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6Properties of Dry Ice Dry ice is unusual in that it sublimes at normal atmospheric pressuresâthat is, it passes directly from the solid to the gaseous state. This behavior occurs because the triple-point pressure is above atmospheric pressure. The phenomenon of sublimation is discussed in more detail in standard thermo- dynamic treatments of the properties of pure substances.1 The first pound of dry ice for commercial refrigerating purposes was sold by the Dry Ice Corporation of America in 1925,2 and by 1929, production was 15,000 tons per year. Dry ice has remained popular for the refrigeration of perishable goods during shipment. Some properties of carbon dioxide and of dry ice are shown in Table 1. Because dry ice is made by compressing carbon dioxide snow, the bulk properties, particularly the density, are somewhat dependent on the details of manufacture and in particular on the degree of compression. Table 2 shows how some properties of solid and gaseous carbon dioxide that are important in heat transfer calculations vary with temperature. Use of Dry Ice in Commerce Forms of Dry Ice Used in Commerce In commerce, dry ice is available in various forms. Fig- ure 1 shows some of these forms. The ½-in. diameter pel- lets are commonly used for shipping packages. Cut blocks of dry ice are used for keeping the food and beverages served on aircraft cold. Types of Air Cargos that Use Dry Ice Shipments by air of cargo that uses dry ice tend to fall into two categories: foodstuffs and medical products. Dry ice is often used for shipments of foodstuffs that must be kept cold, particularly steaks, ice cream, and other frozen or high-value foodstuffs. Sometimes these foodstuffs are bulk shipments of food from producers to distant markets for dis- tribution. In other cases, these are mail order shipments of gourmet foods from vendors to individuals. Because some vendors of mail order foods use large, centralized warehouses, large numbers of packages containing dry ice can be shipped from one airport, particularly during busy holiday seasons. Thus, it is possible that individual cargo aircraft may be called upon to ship large numbers of packages containing dry ice. Dry ice is also extensively used for medical shipments, such as tissue samples and frozen or temperature-sensitive pharmaceuticals. Packaging for Dry Ice Shipments A variety of packaging is in common use for shipments containing dry ice; these are discussed in the following sec- tions. A common element is the inclusion of insulation to help maintain the cold temperatures. However, the shipments differ not only in size and construction but also in the way the temperature is controlled. Passively cooled shipments simply contain the cargo packed in dry ice and maintain the cargo at or near the sub- limation temperature of dry ice (-78°C). Passive cooling is relatively simple and inexpensive to implement. However, passive cooling cannot be used for materials that would be harmed by cooling to -78°C. Actively cooled unit load devices (ULDs) are available that control the temperature inside a container by placing the dry ice, frequently in block form, in a separate section in the con- tainer and using a temperature-controlled fan to circulate a mixture of carbon dioxide and air from the dry ice chamber to the cargo section of the container, thereby maintaining the temperature of the cargo at a set temperature. This tempera- ture may be well above the temperature at which the dry ice is subliming (-78°C). Medical products are often shipped in actively cooled packages because, while they must be main- tained close to 0°C to maintain their shelf life, exposure to C h a p t e r 2 Dry Ice
7 -78°C would damage them. For example, many vaccines are recommended to be kept between 2°C and 8°C. Insulated Cardboard Cartons Insulated cardboard cartons are commonly used for ship- ping goods cooled with dry ice. They consist of a cardboard carton with an inside layer of expanded polystyrene (EPS) foam. Figure 2 shows a typical insulated shipping carton. Figure 3 shows an inside view and the EPS insulation. The cardboard carton is not an essential element of the pack- age, and some packages do not have a cardboard carton. The major purpose of the cardboard carton is to protect the more fragile EPS. The insulation value of EPS foam is known to vary with density, temperature, and the exact formulation. A manu- facturer of EPS packaging has provided the data shown in Table 3.