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Technical Assessment of Dry Ice Limits on Aircraft (2013)

Chapter: Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft

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Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
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Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
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Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
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Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
×
Page 38
Page 39
Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
×
Page 39
Page 40
Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
×
Page 40
Page 41
Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
×
Page 41
Page 42
Suggested Citation:"Chapter 10 - Measurements of Carbon Dioxide Concentrations on Aircraft." National Academies of Sciences, Engineering, and Medicine. 2013. Technical Assessment of Dry Ice Limits on Aircraft. Washington, DC: The National Academies Press. doi: 10.17226/22651.
×
Page 42

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35 Results of Previous Studies Reviewing the studies mentioned previously: • The observed carbon dioxide concentration in the passen- ger cabin varied over a 700- to 2,000-ppm range but was typically about 1,200 to 1,500 ppm. • There was not always sufficient information on passen- ger loading to estimate carbon dioxide production from human metabolism. Moreover, human production of car- bon dioxide depends not only on the number of people but also on their size and activity level. • Even though there is a high circulation rate of air in the cabin, sampling location may also play a part. • It is not clear whether the range of observed concentra- tions is due to variations in passenger loading and metabo- lism, sampling errors, or variations in the operation of the aircraft ventilation system. • Because of these uncertainties, a safety factor may need to be applied to the ventilation rate when setting dry ice lim- its. Additional work would be needed to resolve this issue. Measurements Made for This Study As part of the HMCRP Project 09 study, measurements of carbon dioxide concentrations were made in the passenger cabin of a Boeing 777 aircraft flying a transatlantic route from a U.S. hub to London. On the outbound trip, the aircraft had approximately 680 kg of dry ice in the cargo compartment; on the return trip, it had none. In terms of passenger load, both flights were essentially full, with fewer than five empty seats. Experimental Instrumentation The study used two Li-Cor LI-820 CO2 monitors. This instru- ment is a non-dispersive infrared (NDIR) gas analyzer. As con- figured for this study, one monitor had a 14-cm cell length and a Review of Previous Measurements Studies of Carbon Dioxide Concentrations in Aircraft Passenger Cabins There have been several studies on the concentration of carbon dioxide in the air in the cabins of passenger aircraft. In 1991, Nagda et al. reported carbon dioxide levels in the passenger cabin on 92 commercial airliner flights and found that the average for narrow-body aircraft was 1,700 ppm and the average for wide-body aircraft was 1,200 ppm.54 The same year O’Donnell reported carbon dioxide concentrations, temperature, and humidity on 45 commer- cial airliner flights; the mean carbon dioxide concentration was 719 ppm.55 In 1999, Pierce et al. reported carbon dioxide concentra- tions on eight Boeing 777 flights.56 The average for domes- tic flights was 1,600 ppm, while the average for international flights was 1,400 ppm. Hocking has compared these and other data with calculated carbon dioxide concentrations based on information about cabin air supply.57 Also in 1999, Lee et al. measured carbon dioxide levels in aircraft cabins on 16 flights and found an average of 935 ppm58 More recently, Murphy reported calculated ventilation rates based on measured carbon dioxide concentrations on board passenger aircraft and found apparent ventilation rates of 9 to 16 air changes per hour.59 Studies of Carbon Dioxide Concentrations in Aircraft Cargo Compartments No published information was found that describes the results of measurements of carbon dioxide concentrations in aircraft cargo compartments. There is anecdotal infor- mation that some air carriers may have performed such tests, but the results have not been published or made avail- able publicly. C h a p t e r 1 0 Measurements of Carbon Dioxide Concentrations on Aircraft

