Technical Assessment of Dry Ice Limits on Aircraft (2013)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

43 cargo hold is unventilated, then the void volume‡ and maximum flight duration (perhaps based on the amount of fuel loaded on the plane) must be estimated in order to verify that the concentration limit established in Step 1 is not exceeded during the flight. 4. Calculate the total surface area of the packages that could be placed on the airplane and still not exceed the maxi- mum allowable carbon dioxide concentration in the cargo compartment. 5. Compare the total surface area of the dry ice packages being shipped with the surface area limit calculated in Step 4. 6. If the total dimensional area of the dry ice packages being shipped is less than the limit established in Step 4, then the concentration limit established in Step 1 will not be exceeded.* If the cargo compartment is unventilated, then the proce- dure is somewhat different. The first two steps are the same, but for Step 3 and Step 4 the calculation differs: 3. Estimate the total free volume of the cargo compartment, and from the flight data, estimate the duration of the flight. A void fraction of 50% is suggested if no other data are available. It is also suggested that the flight duration be based on the duration assumed when calculating the fuel loading. The project task assignment called for constructing a deci- sion tool that would allow air carriers to determine the maxi- mum amounts of dry ice that can be safely carried* and for explaining and justifying the amounts of dry ice that the deci- sion tool would recommend. The first section of this chapter provides a step-by-step list- ing of the decision tool; the tool uses an area limit to establish the maximum number and size of dry ice packages that can be placed on an airplane. This is followed by a more detailed discussion of the technical considerations for developing the decision tool and recommendations for shipping guidelines that would need to be developed to implement area-based dry ice limits. Summary of Steps to Determine the Quantity of Dry Ice Packages Allowed in an Airplane The proposed limits for determining the allowable quan- tity of dry ice packages† are derived from a six-step process: 1. Establish a concentration limit for carbon dioxide in cargo holds. This document suggests that 30,000 ppm is consis- tent with the ACGIH STEL and Boeing guidelines. 2. Establish a dry ice sublimation rate. This document sug- gests an area-based rate of 250 g/m2 ? hr. This rate assumes that the carrier requires the use of insulated packages with a thermal resistance equivalent to 38 mm (1.5 in.) of EPS. 3. Consider the ventilation flow rates in any cargo compart- ment containing dry ice packages. If the flow rate is not known, the airframe manufacturer can provide it. If the C h a p t e r 1 1 Development of Dry Ice Limit Guidelines *This work is not intended to make or propose regulations. †In this context, the term “packages” includes insulated ULDs and pal- lets as well as cardboard cartons. ‡The void volume is the total compartment volume less the volume of the cargo. In the case of tightly packed cargo compartments or those with multiple ULDs, the void volume may be much less than the com- partment volume. *If there are also live animals in the compartment, the calculation of dry ice package limits must take into account the carbon dioxide produced by the animals. This additional calculation is beyond the scope of this report.

44 4. Estimate the total surface area of the dry ice packages that would produce a concentration in the free volume of the cargo compartment that equals the concentration limit used in Step 1. 5. Same as previous Step 5. 6. Same as previous Step 6. These steps can be applied to both passenger and cargo airplanes. The steps shown will be examined in greater detail in the guidelines section that follows. Development of Guidelines for Establishing Dry Ice Limits on Airplanes Selection of Target Carbon Dioxide Concentration Limit For areas “normally occupied by passengers or crew mem- bers” [14 CFR 25.831(b)(2)], the target carbon dioxide concen- tration is specified by the FAA as 5,000 ppm. However, cargo compartments do not fall in the “normally occupied” category. Referring back to Figure 9, it can be seen that the ventila- tion for the cargo compartment’s air flows from the passenger cabin (or from the main deck in the case of an all-freight air- craft) to the cargo compartments and that exhaust air from the cargo compartments is then dumped overboard. Thus, carbon dioxide generated in the below-deck cargo compartments should have no effect on concentrations in the flight deck or the passenger cabin/main deck. (On the other hand, air that is exhausted from the passenger compartment already contains elevated amounts of carbon dioxide; this level would become the baseline for any increase in carbon dioxide concentration from dry ice cargo in ventilated cargo compartments.) However, there are additional considerations. The first is cargo compartment safety during loading and unloading. One approach would be to set a cargo compartment carbon dioxide concentration limit equal to the 30,000-ppm STEL for carbon dioxide that has been established by ACGIH. They define short-term as a 15-min average. Such a limit would offer protection during initial entry into the compartment. After initial entry, the