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I
NBSIR 73-308

Insulation of Liquid Oxygen Dewars

C. F. Sindt

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

April 1973

Final Report

Prepared for

Naval Air Engineering Center

Philadelphia, Pennsylvania 19112
J,
NBSIR 73-308

INSULATION OF LIQUID OXYGEN DEWARS

C. F. Sindt

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

April 1973

Final Report

Prepared for

Naval Air Engineering Center

Philadelphia, Pennsylvania 19112

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

CONTENTS

Page

1. 0 Introduction . , o 1

2. 0 Experimental Apparatus and Procedure 2

3. 0 Experimental Dewars , 4

3, 1 Foam Insulated Dewars . 7

3. 2 Glass Bubble Insulated Dewars 22

4. 0 Proposed Techniques for Extending Service Life of

Dewars , . 28

4. 1 Techniques for Existing Liquid Oxygen Dewars . 2 8

4. 2 Techniques for New Liquid Oxygen Dewars . . 3 5

5. 0 Recommendations for Further Investigation of

Insulation for Liquid Oxygen Dewars 3 6

6. 0 References 38

iii
LIST OF FIGURES

Page

Figure 1. Schematic of Experimental Apparatus .... 3

Figure 2. Schematic of Dewars Insulated With Foam ... 9

Figure 3, Voids in Foam Insulation at the Dewar Neck . , H

Figure 4, Frost Formation on the Foamed Dewar

Failure 13

Figure 5, Crack in Foam Insulation 14

Figure 6. Thermal Conductivity Versus Temperature for

Polyurethane Foam 15

Figure 7, Failure of Foam at the Dewar Neck 17

Figure 8. Foam Insulated Dewar With a Repair of the

Neck 18

Figure 9. Schematic of Dewars Insulated With Glass

Bubbles 24

Figure 10. Thermal Conductivity Versus Temperature for

Glass Bubble Insulation 26

Figure 11, Performance of Spherical Dewars Insulated With

Glass Bubbles 30

Figure 12. Performance of Cylindrical Dewars Insulated

With Glass Bubbles 31

Figure 13. Performance of Spherical Dewars Insulated

With Polyurethane Foam 33

Figure 14. Performance of Cylindrical Dewars Insulated

With Polyurethane Foam . 34

iv
LIST OF TABLES

Page

Table 1, Average Thermal Conductivity of Foam

Insulated Dewars 20

Table 2. Properties of Foam Insialation 21

Table 3. Average Thermal Conductivity of Glass

Bubble Insulation 28

V
ABSTRACT

The Navy has experienced failure of vacuum insulation in dewars used for storage and handling of liquefied

breathing oxygen for aircraft pilots. Because of the vacuum insulation failures, a search was made for a more

rugged insulation that has thermal performance similar

to the currently used vacuum with multilayer or powder.

No system was found that compared in thermal performance and did not require a vacuum. Two systems were

experimentally evaluated that did not require vacuum.

One was polyurethane foam with an intermediate fiber

glass shell and the other was glass bubbles in argon gas

at one atmosphere pressure. The polyurethane foam

system was successful in that no cracks penetrated to

the outside surface; however, the average thermal conductivity was 160 ji W/cm-K which is about 15 times

greater than vacuum and powder. The glass bubbles in

argon gas was also successful since the argon gas pressure always remained high enough to prevent air and

moisture from entering the insulation through small

leaks in the outer shell. The thermal performance

was poorer than the polyurethane foam. The average

thermal conductivity was ZIZ W/cm-K or about 20

times greater than for the same glass bubbles in a vacuum.

Key words: Cryogenic insulation; insulation; LOX dewars

microspheres; polyurethane foam.

vi
INSULATION OF LIQUID OXYGEN DEWARS

Charles F. Sindt

1. 0 Introduction

Breathing oxygen for the crews of military aircraft is supplied

by vaporizing liquid which is stored on board the aircraft in a cryogenic

container. These cryogenic liquid oxygen containers (converters) are

filled prior to each flight by either removing the oxygen supply unit for

filling, or in the earlier models, by filling the unit in place. The aircraft containers are filled from portable dewars ranging in size from

3 3

0. 19 m (50 gallon) to 1. 9 m (500 gallon) capacity. The portable dewars

are used to transport the liquid oxygen from a larger storage dewar to

the flight deck or flight line to service a number of aircraft. The large

3

storage dewars are usually about 15 m (4000 gallon) capacity and are

serviced from a transport truck or ground based liquefier or from a shipboard liquefier on the aircraft carriers.

All of the current liquid oxygen containers require vacuum in the

insulation spaces. The airborne containers are insulated with vacuum

and some also include alternate layers of good reflectors separated by

poor conductors commonly referred to as multilayer (MLI) or super (SI)

insulation. Either system requires a vacuum of 1 mN/m for best

performance. The portable vessels are also vacuum insulated, some

with MLI and some with powder in the insulation space. The powder is

usually perlite. Powder and vacuum insulation is always used in the

large storage dewars. Powder systems also require a good vacuum,

2

about 0. 15 N/m or less.

^ 2 2 2

1 mm Hg = 133. 32 N/m ; 1 atm = 101 325 N/m ^ 101 kN/m .
In the past, the Navy has experienced a significant failure rate in

the vacuum insulation in all dewar sizes. Vacuum failures usually require

extensive repair by highly skilled craftsmen using specialized equipment.

