U.S. patent number 3,675,809 [Application Number 05/044,678] was granted by the patent office on 1972-07-11 for capillary insulation.
This patent grant is currently assigned to Martin Marietta Corporation. Invention is credited to John P. Gille, Jay L. McGrew.
United States Patent |
3,675,809 |
McGrew , et al. |
July 11, 1972 |
CAPILLARY INSULATION
Abstract
The internal capillary insulation comprises cellular material
secured to the internal wall of a vessel which is to contain liquid
having a boiling temperature lower than the ambient temperature of
the vessel. The cellular material disclosed provides a plurality of
discrete cells. Each cell provides for establishing a column of gas
therein between the tank wall and the liquid body. A capillary
cover substantially closes the liquid side of the cells and has at
least one capillary opening per cell designed to form a stable
capillary gas-liquid interface or membrane at the capillary
opening. The gas columns having a stable gas-liquid interface
insulate the liquid from the vessel and in addition, support the
liquid in the vessel thereby permitting fabrication of the
insulation from materials which have low strength and weight and
low thermal conductivity.
Inventors: |
McGrew; Jay L. (Littleton,
CO), Gille; John P. (Littleton, CO) |
Assignee: |
Martin Marietta Corporation
(New York, NY)
|
Family
ID: |
21933708 |
Appl.
No.: |
05/044,678 |
Filed: |
June 9, 1970 |
Current U.S.
Class: |
220/560.13;
220/560.14 |
Current CPC
Class: |
F16L
59/147 (20130101); F17C 3/06 (20130101); F16L
59/06 (20130101); F17C 2201/0128 (20130101); F17C
2203/03 (20130101); F17C 2203/013 (20130101); F17C
2223/0153 (20130101) |
Current International
Class: |
F16L
59/00 (20060101); F16L 59/06 (20060101); F17C
3/00 (20060101); F17C 3/06 (20060101); F16L
59/147 (20060101); B65d 025/18 () |
Field of
Search: |
;220/9LG,9D,10,9A,9B,15
;52/249,618 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
842,719 |
|
Jul 1960 |
|
GB |
|
102,073 |
|
Aug 1962 |
|
NL |
|
Primary Examiner: Leclair; Joseph R.
Assistant Examiner: Garrett; James R.
Claims
What is claimed is:
1. Insulation for reducing heat transfer from the surface of a
container for liquid to liquid being contained therein, comprising
first means adapted to be attached to an internal surface of the
container and providing a plurality of discrete cells adapted to
contain insulating gas, means closing the ends of said cells
adjacent the container surface, and second means for providing a
stabilized capillary gas-liquid interface closing the opposite ends
of said cells when in contact with the contained liquid with none
of the liquid penetrating the cells during steady state
conditions.
2. Insulation as defined in claim 1, wherein said first means
comprises structural material having thermal conductivity of 1 (Btu
- ft)/(ft.sup.2 -.degree.F-Hr) or less when measured at 70.degree.
F.
3. Insulation as defined in claim 1, wherein said first means
comprises a structure such that the ratio of cross sectional area
of the solid portion of the structure to the total area of the
surface being insulated is equal to or less than 0.09.
4. Insulation as defined in claim 1, further including means
associated with each cell for reducing free convection within each
cell.
5. Insulation as defined in claim 1, further including means
associated with each cell for reducing radiation in each cell.
6. Insulation as defined in claim 1 wherein the contained liquid
has a boiling temperature corresponding to pressure imposed on the
liquid below the container surface temperature.
7. Insulation as defined in claim 1 wherein the bulk of the
contained liquid is at a temperature below the boiling temperature
corresponding to the pressure imposed on the liquid and wherein the
cell is partially filled with a gas other than the vapor of the
contained liquid, so as to permit a stable liquid gas capillary
interface to be maintained at a temperature lower than the boiling
temperature corresponding to the pressure imposed on the
liquid.
