Technical Assessment of Dry Ice Limits on Aircraft (2013)

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1 Technical Assessment of Dry Ice Limits on Aircraft The objectives of this project were to develop an understanding of the parameters affect- ing the buildup of carbon dioxide in both passenger and cargo aircraft and to develop a decision tool for air carriers to use in determining the maximum quantity of dry ice (solid carbon dioxide) that can be safely carried on board aircraft. To accomplish this objective, a literature review was performed to identify pertinent information developed by others regarding the handling of dry ice on aircraft. By consider- ing the physical characteristics of dry ice sublimation and the engineering characteristics of modern aircraft, the parameters that affect the buildup of carbon dioxide in the various compartments of the airplane were identified and assessed. A topology of dimensionless parameters was developed in an attempt to reduce the number of variables controlling the sublimation of dry ice and the subsequent transport of carbon dioxide in aircraft and to generalize their relationship. The information gained from the literature review and dimensionless analyses was then used to identify knowledge gaps. The review found that there was little experimental data on the sublimation rate of dry ice in packages. Accordingly, several insulated packages used to transport items refrigerated with dry ice were obtained and filled with dry ice, and the sublimation rate of the dry ice was measured until most of the dry ice had sublimed. For actively cooled unit load devices (ULDs), vendors supplied data on the performance and dry ice usage of their cooled containers. The major aircraft manufacturers were contacted to obtain the guidance that they provide to air carriers regarding the operation of ventilation systems and on establishing dry ice lim- its. Airbus and Boeing provided valuable information that filled in many of the knowledge gaps regarding the ventilation systems. Others, particularly the manufacturers of regional jets, did not provide information, and the details regarding the operation of their ventilation systems remain a knowledge gap. From the literature review, the heat transfer experiments, and the data provided by Boeing and Airbus, a reasonable understanding of the parameters that govern the sublimation of the dry ice within the insulated packages and the subsequent increase of carbon dioxide con- centrations in airplane compartments was developed. To demonstrate that it was possible to model and predict the performance of dry ice during flight conditions, a large quantity of dry ice (680 kg) was placed in a ULD and loaded onto a Boeing 777, and the carbon dioxide concentration in the passenger cabin was monitored during the flight. To identify any dif- ferences, the carbon dioxide concentration was also monitored on the return flight, which contained no dry ice in the cargo compartment. These reviews, analyses, and tests resulted in the following key findings: • Past efforts have been directed at establishing and regulating dry ice limits on commercial aircraft based on the mass of dry ice carried (e.g., 200 kg per compartment on a certain air- craft). The technical and dimensional analyses found no theoretical basis for such mass limits. S U M M A R Y