Therefore, since vacuum failure usually renders the cryogenic vessel

unusable and vacuum failures normally require repairs that cannot be

made in the field or on shipboard, a failure may result in loss of the

equipment.

Because of the number of failures occurring in the vacuumjacketed vessels, the Naval Air Engineering Center has requested a review of insulation systems to find a system that is more reliable and

less fragile and, if feasible, a method of repairing existing dewars.

This report covers the experimental program to evaluate several

candidate insulation systems and resiilting recommendations,

2. 0 Experimental Apparatus and Procedure

The experimental apparatus for measuring the average thermal

conductivity and the temperature profiles in the evaluated insulation

systems is shown schematically in figure 1. The gas vent from the

experimental vessel was connected to a barostat which maintained the

2

vessel pressure constant to ± 260 N/m . The discharge of the barostat

flowed through a dry gas meter which measured the volume of gas vented.

Instrumentation for the experiment consisted of temperature

measurements in the insulation, in the liquid, and in the venting gas at

the dry gas meter. Temperatures were measured with type T thermocouples with the reference thermocouple in the bottom of the experimental vessel. Pressure measurements were taken at the dewar vent

and at the inlet to the dry gas meter. Pressures were measured vdth

variable reluctance type pressure transducers. The gas flow measurement was made using a dry gas meter which gave an electrical pulse

2
Barostat

7

Dry Gas Meter

EXPERIMENTAL DEWAR

Figure 1. Schematic of experimental apparatus.

3
for each 2, 83 x 10 m of gas flow. The signals from the thermocouples, the pressure transducers, and the dry gas meter were automatically interrogated and recorded with a computerized data acquisition

system. Data reduction was performed on-line by the computer, and

temperatures, pressures, and total heat leak to the experimental vessel

were printed out at the desired time intervals. The overall accuracy

of the heat leak measurement is estimated to be within ± 4%.

The experimental procedure started by filling the experimental

vessel with liquid nitrogen and connecting the vent to the barostat. The

barostat was set to maintain the desired pressure. The computer was

then started and data were taken at 1 or 2 hour intervals. When the

heat leak was nearly constant for several hours, thermal stability was

assumed and the test was terminated. For the vessels tested, the time

to reach stability varied from 12 to 36 hours.

, 3. 0 Experimental Dewars

A search of the literature for insulation schemes that would be

applicable for liquid oxygen did not reveal any system that was as good

as vacuum insulation systems. No systems were discovered that were

rugged and that approached the performance of the currently used vacuum and MLI or vacuum and powder. One article was published during

the latter stage of our work that described a system that was comparable

in performance to some of the evacuated powders and appeared to be

more rugged as well as compatible with the environment on aircraft

carriers [Haynes, 1972]. The system consisted of polyurethane foam

with fiber glass reinforcement. The only information given on the

thermal insulation performatnce was that the nominal thermal conductivity between 110 K and ambient temperature was 14 u,W/cm-K.

This system had not been put into the intended liquefied natural gas

service at the time the article was printed.

4
A second application using urethane foam as an insulation at

cryogenic temperature was reported by F. Mac and M. Smith [1972],

This application with liquid hydrogen and that reported with liquefied

natural gas indicated that the urethane foam may be a good candidate

for a rugged insulation that would not be susceptible to failure because

of small shallow punctures. Also, the urethane foam would be easily

repaired in the field or onboard ship.

For our tests, a urethane foam was selected that was very similar

to that used in the liquid hydrogen application; two significant differences

were that the selected foam was a pour-type foam instead of a spray-on

type foam and the thickness of the foam was 9. 5 cm instead of the 1, 9 cm

used in the liquid hydrogen application. The pour-type foam was selected

for two reasons. First, special spray equipment was not required for

application and second, if successful, the foam would be especially

attractive to use for repairing existing dewars as well as repairing new

foam insulated dewars.

The second insulation method selected for evaluation was hollow

glass microspheres (bubbles) similar to those described by Cunnington

[1972], The glass bubbles selected were not aluminum coated as the

current cost and supply of coated bubbles was not well established and

availability for experimental use was on a long term delivery. The

glass bubbles were selected as they did represent a new insulation

material and, also, reported performance was good. Since the reported

performajice was in a vacuum environment and was similar to perlite

in a vacuum, the decision was made to measure performance of the

glass bubbles in gases at one atmosphere pressure. The reasoning

was that under one atmosphere pressure, no significant insulation

degradation due to vacuum leaks would occur while the vessel sat idle

and warm. The gases selected for the insulation environment were

nitrogen, argon, and carbon dioxide,

5
The glass bubbles selected contain a gas that condenses and

freezes at liquid oxygen temperatures. Therefore, the thermal conductivity of the colder glass bubbles and the gas surrounding them near

the inner dewar wall should be nearly the same as the thermal conductivity of the surrounding gas since the glass bubbles have thin walls

and point contact. The wall thickness of the glass bubbles is estimated

to be 1 |im from the manufacturer's data which specify diameters

3 from 60 to 120 |j,m, bulk density of 0. 076 g/cm , and true density

3

of 0. 155 g/cm . The nitrogen, argon, and carbon dioxide gases were selected

because they are all readily available, and they are inexpensive. Argon

was selected because the thermal conductivity of argon gas in the temperature range of 90 K to 300 K is lower than any other fluid that remains

gaseous in this temperature range and at one atmosphere pressure.

