The vulnerability of electronic equipment to carbon fibers is studied. The effectiveness of interfaces, such as filters, doors, window screens, and cabinets, which affect the concentration, exposure, or deposition of carbon fibers on both (internal and external) sides of the interface is examined. The transfer function of multilayer aluminum mesh, wet and dry, polyurethane foam, and window screen are determined as a function of air velocity. FIlters installed in typical traffic control boxes and air conditioners are also considered

Taback, I.

English

NASA Technical Reports Server

TRANSFER FUNCTIONS 
7 
Israel Taback 
The Bionetics Corporation 
There are various interfaces, such as filters, doors, window 
screens, and cabinets, which affect the concentration, exposure, 
or deposition on the two sides of an interface. The dimensionless 
ratio of these quantities as they challenge and breach the interface 
is called the transfer function. 
Before transfer functions are discussed, it is useful to re- 
view the overall problem of doing a vulnerability assessment. I 
will talk in general about single fiber lengths, but when overall 
vulnerability assessment is done, we must remember that the fibers 
released from the source are of various lengths and, in general, 
would have a distribution as illustrated in figure 1. It is be- 
lieved that the shorter ones will be numerous and the longer ones 
relatively infrequent. This would be designated as a source char- 
acteristic. At a specific location away from the source, that dis- 
tribution can very well change, depending upon how those various 
fibers fly, the fall rates, and perhaps some of the fiber lift char- 
NUMBER RELEASED 
FROM SOURCE 
EXTERNAL EXPOSURE 
AT SPECIFIC 
LOCATION 
TRANSFER FUNCTION, 
EI/E 
0 
EXPOSURE AT 
EQUIPMENT 
FIBER LENGTH SPECTRA 
SOURCE 
CHARACTERISTICS 
DISSEMINATION 
EFFECTS 
FIBER LENGTH 
Figure 1 
99 
https://ntrs.nasa.gov/search.jsp?R=19790014031 2020-03-20T19:01:43+00:00Z
acteristics. The phenomena would be included in dissemination ef- 
fects. We then get to the external exposure outside a piece of 
equipment. The transfer functions between the external and inter- 
nal world must be evaluated, and we find that the transfer functions 
for the shorter particles are larger than the transfer functions 
for the larger ones. To get the exposure for a piece of equipment, 
all of these factors are evaluated at a fixed fiber length. The 
installed vulnerability of the specific equipment is evaluated by 
determining its local exposure at a fixed length and then inte- 
grating its vulnerability over the complete length spectrum. 
Figure 2 illustrates a number of typical values for filter 
transfer functions. This work was done with fibers about 7 milli- 
meters in length. These are a limited number of samples, there 
are more in the literature. The first one illustrated is one 
thatis used quite often in ground support equipment. It is a multi- 
layer aluminum mesh called Air Maze (trade name), tested dry. In 
its normal mode, this material is used coated with a sticky liquid. 
We tested it dry in order to get some idea of what would happen if 
the filter were poorly maintained. The figure illustrates the 
transfer function at a velocity of 100 feet per minute, which is 
approximately the velocity at which the filter is used. 
FILTER TRANSMISSION 
MULTI LAYER ALUMINUM ESH, 
AIR MAZE R-82, UNCOATED (DRY) 
MULTI LAYER ALUMINUM ESH, 
AIR MAZE R-82, COATED (WET> 
3/b-INCH-THICK OPEN FOAM 
POLYURETHANE 
WINDOW SCREEN 
100 
(MEDIUM LENGTH FIBERS) 
AIR VELOCITY IN FEET/MIN 
100 400 800 
1% 2% 4% 
OJ% 
O,l% 
1% 
Figure 2 
0,2% 
0,9% 
10% 
0,5% 
104% 
15% 
Transmissivity factors of the order of 1% may occur, and as 
the air velocity through that filter is increased, we find that the 
percentage passed through that filter also increases. These tests 
were done at 100, 400, and 800 feet per minute. When this same fil- 
ter is coated, which is the condition in which it is normally used, 
it has transmissivity of the order of tenths of a percent at all 
velocities. Its transmissivity is still a function of air veloc- 
ity. Another commonly used filter is open foam polyurethane. 
This filter was 3/8-inch-thick open foam. Again the transmissiv- 
ities ranged from about a tenth of a percent to 1.4 percent. We 
also tested one very interesting interface, a window screen, which 
is not normally regarded as a filter. 
At air velocities of about 100 feet per minute (which is about 
a mile an hour), it had a transmissivity of 1% and then rose very 
rapidly at higher speeds to about 10 to 15%. One other filter was 
tested with material of rather long lengths, compared to the lengths 
of the fibers just discussed. It is another piece of polyurethane 
filter, and the results are shown on figure 3. It has a trade name, 
Scott Foam, and is 3/4" thick. It did not react the way the other 
filters did in terms of its response to various air velocities. 
,008 - 
FILTER DATA 
SCOTTFOAM, 3/4" THICK 
16 MM INPUT 
24 MM PASSED 
I I I I I 
300 400 500 600 
VELOCITY, FPM 
Figure 3 
101 
When the 16-millimeter material was passed through the filter, 
