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OCR for page 39
2
ENVIRO~:BTAL CONTROL SYSTEMS
ON CO~RCT~ PASSENGER AIRS
Although the variety of airplanes operating
throughout the world is large, the basic designs of the
environmental control systems (ECSs) used on most
aircraft in commercial service are remarkably similar.
In simplified terms, air is first compressed to high
pressure and temperature and then conditioned in an
environmental control unit (ECU), where excess moisture
is removed and the temperature necessary for heating or
cooling the airplane is established. The conditioned
air is then delivered to the cabin and cockpit to
maintain a comfortable environment.
DESCRIPTION OF ENVIRONMENTAL CONTROL SYSTEMS
COMPRESSED-AIR SOURCES
On the ground, compressed air for the ECS can be
obtained from an auxiliary power unit (APU), a special
ground cart (GCU), airport high-pressure hydrants, or
the aircraft engines. In flight, however, compressed
air is obtained almost exclusively from the compressor
stages of the aircraft engines.
In most respects, the composition of ambient outside
air will not be changed in the compression cycle.
Contaminants will in general be neither removed nor
added. Some particles can be removed by centrifuging in
the port through which air is extracted from the
engine. One contaminant that can be affected by the
heat of compression is ozone. In supersonic flight of
the Concorde, the compressed-air temperatures are so
high that nearly all the ozone is destroyed in the
engine, and no further treatment with catalysts or
filters is needed. In all other commercial aircraft,
the normal temperature of the compressed air taken from
39
OCR for page 40
40
the engine for air-conditioning is not adequate to
reduce the free ozone concentration substantially.
Oil seal leaks have sometimes permitted engine oil
to leak into the compressors, and oil can then enter the
bleed air in the form of vapor or, in extreme cases,
mist. In recent years, oil seal failures have not been
a problem. Where engine seal design does not prevent
oil vapors from entering the system, turbo-driven or
engine-driven compressors are installed. The use of
separate compressors increases weight, decreases
reliability, and imposer additional maintenance
requirements.
For ground air-conditioning, high-temperature
compressed air can be supplied to the cabin through the
ECU from an onboard APU or from a portable ground cart.
These units operate much like the main engines in
generating compressed air; however, the design is
usually optimized for efficient delivery of compressed
air, rather than propulsive thrust. The air supplied by
these units is taken from the ramp area and contains
whatever contaminants are present in that area.
High-pressure air can also be supplied from airport
facilities. Because of the lower operating cost of
fixed electrically driven generating and compressor
units and the reduction in ramp contamination and noise,
the use of high-pressure ground air facilities is
lacreasing.
Preconditioned low-pressure air, which is the
lowest-cost source of heating and cooling, can be
supplied directly to the airplane air distribution
system through ground connections from portable
air-conditioning units or from central airport
facilities. The air supplied is taken from the ramp
area or the terminal and contains contaminants-typical
of those areas.
THE ENVIRONMENTAL CONTROL UNIT
In flight, high-pressure, high-temperature air is
conditioned by processing through the ECU before delivery
to the cabin. The ECU (or "pack") usually consists of an
air-cycle machine (ACM) and one or more heat exchangers.
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41
A simplified schematic, Figure 2-1, shows how air is
conditioned in cruise and delivered to the cabin to meet
heating, cooling, ventilation, and pressurization
requirements.
Normally, ambient temperatures at cruise altitudes
are low enough for bleed air to be cooled adequately by
the heat exchangers alone, and the ACM is completely
bypassed. On the ground, at lower altitudes, in
"hot-day" conditions, and during low-speed flight, the
ACM will be used to cool the air further to meet cabin
requirements.
Mechanical water separators are used for ground and
low-altitude operation to remove water droplets from the
outside air. These droplets are formed when the air is
expanded and cooled in the ECU turbine and are very
fine, about 5 IBM in diameter. The mechanical water
separator contains a bag (usually of finely woven Dacron
or frayed Teflon) that coalesces the fine droplets and
permits them to be centrifuged out in the downstream
section of the water separator.
The efficiency of the water separator generally is
80-90%. Water not removed enters the cabin ducting,
where it absorbs heat from the distribution system and
is vaporized. The liquid droplets sometimes appear an
fog emanating from the outlet grilles.
To prevent freezing of the water separator, ACM
discharge temperatures must be limited to about 35°F.
Recent developments have led to the use of
high-pressure water separators that condense and remove
moisture from the bleed air before it expands in the
turbine. This design, which is currently in use on
B-757s and B-767s,~8 permits the moisture content of
air entering the turbine to be less than 15 grains/lb
(2 g/kg), which in turn permits the ECU to discharge air
from the turbine at temperatures well below freezing.
