The hypergolic propellant nitrogen tetroxide (N2O4 or NTO) is routinely used in spacecraft launched at Kennedy Space Center (KSC) and Cape Canaveral Air Station (CCAS). In the case of a catastrophic failure of the spacecraft, there would be a release of the unspent propellant in the form of a toxic cloud. Inhalation of this material at downwind concentrations which may be as high as 20 parts per million (ppm) for 30 minutes in duration, may produce irritation to the eyes, nose and respiratory tract. Studies at both KSC and CCAS have shown that the indoor concentrations of N2O4 during a toxic release may range from 1 to 15 ppm and depend on the air change rate (ACR) for a particular building and whether or not the air conditioning (A/C) system has been shut down or left in an operating mode. This project was initiated in order to assess how current A/C systems could be easily modified to prevent personnel from being exposed to toxic vapors. A sample system has been constructed to test the ability of several types of filter material to capture the N2O4 vapors prior to their infiltration into the A/C system. Test results will be presented which compare the efficiencies of standard A/C filters, water wash systems, and chemically impregnated filter material in taking toxic vapors out of the incoming air stream

Lueck, Dale E.

Mcnulty, R. J.

Meneghelli, Barry

Springer, Mike

English

NASA Technical Reports Server

(NASA-CR-199958) DEVELOPMENT OF AN N96-16912
IN-LINE FILTER TO PREVENT INTRUSION
OF N02 TOXIC VAPORS INTO A/C
SYSTEMS (I-NET) 9 p Unclas
G3/28 0091296
https://ntrs.nasa.gov/search.jsp?R=19960009746 2020-06-16T06:05:22+00:00Z
NASA-CR-199958
DEVELOPMENT OF AN IN-LINE FILTER TO
PREVENT INTRUSION OF NO2 TOXIC VAPORS
INTO A/C SYSTEMS
by
Barry Meneghelli, R.J. McNulty, and Mike Springer
I-NET Space Services, INI-15
Kennedy Space Center, FL, 32899
Dale E. Lueck
NASA, DL-ICD-A
Kennedy Space Center, FL 32899
ABSTRACT
The hypergolic propellant nitrogen tetroxide (NjO* or NTO) is routinely used in spacecraft launched at
Kennedy Space Center (KSC) and Cape Canaveral Air Station (CCAS). In the case of a catastrophic failure of the
spacecraft, there would be a release of the unspent propellant in the form of a toxic cloud. Inhalation of this material
at downwind concentrations which may be as high as 20 parts per million (ppm) for 30 minutes in duration, may
produce irritation to the eyes, nose and respiratory tract.
Studies at both KSC and CCAS have shown that the indoor concentrations of N2O< during a toxic release
may range from 1 to 15 ppm and depend on the air change rate (ACR) for a particular building and whether or not
the air conditioning (A/C) system has been shut down or left in an operating mode.
This project was initiated in order to assess how current A/C systems could be easily modified to prevent
personnel from being exposed to toxic vapors. A sample system has been constructed to test the ability of several
types of filter material to capture the N204 vapors prior to their infiltration into the A/C system. Test results will be
presented which compare the efficiencies of standard A/C filters, water wash systems, and chemically impregnated
filter material in taking toxic vapors out of the incoming air stream.
"^ INTRODUCTION
The Air Infiltration Studies completed in both KSC and CCAS facilities were initiated to determine the
sheltering effectiveness of a variety of structures in the event of a release of toxic materials such as hypergols from a
Titan launch abort. The facilities selected were intended to be representative of the structures at KSC/CCAS with
respect to location, facility size, materials of construction, and number of employees occupying the structure.
During the KSC/CCAS studies, sulfur hexafluoride (SF6), a nontoxic, invisible, and odorless gas, was used as the
tracer gas. The Tracer Gas Decay Technique was used to determine the air changes per hour (ACH) for a building
per ASTM E-741-93 "Standard Test Method for Determining Air Changes in a Single Zone by Means of a Tracer
Gas Dilution". Infrared (IR) gas analyzer instruments calibrated for SFe were used for all testing. Results of the
studies showed that existing heating and air conditioning systems (HVAC) were a major factor in the effectiveness "f
a shelter. The air exchanges per hour for buildings varied widely depending on the HVAC system, but correlated
well with the outside air intake from the HVAC design specifications. It was recommended that in the case of
contigency, the exhaust systems should be secured in order to further lower the air exchanges per hour. The study
