Technical Assessment of Dry Ice Limits on Aircraft (2013)

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31 concentration from NASA test aircraft flights show a simi- lar amount of variation.43 Although geographic variations in carbon dioxide concentration have been modeled and dis- played graphically,44 for the purposes of assessing potential carbon dioxide levels inside aircraft, the variation in carbon dioxide concentration in the outside air with time or location is not significant. Compared with a typical outside concen- tration of about 390 ppm, this variation is only about 1%, and for our purposes, the concentration of carbon dioxide in outdoor air may be taken as a constant. Carbon Dioxide Produced by Human Metabolism People produce carbon dioxide as a product of metabo- lism. The rate of generation of carbon dioxide from people varies with the size of the person* and his or her level of activ- ity. The estimation of human production of carbon dioxide is discussed in detail in the ASTM standard for using carbon dioxide concentrations to evaluate indoor air quality.45 Based on estimates developed by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) for evaluating indoor air quality in buildings46, an average-sized adult who is seated and engaged in reading/ writing activities produces 0.0043 L/s of carbon dioxide. Based on a density of carbon dioxide of 1.799 g/L (see Table 1), this is 0.00773 g/s, or 28 g/hr of carbon dioxide per passenger. As an aside, if we were to use the 2% per hour dry ice sublimation rate quoted in the FAA report as a basis for cal- culation,47 we could estimate that from a carbon dioxide pro- duction point of view, each passenger would be equivalent to a block of dry ice weighing 1.4 kg. Continuing this analogy, and using it to place dry ice carriage limits in perspective, the approximate dry ice equivalent of a full passenger load of Carbon dioxide concentrations on aircraft are determined by the amount of carbon dioxide introduced into the aircraft and the amount of ventilation. Carbon Dioxide Sources Before considering dry ice limits for aircraft, we must iden- tify all the sources of carbon dioxide. There are four sources of carbon dioxide that can contribute to the concentration of carbon dioxide in the air inside an aircraft: 1. The carbon dioxide already present in fresh air from outside. 2. Carbon dioxide produced from human metabolism by the crew and the passengers, if present.* 3. Carbon dioxide produced from metabolism by any ani- mals present on the aircraft. 4. Carbon dioxide produced by the sublimation of dry ice. The dry ice could be from (a) packages carried on board by passengers, (b) dry ice used to keep food cold that is served on board, and (c) dry ice used to keep cargo cold. Information on levels of carbon dioxide in the ventilation air and on the amounts of carbon dioxide produced through human metabolism is reviewed briefly here. The balance of this section focuses on carbon dioxide produced through the sublimation of dry ice. Carbon Dioxide in Ventilation Air The air outside the aircraft contains a certain amount of carbon dioxide—about 390 ppm. Measurements made out- side commercial aircraft show that the range of variation in the carbon dioxide content of the outside air at altitude is on the order of 5 ppm.42 Measurements of carbon dioxide C h a p t e r 9 Factors Affecting Carbon Dioxide Concentrations on Aircraft *Cargo aircraft generally have no passengers. *The rate of carbon dioxide production is best correlated with the body surface area of the person. In this respect people are similar to packages.

