Environmental Engineering Reference
In-Depth Information
The best approach to reduce the aerodynamic drag is to reduce the gap between carts,
which can be accomplished by matching the cargo with the cart. For instance, a 40-foot con-
tainer should be loaded into a 40-foot car rather than into a 48-foot well car, and a 48-foot
trailer on a 48-foot spine car instead of using a 53-foot car (Lai and Barkan, 2005). A well-
matched train can reduce its aerodynamic resistance up to 27 percent and consequently reduce
its fuel consumption by 1 gallon per mile per train (Lai et al., 2008).
REDUCING THE IMPACT OF REFRIGERATED
TRANSPORT
To reduce the environmental impact of the refrigeration part of cold chain logistics energy
consumption and refrigerant leaks are the two aspects that need improvement. It is estimated
that in refrigeration equipment, 20 percent of the emissions come from direct refrigerant
leaks and 80 percent from the use of energy (International Institute of Refrigeration [IIR],
2005); so reduction of energy consumption, which also cut carbon dioxide emissions, must
be a priority.
Because of its mobile nature, refrigerated transport is likely to find different types of
environmental conditions while in transit; therefore, the cooling power of the refrigeration
equipment is designed for the worst environmental conditions they may encounter. However,
refrigerated transports are most of the time subjected to more benign temperatures and the
cooling power needs reduction to achieve the optimal temperature for chilled products, which
is normally narrow. As mentioned previously, refrigerated transportation requires a high
degree of temperature control (particularly for chilled cargo) that is usually achieved by
modulation of the refrigeration power, at the expense of additional energy use (Heap, 2003).
However, technical alternatives are available to reduce energy consumption while still
maintaining a high temperature control.
The most efficient approach to modulate refrigeration is the use of variable capacity
compressors, especially speed scroll compressors, which work in stages or by using varia-
ble controlled motors. Controlling the speed of the compressor can provide an accurate
temperature control and at the same time save energy and increase the life of the compres-
sor. Power consumption in electric motors is proportional to the cube of the rotational speed,
so a reduction of speed translates into lower power consumption. This principle applies not
only to compressors but also to fans used for air circulation inside refrigerated transport,
which can benefit from using variable speed motors and their speed reduced when there is no
need for full blast (see Chapter 11 for more on this topic).
Compressor unloading is the other alternative to modulate compressor power. Particularly
big-capacity compressors are equipped with systems to engage and disengage their many
cylinders, rotors, or scrolls in compressors with multiple numbers of these devices as
needed.
Refrigerant leaks in refrigerated transport
The mobile nature of refrigerated transport systems and the high number of units in the whole
world increase the risk of venting and refrigerant leaks. Mobile refrigeration equipment is
subjected to harsh conditions and vibrations, which make them more susceptible to leaks.
Careless or improperly trained technicians can unintentionally vent refrigerants before servic-
ing the equipment. As mentioned previously, high-global-warming-potential HFCs and
HCFCs are the refrigerants of choice for refrigerated transport, but still some old equipment