36 range of 0 to 2,000 ppm of carbon dioxide, and the second had a 5-cm path length and a range of 0 to 5,000 ppm carbon dioxide. An SKC “Grab Air” sample pump was used to draw a sample of cabin air through the Li-Cor at a nominal rate of 1 liter per minute. A particulate filter was used upstream of the sample inlet to remove any dust particles. Because of the duration of the flights (9 to 10 hours), the seat-based power ports were used to power the gas analyzer* and the notebook computer. The sample pump was powered by a standard 9-volt alkaline battery. Figure 10 shows the gas analyzer and sample pump. There were two sets of instruments, and measurements were planned to be made at two locations in the cabin on both the outbound and the return flights. However, one of the instrument packages, the one with the greater range, was disabled at the outset of the outbound flight by a voltage surge or sag in the power provided to the seat-based power ports. The trouble was subsequently traced to a blown fuse on an internal circuit board, and the instrument was repaired in time for use on the return flight. Procedures Two flights were tested. The outbound flight was from a U.S. hub to London; the return flight was from London back to the same U.S. hub. Carbon dioxide concentration measurements were made at one sample location on the outbound trip and two sample locations on the return trip. Both sample locations were at normal passenger seats. The equipment was placed under- neath the seat in front, and the inlet to the sample pump was positioned about 40 cm above cabin floor level. For the out- bound flight, the sampling location was in the business class section. For the return flight, one sampling location was in the business class section and one was in the coach class section. Data were recorded at 2-second intervals and stored on a notebook computer. The data recorded included time, car- bon dioxide concentration, cabin pressure, and the cell tem- perature inside the NDIR instrument.* Dry Ice Load In cargo compartments. The dry ice test load in the cargo compartment* on the outbound flight consisted of 680 kg† of dry ice in 60-lb blocks‡ loaded on a wooden pal- let that was in turn placed inside a ULD. The blocks were wrapped in shrink wrap. Figure 11 shows the configuration of the dry ice load. Exact weights were obtained for the dry ice just prior to departure and again just after landing in London. The return flight did not contain any dry ice in the cargo compartment. In passenger cabin. Dry ice is used in the passenger cabin to keep food/beverage carts cold. As shown in the left photo in Figure 12, for a large aircraft like a Boeing 777, a *The power needed was about 4 watts. An auxiliary battery pack with 8 D-cell batteries could power the instrument for about 4 hours; the battery pack was used for measurements during boarding when seat power was not available. *The sample cell in the NDIR instrument is heated and kept at a nomi- nal temperature of 50°C in order to eliminate the effect of variations in gas density caused by variations in temperature. The instrument contains a built-in pressure sensor and uses a correction algorithm to account for variations in ambient pressure. *Cargo loading was managed such that the test load was the only dry ice cargo on this particular flight. †This is the airline’s legal limit for this type of aircraft. ‡Dry ice blocks typically measure 10 in. × 10 in. × 12 in. and weigh 60 lbs. Figure 10. Carbon dioxide meter. Battelle photo Figure 11. Dry ice load placed on outbound flight.

37 considerable amount of food must be stowed, and dry ice is used to provide additional cooling.§ As the right photo in Figure 12 shows, the food in some of these carts is covered with slabs of dry ice. Results of Measurements The data collected on the outbound flight are shown in Fig- ure 13; Figure 14 and Figure 15* show the data collected on the return flight. For both the outbound and return flights, mea- surements were made while the aircraft was on the ground and passengers were boarding, but were suspended during takeoff and did not resume until the aircraft reached an altitude of 10,000 ft, resulting in a gap in the data near the beginning of the flight. Data collection was ended as the plane descended through the 10,000-ft level on landing. Other small gaps resulted from minor power interruptions. During the time labeled as Note 1 on Figure 15, the sample tube became dis- connected and air was not entering the analyzer cell.† General Observations Looking at the results for the outbound flight in Figure 13 and the return flight in Figure 14 and Figure 15, it can be seen that: Over 100,000 individual measurements of time, carbon diox- ide concentration, and cabin pressure were taken. The FAA carbon dioxide limit of 5,000 ppm was never exceeded or even approached. Note that in Figure 13, the 2,000-ppm range was briefly exceeded once the instrument was turned on after the airplane reached 10,000 feet and electronic instruments could be used. In the absence of temporary excursions, the carbon dioxide concentration on the outbound flight averaged about 1,300 ppm. On the return flight, the carbon dioxide concentration averaged about 1,250 ppm at the business class location and 1,300 ppm at the coach class location. This difference is not considered to be significant. The general range of carbon dioxide concentration values observed is consistent with previous studies. The spikes above the general background were associated with food/beverage service and tended to occur when a food/beverage cart was near the sample inlet. There was a spike at the beginning of the outbound flight that exceeded the 0 to 2,000 ppm range of the instrument. The period was brief and clearly did not approach 5,000 ppm at the location where the instrument was positioned. At this time the flight attendants were busily opening the many food carts, each of which had kilogram quantities of dry ice that had been stowed during passenger loading and takeoff. It is believed the carbon dioxide that had accumulated in the carts during pas- senger loading and takeoff was now being emitted as the carts were being opened. There can be no other explanation for the transient in Figure 13. The transient on the return flight, shown in Figure 14, is smaller because less dry ice was being carried in the carts and because the carts were opened at a slower pace, reflecting the dif- ference between doing a full dinner service in first and business class for the outbound evening flight and a more leisurely brunch being served on the return flight that left mid-morning. Looking at the steady-state portions of Figure 14 and Figure 15, the data show that the carbon dioxide concentration did not vary significantly with the seat location. Figure 12. Food carts showing use of dry ice. §Many aircraft are also equipped with chiller systems, thus reducing the amount of dry ice required for refrigeration of food and beverages. *The effect of using the instrument with the shorter path length can be shown by comparing the traces in Figure 15 with those in Figure 14. The data in Figure 15 show more noise. †This was one of several experimental challenges related to working in the close quarters of a coach seat space.