8. Insulation as defined in claim 1, wherein said second means
includes a member capping said opposite ends of said cells and
having a capillary opening in communication with each cell.
9. Insulation as defined in claim 8, wherein the projections of the
edges of said member defining said openings includes angles.
10. Insulation as defined in claim 8, wherein said openings are
located relative to the portion of said first means that define
said cells, so that capillary ducting of the liquid into the cell
is prevented.
11. Insulation as defined in claim 8, wherein each portion of said
member defining said openings has a continuous curvature.
12. Insulation as defined in claim 11, wherein the maximum
dimension of said openings is determined by the smaller dimension
(d) provided by the following relationships:
d.sub.ft = (K.sub.1 S/RG).sup.1/2 (a)
where
K.sub.1 = 3.36 (a dimensionless, empirical constant for circular
holes)
S = surface tension of the liquid gas interface under operating
conditions (1b.sub.force / ft)
R = difference between density of the liquid and the density of the
gas (lb.sub.mass / ft.sup.3)
G = magnitude of the gravitational vector imposed by gravitational
attraction and/or acceleration of the container wall (no. of std.
earth gravity units with dimensions of lb.sub.force /lb.sub.mass
)
and
d.sub.ft = K.sub.2 S/RGH (b)
where
K.sub.2 = 4 (a dimensionless constant for circular holes)
H = maximum distance between any two openings in the same cell in
the direction of the gravitational vector (ft)
13. Insulation for reducing heat transfer from the surface of a
container to liquid contained therein, comprising means for
providing a plurality of discrete columns of gas between the liquid
and the container surface and second means for providing a stable
capillary interface between said gas columns and the liquid when
said gas columns are in contact with the liquid to support the
liquid away from the container while preventing the liquid from
penetrating the gas columns during steady state conditions.
14. Insulation as defined in claim 13, further including means
associated with each cell for reducing free convection within each
cell.
15. Insulation as defined in claim 13, further including means
associated with each cell for reducing radiation in each cell.
16. Insulation as defined in claim 13, wherein said means for
providing a plurality of discrete columns of gas includes cellular
material having a plurality of discrete cells or voids therein
within which the gas columns are provided.
17. Insulation as defined in claim 16, wherein said cellular
material has thermal conductivity of 1 (Btu - ft)/(ft.sup.2 -
.degree.F-Hr) or less when measured at 70.degree. F.
18. Insulation as defined in claim 16, wherein said cellular
material has a ratio of cross sectional area of the solid portion
thereof to the total area of the surface being insulated equal to
or less than 0.09.
19. Insulation as defined in claim 13, wherein second means
comprises a capillary cover which substantially closes the liquid
side of each cell, and capillary openings in said cover above each
cell to provide communication between the gas columns and liquid to
permit the stable capillary interface to be established.
20. Insulation as defined in claim 19, wherein said openings are
configured of a continuous curvature.
21. Insulation as defined in claim 20, wherein the maximum
dimension of said openings is determined by the smaller dimension
(d) provided by the following relationships:
d.sub.ft = (K.sub.1 S/RG).sup.1/2 (a)
where
K.sub.1 = 3.36 (a dimensionless, empirical constant for circular
holes)
S = surface tension of the liquid gas interface under operating
conditions (ob.sub.force / ft)
R = difference between density of the liquid and the density of the
gas (lb.sub.mass /ft.sup.3)
G = magnitude of the gravitional vector imposed by gravitational
attraction and/or acceleration of the container wall (no. of std.
earth gravity units with dimensions of lb.sub.force
/lb.sub.mass)
and
d.sub.ft = K.sub.2 S/RGH (b)
where
K.sub. 2 = 4 (a dimensionless constant for circular holes)
H = maximum distance between any two openings in the same cell in
the direction of the gravitational vector (ft)
22. A method for thermally insulating contained liquid to reduce
heat transfer from the container to the liquid comprising the steps
of providing a plurality of discrete columns of gas interposed
between the liquid and the container and providing the ends of the
gas columns in contact with the liquid with a stable capillary
interface with the liquid to support the liquid away from the
container while preventing the liquid from penetrating the gas
columns during steady state conditions.
