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 2.4 m wide, 2.4 to 2.9 m high, and6.1 or 12.2 m long (Figure 1). They have hinged doors in one endfor cargo loading and other access to the interior. The machinerycomprises the opposite end, so it must also provide structural rigid-ity and insulation. As shown in Figure 1, containers have
7、standard-ized corner fittings to secure them to vessels, railway cars, andhighway vehicles. Standards also govern their exterior dimensions.(See ANSI Standard MH5.1.1.5 and ISO Standard 668.)Railway refrigerator cars are insulated boxcars, usually 15 to20 m long (Figure 2). As illustrated, they may
8、have a machinerycompartment at one end.Trailers range in size from 2.4 to 2.6 m wide, 3.7 to 4.1 m high,and 7.3 to 16.8 m long. Their doors are usually hinged, but they mayhave insulated roll-up doors if used for multistop delivery service.Some include a curbside door in addition to rear doors. Seve
9、ral inte-rior compartments 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
10、 configured 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
11、 several 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 BarrierEnve
12、lope design 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: therm
13、al conductivity, 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 expenseW
14、hen applied 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 w
15、eight. The 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 barrie
16、rsfrom vehicle 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 HandbookRefrigeration (SI)Closed-cell foamed-in-place insulation, such as polyure
17、thane,is generally recommended to achieve an approximate thermalconductivity k of 0.022 W/(mK). It also helps limit air and watervapor infiltration. Buyers often specify the UA or maximum heattransfer rate, usually at 38C and 50% rh outside and 18C inside,expressed as W/K for the entire vehicle.Envi
18、ronmental 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 characteristic
19、s such 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 container
20、s usually have polyurethane insulation at 75 mmthickness in walls and floors, and 100 mm in ceilings. Rail cars oftenuse 75 to 150 mm in walls, and 125 to 200 mm in floors and ceilings.Trailers and trucks generally use 35 to 100 mm in walls, floors, andceilings for frozen loads, and 25 to 65 mm in w
21、alls, floors, and ceil-ings for nonfrozen loads. Vehicle front walls are sometimes thickerto resist cargo shifting and support equipment.As mentioned previously, exterior dimensions are restricted andshippers want maximum cargo space. Increasing insulation thick-ness from 75 mm to 100 mm in a 12 m l
22、ong trailer decreases cargospace by 2.8 m3, or about 4%. However, the vehicles UA willimprove, affecting equipment selection and improving operatingeconomy. This exemplifies the need to regard the vehicle and itsequipment as a system.Floors in all vehicles must support cargo and cargo-handlingequipm
23、ent. They frequently 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 150 mm up walls may be needed tocontrol water running down walls and collecting on
24、 the floor. 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 trucka
25、nd trailer 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 void
26、s and breaks) 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 requ
27、ire air leakagetests. 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 p
28、ressure in a 12 m long container drop from750 to 500 Pa in not less than 8 min, for a leakage rate of approxi-mately 1.4 m3/h. A 14.6 m trailer for general refrigeration servicemay be tested at 125 Pa with a leakage limit of 3.4 m3/h.Infiltration into insulated vehicles occurs even when they are sta
29、-tionary, probably because of stack effect caused by the inside-to-outside temperature difference. The infiltration driving force for avehicle 2.4 m high with a 55 K difference is about 7.5 Pa (Phillipset al. 1960). Openings with an aggregate area of 645 mm2each at theFig. 2 Mechanical Railway Refri
30、gerator CarFig. 2 Mechanical Railway Refrigerator CarCargo Containers, Rail Cars, Trailers, and Trucks 25.3top and bottom allow infiltration of about 3.4 m3/h if assumed to bethin-plate orifices.Eby and Collister (1955) discuss the infiltration load from airentering through cracks in the front of a
31、moving vehicle. The ramair pressure is 300 Pa at 80 km/h, and an exposed 645 mm2openingcan allow 32.6 m3/h of air to enter. At ambient conditions of 38Cand 50% rh and a vehicle temperature of 18C, the extra infiltra-tion load is approximately 1115 W. Figure 3 illustrates heat gaininto a 18C vehicle
32、resulting from infiltration of ambient air atvarious 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 surfaces must have channels for flo
33、w 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 havefixed longitudinal T-bars
34、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 equipment (see Equipment Selec-
35、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 cargo(normally used in cargo co
36、ntainers). 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 system. In a bottom air delivery
37、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 circula-tion around the cargo to pre
38、vent 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 capable of secure support under al
39、l 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 frontwall of cargo containers
40、), 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 water, disinfectants,detergents
41、 (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 HeatingRefrigerated cargo containers typically
42、 have unitary equipmentthat comprises the entire front wall of the container. The refrigera-tion unit depth is approximately 400 mm and provides structure andinsulation to the container front wall. Figure 5 illustrates a typicalunit. The equipment has a vapor compression refrigeration systemand uses
43、 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 Delivery Systems (A) Top and (B)
44、 BottomFig. 4 Air Delivery Systems (A) Top and (B) Bottom25.4 2010 ASHRAE HandbookRefrigeration (SI)detachable 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 engine-generator set, condens
45、ing 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 space. Electricheaters i
46、n 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 unitary equipment that consi
47、sts 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 and fan, asshown in Figure
48、 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 large trucks, or field-inst
49、alledcomponents. 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 Containers, Rail Cars, Trailer
