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 27.4IR freight service is provided by all-cargo carriers and passengerA airlines. The latte
2、r companies 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 regul
3、ar ter-minals 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 Regulation
4、s. Prospective 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 temp
5、era-ture. 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 temp
6、eratures at high (cold) alti-tudes. This mode of refrigeration is not available at low altitudes oron the ground, where skin temperatures can exceed the compart-ment temperature significantly. Refrigeration techniques for aircraftrely primarily on precooling, insulated containers, dry-ice-chargedcon
7、tainers, quick handling, and shortening exposure to adverse con-ditions. Airports seeking to expand cargo operations are addingrefrigerated warehouses internationally. The availability of refriger-ated warehouses is generally the result of specific market demandsand competition.Fruits and vegetables
8、, flowers and nursery 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 t
9、ransit,making air movement 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 i
10、n better condition than they would other-wise, so the extra transportation cost is justified. Flowers areshipped on a regular basis from Hawaii to the mainland UnitedStates and from California and Florida to large midwestern and east-ern cities. Air movement of strawberries has increased tremen-dous
11、ly, including direct shipments to global destinations. Papayasare shipped from Hawaii almost exclusively by air.When carefully handled, ice cream is shipped successfully tooverseas markets from the United States; however, some unsuccess-ful shipments have occurred because customs inspectors haveopen
12、ed containers for inspection and have 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
13、on the conditionsThe preparation of this 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 HandbookRefrigera
14、tionthey can tolerate. They remain 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
15、internal changes associatedwith 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
16、 air shipment.AnimalsDesign of aircraft 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
17、 of all sizes are included in thesedocuments. 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 commodi
18、ties is basedon (1) time and (2) 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. The market qua
19、lity of perishable items is definitely controlledby 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.Welby and McGregor (2004) lists desirable transit
20、 environmentsfor most perishable horticultural commodities. 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 i
21、n fruit temperatures ranging from 50 to60F instead of the desired 32 to 34F. These berries were com-pared with those shipped by rail in 4.5 days with temperatures aver-aging 38F for the transit period. Appearance and decay ondelivery were about the same for both lots. Thus, the advantage ofthe short
22、 22 h air movement was offset by a loss in quality causedby unfavorable temperature.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 re
23、ndered completely ineffective or toxic ifnot kept at the specified low temperature.Some flowers, fruits, and vegetables respond favorably to reducedoxygen levels, increased amounts of carbon dioxide, or both, whichcould be maintained by gastight packaging or containers.Maintaining temperatures near
24、freezing is not desirable for allproducts because some are subject 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, and orchids. Temperatures above 55F are usually safe fo
25、rcold-sensitive commodities. Other items, such as cut flowers, requiretemperatures between 32 and 55F. For specific storage tempera-tures and conditions for fruits and vegetables, see Chapters 35 to 37.Certain fruits and vegetables require humidity control. Humidityshould be kept between 85 and 95%
26、to prevent wilting and generalloss of water. The relative humidity in the cabin of an airplane flyingat about 40,000 ft is generally less than 10%. However, respirationof fruits and vegetables, placed in closed containers with recircu-lated cooling air, should produce the required humidity level wit
27、h noadditional water added.Some vegetables, such as peas, broccoli, lettuce, and sweet corn,have high respiration rates and may produce heat equivalent to250 lb or more of ice meltage per ton of vegetables per day at 60F.In designing the refrigeration systems for aircraft containers, theadditional e
28、vaporator capacity required to handle the heat of respi-ration should be considered.DESIGN CONSIDERATIONSA 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, whichmake
29、s it difficult to use one optimum refrigeration system for allkinds of cargo. For example, frozen foods should have a tempera-ture of 0F or lower, fresh meat and produce 30 to 45F, and liveanimals generally require temperatures in the same comfort range asfor humans. Many commercial jet cargo planes
30、 operate with themain cabin divided between cargo compartments and passengercompartments, and they are 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 t