If air were introduced into the cabin at subfreezing
temperatures, draft and local cold areas would be
created. Therefore, recirculated cabin air is mixed
with the cold ECU discharge air at a ratio of about 1:1
to achieve a minimal temperature of 35-40°F, the minimal
temperature to prevent icing. Through this mixing and
operation of the ECUs to produce very low discharge
OCR for page 42
42
<
,
If high-pressure
water separators
are used, they are
installed here.
ACM
1P
\
~ Bypass
T ,- . ~l
'a ~
Outside air at ~65° f and 3.5 psia
enters the jet engine and passes
through multistage compressors,
where the temperature and pressure
are increased.
A portion of the air passing through
the engine is extracted from the
intermediate stages of the compressors
at about 400° F and 35 psia.
The air is ducted to a flow-control
valve, which regulates the flow of
outside air to the cabin. (It can have
dual flow schedules selectable by
the crew.)
Pressure will be about 20 psia, and
temperature about 390° F.
The bleed air is passed through
heat exchanger(s), where it is
cooled by outside air to the required
temperature for cabin air~onditioning.
At cruise conditions, air will normally
bypass the air~ycle machine entirely.
However, under unusual conditions, air
can be cooled further by expanding
through the turbine.
At low altitudes, where humidity is high,
a mechanical water separator is used to
remove liquid water from the air. No
moisture is removed at cruise conditions.
Conditioned outside air is supplied to
the flight station and cabin at 12 psia
and at the desired temperature.
FIGURE 2-1 Operation of aircraft environmental
control unit in cruise conditions at 3S,OOO ft.
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43
temperatures, the cooling capacity of each pound of
outside air is almost doubled, compared with that in
systems that use conventional water separators.
AIR DISTRIBUTION
Outside air, conditioned by the ECU to the proper
temperatures, is usually mixed in a plenum and then
distributed to the cockpit and the cabin zones. A
large, wide-body aircraft might have as many as six
individual temperature-controlled zones, each with its
own supply ducting system, whereas a smaller,
narrow-body aircraft usually has only two such zones,
one for the cabin and one for the cockpit. Airflow to
each zone is established by the cooling requirement of
the zone. The total cooling requirement is met by
supplying a quantity of air to the zone at the
low-temperature limit (40°F). Because passenger and
crew heat loads account for only 40-50X of the total
cooling requirement--whereas the remaining 50-60%
(lighting, solar loads, and conduction through cabin
structure) is determined by cabin areas, rather than by
number of passengers--outside air will not be
distributed strictly on a Der-nas~en~er hack ~
First-class and business sections of the cabin might
have 2-3 times an high a ventilation rate per occupant
as the economy section.
O ~
Because of the larger solar and electronic cooling
loads in the cockpit, ventilation per flight crew member
might be 10 times as high as that in the cabin, or even
higher.
The distribution of outside air (or outside and
recirculated air) to the cabin is usually fixed by the
ducting design and flow-balancing orifices. However,
some combi-aircraft (aircraft modified to carry
passengers and cargo in the main cabin) have provisions
to reduce airflow to the aft section when cargo is
carried in that section ot the cabin.
Outlets (or gaspers) ~ ~ ~~
~ ~ Individual air
that can be adjusted by the
passenger for air flow and direction can be supplied
with cold air taken from the ECU discharge or with air
from the main supply ducts in the cabin. Thus, the air
can be fresh, a mixture of fresh and recirculated air,
or, as in the case of the DC-10 seat-mounted Raspers,
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44
only recirculated air. Gaspers are generally being
phased out in the newer and wide-body aircraft.
The main supply air enters the cabin through fixed
outlets, which con be in the ceiling or in the sidewalls
below the overhead storage bins. Some aircraft have
both types of outlets, and the selection of a system to
use is based on whether the aircraft is being heated or
cooled.
EXHAUST SYSTEMS
Air is normally exhausted from the cabin through
floor-level grilles, which run the length of the cabin
on both sides along the sidewall. The exhaust air is
directed alongside or through the lower-lobe cargo
compartments, where it can provide some heating or
cooling. The air is then exhausted overboard through
outflow valves controlled to maintain the desired cabin
pressure. Figure 2-2 illustrates typical passenger
cabin airflow patterns. Cabin exhaust air is also used
to cool avionics and electric equipment and then
discharged overboard through the outflow valves.