the amount of l&millimeter material which came through the filter 
varied with air velocity from about 0.004 (note this is not per- 
cent, this is in mass ratio) to a value about 0.001 at high veloc- 
ity. On the other hand, if you look at the mass transfer ratio 
without regard to the length of the material passed (the upper 
solid line), this was relatively invariant with velocity through 
the filter and, in fact, decreased as the velocity through the fil- 
ter increased. The ratio of all lengths passed was about 2 to 3 
times the amount of material that was passed at the 16-millimeter 
length. Very much smaller ratios were secured for the 24-millime- 
ter material, and again there was a difference between the total 
amount of material passed, regardless of length and the material 
that was passed at the original length. These numbers, by the way, 
seem to be somewhat typical of most of the filters that we have 
tried. A transfer function of approximately a few tenths of one 
percent seems to be a number that we come across many times. 
One typical enclosure, sketched on figure 4, that we encoun- 
tered in some of our survey work was evaluated for its overall 
transfer function. This is a rather typical traffic control box 
somewhat similar to the kind that exist on many street corners. 
It contains electronic equipment which controls traffic signals. 
That equipment not only controls the traffic signals, but also has ' 
TRAFFIC CONTROL ENCLOSURE TRANSFER FUNCTION 
VOLUME, V = 1 METER3 
STEADY STATE: 
FLOOR AREA, A = Z/3 METER* 
T.F,c = yi = F = ,0014 C 
0 
FLOW RATE, 61 = l/30 METERhC c 3 
l+V, 
G! 
Ci DT 
C I 
TIME 
FILTER T,Fn, F = ,002 
s 
FIBER SETTLING VELOCITY, V, = 802 M/S s 
= T.F, = a0014 
co DT 
Figure 4 
102 
within itself a conflict sensor, such that in the event of an elec- 
tric fault, it will put all the lights on that corner onto red. One 
electronic failure in this box, generally, is not of large concern 
with regard to causing accidents. The volume of this box is about 
one cubic meter, and the floor area is 2/3 of a square meter. The 
-enclosure is ventilated by a flow into the box of about a 30th of a 
cubic meter per second, so that it takes about 30 seconds to venti- 
late the entire box. 
We have tested the specific filter that is used in these box- 
es, at least in the city of Newport News, and find that these fil- 
ters have a transfer function of about 0.002. The fibers that were 
considered here are single fibers, and their fall velocity is of 
the order of about 0.02 meters per second. If one hypothesizes 
that there is a square pulse of concentration outside the box and 
looks at the inside of the box, the concentration inside the box 
comes up slowly with time and reaches a steady-state value. 
At the end of the pulse, the concentration drops off at a rate 
that is shown in the figure consistent with the ventilation rate of 
the box. If the internal concentration is computed with respect 
to the external, it turns out to be a function of the filter fac- 
tor divided by one plus a correction term. This term consists of 
the fall velocity times the area of the box divided by the flow 
rate into the box. This fall rate, this settling term as it is 
called, is the settling that would occur if the internal mixing in 
the box was fairly uniform so that the concentration throughout 
the box was constant. 
The overall-steady state transfer function for concentration 
turns out to be 0.0014, somewhat smaller than the filter function 
of 0.002. The transfer function for exposure is the ratio of the 
integrals for concentration with time. For the external exposure, 
the integral takes the form of a square pulse. For the internal 
exposure the integration must include the effect of buildup and 
the effect of drop-off. In performing the integration, the two 
effects balance, and the transfer function for exposure is exactly 
the same number as computed for the steady-state ratio of 
concentrations. 
There is one interesting thing about this filter, as well as 
all others. (1'11 present this in inches and you can turn it into 
meters). If the height of that filter were about 6 inches and 
there was a little bit of a gap at the top of the filter (which we 
have observed in many installations) of perhaps an eighth of an 
inch, one would find that perhaps ten times as many fibers entered 
that box due to the poor installation of the filters. The most 
important factor with regard to the number of fibers that may be 
ingested into this box may not be the filter itself but how well 
it is inserted into its holder. 
103 
The next illustration, figure 5, is for a typical room. It is 
hard to recognize in meters but it is a 9- by 12-foot room with an 
8-foot ceiling. The infiltration factor which I have assumed is 
0.1. The flow rate into the room is such that the ventilation 
time for the room would be of the order of about 3 hours. That is 
fairly realistic. It could be as little as 2 and as large as 6 de- 
pending on how well built the house was. We assumed that the room 
has an air conditioner with a filter having a transfer function of 
0.005 with a flow rate such that it could ventilate the room in 
about 8.6 minutes. 