Carbon dioxide was selected because it was a gas that was readily

available and inexpensive, and because it would freeze out on the dewar

walls at liquid oxygen temperatures. The liquid oxygen temperature is

105 K at the maximum dewar operating pressure of 374 kN/m . Carbon

2

dioxide has a vapor pressure of less than 0. 13N/m atl05K and the

pressure drops rapidly with lower temperatures [Honig and Hook, I960].

This pressure is compatible with that required for powder or glass

bubbles to give maximum insulation.

A second reason for selecting the glass bubbles was that handling

may be easier than perlite or other fine particle powders simply

because of the spherical shape.

Performance tests were conducted on each of the dewars. Test

data consisted of temperatures at radial distances 0. 65 cm apart in the

insulation, dewar pressure, gas meter pressure and temperature, and the

heat leak of the dewar. The heat leak for all of the tests was calculated

6
from the volume of boil-off gas, from the gas temperature and pressure

at the gas meter inlet, and from the calculated heat of vaporization of

the liquid nitrogen. The nitrogen in the dewar was assumed to be at

constant temperature and in thermal equilibrium with the ullage pressure.

The heat leak of the dewar was measured after a minimum of 12 hours

thermal stabilization time and the inner vessel was made of copper;

therefore, the assumption of equilibrium conditions in the dewar is

substantiated.

The thermocouple used as a reference for all temperature measurements was located at the bottom of the experimental dewar. With thermal

eq\iilibrium in the dewar, this temperature must be within 0. 3 K of the

liquid saturation temperature as calculated from the measured dewar

pressure.

3. 1 Foam Insulated Dewar s Four dewars were made using foam insulation. A total of nine

tests were conducted on these dewars. All of the foam dewars were

made using 30. 5 cm diameter copper hemispheres for the cold vessel.

These hemispheres were soldered together and a 1. 27 cm diameter tube,

45 cm long, was soldered to a center hole in the upper hemisphere. This

tube was used for the liquid fill and gas vent line. Three of the four

foam insulated dewars had an intermediate fiber glass shell in the

foam insulation. The other dewar insulation was all foam. The intermediate fiber glass shells were all 40 cm in diameter and the outside

diameter of the outer foam layer was 49. 5 cm.

Dewar A was made by pouring the p remixed foam into the fiber

glass shells. On dewar A, the external surface of the copper inner

vessel was cleaned with trichloroethane and wiped dry with clean paper

7
towels prior to pouring the foam. The foam was poured into the lower

fiber glass hemisphere first. Then the foam was trimmed to the top

of the fiber glass shell. The upper fiber glass hemisphere was attached

to the lower one with fiber glass tape and resin. Next the foam was

poured into the upper hemisphere. Construction of this vessel was

our first attempt at using the selected pour-type foam and the quality

of the foam was very poor. The poor quality was caused by inadequate

mixing of the two liquid components. These two components must be

thoroughly mixed in less thaji 30 seconds as the foam starts to rise at

approximately 30 seconds. After the rise has started, the mixture

cannot be poured from a container. The mixing problem was resolved

after consulting with the manufacturer and other users of the foam.

The poor quality of the foam was evident by color streaks, surface

voids, and sub- surface voids which could be detected by pressing on

the foam surface. Even though the first foam layer was of poor quality,

the second layer was poured. This second layer was poured similar

to the firstexcept that mixing was improved and 49. 5 cm diameter steel

hemispheres were used for the outer molds. The inner surfaces of

these shells were coated with silicone grease prior to pouring the foam.

The grease was a releasing agent and the steel shells were removed

after the foam had cured. The upper steel hemisphere had a centrally

located hole, 10 cm in diameter. A 10 cm diameter cylinder was

attached at the hole and was used as a mold to form a 40 cm long

cylinder of foam around the fill and vent line. This foam was formed

when the upper hemisphere filled with foam and the foam continued

to rise. A schematic of the foam dewars is shown in figure 2,

Performance tests were made on dewar A even though the foam

quality was poor. However, the insulation quality of this foam was

8
Fill and

Vent Tube

(Not used on Dewar B

Figure 2, Schematic of dewars insulated with foam.

9
not much different than the other dewars. The thermocouple data for

temperatures within the foam insulation were not valid for this test as many were located in common voids in the insulation.

Insulation on dewar B was all foam with no fiber glass shell.

The copper inner vessels of dewars B, C, and D were primed with

a polyurethane primer prior to pouring the foam. The primer was

used to improve bonding between the foam and the cold wall. The

improved bonding was considered best because the stress in the foam

due to differential thermal contraction between the copper shell and

the foam would then be evenly distributed in the foam. Sectioning after

the test indicated the bond between the primer and the metal and the

primer and the foam was excellent. No bond failure was visible and

removal of the primer and foam from the copper shell required buffing

with a wire brush.

The 49. 5 cm diameter steel shells were used as the mold for

dewar B and the foam was made in two pours, one for each hemisphere.