Figure 13. Cabin carbon dioxide concentration on outbound flight.

Figure 14. Cabin carbon dioxide concentration on return flight, location 1.

Figure 15. Cabin carbon dioxide concentration on return flight, location 2.

41 The overall levels of carbon dioxide were somewhat higher on the outbound flight, particularly near the beginning of the flight. While the difference in the steady-state concentration between the outbound and return flight might not be statistically signifi- cant, the 50 ppm difference in steady-state values, as well as the higher levels observed at the beginning of the outbound flight, could be attributed to (a) differences in the numbers of passen- gers (the return flight had four more passengers), (b) differences in the food service (the outbound flight began with a dinner ser- vice; the return flight started with a smaller lunch service), or (c) infiltration of carbon dioxide from the dry ice carried on the outbound flight into the passenger cabin from the cargo hold. The likelihood and importance of these possible sources are dis- cussed in the following. Analysis of the Relative Importance of Carbon Dioxide Sources The sources of carbon dioxide include (a) the outside air, (b) the passengers and crew, (c) any dry ice used for cooling food and beverages to be served on board, and (d) carbon dioxide that may have originated from dry ice in the cargo compartments and subsequently entered the passenger cabin. In order to place these carbon dioxide sources in perspec- tive, calculations were performed using some very simple assumptions. The results of the calculations are presented in Table 9; the assumptions are discussed in more detail in the following. Carbon Dioxide from Passengers and Cabin Crew The amount of carbon dioxide generated by the passen- gers and cabin crew depends on their metabolic activity level, stated in METS. Inasmuch as the passengers are mainly at rest, we assume an activity level for the passengers of 1.0 MET.* Based on literature values, the airline flight attendant activity level was assumed to be 3.0 MET.61 The air carrier supplied both passenger counts and crew staffing levels for the test flights. The total people-generated carbon dioxide was estimated using parameters from ASHRAE and the method described by Murphy.62 For the test flights, this source of carbon dioxide amounted to about 9 to 10 kg/hr. Carbon Dioxide from Ventilation Air Information on the actual ventilation rate for the test flight was not available. However, the body of published work, as well as available data from aircraft manufacturers, indicates that typical aircraft ventilation rates range from 10 to 20 ACH. Based on published information on aircraft specifications, the cabin volume of the Boeing 777-200 aircraft was estimated to be 480 m3, and, based on data for other wide body aircraft, the air change rate was assumed to be 14.7 ACH. Assuming that the ventilation air contains 390 ppm of carbon dioxide* and using the ventilation rate data supplied by Boeing for their 777 aircraft, about 6 kg/hr of carbon dioxide enters with the ventilation air. Carbon Dioxide (CO2) Source Estimated CO2 Production Rate, kg/hr Percent of Total Percent Added CO2 Calculated ppm CO2, Outbound Calculated ppm CO2, Return People 14 56 74 689 699 Ventilation air 6 24 — 390 390 Food service 4 16 21 303 241 Dry ice in cargoa 1a 4 16 55 0 Totals 25 100 100 1,427 1,321 a The amount of air leakage from the cargo compartment to the passenger compartment is not known. However, if more than 10% of the carbon dioxide produced from dry ice in the cargo compartment entered the passenger cabin, the leakage would have been detected by measuring the difference in carbon dioxide concentration during the outbound flight, which had 680 kg of dry ice in the cargo hold, and the concentration during the return flight with no dry ice in the cargo hold. It is not suggested that such in-leakage occurs; it is included only to compare magnitudes of possible sources. Boeing does note that “carriage of animals or other odiferous cargo in the forward cargo compartment may result in odors in the main cabin and flight deck.”60 This indicates that some small amount of air flow from this cargo compartment into the cabin is possible. Table 9. Relative importance of carbon dioxide sources on transatlantic flight. *People at rest have an activity level of 1.0 MET. Various activities are considered as multiples of the resting value; the sleeping level is 0.9 MET; jogging is about 7 MET. *Information from NOAA and from NASA test flights shows that, at altitude, the concentration of carbon dioxide in the air does not vary more than ±3 ppm from this value.

