Chapter 10 - Capillary Condensation in Non-Waxed Cardboard Boxes

Abstract

When the pressure was controlled by a vacuum breaker and water-saturated low-pressure air changes were flowed, throughout 3 days the humidity could not be elevated above 92% in a laboratory hypobaric storage chamber containing an empty nonwaxed cardboard box. After the box was removed, the humidity immediately reached saturation. The cardboard was lowering the humidity. This same behavior occurred in a mechanically humidified Vivafresh hypobaric warehouse in which the pressure was controlled by a vacuum breaker. Even though mechanical humidification with sufficient moisture to saturate the air was supplemented with transpired moisture, 12.5 days passed before the warehouse’s humidity reached saturation. Cardboard is a capillary-porous colloidal cellulosic material containing randomly distributed micropores and macropores. It is highly hygroscopic, absorbing large amounts of adsorption-bonded moisture because its polymeric molecular chains contain hydroxyl groups. Moisture diffuses through cardboard micro- and macropores 20- to 50-fold faster in a vacuum than at atmospheric pressure, and pure water vapor is absorbed 50- to 150-fold more rapidly. Dry wooden field crates and corrugated fiberboard containers take up moisture and increase in weight during apple storage at atmospheric pressure, but in a vacuum the increase in box weight occurs 100-fold faster. Regardless of whether boxes in a Vivafresh warehouse were empty or filled with roses, water condensed in the cardboard, weakening it and increasing its weight by 20.1% in 6 weeks. By the fourteenth day, each cardboard box had released 594cal of latent heat per gram of condensed water, a total of 115.2kcal, an amount capable of raising the cardboard’s temperature by 213°C. Most of this heat was transferred to the flowers since the box surface area moving heat inward by radiation and convection was 22.8-fold larger than that radiating heat from one end of the box to the warehouse wall and 11.4-fold larger than the area transferring heat by outward convection from both ends of the box into the warehouse air. After the warehouse was vented and again pumped down on the fourteenth day, no significant temperature rise occurred in the flowers because water condensation in the cardboard had by then approached its end point. During the initial 2 weeks, water did not condense inside the warehouse, but after 17 days, when the humidity became supersaturated, condensation began. A thermistor probe was imbedded in the cardboard and a second probe inserted among the flower bunches contained in a box. Pump-down decreased the chamber relative humidity (RH) to 93%, and by the second day, heat generated by water condensation in the cardboard had warmed it to 4°C and the roses to 2.9°C, while the chamber RH rose to 98.5%. During the initial 12 days, because the flowers remained colder than the cardboard, they received heat from it by radiation, convection, and conduction, and the roses emitted this heat as well as metabolic heat by evaporative cooling. Heat generated by water condensing in the cardboard caused 55% of the total floral weight loss, and metabolic heat the remainder. Mylar® was 70% effective blocking floral weight loss. The evaporative weight loss caused by metabolic heat was less than 2.8% during 35 days. Plastic boxes eliminated the entire floral weight loss caused by water condensation in the cardboard.