Center Stowage B in ~
r Conditioned Air Distribution Duct
if , Conditioned Air Outlet
/~: ?: \\
l ~
J
~_ ~
a, .
ret ~
Cabin Air Exhaust
I__
FIGURE 2-2 Typical passenger cabin airflow
patterns. Reprinted from Lorengo and Porter.9
OCR for page 45
45
Lavatories are ventilated with cabin air drawn
through them. About 3-5 cfm is supplied with an
individual air outlet. The ventilation air is exhausted
overboard, either directly through a port in the skin of
the aircraft or through ducts that direct the air toward
the outflow valves.
Air is exhausted from galleys to exhaust moisture
and food odors and to prevent their diffusion into the
cabin. Galley ventilation air can be ducted directly
overboard or to the outflow valves. Galleys and
lavatories are often exhausted through a common duct
system.
RECIRCULATION SYSTEMS
Recirculation systems have been used on the early
Convair 880 and 990, B-707, DC-8, Lockheed Electra, and
many other older aircraft that used vapor-cycle cooling
systems. The use of air recirculation systems in modern
aircraft has recently increased with the advent of
higher engine bypass ratios, higher jet-fuel costs, the
design of "stretched" versions of production aircraft,
and the development of advanced ECUs that use high-
pressure water separators. The bypass ratio is the
ratio of fan air flow to high-pressure or engine-core
air flow. The fuel and performance penalties associated
with bleed-air extraction increase as the bypass ratio
increases. As aircraft are "stretched" to increase
seating capacity, recirculation systems are added to
improve air distribution and circulation. To use the
greater cooling capacity of ECUs equipped with high-
pressure water separators, warm cabin air must be mixed
with outside air to raise the temperature of air
supplied to the cabin. The very cold ECU discharge air
would cause condensation and local draft problems if
introduced into the cabin without mixing.
In 1985, about 30% of the seat-hours flown by U.S.
airlines were on aircraft with recirculation systems.
By 1990, this percentage will have increased to 40X, as
more of the newer, fuel-efficient aircraft enter service.
Air for recirculation can be taken from the general
space above the ceiling (B-747), from slotted openings
in the ceiling (DC-10), or from underfloor spaces. In
OCR for page 46
46
about 75% of the aircraft with recirculation systems in
1985, the recirculated air was returned to the same
zone. In the remaining 25%, recirculated air was mixed
with outside air and distributed throughout the cabin
and, in the B-767 and some models of the B-757, to the
cockpit. By 1990, the percentage in which air is
totally mixed with outside air will increase from 25% to
about 45%.
Recirculation air can be filtered to remove lint,
aerosols, and gaseous tars from tobacco smoke. Although
the technology is well developed for removing gases with
charcoal, only some models of the B-757 and B-767
currently use this method in the recirculation system.
These aircraft have charcoal filters available as an
airline option. Particle filters that remove
particles as small as 0.3 Am are installed in 80% of
the aircraft with recirculation systems.
Some aircraft manufacturers and filter manufacturers
are conducting research to improve equipment for
removing particles and gases from recirculated air.
Programs begun in 1985 are investigating the use of
electrostatic precipitators in aircraft to remove
particles (McDonnell Douglas, personal communication,
1985; Boeing, personal communication, 1985~.
TEMPERATURE CONTROL
Temperature in each zone of the aircraft is
controlled to a value selected by the flight crew,
usually between 65 and 85°F. Turbine bypass and
heat-exchanger airflow valves are typically used to
establish the ECU discharge temperature and a zone
reheating system to establish supply temperatures for
each zone. Where discharge air from the ECUs is mixed
in a plenum, the ECU discharge temperature is controlled
to meet the demands of the zone that requires the coldest
air, and a reheating system is used to add hot bleed air
to the other zones, which need less cooling or more
heating. Operation of the zone reheating system does
not substantially affect air flow and distribution to
the zones.
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47
PRESSURE CONTROL
An automatic pressure control system establishes the
cabin pressure as a function of altitude and controls
the rate of change of cabin pressure during climb and
descent. Cabin pressures during cruise are based on the
allowable pressure difference between the cabin and the
outside. The allowable difference varies with aircraft
design and is a structural limit. Figure 2-3 shows the
relationship between cabin and flight altitudes for
typical commercial aircraft. The maximal cabin altitude
cannot exceed 8,000 it for normal operation up to the
certified aircraft altitude.
10
-
° 8
to,
-
C:
~6
-
J
~4
U'
UJ
if
-
CO
SL
G Max. Cruise Altitude
M n.Rn
U0 ~ O. ~
...,,g God,..)
