1、25.1CHAPTER 25CARGO CONTAINERS, RAIL CARS, TRAILERS, AND TRUCKSVehicles . 25.1Vehicle Design Considerations . 25.1Equipment . 25.3Equipment Design and Selection Factors . 25.6Qualification Testing. 25.8System Application Factors. 25.8Operations. 25.10RANSPORT of commodities may be as simple as direc
2、t deliv-Tery of fresh vegetables from garden to market in a wagon. How-ever, travel time, ambient temperature, and risk of spoilage oftenmake temperature-controlled transport necessary. Because somecommodities are sensitive to the relative humidity and chemicalcomposition of their surrounding atmosp
3、here, these conditions mayalso need to be controlled. Today many commodities travel to dis-tant markets intermodally (i.e., by some combination of highway,ocean, and railroad). This chapter discusses the vehicles, equip-ment, and related factors that combine to preserve temperature-sensitive commodi
4、ties as they travel.Users are urged to regard the vehicle and its equipment as a sys-tem, particularly when making insulation and equipment sizingdecisions.VEHICLESVehicles used for temperature-controlled transport are similar inconstruction and outward appearance to those in general freight ser-vic
5、e, but have three fundamental differences: they have (1) insula-tion that is usually foamed in place, (2) provisions for conditionedair circulation through and around the cargo, and (3) machinery forcooling and/or heating. A brief description of the four main vehicletypes follows.Cargo containers ar
6、e usually 8 ft wide, 8 to 9.5 ft high, and 20or 40 ft long (Figure 1). They have hinged doors in one end forcargo loading and other access to the interior. The machinery com-prises the opposite end, so it must also provide structural rigidityand insulation. As shown in Figure 1, containers have stan
7、dardizedcorner fittings to secure them to vessels, railway cars, and highwayvehicles. Standards also govern their exterior dimensions. (SeeANSI Standard MH5.1.1.5 and ISO Standard 668.)Railway refrigerator cars are insulated boxcars, usually 50 to70 ft long (Figure 2). As illustrated, they may have
8、a machinerycompartment at one end.Trailers range in size from 8 to 8.5 ft wide, 12 to 13.5 ft high, and24 to 53 ft long. Their doors are usually hinged, but they may haveinsulated roll-up doors if used for multistop delivery service. Someinclude a curbside door in addition to rear doors. Several int
9、eriorcompartments for different temperatures may be provided by aninsulated bulkhead to separate the different zones. For hanginguncut meat, overhead rails are used. Specially designed trailers rid-ing on railway flat cars are quite common. Another design can bemounted directly on specially configur
10、ed railway bogies and pulledby a locomotive in a train of similar trailers.As with ordinary trucks, those built for temperature-controlledduty come in a wide variety of designs and sizes. Their bodies mayhave insulated hinged or roll-up doors on the sides and rear. Truckbodies also may have several
11、interior compartments for differenttemperatures, similar to trailers, with an insulated bulkhead separat-ing the different zones. Smaller vehicles may include a refrigerationcompressor as an engine-driven accessory (see Figure 7).VEHICLE DESIGN CONSIDERATIONSInsulation and Vapor BarrierEnvelope desi
12、gn factors to be considered are similar to those forstationary refrigerated facilities, and include the following: Extremes of exterior conditions: temperature, relative humidity,wind, and solar effectDesired interior conditions: temperature and relative humidityInsulation properties: thermal conduc
13、tivity, moisture permeabilityand retention, chemical and physical stability, adhesion, unifor-mity of application, fire resistance, cost of material and applica-tion, and presence of structural membersInfiltration of air and moistureTradeoffs between construction cost and operating expenseWhen appli
14、ed to refrigerated vehicles, these five factors are com-plicated by others unique to transportation. Exterior dimension con-straints are imposed by domestic or international standards andregulations, and shippers want maximum cargo space (which limitsinsulation thickness) and minimum tare weight. Th
15、e frequency andduration of door openings may be considerable. Long trips at high-way or railway cruising speeds affect infiltration. Physical deteriora-tion from the shock and vibration of travel and cargo shifting is likely.Also, there is potential for damage to insulation and vapor barriersfrom ve
16、hicle accidents and cargo handling mishaps.The preparation of this chapter is assigned to TC 10.6, Transport Refrigeration.Fig. 1 Refrigerated Cargo ContainerFig. 1 Refrigerated Cargo Container25.2 2010 ASHRAE HandbookRefrigerationClosed-cell foamed-in-place insulation, such as polyurethane,is gener
17、ally recommended to achieve an approximate thermalconductivity k of 0.15 Btuin/hft2F. It also helps limit air andwater vapor infiltration. Buyers often specify the UA or maximumheat transfer rate, usually at 100F and 50% rh outside and 0Finside, expressed as Btu/hF for the entire vehicle.Environment
