NBSIR 77-857

FILE COPY

DO NOT REMOVE

CRYOGENIC DESIGN AND SAFETY REVIEW

NASA-LANGLEY RESEARCH CENTER

0.3 METER TRANSONIC CRYOGENIC TUNNEL

T.R. Strobridge

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder , Colorado 80302

April 1977

Prepared for: Dr. Robert A. Kilgore

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23665

R.O. Voth

NBSIR 77-857

CRYOGENIC DESIGN AND SAFETY REVIEW

NASA-LANGLEY RESEARCH CENTER

0.3 METER TRANSONIC CRYOGENIC TUNNEL

R.O. Voth

T.R. Strobridge

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder , Colorado 80302

April 1977

Prepared for:

Dr. Robert A. Kilgore

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23665

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Dr. Betsy Ancker-Johnson , Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1.0 INTRODUCTION 1

2.0 COMPONENT REVIEW 2 2.1. Storage Tank Fill Transfer Line 2 2.2. Liquid Nitrogen Storage Tanks 2 2.2.1. Vacuum space relief valves 2 2.2.2. Storage tank schematic drawing . 2 2.2.3. Vacuum leak - tank A 4 2.3. Liquid Nitrogen Transfer System to the Tunnel 4 2.3.1. Replumbing of liquid return line 4 2.3.2. Direction of the discharge from the main storage

tank relief valves 4 2.3.3. Protective clothing for use in the nitrogen

storage area 6 2.4. Tunnel Structure 6 2.5. Tunnel Insulation 6 2.6. Tunnel Vent Piping and Pressure Relief 7 2.6.1. Cryogenic valve stem position 7 2.6.2. Carbon liquid sensor 7 2.7. Controls and Instrumentation 13

2.7.1. Remote storage tank liquid level and pressure gauges. 13

2.7.2. House air supply - receiver 13

2.7.3. Digital valves 13

2.7.4. Control system for tunnel pressure and temperature. . 13

2.8. Safety Systems 14

2.8.1. Oxygen analysis 14

2.8.2. Fire control equipment 14

2.9. Physical Plant Site 14

3.0 SUMMARY OF RECOMMENDATIONS AND SUGGESTIONS 16

4.0 REFERENCES 17

APPENDIX A. COST OF BOILOFF LIQUID NITROGEN AS A FUNCTION OF

INSULATION SPACE PRESSURE 18

APPENDIX B. ESTIMATED COST OF RECOVERING NITROGEN GAS AND

REFRIGERATION FROM THE 0.3 m TRANSONIC CRYOGENIC

TUNNEL VENT 19

CRYOGENIC DESIGN AND SAFETY REVIEW

NASA-LANGLEY RESEARCH CENTER 0.3 METER TRANSONIC CRYOGENIC TUNNEL

R. 0. Voth and T. R. Strobridge

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302^ .

The findings of a Cryogenic Design and Safety Review of the

NASA-Langley 0.3 m Transonic Cryogenic Tunnel are presented in

this report. The tunnel working fluid and coolant is nitrogen.

The nitrogen, supplied as liquid, is exhausted as a low temperature gas. The use and storage of liquid nitrogen at the facility

presents several potential hazards to personnel and equipment.

An appropriate cryogenic design minimizes these risks, and provides for convenient tunnel operation and for the economical use

of the liquid nitrogen. The tunnel and ancilliary systems are generally well designed but several recommendations to improve

the cryogenic systems are made. The cost of recovering the cold

vent gas is compared to the cost of producing the required liquid

nitrogen using a captive air separation plant. Although the

economic analysis is preliminary, it shows that because of the

periodic operation of the tunnel, a captive air separation plant

has a lower annual operating cost than the vent gas recovery

systems considered.

Key words: Cryogenic; design; nitrogen; safety; wind-tunnel.

1.0 INTRODUCTION

The Cryogenics Division of the National Bureau of Standards has completed

a Cryogenic Design and Safety Review of the NASA-Langley 0.3 m Transonic Cryogenic Tunnel. The tunnel is operated at temperatures as low as 80 K and pressures as high as 5 atmospheres absolute. The working fluid and coolant is

nitrogen supplied to the tunnel as liquid and exhausted as low temperature

gas. Typically the use and storage of liquid nitrogen presents the following

potential hazards to personnel and equipment:

1) Freezing human tissue,

2) Asphyxiation due to oxygen displacement,

3) Failure of materials not suitable for low temperature service,

4) Overpressure failures caused by trapped liquid,

5) Failure of structures due to thermal contraction,

6) Fire in flammable materials compounded by oxygen enrichment

of liquid air, and

7) Fogging or icing of the adjacent area. With regard to the above potential hazards, we find the tunnel and ancillary

systems generally well designed for low temperature service and the risks

eliminated or reduced to acceptable levels. Recommendations to further reduce

risks or improve operation are made in subsequent sections of this report. At

the time of our inspection, September 27 and 28, 1976, the tunnel had accumulated some 600 hours of operation without an accident or injury.

