The first thing to consider is the location of the building. Is this structure on a class A road and in a location that minimizes freight into or out of the facility? In the spring of the year, when the frost begins to break in many potato storage regions and load restrictions are enforced, it is critical to have access to ship potatoes from the storage as the market or the potato quality demands. Also, is the structure easy to fill and unload? A building that is connected to an existing washing/packing/loading facility has a higher long-term value to the storage owner than remote facilities that require trans-loading to ship potatoes throughout the storage season.
The second consideration is the physical structure of the building. Are the structural members solid and in good condition? If the structure is in great shape and the building is in a good location, there is significant reason to continue considering a ventilation system upgrade. However, if the building has structural issues that will put your potatoes at risk from the elements, this brings a “proceed with caution” message.
The type and quantity of insulation in the storage unit is another critical variable that will help determine the success of your potato storage. Poorly insulated storages will result in condensation on the storage walls and ceiling surfaces in the cold storage climates of North America, resulting in rotten potatoes. Consult your local construction and insulation professionals to assure that proper insulation materials and installation methods are followed to protect your potatoes and the structure from deterioration due to condensation.
Once these questions are answered and it is determined that there is sufficient value in the structure to merit investment in remodeling the ventilation system, the next step is to evaluate your existing ventilation system.
The airflow rate in the storage is the first thing to consider. Airflow rates are expressed in cubic feet per minute (cfm) per ton or hundredweight of potatoes. North American ventilation rate recommendations range from 20 cfm/ton (1 cfm/cwt) to 40 cfm/ton (2 cfm/cwt) depending on the intended market for your crop and the local climate for cooling air available during the harvest season.
To determine the capacity of your fan, record the fan manufacturer and model number, the diameter of the propeller, the number of blades on the propeller and the motor hp and rpm along with number of fans in the storage. The fan manufacturers’ fan curve provides the capacity of a fan based on the back pressure that fan will experience. Fans are typically selected at 0.75 to 1.25 inches of static pressure depending on the equipment in a ventilation system. This fan curve will tell you the airflow of your fan in cfm at your chosen static pressure. Make sure that you use an altitude-corrected fan curve for high-altitude storage locations. Next, take this airflow value and divide it into the volume of potatoes that you put into your storage before shrink. This will let you know if your storage has enough fan capacity to properly ventilate the crop for successful storage.
After determining the fan capacity, the air duct system needs to be evaluated. Typical ventilation duct systems are made up of a main plenum where the air travels after it initially leaves the fan, a series of lateral ducts that split off the main plenum and air slots that allow the air to exit the ventilation system and enter the potato storage (either bulk or box building). Each of these components needs to be properly sized to assure that there is uniform air distribution. View this like a funnel. The plenum must be properly sized and have a cross-sectional area at least 10 percent larger than the total cross-sectional area of the lateral ducts that it supplies air into. Likewise, each lateral duct must have a cross-sectional area that is at least 10 percent greater than the slot area that is leaving the duct.
These ratios assure that as the air moves through the air system, sufficient back pressure will be generated to accomplish uniform distribution. It is critical that the air slot velocity—calculated by dividing the total airflow of the storage by the total slot area in the ventilation system—does not exceed 1,200 feet per minute to assure that ventilation air can maintain a high level of humidity as it enters the potato environment. High speed air, with a velocity greater than 1,200 feet per minute, will lose valuable water as it passes through the discharge slots and then remove water from the potatoes, resulting in weight loss and pressure bruise. When calculating the slot area, make sure that you allow for potato blockage if the slots are not “guarded.” A guarded slot is one that provides more than twice the slot area for the air to leave the slot and enter the pile without restriction.
The final step in evaluating the existing ventilation system is to assure that the fresh air intake, return air openings and exhaust louvers or fans are properly sized and located. The main ventilation fan needs the proper area to feed it air without creating high suction pressure and reducing the fan’s capacity. The area of the air feeding the fan should allow for intake velocities of 1,400 feet per minute or less when the fans are pulling full fresh air for cooling. Return air openings should be equally sized or slightly larger and assure that there is a path to the return air without restriction.
A simple way to determine if the intake and return air openings are properly sized is to see how hard it is to open the man-door on the suction side of the fan. A ventilation system with undersized intake or return air openings will require excessive force to open this door, and that is an indication that your fan is starved for air. Exhaust air openings or exhaust fans are critical to the proper ventilation of the storage. Air cannot come into a building unless there is a clear exhaust path for it to leave. Make sure that the exhaust louvers are properly sized and located to assure that this can happen.
Finally, the control system should be evaluated to determine if there is a need for taking advantage of new technology to assist the storage manager in making key decisions regarding the proper treatment of the potatoes in storage. The control system should have accurate measurement of the potato temperature, the plenum temperature and relative humidity, and for processing potatoes, the storage carbon dioxide levels. These values should be easy to access as decisions are made on the status of the crop in storage and the operation of the ventilation system. Determine the type of internet service that is available to assure that information from the storage is accessible from your storage manager’s mobile devices and computers. This will provide alert communication and remote monitoring and management of the storage ventilation equipment.
After you have determined if there are any weaknesses in the ventilation system that is in the existing storage, it is time to begin to begin designing the new ventilation system. The guidelines given above will assist you in your initial storage ventilation system analysis. Consult your local ventilation professional to assist in the design process to assure that your new ventilation system will uniformly deliver the air to your potatoes in storage.
Weather your potato storage just needs some insulation, a new fan or VFD, or a full blown redesign of the ventilation and control system, remember that continuous investment and improvement of your potato storage has a direct return to your bottom line. Once the crop is in storage, all the investments of production have been made and production cost are in full view. Maximizing sold tonnage or cwt per acre is a reflection of your storage performance, and successful storage facilities and managers make for successful potato growing operations.