· /, / /
..,, / /
..,; / /
B 737 ,, ~ ;011, 8-727
e-X //'
~ B 747
SL 5 10 15 20 25 30 35 40 45
AIRPLANE ALTITUDE, 1,000 ft
FIGURE 2-3 Cabin-pressure altitude at maximal
differential pressure. SL = sea level. Data from
Lorengo and Porter.9
The rate of pressure change is controlled during
climb and descent to meet criteria for passenger comfort
and pressure-difference limits of the aircraft. The
recommended rates of change of pressure for passenger
comfort are 500 ft/min (-0.256 psi/min) during climb and
300 ft/min (+0.154 psi/min) during descent.1 2
The crew can select higher or lower rates of change,
but the controls are normally set at the recommended
value, which is usually identified by an index mark on
the pressure control panel.
OCR for page 48
48
PERFORMANCE OF ENVIRONMENTAL CONTROL SYSTEMS
A mayor aspect of aircraft ECS performance, And one
that was considered of primary importance by the
Committee in the study of cabin air quality, is the -
ventilation rate. Aircraft ventilation rate is defined
as the amount of outside air supplied to the passengers
and crew in cubic feet per minute per occupant and is
determined by dividing the outside air supplied at
design conditions by the passenger and crew seats.
Normal conditions include full passenger load, operation
of all ECUs at rated flow, and steady-state cruise.
Airlines may increase the passenger capacity above what
is shown in Table 2-1, and that would reduce passenger
ventilation rates. In addition, the operation of the
ECU can affect the ventilation rate. Minimal airflow
must be adequate to meet heating, cooling, and
TABLE 2-1
Effect of Flow Options and Seating Density
on Passenger Ventilation Ratea
ECU Oneration Outside Air Per Passenger, cfm
Aircraft No. Flow First Overall Economy
Model Passengers No. Schedule Class Averazeb Class
DC-10 290 3 High -- 18.5 -
290 2 Low -- 6.9 -
274 3 High -- 21.0 -
274 2 Low -- 8.4 -
L-1011-1 279 3 Normal 28.7 -- 19.5
279 2 Normal 17.3 -- 11.6
L-1011-500 235 3 Normal 51.0 -- 18.6
235 2 Normal 30.5 -- 11.2
B-747-200 381 3 Normal 40.4 -- 17.1
381 2 Normal 26.5 -- 8.5
300C 3 50% 11.4 -- 10.6
265C 2 50% 12.7 -- 10.6
B-767-200 217 2 Normal 10.0 -- 9.9
217 2 Optional 20.0d __ l9.8d
filters
installed
1
a Based on data from Aerospace Industries Association of America.
b Section data not available.
c Recommended maximal number of passengers (Boeing).
d Includes treated air.
OCR for page 49
49
pressurization requirements throughout the aircraft.
Variation in seating density between first-class and
economy sections causes a variation in the outside-air
ventilation rates in these areas.
If the ECS is made up of three independent ECUs, the
operator might be permitted to dispatch the aircraft
with one ECU inoperative or, if the aircraft was
dispatched with all ECUs operating, to shut down one of
the units. In either case, the ventilation rate can be
less than originally specified by the manufacturer.
Crew options also include selection of individual
ECU flow rates. High and low flow schedules are
sometimes incorporated into the ECU flow control valve,
to permit crew operation of each unit at normal or
reduced flow. Reduced-flow schedules are usually
one-half to two-thirds of normal. Operators of the
B-747 and DC-10 also have access to dual-schedule flow
control valves that permit selection of ventilation flow
in increments of less than one full ECU. This design is
available an an airline option. The option of reducing
flow by shutting off a pack is now available only on the
B-747, DC-10, and L-1011 aircraft, all of which have
three independent ECUs.
Because in normal cruise conditions the ECUs have
more than adequate heating and cooling capacity,
ventilation can be reduced with no substantial effect on
cabin temperature or pressure. Airlines are therefore
financially motivated to save fuel by reducing the amount
of ventilating air that is taken from the engines.
A NASA-sponsored study in 19801° showed that about
62,000 gal of fuel, or about 1% of the annual total,
could be saved per year per DC-10 if the flight crew
reduced the ventilation flow from 18 cfm to 8 cfm per
passenger.
The combined effect on passenger ventilation rate of
reducing ventilating air flow and variations in seating
density is shown in Table 2-1.
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53
40
a:
O 30 _
-
u"
20 _
of
us
co
u)
~5
o
10 _
go
C)
cr:
or
O
r~~
20
_-- 1 990
Minimal Ventilation
tCFM/pas~engerl
Ave. Std. Dev.