The foam quality appeared good from the outer surface. Sections of

the foam tcLken after tests showed that the quality was good everywhere

except at the transition from the sphere to the cylindrical neck. In this

area, gas bubbles were trapped during the foam rise and voids occurrec

These voids and the poor quality foam surrounding them are shown in

figure 3. The data for foam insulated dewar B appeared to be good. Even

though some voids in the foam occurred around the neck, the effect of

these voids on the overall dewar performance appeared small since

data indicated that the temperature of the gas in the dewar neck was

within 2 K of the temperature of the liquid at the bottom of the dewar.

The performance of dewar B deteriorated with each test as the

total heat leak for tests one, two, and three was 16. 9, 17. 7, and 1 8. 4

10
11
watts, respectively. This deterioration of pe rformance was probably

not due to general deterioration of the foam insulation but rather to

specific failures of the foam. The failure during test two was around

the neck and frost formed at this failure. After test two, the dewar

was covered with fiber glass and resin on the outside of the foam„ On

test three, the foam failed in the lower hemisphere. Figure 4 shows

the frost formation on the outer fiber glass shell. The frost formed

at each end of a large crack. This crack is shown in figure 5 which

is a photograph of a section of the foam taken after the test.

Data taken from test one for dewar B should be quite accurate

for determining the thermal conductivity versus the temperature for

the foam insulation. The results of this test are shown in figure 6,

Dewars C and D were both made with the intermediate fiber

glass shell using a procedure similar to that used for dewar A. The

fiber glass shell was used because the dewar without the intermediate

fiber glass shell failed. The reasoning was, if the inner layer of

foam cracked, the cracking would stop at the fiber glass shell, and

since the temperature would be much higher at the fiber glass than at

the inner wall, the probability of this crack propagating through the

fiber glass was very small.

In addition to the fiber glass shell, a second feature was added

to these two dewars. The feature was to seal off the inner foam by

closing the fiber glass around the fill and vent line. This was accomplished by soldering a 2. 5 cm diameter stainless steel transition tube

10 cm long to the fill and vent line. The steel tube was then joined to

the fiber glass shell with a fiber glass joint. Figure 2 shows a schematic of this construction.

To help alleviate the problem of trapped gas at the sphere-to-neck

transition during foam rise, holes were placed in the upper hemispheres.

12
13
14
1 1 1

CO in Tj- 8

>i.uJO/M^ • AilAliOnaNOO nVIAId3Hi

Figure 6. Thermal conductivity versus temperature for

polyur ethane foam.

15
Six holes about 1. 5 cm in diameter were evenly spaced about 8 cm away

from the neck hole in the fiber glass and in the steel shell molds for

the outer foam layer. The holes in the fiber glass were patched before

the outer foam layer was added. These holes are shown in figure 2.

Although the holes helped reduce the undesirable condition shown

in figure 3, some voids still occurred in the outer foam layer. These

voids caused subsequent failure at the neck-transition on the first

cooling of both dewars. This failure is shown in figure 7. However,

no frost appeared at the failure, so the fiber glass shell with the seal

around the fill and vent line must have provided sufficient insulation to

prevent frost formation. The failure in dewar D was repaired by

removing all of the foam that contained voids and then refoaming the

neck transition. This dewar is shown in figure 8. Dewars C and D were slightly different in that the inner wall of

the fiber glass shell on dewar D was coated with silicone grease prior

to pouring the inner foam layer. The silicone grease was used to allow

release of the inner foam layer from the fiber glass shell. This difference in fabrication did not produce any significant differences in

dewars other than a small difference in the performance on the first

cooldown.

The tests conducted on foam dewars C and D did not include temperatures in the insulation. Tests were made for heat leak after the

first cooldown and after the 17th cooldown. Both dewars were cooled

thirty times from ambient to 77 K. No surface failure occurred except

as previously noted, which was at the neck-to- sphere transition. Dewar D,

which was repaired, did not show any surface failures after repair. The

average thermal conductivity of the foam on the two vessels was different

during the first tests. The average thermal conductivity for the dewar

with the foam bonded to all surfaces, dewar C, was greater than for the

dewar with the silicone grease on the inner wall of the fiber glass shell.

16
17
Figure 8. Foam insulated dewar with a repair of the neck.

18
The average thermal conductivity after the 17th cooldown was larger

for both dewars but there was practically no difference between the

dewars. Table 1 shows the thermal conductivity of the foam dewars.

The subscript on k is the number of the cooldown when k was measured.

Some of the properties of foam samples taken from dewars A and

B are given in table 2. The samples from dewar A are from the outside

layer of foam. As is indicated in the table, the samples from dewar A

and dewar B are of different density. This density difference was probably caused by a difference in temperature of the foam components when

poured, or a difference in temperature of the metal molds at pouring

time. The molds were heated but the temperature was not closely

controlled.

The large difference in thermal contraction between samples is

related to the cell orientation in the foam. Foam cells were in general

elongated in the direction of foam rise and the elongation was as much

as two to one for many of the cells. Cell elongation was also affected by

the geometry of the annulus being foamed. If the annulus was increasing

in cross section normal to rise the cells are less elongated.