18、al considerations affect and are affected by vehicleinsulation and vapor barrier choices. Mandated changes to insula-tion frothing agents with little or no adverse environmental impactmay increase insulation k value, and moisture permeability andretention. Chemical and physical characteristics such
19、as adhesion,durability, and stability may also be degraded. Because reducedinsulation effectiveness increases energy use, it adds to air pollu-tion and global warming concerns. Finally, the potential for materi-als recycling at the end of useful vehicle life must be considered.Cargo containers usual
20、ly have polyurethane insulation at 3 in.thickness in walls and floors, and 4 in. in ceilings. Rail cars often use3 to 6 in. in walls, and 5 to 8 in. in floors and ceilings. Trailers andtrucks generally use 1.5 to 4 in. in walls, floors, and ceilings forfrozen loads, and 1 to 2.5 in. in walls, floors
21、, and ceilings for non-frozen loads. Vehicle front walls are sometimes thicker to resistcargo shifting and support equipment.As mentioned previously, exterior dimensions are restricted andshippers want maximum cargo space. Increasing insulation thick-ness from 3 in. to 4 in. in a 40 ft long trailer
22、decreases cargo spaceby 100 ft3, or about 4%. However, the vehicles UA will improve,affecting equipment selection and improving operating economy.This exemplifies the need to regard the vehicle and its equipment asa system.Floors in all vehicles must support cargo and cargo-handlingequipment. They f
23、requently include rigid polystyrene or polyure-thane foam to eliminate beams. Floors must be watertight andjoined to walls to exclude water from insulation; a skirt bonded tothe floor and extending at least 6 in. up walls may be needed to con-trol water running down walls and collecting on the floor
24、. Floordrains, if used, must be trapped or capped to prevent infiltration ofoutside air.Infiltration of moisture and air is affected by the integrity of avehicles exterior surfaces (usually sheet metal with riveted joints).The molded glass-fiber-reinforced plastic sometimes used for truckand trailer
25、 exteriors is quite effective. There is some experimenta-tion with composite materials for cargo container bodies. Inside, itis common to use a vapor barrier, such as aluminum foil coated withplastic binder and sealed at joints. Integrity of foamed-in-place insu-lation (the absence of voids and brea
26、ks) is also important. Otherphysical contributors include the effectiveness of door gaskets andsealing around all exterior-to-interior penetrations. Operational fac-tors are vehicle travel speed and the frequency and duration of dooropenings.Purchasers of new refrigerated vehicles may require air le
27、akagetests. Purchasers criteria for these tests vary, depending on vehiclesize and intended use. A cargo container for modified-atmosphereservice (see the discussion on Controlled and Modified Atmospherein the Equipment section) must be especially tight. The purchasermay specify that air pressure in
28、 a 40 ft long container drop from 3 to2 in. of water in not less than 8 min, for a leakage rate of approxi-mately 48 ft3/h. A 48 ft trailer for general refrigeration service maybe tested at 0.5 in. of water with a leakage limit of 120 ft3/h.Infiltration into insulated vehicles occurs even when they
29、are sta-tionary, probably because of stack effect caused by the inside-to-outside temperature difference. The infiltration driving force for avehicle 8 ft high with a 100F difference is about 0.03 in. of water(Phillips et al. 1960). Openings with an aggregate area of 1 in2eachFig. 2 Mechanical Railw
30、ay Refrigerator CarFig. 2 Mechanical Railway Refrigerator CarCargo Containers, Rail Cars, Trailers, and Trucks 25.3at the top and bottom allow infiltration of about 120 ft3/h if assumedto be thin-plate orifices.Eby and Collister (1955) discuss the infiltration load from airentering through cracks in
31、 the front of a moving vehicle. The ramair pressure is 1.21 in. of water at 50 mph, and an exposed 1in2opening can allow 1150 ft3/h of air to enter. At ambient condi-tions of 100F and 50% rh and a vehicle temperature of 0F, theextra infiltration load is approximately 3800 Btu/h. Figure 3 illus-trate
32、s heat gain into a 0F vehicle resulting from infiltration ofambient air at various conditions.Air CirculationTo avoid spoilage during transport,Surround the cargo with a flow of conditioned air sufficient toremove heat that enters the vehicle by conduction and infiltration.To do this, interior surfa
33、ces must have channels for flow of con-ditioned air. There may be space between the top of the cargo andthe ceiling, or flexible duct(s) in that space, or a fixed duct (falseceiling). Walls may have batten strips, or channels formed into thewall surfaces, or fixed ducts (false walls). The floor may
