| | Storage structure | Insulation | Ventilation | Refrigeration
Return to the Guide to Commercial Potato Production on the Canadian Prairies.
.Stored tubers are living organisms, which produce heat through respiration and lose moisture (shrink) through respiration and evaporation. An ideal storage environment must be provided if the tubers are to be stored up to 10 months. Tubers go through four different storage phases (curing, cooling, long-term storage and marketing), each requiring a different environment. To meet all of these requirements the potato storage must be designed to:
- Maintain tubers at a desired temperature by exhausting the heat of respiration and circulating cool fresh air through the pile.
- Maintain a high relative humidity to promote wound healing at harvest and to prevent tuber desiccation (shrink)
- Provide oxygen for tuber respiration
- Remove carbon dioxide, the by-product of respiration and other deleterious gasses, which affect tuber quality.
- Deal with adverse storage conditions where the tubers are wet, rotting, chilled, frozen or too warm.
There are four factors to consider when choosing a potato storage design:
- Style of structure
- Insulation
- Ventilation and humidification
- Options such as auxiliary heating or refrigeration.
Storage Structure
Almost any type of building can be adapted to store potatoes, however, commercial rigid frame steel buildings are not normally used because the exposed steel beams and columns are difficult to insulate. The most common storage buildings are concrete, wood stud and pole frame, and metal quonset. The factors that vary between various building types are capital cost, durability and longevity, and the type of insulation required for the exterior building envelope.
Regardless of the type of building, the design should be undertaken by a Professional Engineer to ensure that the structure can withstand the forces exerted by the stored potatoes, wind and snow. Design and construction can typically require three to four months or more; therefore early planning is required to ensure the storage is ready at harvest time.
The size of individual bins and storage systems vary widely with the needs of individual producers and the cost of construction. Bin sizes in modern buildings normally range from 40,000 to 80,000 cwt. Potatoes from individual fields often behave differently in storage, requiring individual management. Ideally, the potatoes from each field should be stored in a separate bin, but since smaller bins cost more per hundredweight, storage management is compromised when storing potatoes from different fields in large bins.
An enclosed loading area attached to the storage bin(s) is recommended. This will allow workers to comfortably load potatoes in severe weather and minimize potential problems associated with chilled tubers.
Insulation
Storages must be properly insulated and sealed in order to maintain the environment required to keep stored potatoes healthy. Besides reducing heat loss and thus helping to maintain the desired storage temperature, insulation is also critical in preventing condensation. Condensation water, dripping onto the tubers, will encourage the development of soft rots and can significantly impact potato quality. It is recommended that enough insulation be installed to achieve a minimum thermal resistance (RSI) of 6.1 (R35). This is equivalent to 10 inches (250 mm) fiberglass or 6 inches (150 mm) of polyurethane insulation. Ceiling fanshave also proved beneficial in reducing free moisture on ceiling surfaces and/or the top of the potato pile.
The amount of insulation also impacts interior air quality. Insulation decreases heat loss through the walls and ceilings, resulting in more heat of respiration exhausted from the building via the ventilation system. In extremely cold weather, this allows the ventilation system to bring additional fresh air into the building, thus maintaining adequate levels of oxygen and reducing the level of deleterious gasses such as carbon dioxide.
Ventilation
Ventilation is the most important factor for maintaining correct temperature, relative humidity, and air quality in the storage. It is also essential for managing potential storage problems caused by disease or frost. The basic ventilation system design is similar, regardless of the type of storage structure. A typical ventilation system consists of intake door(s), fan(s), air plenum(s), ducts, exhaust louvers and a control system.