The equation in the upper right of the figure gives the same 
relationship that was shown in the previous figure: that is, the 
ratio of the internal to the external concentration. Again, the 
integral is really the same ratio that would be secured in the 
steady state (if indeed the pulse lasted long enough to achieve 
steady state). The ratio is always correct regardless of the 
length of the pulse. 
Again, we find that the predominate factors that determine the 
ratio of internal to external transfer function consist of the filter 
factor, the filter infiltration factor, and three terms in the denom- 
inator: the settling velocity times the area of the room, the filter 
factor of the air conditioner and its flow rate, and both divided 
TYPICAL ROOM TRANSFER FUNCTIONS 
ROOM DATA 
V VOLUME = 23,3 M3 
A FLOOR AREA = 9,72 M2 
Fi INFILTRATION T,F, = On1 
Qi FLOW RATE = ,0022 M3/SEC 
VENTILATION TIME = 3 HRS 
AIR CONDITIONER DATA 
F AC AIR CONDITIONER T,F, = ,005 
Q AC FLOW RATE = .04 M3/SEC 
VENTILATION TIME = 8,6 MINS, 
% FIBER SETTLING VEL0C.s 002 M/S 
AIR CONDITIONER RECIRCULATING: 
Ci DT 
= F 
co or 1+ 
V, A + (1 - F,,) QAC 
Qi 
5 0,l = 10-s 
l+ 
a19 + ,045 
,0022 
AIR CONDITIONER INTAKE: 
s 
Ci DT 
n = 
Q; Fi + QAc F,, 
3 co DT Qi + Q,, + V, A 
2,2 x 10-4 + 2.2 x IO-4 = = 2 x 10-3 
80022 + ,047 + 019 
Figure 5 
104 
by the infiltration volume flow. There are really only two impor- 
tant terms here: one is the filter infiltration factor, and the 
second is the fall rate of the fiber. The characteristics of 
the air conditioner, as such, do not strongly affect the ratio of 
internal to external exposure. That ratio, the room transfer func- 
tion, turns out to be about 10m3. One can recompute this with 
the air conditioner bringing in air from the outside. 
We still have an infiltration factor which is assumed to be 
the same, although it may change somewhat with the air conditioner 
running. The input-flow volume of the air conditioner and its fil- 
ter factor appear, and when that is evaluated, it turns out that 
the fact that the air conditioner is taking in air and passing it 
through its filter has only raised the internal to external rela- 
tionship by just a factor of two. 
Of further interest is that if one shuts off the air condi- 
tioner entirely (that is, let the second term be zero), one gets the 
same answer as in the first case or about 10D3. So although I did 
not choose these numbers they are typical of a room. For all three 
conditions of operating that air conditioner, there is not an enor- 
mous difference between the ratios of internal to external exposure 
(time integrals of the concentration). 
Figure 6 lists the work we have planned on transfer functions. 
PLANNED TRANSFER FUNCTION WORK 
ADDITIONAL FILTER TESTS: SHORTER LENGTHS 
COARSE, MEDIUM, FINE 
USED FILTERS 
SPECIAL FILTERS: AIRCRAFT AIR CLEANERS 
AIRCRAFT WATER SEPARATORS 
GENERIC CABINET T,F, TESTS 
FILTER T,F, MODELING 
VERIFICATION OF TYPICAL ROOM/EQUIPMENT PREDICTIONS 
Figure 6 
105 
I suspect its elements are rather obvious; do more filter tests 
at lengths down to the order of 1 millimeter or shorter. We did 
want to test down to this value somewhat earlier in time, but have 
been limited until very recently, both in terms of being able to 
make fibers of that length in a practical way and in sensing those 
fibers in the laboratory. I believe we are now in a position of 
running our tests down to about 1 millimeter. Various filters of 
different fineness will be tested. Special aircraft filters will 
be tested because of the specific responsibility NASA has regarding 
aircraft. 
Generic cabinet transfer function tests are planned. These 
tests are on cabinets with louvers and on appliance cases with air- 
flow which is fairly typical of that encountered in practice. 
Filter transfer function modeling will be attempted in order 
to predict transfer functions, and although the computations shown 
on the previous figures seem to be straight forward, there is a need 
to verify the settling terms in those equations. These may be some- 
what dependent upon the amount and the scale of the turbulence that 
might exist within a given enclosure, and it is planned to check 
this under one or two-sets of conditions. 
Figure 7 lists some preliminary conclusions. I believe that, 
PRELIMINARY CONCLUSIONS 
1, 
2, 
3, 
A BODY OF ANALYTIC AND TEST DATA EXISTS WHICH ALLOWS COMPUTATION 
OF TRANSFER FUNCTIONS TO BE MADE FOR SPECIFIC INSTALLATIONS, 
ADDITIONAL TEST DATA AND MODELING IS NEEDED REGARDING ENERAL 
TYPES OF ENCLOSURES FOR PERFORMING THE NATIONAL RISK ESTIMATE, 
VERIFICATION BY TEST OF COMPUTED TRANSFER FUNCTIONS FOR TYPICAL 
INSTALLATIONS HAS YET TO BE ACCOMPLISHED, 
Figure 7 
106 
if we have any specific installation and understand its filter, 
the infiltration flow rates, and forced ventilation rates, there 
is no question that the transfer function can be computed. How- 
ever, to generalize these relationships for particular industries, 
homes, and other facilities in the country, additional modeling 
work must be done so that the national risk can be computed. 
107 


Transfer functions

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