Chapter 6 - Humidity Control

Abstract

During hypobaric storage a “metabolic humidification system” evaporates sufficient cellular water to transfer respiratory heat, fermentative heat, and any environmental heat plant matter may acquire. In LP warehouses and laboratory systems, the metabolic humidifier is supplemented with a mechanical humidification system that utilizes electric heat to evaporate sufficient ancillary water to saturate the incoming air changes. The advantage gained with a mechanical humidifier is that a full commodity load, partial load, even a single fruit can be stored, certain that the humidity will always be saturated to minimize commodity water loss. A chilled-mirror dew-point sensor was tested for humidity control in a Grumman/Dormavac hypobaric container at the sensors highest reliable noncondensing upper limit of 95% relative humidity (RH), where its accuracy was ±1.25% RH. Whenever the humidity decreased below 95% RH, a low-pressure water boiler’s electric immersion heater energized to inject cold steam into the incoming low-pressure air change. Without cargo present the system worked as envisioned, but after the container was filled with 30,000 pounds of plant matter the humidification heater failed to energize because metabolic heat evaporated enough commodity water to keep the chamber RH above 95% and prevent the boiler’s water heater from energizing. Because water vapor diffusion from plant matter is accelerated at a low pressure, the RH must be controlled at a higher value than 95%, close to 99.5–99.8% to minimize commodity water loss. This is accomplished measuring the humidity with wet- and dry-bulb thermistors shielded from radiation with Mylar®. The thermistors should be glass-encapsulated with ±0.05°C accuracy to measure the RH ±0.1% at 99.8% RH with optimal reliability. VacuFresh LP containers use metabolic humidification and do not need a mechanical humidifier because they only ship full loads. This eliminates the necessity to carry water, lowers energy consumption, and reduces cost. In modern VacuFresh containers, a vacuum regulator prevents condensation and continuously controls the pressure by regulating the pumping speed while the air-change rate is adjusted with a thermal mass flow controller. Atmospheric air enters the vacuum tank through a pneumatic air horn’s collar, pressurizing the air horn’s jets, inducing circulation of up to 40 volumes of low-pressure chamber air per volume of expanded incoming dry air. Discharge from the air horn flows though a duct to the opposite end of the container and returns to the air horn’s suction after passing through cargo boxes stagger-stacked to facilitate longitudinal airflow. The return air, saturated by transpired commodity water, mixes with expanded dry incoming air in the pneumatic air mover, nearly saturating the RH in the discharge. Without producing mechanical heat or consuming additional energy, the pressure difference created by the vacuum pump powers the air horn, causing it to emit up to 2500cfm of low-pressure humidified cargo air at sonic velocity.