32 various aircraft at common seating arrangements would be as shown in Table 7. Or, assuming an area-normalized sublimation rate of 170 g/m2 ? hr, the carbon dioxide produced from boxes for which data are shown in Figure 6 would be equivalent to the carbon dioxide produced by two passengers. Carbon Dioxide Produced by Animal Metabolism Consideration of this source of carbon dioxide is not within the scope of the present study. We note that Boeing has provided guidance for the carriage of animals,48 and that carbon dioxide emissions are an important part of this guid- ance, as is the need to manage heat and humidity. Volume of Carbon Dioxide Produced by Sublimation of Dry Ice The production of carbon dioxide from dry ice carriage has been discussed previously, and the area-normalized dry ice loss rate has been suggested as the best way to estimate the amount of carbon dioxide produced from the sublimation of dry ice. Experimentally, the sublimation of dry ice is conveniently studied on a mass basis: the mass of dry ice sublimed is related to the amount of heat transferred, and therefore the loss of dry ice as carbon dioxide can be determined by using a scale. Ventilation, on the other hand, is conveniently studied on a volume basis. The size of cargo compartments on airplanes is measured in terms of volume, air flows are often measured in volumetric flow units (e.g., L/s, ft3/min), and carbon dioxide concentrations are generally stated in volume ppm or volume percent, both of which are volume ratios. Thus, there needs to be a bridge between the measurement of the mass of dry ice and the volume of carbon dioxide gas produced. That bridge is the gas density, defined as the mass per unit volume. Gas density has SI units of kg/m3, and is usually represented by the symbol r. For an ideal gas, ρ := Press MW R Tempgas i i where Press = gas pressure—standard atmospheric pressure is 101.325 kPa, MW = gas molecular weight—44.01 kg/kmole for carbon dioxide, Rgas = universal gas constant—8.314 kPa ? m3/kmole K, and Temp = the absolute temperature of the gas—K. This means that we must know the pressure and tem- perature to calculate the gas density. Table 8 shows various gas densities used in the literature related to carbon dioxide concentrations in aircraft cabins, as well as the assumed tem- perature and pressure associated with those gas densities. Clearly, as Table 8 shows, within the range of reports or regulations related to carbon dioxide and dry ice on aircraft, there have been differing opinions as to the proper condi- tions to use for calculating the specific volume of carbon dioxide gas. Also, clearly these differences are quite small in comparison to the other uncertainties in the estima- tion of dry ice sublimation rates and associated ventilation requirements. In the interest of consistency, we choose to use the value of r = 1.799 kg/m3 since this is the value legally adopted by the FAA in their Final Rule on Allowable Car- bon Dioxide Concentration in Transport Category Airplane Cabins, wherein the FAA lists their standard conditions as a pressure of 101.325 kPa and a temperature of 25°C.49 This density of 1.799 kg/m3 was used in the calculations shown in this report. Carbon Dioxide Removal: Ventilation Air Flows Ventilation is the process of introducing fresh air into an enclosed space. On aircraft the ventilation process is neces- sarily accompanied by air conditioning, the process of chang- ing the temperature and humidity of the air to meet human comfort requirements. Ventilation may also be accompa- nied by the process of air purification, which has the goal of removing gaseous and particulate contaminants. Aircraft Ventilation Requirements Aircraft require ventilation, air conditioning, and air puri- fication. Ventilation is required to provide air with sufficient oxygen for people (and any animals that may be on board) to breathe and for removal of air contaminants. Air condition- Aircraft Typical No. Passengers Equivalent Dry Ice, kga Equivalent Number of Boxesb 737-300 124 174 36 737-800 157 220 46 767-400 256 358 75 777-200 285 399 84 a Based on 1.4-kg dry ice equivalent per passenger. b Based on 12-in. x 10-in. x 16-in. boxes with 1.5-in. EPS insulation. Table 7. Dry ice equivalents of aircraft passenger loads for selected aircraft types.

33 ing is required because, in general, the air outside the aircraft is too hot or too cold for comfort. Air purification is required for gaseous contaminant* and particulate matter† removal. In addition, at all but the lowest altitudes, the aircraft is required to be pressurized because the ambient air pressure at altitude is too low. Although the electronic equipment on an aircraft does not require air to operate, such equipment does need to be kept within operating temperature limits, and air cooling is required. Thus, conditioned ventilation air must be supplied to the aircraft electronic equipment as well. Understanding of Aircraft Ventilation Air Flows Based on discussions with major airframe manufactures Boeing and Airbus, we may visualize the ventilation air flows to the various compartments as follows: The fresh air supply is extracted off the compression sec- tion of the jet engine and conditioned, with a small amount being sent directly to the flight deck and the balance sent to a mixing box. In the mixing box, the incoming air combines with recycled air from the cabin that has been filtered. This air is then sent to the passenger cabin. In most new aircraft, the supply of fresh air amounts to about half the air flow supplied to the cabin. The air displaced by the