1、27.1CHAPTER 27AIR TRANSPORTPerishable Air Cargo 27.1Perishable Commodity Requirements. 27.2Design Considerations 27.2Shipping Containers 27.2Transit Refrigeration. 27.3Ground Handling 27.4Galley Refrigeration 27.4IRfreightserviceisprovidedbyall-cargocarriersandpassengerAairlines. The latter companie
2、s also have all-cargo aircraft.Wide-body aircraft have a passenger and cargo mix on the maindeck, increasing cargo capacity (Figures 1 and 2). All lines maintainregularly scheduled flights so shippers may adequately plan deliv-ery time. Special charter flights are also available from regular ter-min
3、als and from airports located close to producing areas. Payloadrange comparisons of wide-body jets are shown in Figure 2.The industry-recognized standard for shipping temperature-sensitive goods is International Air Transport Associations (IATA,updated annually) Perishable Cargo Regulations. Prospec
4、tive ship-pers should review the IATA regulations, and contact airlines serv-ing their locality to obtain specifics for handling perishableshipments.PERISHABLE AIR CARGOSome aircraft have cargo compartment temperature control withoptions ranging from just above freezing to normal room tempera-ture.
5、Most compartments have a single temperature control. Thecontrol is achieved by balancing skin heat loss with the supply ofexpended passenger cabin air and, when necessary, introduction ofhot jet engine bleed air through eductors. Skin heat exchangers areused to help maintain the lower temperatures a
6、t high (cold) alti-tudes. This mode of refrigeration is not available at low altitudes oron the ground, where skin temperatures can exceed the compart-menttemperaturesignificantly.Refrigerationtechniquesforaircraftrely primarily on precooling, insulated containers, dry-ice-chargedcontainers, quick h
7、andling, and shortening exposure to adverse con-ditions. Airports seeking to expand cargo operations are addingrefrigerated warehouses internationally.Theavailabilityof refriger-ated warehouses is generally the result of specific market demandsand competition.Fruits and vegetables, flowers and nurse
8、ry stock, poultry andbaby chicks, other live animals, hatching eggs, meats, seafoods,dairy products, whole blood, body organs, and drugs (biologicals)are transported by air. Items are generally so perishable that slowermodes of transportation result in excessive deterioration in transit,making air m
9、ovement the only possible means of delivery. Certainearly-season and specialty fruits and vegetables can be flown to dis-tant markets economically because of the high market prices whenthere is a short supply. Some items, such as cut flowers and papayas,arrive at distant markets in better condition
10、than they would other-wise, so the extra transportation cost is justified. Flowers areshipped on a regular basis from Hawaii to the mainland UnitedStatesandfromCaliforniaandFloridatolargemidwesternandeast-ern cities. Air movement of strawberries has increased tremen-dously, including direct shipment
11、s to global destinations. Papayasare shipped from Hawaii almost exclusively by air.When carefully handled, ice cream is shipped successfully tooverseasmarketsfromtheUnitedStates;however,someunsuccess-ful shipments have occurred because customs inspectors haveopened containers for inspection and have
12、 taken too much time.Lowered trade barriers have reduced this risk.Fruits and VegetablesAll fresh fruits, vegetables, and cut flowers remain livingthroughout their entire salable period. Being alive, they respond totheir environment and have definite limitations on the conditionsThe preparation of t
13、his chapter is assigned to TC 10.6, Transport Refrig-eration.Fig. 1 Flexible Passenger/Cargo MixFig. 1 Flexible Passenger/Cargo MixFig. 2 Payload-Range Comparison for Wide-Body JetFig. 2 Payload/Range Comparison for Wide-Body Jet27.2 2010 ASHRAE HandbookRefrigeration (SI)they can tolerate. They rema
14、in alive through respiration, whichbreaks down stored foods into energy, carbon dioxide, and water,with the uptake of atmospheric oxygen. Respiration, together withaccompanying chemical changes, results in quality changes and theeventual death of the commodity. These internal changes associatedwith
15、life cannot be stopped but should be retarded if high quality isto be retained for a prolonged period.SeafoodSeafood and fish also benefit from the speed of air freight. Theabundance of fresh fish at restaurants and markets throughout theUnited States is the result of air shipment.AnimalsDesign of a
16、ircraft cargo compartments for animals is based onSociety of Automotive Engineers (SAE) Standard AIR 1600 andthe U.S. Code of Federal Regulations (CFR, updated annually),Title 9. Temperature and ventilation regulations as well as recom-mendations for birds and animals of all sizes are included in th
17、esedocuments. Air transportation limits exposure to the extremes thatwould otherwise require special handling and additional cost foranimal safety in accordance with the regulations.PERISHABLE COMMODITY REQUIREMENTSJustification for air transport of perishable commodities is basedon (1) time and (2)
18、 the delivery of a higher-quality product than ispossible by other modes of transportation. Better delivered qualityincreases returns to the shipper. This not only offsets the addedtransportation costs but also increases consumer demand and accep-tance.Themarketqualityofperishableitemsisdefinitelyco
19、ntrolledby a time and temperature relationship. Temperature cannot beignored even for the few hours now required for transcontinental airmovement. Proper temperature and humidity must be maintained atall times.WelbyandMcGregor(2004)listsdesirabletransitenvironmentsfor most perishable horticultural c