34、havefixed longitudinal T-bars or “hat” sections, or movable racks.For commodities that respire or require in-transit cooling (e.g.,fresh fruits, vegetables, flowers), provide an adequate flow of con-ditioned air between and through packages. This process relies onthe air circulation ability of the e
35、quipment (see Equipment Selec-tion in the section on System Application Factors), and commod-ity loading practices (see Vehicle Use Practices in the section onOperations).Conditioned air may enter the cargo space over the top of cargo(normally used in rail cars, trailers, and trucks), or under the c
36、argo(normally used in cargo containers). Figure 4A shows an example ofa trailer with a top air delivery system. Air gaps between the cargoallow conditioned air to flow sufficiently around the load and alongthe side. Figure 4B shows an example of a cargo container systemwith a bottom air delivery sys
37、tem. In a bottom air delivery system,it is important to maintain air pressure under the load. Respiringcommodities should be packed in boxes with aligned top and bot-tom vent holes so conditioned air can flow through and remove heat.Proper loading technique is critical to maintaining good air circul
38、a-tion around the cargo to prevent spoilage. For additional informa-tion about loading techniques, see Ashby (2000).Equipment Attachment ProvisionsAll components must be securely fastened to the vehicle to resistshock, vibration, and vandalism. Vehicle-to-equipment interfacesmust have structure capa
39、ble of secure support under all conditionsof dynamic loading caused by vehicle travel and equipment opera-tion (e.g., engine and compressor vibration). Suitable fastening pro-visions (mounting holes, studs, or captive nuts) are needed. Wallopenings for equipment, which may be large (e.g., the entire
40、 frontwall of cargo containers), must have provisions to limit infiltration,using gaskets or other sealing methods.SanitationVehicle internal cleanliness is enhanced by eliminating interiorcrevices where fungi and bacteria can grow, and by using surfacesthat tolerate cleaning materials such as hot w
41、ater, disinfectants,detergents (including harsh cleaning solutions), and metal brighten-ers. Vehicle interiors should enable access to equipment componentsexposed to conditioned air (e.g., fans, cooling coils, and condensatepans) for periodic cleaning.EQUIPMENTMechanical Cooling and HeatingRefrigera
42、ted cargo containers typically have unitary equipmentthat comprises the entire front wall of the container. The refrigera-tion unit depth is approximately 16 in. and provides structure andinsulation to the container front wall. Figure 5 illustrates a typicalunit. The equipment has a vapor compressio
43、n refrigeration systemand uses an external source of electricity for its compressor and fanmotors, resistance heaters, and operating controls. It usually usesbottom air delivery, as shown in Figure 4B. The unit may have aFig. 3 Heat Load from Air LeakageFig. 3 Heat Load from Air LeakageFig. 4 Air De
44、livery Systems (A) Top and (B) BottomFig. 4 Air Delivery Systems (A) Top and (B) Bottom25.4 2010 ASHRAE HandbookRefrigerationdetachable diesel engine-generator set (with integral fuel tank)accompany it while traveling by land.Rail cars may have field-installed components. A three-phase acdiesel engi
45、ne-generator set, condensing unit, and refrigerant andelectrical operating controls are usually located in a machinerycompartment at one end of the car. An evaporator fan-coil package,or separately mounted evaporator and fan, is typically adjacent tothe machinery compartment but inside the insulated
46、 space. Electricheaters in or under the evaporator are used for heating and defrost.This equipment usually uses top air delivery, as shown in Figure 4A.Fuel tanks are generally located under the car. Newer rail cars mayuse end-mounted unitary equipment similar to trailers.Trailers typically have uni
47、tary equipment that consists of a dieselengine with battery-charging alternator, compressor, condenser andengine radiator with fan, evaporator with fan, and refrigerant andelectrical controls. It is installed on the front of the vehicle near thetop, over an opening that accommodates the evaporator a
48、nd fan, asshown in Figure 6. Top air delivery is usually used as shown hereand in Figure 4A. The fuel tank is mounted under the trailer.Large trucks typically have unitary equipment that is similar totrailer equipment, but more compact. Small trucks may have uni-tary equipment similar to that for la
49、rge trucks, or field-installedcomponents. The latter, as shown in Figure 7, have a truck-engine-driven compressor. Also included is a condenser and evaporatorfan-coil package. The unit is installed at the front top over an open-ing that accommodates the evaporator and its fan(s). (So it can beseen in this figure, the evaporator is shown shifted rearward.) MostFig. 5 Container Refrigeration UnitFig. 5 Container Refrigeration UnitFig. 6 Trailer Unit InstallationFig. 6 Trailer Unit InstallationFig. 7 Small Truck Refrigeration SystemFig. 7 Small Truck Refrigeration SystemCargo Co
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