carbon dioxide concentration in the compartment would be reduced through flow through the open cargo compartment door. The second consideration is that many airlines carry both live animals and dry-ice–cooled cargo, and therefore the limit must be chosen so that there is no conflict between the two. In a guidance document,63 Boeing suggests a carbon dioxide concentration of a 30,000-ppm (3%) maximum for compart- ments in which there is live animal carriage. It is convenient that this limit is the same as the STEL suggested by ACGIH. A further caution is the need to know the degree to which the cargo compartments are completely isolated from the flight deck and the passenger cabin. Although the cargo com- partments are to be maintained at a lower pressure so that during normal operation any leakage is into, rather than out of, the cargo compartment, abnormal conditions may occur. It is not certain that the cabin floor provides an airtight seal between the cabin and the cargo compartments, and there is a concern that off-normal conditions could cause carbon- dioxide–laden air from the cargo compartment to enter the passenger compartment or flight deck. Additional study of this issue is needed, and special procedures may be required to address these off-normal conditions. Estimation of Sublimation Rates As has been discussed, the sublimation rate is best esti- mated using the dimensional area of the package. Observed sublimation rates ranged from 120 g/m2 ? hr to 200 g/m2 ? hr, with an average of 170 g/m2 ? hr. Packages that were in a stack and had no air-exposed surfaces were at the lower end of the range. Based on these results, and including a safety factor of 1.5, a sublimation rate of 250 g/m2 ? hr is suggested. The sublimation rate used could be different, depending on safety factors used by the air carrier. Consideration of Ventilation Air Flow Information on the design amount of ventilation for a given compartment, as well as the compartment volume, is available from the aircraft manufacturer. However, the actual amount of ventilation may depend on how the aircraft systems are operated; for some newer aircraft there are even computer- ized ventilation control strategies that can adjust the amount of ventilation depending on the number of passengers. Calculation of Maximum Allowable Dimensional Area For a ventilated compartment, the maximum allowable package area may be calculated from the following formula:* PkgArea VentRate CO COLimit In CO: • • • = −( ) −2 2 10 6 ρ 2 k T SubNum t insul insul dryice • • • ∆ λ where† PkgArea = total allowable dry ice package area, m2, VentRate = ventilation rate, m3/s, *An Excel spreadsheet is available on the CD-ROM that accompanies this report that incorporates this formula and shows an example calculation. †SI units are shown here, but any consistent set of units may be used, or appropriate conversion factors may be added.

45 CO2Limit = carbon dioxide ceiling concentration, ppm, CO2In = carbon dioxide concentration in the incoming air‡, ppm, rCO2 = density of carbon dioxide, kg/m3, DT = the temperature difference between the dry ice and the external environment, 100 K, SubNum = sublimation number, dimensionless, tinsul = insulation thickness, suggest 0.038 m, and ldryice = heat of sublimation of dry ice, 573 kJ/kg. For an unventilated compartment: PkgArea CO CO Vol unvent Limit In compart : • = −2 2 106 ment COVoidFrac SubRate FlightTime i • • ρ 2 where the variables are same as the previous equation, except for: Volcompartment = compartment volume, m3, VoidFrac = fraction of compartment not occupied by cargo, dimensionless, SubRate = area-based sublimation rate, kg/m2 ? hr, and FlightTime = duration of flight, including a safety factor for delays, hr. Guidelines for Implementation of Dry Ice Limits Packaging The development of dry ice quantity limits is predicated on the assumption that we know about the heat transfer to the dry ice. But, as of now, there is no standard that requires adequate insulation or even any insulation at all. Insulation Standards for Packages For packages, the following is suggested: • Cardboard cartons could be dry ice certified. Dry-ice– certified cartons should have a minimum thickness of 35 mm of EPS foam protected by a cardboard carton. Note that currently there is no minimum amount of insulation specified or required. (Other packaging could be accept- able if the heat transfer performance could be shown to be equivalent or better according to the procedures of the ASTM D3103 standard.) • It is also possible that less insulation could be accepted, but then the package should have an associated penalty fac- tor that would increase the effective dimensional area of the package for the purpose of calculating dry ice package limits (and perhaps for calculating shipping costs as well). • The heat transfer analysis suggested that the sublimation number could be used as a package design criterion if the package standard adopted does not specify a minimum EPS foam thickness, but more study is needed to validate the use of such a criterion and to assess its value in practice. Insulation Standards for Insulated ULDs For insulated ULDs: • Specifications for insulated ULDs could require insulation and a maximum average U value of 0.35 W/m2 K. Note that even though insulated ULDs from responsible ven- dors already meet this specification,* right now there is no insulation