20、ommodities. Figure 4 shows theresult of a test of air shipments of strawberries from California toChicago in a refrigerated but uninsulated container. The shipmentswere exposed to high ambient temperatures during ground han-dling at origin, resulting in fruit temperatures ranging from 10 to15C inste
21、ad of the desired 0 to 1C. These berries were comparedwith those shipped by rail in 4.5 days with temperatures averaging3C for the transit period. Appearance and decay on delivery wereabout the same for both lots. Thus, the advantage of the short 22 hair movement was offset by a loss in quality caus
22、ed by unfavorabletemperature.Top quality of many of the most perishable commodities can besignificantly reduced by only a few hours exposure to unfavorablyhigh temperatures. Many drugs (biologicals) and other items, suchas whole blood, can be rendered completely ineffective or toxic ifnot kept at th
23、e specified low temperature.Someflowers,fruits,andvegetablesrespondfavorablytoreducedoxygen levels, increased amounts of carbon dioxide, or both, whichcould be maintained by gastight packaging or containers.Maintaining temperatures near freezing is not desirable for allproducts because some are subj
24、ect to chilling injury, even at temper-atures well above the freezing point. Chilling injury is most pro-nounced in tropical products, such as bananas, tomatoes, cucumbers,avocados,andorchids.Temperaturesabove13Careusuallysafeforcold-sensitivecommodities. Otheritems,suchascutflowers,requiretemperatu
25、res between 0 and 13C. For specific storage temperaturesand conditions for fruits and vegetables, see Chapters 35 to 37.Certain fruits and vegetables require humidity control. Humidityshould be kept between 85 and 95% to prevent wilting and generalloss of water. The relative humidity in thecabin of
26、an airplane flyingat about 12000 m is generally less than 10%. However, respirationof fruits and vegetables, placed in closed containers with recircu-latedcoolingair,shouldproducetherequiredhumiditylevelwithnoadditional water added.Some vegetables, such as peas, broccoli, lettuce, and sweet corn,hav
27、e high respiration rates and may produce heat equivalent to125 kg or more of ice meltage per tonne of vegetables per day at15C. In designing the refrigeration systems for aircraft containers,the additional evaporator capacity required to handle the heat of res-piration should be considered.DESIGN CO
28、NSIDERATIONSA refrigeration or air-conditioning system for a cargo airplane orairborne cargo containers has conflicting design temperaturerequirements, depending on the type of cargo to be carried, whichmakes it difficult to use one optimum refrigeration system for allkinds of cargo. For example, fr
29、ozen foods should have a tempera-ture of 18C or lower, fresh meat and produce 1 to 7C, and liveanimalsgenerallyrequiretemperatures in thesamecomfort rangeasfor humans. Many commercial jet cargo planes operate with themain cabin divided between cargo compartments and passengercompartments, and they a
30、re supplied by a single air system con-trolled to the comfort of the human occupants. In this case, perish-able cargo must be packed in containers, insulated, and iced orprecooled.The design ambient temperatures that an airplane experiences inflight are given in Chapter 10 of the 2007 ASHRAE Handboo
31、kHVAC Applications. A cargo jet cruising close to Mach 0.9 has anincrease in skin temperature over ambient of about 28 K. With anall-cargoload,thebasicair-conditioningsystemscanmaintainmaincargo compartment temperatures on a design hot day from 4C at9000 m to 1C at 12000 m.Air-conditioning systems a
32、re equipped with controls to preventfreezing of moisture condensed from the air at low altitudes. Withthe extremely dry air prevailing at cruise altitudes, an override ofthese anti-icing controls would allow an even lower cabin tempera-ture, although it is doubtful that storage temperatures for froz
33、engoods could be met. Thus, some insulation is still required in frozenfood containers. Also, airplanes are often required to hold at rela-tively low altitudes of 6000 m or less (because of heavy traffic atbusier airports) for 30 min or more.Permanently attaching a mechanical refrigeration system to
34、 acargo container may not be desirable for several reasons: increasedload, reduced usable volume, and difficulty in rejecting the con-densing unit heat load overboard. These objections are particularlyapplicable to containers carried in the main cargo hold. On the otherhand, permanently attached uni
35、ts allow refrigeration of just part ofthe cargo load while the remainder is held at temperatures in theFig. 3 Temperature of Strawberries Shipped by Air and RailFig. 3 Temperature of Strawberries Shipped by Air and RailAir Transport 27.3normal human or animal comfort zone. Temperature control ofprod
36、ucts requiring widely differing transit and storage temperatureswould be more feasible with onboard refrigeration units.SHIPPING CONTAINERSFruits and vegetables are generally shipped in the same contain-ers used for surface transportation: wooden boxes, veneer crates ofvarious types, or fiberboard c
37、artons. Most flower containers areconstructed of either plain or corrugated fiberboard materials.Wooden cleats are used for bracing, generally as dividers or cornerbraces inside the cargo box. Where lading may be exposed to verycold surfaces, external cleats may be used as spacers to preventdirect c
38、ontact. Certain flowers, such as gardenias and orchids, maybe packaged in individual cellophane-wrapped boxes or trays andplaced in a master container. Any tightly sealed film wrap must beperforated by at least one small hole to allow release of air from thecontainer during ascent to high altitudes.