Since the 0.3 m Transonic Cryogenic Tunnel uses significant quantities of

liquid nitrogen, a vent gas recovery system may be more economical than purchasing the liquid. A preliminary economic analysis of several recovery systems is presented in Appendix B. The recovery systems considered had an
equal or higher annual cost than a captive air separation J)lant and nitrogen

liquefier that ran continuously. The periodic vent flow and the low availability of the vent gas were the principle reasons the recovery systems were

more costly.

2.0 COMPONENT REVIEW

Technical documents and drawings supplied by the facility staff were

studied and a two day inspection was augmented by helpful discussions with

the staff scientists, engineers and technicians. The tunnel was partially

disassembled during the inspection. The elements reviewed and studied were

storage tanks, tunnel structure, tunnel insulation, vent valves and piping,

controls and instruments, safety systems, the physical plant site, and the

liquid nitrogen transfer systems both from the truck loading station to the

storage tanks and from the storage tanks to the tunnel.

2.1. Storage Tank Fill Transfer Line

Liquid nitrogen is delivered in semi-trailer tankers. The truck does

not enter the site proper but fills the storage tanks through an insulated

transfer line penetrating a cement block structure partially surrounding the

storage tanks on two sides. The transfer line, figure 1, extends almost to

the street and is firmly anchored to a stanchion. Following a horizontal run

to the storage tanks, there is a right angle bend to a vertical section con- necting to the larger diameter storage tank liquid manifold. Although expansion loops and bends provide stress relief for the liquid manifold, the smaller

liquid fill line is restrained at both ends. Estimating a 7.6 m transfer

line length, the calculated free thermal contraction is 2.3 cm; if the line were fixed firmly at both ends, the stress would be above yield.

Even though the line has been in service for some time, we recommend that

calculations be made, considering the restraints and contraction of the storage

tank liquid manifold, to determine the stress in the fill transfer line. If

these stresses are not acceptable, then design changes should be made and the

piping modified. Note that the forces imposed by making and breaking the truck

connection should not be reacted by the transfer line.

2.2. Liquid Nitrogen Storage Tanks

Liquid nitrogen is stored in two conventionally designed, evacuatedpowder insulated 28,000 gallon tanks. A pressure building vaporizer and the

liquid nitrogen pump to supply the tunnel are located in the storage tank

area

.

2.2.1. Vacuum space relief valves

The storage tanks schematic drawing shows a relief device for the evacuated insulation space. There were no relief devices on the vacuum space at

the time of our inspection.

We recommend that appropriate relief valves be installed.

2.2.2. Storage tank schematic drawing

We recommend that easily read schematic drawings or drawing of the

storage tanks and manifolding be prepared that show at least:

1) Tank internals,

2) All penetrations and their schematic positions,

3) All piping,

4) All valves and their numbers,

5) All safety devices,

6) The pump and pressure building vaporizer.

2
3
7) All transfer lines, and

8) Instruments.

Such drawings will be valuable in training new facility personnel and

truck drivers or for reference by those who only occasionally operate the

system.

2.2.3. Vacuum leak - tank A

We understand that the best vacuum ever attained in the A tank insulation

space was 9.2 x lO"^ atm (7 mmHg) . The pressure was 0.132 atm (100 mmHg) at

the time of our inspection and the vessel was being pumped. The insulation

performance at 0.132 atm (100 mmHg) is essentially the same as if the space were at atmospheric pressure. In addition to being costly, see Appendix A,

certain combinations of the leak location, operating sequence, and lack of

vacuum relief could lead to collapsing the inner vessel.

We recommend that all steps necessary be taken to repair the leak and

return the insulation to serviceable condition.

2.3. Liquid Nitrogen Transfer System to the Tunnel

The liquid nitrogen transfer lines are all foam insulated and covered

with a conventional vapor barrier. Nitrogen is pumped from the storage tanks

to the tunnel; excess liquid not used in the tunnel is returned to the storage tanks. Sufficient plumbing is provided so each storage tank can be used

or filled independently; however, the returning excess liquid lines connect

into the storage tank vent lines inboard of the vent valves. Thus, if a tank

vents while flow is being returned to it, the returning liquid is forced out

the tank vent. The tunnel is operated using both storage tanks to reduce

the amount of liquid vented. The liquid is withdrawn from one tank while

the excess liquid is returned to the other tank that is initially at low

pressure

.

We feel that the liquid nitrogen transfer system is adequately protected

from overpressure failures due to trapped liquid and excluding the liquid

fill line, the system is adequately protected from excess stress due to thermal

contraction. Relief valves are present for each section of pipe that can be

isolated and pipe loops are used to relieve the thermal stresses in the longer runs of piping. Because the plumbing system is located outdoors and

the probability of a pipe rupture is remote, the danger of oxygen depletion

in the storage area is remote.