requirement. • The information available on dry ice sublimation rates for insulated ULDs is still quite limited, and there are no data available from third-party tests. Tests with several types and brands of ULDs would provide increased confidence that their thermal performance can be bounded. Another issue is the degree to which the amount of dry ice added to an insulated ULD is actually accurately known—it is possible that some shippers just use a standard value for all shipments or just use the amount needed to fill the bunker that is found in the product literature. Actual mea- surements of carbon dioxide production rates would be helpful here. Standards for Uninsulated ULDs • Shippers using uninsulated ULDs (either with or without thermal blankets over the cargo) could be required to state the dry ice loss rates and to justify them based on actual third-party tests or on tests reviewed by a third party. The results of such tests could be used to develop an equivalent area-based dry ice loss rate. In the absence of such tests, uninsulated ULDs should be subjected to a much higher assumed dry ice loss rate.* ‡The carbon dioxide concentration in the incoming air can be assumed to be 390 ppm if outside air is used for ventilation. However, if air to the cargo compartment has previously been in the passenger cabin, a higher value, such as 2,000 ppm, should be used. *If necessary, ASTM D3103, Standard Test Method for Thermal In- sulation Quality of Packages, could be extended to cover the testing procedures. *Recall that the dry ice loss rate for the test flight described in Chapter 10 was over 400 g/m2 ? hr based on the area of the entire LD3 container and over 1,600 g/m2 ? hr based on the area of the dry ice blocks themselves, compared to values of 120 to 200 g/m2 ? hr for insulated packages and ULDs.

46 Dimension and Area Data Basing the estimated sublimation rate on dimensional area should be feasible. Major freight carriers already collect pack- age dimensions from customers in order to calculate a dimen- sion weight, which is then compared with the actual weight to determine the shipping charge. Knowing the package dimen- sions, the calculation of package area is straightforward. For ULDs there are a few standard sizes, and their areas need to be determined only once. After that, the area associated with the type of ULD could even be stenciled on the ULD as the tare weight is now. We believe that for other cargo (e.g., pallets), it is reasonable to ask the shipper to provide this information. Guidelines for Specific Locations and Situations Guidelines are presented in the following for specific loca- tions on the aircraft. Guidelines for Passenger Compartment For the passenger compartment: The decision tool could include a nominal limit for the number of packages contain- ing dry ice based on the cabin volume, the ventilation rate, an estimate of the carbon dioxide emissions from the passenger cabin occupants based on the potential number of passengers and crew, and an estimate of the carbon dioxide sublimation rate from packages with dry ice based on a 300 g/m2 ? hr car- bon dioxide emission rate. (This sublimation rate is higher to account for the possibility that some passengers may use substandard packaging.) The limit should also take into con- sideration that there may be some uncertainty in passengers’ estimates of the amount of dry ice actually carried on board. In keeping with the discussion in Chapter 3, the guidelines could be based on a 5,000-ppm carbon dioxide concentration limit because the cabin is an occupied space. However, given existing restrictions on the number and size of carry-on items that passengers may bring on board, the need for passengers to stow other luggage, and the overall lack of space for carry-on packages in the passenger cabin, the number of parcels with dry ice will be few, and it is likely that dry ice packages in the passenger compartment will never be an issue. Guidelines for Ventilated Cargo Compartments For a ventilated cargo compartment: The limit for dry ice carriage could be based on dimensional area of the cargo and a normalized dry ice loss rate, either 250 g/m2 ? hr or a higher value for an increased safety factor, as well as the cargo com- partment volume, the ventilation rate, and a 30,000-ppm car- bon dioxide concentration limit. Guidelines for Unventilated Cargo Compartments In the case of unventilated cargo compartments, the carbon dioxide concentration will continually increase with time, so the amount of dry ice that can be tolerated is smaller and must be strictly controlled so that the carbon dioxide concentration limit is not reached prior to the end of the flight. Airbus suggests that for unventilated cargo compartments, the total volume of carbon dioxide generated by the dry ice packages over the duration of the flight must be less than the volume of the cargo compartment times 0.005, which is equivalent to establishing a carbon dioxide concentration limit of 5,000 ppm. Considering that a good argument may be made for a 30,000-ppm limit, the Airbus limit might be too restrictive. However, it should also be noted that much of the cargo compartment volume may be occupied by the volume of cargo or ULDs, and so the free volume of air to absorb the carbon dioxide may be considerably less than the volume of the empty compartment. It is this void volume that should be used for any calculation