1 985 22.6 1 0.4
20.9 10.1
10
OUTSI DE Al R. cfm/passenger
30 40 50-50+
FIGURE 2-6 Ventilation rate distribution, minimal flow,
for mayor U.S. domestic airlines. Passenger flight-hours
= (number of passengers)(flight duration, hours). Based
on data from U.S. FAAl9 and ATA.3
40
u,
3 So
-
u"
us 20
z
C
'_ 1 0
UJ
C:
UJ
: _
o _1
1985
1990
Maximal Ventilation
(CFM/passenger)
Ave. Std. Dev.
26.? 10.3
24.8 1 0.4
0 10 20 30 40 50
OUTSIDE AIR, ctmtpassenger
FIGURE 2-7 Ventilation rate distribution, maximal flow,
for major U.S. domestic airlines. Passenger flight-hours
= (number of passengers)(flight duration, hours). Based
on data from U.S. FAAl9 and ATA.3
OCR for page 54
54
70%, MF = 1.43. The load-factor frequency was taken
from Figure 2-5, with the percent unaccommodated demand
arbitrarily assigned to 100% load factor. The actual
ventilation rate (AVR) then was summed for each aircraft
type, on the basis of the percent of seat hours flown by
that aircraft and the load-factor frequency. For
example, in 1984, B-727s flying 27.7% of the total U.S.
fleet seat-hours would dispatch 16.6% of the flights
with a load factor between 20 and 40%. The load-factor
mean of 30% was used, the multiplier was 3.3, and the
AVR was 57e8 cfm/passenger. The total number of seat-
hours at 57~8 cfm/passenger then was 0.166 x 0~277 =
0.046 (4.6%~. To convert seat-hours to passenger-hours,
this value was multiplied by the load factor for this
segment (30%~. Thus, B-727s provided 1.38X of passenger
flight hours, with an AVR greater than 50 cfm/passenger.
The values for each airplane and each load factor segment
(Figure 2-5) were summed to generate Figure 2-6. Figure
2-7 was generated in the same way, except that minimal
ventilation rates were used. The ventilation rates used
in preparing Figure 2-6 were based on the flight crew' 8
use of minimal flow permitted by the aircraft design.
The frequency of use of low-flow options by flight crews
is unknown. The effect of crew use of maximal flow on
ventilation rate is shown in Figure 2-7. However, the
trend toward lower ventilation rates is expected to
continue. This will occur through the addition of
recirculation systems to the existing fleet, the
increased use of low-flow options, and the introduction
into the U.S. airline fleet of more aircraft that use
higher percentages of recirculated air (B-767, B-757,
B-737-300, and MD-80.
EFFECT OF VENTILATION ON TOTAL CABIN ENVIRONMENT
Outside-air ventilation is the prime variable
affecting contamination in the aircraft cabin. At high
outside-air ventilation rates, passenger well-being is
increased with respect to carbon monoxide and carbon
dioxide, contamination due to smoking, and odor.
Increasing total cabin airflow (with either outside or
recirculated air) also increases movement of air, which
creates a feeling of freshness and reduces temperature
stratification.
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55
Higher outside-air ventilation rater lower cabin
relative humidity. In addition, when the aircraft is
operating in regions of high ambient ozone, cabin ozone
is also increased by the increased use of outside air.
An increase in total cabin airflow, with either outside
or recirculated air, creates a potential for local high
velocities and drafts, adds a direct fuel cost, and
potentially involves costs of equipment weight and
maintenance.
VENTILATION AND CONTAMINATION
Cabin ventilation provides air for dilution of
contaminants and supplies oxygen for passengers and
crew. As shown in Table 2-1 and Figures 2-6 and 2-7,
outside-air ventilation rates can vary widely. Oxygen
requirements for sedentary adults can be met with only
0.24 cfm. 4 Thus, even at the lowest ventilation
rates on aircraft, there is no significant reduction in
the percentage of oxygen in the cabin. Contamination
with carbon dioxide varies inversely with ventilation
rate, because carbon dioxide production by passengers is
nearly constant. However, the amount of contamination
with tobacco smoke (carbon monoxide and particles)
depends on ventilation rate, number of smokers, and
smoking rate.
Smokers on airplanes are estimated to make up 33% of
the total passenger load. The average smoking rate has
been estimated at 1.25-2.2 cigarettes/in per smoker.
Halfpenny and Starrett7 measured 1.25 cigarettes/in
per smoker on 33 2-h flights. Cain et al. 5 used a
rate of 2 cigarettes/in per smoker in 1982 odor studies,
and Thayerl 6 calculated an average smoking rate of
2.2 cigarettes/in on the basis of the total number of
cigarettes produced, 33% of the population aged 18 and
over being smokers, and a 15-h smoking day.