Description
BACKGROUND
Cryogenic and low temperature boiling point liquids have been used
extensively by aerospace industries and are gaining in application
in other industries. Examples of useful liquids are liquefied
natural gas and liquid nitrogen, hydrogen, and oxygen. These
liquids have several attractive features which contribute to their
gain in application. However, the attractive features are offset by
the problems associated with storing these fuels.
Insulation problems associated with storing low temperature liquid
must be solved before widespread use can be made of these low
temperature liquids. Specifically, the insulation must be thermally
effective, relatively lightweight, inexpensive, and reliable over
long periods, to mention a few. The present invention provides an
insulation which provides the aforementioned features.
Vacuum jacket-type insulation is thermally the most effective
insulation developed to date for insulating cryogenic and low
temperature liquids. Basically, this insulation is provided by a
container having inner and outer spaced walls and a vacuum is
established between the walls. Such insulation has a very low
thermal conductivity and consequently is a good insulator. However,
the vacuum jacket-type insulation, even though effective as an
insulator, has many problems, such as, making the walls air-tight
so that a vacuum can be maintained without leakage, supporting the
double wall tank, using special material for the inner tank to
enable it to withstand the stress induced therein as a result of
the large temperature differential on opposite sides thereof, and
expending a large part of the liquid to be stored to cool down the
inner tank wall. Hence, the vacuum jacket insulation has limited
application because of the cost associated in overcoming these
problems.
The present invention has for its objectives to provide an
insulation which is lightweight, inexpensive, reliable, effective
to insulate the liquid from the warmer environment it will
experience during storage, which does not rely on vacuum and
requires a minimum amount of stored liquid to cool down the
insulation.
These objects are realized by the present invention by interposing
and maintaining a plurality of discrete columns of gas between the
wall of the tank to be insulated and the liquid stored therein. The
gas columns are defined by lightweight cellular material having a
low thermal conductivity and which can be of low strength because
it is not required to support the liquid. The support function is
assumed by the tank wall through the gas columns.
The discrete gas columns are established by gas trapped in voids in
the cellular material and are maintained by utilizing surface
tension herein referred to also as capillary forces which cooperate
according to the present invention to establish a stable liquid-gas
interface at the liquid end of the gas columns to prevent the
liquid from penetrating the gas columns.
The stable capillary interface can be characterized as a membrane
stretched across the top of the gas columns where the columns come
into contact with the liquid and provide a physical barrier which
prevents the liquid from penetrating the gas columns. Surface
tension is that property that causes the interface to behave as a
stretched membrane. A stable membrane permits the pressure of the
gas to in effect push against the liquid to support the liquid
independent of the cellular material and cover while insulating the
liquid from the warmer tank wall.
Establishment of a stable membrane is dependent on a number of
parameters such as the diameter of the opening which provides for
communication between the gas column and liquid, the contact angle,
surface tension and density of the liquid, and gravity. When the
relationship between these parameters if controlled, it has been
discovered that a stable membrane can be provided and maintained so
that the gas columns can function to insulate and support the
liquid from the tank wall. Furthermore, when the relationship
between the parameters is so controlled, the stable membrane is
independent of its orientation relative to the gravity vector.
To better understand the effect the above mentioned parameters have
on establishing and maintaining the stable capillary membrane at
each gas column, consider the following example. It can be observed
that when a finger closes one end of a soda straw immersed in
liquid and the straw is removed, the liquid is suspended in the
straw. This is due to the fact that a stable membrane is
established so that the atmospheric pressure can act on the exposed
end of the liquid and exert sufficient force on the liquid to
support the liquid. Atmospheric pressure is sufficient in this
example to support the liquid column because a partial vacuum is
established in the straw by closing off one end which contributes
to the support of the liquid.
