction pipe 4a. When the liquefied working fluid returns to evaporator
1, the
working fluid comes in contact with a heat absorption portion 1 a and/or the
internal fin
(not shown) of the evaporator 1 so as to exchange heat. In this manner,
natural
circulation of the working fluid is attained and the heat of Stirling cooler
11 is
transferred to the outside air.
The operation of Stirling cooler 11 serves to lower the temperature of low-
temperature portion 12, and the secondary refrigerant in the heat exchange
system
flowing through the low-temperature portion is deprived of heat. On the other
hand,
the secondary refrigerant in the low-temperature side heat exchange system
absorbs
heat from the air inside the refrigerator in the refrigerator cooling
apparatus by
rotation of a cooling fan 16 on which a damper 17 is arranged. In the present
embodiment, the secondary refrigerant in the low-temperature side heat
exchange
system attains natural circulation by gravity. Alternatively, circulation may
naturally
be attained by circulation means using a pump. As described above, the cold of
Stirling cooler 11 is continuously provided to the air inside the
refrigerator.
In addition, drain water resulting from defrosting of refrigerator cooling
apparatus 15 is discharged from a drain water outlet 18.
Figure 4 shows temperature fluctuation of the high-temperature heat source
when a loop-type thermosiphon according to a third embodiment of the present
invention is employed. The loop-type thermosiphon in the present embodiment is
obtained merely by varying the manner of return of the liquid to the
evaporator in the
conventional loop-type thermosiphon shown in Fig. 6. In other words, the loop-
type
thermosiphon is structured such that the condensed working fluid is returned
so as to
contact the heat absorption portion which is not in contact with the liquid
pool, instead
of being directly introduced into the liquid pool.
The variation with time of the temperature of the high-temperature heat source
shown in Fig. 4 exhibits the effect obtained under the condition of when the
heat load
is the same as in the conventional loop-type thermosiphon. As compared with
the
large temperature fluctuation of the heat source in the conventional loop-type
thermosiphon shown in Fig. 7, stable temperature transition can be achieved.
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Examples, including those mentioned in the first to third embodiments of the
present invention, will comprehensively be explained, referring to the effects
of the
loop-type thermosiphon and the refrigerator in each embodiment of the present
invention.
In one embodiment of the present invention, a loop-type thermosiphon
transferring heat from a high-temperature heat source having a heat
dissipation
surface includes an evaporator depriving the high-temperature heat source of
heat, a
condenser arranged above the high-temperature heat source, and a pipe
connecting
the evaporator and the condenser so as to form a loop. The loop-type
thermosiphon
contains a working fluid, and drops the liquid of the working fluid from the
condenser
on a heat absorption portion when it is introduced into the evaporator, in
order to
exchange heat. Therefore, a loop-type thermosiphon is capable of maintaining a
stable temperature for the high-temperature heat source can be provided.
In addition, in one embodiment according to the present invention different
from that described above, an internal fin is provided in the heat absorption
portion in
the evaporator constituting the loop-type thermosiphon. The liquid of the
working
fluid condensed in the condenser is introduced into the evaporator through the
upper
portion thereof, in order that the liquid of the working fluid falls on the
heat absorption
portion or the internal fin in the evaporator. Here, the evaporator may have a
box-
shape, or may have an annular shape by combining semi-annular portions.
Alternatively, combination of portions of another shape may be employed. The
heat
absorption portion may be of a cylindrical shape or formed like a hole so as
to receive
the high-temperature heat source. According to the structure described above,
the
heat dissipation amount from an upper half of a cylindrical heat dissipation
surface of
the high-temperature heat source is not as large as that in a lower half
thereof.
Therefore, the liquid of the working fluid can be preheated and a uniform and
stable
temperature of the high-temperature heat source in the evaporator can be
achieved.
In an arrangement of a loop-type thermosiphon according to another
embodiment of the present invention, flow resistance of the gas pipe guiding
vapor
produced in the evaporator to the condenser is made smaller than that of the
liquid
pipe guiding the liquid condensed in the condenser to the evaporator.
According to
such an arrangement, backflow of the working fluid and hard starting in the
thermosiphon can be prevented.
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Moreover, in another embodiment of the present invention which is dependent
on the amount of heat transferred from the high-temperature heat source, the
flow
resistance of the pipe is made smaller if the amount of transferred heat is
large, and
it is made larger if the amount of transferred heat is small. If the diameter
of the pipe
is determined based on such an arrangement, more stable circulation flow rate
of the
working fluid can be achieved. Here, a reference value of magnitude
corresponding
to the amount of transferred heat, for example, may be set to 75% of the
designed
load. That is, if the amount of heat generation from the heat source is not
larger than
75% of the designed load, the flow resistance of the pipe is made larger, and
if it
exceeds 75%, the flow resistance of the pipe is made smaller. Alternatively,
another
reference value such as 50% of the designed load may be implemented.
In a loop-type thermosiphon according to another embodiment of the present
invention, the contained amount of the working fluid can be set to the mass of
the
working fluid of which fills 1/3 to 2/3 of the total, possible volume of
liquid pool in the
condenser at an operating temperature, the volume of the liquid pipe (the
pipe) and
the volume of the evaporator, and of which saturated vapor fills a remaining
volume
at the operating temperature. Accordingly, a disadvantage resulting from a
contained
amount of the working fluid can be eliminated.
A loop-type thermosiphon according to yet another embodiment of the present
invention employs a natural refrigerant such as carbonic acid gas, water,
hydrocarbon, or the like as the working fluid, and can provide an
environmentally-
friendly heat exchange process. Particularly when water is employed as the
working
fluid, a safe loop-type thermosiphon free from toxic or flammable properties
can be
obtained. Addition of ethanol by not larger than 60% can expand a range of
temperature in an environment in which the loop-type thermosiphon employing
water
as the working fluid can operate.
In a refrigerator equipped with a Stirling cooler employing the loop-type
thermosiphon according to any one of the embodiments of the present invention
described above, the evaporator of the loop-type thermosiphon described above
exchanges heat with the high-temperature portion of the Stirling cooler.
Specifically,
both of these components are brought into close contact with each other. In
addition,
the condenser can be arranged in a position higher than that of the high-
temperature
portion of the Stirling cooler of the refrigerator. According to such an
arrangement,
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even when the heat load of the Stirling refrigerator is varied, the Stirling
cooler can
achieve stable operation. In addition, as the working fluid achieves natural
circulation
by gravity, it is not necessary to provide a pump. Therefore, high reliability
and
efficiency can be effectively achieved.
The effects in each embodiment of the present invention have been
enumerated and explained. In the present invention, however, a loop-type
thermosiphon according to an embodiment covering a broadest scope does not
have
to attain all effects in each embodiment described above. The loop-type
thermosiphon in the embodiment covering the broadest scope should only achieve
a
stable operation adapted to fluctuation of load of the heat source.
Although the present invention has been described and illustrated in detail,
it is
clearly understood that the same is by way of illustration and example only
and is not
to be taken by way of limitation, the spirit and scope of the present
invention being
limited only by the terms of the appended claims.
Industrial Applicability
The loop-type thermosiphon according to the present invention can absorb
fluctuation of heat load of the heat source and attain a stable operation.
Therefore,
the loop-type thermosiphon described above is used for cooling the high-
temperature
portion of the Stirling cooler in the refrigerator employing as a cooling
apparatus the
Stirling cooler, without using CFC and free from greenhouse gas emission. The
loop-
type thermosiphon is expected to contribute to ensuring stable freezing
performance
throughout a year.
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