presented in this paper was undertaken to examine the possibility of using a filter in-line with or in place of the
existing A/C filters which would decrease the incoming NO? concentration to acceptable levels and further protect
personnel.
* This work was performed under contract No. NAS10-11943 with NASA, John F. Kennedy Space Center, Florida.
Approved for public release; distribution is unlimited.
HARDWARE
The testing of air conditioning (A/C) coils and potential filter material was completed in the Toxic Vapor
Detection (TVD) lab, complex 34, CCAS. The basic test system, shown in figure 1, was fabricated inside of a
certified fume hood running at approximately 1000 cubic feet per minute (CFM). The dew point of the incoming air
stream was measured using a General Eastern Model 1211H dew point sensor. The velocity of the air within the
polyvinylchloride (PVC) sections of piping was monitored by an Alnor CompuFlow Model 8525
Thermoanemometer. Generation of NOj concentrations between 40 to 50 ppm was accomplished by allowing an
undiluted vapor sample of NOj to mix with the incoming air. The resulting air/NOj sample was homogenized
using an 18 inch section of piping downstream of the Alnor Thermoanemometer filled with polypropylene mixing
balls. The heat exchangers were manufactured by Super Radiator Coils and were all dimensioned with 3" by 3"
openings containing 1, 2, and 4 rows of 3/8" outside diameter (o.d.) copper tubes, respectively. The three different
coils were fabricated to test the effects of increasing the surface area of the coil for air contact. Drains were placed
both upstream and downstream of the A/C coils in order to catch any moisture which might accumulate for
subsequent analysis. The blower used during all testing was a PAR Model 35400-0000, with a capacity of 250
CFM. This blower was run using a DC motor controller operating at 12 volts and 5 amps . The concentration of
NOj generated within the testing assembly was monitored using Energetics Sciences Incorporated (ESI)
electrochemical cell analyzers and a DIONEX Ion Chromatograph. The ESI analyzers were calibrated prior to the
start of A/C coil testing and recalibrated on a monthly basis to assure analyzer integrity. The stability of the
DIONEX system was checked using internal standards during each sample run.
RESULTS
Initial Tests (Standard A/C Coils)
The initial task of the A/C coil study was to examine the potential removal efficiency of the coils using
humid air flowing at 500 feet per min (ft/min) across 1,2 or 4 coil heat exchangers. The measured flow path lengths
for the 1,2, and 4 coil heat exchangers are 1,2, and 3.5 inches, respectively. During testing, the coil temperatures
were held below the dew point of the inlet air so that a water film would form on the surfaces of the individual coils.
There are two factors in gas/liquid scrubbers that have to be considered when one is computing the
efficiency of a scrubber system. The first factor is the residence time. This time must be long enough for the
interaction between the reacting gas (N02) and the scrubbing liquid (H:O) to occur. The second factor is the
calculated ratio of the scrubber liquor mass flow (L) to the total gas mass flow (G), and is represented by L/G. The
scrubber liquor mass flow rate for typical oxidizer scrubbers is the product of the liquid pumping rate (50 gallons per
minute gal/min) and the density of the scrubber liquor (10.6 pounds per gallon, Ib/gal). The calculated mass flow
rate for the oxidizer scrubbers is the product of the volumetric flow rate in standard cubic feet per minute (serin) times
the standard density of the nitrogen carrier gas, 0.078 pounds per standard cubic foot (Ib/scf). The data presented in
table 1 is a summary of the efficiency of a typical oxidizer scrubber. The oxidizer scrubber towers are 30 inches in
diameter with approximately 4 feet of packing and the scrubbers have four towers connected in series. The scrubber
liquor, 25% sodium hydroxide (NaOH), is pumped at a total of 200 gallons per minute (gal/min) with an equal split
in all four towers. The data displayed in table 1 was taken with a constant pumping rate in the scrubber while the
total gas flow rate through the scrubber was varied. In the case of the standard A/C coils, where humidified outside
air is used to deposit a water film on the heat exchanger coils, the liquid mass flow can be approximated as the
difference between the mass of the water vapor in the incoming humidified air and the mass of saturated water which
can be found at a known coil temperature (table 2).