2.3.1. Replumbing of liquid return line

The operational complexity resulting from the excess liquid returning to

the storage tank inboard of the tank vent valves could be reduced by directing the return flow to the now unused ullage connection shown in figure 2.

We recommend this change be made to allow completely independent operation of the two tanks and to eliminate the venting of liquid nitrogen.

2.3.2. Direction of the discharge from the main storage tank relief valves

The discharge from the main relief valves for the storage tanks is cur- rently located so that liquid could impinge on the carbon steel outer shell

of the adjacent tanks. A liquid discharge is possible with the current liquid

return plumbing or by overfilling.

We recommend directing the vent valve discharge so that liquid cannot

contact the adjacent tanks.

4

2.3.3. Protective clothing for use in the nitrogen storage area

Protective clothing is suggested by most safety manuals for use when

handling liquid nitrogen. The clothing includes: goggles or face shield,

gauntlet leather gloves, and boots or high-top shoes covered by cuff less,

clean, tightly woven white cotton coveralls. However, this type of clothing

is considered to be quite conservative for the protection it provides from

the cold burns resulting from large scale liquid nitrogen spills. In a system such as the wind tunnel liquid nitrogen storage area — where the

plumbing is hard and has no quick disconnects, etc. — the chance of a line

rupture is remote and the chance of a large spill where the protective

clothing would serve a useful function is even more remote. Therefore, it

is reasonable to relax the protective clothing requirements after some ex- perience is gained with the storage system. The best assurance of personnel

safety lies in the safety-education of the operators themselves and in a properly designed system that is in good order. We believe that the equipment meets those criteria and that the operators are responsible and knowledgeable. Therefore, the requirement that they wear all the suggested protective clothing appears to be ultra-conservative. Nonetheless, the clothing

should be available and easily accessible for use in emergencies.

2.4. Tunnel Structure

The tunnel shell is 6061-T6 aluminum and the support structure that may

be cooled is 347 stainless steel; both suitable for low temperature service.

Warm support members are A-36 carbon steel. Thermal contraction allowance

is provided by elongated mounting holes and teflon slide pads. The tunnel

structure seems suitably designed and we make no recommendations for change.

2.5. Tunnel Insulation

We were told that the tunnel insulation (urethane foam with fiberglass

epoxy vapor barrier) is sufficient to keep the outside of the insulation warm

and dry during operation. However, flange areas and penetrations may get cold

enough to condense air.

The liquid condensed from air on a liquid nitrogen temperature (77 K)

surface contains 50 mol percent oxygen. Further, as the liquid evaporates,

the oxygen concentration increases. Certain fuels become more flammable or even explosive when combined with oxygen rich liquid air. At one atmosphere

pressure, air will not condense at a temperature above 81.4 K; nitrogen vapor

pressure at 81.4 K is about 1.6 atmospheres.

The normal tunnel operating temperature is above 81.4 K and therefore

above the air condensation temperature with the exceptions below:

1) During cooldown if the tunnel static pressure is below 1.6 atmosphere

the structure could be below 81.4 K.

2) If liquid nitrogen collects at the tunnel low point and is vented,

the temperature in the uninsulated vent lines will be below 81.4 K.

3) During tunnel operation, liquid nitrogen is entrained in the flow

stream between the nitrogen injection point and the fan. At static pressures below 1.6 atmospheres, the tunnel wall in this section could be

below 81.4 K. Therefore, air could condense between the outer tunnel

wall and the urethane insulation if the fiberglass vapor barrier is not

tight. Urethane is not a liquid oxygen compatible material.
We recommend reducing or eliminating the possibility of condensing air

and concentrating oxygen by:

1) Controlling the cooldown and normal operation to keep all temperatures above 81.4 K insofar as possible,

2) Installing temperature sensors on the outside tunnel wall beneath

the insulation, particularly between the liquid nitrogen injection nozzles

and the fan,

3) Sampling the oxygen concentration beneath the insulation, particularly during warm up,

4) Maintaining the vapor barrier to exclude as much air as possible,

5) Installing drip trays in all areas where liquid air is known to

collect, and by arranging them to evaporate the liquid quickly and prevent accumulation,

6) Installing a liquid sensor in the low point of the tunnel to prevent liquid nitrogen accumulation. See Section 2.6.2 for a suggested

sensor,

7) Keeping the areas that liquid air may contact free of grease or

fuels. Figure 3 shows material collected under the tunnel during a maintenance period. We were assured the area is clean during experiments , and by

8) Purging the tunnel insulation space with nitrogen at slightly above

atmospheric pressure to exclude air.

2.6. Tunnel Vent Piping and Pressure Relief

Nitrogen is vented from the tunnel through three control valves. Fog,

produced by the cold gas, tends to settle to the ground. Ejectors, figure 4,

installed on the stainless steel vent stacks have eliminated that problem.

However, liquid which is occasionally vented during cooldown, may strike the

ground. Tunnel overpressure protection is provided by an appropriate relief

valve and a rupture disk.