Elasticity of foam is also affected by cell structure [Reed, 1972],

Elasticity of urethane foams may be several times greater normal to the

cell elongation, especially at cryogenic temperatures. The combination

of elasticity and thermal contraction will determine if the foam will fail

at the bond to the cold wall or in the bulk insulation due to the large

thermal gradient. Since cell geometry affects thermal contraction, and

elasticity and cell geometry is a function of confinement during the foam

rise, spray foam may be less susceptible to cracking from differential

thermal contraction.

Experiments in making and testing the four foam dewars has shown

that cryogenic dewars can be made of foam. Special techniques are

required to prevent cracks that may form in the insulation at the cold

wall from progressing through to the outside during thermal cycling and

to assure that the foam is of nearly uniform density.

19
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Table 2. Properties of foam insulation

Dewar Sample No. Density Thermal Expansion Cells orientation

g/cm -^293 " 77 *° expansion

L measurement

A 1 0, 0276 0. 029 Perpendicular

2 0. 0283 0. 009 Parallel

3 0. 0274 0. 025 Perpendicular

B 1 0. 0252 0.010 Parallel

2 0. 0256 0.020 30° to Parallel

3 0. 0253 0. 018 1 5

° to Parallel

As has been previously discussed, voids may form in poured

foam. The voids may be eliminated by venting and control of the geometry of the pour molds. To assure that the correct and consistent density

foam results in the dewar insulation, each vessel configuration should

be experimentally evaluated. Temperatures during foaming, foam

density and insulation quality should be measured from dewar to dewar

to determine the best foaming technique.

For the reasons previously stated, spray-type foam is probably

superior. Spray foam has been used in an experiment to evaluate the

possibility of replacing vacuum and powder insulation on storage dewar

3

[R. W. Arnett, 1972], The experiment was made on an existing 7. 5 m

(2000 gallon) liquid oxygen vessel. Spray foam has the disadvantage of

requiring good access to all surfaces to be foamed but has the advantage

of consistency in application. Three other features are apparently

needed to assure dependable foam insulation. The first is that at least

one intermediate shell of fiber glass and resin is required to stop crack

propagation in the cold foam. Second, this fiber glass shell should be

21
gas tight to prevent air and moisture from entering at insulation penetrations such as fill and vent tubes. Third, the cold metal surface should

primed to assure good uniform bonding of the foam and the metal.

The requirements for foam insulation, especially spray foam,

should be investigated more thoroughly to determine if the primer is

really the best system or if the cold surface should be treated for total

release. In conjunction with total release, some form of seal would

need to be made around the foam and to all penetrations of the foam to

keep air and moisture from the cold surfaces. The seal could be the

fiber glass shell with metal-to-fiber glass transitions for cold surfaces.

Further investigation of spray foams should also include the effects of

the cell orientation on the resistance to cracking. This investigation

should include determination of basic foam properties, such as thermal

contraction, elasticity, density and thermal conductivity of the selected

spray foam.

The conclusions from the foam insulation experiments are 1) that

foam systems such as those used in dewars C and D will work for cryogenic applications but more development of foaming technique for each

specific vessel will be required, 2) spray foam is more desirable

because voids caused by gas entrapment can be eliminated, and 3)

foam with fiber glass reinforcing layers and a fiber glass cover is a

potentially rugged insulation system.

3. 2 Glass Bubble Insulated Dewars

The dewar for the glass bubbles used the same size inner vessel

made of the 30. 5 cm diameter copper hemispheres. The outer vessel

was made of two 56 cm diameter steel hemispheres. The fill and vent

tube was 1. 27 cm in diameter and 45 cm long. The insulation for the

fill and vent line was provided by joining to the steel hemispheres

22
a 11. 5 cm diameter cylinder 30 cm long. The pumping line and valve

and an instrumentation vacuum feed-through were installed in the neck.

A schematic of the dewar is shown in figure 9. The glass bubbles were

poured into the insulation space through the vacuum valve using a funnel

and a 1. 3 cm diameter tube that extended through the valve opening.

The insulation space was filled to the level of the vacuum valve which

was 19. 5 cm up from the outer hemisphere. After filling the insulation

space with glass bubbles, it was pumped to 0. 1 3 N/m pressure.

Seven tests were conducted on the dewar with glass bubbles as

insulation. Two tests were made with a total pressure in the insulation

2 2

of 0. 1 3 N/m ; one test was made with nitrogen gas at 101 kN/m , two

2

with 80% argon and 20% nitrogen gas mixture at 101 kN/m , and two

2

with carbon dioxide at a pressure of 101 kN/m (with the dewar warm).

The tests were all conducted as described in section 2. The test data

included temperatures in the insulation, temperature and pressure at

the gas meter, pressure in the dewar, and heat leak to the dewar.

The first test with glass bubbles, which was conducted with a vacuum in the insulation space, was made after the insulation space

was pumped for the first time.

After completing test one, the insulation space was filled to an

absolute pressure of one atmosphere with dry nitrogen gas. Test two

was conducted with this configuration.

For tests three and four, the insulation space was again pumped

2

to 0. 13 N/m pressure, then filled to one standard atmosphere with a mixture of 80 mole percent argon and 20 mole percent nitrogen gas.