42 Carbon Dioxide from Dry Ice Used for Food and Beverage Cooling Some airlines use dry ice for keeping food and beverages cold that will be served on board. The amount of carbon dioxide from this source varies widely and is dependent on the type of meal service provided, the length of the flight, and the way that the dry ice is used. Dry ice was used for food and beverage cooling on the flights that we surveyed. It is esti- mated that the amount of carbon dioxide generated from this source could be from 1 to 5 kg/hr. Carbon Dioxide from Sublimation of Dry Ice in Cargo Compartment As part of this test, on the outbound flight, 680 kg† of dry ice blocks was placed on a wood pallet in an uninsulated ULD and loaded into the Boeing 777 cargo compartment. Based on the actual weights of this ULD at the beginning and end of the flight, the estimated carbon dioxide produc- tion rate from the dry ice in the cargo compartment‡ was 7.3 kg/hr. Figure 9 is a schematic of the air flow on a typical airplane. Based on this diagram, all of the carbon dioxide produced from the presence of dry ice in the cargo compartment would be expected to be exhausted from the aircraft via the out- flow valve. However, some of this carbon dioxide might enter the passenger cabin, either from leakage through the cabin floor or through intermixing of the various exhaust streams with recirculated air just prior to the outflow valve. Assuming that the amount of carbon dioxide that enters the passenger cabin amounts to 10% of the total amount formed in the cargo compartment, the estimated amount of carbon dioxide entering the passenger cabin is 0.73 kg/hr. Ventilation Model for Onboard Test Based on the carbon dioxide generation rates (discussed previously), the volume of the passenger cabin, and the ven- tilation rate (obtained from Boeing), the expected increase in carbon dioxide concentration above the 390-ppm natu- ral background for each of these sources has been calculated using an estimated cabin volume of 482 m3 (17,030 ft3), and a ventilation rate of 14.7 air changes per hour.§ Based on these data and assumptions, the total carbon dioxide concentration and the contributions of each of the carbon dioxide sources to the total are shown in Table 9. Comments on Dry Ice Sublimation Rate Using the weights of the ULDs containing the dry ice, and the known initial weight of dry ice prior to placing it on the ULD of a 680-kg test load of dry ice and the measured area of the stack of dry ice blocks, the area-normalized loss rate was about 1,600 g/m2 ? hr. Using the measured surface area of the LD3 container (a standard type of container used on 777 and other aircraft), the estimated area-normalized loss rate was about 400 g/m2 ? hr. Comparing these numbers with the results summarized in Figure 8, it can be seen that the lack of insulation results in a much higher sublimation rate, as expected. Note also that the rate of carbon dioxide pro- duction for the test load was the equivalent of perhaps 80 insulated cartons like the one shown in Figure 2. This discussion points to the need for a packaging standard that specifies a minimum heat transfer resistance. Without that standard, packages with vastly different sublimation rates could be presented by shippers for carriage on airplanes. Importance of Various Sources • Looking at these estimates and comparing them to the observed results, it is clear that a small fraction of the carbon dioxide produced from the dry ice in the cargo compartment enters the passenger cabin, and the fraction that does enter is small compared to the carbon dioxide from other sources. • Given the magnitude of the carbon dioxide produced by human metabolism of the passengers and crew, a 10% or 20% change in the assumed activity level of the passengers and crew would likely be more significant than the carbon dioxide derived from assuming some leakage of carbon dioxide produced by dry ice in the cargo compartment. • Dry ice used for food and beverages is an important source and likely to be more important than dry ice carried in the cargo compartments. †This is the airline’s limit for dry ice carriage on this type of aircraft. ‡The dry ice in this ULD was the only dry ice in cargo on this particular flight. §However, it is noted that a paper by Hocking gives a Boeing 777 (model not specified) passenger cabin volume of 620 m3 and a ventilation rate of 10.4 ACH. Hocking states in his paper that this information was confirmed by Boeing.

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TRB’s Hazardous Materials Cooperative Research Program (HMCRP) Report 11: Technical Assessment of Dry Ice Limits on Aircraft describes a technical approach to determining the maximum quantity of dry ice that may be safely carried aboard aircraft.

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