2.6.1. Cryogenic valve stem position

Two of the three vent control valves are mounted with the stems pointing

down at an angle, figure 5, the third is horizontal. These valves depend on

near vertical installation to keep the stem seal warm and functional. Occasionally the non-vertical valve stems freeze and the tunnel pressure control

is lost. In this event the alternatives are to warmup the tunnel or to have

an operator enter the room to free the valves. The latter option has been

chosen on occasion as evidenced by the hammer indentations on the valve

packing area in figure 6.

We recommend mounting all cryogenic valves with the stems at least 45°

above horizontal.

2.6.2. Carbon liquid sensor

A liquid sensor in the tunnel low point would assist in preventing significant liquid nitrogen accumulation. A simple sensor, unaffected by temperature, can be made using an 1/8 watt carbon resistor (R ) in the circuit

shown in figure 7. Carbon resistance is inversely proportional to temperature.

The small current passing through the resistor heats the element above the

temperature of the surroundings. Given liquid and vapor at the same temperature, the element will be warmer in gas than liquid because of the different

7
C

-HU

0) •H

5h J2

0)

0) -P

fO iHPgUO

Cn HOw

(U

O

-H

03 CU

+Jmo^>

Q)U

•H

fa 8

10
C

•H

15O>> ' c

0)>o

0)

t7« • C CO

rH

O

-P Cn fd C 4-> H C

(U

O Xi

(13

C

1^

•H

Em

11
Output

Meter

(V)

Battery or Constant (E=20 V)

Voltage

Source

At ambient temperature R^=R2=R3=R^=1000 O

R^=R2=R3=]^ watt resistor

R,=V8 watt resistor liquid sensor

output voltage(V)

0 V

.22 V

.30 V

condition of Rg^

Ambient Temp

Sat LN Vapor

Sat LN Liquid

resistance of R|^

1000 ohms

1045 ohms

1062 ohms

Figure 7. Schematic of liquid level detector.

12
natural convection heat transfer coefficients. The resulting change in

resistance is easily detected as a step change in the bridge voltage.

Since the sensor depends on a different heat transfer coefficient in the

two phases, it must be located where the flow velocities are quite low. The

mounting must also minimize heat transfer from the attachment point.

2.7. Controls and Instrumentation

The liquid nitrogen controls and instrumentation that are not directly

associated with the control of pressure and temperature in the wind tunnel

are simple and adequate in their functions. Liquid level and pressure indicators for the storage tanks, manually operated vent and liquid supply valves,

and a pump on-off switch constitute the liquid nitrogen control system.

We understand that the control system for the tunnel pressure and temperature is not operational except, perhaps, over a very narrow band of operating conditions. However, a new digital controller and digital valves were

being installed to regulate the tunnel liquid nitrogen supply and installation

of a digital controller and valves for the pressure controlling vent valves

are planned in the near future to improve the control capabilities.

2.7.1. Rem.ote storage tank liquid level and pressure gauges

We suggest that remote indicator gauges for liquid level and storage

tank pressure be installed in the control room. Although this installation

would not increase safety or utility, the remote gauges would reduce the re- quired operator attention to gauges that are now located outside the control

room. Remote gauges for liquid level and storage tank pressure could also

be installed at the liquid nitrogen fill station. These together with re- motely controlled liquid fill and vent valves would allow liquid nitrogen

delivery without the truck driver entering the fenced area. Appropriate

locks could be used to insure that only the truck driver had access to the

remote switches.

2.7.2. House air supply - receiver

We suggest that a receiver and appropriate check valve be installed on

the house air supply to insure an air supply sufficient to at least shutdown

the tunnel in the event of a air supply failure. The failure mode of the new

digital valves should be incorporated into the design of the receiver system.

The vent valves should fail open while the liquid nitrogen supply valves

should fail closed. Instrumentation failure should be alarmed.

2.7.3. Digital valves

We are skeptical about the successful operation of the digital control

valves being installed in the system.. However, we do recognize that the

cycling tests being conducted while we were making the inspection should

indicate the acceptability of the valves if the tests are conducted properly.

For instance, the valves should be tested in the same orientation as they

will be installed and the valves should be tested at variable cycle rates.

Leaving the valves open or closed for extended periods of time while they are

cold would show any possibility of their freezing in position.

2.7.4. Control system for tunnel pressure and temperature

We also suggest that a study be conducted of the time constants associated with the tunnel pressure and temperature control system. A good control

13
system will achieve the desired operating conditions in less time reducing

the consumption of liquid nitrogen while reducing the required operator

attention. The digital control valves currently being installed may help

but our intuitive feeling is that the time constant of the tunnel is very

long thus special long time constant reset and proportional controllers may

be required to achieve stable control. If this is the case, the current

control valves are sufficiently sensitive to achieve acceptable control and

only the valve controllers need to be altered.