When the finger is removed, the partial vacuum is replaced by
atmospheric pressure which acts along with gravity to overcome the
atmospheric pressure supporting the liquid in the straw causing the
liquid to drain from the straw. Notwithstanding the fact that a
pressure differential contributed to the support of the liquid in
the example, no quantity of air pressure will support the liquid in
the absence of a stable membrane because there is nothing for the
pressure to act against.
To demonstrate this latter statement, consider a second example
where the same procedures of the first example are followed except
that the straw selected has a diameter several magnitudes greater
than the ordinary soda straw. The same pressures are acting on the
liquid but the liquid drains from the larger straw even with one
end closed.
The reason for the different result is that a stable membrane does
not form in the second example so that the atmospheric pressure has
nothing to act against. The larger straw diameter limited the
membrane to a small rate of curvature in order to span the
diameter. The smaller the rate of curvature of the membrane, the
weaker the membrane. Conversely, the greater the rate of curvature
of the membrane, the stronger the membrane. When the rate of
curvature of the membrane is small as in the second example, the
membrane is not stable and gas flows up the tube while liquid flows
down until the tube is drained.
The parameters controlling the maximum diameter across which a
stable membrane can form are the surface tension and density of the
liquid, magnitude of the gravitation field, and the contact angle
of the liquid. The contact angle of the liquid is the angle formed
through the liquid at the line of intersection of liquid, gas and
solid. The contact angle depends on the particular liquid, gas and
the material of the solid surface involved.
PREFERRED EMBODIMENT OF THE INVENTION
Having described the background of the invention, a preferred
embodiment of the invention will be described in conjunction with
the drawing in which:
FIG. 1 is a side elevational view of a tank having a portion cut
away to expose the insulation according to the present
invention;
FIG. 2 is a perspective view of the insulation of FIG. 1 with parts
broken away to more clearly illustrate details of construction of
the insulation;
FIG. 3 is a cross sectional view taken approximately along the line
3--3 of FIG. 2 but on a larger scale showing the stable capillary
interface of the liquid and gas;
FIG. 4 is a cross sectional view of one cell of the insulation
illustrating packing means for the cell to reduce gas convection
and radiation within the cell; and
FIG. 5 is a view similar to FIG. 4 but illustrating still another
packing means for the cell for reducing convection and more
effectively reducing radiation through the gas column.
Referring to the drawings and initially to FIG. 1, a tank indicated
generally as 10 is disclosed for storing low temperature boiling
point liquids 12. The liquid 12 may be liquefied natural gas or
liquid nitrogen, oxygen, hydrogen, etc. The tank 10 has internal
insulation 16 applied to the interior surface 11 of the tank 10, in
any appropriate manner such as by bonding, etc. The bonding agent
must be effective to bond the particular material selected for the
insulation and tank and be compatible with the liquid to be
stored.
The insulation 16 includes a cellular structure 18 best illustrated
in FIG. 2 which may be fabricated of any lightweight material which
is compatible with the liquid being insulated and has a low thermal
conductivity, such as plastic. It has been discovered that
materials having thermal conductivity of 1(Btu -ft)/(ft.sup.2
-.degree.F-Hr) or less are suitable. Examples of some plastic
materials which comply with this criteria for use as the cellular
structure 18 are polyimide, Mylar, Nomex, nylon, and plastic
impregnated Kraft paper. Studies to date indicate polyimide is a
preferred material for high tank temperature applications (up to
700.degree. F) and Mylar is a preferred material for lower
temperature applications. Both have excellent low temperature
compatibility, low thermal conductivity, high shear strength,
ductility at low temperature, and commercial availability.
The cellular material 18 can be of various geometric forms but the
illustrated embodiment comprises a honeycomb structure defining a
plurality of hexagonal cells 20 therein.