Recommended ventilation rates.
 | L/s/t1 | cfm/cwt2 |
| Seed | 75. to 10 | .75 to 1.0 |
| Processing | 12.5 to 15 | 1.25 to 1.5 |
1 Litres/second per tonne of potatoes
2 cubic feet/minute per hundred weight of potatoes
A qualified individual should design the ventilation system. The size of the intake doors, air plenums, ducts, duct outlets and exhaust louvers must be carefully selected to ensure that the ventilation air is evenly distributed throughout the storage. Intake dampers should be designed to close return air supply proportionally as the intake door is opened. As more fresh air intake is required, return air carrying excessive heat or humidity will be forced out of the building through the exhaust vent(s). This can be particularly helpful when trying to remove excessive field heat at harvest. The addition of refrigeration coils, humidification units and light traps in the exhaust louver
all impact the resistance to airflow and must be considered when selecting the ventilation fans. Controllers for varying the speed and airflow of the fans are recommended. Contact a potato processor for the name of someone qualified to design the ventilation system.
Most control systems utilize a single insulated damper to control the blend of fresh and return air. A heating system, in the perimeter of the damper, is used to prevent freezing of the damper in cold weather. Heavy-duty screw type actuators are used to adjust the position of the damper.
Ventilation controllers vary in complexity, depending on the number of control strategies. The simplest strategy involves running the fans continuously. The volume of air is manually adjusted through the number of fans operating or by adjusting the speed of the fan(s). In this situation, the minimum sensors required are: the temperature control sensor that modulates the mixing damper; a low limit in the supply air to prevent accidental chilling or freezing of the tubers; and an outdoor sensor to prevent outdoor air warmer than the pile from entering the storage.
More advanced systems have carbon dioxide (CO2) sensors that will activate the damper to bring in fresh air whenever CO2 levels exceed a preset limit. Few ventilation control systems include a humidistat because they are inaccurate when the relative humidity is >90% and have been of limited value controlling humidity in a potato storage.
Under normal storage conditions, the relative humidity of the supply air should be maintained near 98 percent (see the section on Storage Cycle). Humidifiers should be installed immediately downstream from the fan(s). Three types of humidifiers are commonly used: high-pressure nozzles, centrifugal spinning disk, and water-saturated fibrous media. The first two types are used less frequently as they are difficult to regulate, resulting in either too much or not enough water added to the air streams. The third type humidifies the air as it passes through a fibrons media without the pressure of free water droplets that can effect potato quality. This type of humidification unit will create resistance to the airflow and must be sized accordingly. The design of a humidifier is critical and must be undertaken by someone with expertise in ventilation and humidification.
Heat requirements
Heat from tuber respiration must be removed from the storage to keep the potato pile at the appropriate temperature. Warm air is exhausted and a small amount of cool fresh air is blended with return air to keep the potato pile cool. The quantity of cool fresh air required for temperature control is dependent upon outside temperatures. Under normal storage conditions, no auxiliary heat is required to maintain a constant pile temperature. There are circumstances however where cool outside air in excess of that required for temperature control, is brought into the storage. This situation occurs when:
- Excess moisture must be removed from the storage.
- Moisture is released into the storage pile when tubers break down from disease or frost; or
- Outside temperatures are extremely cold. Fresh air is brought into the storage to exhaust carbon dioxide and to replenish oxygen.
In these situations supplemental heat is required to warm the incoming air. A heater capacity of approximately 7.5 kilowatts per 10,000 cwt (500 tonnes) of potatoes is normally adequate. Propane or natural gas heaters must be vented to eliminate the possibility of depleting oxygen and increasing carbon dioxide gases.
Refrigeration
Refrigeration should be available for any potatoes stored through the late spring and summer, and can be an asset when trying to remove field heat during a hot harvest season. The basic system consists of an evaporator coil, located in the return air stream, and a compressor/condenser unit located outdoors. The design of a refrigeration system is very complex and should only by undertaken by a person specializing in this area.
The very high humidity in a potato storage requires an evaporator coil with a large surface area to prevent desiccation (drying) of the air, or even worse, frosting of the coil. A minimum capacity of one ton of refrigeration per 2,000 cwt (0.4 kW per tonne) of potatoes is recommended. Note this capacity is only adequate for healthy, high quality potatoes. It may not maintain potatoes that are diseased or have broken dormancy.
Written by D. Small and K. Pahl |
|