2.8. Safety Systems

2.8.1. Oxygen analysis

The nitrogen in the tunnel at five atmospheres and 100 K would occupy

17 times the tunnel volume at standard temperature and pressure. If the

tunnel ruptured, the nitrogen would displace the room air and could reduce

the oxygen concentration to seriously low levels. The room is equipped with

louvered vents and fans. An oxygen analyzer, sampling the room air activates

an alarm system in the building and at the local fire station when the

oxygen concentration is below 19 percent. The analyzer is calibrated weekly.

Normal shutdown procedure is to warm the tunnel and circulate fresh air.

The tunnel fluid is analyzed to assure an adequate oxygen level prior to shutdown.

Both the systems and procedures appear to be sufficient to prevent

asphyxiation

.

2.8.2. Fire control equipmerit

Fires in the wind tunnel area would be Class A fires — fires in ordinary

combustible materials such as wood, paper and rubbish. The worst conditions

would be a fire in the tunnel insulation with oxygen enriched air in the foam.

The best fire extinguishing agent for oxygen enriched fires is water or waterfog spray which extinguishes the fire by cooling and eliminating other oxygen

from the fire by the steam formed [1]. The water could be applied by automatic or remotely controlled sprinkler heads which would not require operators

to enter the room and be exposed to noxious fumes from the burning insulation

or asphyxiation from lack of oxygen. Although water is the most effective

for large fires, other more portable or dry chemical extinguishers should

be available for smaller fires.

2.9. Physical Plant Site

The 0.3 m Transonic Cryogenic Tunnel is completely enclosed by walls

or fencing and is inside NASA-Langley property. The probability of intrusion

and damage to the low temperature equipment is low. The storage tanks are separated from the adjacent street by a wall taller than the tanks which would

deflect liquid sprayed from most plausible ruptures. Possibly a ruptured

liquid line could spray into the street but the probability is low.

Even though unlikely, the consequences of a major spill should be con- sidered . We recommend that:

1) The plot and surrounding drainage should be studied to determine

if any unacceptable damage would be caused by a major spill. Dikes

could be considered.

2) The effects of cold shocking the utilities (if any) in the covered

trench, figure 8, between the storage tanks should be studied.

14
Hooo

00

CN

0)

-p

0)

cu c

(J -p

0) X! Q) U M

c: o

S-lwc

>i 0) P tn •H o

•H -P

-P -H D C

00

0)

•H

15
3.0 SUMMARY OF RECOMMENDATIONS AND SUGGESTIONS

The recommended and suggested changes to the tunnel elements made in the

preceding sections can be separated into four categories: 1) Those effecting the safety of operating personnel, 2) Those preventing or reducing the

possibility of equipment damage, 3) Those increasing tunnel operating ease

and decreasing operator attention, and 4) Those decreasing tunnel operating

costs. The categories are arranged in order of their importance; number 1 is the most important and number 4 least important. Some recommendations

fit in several categories but we have placed them where we feel they have the

most impact.

Category 1. Operator Protection :

1. Make protective clothing available for emergencies.

2. Install liquid air drip and evaporation trays, if necessary, based on observations of the tunnel operation by NASA-Langley personnel.

3. Keep tunnel area free of hydrocarbons or other fuels while operating.

4. Keep insulation barrier in good repair.

5. Operate tunnel walls above 81.4 K.

6. Install temperature sensors on tunnel walls.

7. Sample oxygen concentrations in insulation space adjacent to tunnel walls.

8. Purge tunnel insulation with nitrogen.

Since oxygen accumulation in the tunnel insulation can have serious con- sequences, recommendations 5 through 8 are redundant. For instance, if all

the tunnel wall is above 81.4 K, no air is condensed and recommendations 7 and 8 are unnecessary. Likewise, if the tunnel insulation is completely

purged with gaseous nitrogen, no oxygen will accumulate so recommendations

5, 6, and 7 are unnecessary. However, because of the possible serious con- sequences resulting from the condensation of air and the difficulty of assuring the absolute success of any single recommendation, we recommend the re- dundant approach. Since all recommendations in this category concern operator

safety, we strongly recommend that all points in this category except perhaps

number 5 be implemented. Implementing number 5 limits the tunnel static

operating pressure to more than 1.6 atmospheres, however, if numbers 6, 7,

and 8 are implemented, operating the tunnel at lower temperatures and pressures is acceptable.

Category 2. Equipment Protection :

1. Calculate fill transfer line stresses.

2. Install storage tank vacuum relief valves.

3. Redirect storage tank relief valve discharge.

4. Install and alarm an instrument air receiver and check valve.

5. Prepare storage tank and piping schematics.

6. Study site drainage and possible damage to underground utilities or other property.

Of all the recommendations in Category 2, we strongly recommend implementing number 2

.