addition of fresh air is exhausted. Some air from the cabin is exhausted directly from galley and toilet areas to the outside. Some air that was previously in the passenger cabin is used to provide cooling air to electronics; this air is also exhausted. And some air leaves the passenger cabin to flow to the cargo compartment(s). These flows are shown schemati- cally in Figure 9. It is possible for an air carrier to order aircraft equipped with separate air conditioning systems for one or more of the cargo compartments; this would be done if the carrier expects to carry particularly temperature-sensitive cargo. In discussions with some passenger air carriers, their loading schedule was to place any live animals in one of the air- conditioned compartments and the dry ice packages in the other, presumably non-air-conditioned compartment. There is also the possibility that a cargo compartment may not receive any ventilation air.* Information on Ventilation Rates Airbus and Boeing supplied information on the ventilation rate for various models and configurations of their aircraft. Although presented somewhat differently, the information from both manufacturers describes the amount of fresh air introduced into the aircraft and into various compartments.*Such as ozone. †Particulate contaminants include both nonviable particulate matter, such as dust, and viable particles, such as bacteria and viruses. Outside air at altitude is generally free of both types of particulate matter, but inasmuch as recirculated air is mixed with the fresh air supply, air filtra- tion is necessary. Table 8. Various statements of the density and specific volume of carbon dioxide gas. ρ, kg/m3 Specific volume, ft3/lb Temp., °C Pressure, kPa Comments 1.799a 8.904 25 101.325 Calculated for conditions in FAA standard in 61 FR 63951 1.808b 8.862 25 101.325 NIST data for carbon dioxide 1.820c 8.8 Not stated Not stated Density used in FAA sublimation rate study50 1.842a 8.695 18 101.325 Perhaps a more realistic cargo compartment temperature 1.884c 8.5 Not stated Not stated Used in: Boeing dry ice carriage document51 Airbus SIL 00-08152 FAA Advisory Circular (AC) No. 91- 7653 a Calculated from ideal gas law. b Based on NIST database. For real gas. c Calculated from specific volume listed at right. *For example, the aft cargo compartments in the Airbus A300, A310, A300-600, and A318 aircraft are unventilated.

34 Calculation of Expected Carbon Dioxide Concentrations With information on ventilation (fresh) air flows and with information about the volumetric rate of production of carbon dioxide gas from dry ice and the target carbon dioxide concen- tration limit, the calculation of the maximum amount of dry- ice–containing cargo that can be carried is straightforward. The concentration of carbon dioxide in a steady flow situ- ation is given by the relationship: CO2Concentration Volume CO2 in Ventilation = Air Volume CO2Generated Volume of Ventila + tion Air Examples of Carbon Dioxide Concentration Calculations Two examples of carbon dioxide concentration calcula- tions follow. They are intended to illustrate the general mag- nitude of cargo compartment carbon dioxide concentrations that might be expected. For these calculations it is assumed that the air supplied to the cargo compartment was previously in the passenger cabin. Passenger cabin air has been found to have from 1,000 to 2,000 ppm of carbon dioxide.* Here a value of 2,000 ppm is used as the concentration of carbon dioxide in the ventila- tion air entering the cargo compartment. It is also assumed that the air in the cargo compartment is well-mixed. Based on the experimental tests with packages and the analysis of industry information on insulated ULDs, a con- servative value of 250 g/m2 ? hr for the rate of carbon dioxide production from dry ice is used.† Example 1 Consider a passenger aircraft with a forward cargo com- partment volume of 3,742 ft3 and a ventilation rate of 992 ft3/ min. If there were packages or ULDs containing dry ice with a dimensional area of 75 m2, the concentration of carbon di- oxide would be expected to be about 8,200 ppm. (Note that the compartment volume is not needed for the calculation.) Example 2 Assume an aircraft carrying four type RKN-insulated ULDs (with a total area of 67.7 m2) and 200 EPS-insulated cartons that are 290 mm × 240 mm × 200 mm (with a collective area of 70.2 m2). The total dimensional area would then be 138 m2. Assume that this cargo is carried in a compartment with a vol- ume of 3,613 ft3 and a ventilation rate of 21.8 air changes per hour, leading to a ventilation flow rate of 1,313 ft3/min. For these conditions, the total allowable dimensional area would be 450 m2, so the amount of dry-ice–containing cargo is well within limits. These two calculations show how the dimensional-area method may be used to obtain information relative to carbon dioxide buildup from packages containing dry ice once the surface area of the dry ice packages is specified. A spreadsheet for doing such calculations is available on the CD-ROM that accompanies this report. Figure 9. General ventilation air flows in an aircraft. P a s s e n g e r C a b i n E l e c t r o n i c s G a l l e y T o i l e t s C a r g o C o m p a r t m e n t ( s ) A i r C o n d i t i o n i n g U n i t s ( “ P a c k s ” ) M i x i n g B o x F l i g h t D e c k T o E x h a u s t To Exhaust B l e e d A i r f r o m E n g i n e *See discussion in Chapter 10 for a review of these measurements. †Actual measured values were typically 140 to 170 g/m2 ? hr. The value of 250 g/m2 ? hr includes a safety factor of 1.5.