In the illustrated embodiment, the cells 20 are closed on one side
by the inner surface 11 of the tank wall. However, in certain
instances it may be desirable to bond the cellular material to
another member which in turn is secured to the tank wall.
The cells 20 are substantially closed on the liquid side by a
capillary cover 22. Cover 22 may be made from material such as
1-mil Mylar film, plastic impregnated fiberglas cloth, or 1-mil
Kapton film. However, Mylar film is preferred because of its
strength, ductility, and flexibility at very low temperatures. The
cover 22 is secured to the liquid side of the cellular material 18
in any suitable manner consistent with the intended use such as by
adhesive bonding.
The cellular material 18 cooperates with cover 22 and the inner
surface 11 of tank 10 to define substantially confined areas or
voids designated herein as cells 20 within which gas can accumulate
and form a plurality of discrete gas columns extending between the
surface 11 of tank wall 10 and the liquid 12 being insulated. It is
very important to the present invention to avoid communication
between cells because such communication would destroy the
integrity of the gas columns.
Each cell 20 communicates with the liquid 12 through at least one
capillary opening or hole 24 provided in cover 22. The opening 24
is generally smaller than the width of its associated cell and is
sized to permit a stable capillary interface or membrane 26, best
illustrated in FIG. 3, to form at the interface of the gas columns
and liquid 12. Membrane 26 prevents liquid 12 from penetrating the
gas columns in cells 20 so that the gas columns remain intact to
function as insulators for the liquid. Also membranes 26 provide
surfaces against which the pressure of the gas columns can act to
support the liquid 12 apart from the insulation 16. Ultimately, the
liquid 12 is supported by tank 10 through the gas columns. Hence,
the insulation 16 can be constructed of relatively low strength
material since it is not required to bear substantial loads.
Holes 24 can be provided by selecting material for cover 22 which
has openings therein of the required size and distribution. Woven
filter cloth and screen are examples of this type of material. Care
must be taken when using this type of cover to assure proper
distribution of the openings relative to the cells as will be more
fully described hereafter. For this reason, it is preferable to
select a material such as Mylar which is not porous and physically
make the appropriate sized holes therein after the cover 22 has
been bonded to the cellular material 18. This assures that the
proper number of openings are provided at the proper location
relative to the cells. The holes can be formed through the cover
approximately at the cell centers by use of an appropriate
tool.
The location of the hole 24 relative to the cells 20 and the shape
and diameter of the holes are parameters which are critical to
establishing the stable membrane 26. Each hole 24 must be located
relative to its associated cell so that it is not adjacent to one
of the cell's corners. If a hole is located over a corner there is
a danger that a wicking condition will develop causing the liquid
to be drawn or wicked along the corner into the cell and gas
column. When liquid enters the cell the thermal conductivity of the
cell increases.
Holes 24 preferably are circular or have a continuous curvature. A
sharp angle in the surface of the material defining the hole is
permissible, but will require a smaller maximum hole size.
The size of holes 24 is very important in establishing the stable
membranes 26. It has been discovered that the largest permissible
circular hole diameter (d) which will establish and maintain stable
membranes is the lesser diameter derived by the following
relationships:
d.sub.ft = (K.sub.1 S/RG).sup.1/2 (a)
where
K.sub.1 = 3.36 (a dimensionless, empirical constant for circular
holes)
S = surface tension of the liquid gas interface under operating
conditions (1b.sub.force/ ft)
R = difference between density of the liquid and the density of the
gas ( 1b.sub.mass / ft.sup.3 )
G = magnitude of the gravitational vector imposed by gravitational
attraction and/or acceleration of the container wall (no. of std.
earth gravity units with dimensions of 1b.sub.force / 1b.sub.mass
)
and
d.sub.ft = K.sub.2 S/RGH (b)
where
K.sub.2 = 4 (a dimensionless constant for circular holes)
H = maximum distance between any two openings in the same cell in
the direction of the gravitational vector (ft)
The smaller diameter obtained from these relationships defines the
ideal maximum hole size for the cells. In practice a hole diameter
is selected which is smaller than the hole diameter provided by the
above relationship to provide a safety factor.