16
Category 3. Operating Ease ;

1. Replumb return liquid line.

2. Install liquid sensor in tunnel low point or where liquid may collect.

3. Install tunnel vent control valves with stems vertical.

4. Install remote storage tank liquid level and pressure gauges.

We recommend numbers 1, 2 and 4 in this category as low cost ways to

reduce operator attention during tunnel operation. If a major overhaul

of the tunnel is undertaken, we also recommend implementing number 3.

Category 4. Economics ;

1. Repair vacuum leak. Tank A.

2. Study tunnel control system.

The vacuum leak in Tank A is costly in terms of consumption and we believe the repair will be less costly in the long term — especially if the

leak is in the outer tank. We, therefore, recommend number 1. We also recommend a further study of the tunnel control systems — number 2 — to reduce

the cost of tunnel operation.

4.0 REFERENCES

1. Best's Safety Directory, A. M. Best Company, Oldwick, New Jersey, 1976,

pp. 742-744.

2. Scott, R. B. , Cryogenic Engineering, Book, D. Von Nostrand Company, Inc.

New York, NY, 1959, pp. 221-223.

3. Moeller, C. E, J. B. Loser, W. E. Snyder, and V. Hopkins, Thermophysical

properties of thermal insulating materials. Technical Documentary Report

No. ASD-TDR-62-215 , July 1962.

17
APPENDIX A. COST OF BOILOFF LIQUID NITROGEN AS A FUNCTION OF

INSULATION SPACE PRESSURE

The daily liquid nitrogen boiloff for tank A can be determined quite

simply if we assume that the heat leak to the tank is proportional to the

thermal conductivity of the insulation. This assumption is not entirely

accurate since the heat conducted through the inner tank supports, and con necting piping does not change with insulation space pressure. Typically,

the insulation heat leak is 85 to 95 percent of the total tank heat leak s using the thermal conductivity of the insulation to calculate the loss of

liquid gives acceptable results.

According to Scott [2], a storage container of this size should have

loss rate of 0.085 percent of its total capacity per day when used with

liquid oxygen or about 0.1 percent per day when used with liquid nitrogen.

If we assume that liquid nitrogen costs $0.22/gallon and that the thermal

conductivity of evacuated perlite insulation varies with pressure as given

by [3] , the total cost of the boiloff liquid nitrogen can be calculated as shown in Table Al.

Table Al. Boiloff liquid nitrogen costs as a function of pressure

for storage tanks A &. B

Vacuum Space Pressure

atm. mmHg

Thermal Conductivity

of Perlite Insulation

Watt/cm K

Liquid

Lost

Gallons/day

Cost

$/day

1.3 X 10

1.3 X 10

1.3 X 10

1.3 X 10

1.3 X 10

-1

-2

-3

-4

-5

100

10

10.1

0. 01

3.46 X 10

2.59 X 10

1.38 X 10

3.46 X 10

1.73 X 10

-4

-4

-4

-5

-5

560

419

223

56

28

123.20

92.18

49.06

12. 32

6.16

18
APPENDIX B. ESTIMATED COST OF RECOVERING NITROGEN GAS AND

REFRIGERATION FROM THE 0.3 n TRANSONIC CRYOGENIC TUNNEL VENT

INTRODUCTION

The cost of liquid nitrogen used in the 0.3 m Transonic Cryogenic Wind

Tunnel is sufficiently large to consider recovering the nitrogen vent gas.

The two losses incurred by venting are the work expended to separate the

nitrogen from air and the wasted refrigeration potentially available in the

cold vent gas. The relative magnitude of the losses may be estimated from

the ideal work of separation and the thermodynamic availability of the vent

gas at 100 K (the assumed vent temperature) . These works are 58 J/g and 136 J/g,

respectively, and the ideal work required to liquefy nitrogen is 769 J/g.

Thus the maximum savings that can be realized by a recovery system is 2 3 percent of the original work required to separate and liquefy the nitrogen.

Therefore, the recovery system must have a relatively high efficiency before

recovering the vent gas becomes less costly than producing the liquid using

a captive air-separation plant.

We have estimated the annual cost of seven recovery systems using a number

of simplifying assumptions*. Even though the estimates must be refined, they

show that since venting occurs periodically, recovering the gas and saving

the refrigeration is expensive; in fact, nearly the same or more expensive

than separating and liquefying the nitrogen supply directly from air. The

estimated annual cost for each case relative to a captive air separation

plant is shown in Table Bl.

Table Bl. Estimated annual cost relative to a captive air separation plant.