Table 1. Summary of Flow Rate vs. Efficiency, Residence Time, and L/G Ratio
Total Flow Rate
scfm(1)
108
202
215
308
408
508
Packing Depth
f t
16
1 6
. 1 6
1 6
1 6
1 6
Gas Velocity
ft /min
22.0
41.2
43.8
62.8
83.2
103.5
Residence Time
min
0.73
0.39
0.37
0.25
0.19
0.15
Efficiency*1'
100
94.2
92.2
81.7
72.9
70.6
L/G
Ratio(1)
61
32
31
21
16
1 3
Table 2. L/G Ratio for 1, 2, and 4 Coil Heat Exchangers
Heat Exchanger
1 Coil
1 Coil
1 Coil
1 Coil
2 Coil
2 Coil
2 Coil
4 Coil
4 Coil
4 Coil
4 Coil
Air Weight
Ib/min
3.52
3.52
3.52
3.52
3.52
3.52
3.52
3.52
3.52
3.52
3.52
Water Content
Inlet
Ib/min
0.0490
0.0490
0.0571
0.0486
0.0476
0.0553
0.0645
0.0472
0.0677
0.0641
0.0553
Water Content
Outlet
Ib/min
0.0183
0.0282
0.0197
0.0183
0.0282
0.0197
0.0211
0.0335
0.0388
0.0187
0.0211
Water Weight
Ib/min
0.0307
0.0208
0.0374
0.0303
0.0194
0.0356
0.0433
0.0137
0.0289
0.0455
0.0342
L/G
Ratio
0.0087
0.0059
0.0106
0.0086
0.0055
0.0101
0.0123
0.0039
0.0082
0.0129
0.0097
During testing of the A/C coils, there was no measurable decrease in the concentration of NO* when a
nominal 50 ppm concentration sample was passed through the 1,2, or 4 coil exchangers as measured by the ESI
electrochemical analyzers. This implies that the efficiencies for these tests would be zero. A summary of the heat
exchanger efficiency studies can be found in table 3.
Table 3. Efficiency Study with 1, 2, and 4 Coil Heat Exchangers
Gas Velocity
f t /min
500
500
500
Heat Exchanger
1 Coil
2 Coil
4 Coil
Path Length
f t
0.08
0.17
0.29
Residence Time
min
0.00017
0.00033
0.00058
Efficiency
0
0
0
The data presented in tables 2 and 3 clearly show that the residence times and L/G ratios are too low to
produce any measurable change in the NOj concentration by using the A/C coils alone. In comparison, the
residence times for NO2 in the hypergolic scrubber study shown in table 1 are 1000 times longer than the residence
times found in the A/C coil study presented in table 3. In addition, similar differences were found with the L/G
ratio for the scrubber study (table 1) as compared with the A/C coil study (table 2). These large differences suggest
that the path length for any solid adsorbent filter media to be tested must be large in order to achieve an efficiency of
90% or greater(^ .
Water Spray Testing
Hollow Cone Spray Nozzle
In an attempt to increase the gas residence time within a "scrubber" system, a hollow cone spray nozzle
(UNIJET Spray Tip, TNI) was procurred and placed into the test assembly. The inlet water pressure to this nizzle
was set for 100 psig. At this pressure the nozzle is designed to deliver water droplets with a size range of 60 to 70
microns. This droplet size was chosen in an attempt to maximize the ability of the water spray to mix with the
NOj in the vapor sample. The first test point for the water spray system was downstream of the polypropylene
mixing balls at a point perpendicular to the air flow. This position is designated as SPJ on figure 1. The flow of
the spray head was measured to be 1.0 gallons per hour (GPH). No decrease in the N02 concentration was observed
when comparing the output values from two independent ESI analyzers positioned upstream and downstream from
the injection point of the water spray. Subsequently, the water spray injection point was moved to SP2 on figure 1.
This sample point was chosen since the water spray flow would be countercurrent to the air flow, affording more
contact time between the scrubber liquor, H20, and the air stream. This countercurrent orientation of the scrubber
liquor and air stream is consistent with typical scrubbers. In conduction with the movement of the water spray