Tests three and four were then conducted using liquid nitrogen at

2

223 kN/m pressure. At this pressure, the equilibrium temperature

23
24
of liquid nitrogen is slightly over 84 K. This is near the condensation

temperature of argon at 0. 8 atmosphere pressure which was the partial

pressure of argon in the insulation space. The pressure in the insulation

2

space dropped 10 kN/m after the dewar was at the controlled pressure

and boil-off had stabilized for several hours. The argon-nitrogen gas

mixture was used instead of pure argon gas because liquid argon was

not available to use as the cryogenic fluid; therefore, liquid nitrogen

2

was used. The system was pressure limited to 223 kN/m , so the test

temperature was limited to slightly less than 85 K. Pure argon gas at

one atmosphere pressure would have condensed and run down onto the

warm surface of the outer shell causing reflxixing and high heat transfer.

The 0. 8 atmosphere partial pressure of argon prevented the refluxing.

The argon-nitrogen gas mixture data were extrapolated to get

estimated data for pure argon. The extrapolation was made assuming

conductivity follows a linear relationship between pure nitrogen and

pure argon. This extrapolation is believed to be as precise as the

experimental data.

Tests five and seven were conducted using carbon dioxide in the

2

insulation space. The insulation space was pumped to 0. 4 N/m pressure before the carbon dioxide gas was added. The carbon dioxide gas

pressure was one standard atmosphere with the vessel at ambient

temperature. Between tests five and seven, the insulation space was

2 pumped to 0. 1 N/m and a second test was run with vacuum. The

insulation space was then refilled with carbon dioxide gas and test

seven was run, A difference in dewar performance was apparent

between test five and seven. Curves showing thermal conductivity

versus temperature for the tests are given in figure 10. The data

presented are for thermal equilibrium conditions.

25
o

OOOQOOOO O Q O O O O OO—

h- CD in ^ rO CJ —

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Figure 10, Thermal conductivity versus temperature for glass bubble

insulation.

26
The thermal conductivity for test five, the first test with carbon

dioxide, is obviously in disagreement with the data from test seven.

This difference in conductivity is assumed to have been caused by the

residual argon and nitrogen gas. This assumption cannot be confirmed

because the pressure in the insulation space was not measured when the

, 2

dewar was cold. The insulation was pumped to 0. 1 3 N/m prior to test

six which was the second test with vacuum and glass bubbles. The

2

carbon dioxide was again added to a total pressure of 101 kN/m and

test seven was run. A vacuum gauge was installed and for this test

2

the dewar insulation space dropped to 0. 15 kN/m after the dewar had

been cold for about 2 hours. The performance was nearly the same as with a vacuum this time. The average thermal conductivities for the

seven tests and the estimated value for pure argon are given in table 3. Oxygen compatibility tests were made on the glass bubbles used

in the seven thermal performance tests and on two other batches of glass

bubbles. The first of the other two batches of glass bubbles were identical to those used for thermal tests except that they were not treated with

a hydrocarbon washing compound. The second batch were half aluminum

coated glass bubbles. The oxygen compatibility tests were impact tests

and were performed by NASA, Marshall Space Flight Center, Huntsville,

Alabama, The uncoated glass bubbles were tested in liquid oxygen at

one atmosphere pressure. The aluminized bubbles were tested at

2

1 atmosphere but they were also tested at pressures up to 10. 3 MN/m

in liquid and gaseous oxygen. No reactions occurred in any of the tests.

The conclusions from the seven tests with glass bubbles as insulation are that: 1) the performance of the glass bubbles in a vacuum

is very similar to -80 mesh perlite in a similar vacuum, 2) Carbon

dioxide can be used in the insulation space with the glass bubbles without

affecting performance and the carbon dioxide will prevent air and moisture

Personal communication with John G, Austin
from entering the insulation while the dewar is warm, 3) Argon gas

used with the glass bubbles for insulation of liquid oxygen vessels will

degrade performance (about 20 times worse than with a vacuum),

4) Glass bubbles aluminum coated and plain are compatible with oxygen.

Table 3, Average thermal conductivity of glass bubble insulation.

Insulation Pressure

Test Gas Warm Cold K(77 to 295 K)

1 vacuum 0. 13 N/m^ 0. 13 N/m^ 11 |iW/cm-K

2 101 kN/m^ 87 kN/m^ 259 ^iW/cm-K

3 80% Ar, 20% ^2

2

101 kN/m 91 kN/m 221 ^iW/cm-K

4 80% Ar,

100% Ar

20% ^2

2

101 kN/m 2

91 kN/m

(estimated)

220 ^W/cm-K

212 ^LW/cm-K

5 C°2 101 kN/m^ 63 p, W/cm-K

6 vacuum 0. 13 N/m^ , 2

0. 13 N/m 11 |j,W/cm-K

7 101 kN/m^ 20. 15 N/m 13 |j,w/cm-K

4. 0 Proposed Techniques for Extending Service Life of Dewars

4. 1 Techniques for Existing Liquid Oxygen Dewars

Dewars that are vacuum insulated or vacuum-and-powder insulated

and have developed a vacuum leak can be treated to extend their service

life and reduce the necessity of vacuum pumping in the field. The treat2

ment is to be used if the vacuum leak is small, < 0, 1 N/m pressure

rise per 30 days and if the dewar is used less than 10 percent of the

time. The technique consists of pumping the vacuum jacket to as low

2 a pressure as possible, ^ 0, 13 N/m is desirable; then, the vacuum

2 jacket is filled to approximately 14 kN/m gauge pressure with carbon

dioxide gas while the dewar is warm. Caution, Do not overpressure

the insulation space as the inner vessel may be collapsed. Continued

28
leakage of air and water into the insulation is prevented while the dewar

is not in use, but the carbon dioxide will freeze out on the cold walls

(cryopump) when the dewar is cooled and result in a low pressure insulation that performs as well as a vacuum. Of course, this salvage is

temporary and not acceptable for dewars in continual use. It can be

used to keep a dewar in service for an extended period of time while

awaiting repair.