Relative Annual

Cost

Case 1: Purchase liquid 2.3

Case 2: Captive air separation plant Base line

Case 3: Large nitrogen reliquefier 1.8

Case 4: Refrigerator and triple point storage 1.6

Case 5: Refrigerator and slush storage 1.5

Case 6: Nitrogen reliquefier and high pressure storage 1.5

Case 7: Nitrogen reliquefier and low pressure storage 2.1

The following tunnel operating profile is assumed in the analysis:

1) liquid nitrogen flow rate 15 0 gallon/minute

2) vent gas temperature 100 K

3) vent gas pressure 1.0 atm

4) experiment duration 5 hour/day

5) frequency of experim.ents 4/week

6) time elapsed between experiment starts 24 hours

7) number of weeks experiments conducted 52/year

* We ass'umed a high nitrogen use rate (corresponding to maximum tunnel test

Reynolds numbers) so the recovery system costs are the lowest per unit of

nitrogen used. The actual tunnel operating profile uses nitrogen at 1/4 to

1/8 the assumed rate. A recovery system not attractive at the assumed high

use rate will be even less attractive at the actual lower nitrogen use rates

19
SYSTEM DESCRIPTIONS AND REMARKS

Case 1; Purchase liquid . Purchase the required liquid at $0 . 22/gallon.

Case 2; Captive air separation plant . Purchase and continuously operate, an air separation-nitrogen liquefier

dedicated to the 0.3 m Tunnel.

Case 3: Large nitrogen reliquefier . Purchase a nitrogen reliquefier large enough to reliquefy the nitrogen

as it is vented. The reliquefier would be large and impractical to start and

stop for each experiment.

Case 4; Refrigerator with triple point liquid storage .

In this system, a refrigerator runs four days a week to cool stored

liquid nitrogen to the triple point. At the beginning of a tunnel run, the

storage vessel is 70 percent full of triple point liquid. During the run,

the vent gas is bubbled into the liquid and condensed. At the end of a five

hour run the vessel is nearly full of normal boiling point liquid. Liquid

is then drawn off to the 70 percent level as the refrigerator continues to

run and the cycle is repeated. Although the refrigerator operates with a nearly continuous load, the irreversibility of mixing and condensing the

100 K vent gas in the 63 to 77 K liquid lowers the process efficiency and

increases the refrigerator size. The cost incurred by the irreversibilities

and the large storage tank make this system more expensive than the baseline.

Case 5; Refrigerator and slush storage . This system is different from Case 4 in that a 50:50 mixture of triple

point solid and liquid (slush) is used to condense the vent gas. The storage

tank is smaller than in Case 4 because the latent heat and density of the

slush are higher than for liquid. Opposing the lower storage vessel cost is

a higher refrigerator cost due to the higher average temperature difference

between the vent gas and the slush. No economic advantage is apparent in

this system.

Case 6: Nitrogen reliquefier and high pressure storage .

In this system, the vent nitrogen is compressed into 140 atm cylinders

during a tunnel run. The stored gas is continuously liquefied in preparation

for the subsequent experiments. The available refrigeration is lost and the

compression and storage equipment is expensive. The system is not economically

attractive

.

Case 7: Nitrogen reliquefier and low pressure storage .

In this, the most expensive system, a low pressure gas holder collects

the vent gas which is then reliquefied. The cost of the gas holder is dominant.

PERFORMANCE AND COST ESTIMATES

The following values were used for the performance and cost of the system

components and for power.

20
1) Refrigerators and liquefiers

A. Efficiency

A thermodynamic efficiency of 30 percent of Carnot was assumed

for all systems considered.

B. Capital Cost

0 7 Cost in dollars = 7260 x (input power, kW)

2) Capital Cost of Storage

A. Liquid and Slush Nitrogen Storage Vessels

Total capital cost = $10/gallon

B. High Pressure Storage Cylinders

$24/kilogram of stored ambient temperature nitrogen

or $ll/lb of stored ambient temperature nitrogen

C. Low Pressure Gas Holder

$600/kilogram of stored ambient temperature nitrogen

or $272/lb of stored ambient temperature nitrogen.

3) Power Cost

$.02 5/kW-h

4) Compressor for High Pressure Storage

A. Capital Cost

Compressor - $350/kW input power

Electric Drive - $54/kW input power

5) Annual Cost

The annual costs are 18 percent of the total capital costs for the

liquefiers, refrigerators and storage containers. About 15 percent

is the annual cost of the capital and the remaining 3 percent is

operating and maintenance expense.

Table B2 shows a summary of the total annual costs of recovering the venting

nitrogen from the 0.3 m Transonic Cryogenic Wind Tunnel. The lowest cost system

is Case 2 — a captive air separation plant. Even though 23 percent of the

original work required to separate and liquefy the nitrogen is still available

in the vent gas; the periodic nature of the tunnel operation results in a high

recovery cost. Our assumption of operating the tunnel 52 weeks a year is

conservative because reduced operation will decrease only the power costs of

Cases 2 through 7 while the annual capital costs remain constant. If the

tunnel operates less than 12 weeks a year then purchasing the liquid nitrogen

at $0.22/gallon becomes the least costly (Case 1).

We assumed a temperature of 100 K for the venting nitrogen, but the

tunnel will also operate at higher temperatures resulting in even higher re- covery costs. Although the recovery systems are not currently competitive

in cost with a captive air separation and nitrogen liquefaction facility, a higher electrical power cost would change this comparison. The electrical

power cost needs to rise to a value six times its current cost ( $ . 02 5/kW-h)

,

however, before Case 4 would be competitive with the captive air separation

plant.