With a generally constant smoking rate, the
concentration of tobacco smoke depends on the flow of
outside air into the cabin. Passengers perceive
tobacco-smoke contaminants in the form of odor and
irritation of eyes and nasal passages. Acceptance of
air contaminated with tobacco smoke has been measured in
Juries of smokers and nonsmokers in odor test rooms and
in an airplane mockup. The results of three studies are
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56
shown in Figure 2-8. The difference in Jury acceptance
of contamination shown in Figure 2-8 is due to the
evaluation criteria used by the investigators. The
results obtained by Cain et al.5 were based on odor
evaluations by active smokers, and the high degree of
acceptance by the occupants, compared with that reported
by the nonsmoking visitors, represents odor adaptation.
Halfpenny and Starrett7 and Thayer~6 evaluated odor
and occupant irritation. Because people do not adapt to
the irritants in tobacco smoke--rather, the degree of
irritation increases with duration of exposure, reaching
a peak after about 15 min and then remaining relatively
constant7--the acceptance of odor and irritation
shown is lower than acceptance of odor alone.
100
o
~ 80 _
-
O 60
o
z
c~ 40
Cot
At
20
LU
O _
- e.~. ·
o
O ,' ~
/fW
0 O t//
/
/
l I
l
o
· Odor evaluations by occupants. Data from Cain et al.5
O Odor evaluations by visitors. Data from Cain et al.5
- Irritation evaluations by smokers. Data from Thayer.15
Irritation evaluations by nonsmokers. Data from Thayer.15
Irritation evaluations. Data from Halfpenny and Starrett.6
0 10 20 30 40 50 60 70 80
V E NT I LAT I O N R ATE, of m/smoker
FIGURE 2-8 Relationship of ventilation rate to
acceptability by smokers and nonsmokers of
tobacco smoke odor/irritation. The Cain et al.
data--outside-air flow (L/s) and number of
cigarettes smoked--are converted to cfm/smoker,
according to [(L/s)~2.118~/~(cigarettes/h)~2~.
In their studies, the air had 50X relative
humidity. Data from Cain et al., 5 Halfpenny
and Starrett, 7 and Thayer.l 6
OCR for page 57
57
All the data shown in Figure 2-8 were taken at
relative humidities of 30-75%, which are much higher
than are normally encountered in airplanes. Kerka and
Humphrey showed that, in general, increased
humidity tended to decrease sensory response to odors
and irritants. Cain et al.5 showed that "high
humidity" (75%) generated a more intense odor response
than "moderate humidity" (50%~. However, the degree to
which low humidity typical of aircraft cabins (usually
5-10%) can affect response to odor and irritation has
not been investigated.
The contamination at various ventilation rates
encountered in airplane smoking sections and the average
contamination in the cabin when air in smoking and
nonsmoking sections is fully mixed are also shown in
Figure 2-8.
Contamination in the form of tars can affect
aircraft systems where cabin air is used for cooling.
Avionics components that are usually cooled by cabin air
are adversely affected by a buildup of tars and lint,
which reduces component cooling. Particularly
vulnerable are temperature control sensors that respond
to a flow of cabin air. Tars and lint cause slow sensor
response, which results in unstable cabin temperatures.
Axial-flow fans have become so contaminated with tobacco
tars that fan blades are stuck to the housing, causing
motor overheating and premature bearing failures. The
actual increase in maintenance costs due to tobacco
smoke was not available; however, it is generally felt
by airliner maintenance personnel that they are
significant. \5
AIR VELOCITY AND CABIN FLOW PATTERNS
Circulation of air in the passenger area at
velocities of 10-60 ft/min is necessary to prevent local
stagnation and temperature stratification. A minimal
velocity of 10 ft/min (0.05 m/s) is necessary to avoid
the sensation of stagnation, whereas velocities above 60
ft/min (0.3 m/s) can create a draft sensation on the
neck. is Aircraft distribution systems normally
provide adequate circulation when the ECS is operated at
full rated flow. However, when total outside air is
reduced and there is no compensating recirculated air,
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58
/
stagnation can be created, and normal flow patterns in
the cabin can be affected. Operating with reduced
outside-air flow sometimes causes air from the smoking
areas to be drawn into nonsmoking areas. This can occur
if bleed flow is reduced to the point where controlled
exhaust through outflow valves is very low and the bulk
of the exhaust is through leakage paths. This can create
fore and aft flow in the cabin which can spread tobacco
smoke into nonsmoking zones.