* This table shows the effect of EPS density and tem- perature on the thermal conductivity of EPS foam. Note that although there is some variation in thermal conductivity of EPS with both density and temperature, the variation is about 15% to 25%. Considering the other uncertainties in thermal conditions during transport, all EPS was consid- ered to have approximately the same insulating value, and a value of 0.035 W/m K was used in all the thermal analyses presented in this report. Cartons insulated with polyurethane foam are also avail- able. These are more expensive but offer a lower rate of heat transferâabout 30% less than for EPS. Figure 4 shows a shipping carton insulated with polyurethane foam insulation. ULDs Larger shipping containers for cold cargos are also available. These are generally based on the standard ULDs that are used on larger aircraft to load baggage and freight. ULDs come in dif- ferent sizes to accommodate different amounts of cargo and to fit the interior spaces in different aircraft. These ULDs may be passively or actively cooled. The passively cooled ULDs typically consist of a ULD with insulation inserted between the structural members; dry ice is packed with the cargo to provide cooling. If active cooling is used, the ULD is insulated and divided into an auxiliary chamber and a main chamber. The auxiliary chamber contains blocks of dry ice; the other, larger chamber Property Value Color White or translucent white Molecular weight 44.01 3 Density of gas (kg/m3 at 1 atm, 25°C) 1.799 4 Specific gravity of gas (air = 1) 1.53 5 Density of solid (kg/m3) 1355 6 1400 to 1600 7 Specific gravity of dry ice (water = 1) 1.35 8 Sublimes at (°C) -78.48 9 Latent heat of sublimation at -78°C (kJ/kg) 573 10 Table 1. General properties of carbon dioxide and dry ice. Temp., ° C Temp., K Pressure, kPa Density, kg/m 3 Enthalpy, kJ/kg Specific Heat, kJ/kg K Thermal Conductivity, mW/m K Kinematic Viscosity, µPa - s Thermal Diffusivity, cm 2 /s Prandtl Number - 55 218.15 101.33 2.494 440.8 0.780 10.78 10.97 0.0554 0.793 - 50 223.15 101.33 2.436 444.7 0.783 11.11 11.22 0.0582 0.791 - 40 233.15 101.33 2.327 452.5 0.791 11.77 11.72 0.0640 0.787 - 30 243.15 101.33 2.228 460.5 0.799 12.46 12.22 0.0700 0.784 - 18 255.15 101.33 2.120 470.1 0.810 13.32 12.82 0.0776 0.779 - 10 263.15 101.33 2.054 476.7 0.817 13.92 13.22 0.0829 0.776 0 273.15 101.33 1.977 484.9 0.827 14.67 13.71 0.0898 0.773 10 283.15 101.33 1.906 493.2 0.836 15.45 14.20 0.0969 0.769 20 293.15 101.33 1.839 501.6 0.846 16.24 14.69 0.1044 0.765 30 303.15 101.33 1.778 510.1 0.856 17.05 15.17 0.1121 0.762 40 313.15 101.33 1.720 518.7 0.865 17.86 15.66 0.1200 0.758 50 323.15 101.33 1.666 527.4 0.875 18.69 16.13 0.1282 0.755 Table 2. Properties of carbon dioxide vapor at atmospheric pressure.11 *The source data have been converted to SI units.
8High-Density Dry Ice Pellets ⢠1/ 8- in . di am et er dr y ic e pe lle ts fo r us e in dr y ic e bl as t cl ea ni ng sy st em s. Dr y ic e pe lle ts ar e av a ila bl e wr a ppe d in po ly or br ow n Kr af t pa pe r. Dry Ice Rice Pellets ⢠1/ 4- in . di am et er dr y ic e pe lle ts . ⢠Us ed pr im ar il y in fr oz en f ood a pplic at io ns . Dry Ice Standard Pellets ⢠1/ 2- in . di am et er dr y ic e pe lle ts . ⢠Us ed pr im ar il y in fr oz en f ood a pplic at io ns . ⢠Us ed in me at pr oc e ssi ng pl an ts . ⢠Us ed fo r tr an sp or ta ti on of bl ood pl as ma an d la b sp ec im en s. Cut Blocks of Dry Ice ⢠Dr y ic e av ai la bl e in cu st om si ze s pr oc e sse d an d pa ck ag ed to cu st om er sp ec ific at io ns . ⢠Dr y ic e ma y be wr a ppe d in Kr af t pa pe r or in po ly . Cu t bl oc ks of dr y ic e ar e av a ila bl e wr a ppe d in po ly or br ow n Kr af t pa pe r. Block of Dry Ice ⢠St an da rd dr y ic e bl oc k me as ur es 10 in . x 10 in . x 12 in . ⢠A ppr ox im at el y 60 lb s ea ch . ⢠Us ed fo r fr oz en f ood a pplic at io ns , es pe ci a lly in sh i ppi ng ic e cr ea m. ⢠Us ed in groc er y wa re hous es . Figure courtesy Continental Carbonic, Inc. Used with permission. Figure 1. Forms of dry ice available for sale.12
9 Battelle photo Figure 2. Typical insulated shipping carton. Battelle photo Figure 3. Inside view of insulated shipping carton. Density, kg/m3 Temperature, °C k, W/m K 16 -18 0.0317 16 4 0.0346 16 24 0.0375 16 38 0.0404 32 -18 0.0288 32 4 0.0303 32 24 0.0332 32 38 0.0361 Table 3. Thermal conductivity of expanded polystyrene foam.13 Battelle photo Figure 4. Insulated shipping carton with polyurethane foam insulation.
10 The ability to consolidate many cold cartons within a single ULD, even if it is not insulated, can also be an impor- tant strategy for limiting overall heat transfer to the cartons: keeping the cold packages together reduces the overall surface area that is available for heat transfer. contains the cargo. An electronic temperature controller and an associated battery-powered fan are used to monitor the tem- perature of the cargo in the ULD and to circulate cold gas from the dry ice chamber into the cargo section to maintain a desired temperature set point. Figure 5 shows one such insulated ULD. Figure 5. Photo of an insulated ULD.
