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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
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.
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
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.
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,
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
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
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.
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.
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.
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.
50 LOAD FACTORS The load factor, or percentage of aircraft seats occupied, is a vital statistic in airline operation. A measure of potential profitability, it is used by airlines in route analysis, equipment assignment, and decisions regarding purchase of new equipment. Load factor also has a mayor effect on the ventilation rate of a flight. Just as the number of seats cannot be tailored to each flight, the ventilation system on newer aircraft has only limited variability. Thus, the ventilation rate is much higher on low-load flights than when the aircraft is full. The average load factor in the United States declined in fiscal 1984 to 57.8%. FAA projects a rise to 59.1% in 1985 and, after a slight decline in 1986, a slow rise to 63.8X in 1996.19 The effects of load factor on ventilation rate and resulting air quality are to a large extent buried in the averaging process. To measure and evaluate ventilation rates, it is necessary to examine individual flights. An ATA study of individual flight data that had been collected by CAB in 1975-1976 showed that the frequency distribution of load factors can be well represented by a normal curve at the lower end. The upper-end or right- hand extension of the curve is cut off by the physical limit on the available seats. The extension of the normal curve past the 100% load factor is called "unaccommodated demand" by ATA. 3 Unaccommodated demand occurs when the number of requests for passenger seats is greater than the capacity. Therefore, although passenger demand follows a normal distribution, the flight load-factor distribution is a truncated normal curve, as shown in Figure 2-4. The Boeing Commercial Airplane Company studied the relationship between average passenger load factor and unaccommodated demand (the percentage of passengers who cannot be accommodated at their desired departure times) and developed a program that defines this relationship. 3 The relationship is shown in Table 2-2. On the basin of average load factors, 1 9 the resulting unaccommodated demand (Table 2-2), the truncated normal distribution curve (Figure 2-4), and the assumption that the
51 unaccommodated demand will represent the percentage of flights at 100% load factor, a load-factor distribution histogram was prepared for the years 1985 and 1990. It is shown in Figure 2-5. A ventilation distribution was then calculated by the Committee on the basis of the load factors in Figure 2-S, the ventilation rate of each major aircraft model, and the percentage of seat-hours flown by that model (Table 2-3~. These ventilation distributions (Figures 2-6 and 2-7) were calculated as follows: A ventilation rate multiplying factor (MF), based on a load factor, was used to modify the basic ventilation rate for each aircraft type (Table 2-3~. For a load factor of 50%, MF = 2; for a load factor of 100% Capacity ~ i ~ At UJ a LO \ Unaccommodated nominal Average LOAD FACTOR FIGURE 2-4 Demand distribution at various load factors. Adapted from ATA.3 TABLE 2-2 Unaccommodated Demand vs. Load Factora Average Load Factor J fib 80 70 60 50 Unaccommodated Demand J ~XC 64 21 2 a Based on data from ATA.3 b Average year-round load factor for U.S. airline industry. Percentage of passengers who cannot be accommodated at their desired departure times.
52 30 En I 20 11 10 1 ~ 1 ! I 1 o ., 0 20 40 60 80 1 00 LOAD FACTOR, % Load Factor (percent} Ave. Std. Dev. 25.7 _ 1985 59.1 _ _ _ 1 990 62.0 27.0 8% at 100% LF-1990 f 5.6% at 100% LF-1985 1 FIGURE 2-5 Load-factor distribution. Based on data from U.S. FAAl9 and ATA.3 TABLE 2-3 Seat-Hours Flown and Ventilation Rates, by Aircraft Typea Proportion of Total Seat-Hoursb Aircraft Flown Annuallv. % Outside Air, Model 1984 l_955C 1986C 1981-1990c cfm/occucantd B-727 27.7 24.8 24.2 22.9 17.5 B-747 18.8 19.5 19.1 18.0 8.5e-40.4 DC-9/MD-80 12.8 16.0 16.2 15.9 13.7 DC-10 11.7 10.8 10.9 10.4 6.9e-18.5 B-737-200 8.4 8.0 7.9 7.7 15.3 L-1011 7.1 9.1 8.9 B.5 11.6e B-767-200 3.3 3.4 3.3 4.0 9.9e-18.8f A-300 2.2 2.5 2.4 4.0 14.3 DC-8 3.1 2.2 2.0 1.9 13.0 B-757 0.8 1.8 2.7 3.8 8.8e-18.7f B-737-300 0.4 1.8 2.4 2.9 8.6 a Based on data from Lorengo and Porter.9 b Seat-hours = (number of seats installed)(flight duration, hours). c Projection. d At lOOX passenger load factor, including normal complement of cabin crew. e At minimal flow. f Incorporates optional particle filters and charcoal filters to remove gaseous and aerosol contaminants at 90X efficiency. Higher flow rate shown is with charcoal filters installed.
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
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.
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
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
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,
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
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
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
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.
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.
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.
