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By John Klimes, Product Quality Specialist, Agri-Stor Companies
In the early days of potato storage, ventilation systems were designed with enough air to allow for light air exchange and application of products like CIPC. These systems were a step up from natural convection of heat rising through the pile. Natural convection piles were kept to a minimum size to allow heat to escape. With the advent of crop protection products, it became necessary to use mechanical methods of forcing air through the piles.
This mechanical ventilation allowed potatoes to be stored in larger piles, providing a more efficient use of storage space. More recent applications of the ventilation system are intended to also control climate in the storage to maintain the quality of the potatoes.
Systems should be designed to remove field heat, remove the heat of respiration, remove carbon dioxide, supply appropriate humidity, apply crop protection products, and maintain the ideal storage climate for potatoes with a balance of conditioned air to all parts of the storage.
Proper and appropriate system design is critical for all these criteria to be met.
Research in the United States and Europe has concluded that continuous low-rate ventilation with high humidity provides the best climate for potatoes and reduces risk within the storage when the system is properly designed. In contrast, systems that operate intermittently allow heat to build and allow for increased carbon dioxide. They also disrupt cooling fronts and place the storage at high risk for condensation and can increase transpiration loss. These, as well as less upfront equipment expenses, are the critical arguments for continuous ventilation. However, it also has been determined that an effort to continuously ventilate at a rate that exceeds 1,180 feet per minute can increase back pressure in a system and cause humidity to be dropped out of the air. The question then is what is the ideal ventilation rate?
With continuous ventilation, the fans run constantly, decreasing the need for excessive ventilation equipment. The amount of ventilation air needed to remove the heat of respiration for potatoes stored between 45 and 48 degrees Fahrenheit is about 8 cubic feet per minute (cfm) per ton. Ventilation needed to remove field heat requires an additional 10 cfm per ton. If you add these two together, the total need for ventilation in a continuous ventilation system is 18 cfm per ton. This will allow for a complete air exchange in a storage to occur approximately every two to three minutes when maximum fresh air is available.
Following the removal of field heat, the need for air exchange is reduced to removing the heat of respiration. To determine the exact ventilation rate, we ask the potatoes. We measure the difference in temperature from the bottom of the pile to the top (Delta T) and monitor the carbon dioxide levels. This combination is also the reason that variable frequency drives are effective in storages over time, as they allow for ventilation to be reduced or increased as needed.
To control the climate in a potato storage, the fan output is not the only determining factor.
The entire system must be designed, built and maintained to specifications. The fans are the heartbeat of the system, but the entire system must be designed to deliver the air provided in an even and balanced manner. The plenum, ducts, duct outlets, return air openings, exhaust outlets, fresh air intake, cell size, cooling coil size, and any other obstacle that the air passes through must be sized appropriately to ensure proper and even distribution of conditioned air.
Overbuilding a system for a “just in case” scenario seems like a good decision, but it can lead to logistical issues and add significant construction cost. If you build a 30 cfm building and all systems are designed correctly, what are the concerns? Yes, you can exchange air faster and you may be able to store other crops that require more air. However, one logistical concern is the ability to slow ventilation to that 8 cfm without harming motors. You could block fans like in the old days before the existence of variable frequency drives, but this leads to the next concern: when running refrigeration, a minimum velocity is needed to prevent icing. If the coils are designed for the 30 cfm system, the fans would have to be put back into service and over-ventilation can’t be avoided. And a third concern is slowing fans to appropriately apply crop protection products. Most crop protection products must be applied at a significantly lower air velocity that normal ventilation settings because the product needs to “remain” in the pile. To rush these applications with excessive air velocity can lead to uneven distribution in the building.
If you are considering building storage facilities, do a lot of homework. Look at the designs and know your goals. Ask a lot of questions. Make your salesperson squirm a bit. Dig deep to learn what you don’t already know. Decisions that you make now – during the planning process – can affect your bottom line and product marketability for the life of the structure.