RELATIVE HUMIDITY
Relative humidity in aircraft cabins in cruise is
seldom controlled and depends entirely on the moisture
given off by passengers and crew in the form of
respiratory vapor and perspiration. The amount of
moisture given off depends on the extent of activity and
cabin temperature. A sedentary passenger normally emits
about 0.7 g/min, and a cabin crew member, about 2 g/min.
Because outside air is essentially dry (moisture at less
than 100 ppm), cabin relative humidity varies inversely
with ventilation rate (see Figure 2_9~.12
40
30
o
I 20
UJ
-
Cl
LU
CC
10
o
\` 65 F
\ \ Air/ 70 F
- ~75-F
Cabin Temp
iO
OUTSIDE AIR, cfm/passenger
20 25 30
FIGURE 2-9 Relation of relative humidity and outside-
air ventilation rate. Equivalent cabin altitude,
6,500 ft. Data from SAE.12
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s9
OZONE
Ambient ozone is present above the tropopause, whose
height varies with latitude and season. It normally
exists at an approximate altitude of 11 km (36,000 It) in
then mi 111 ~ 1~1 tulips in cow
~ . Ozone enters the cabin
with outside air through the engines and ECU. Residual
cabin ozone concentration is a function of the outside
concentration, the design of the air distribution
system, the use of catalysts or adsorbers, and the total
airflow. Each airplane has a characteristic cabin ozone
retention factor, which is the ratio of the ozone
concentration in the cabin to the ozone concentration in
outside air after it has passed through the ECU.
Normally, the retention ratio is from 0.75:1 to 1.00:1
without any recirculation, but it can be as low as 0.4:1
with recirculation. 20 Where the retention ratio is
too high to limit cabin ozone to the FAR 121 maximum,
alternative treatment of the outside air is required.
Noble-metal catalysts are used to remove a portion of
the ozone before it enters the cabin. These units have
~~ ~ (See Chapter 5 for
removal etticlency of 90-95%.°
additional details on ozone.)
EFFECT OF RECIRCULATION ON CONTAMINATION
Cabin recirculation systems on most airplanes result
in partial or complete mixing of air in the smoking and
nonsmoking sections. Recirculation air is often taken
from a plenum near the outflow valve where exhaust air
from all cabin sections is collected and then distributed
to all sections and in some cases to the cockpit. This
negates to some extent the nonsmoking/smoking sectioning
of the cabin. The flow model developed by the Committee
has been used to evaluate contamination in all sections
as a result of recirculation designs (see Appendix A).
COST OF VENTILATION
The direct cost of supplying outside air to
passengers and crew includes the loss of aircraft thrust
due to the extraction of high-pressure air from the
engine compressors, the power loss due to the extraction
of fan air for precooking, and the rem drag incurred in
ECU heat-exchanger cooling. All this power loss must be
OCR for page 60
60
compensated for by increasing engine power settings,
which increases fuel consumption.
The net cost of ventilation is reduced somewhat by
the use of thrust-recovery exhaust valves, which
discharge exhaust air aft and produce positive thrust.
The weight penalty for basic ECS equipment should not
be charged to the design ventilation air flow, because
the equipment is normally sized to meet design cooling
requirements, which are based on hot-day conditions at
sea level. However, if the ventilation rate were
increased above the flow required for cooling as
designed, then the weight penalty of the added ECS
equipment (large ducts, valves, heat exchangers, etc.)
would constitute an added ventilation cost.
Studies by aircraft manufacturers to establish
ventilation costs have shown significant variation in
those costs. The Boeing Commercial Airplane Company
estimated a fuel-burn penalty of 0.015 gal/in per cubic
foot per minute (gph/cfm) for the B-727 and B-747,11
whereas McDonnell Douglas estimated O.OO9 gph/cfm for a
DC-10 in a NASA-funded fuel-reduction program.~°
These variations are due in part to the stage length
used in the analyses and the ambient conditions; fuel
penalty is higher in climb and on hot days. The
greatest variation, however, is due to the drag
coefficients used.
The range of fuel costs in gph/cfm per passenger
based on these analyses is shown in Figure 2-10. To
place the cost of aircraft ventilation in perspective,
it can be compared with the cost of providing equivalent
fresh air in commercial or residential buildings. The
cost of providing outside air for an airplane is 22-37
times the cost of providing the same amount of air in
Washington, D.C., during the coldest month, January. \7
Fuel costs constitute a substantial percentage of
operating costs. At the current price of 76-86
cents/gallon, fuel costs for the wide-body fleet (B-767
B-747, A-300-B4, DC-10-10, and L-1011) in the quarter
ended September 30, 1985, ranged from 52 to 68X of the
cash operating cost and from 37 to 57X of the total
aircraft operating expenses. 2
OCR for page 61
61
0.8
0.6
0.4
0.1
O
10 15 20 25
OUTSIDE AIR, cfm/passenger
30 35
FIGURE 2-10 Fuel required for ventilation with outside
air. Data from Reese.
