Environmental Engineering Reference
In-Depth Information
Refrigerated transport
Refrigerated transport is the link between production, processing, distribution and retailing of
perishable foods that need to be handled at cold temperatures, in either chilled or frozen state.
Refrigerated transport comes in several modes including refrigerated trucks, refrigerated rail
cars, reefer containers, reefer ships, planes, and a combination of two or more (intermodal
transport). There are around 1 million refrigerated trucks and 400,000 refrigerated container
units in the world (Ruiz-Garcia et al., 2007).
With exception of planes, most refrigerated transports use vapor-compression refrigeration
equipment, which are preferably run with hydrofluorocarbon refrigerants (i.e., R410A,
R407C, R404a, or HFC-134a), but numerous equipment still runs on CFCs and HCFCs.
A number of refrigeration systems are based on the one-time use of solid or liquid refrigerants,
such as solid or liquid carbon dioxide (i.e., R-744) and liquid nitrogen. Because these refriger-
ants evaporate as they absorb heat, they are lost after use.
Most commonly, refrigerated trucks operate their vapor-compression refrigeration equip-
ment with small dedicated diesel engines, but there are systems that run the refrigeration
equipment with the truck's engine. Railroad cars can run their refrigeration equipment with
diesel engines or electric motors with electricity supplied from a generator car. In maritime
transportation, refrigeration equipment is electrically run with electricity generated by a
generator driven by the vessel's engine or by an auxiliary diesel power generator.
Intermodal refrigerated transport is based on the use of “refrigerated containers” or
“reefers” that can be transported by ship, railroad, or truck. There are two types of containers
to carry refrigerated products: the integral unit and the porthole containers. Integral containers
are insulated shipping containers of standard dimensions equipped with electric refrigeration
equipment. When on board-ships, integral containers are plugged into the vessel's power
supply or to individual generators when necessary. Once the container is on a rail car or a
truck, generators attached to the end of the container or located under the trailer provide
electricity to the refrigeration equipment.
Porthole containers are insulated containers with no permanent refrigeration units attached
to them. Low temperatures are achieved by circulating cold air that is obtained from a central
cooling through two ports (one for supply and another for return) plant, which are present at
some container ships and terminals. When there are no central cooling plants, portholes are
refrigerated by portable “clip-on units” that are attached at the end of the container or by port-
able tower units that can serve more than one container. Trends indicate that porthole contain-
ers are being phased out in favor of more versatile integral units, which as a disadvantage have
higher energy consumption during transportation below deck and higher maintenance costs
(Container Handbook, 2010).
Energy consumption in refrigerated transportation
Refrigerated transportation has two main impacts: energy consumption and refrigerant
leakage. In reefers, it is important to make a distinction between the way refrigeration is
operated at freezing and chilling temperatures. For temperatures below −10°C small fluctua-
tions in temperature are not important, so on/off control types are appropriate. Temperature is
maintained in a band of +/− 0.2°C from the nominal value and the compressor is turned off
during the off cycle, while the circulation fans are turned to low speed to save energy. It is
estimated that during the off cycle, the consumption of energy is lowered by a factor of 8
(Container Handbook, 2010). Typical power consumption at freezing temperatures in a reefer
is around 2 kW.
