Chapter 7 - Water Condensation in Hypobaric Chambers

Abstract

Water transpired by plant matter supersaturates a hypobaric storage chamber when the incoming air change has been saturated by a mechanical humidifier at the storage temperature. The system’s pressure control device, pressure measuring transducer, and vacuum pump are operated at 25°C to ensure that saturated process water in the 0–15°C low-pressure air does not condense in these instruments and interfere with their operation. The high buoyancy of the chamber’s supersaturated low-pressure mixture causes it to rise, and when a vacuum breaker is controlling the pressure, the excess water vapor condenses on the storage chamber’s roof and walls. Unexpectedly, transpired water did not condense and accumulate in laboratory hypobaric chambers during 8-week tests with full loads of plant matter when a mechanical humidifier saturated the incoming low-pressure air change and the pressure was controlled with a vacuum regulator operating at 25°C. In the same laboratory chamber at an identical storage temperature, pressure, and mechanically humidified air-change rate, a substantial quantity of transpired water condensed when the pressure was controlled with a vacuum breaker operating at 25°C. If low-pressure supersaturated air escaped before condensing in a hypobaric storage chamber, the rarified air’s low heat capacity would cause it to rapidly warm and the pressure to increase upstream of a vacuum regulator operating at 25°C. The regulator would respond by enhancing the pumping speed to offset the pressure rise. Water vapor would continue entering the storage chamber at the initial rate determined by the humidification system’s wattage setting, and air would enter at the initial rate determined by choked flow through a preadjusted flow-regulating restriction. The system equilibrates when the increased rate of air and moisture flow from the chamber to the vacuum regulator and vacuum pump decreases the chamber RH to saturation, preventing condensation and stabilizing the pressure. This explanation was confirmed by measurements demonstrating that when one saturated 15mmHg, 13°C air change per hour entered a hypobaric chamber filled with mangos, a vacuum regulator operating at 25°C increased the rate at which the low-pressure mixture flowed from the chamber to the vacuum pump by 1.54-fold. Vacuum breakers control the rate at which air enters the storage chamber. If air supersaturated by commodity transpiration and mechanical humidification escaped from an LP storage chamber in which the pressure was controlled with a vacuum breaker, and this mixture warmed to 25°C, elevating the pressure upstream of the vacuum pump, the breaker would sense the pressure rise which back-fed into the storage chamber and try to decrease the pressure by slowing the rate at which air entered the chamber. The initial amount of water vapor will continue entering accompanied by less air, and the excess moisture condenses in the chamber. Commodity weight loss during VacuFresh shipments can be decreased by substituting a vacuum regulator operating at 25°C in place of the vacuum breaker used in the original VacuFresh design. The regulator causes the pumping speed to slow in response to the progressive decrease in metabolic heat production and transpiration that occurs during many weeks after harvest. This prevents evaporative cooling from decreasing the commodities temperature and causing heat to radiate to the plant matter from the chamber wall. Eliminating access to environmental heat allows the commodity weight loss to decline in proportion to the progressive decrease in metabolic heat production that occurs during storage.