REFERENCES
1. Aerospace Industries Association of America, Inc.
Airplane Air Conditioning System Configuration and
Air Flow Data for Selected Boeing, Douglas, and
Lockheed Aircraft. (unpublished document,
September 17, 1985)
2. Aircraft operating data. Air Transport World
23~5~:188, 1986.
3. Air Transport Association of America, Economics
and Finance Department. The Significance of
Airline Passenger Load factors. Washington, D.C.:
Air Transport Association of America, 1980.
4. American Society of Heating, Refrigerating, and
Air-Conditioning Engineers, Inc. ASHRAE Standard
Ventilation for Acceptable Air Quality. ASHRAE
62-1981. Atlanta, Gal: American Society of
Heating, Refrigerating, and Air-Conditioning
Engineers, Inc., 1981.
OCR for page 62
62
Cain, W. S., B. P. Leaderer, R. Isneroff, L. G.
Berglund, R. J. Huey, E. D. Lipsitt, and D.
Perlman. Ventilation requirements in buildings,
I: control of occupancy odor and tobacco smoke
odor. Atmos. Environ. 17:1183-1197, 1983.
6. Engelhard Special Chemicals Division. Ozone
Converter. Specification SIC-3. Service
Union, N.J.: Engelhard,
Information Letter SIL-3.
1984.
7. Halfpenny, P. F., and P. S. Starrett. Control of
odor and irritation due to cigarette smoking
aboard aircraft. ASHRAE J. 3~3~:39-44, 1961.
8. Kerka, W. F., and C. M. Humphreys. Temperature
and humidity effect on odor perception. ASHRAE
Trance. 62:531-552, 1956.
9. Lorengo, D. E., and A. Porter. Aircraft
Ventilation Systems Study: Final Report.
DTFA-03-84-CO-0084. Atlantic City, N.J.: U.S.
Federal Aviation Administration Technical Center,
1985. (draft)
10. Newman, W. H., and M. R. Viele. Engine Bleed Air
Reduction in DC-10. NASA-CR-159846. Long Beach,
Cal.: Douglas Aircraft Company, 1980.
Reese, J. Statement, pp. 51-81. In U.S. Senate
Committee on Commerce, Science, and
Transportation, Subcommittee on Aviation (97th
Congress, 2nd Session). Airliner Cabin Safety and
Health Standards: Hearing on S. 1770, May 20,
1982. Serial No. 97-122. Washington, D.C.: U.S.
Government Printing Office, 1982.
_
12. Society of Automotive Engineers. Aero-Space
Applied Thermodynamics Manual. ARP-1168. New
York, N.Y.: Society of Automotive Engineers, 1969.
13. Society of Automotive Engineers. Air Conditioning
Systems for Subsonic Airplanes. ARP-85E.
Warrendale, Pa.: Society of Automotive Engineers,
1986.
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63
14. Society of Automotive Engineers. Aircraft Cabin
Pressurization Control Criteria. ARP-1270.
Warrendale, Pa.: Society of Automotive Engineers,
1976.
15. Society of Automotive Engineers. Environmental
Control System Contamination. AIR-1539.
Warrendale, Pa.: Society of Automotive Engineers,
1981.
16. Thayer, W. W. Tobacco smoke dilution
recommendations for comfortable ventilation.
ASHRAE Trans. 88~2~:291-306, 1982.
17. U.S. Bureau of the Census. Statistical Abstract
of the United States, 1985, p. 217. 105th ed.
Washington, D.C.: U.S. Government Printing Office,
1984.
18. U.S. Federal Aviation Administration. Aircraft
Air Conditioning and Pressurization System
Documentation. (unpublished compilation, 1985.)
19. U.S. Federal Aviation Administration. FAA
Aviation Forecasts, Fiscal Years 1985-1996.
FAA-APO-85-2. Washington, D.C.: U.S. Federal
Aviation Administration, 1985.
20. U.S. Federal Aviation Administration. Transport
Category Airplanes Cabin Ozone Concentrations.
Advisory Circular 120-38. Washington, D.C.: U.S.
Federal Aviation Administration, 1980.
Representative terms from entire chapter:
ventilation rate
