Onion Store Drying & Curing

In short

Onions coming into store have to be dried, then cured, then held in good condition until they are sold. We supply the equipment for each stage: drying plenums and fully modulating gas burners to dry and cure, motorised louvres, fans and refrigeration to hold the crop, condensation drying when the weather will not help, and Cropscan onion control to run it all.

Drying, curing and holding onions

An onion store does three jobs in turn. It dries the crop as it comes in, it cures it, and then it holds it until it leaves the store. Each stage means moving air through the crop in a controlled way, and the right equipment depends on whether you store in boxes or bulk and on the building you have.

Our three onion store layouts show how the pieces fit together, from a drying plenum and gas burner with a compressor housing, to a box store that combines a packaged ambient unit, a packaged refrigeration unit, a humidifier, a suction wall and air bags. This page explains what each piece does at each stage.

Cutaway illustration of a box onion store with its ventilation and drying equipment

Drying plenums

For drying or warming a small batch of boxed crop, our moveable drying plenum is a useful option. The single-sided unit serves 2 boxes wide by 6 boxes high and up to 9 boxes out (108 boxes). Its 2.7kW three-phase axial fan produces 12,500 cfm of air, a positive ventilation rate of 115 cfm per ton.

The double-sided version serves 2 boxes wide by 6 high and up to 10 boxes out. Its 4.4kW axial fan is frequency inverter controlled and produces up to 20,500 cfm, up to 170 cfm per ton, so the airflow can be fine-tuned to the produce being dried. Internal outlet flaps open or close in any combination for the number of boxes, and one complete side can be closed off.

From the side-mounted control panel the operator can run the fan continuously or set a number of hours, after which it stops. Built-in forklift pockets make the plenum easy to move, it is made in sections that simply locate together, and multiple plenums can be stacked. It is built from 40mm box section with 20mm plywood infill panels to survive day-to-day use.

Moveable drying plenum box for forced drying of boxed crops

Gas burners for drying and curing

Our FE ACE gas burner is a fully modulating gas heater with a 40 to 1 turn-down, made for onion drying and curing as well as grain drying. A dedicated combustion fan with a motorised inlet iris adjusts the combustion air in proportion to the gas flow, and a PID controller with autotune controls the burner on duct temperature or duct humidity.

Outputs run from 400,000 Btu/hr (117kW) to 3,500,000 Btu/hr (1,025kW). The STA range stands away from the main drying fans with its own distribution fan, for use with duplex or triplex large-diameter axial and centrifugal drying fans. Our direct-coupled modulating gas heaters can be supplied single, duplex or triplex to suit the fans.

Condensation drying, whatever the weather

Drying with outside air depends on the weather. Condensation drying does not. Moist air is drawn through the evaporator and cooled, so its moisture condenses out, and the cooled air is reheated with the heat released by the cooling process.

It runs continuously and entirely on electricity, dries the product faster, helps prevent fungal growth and reduces gas costs, which makes it even more interesting for businesses that generate their own energy. An additional module brings condensation drying to Tolsma-Grisnich indirect cooling systems, and the Dry 50 climate unit ventilates, cools, heats and dries by condensation in a single plug-and-play unit.

Holding the crop: louvres, fans and refrigeration

Once the crop is dried and cured it is held in store until it is sold. Motorised louvres give positive air control for both ambient ventilation and refrigeration. They are insulated on the reverse face to minimise cold bridging, sealed with brush strips on the blade ends and have individually sprung blades, so air leakage is minimal.

Layout 2 on our onion storage page uses a packaged ambient unit, which cools with outside air, alongside a packaged refrigeration unit for when outside air is not cold enough. Layouts 1 and 3 use a compressor housing, whose compressor and refrigeration controls link back to the Cropscan touchscreen panel, and layout 1 adds evaporator coils and evaporator fans. An ambient temperature and humidity sensor tells the controller what the outside air is doing. See our full refrigeration range.

Cropscan onion control

Cropscan scans crop temperature continuously and selects ambient ventilation or refrigeration automatically. Its drying control guards against the duct air reaching dew point, timed recirculation keeps air moving between cooling periods, and a timed CO₂ purge stops carbon dioxide building up.

The Cropscan TS7 Onion Controller has its own operator manual, free on our Manual Downloads page. Connected to the internet, a Cropscan can be viewed and adjusted from a phone or tablet app and can email you about faults. Compare the options on our store controllers page.

Questions about onion drying

What equipment do I need to dry onions in store?

It depends on the store. Our onion layouts use drying plenums and modulating gas burners to dry and cure, with motorised louvres, fans, refrigeration and Cropscan control to hold the crop. Condensation drying is an electric alternative that does not depend on the weather.

Yes. Condensation drying removes moisture by cooling the air through the evaporator and reheating it with the heat from the cooling process, so it runs continuously on electricity whatever the weather.

On duct temperature or duct humidity, by a PID controller with autotune. The burner modulates with a 40 to 1 turn-down, and a motorised inlet iris matches the combustion air to the gas flow.

Yes. Built-in forklift pockets make it easy to move, it comes in sections that locate together, and the fan cable unplugs so the upper box can be moved separately. Several plenums can be stacked.

The Cropscan TS7 Onion Controller. Its operator manual is free to download from our Manual Downloads page.

Next step: get your onion store specified

Send us your tonnage, whether you store in boxes or bulk, and your building details through our contact page, and we will work out the drying and cooling your store needs. You can compare our onion store layouts first.