injection nozzle to SP2. the concentration of NO2 in an integrated water sample, measured as nitrate and nitrite, was
done using ion chromatography (1C). The nitrite and nitrate found in solution are the result of both NO: and N20*
dissolving in water to form nitrous and nitric acid. During subsequent testing of the water spray method, the ESI
electrochemical analyzers were used to monitor the relative concentration of the NOj, while depending on the results
of the 1C for the analytical concentrations.
1C Instrumentation and Methodology
The Ion Chromatograph used for analysis of water samples was a DIONEX LC20 1C System. The setup
parameters for the system during operation are listed in table 4. A QC sample with a concentration of 500
nanograms/milliliter (ng/ml) of sodium nitrite (NaNO2 ) was analyzed prior to quantifying any samples from the
A/C coil test system as an internal check of the instrument stability.
Table 4. DIONEX 1C System Parameters
1C Parameter
Detector
Pump
Eluent
Temperature
Sample loop
Pressure
Description
ED40 Conductivity Detector
GP40 Gradient Pump
1.8 mM Na2COyi.7mM NaHCO3
Ambient
100 microliters (ul)
1000 pounds per square inch gauge (PSIG)
12" PVC Test Bed
A secondary test section of PVC was constructed using a 4 ft section of 12" i.d. PVC in place of the 4" i.d.
PVC used during previous testing. Two water flow configurations, shown in figure 2, were used during the testing.
A water sample was collected for 10 minutes from the labeled drainage points and analyzed by 1C for each of the
water flow configurations.
4" PVC/Poiypropylene Ball Test Bed
The third configuration used during the testing of the water spray is shown in figure 3. The 50" section of
PVC is filled with polypropylene balls in a manner similar to the mixing section illustrated in figure 1. The water
spray is again pointed down with the air flowing in a countercurrent direction to maximize the contact area between
the air stream and scrubber liquor.
CO
C
_o
m
3
CO
^
O
u
u
in
O
00
I
a.
§,
C
JO
?I
O)
u=
O
u
From NO2
Gen.
PVC PIPE:
4" i.d.
50" length
ON/OFF
Valve
water Inlet
Air Flow
Polypropylene Mixing Balls
To Peristaltic
Pump
Figure 3. Water Spray Test Configuration (4" PVC)
Results
The data collected during the testing of the hollow cone water spray nozzle is summarized in table 5. Note
that the L/G ratios for all of the water testing are quite low compared to the typical ratios for standard scrubber
systems shown in table 2. At the measured air flow of 500 linear feet per minute (Ifm) the calculated residence time
for any sample within the water spray area is very short and precludes obtaining a high percentage of NO2 dropout.
Table 5. Hollow Cone Water Spray Nozzle Test Results
Pipe
Length (ft)
1.5
4
4.17
4.17
Pipe Size
(i.d. in
inches)
4
4
12
12
Mixing
Balls
Yes
Yes
No
No
%NO2
Dropout
0.93
1.01
1.72
2.12
L/G
Ratio
0.026
0.026
0.026
0.026
Air Flow
Direction
Up
Up
Down
Up
Water Flow
Direction
Down
Down
Up
Down
Residence Time
(min)
0.003
0.008
0.075
0.075
Ultrasonic Atomizing Nozzle
Further testing using water as the scrubber medium was continued using an ultrasonic atomizing nozzle
(Sono-Tek, Model 8700-48MS). This nozzle is designed to atomize liquid as it comes in contact with the
vibrating nozzle atomizing surface. The atomizing nozzle was positioned at SP2 (figure 1) and used to generate a
fine mist of water droplets countercurrent to incoming air flow. The mist was intended to provide a potentially
larger surface area for interaction with the incoming air sample. Water samples were pumped to the nozzle by means