31、emperatures that an airplane experiences inflight are given in Chapter 10 of the 2007 ASHRAE HandbookHVAC Applications. A cargo jet cruising close to Mach 0.9 has anincrease in skin temperature over ambient of about 50F. With anall-cargo load, the basic air-conditioning systems can maintain maincarg
32、o compartment temperatures on a design hot day from 40F at30,000 ft to 30F at 40,000 ft.Air-conditioning systems are 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
33、 controls would allow an even lower cabin tempera-ture, although it is doubtful that storage temperatures for frozengoods 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 20,000 ft or less (beca
34、use of heavy traffic atbusier airports) for 30 min or more.Permanently attaching a mechanical refrigeration system to 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 objection
35、s are particularlyapplicable to containers carried in the main cargo hold. On the otherhand, permanently attached units 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
36、Strawberries Shipped by Air and RailAir Transport 27.3normal human or animal comfort zone. Temperature control ofproducts requiring widely differing transit and storage temperatureswould be more feasible with onboard refrigeration units.SHIPPING CONTAINERSFruits and vegetables are generally shipped
37、in the same contain-ers used for surface transportation: wooden boxes, veneer crates ofvarious types, or fiberboard cartons. Most flower containers areconstructed of either plain or corrugated fiberboard materials.Wooden cleats are used for bracing, generally as dividers or cornerbraces inside the c
38、argo box. Where lading may be exposed to verycold surfaces, external cleats may be used as spacers to preventdirect contact. 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 mu
39、st beperforated by at least one small hole to allow release of air from thecontainer during ascent to high altitudes.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 Transp
40、ortAssociations (IATA, updated annually) Unit Load Devices (ULD)Technical Manual. Containers for aircraft, except for belly cargoholds, are not shaped to make maximum use of the interior volumeof the airplane. One reason for this is that the individual packagesfilling the containers are generally re
41、ctangular anyway. Another rea-son is to allow easier intermodal transport (e.g., from motor truck tothe airplane and vice versa). Because of the size of aircraft loadingdoors and irregular aircraft cross sections compared to surface vehi-cles and vessels, containerization may require this compromise
42、.Containerization is a system of moving goods in sealed, reus-able freight containers too large for manual handling and withoutpermanently attached wheels. Advantages include far less cargodamage and pilferage, lower packaging costs, minimized handling,and lower shipping rates. Containers may be loa
43、ded at the airfreight terminal or loaded at the shippers facility and transportedby flatbed truck-trailer or railroad, or both, to the air terminal.The critical condition for design of insulation and refrigerationsystems (detachable plug-in type or permanently installed) forcargo containers is the t
44、ime that the container is on the dock in thehot sun waiting for shipment. For this condition, an ambient tem-perature of 100F db is assumed. The average outside skin temper-ature of an unpainted metal container is about 115F.Under these conditions, the 8 by 8 by 10 ft container with 0.5 in.of high-e
45、fficiency insulation (recirculating the air and consideringno latent load) requires about 18,000 Btu/h of refrigeration to main-tain 35F inside and about 24,000 Btu/h to maintain 0F inside. Forquick pulldown to these temperatures of the container and fresh per-ishable contents (assuming prefreezing
46、of frozen products), capac-ities should increase by about 50%.Fresh fish, shrimp, and oysters may be packed in boxes, barrels, orspecial containers. Precautions must be taken to prevent drippagefrom melted ice into the cargo space. Live lobsters are packed ininsulated containers with saltwater seawe
47、ed. Frozen foods arealways packed in insulated containers. Whole blood is shipped inspecially developed containers. Insulated bags are also used.The configurations and dimensions of two insulated containers areshown in Figure 4. Insulated with closed-cell, rigid plastic foam, thecontainers are a fab
48、ricated sandwich structure and are sized to fit con-ventional pallets and materials handling systems. The heat transferrate for the entire standard container is 28 Btu/hF, and 32 Btu/hFfor the commercial size. More recent aircraft, such as the MD-11depicted in Figure 1, use pallets 125 in. wide by 6
49、4.4, 88, and 96 in.,which may be loaded to 64 in. high and retained by straps. Loadcapacities are 6700, 10,000, and 11,000 lb, respectively. Containersare LD-3 (half width) and LD-6 (full width), the latter having twice thecapacity and width. Both are 60.4 in. deep and 64 in. high. The LD-3width is 79 in., the volume is 158 ft3, and the capacity is 3300 lb. 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