39、Containers, may be built on pallets and shaped to make maxi-mum use of the interior airplane volume. Airline containers pres-ently in use are described in the International Air TransportAssociations (IATA, updated annually) Unit Load Devices (ULD)Technical Manual. Containers for aircraft, except for
40、 belly cargoholds, are not shaped to make maximum use of the interior volumeof the airplane. One reason for this is that the individual packagesfillingthecontainersaregenerallyrectangularanyway.Anotherrea-son is to allow easier intermodal transport (e.g., from motor truck tothe airplane and vice ver
41、sa). Because of the size of aircraft loadingdoors and irregular aircraft cross sections compared to surface vehi-cles and vessels, containerization may require this compromise.Containerization is a system of moving goods in sealed, reus-able freight containers too large for manual handling and witho
42、utpermanently attached wheels. Advantages include far less cargodamage and pilferage, lower packaging costs, minimized handling,and lower shipping rates. Containers may be loaded at the airfreight terminal or loaded at the shippers facility and transportedby flatbed truck-trailer or railroad, or bot
43、h, to the air terminal.The critical condition for design of insulation and refrigerationsystems (detachable plug-in type or permanently installed) forcargo containers is the time that the container is on the dock in thehot sun waiting for shipment. For this condition, an ambient tem-perature of 38C
44、db is assumed. The average outside skin tempera-ture of an unpainted metal container is about 45C.Under these conditions, the 2.4 by 2.4 by 3.0 m container with13 mm of high-efficiency insulation (recirculating the air and con-sidering no latent load) requires about 5 kW of refrigeration tomaintain
45、1.7C inside and about 7 kW to maintain 18C inside. Forquick pulldown to these temperatures of the container and fresh per-ishable contents (assuming prefreezing of frozen products), capac-ities should increase by about 50%.Freshfish,shrimp,andoystersmaybepackedinboxes,barrels,orspecial containers. P
46、recautions must be taken to prevent drippagefrom melted ice into the cargo space. Live lobsters are packed ininsulated containers with saltwater seaweed. Frozen foods arealways packed in insulated containers. Whole blood is shipped inspecially developed containers. Insulated bags are also used.The c
47、onfigurations and dimensions of two insulated containersare shown in Figure 4. Insulated with closed-cell, rigid plastic foam,the containers are a fabricated sandwich structure and are sized to fitconventional pallets and materials handling systems. The heat trans-fer rate for the entire standard co
48、ntainer is 15 W/K, and 17 W/K forthe commercial size. More recent aircraft, such as the MD-11depicted in Figure 1, use pallets 3.2 m wide by 1.6, 2.2, and 2.4 m,which may be loaded to 1.6 m high and retained by straps. Loadcapacities are 3000, 4500, and 5000 kg, respectively. Containers areLD-3 (hal
49、f width) and LD-6 (full width), the latter having twice thecapacity and width. Both are 1.5 m deep and 1.6 m high. The LD-3width is 2 m, the volume is 4.5 m3, and the capacity is 1500 kg. Aplug-in portable mechanical refrigerating unit may be positioned inthe doorway for standby operation. Tight construction allows con-trolled atmosphere application. A smaller shipping unit, insulatedwith a foamed plastic, has inside dimensions of 1.1 by 0.5 by 0.6 m(i.e., a total area of 3.0 m2and a capacity of 0.33 m3). The heat trans-fer rate of the entire container is 1.5 W/K.TRANSI