of a Watson Marlow 505S variable speed peristaltic pump fitted with 6 roller pump heads and minicartridges. The
minicartridges housed Pharmed™ tubing able to dispense water at flows ranging from 1 to 25 seem. The speed of
the pump was fixed at 50 revolutions per minute (rpm). No decrease in the concentration of NO2 was observed
during testing of the ultrasonic nozzle.
Chemical Filter Testing
MSA Canister
The use of chemical filters as potential absorbents of NO: was initiated based on the minimal dropout of NO2
during the water spray testing. An MSA gas mask canister, type GMN-SSW, targeted for respiratory protection
against NO2) was initially tested at a flow of 15 sLpm and found to totally absorb NO2 from an air stream at a
concentration of 40 parts per million (ppm). The canister continued to absorb NO2 at concentration levels as high as
125 ppm and air flow rates as high as 370 sLpm. The concentration of NO2 was measured both upstream and
downstream of the canister by ESI electrochemical detectors. The canister is composed of soda lime, a mixture cf
calcium oxide and sodium hydroxide, along with hopcalite. The hopcalite is a mixture of the oxides of copper,
cobalt, manganese and silver, designed as a catalyst to convert carbon monoxide to carbon dioxide. The chemical
compoosition of the canister effectively scrubbs out the NO2 present in the incoming air sample in a manner similar
to the 25% NaOH rirubber liquor discussed earlier in this paper. The canister is rated as being able to protect
against concentrations of NO2 as high as 2% by volume for up to 12 minutes. During a catastrophic failure of a
spacecraft containing NTO, the concentration of levels of toxic material at building sites is projected to be in the 25
to 50 ppm range. This canister could be used as personnel protection for several hours at these low concentration
levels.
Charcoal
A secondary test of chemical filters was done using charcoal as the medium. A 12" section of 4" PVC was
filled with charcoal and packed at either end with standard A/C filter material and screening to prevent movement of
the charcoal during testing. This test bed decreased a 40 ppm vapor sample by 10% compared to an unfilled section
of PVC. This is a significant decrease in concentration, but not high enough to consider the charcoal as an effective
filter medium.
Conclusions
1. Water is not an effective medium for capturing NOj toxic vapors in a scrubber-like system at the high flow
rates typical of commercial HVAC systems.
2. Soda lime based materials are potential filter media for eliminating NOz toxic vapors, but further
experiments at high linear velocities are required to show feasibility.
Continuing Work
1. Testing of soda lime based materials at high linear velocities to show feasibility of filter media.
2. Sizing, fabrication and testing of an in-line filter using soda lime based materials as the filter media. The
sizing of the filter should approximate one which could be inserted into an existing A/C system.
3. Investigation of powder spray techniques as potential methods for removing NO2 toxic vapors.
Acknowledgements
The authors wish to thank members of the TVD/CML for their help in fabricating test fixtures and
suggesting methodologies for testing various filter media.
Bibliography
1. Parrish, C. and R. Barile, "Hypergolic Oxidizer and Fuel Scrubber Emissions Report", KSC-DL-3332,
September 30, 1994.
2. TVD/CML Internal Technical Memo, #95-005, July 25, 1995.


Development of an in-line filter to prevent intrusion of NO2 toxic vapors into A/C systems

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