2• This project showed that there is a sound theoretical basis for limiting the number of pack- ages containing dry ice in a cargo compartment using an area-normalized sublimation rate (which can be thought of as a mass flux). • Heat transfer experiments showed that for packages having an insulation thickness of 38 mm (1.5 in.), the average area-normalized sublimation rate was 170 g/m2 ? hr. This sublimation rate can also be used for insulated ULDs with active ventilation systems. • The use of a dry ice package limit based on surface area represents a change in industry practice. Thus, provision would need to be made to develop the procedures and standards necessary. • Currently, there are no standards for the amount of insulation required in dry-ice– containing packages or ULDs. Because sublimation rates can vary by a factor of 10 or more depending on the type and thickness of insulation (or lack thereof), the industry needs to consider implementing insulation standards regardless of any other changes. For most shippers, these insulation standards would merely codify common practice, but standards are needed to prevent hazardous situations. Additionally, the project resulted in findings that support the key findings listed previously: • A one-dimensional heat transfer model can adequately predict the sublimation rate of dry ice in both actively and passively cooled packages. • Using the area-normalized sublimation rate and available ventilation data, it is possible to estimate the amount of dry-ice–cooled cargo that could be stowed in a ventilated cargo compartment without exceeding a carbon dioxide concentration ceiling. A carbon dioxide concentration limit of 30,000 ppm could be used for this purpose. A 30,000-ppm limit would meet the American Council of Governmental Industrial Hygienists (ACGIH) short-term (15-min) limit for human exposure, thus protecting baggage handlers; also, this limit is currently used for transport of animals in aircraft cargo compartments. • Since packages with dry ice are frequently shipped in stacks with other packages, the area-normalized sublimation rate was determined for a package with dry ice surrounded by other packages containing dry ice and also for a package with dry ice surrounded by packages not containing dry ice. For an array of packages all containing dry ice, the array could be treated as one large package using the outer surface area for determination of the sublimation rate. When a package containing dry ice was surrounded by other packages not containing dry ice, the sublimation rate was about 20% less than the sublimation rate observed for an individual package containing dry ice. Thus, it would be conservative to use the sublimation rate for single packages even when they are shipped in stacks contain- ing other packages containing no dry ice. • During this project, it was not possible to measure the in-flight concentration of carbon dioxide in the cargo compartment containing dry ice packages. However, in-flight measure- ments in the passenger cabin of a Boeing 777 verified the manufacturer’s statement that there is very little transport of ventilation air from the cargo compartment to the passenger com- partment. These measurements showed that the passengers were not exposed to high levels of carbon dioxide when a significant quantity of dry ice was stowed in the cargo compartment. • If packages containing dry ice are stowed in an unventilated cargo compartment, then the allowable quantity analysis should take into consideration the concentration buildup during the flight compared to the 30,000-ppm limit, and the analysis should also take into consideration the void fraction of the cargo compartment and the flight duration. A cargo compartment void fraction greater than 50% should not be assumed unless it can be demonstrated that there is actually that much void space in the cargo compartment. This report suggests using the flight duration used to estimate fuel requirements when assessing the carbon dioxide buildup in unventilated cargo compartments.

3 • This analysis and associated findings have not considered the off-normal situation where there is a loss of ventilation. In such circumstances, the cargo hold containing the dry ice would become equivalent to an unventilated cargo hold. Air carriers would have to address this situation in their off-normal operating procedures and in the extreme might have to establish the carbon dioxide limits assuming the cargo hold is unventilated. • Periodic reviews of the rationale should be conducted to account for possible changes in aircraft configuration and operation. • This analysis did not consider the carriage of dry ice on the smaller regional jets because the aircraft manufacturers did not provide any information on their ventilation systems. • Even if carriers continue to use mass-based limits for dry ice carriage, it is important that they also implement package standards that establish a minimum heat transfer resistance for dry ice packaging being carried on their airplanes. • In the dimensional analysis, a dimensionless term that was called the sublimation number was identified. It is defined as follows: Sublimation Number = ∆T R raλ where R is the R value of the insulation, m2 K/W, l is the heat of sublimation, kJ/kg, DT is the temperature difference, K, and ra is the sublimation rate expressed as kg/m2 ? hr. For the insulated packages that were modeled experimentally, the observed sublimation number was 3.4. A value similar to that number might be used as a package design standard to ensure that the package had sufficient insulation. Including a safety factor of 1.5, a sublimation number of 2.3 might be used for calculating compartment limits. Important findings from HMCRP Project 09: • The production of carbon dioxide by sublimation is controlled entirely by trans- fer of heat to the dry ice, and the rate of heat transfer in turn is controlled by the temperature difference, the area available for heat transfer, and the R value of the insulation (the reciprocal of thermal conductivity of the insulation divided by its thickness). • Since mass does not appear in any of the equations for heat transfer, the cur- rent mass-based sublimation rates are based on rules of thumb and cannot be derived from any theoretically developed set of equations. • A dry ice sublimation rate based on package dimensional area is technically sound and is a good predictor of carbon dioxide production from packages containing dry ice. • Thermal performance (amount of insulation) of packages and ULDs is important, and a specification for the thermal performance of both packages and ULDs is needed.