Chapter 13 - Cost-Effective LP Intermodal Container

Abstract

All intermodal containers are depreciated in 5–6 years. A hypobaric intermodal container’s strong and durable structure assures that it will realize the tank container industry’s standard 20-year life expectancy. Because of their long life expectancy, the increased fabrication cost of tank containers can be offset by financing them for a longer term at a lower yearly expense, while benefiting from a tax advantage resulting from rapid depreciation of the higher fabrication cost. A brazed-plate heat exchanger and jacketed refrigeration system remove heat transmitted through a VacuFresh tank’s insulation before it enters the storage area, and the leak-tight structure prevents ambient heat from infiltrating except in controlled air changes. Due to an LP air change’s low density, the pounds of air and kilocalories of sensible heat introduced in it are extremely low. The incoming air does not reach its dew point after expanding and drying during entry, so no latent heat is released. At an 80% RH, 38°C ambient condition, 1ton of refrigeration is required to cool two air changes per hour flowing at 10°C and atmospheric pressure through a conventional refrigerated intermodal container, and only 0.004tons to cool the same air change flowing through a VacuFresh container. In VacuFresh, the refrigeration compressor’s capacity is balanced versus heat generated by the vacuum pump to insure that the compressor operates continuously in an unloaded state. The entire surface of the vacuum tank remains at a constant modulated temperature ±0.2°C at an ambient temperature of 49°C. Additional heat is not needed in cold weather, and defrost is not required since secondary glycol coolant is used. In CA and NA systems, metabolic heat is removed by refrigeration, consuming power, but in LP most of the heat produced by respiration is transferred by evaporative cooling and the water vapor is evacuated, independent of refrigeration. VacuFresh does not have energy-consuming, heat-producing evaporator fans. Instead, ventilation is provided by a pneumatic air mover that produces no heat, consumes no additional power, and is operated by the pressure difference created by the vacuum pump between the ambient atmosphere and interior of the vacuum tank. VacuFresh has a 2kW vacuum pump, CA uses a 2.5kW air compressor to produce N2 gas. The power requirement and cost was reduced in the original VacuFresh design by using “metabolic” instead of mechanical humidification, and the LP system has been redesigned to further improve reliability, reduce cost and energy consumption, and decrease commodity weight loss. The vacuum breaker has been replaced with a vacuum regulator, and the new system uses a mass flow controller, scroll compressor, and aluminum microchannel condenser. A spiral tank-stiffening ring eliminates the straight pipes, short radius elbows, entrance effects, and sudden enlargements associated with circular stiffening rings and interring flow connections in the original design. The spiral reduces the “head” needed to flow glycol by 42%, allowing a 1/2 HP glycol pump to provide the required glycol flow, in place of the original 1.5 HP pump. Reducing the glycol pump HP lowers the amount of heat introduced into the flowing glycol by 61%. A conventional refrigerated intermodal container’s 3/4 HP evaporator fans introduce 3.9 times that amount of heat. The respiratory heat load in VacuFresh is decreased by 90% due to the respiratory inhibition caused by the ultra-low [O2] at the storage pressure, and most of the respiratory heat is transferred by evaporative cooling and evacuated independent of refrigeration. HP consumption is reduced by 37% in the redesigned VacuFresh. The container’s price has been decreased by eliminating the material and labor costs associated with installing piping required for series glycol flow between circular stiffening rings, and by deleting heavy aluminum gussets needed to reinforce each circular stiffening ring where its continuity was broken in the original design. Shelving has been deleted, eliminating the expense of the aluminum and its installation cost. The price for LP and CA shipments is approximately the same, and much less than by air transport.