21
RECOMMENDATIONS

Recognizing that the maximum recovery system saving is 23 percent of the

original energy required to separate and liquefy the nitrogen and that re- covery system irreversibilities are inevitable, we nonetheless feel that

further study of recovery systems is prudent and may be fruitful. Certainly

concepts other than those in this report should be examined with regard to

cost and engineering practicality. One of the first tisks should be to refine

the costs given in this report which are estimates not based on manufacturer

contact for the specific items.

22
Table B2 . Summary of estimated costs to recover the nitrogen vent

gas from the 0.3 m Transonic Cryogenic Tunnel

CASE 1: PURCHASE LIQUID

9,36 X 10 gallons/year 5 $0.22/gallon

Total annual cost $2,059,000.00

CASE 2: CAPTIVE AIR SEPARATION PLANT

Air Separation Plant , Capital Cost — $1,936 x 10

Power Cost

Storage Container

(Use existing storage)

Total annual cost

Annual Cost

348,500.00

547,000.00

$895,500.00

CASE 3: LARGE NITROGEN RELIQUEFIER

Reliquefier , Capital Cost — $6.4 x 10

Power Cost

Total annual cost

Annual Cost

1,152,000.00

418 ,000.00

$1,570,000,00

CASE 4;

A,

REFRIGERATOR AND TRIPLE POINT STORAGE

,6 Refrigerator

Capital Cost — $2.19 x 10'

Power Cost

Storage Container

356,813 gallons.

Cost $3.57 X 10

Total annual cost

Annual Cost

394,200. 00

434,200. 00

642 , 600 . 00

$1,471,000. 00

CASE 5:

A.

REFRIGERATOR AND SLUSH STORAGE

,6 Refrigerator

Capital Cost — $2.26 x 10'

Power Cost

Storage Container

273,690 gallons , Costs $2.74 x 10

Total annual cost

Annual Cost

406 , 800.00

454,200. 00

493,200.00

$1,354,200.00

CASE 6: NITROGEN RELIQUEFIER AND HIGH PRESSURE STORAGE

Reliquefier , Capital Cost — 1.8 x 10

Power

Compressor to fill high pressure storage

Capital Cost — 2.29 x 10^

Power

High Pressure Storage Cylinders

Capital Cost -- . 056 X 10

Total annual cost

Annual Cost

324 , 000. 00

508,600.00

412,200.00

72,800. 00

10,100.00

$1,327,700.00

23
Table B2 . Summary of estimated costs to recover the nitrogen vent

gas from the 0.3 m Transonic Cryogenic Tunnel (continued)

CASE 7: NITROGEN RELIQUEFIER AND LOW PRESSURE STORAGE

Reliquef ier

Capital Cost — $1.8 x 10

Power Cost

Low Pressure Gas Holder

Capital Cost -- $5.94 x 10" 1,069,200.00

Total annual cost $1,901,800.00

Annual Cost

324,000.00

508,600.00

24

USCOMM ' ClU.
BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-857

.2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

CRYOGENIC DESIGN AND SAFETY REVIEW NASA-LANGLEY

RESEARCH CENTER 0.3 METER TRANSONIC CRYOGENIC

TUNNEL

5. Publication Date April 1977

6. Performing Organization Code

275.05

7. AUTHOR(S)

R. 0. Voth and T. R. Strobridge

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2750558

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Aeronautics and Space Admin.

Langley Research Center

Hampton, VA 2 3 365

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16, ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The findings of a Cryogenic Design and Safety Review of the

NASA-Langley 0.3 m Transonic Cryogenic Tunnel are presented in

this report. The tunnel working fluid and coolant is nitrogen.

The nitrogen, supplied as liquid, is exhausted as a low temperature gas. The use and storage of liquid nitrogen at the facility

presents several potential hazards to personnel and equipment.

An appropriate cryogenic design minimizes these risks, and provides for convenient tunnel operation and for the economical use

of the liquid nitrogen. The tunnel and ancilliary systems are

generally well designed but several recommendations to improve

the cryogenic systems are made. The cost of recovering the cold

vent gas is compared to the cost of producing the required liquid nitrogen using a captive air separation plant. Although the

economic analysis is preliminary, it shows that because of the

periodic operation of the tunnel, a captive air separation plant

has a lower annual operating cost than the vent gas recovery

systems considered.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Cryogenic; design; nitrogen; safety; wind-tunnel.

18. AVAILABILITY ^Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1 ! For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED 27

1 1 Order From Sup. of Doc, U.S. Government Printin° Washington, D.C. 20402. SD Cat. No. C13

Office 20. SECURITY CLASS

(THIS PAGE)

22. Price

IXX^ Order From National Technical Information Service

Springfield, Virginia 22151 (NTIS)

UNCLASSIFIED $4.00

USCOMM - ERL U SC OMM. D C 29042- P 7 4