If the leak in the vacuum jacket of a powder insulated dewar is

greater than 0. 1 N/m pressure rise per 30 days or the dewar is in

continual use, a temporary fix can also be made but the dewar performance will be degraded. The temporary fix is to pump the vacuum jacket

. 2

to as low a pressure as possible, at least 1 N/m , and to fill the vacuum

, 2

jacket with argon gas to a maximum of 14 kN/m gauge pressure. This

will degrade the insulation but will allow continued use of dewars that

are needed and cannot be repaired in the field. The pressure in the

insulation will need to be monitored as the argon gas will leak out.

When the pressure in the insulation drops below 5 kN/m gauge pres2

sure, argon gas should be added to bring the pressure back to 14 kN/m

gauge. Comparative dewar performance for spherical and cylindrical

vessels, using argon gas, is shown in figures 11 and 12, The comparative performance presented is calculated from the experimentally

measured thermal conductivity and does not include losses due to penetration of the insulation such as fill and vent line or inner vessel supports.

Vacuum insulated liquid oxygen dewars that have small vacuum

2

jacket leaks that exceed the 0, 1 N/m pressure rise in 30 days Ccin

also be salvaged for temporary use. The method will degrade the

performance but in some cases it may be useful to salvage a container

and keep it in service for an indefinite time. The procedure for

29
Figure 11, Performance of spherical dewars insulated with

glass bubbles.

30
E

o

CO

CO

LxJ 2

O

X

<

-J

^00/% * ssol ainon

Figure 12. Performance of cylindrical dewars insulated with glass

bubbles.

31
salvaging such dewars is to fill the available insulation space with

powder such as perlite or with glass bubbles similar to those described

in section 3, Then, the insulation space is pumped to a suitably low

,2

vacuum, 1 N/m or less. After vacuum pumping, the insulation space

2

is filled with argon gas to 14 kN/m gauge pressure. The dewar performance will be degraded similar to the comparisons shown in figures

11 and 12. The argon gas pressure will need to be monitored and

replenished as in the previous 'fix. ' MLI insulated liquid oxygen dewars

that have small vacuum jacket leaks can be salvaged for temporary use

as well. The thermal performance will also be degraded. The method

of salvage is to pump the insulation space to the lowest vacuum possible,

2 2 < 1 N/m , then to fill it with argon gas to 14 kN/m . Again the perform

ance will be similar to that shown in figures 11 and 12.

A method of repairing dewars with bad vacuum jackets is to

remove the vacuum jacket and any associated attaching devices and to

cover the vessel with a polyurethane foam using a spray application.

Performance of foam insulation is better than glass bubbles with argon

gas and has the additional advantage of requiring no vacuum equipment;

foam can be applied to any desired thickness, thereby more nearly

approaching the original overall dewar performance. Comparative

performance of foamed dewars is given in figures 13 and 14, The

curves represent calculated performance based on thermal conductivity

measured experimentally and do not include losses due to penetrations

such as fill and vent lines. The disadvantages of foam are that foam

is not oxygen compatible, the application requires removal of the vacuum jacket, special treatment of surfaces to be insulated is necessary,

and an intermediate fiber glass shell must be applied with special

adaptors for each penetration of the insulation. An exterior cover of

fiber glass or metal is required to protect the foam. A complete

description of a suitable foam insulation system is given in section 3

except that spray application is recommended rather than pouring . 32
o o o o o

If) ^ lO CM — ^00/% * sson ainon

Figure 13, Performance of spherical dewars insulated with

polyurethane foam.

33
If) ^ ro oj

f^^or/o * sson ainon

ure 14. Performance of cylindrical dewars insulated with

polyurethane foam.

34
Foams that can be poured in place may be used in some instances,

thus eliminating the requirement for removal of the vacuum jacket; however, the results of tests on foam dewars A and B described in section 3

indicate that considerable development of pouring techniques may be

required before a pour-in-place foam could be relied upon for insulation.

4. 2 Techniques for New Liquid Oxygen Dewars

Because no new insulation system was conceived that rivals the

performance of the vacuum systems, continued use of the vacuum systems

is recommended in new vessels. If the vacuum jacket of the system subsequently fails in service, the previously described methods of salvage

may be implemented as required. To accommodate these methods, the

recommendation is to enlarge the vacuum space in new vessels, where

possible, to make the salvage method a more effective insulation.

Foam insulation with an intermediate fiber glass shell and an

external protective shell can be considered if the performance is acceptable. The poorer performance of foam may be acceptable for vessels

that are used intermittently and for short durations. Also, if the dewar

is in continual use (always cold) a thick layer of foam can be applied, if

space limitations permit, and the performance is improved. In this case

the performance will more nearly approach that of powder- and -vacuum.