of expected carbon dioxide concen- trations, not the volume of the empty compartment. For an unventilated cargo compartment, what is proposed instead is that the limit be based on the effective cargo com- partment volume, the maximum flight time, the carbon dioxide production rate in the cargo compartment (based on a sublimation rate of 250 g/m2 ? hr), and a 30,000-ppm car- bon dioxide concentration limit. The maximum flight time should be based on the actual expected flight time plus any possible extra flying time according to the amount of extra fuel carried on board; moreover, procedures should be in place to account for ground delays. Guidelines for Regional Jets No information was obtained on ventilation rates for regional jets. Nor did these manufacturers provide any information on air flow paths, ventilation control strategies, or ventilation rates. In view of the lack of information on ventilation flow paths, compartment volumes, and ventilation rates for regional jets, there is no engineering basis for setting dry ice carriage limits. Without any engineering basis of estimate, carriage of dry ice on these aircraft cannot be recommended. Some information on compartment volumes of regional jets may be obtained from published sources or the Internet. It is possible that a dry ice package with a very small area could be carried based on knowledge of the compartment volume alone, without assuming any ventilation. However, such capacity would be minimal. Consider an Embraer 145 regional jet with a cargo com- partment volume* of 9.2 m3 (325 ft3). Assuming no ventila- *Regional jet compartment volumes can be found in various industry reference manuals and Internet sources.

47 tion,† a 1-hour maximum flight time, a 50% void volume, a 5,000-ppm concentration limit,‡ and a sublimation rate of 250 g/m2 ? hr, the total surface area of dry ice packaging that could be placed in that cargo compartment would be 0.24 m2. This is equivalent to one small package having dimensions of about 200 mm (about 8 in.) on a side. Note that for regional jets, because the cargo space is limited, the void volume could be considerably less than 50%, further limiting the allowable number of packages. A void volume of 25% would lower the allowable total surface area of dry ice packages to 0.12 m2, implying that no dry ice shipments should be allowed on regional jets. Summary of Implementation Steps A summary of the dry ice implementation steps is pre- sented in the following. Prerequisites: • Establish an allowable concentration limit for carbon diox- ide in the compartment containing the dry ice packages. • Develop specifications for the insulating ability of dry-ice– certified packaging. • Develop specifications for the insulating ability of insu- lated ULDs. • Ascertain aircraft compartment volumes and ventilation rates. Is dry ice carried on board by passengers? • Advise passengers of 2.5-kg limit. • Advise passengers of need for dry-ice–certified packaging. • Provide information on dry-ice–certified packaging. • It is not believed that any other tracking is necessary inas- much as it does not seem likely that more than a few pas- sengers will have dry ice, and the packages will necessarily be of modest size. Perhaps use a nominal value of 0.15 kg/ hr for each such package given that their size is necessarily limited. For 10 packages, this would be 1.5 kg/hr. Is dry ice carried in cargo compartments? • Advise shippers of need for dry-ice–certified packaging or the use of insulated ULDs with an equivalent insulation value. • Collect package dimension information. • Compute the total dimension-based area of all packages with dry ice. • Compute the total dimension-based area of all insulated ULDs with dry ice. Is conditioned air supplied to the compartment? • If yes, determine the aircraft ventilation rate in volume of air per time (e.g., cfm, m3/hr, L/s). • Compute a total dimensional area limit for each compart- ment from the compartment volume, ventilation rate, allowable carbon dioxide concentration limit, and assumed normalized loss rate. If the ventilation rate does not change, this needs to be done only once for each aircraft type and configuration. • Compare the total dimensional area of packages and insu- lated ULDs containing dry ice to the maximum allowable package area for a given aircraft ventilation rate. If no conditioned air is supplied to the compartment: • Find out the maximum flight duration, including a reserve for possible flight delays. • Use the compartment volume, void fraction, and maxi- mum flight duration to compute the maximum allowable package area. • Compare the total dimensional area of packages contain- ing dry ice to the maximum allowable dimensional area for a given aircraft situation. • Develop procedures to trigger a warning if the assumed flight time is exceeded. • Use special ventilation procedure upon unloading, such as: open hatch, insert canvas air supply hose, wait, then unload. Note that the calculations used to estimate buildup of CO2 in an unventilated compartment could also be used to estab- lish a time limit for cargo handler activity during loading if a normally ventilated compartment was not actually being supplied with ventilation air. †The compartment is assumed to contain fresh air prior to loading. ‡Without knowing the ventilation air paths, we must assume that cargo compartment air may enter the passenger compartment.