Chapter 3 - Gas and Vapor Mass Transfer at a Low Pressure

Abstract

Gas and vapor diffusion is accelerated when the pressure is lowered, and expansion dries and purifies the air in proportion to the pressure reduction that occurs when it enters a hypobaric system. The partial pressures of [O2], [CO2], [C2H4], other gases, volatile organic compounds (VOCs), water vapor, and cloud condensation nuclei (CCNs) present in the air are reduced in proportion to the pressure reduction. Heterogeneous condensation of supersaturated water vapor produced in a hypobaric warehouse by a combination of mechanical humidification and commodity transpiration is unlikely to occur because expansion of the incoming air reduces the concentration of CNNs. At a low pressure, fewer kilograms of air have to be passed through a chamber to provide an air change, and very little sensible heat is introduced. When the expanded low density air is cooled to 0°C, it does not reach its dew point, and therefore no latent heat needs to be removed. The air change rate in a conventional refrigerated intermodal container is limited to one or two per hour to avoid consuming excessive refrigeration capacity, but a much higher air-change rate during hypobaric storage does not create a significant refrigeration load. LP’s ability to drastically decrease both the ambient and intercellular [CO2] levels provides benefits that cannot be duplicated by elevating [CO2] during CA storage. A reduced [CO2] concentration during low-pressure (LP) storage opens stomata in darkness, depresses the growth of bacteria and fungi, prevents the loss of ascorbic acid, inactivates ethylene forming enzyme; limits toxic succinate accumulation; and prevents [CO2] injury