Consider a typical example for determining the maximum diameter
hole which would provide a stable capillary gas-liquid interface
according to the foregoing relationship. Liquid nitrogen is to be
stored at local atmospheric pressure (14.7 psia) in a stationary
storage vessel. The saturated or boiling temperature of the liquid
nitrogen under the foregoing conditions will be approximately
-320.degree. F. At this temperature and pressure, the liquid
density is approximately 50.4 lb.sub.mass /ft.sup.3, the density of
the nitrogen vapor is approximately 0.28 lb.sub.mass /ft.sup.3 and
the liquid-gas surface tension is approximately 0.000597
lb.sub.force /ft.
The maximum permissible diameter of a circular hole at which the
capillary interface is established, is as follows:
dia.sub.ft = (K.sub.1 S/RG ).sup.1/2
where
K.sub.1 = 3.36
s = 0.000597 lb/ft
R = 50.4 - 0.28 = 50.12 lb.sub.mass /ft.sup.3, the difference in
density between liquid and gaseous nitrogen at the operating
conditions (Note: for many cases, including this one, the gas
density can be considered to be negligible without significant
consequence)
G = 1 standard earth gravity unit, since the vessel is stationary
and not subject to significant vibration or acceleration.
Thus,
dia.sub.ft = (3.36 .times. 0.000597/50.12 .times. 1).sup.1/2
=0.00632.sub. ft
or 0.0758 in.,
which is the maximum hole diameter if only a single opening is
provided for each cell.
If, as a second example, two holes per cell are to be used for the
possible purpose of providing redundancy in case one of the holes
in any cell should become plugged, a further calculation is
required. Assume that the maximum vertical separation between the
two holes in any cell is controlled to one-half inch, or 0.0208
ft., then the maximum hole diameter is also limited by the
condition of Equation (b):
dia.sub.ft = K.sub.2 S/RGH
where
K.sub.2 = 4
h = 0.0208 ft. the maximum separation between any two openings in
the same cell in the direction of the gravity vector, i.e., in the
vertical direction for a stationary vessel.
thus
dia.sub.ft = (4 .times. 0.000597/50.12 .times. 1 .times. 0.028) =
0.0023 ft or 0.0263 in
Since the smaller diameter obtained from the two equations
establishes the maximum hole size, the hole diameters must be
0.0023 ft or smaller.
It should be apparent from the foregoing description that the
insulation 16 is effective to insulate the liquid 12 from the tank
wall 11. The heat gradient which is indicated by the arrow marked H
on FIG. 2 is from the tank wall 11 to the liquid 12, and
consequently, the insulation is effective for maintaining the
liquid 12 at a temperature below the ambient temperature of the
tank 10.
It should also be apparent due to the nature of the cellular
material 18 and cover 22, that the weight of the insulation for a
given surface to be insulated is very small. The disclosed cellular
material 18 is of a honeycomb configuration which is available
commercially in sizes as small as 35 cells per foot weighing
approximately from 1.9 to 2.5 pounds per cubic foot to nine cells
per foot weighing less than 1.5 pounds per cubic foot. At the
present time, honeycomb having nine cells per foot has been found
to be satisfactory for purposes of the present invention. However,
it should be understood that various materials and cell
configurations could be used for the cellular material as long as
they satisfy the criteria required of the cellular material.
The cellular material selected should be strong enough to withstand
the shear loading, thermal stresses, and the small loads due to the
pressure acting on the cover. However, these loads are very small
and the structural requirements imposed during fabrication and
installation of the insulation may be more significant and may
ultimately dictate the structural requirements.