In either case, the degradation in performance must be balanced with

the advantages of a foam insulation system. These advantages are

greater durability because of no vacuum requirements, lighter weight,

no support structure in the insulation space, and field rep air ability.

Field repairs can be made with a pour-in-place foam and a fiber glass

and resin patch.

35
A cost comparison was made on a foam insulated liquid oxygen

system versus a currently used MLI system to see if the foam insulated

system had a cost advantage. The cost analysis was provided by a firm

producing liquid oxygen dewars for the military. This firm also has

some experience with foam insulated dewars. The cost comparison

was made on liquid oxygen systems similar to the currently produced

Type TMU 70/m which is Navy part number FSN-RX 3655-1 58-0657-5X-7X.

Both systems considered were identical except for the insulation system.

The cost reduction for a foam insulated system was estimated to be 25 to

30% or $1000 to $1200 less than the $4000 current production contract

costs. These costs are based on an order of 200 items.

A significant disadvantage of the urethane foams is that they generally are not compatible with oxygen- -posing a potential safety hazard .

5. 0 Recommendations for Further Investigation of

Insulation for Liquid Oxygen Dewars

The tests performed have proven that foam can be used as insulation for cryogenic dewars. Since pour foams result in problems in

getting uniform density throughout the insulation, spray foams are recommended. A spray foam insulation system similar to the pour foam system used should be tested for thermal performance, for ruggedness,

and for aging and thermal cycling effects. Combined with this prototype

test should be tests of foam compressive strength, elasticity, thermal

contraction, and oxygen compatibility to assure that the spray foam

selected is not significantly worse in thermal and mechanical properties

than the pour foam we tested.

The use of glass bubbles to replace perlite should be considered

because of greater ease in handling and a small gain in performance

when in a one atmosphere pressure environment. Glass bubbles that are

36
half aluminum coated are reported to improve insulation performance

[Cunnington, 1972] and may be desirable over the complexities of fabrication of MLI even though performance may not be so good as with ML.I.

Current cost of half aluminum coated glass bubbles is prohibitive; however, the process is in the development stage and price of mass produced

bubbles are expected to be competitive with MLI and possibly competitive

with powders.

37
6. 0 References

Arnett, R. W, , Unpublished report, Substituting polyurethane foam for

evacuated perlite insulation in a 2000 gallon liquid oxygen trailer,

prepared for Naval Air Engineering Center by

the Cryogenics Division, Institute for Basic Standards, NBS,

Boulder, Colorado (April 1972).

Cunnington, G. R. , and Tien, C. L. , Heat transfer in microsphere

cryogenic insulation. Paper presented at the Cryogenic Engineering

Conference, Boulder, Colorado, August 10, 1972,

Hayes, R. G. , Cellular plastics for cryogenic insulation. Cryogenic

Technology (July/August 1972).

Honig, R. E. , and Hook, H. O. , Vapor pressure data for some

common gases, R. C. A. Review, Vol. XXI, No. 3, September

I960, David Sarnoff Research Center, Princeton, New Jersey.

Mac, F. , and Smith, M. , High-Performance spray-foam insulation

for application on Saturn S-II stage. Book, Advances in Cryogenic

Engineering 16, Ed K. Timmerhaus, pp. 118, 127 (Plenum

Press, New York, N. Y. , 1970).

Reed, R. , Arvidson, J. , and Durcholz, R. , Mechanical properties

of polyurethane and polystyrene foams from 76 to 300 K,

Paper presented at the Cryogenic Engineering Conference,

Boulder, Colorado, August 10, 1972.

38
FORM NBS-n4A (U71)

1. PUBLICATION OR REPORT NO.

NBSIR 73-308

2. Gov't Accession No.

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

INSULATION OF LIQUID OXYGEN DEWARS

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Charles F. Sindt

8. Performing Organization

NBSIR 73-308

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Boiilder, Colorado 80302

10. Project/Task/Work Unit No. 2750554

11. Contract/Grant No. PO 2-8061

12. Sponsoring Organization Name and Address

Naval Air Engineering Center

Philadelphia, Pa. 19112

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summacy of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The Navy has experienced failure of vacuum insulation in dewars used

for storage and handling of liquefied breathing oxygen for aircraft pilots.

Because of the vacuum insulation failures, a search was made for a more

rugged insulation that has thermal performance similar to the currently used

vacuum with multilayer or powder. No system was found that compared in

thermal performance and did not require a vacuum. Two systems were

experimentally evaluated that did not require vacuum. One was polyur ethane

foam with an intermediate fiber glass shell and the other was glass bubbles in

argon gas at one atmosphere pressure. The polyurethane foam system was

successful in that no cracks penetrated to the outside surface; however, the

average thermal conductivity was 160 |j,W/cm-K which is about 15 times greater

than vacuum and powder. The glass bubbles in argon gas was also successful

since the argon gas pressure always remained high enough to prevent air and

moisture from entering the insulation through small leaks in the outer shell.

The thermal performance was poorer than the polyurethane foam. The average

thermal conductivity was 212 p^W/cm-K or about 20 times greater than for the

same glass bubbles in a vacuum.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Cryogenic insulation; insulation; LOX dewars; microspheres; polyurethane foam.

18. AVAILABILITY STATEMENT

[x] UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Pr

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