It’s no accident that research at university ag schools assumed the task of determining optimum conditions for storage of the famous Russet Burbank, a Fall-crop variety, the primary potato variety of choice initially for French fries. The fact that frozen French fries took-off in the 1960s, and per capita consumption today in the United States is approximately 55 pounds per year, commanded serious attention: The Russet needed to be stored at harvest so that French fry processors could have raw product all year. Consequently, an explosion of raw product potato storage system performance took place during the 1970s.
An appreciation for the reasons this decade of increase in potato storage and system performance took place will help to more-completely understand the subject of Condensation Control, and the reasons for development of the Suberizer Air Envelope Cavity System in 1979.
University research determined that potatoes placed in storage were a living-breathing organism requiring oxygen. This respiration process, actually a combustion process, resulted in heat, water, and CO2 in the potato pile. A look at this respiration process as a function of time, shows that the respiration rate, the heat being generated in BTU per ton of potatoes per day, can be quite high during the first month potatoes are in storage. Several things that affect storage system performance are important:
- The heat and CO2 from respiration must be removed;
- That means the pile temperature must be controlled;
- Supply air must be uniformly distributed through the pile to control pile temperature;
- The graph shows that the rate of heat generation is a function of supply air relative humidity; and,
- Most-important of all: High Relative Humidity supply air, with tightly-controlled temperature, produces an optimum suberizing environment which reduces losses during storage.
A closer, more thorough appreciation of suberization is worthwhile and fundamental knowledge for successful storage management. Suffice, for this publication, to realize that an optimum suberization process significantly reduces weight loss, quality losses, and losses due to spoilage. In fact, research suggestions that every percent of weight loss during storage results in an additional percent of quality loss, has been validated in the real world.
With this in mind, realize that in the 1960s supply air was typically controlled to plus-or-minus 3°F. By 1970 control of potato storage supply air was tightened to plus-or-minus 1.5°F by a mechanical proportional controller. Suberizer’s first control systems in 1972 incorporated three-mode electronic supply air temperature control to plus-or-minus 0.5°F, and by the late 1970s Suberizer panels controlled supply air temperature to plus-or-minus 0.1°F.
Along with tight supply air temperature control, Suberizer initiation of the high-performance blow-through air washer and low-velocity cellular humidification systems minimized weight loss, and made optimum potato suberization a reality.
However, in 1978, this extremely moist, pressurized environment, terrific for potatoes, also resulted in a serious need for control of both ceiling condensation and storage structure.
REGARDING THE SUBERIZER AIR ENVELOPE CAVITY SYSTEM
Typical raw product storage in the 1970s was a metal-sheathed structure, insulated with urethane sprayed on the inside of the storage metal skin. A call in 1978 from an early 1970s customer suggested we take a look at a problem with his storage. Rust spots were visible through the paint on the exterior of the storage skin. It was clear the rust initiated on the inside of the skin, and had developed enough that you easily poke your finger through the metal skin, and into the urethane. The urethane was very wet. The cause of the deterioration was easily determined:
- Heat from the sun during the day slightly expanded the metal skin which caused the metal skin to separate from the urethane foam, leaving a void between the skin and foam;
- The heat also slightly “puffed-up” the urethane foam during the day, causing the foam to slightly expand;
- Each night the urethane contracted.
- This regular expansion and contraction of the urethane weakened the cell structure of the urethane and made the foam more permeable, susceptible to moisture from the pressurized storage environment.
- The pressurized storage environment, along with the temperature difference between skin and storage environment temperatures caused a vapor-drive, forcing moisture to migrate through the foam to the void between foam and skin.
- The moisture migrating through the foam picked up hydrocarbons, allowing a dilute solution of hydrochloric acid to enter the void next to the skin.
- The hot metal sheathing during the day intensified the chemical reaction in the void, rusting the skin.
Top-performing potato storage houses a very caustic, harsh environment: Determination of the above scenario immediately initiated development of the Suberizer Air Envelope Cavity System to address this very “harsh to insulation”, and “harsh to structure” situation.
The Suberizer Air Envelope Cavity System provides an all-important vapor barrier to separate the storage environment from the cavity, insulation, and structure, with provisions for complete control of condensation.
The High-performance humidification systems developed in the 1970s assured a moist potato storage environment. Realizing that potatoes are a living-breathing organism in storage, the system and storage must prevent free moisture from coating the potatoes. Wet potatoes cannot breathe. So, the requirement for a high-humidity storage environment must also prevent accumulation of free water in, and on the potato pile.
Air above the potato pile is always very close to saturation. If the surface temperature of the ceiling is the same temperature as the air above the pile, condensation will not form and drip on the pile. The Suberizer Air Envelope Cavity System separates the insulation from the storage environment, and addresses the three elements that affect ceiling temperature:
- Insulation thermal resistance;
- Convective action next to the ceiling surface; and,
- Radiant energy from the pile.
The first element considered in condensation control is insulation. Appropriate thermal resistance must be chosen for storages in the colder climates associated with Fall crop potatoes. Properly-designed insulation minimizes conductive heat loss from inside the storage. A main feature of the Suberizer Air Envelope Cavity System protects the insulation from the storage environment. Furthermore, the sandwich panel primary insulation incorporated in the system design has been chosen for long-life retention of thermal resistance.
During winter, the air on top of the pile is primarily returning to the Fanhouse. This return air, which is essentially the same temperature as the top of the potato pile, moves too slowly to spoil the “surface Film”, a stagnant layer of air adjacent a smooth ceiling surface. The Condensation Control Coating feature of the Air Envelope Design takes advantage of return air temperature by spoiling the surface film. This increase in convective action from the return air is the second element, and significantly helps the ceiling take-on Return Air Temperature.
The third element that affects ceiling temperature is equally important: Remember, the potato pile is “living and breathing”, which means there’s a “combustion process” going on in storage, and heat is constantly being radiated to the ceiling. The ability of the ceiling surface to absorb this radiant energy is referred to as “emissivity“. The Condensation Control Coating chosen for the system design to increase convective warming, has the ability to absorb virtually all the heat radiated from the pile.
The Suberizer Air Envelope Cavity System has proven to be the most cost-effective technique for control of condensation in potato storage. The system is a key feature of every Suberizer raw product storage kit.
* Bob Hesse is Director of Research and Development at Suberizer, Incorporated, a Washington State Engineering firm noted for design, manufacture and construction of raw product storages and storage system equipment.