It has been discovered that the low structural requirements imposed
on the cellular material permit it to be constructed from a
material having a ratio of the area of the solid material taken in
a plane through the cellular material parallel to surface area 11
being insulated to the surface area 11 insulated thereby of 0.09 or
smaller ratio. The small amount of material required also reduces
the thermal conduction through the solid material and consequently
reduces the thermal conductivity of the insulation. Further, the
small thermal mass of the cellular material and cover requires a
minimum of liquid boil-off to cool the insulation to the operating
temperatures. Liquid boil-off cools the insulation by a process
similar to the evaporative refrigeration process. From the
foregoing, it should be apparent that the insulation is
lightweight, has thermal conductivity approaching that of the gas
columns in the cell, and requires small boil-off losses to cool
down.
A tank 10 having the insulation 16 applied thereto as disclosed in
FIG. 1 will be filled through a conventional cryogenic opening or
inlet 30 disclosed on the top of tank 10. The tank 10 can be
prepared prior to filling by purging with a suitable gas compatible
with the liquid 12 such as nitrogen or helium. The purging gas
assures that all undesirable elements such as water vapor are
displaced by the purging gas. It has been discovered that the gas
in cells 20 will be replaced by the purging gas due to diffusion
through openings 24. Hence the cells 20 will be initially filled
with the purging gas.
After the purging process has been completed, liquid is directed
into the vessel 10 through inlet 30. The liquid 12 contacts the
capillary cover 22 which cools and contracts the gas columns in
cells 20 and permits in most instances a small amount of liquid to
enter the cells. The liquid in cells 20 vaporizes and increases the
pressure in the gas columns until sufficient liquid vaporizes to
equalize the pressure of the gas columns with the pressure of the
liquid which permits the establishment of the stable capillary
membrane 26. The membrane 26 can form at various locations relative
to the openings 24 which positions are illustrated in dotted lines
in FIG. 3. As long as the conditions remain relatively constant,
the gas columns will insulate and support the liquid 12. If the
tanks were to be depressurized after the stable membranes 26 are
established, gas would bubble from the cells 20 to establish a new
state of pressure equilibrium. A pressure increase in the tank
results in a sequence of events similar to those described during
initial cool-down of the system. At no time during normal operation
does the pressure difference across the cover 22 exceed that due to
flow loses through the capillary opening 24, which can be sized to
limit this pressure difference to a small value. Thus, the cells
can be characterized as being self-restoring when liquid enters
them which contributes to the insulation being reliable over long
durations.
Immediately following filling of the tank with a liquid, the cells
20 will be filled with a mixture of the purged gas and vapor of the
contained liquid. The purge gas will ultimately dissipate and the
cells 20 will be totally filled with the vapor of the contained
liquid.
The present invention contemplates providing packing means in each
cell 20 to reduce the convection and radiation within cell 20. This
means is illustrated in FIGS. 4 and 5. In FIG. 4 packing means 31
comprises a filler 32 which is loaded into the cell to fill the
cell prior to placing the cover 22 thereon. The filler 32 must be
such that it will not prevent pressure equalization to occur
throughout the cell, be lightweight, inexpensive, and have low
thermal conductivity. Examples of suitable fillers would be loose
polystyrafoam chips, rock wool batting, Fiberglas, ceramic felt,
shredded paper, and expanded perlite.
FIG. 5 discloses a modified cell packing 34 which includes a
combination of filler 32 sandwiched by a reflective foil material
38 such as aluminum foil, aluminized Mylar, or aluminized Kapton.
The filler 32 reduces the convection while the foil 38 reduces
radiation through the cells.
Another means effective to reduce convection and radiation in cell
20 is to making the dimension across the cell very small in
relation to the length of the cell. However, this approach
increases the weight and thermal conduction through the cellular
material.
It should be apparent that the present invention provides an
effective insulation which satisfies the objects set forth
heretofore namely light in weight, reliable due to the
self-restoring feature provided thereby, easily applied and
utilizes minimum amounts of stored liquid to cool down the
insulation, all of which contribute to an economical insulation for
low temperature liquid.
* * * * *