Vacuum Pump Improves Lettuce Cooling Process

Switching from three oil-lubricated rotary vane vacuum pumps to one frequency-controlled screw vacuum pump, a lettuce grower eliminated maintenance issues and improved process time and energy consumption.

Serving discount stores and supermarket chains throughout Germany, Jörg Heekeren has grown his business, Heekeren GbR, to produce more than 10 million heads of iceberg lettuce each season, along with other lettuce varieties. To give that lettuce the longest possible shelf life without any loss of quality, he’s been relying on vacuum cooling for eight years, transferring the heads of lettuce to a vacuum chamber as soon as they are harvested.

The vacuum chamber has space for 11 pallets, which can be loaded up to almost 3 m high.The vacuum chamber has space for 11 pallets, which can be loaded up to almost 3 m high.It’s a common approach for vegetables and leafy greens, but Heekeren was having difficulties with his vacuum pumps. Given the size of the vacuum chamber—designed to hold 11 europallets stacked with lettuce crates to a height of almost 3 m—three parallel oil-lubricated rotary vane vacuum pumps were connected. The operating fluid oil in the pumps tended to cause issues, requiring the oil and filters to be changed regularly.

Heekeren has now traded out the three vacuum pumps for a single Cobra Plus screw vacuum pump from Busch Vacuum Solutions. Not only has this enabled reduced cool-ing times and energy consumption, its oil-free operation has reduced maintenance efforts.

The cooling process

Placed in crates that are stacked onto pallets, the lettuce is loaded into the chamber, where vacuum is applied. Any moisture in and on the lettuce starts to evaporate and is extracted from the chamber as water vapor. Besides quickly cooling the lettuce to about 37°F, it cools the heads from the inside out, further speeding the process. Also, less moisture is lost than in conventional air cooling. After the required cooling temperature is reached, the vacuum chamber is venti-lated, the lettuce is removed, and it’s transported to a refrigerated warehouse for interim storage.

The challenge comes when the water vapor is suctioned out of the vacuum chamber along with the air. The mixture of air and water vapor is fed from the vacuum pump through a cold trap upstream, where the air is cooled, condensing out the water vapor. This ensures that no water vapor makes its way into the downstream vacuum pump. With Heekeren’s previous setup, however, any water vapor that was not fully condensed out via the cold traps would mix with the operating fluid oil in the vacuum pumps, causing maintenance issues.

Heekeren installed the Cobra Plus at the start of the harvesting season in April 2020. The pump ran for the entire season, through the end of October, without any interruptions and requiring no maintenance work. Because the screw vacuum pump is frequency-controlled, it adapts its pumping speed to actual demand. At the beginning of the vacuum chamber’s evacuation phase, when air needs to be extracted quickly, the motor runs at a high rotation-al speed. Once the pressure in the chamber drops, the pump automatically reduces its speed, thereby reducing energy consumption. 

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